WO1979000528A1

WO1979000528A1 - Internal combustion engine utilising liquefied gaseous fuel

Info

Publication number: WO1979000528A1
Application number: PCT/GB1979/000010
Authority: WO
Inventors: N Payne; T Bedford
Original assignee: T Bedford
Priority date: 1978-01-19
Filing date: 1979-01-15
Publication date: 1979-08-09
Also published as: IT1113746B; ES476954A1; IT7947711A0; EP0009023A1; NO790172L

Abstract

In an air/fuel supply system for enabling an internal combustion engine to utilise liquefied gaseous fuel efficiently, a solenoid operated fuel injector (18) injects the fuel in a liquid state into a mixing region (26) of the induction system. A control system monitors the cooling effect of the vapourising fuel by measuring the temperature difference between a pair of temperature sensors (44, 46), respectively upstream and downstream of the mixing region and compares the measured temperature difference against a preset reference level to derive an error signal. The error signal is applied to control the fuel injector and regulate the fuel supply to maintain the temperature difference and related mixture strength constant at a value set by the reference level. The reference level may be controlled by monitoring other parameters or operating characteristics e.g. intake air humidity, exhaust gas composition or firing voltage values, to determine and hold the optimum setting.

Description

INTERNAL COMBUSTION ENGINE UTILISING LIQUEFIED GASEOUS FUEL

Technical Field

This invention relates to internal combustion engines and to their air/fuel supply systems. More particularly it is concerned with means for enabling an internal combustion engine to utilise liquefied gaseous fuel in an efficient manner.

The term "liquefied gaseous fuel" is used herein to denote a combustible fuel which at atmospheric pressure exists entirely in the vapour phase at least at normal room temperature, 15.6°C (60°F), but which is liquefied by superatmospheric pressure and/or by cooling to lower temperatures.

This term "liquefied gaseous fuel" therefore includes fuels which under atmospheric pressure at 0°C are substantially below their critical pressure and critical temperature so that they can be liquefied by pressure alone. Such fuels, herein referred to as fuels of the liquefied petroleum gas (LPG) type, are commercially available as bottled gas and are generally composed predominantly or wholly of butane, propane and/or propylene.

The term "liquefied gaseous fuel", as defined above, also includes fuels known as "cryogenic fuels" which have a critical temperature substantially below 0°C so that they can only exist in the liquid phase at low sub-zero temperatures. Such cryogenic fuels include natural gas (methane) and hydrogen.

Background Art

LPG type fuels have already been used for internal combustion engines, for example in motor vehicles. The arrangements employed for this purpose, however, have usually involved evaporating the fuel from a tank in which it is stored as a liquid under pressure and delivering the fuel in a gaseous state through a supply line to a modified carburettor, and thence to the engine. The operator controls the speed of the engine by causing the carburettor to deliver more or less fuel as appropriate. Such prior art arrangements, however, are often subject to significant disadvantages including poor control of the fuel/ air ratio of the mixture passed to the engine, and tend to have a relatively low efficiency and high fuel consumption.

Proposals have also been made for adapting internal combustion engines in order to utilise cryogenic fuels of the kind mentioned above, but again the prior art arrangementshave not been entirely satisfactory and, in particular, the operation thereof has in general been subject to difficulties in maintaining a close control of the fuel/air mixture necessary to give and maintain high efficiency.

Disclosure of Invention An object of the present invention is accordingly to provide improved arrangements for adapting internal combustion engines in order to utilise liquefied gaseous fuel, either LPG type or a cryogenic fuel, which can effectively be controlled to operate at relatively high efficiency with an enhanced power output.

According to the present invention, in an internal combustion engine having an air and fuel supply system which includes inlet duct means wherein there is a mixing region in which liquefied gaseous fuel supplied from a storage tank or reservoir mixes with combustion air supplied from an air intake so as to provide a substantially homogeneous combustible fuel/air mixture which passes to the respective combustion chamber or chambers of the engine prior to ignition there is provided in combination:

(a) a fuel supply line arranged to convey the liquefied gaseous fuel in a liquid state from said tank or reservoir to nozzle means through which, in use, the liquefied gaseous fuel is introduced directly into said mixing region where it is subjected to rapid vapourisation and mixing with the combustion air accompanied by cooling of such air;

(b) sensing means arranged to sense the temperature difference between the incoming air upstream of said mixing region and the fuel/air mixture downstream of said mixing region so as thereby to monitor the cooling effect of the vapourisation and mixing of the liquefied gaseous fuel which cooling effect is related to the fuel/air ratio of the mixture, and

(c) control means arranged to respond to the temperature difference sensed by said sensing means and to operate, in use, automatically to control the relative proportions in which the fuel and air constituents of the mixture are mixed, thereby to control the fuel/air ratio of the mixture, in such manner as to establish and maintain said temperature difference at a predetermined particular value which is selectable so as to correspond to the value of said fuel/air ratio of the mixture which provides a desired level of engine performance; said sensing means and said control means together constituting a mixture control system.

Generally, in practical embodiments of the invention, the sensing means and the control means will together in effect constitute a servo control system, with the control means being adapted

(a) to relate or compare an output of the sensing means representative of the actual temperature difference sensed with a preset reference quantity representative of said particular value thereby to derive an error quantity representative of the extent to which said actual temperature difference differs from said particular value, and

(b) to utilise said error quantity through a feedback loop to provide a mixture control output effective to control operation of a metering control device which regulates the supply to said mixing region of one of the constituents of the fuel/air mixture thereby to vary the fuel/air ratio in a sense which causes said actual temperature difference to approach said particular value and so reduce or eliminate said error quantity.

Usually, the metering control device will comprise a valve operable to regulate the supply of the liquefied gaseous fuel which is injected into the mixing region through the nozzle aperture of a fuel injector nozzle device. This fuel supply valve may be disposed in the fuel supply line leading to the nozzle device but preferably it is incorporated in the nozzle device, conveniently in the form of a needle valve adapted to vary the effective size of the nozzle aperture.

The fuel supply valve will generally be associated with an actuator, for example an electrically controlled actuator such as a solenoid, so that the quantity of fuel passing through the nozzle aperture can be varied or regulated by progressive or, advantageously, intermittent periodic opening and closing of the valve. Thus, in preferred embodiments utilizing a fuel injector nozzle device having a solenoid operated needle valve, the solenoid is energised by an electrical drive signal of pulse form derived from a pulse generator so that the valve is caused to open and close intermittently in repetitive cycles of operation at a convenient .predetermined rate, 15 cycles per second for example, the pulse generator being set to provide, at a given bias, a predetermined pulse length. To regulate the supply of the liquefied gaseous fuel as required, the error quantity derived in the control system is utilised to modify or modulate this electrical drive pulse signal so as to vary the pulse length and thereby the ratio of opening time to closing time of the valve within each cycle of operation. Ideally, such pulse signal should approximate to a square wave, and it has been found that this form of control arrangement can give a very reliable and responsive positive control of the operation of the valve.

The sensing means will generally comprise a pair of temperature sensors disposed in the path of the incoming combustion air or fuel/air mixture respectively upstream and downstream of the mixing region. Such temperature sensors may be of any convenient type, for instance, mechanical, electrical such as thermocouples or thermistors, or electro-mechanical such as pneumatic/electrical or hydraulic/electrical. They should, however, be sensitive to small temperature changes, ideally changes of less than 1ºC, and they should have quick response characteristics.

When the temperature sensors are in the form of electrical transducers they are associated with means for deriving from their outputs an electrical signal representative of the difference between the temperatures sensed by each. This signal, forming the output of the sensing means, is then supplied to a comparator element of the control means wherein it is compared with an electrical reference signal of preset value constituting the reference quantity representing the temperature difference which it is desired to maintain between the sensors. The result of this comparison produces the error quantity in the form of an electrical error signal which is processed to provide the mixture control output from the control system for controlling operation of the valve of the fuel injector device or other metering control device so as to vary the fuel/air ratio of the mixture and establish or re-establish the desired temperature difference.

The inlet duct means will generally comprise a duct or conduit from the air intake which incorporates or is connected to the engine manifold leading to the combustion chamber or chambers of the engine and usually, with the control system arranged to control regulation of the fuel supply, it will contain, upstream or downstream of the mixing region, a variable throttle valve associated with remote control means for controlling the supply of the combustion air or fuel/air mixture and thus the speed of the engine.

When the variable throttle valve is operated so the supply of air is varied and, in turn, the control system responds in a manner which tends to maintain the fuel/air ratio of the mixture constant. Thus, in order to increase the engine speed, the operator opens the variable throttle valve. This allows more air to be drawn into .the inlet duct which results in a decrease in the fuel/air ratio and an increase in the temperature sensed by the sensor downstream of the mixing region. This is because the temperature difference between the two sensors is dependent both on the amount of fuel passing through the nozzle and on the amount of air flowing through the inlet duct. To maintain the sensed temperature difference constant, therefore, the control system responds to cause the quantity of fuel passed through the nozzle to be increased in proportion to the air flow increase, with a resulting increase in the fuel/ air ratio tending to restore the latter to its original value.

Such variable throttle valve is conveniently a conventional butterfly valve in the inlet duct, under the direct control of the accelerator pedal of the vehicle in the usual manner.

Although the control system can be designed to respond quickly to changes in the temperature difference following movements of the variable throttle valve, there will generally be a slight delay especially following a rapid or "snap" opening of the throttle. Moreover, following a rapid or "snap" opening of the throttle, it may be desired temporarily to enrichen the mixture to improve acceleration characteristics as with a conventional "acceleration pump". In some cases, it may therefore be desirable to provide additional means arranged to respond to rapid opening of the throttle so as to produce a transient control signal which is utilised to cause the supply of the liquefied gaseous fuel to the mixing region to be increased before sufficient time elapses for control of the fuel/air ratio of the mixture in accordance with the cooling effect of the vapourising fuel on the incoming air to be re-established.

In practice, such transient control signal can readily be provided, for example, by a signal representative of the rate of change or first order derivative

of throttle position obtained by differentiating the output of a position sensor or electrical transducer responsive to the position or setting of the throttle, or, alternativel and sometimes preferably, by a signal produced by the output of a pressure responsive transducer arranged to respond to sudden changes in pressure in the inlet duct which result from rapid opening movements of the throttle during operation of the engine. The mixing region must of course be sufficiently long for complete vapourisation and mixing of the liquid fuel introduced therein before the mixture reaches the downstream temperature sensor. At the same time, however, it is generally desirable to keep the inlet duct means as compact as possible but the extent to which this objective can be achieved is somewhat limited if the inlet duct means is in the form of a straight through duct or conduit from a single air intake opening, with the nozzle device placed along the length of the duct or conduit to inject the fuel directly into the path of the entire airstream because this airstream at this point will generally be moving very rapidly, such that the mixing region downstream of the nozzle device must be of substantial length to ensure complete vapourisation and mixing of the fuel.

In a preferred arrangement in some cases, the nozzle device may therefore be arranged to inject the fuel into the air supply conduit at a place where there is relatively still air, this being achieved by designing a portion of the inlet duct means in the form of an elongate mixing chamber having inlet apertures distributed along its length for the entry, through suitable filter means, of the combustion air from the air intake, one end of the mixing chamber being connected to the engine inlet manifold and the opposite end being closed by an end wall in or adjacent which the nozzle device is located. By this means, a better mixing efficiency with a shorter, more compact, inlet duct or conduit can be obtained, the throttle in this case being conveniently placed downstream of the nozzle, beyond the downstream temperature sensor. This arrangement may also be more convenient when converting an existing engine having a conventional carburettor, where it is desired to retain the carburettor and use the existing carburettor throttle valve so that basically the inlet duct or conduit and fuel supply nozzle device can form a unit which can be attached directly to the existing carburettor air intake. When using an LPG type fuel, the fuel supply line will generally be a pipe adapted to convey the fuel at superatmospheric pressure from a pressure vessel or tank in which it is stored so that the fuel remains in the liquid state right up to the nozzle device, and the rapid vapourisation takes place as soon as the fuel passes through the nozzle aperture by reason of the lower atmospheric or sub-atmospheric pressure in the mixing region of the inlet duct or conduit. It will be appreciated that in this instance the cooling of the intake air which occurs arises directly from the vapourising of the liquid fuel. This cooling effect incidentally assists by increasing the volumetric efficiency and is directly dependent on the quantity of fuel passing through the nozzle.

In order to ensure that the LPG fuel reaches the nozzle aperture in a liquid state even if it should become overheated and partially vapourised at some earlier point within the fuel supply line, it may sometimes be advantageous to arrange for a bight or loop of the fuel supply line immediately adjacent the nozzle device to pass through the mixing region in the path of the vapourising fuel from the nozzle device so that following fuel is pre-cooled before reaching the nozzle aperture.

When using a cryogenic fuel, this will generally be stored in its liquid phase in a thermally insulated cryogenic tank at the boiling point of the liquid relative to the pressure within the tank (usually 0 - 25 p.s.i). As heat leaks into the tank, in which the temperature will be approximately - 160°C at atmospheric pressure in the case of methane for example, the liquid fuel boils and the gas produced is vented to atmosphere via a safety valve which determines the final equilibrium pressure and so the final temperature within the tank. The fuel supply line for the liquid fuel in this case will be a heat insulated fuel pipe leading from below the level of the liquid fuel to the nozzle means which is preferably a fuel injector nozzle device incorporating a needle valve as referred to previously and which will also be heat insulated. Also, when using a cryogenic fuel, provision may need to be made for an additional bypass nozzle to be brought into use to pass extra gaseous fuel on starting an engine that has been standing for some time because, under these conditions, the relatively "hot" fuel supply pipe, which may have warmed up to ambient temperature, will be likely to evaporate a high proportion of the first rush of liquid fuel to emerge from the cryogenic fuel tank such that the main nozzle which is used during normal operation would alone be unable to pass the greatly increased volume of gas.

It will be appreciated that in arrangements as already described in accordance with the invention, the mixture control system acts basically to maintain the temperature difference between temperature sensors placed upstream and downstream of the mixing region substantially constant at a value set by the value selected for the reference signal forming the reference quantity or standard, assuming the latter is maintained constant, and this temperature difference will generally correspond to a constant fuel/air ratio so long as the characteristics of the intake air, particularly the temperature and humidity of the air passing into the intake, do not vary significantly.

However, for best results it is necessary selectively to pre-set the reference signal at a value which will provide a fuel/air ratio giving optimum performance, and under some conditions substantial changes in temperature and humidity of the intake air may occur during operation of the engine which means that to maintain optimum performance, it will be necessary to re-select and re-set the value of the reference signal.

The effect of variation of humidity can be particularly significant at relatively high inlet temperatures of the incoming air and results from the latent heat of vapourisation given up by the water vapour when it cools and condenses following the injection of and vapourisation of the liquefied gaseous fuel. If, at a certain stage, air of higher humidity is taken in, the effect will be to enrich the mixture if an attempt is then made to keep the temperature drop constant since more fuel will need to be injected to maintain such constant temperature drop.

In a further development of this invention which may be utilised if desired, especially when it is anticipated that significant variations in the humidity of the incoming air may occur during running of the engine, additional means may also be provided for finding or selecting the correct value of the reference signal which corresponds to the temperature difference required between the temperature sensors under the particular conditions of operation pertaining, and for adjusting or re-setting this when necessary, for always providing the mixture with the fuel/air ratio needed for optimum performance, this corresponding generally to the chemically correct stoichiometric value.

For example, for a given engine installation, the temperature difference required can be computed or determined empirically for different temperatures and humidities of the inlet air. This data can then be fed and stored in an electronic memory incorporated in the mixture control system, and with the provision also of means for continuously monitoring or measuring the humidity as well as the temperature of the incoming air, the measurements so obtained can be compared with the data in the electronic memory to enable the reference signal to be continually adjusted or reset, either manually through the intervention of an operator or automatically through a servo type control, so as to correspond to the correct value of the temperature difference required.

Thus in this more sophisticated arrangement the additional means is effective to enable or cause the reference signal to be selected and reset as necessary taking into account the humidity and/or temperature of the incoming combustion air from the air intake, and any changes therein. Conventional means for measuring the humidity of the air can be used, such as a so called "damp string" comprising a moisture sensitive fibre or filament of which the tension varies with humidity and which is connected to a device, conveniently an electrical transducer device, sensitive to the tension.

In an alternative arrangement adapted to enable or cause the reference signal to be selected and re-set as necessary to maintain a temperature difference corresponding under working conditions to an optimum fuel/air satio, means can be provided for monitoring the exhaust gas in respect of its carbon monoxide or water content. It has been found that, in practice, a one per cent concentration of carbon monoxide in the exhaust gas corresponds to a substantially stoichiometric fuel/air ratio of the input mixture, so that by measuring the carbon monoxide, for example with a conventional "Luft" cell, a signal representing any deviation from a one per cent concentration can be produced and utilised in a servo control arrangement to select and re-set the value of the reference signal so as to correspond to a temperature difference which reduces such deviation towards zero.

If, alternatively, it is desired to monitor the amount of water in the exhaust gas, measurements can be conveniently made by an infrared absorption device for example, and checks are made periodically to determine whether or not the percentage amount of water in the exhaust is a maximum. In general, when fuel is consumed most efficiently, as when the fuel/air ratio of the mixture is stoichiometric, the largest percentage amount of water in the exhaust is obtained. This can be determined by further providing means which will periodically vary briefly the setting of the fuel supply regulating means, such as by producing and applying a signal intentionally to vary the reference signal value in a predetermined manner, so as temporarily to change the fuel/air ratio over a certain range. Thus, in effect, a "scanning" search is carried out, and the successive measurements of the water content during this search stage will reveal the maximum value from which can be deduced or derived a signal for checking or re-setting the reference signal for a temperature difference which will correspond to the correct mixture composition.

For spark ignition engines, other possible, and perhaps preferred, additional control arrangements for overcoming the problem of possible variations in the temperature and humidity of the incoming air by automatically selecting and re-setting the reference signal to maintain it at a value corresponding to the fuel/air ratio required for optimum performance may rely for their operation upon measurements of the firing voltage characteristics in the spark ignition system.

The firing voltage is the voltage required to ionize the mixture within the spark plug gap in the combustion chamber, so making the mixture conductive and allowing current pass in the form of a spark. As soon as a spark is "struck", the voltage falls to a much lower level for the duration of the spark.

Although the firing voltage depends on a number of factors, including engine speed, compression ratio, mixture swell etc., it is also another parameter which is dependent upon the strength of the mixture or fuel/air ratio and it has been found generally to have its lowest value when the mixture is substantially stoichiometric providin optimum efficiency and is significantly higher when the mixture is either richer or weaker. Thus, with an LPG type liquefied gaseous fuel, the drop in firing voltage at a correct stoichiometric fuel/air ratio is very considerable and is sharply defined so that, in practice, it represents a very satisfactory parameter for measuring the performance and for controlling a reference signal level which determines and maintains mixture strength and engine performance. For example, under a given set of conditions, the firing voltage may typically rise from its lowest value, corresponding to the mixture having a correct stoichiometric ratio, by up to 10,000 volts or more when the engine is running on a weak mixture. It will also rise considerably if the engine is running on a rich mixture, and if the effective composition of the mixture fed to the combustion chamber varies too much from stoichiometric, in extreme cases, the ignition system becomes incapable of supplying the required voltage and the engine misfires or ceases to run. With a control system using the temperature difference sensing arrangement of the form hereinbefore described the correct value of the temperature difference which is required to be represented by the reference signal, for providing optimum performance with a substantially stoichiometric fuel/air ratio, will accordingly be the temperature difference produced in operation when the firing voltage is a minimum.

In practical embodiments, this correct value, or at least the equivalent temperature difference reference signal, can be determined automatically by providing means for periodically varying the value of the reference signal in a periodic scanning sweep or search to determine at what point the firing voltage becomes a minimum, in a manner similar to the already mentioned in relation to determining what value of reference signal gives the maximum percentage amount of water in the exhaust.

The firing voltage may be measured directly or, preferably, the times taken for building up to the firing voltage for different values of the reference signal are measured and compared with one another. The voltage build-up across the spark gap does not of course take place instantaneously but generally follows a sine wave curve and is related to the value of the firing voltage. The time taken to build up to the firing voltage can readily be measured by conventional electronic instrumentation. With a correct stoichiometric fuel/air ratio, the time taken for build-up to the firing voltage may typically be about 30 micro seconds but, in contrast, for incorrect non-stoichiometric fuel/air ratios the time for build-up may typically take about 50 micro seconds.

With this kind of arrangement, the periodic search or systematic scanning variation to determine the correct level or value of the temperature difference reference signal will generally be performed at intervals, for example, of seconds or minutes according to how often it is considered a check should be made in the particular conditions existing. After the correct level or value of the temperature difference reference signal is found in each search or scanning sweep, the actual level or value of the reference signal is adjusted if necessary, preferably automatically through a servo type control arrangement, and is locked or held constant until the next search or scanning sweep is made whereupon it is again adjusted if necessary. The periodic scanning sweeps or searches may be carried out at either regular or irregular intervals, and each may be initiated by an appropriate trigger signal. This could be arranged by a preset timer or clock set to trigger off a scanning sweep or search signal generator at the required intervals but there are of course many othe means which can be employed in practice for providin such trigger signals. For instance, they could also be provided by a circuit under the control of a manually operated switch, or even by a switch or other means responsive directly to movements of the throttle control when this is fitted for controlling engine speed.

Details of suitable electronic equipment and techniques needed will be familiar to persons skilled in the art who should have no difficulty in adapting conventional electronic equipment and circuitry for the practical realisation of the further developments indicated above. In effect, the arrangements described in connection with these developments involving an analysis of the exhaust gases or measurement of the firing voltage characteristics provide periodic checks of the fuel/air ratio in terms of the operating performance of the engine and automatically adjust and maintain the temperature difference reference signal or reference standard for the mixture control system at a value which provides the correct fuel/air ratio for optimum performance, and the problem of the effect of varying temperature and humidity of the intake air is overcome.

In adapting internal combustion engines to utilise liquefied gaseous fuel in accordance with the invention, it will be clear that many advantages arise and there are a number of factors which tend to increase power output and/or to lower fuel consumption including the following: 1. The cooling of the ingoing charge air which, by increasing the density, increases volumetric efficiency.

2. The lowering of the temperature at the commencement of combustion whereby the compression ratio can be increased so improving thermal efficiency and power output.

3. The operation of the temperature sensors in constantly monitoring, in effect, the fuel/air ratio and correcting it to keep it at any preselected optimum for any given engine and liquefied gaseous fuel.

4. A capability for increasing the specific power output so that it is possible to use a smaller engine which is cheaper, lighter, smaller and has low mechanical losses.

5. The increase in power due to (1) above is available at all engine speeds, unlike turbochargers which are usually ineffective at low engine speeds. Brief Description of Drawings

In the accompanying drawings, Figure 1 is a schematic diagram illustrating the manner in which an internal combustion engine may be adapted to utilise liquefied gaseous fuel in accordance with a preferred embodiment of the invention;

Figure 2 is an enlarged detail view showing the fuel injection and mixing means of the arrangement of Figure 1; Figure 3 is a schematic block diagram of the basic mixture control system for the arrangement of Figure 1;

Figure 3a is a detail diagram of the voltage supply arrangement for the system of Figure 3; Figure 3b is a diagram showing a modification of a part of the control system illustrated in Figure 3;

Figure 4 is a more detailed schematic circuit diagram of the position of the control system of Figure 3 to the left of the point marked "X"; Figure 5 is a more detailed schematic circuit diagram of the remaining portion of the control system of Figure 3, to the right of the point marked "X"; and

Figure 6 is a schematic block diagram showing the arrangement of additional control means, relying on firing voltage measurements for operation, which may also be provided as a modification to the basic mixture control system. Preferred Mode for Carrying Out the Invention

In the preferred embodiment which is illustrated by way of example in the drawings, a fuel tank 10 of conventional design is provided for containing LPG type liquefied gaseous fuel. A withdrawal pipe 12 reaches to the bottom of the fuel tank to ensure a liquid take off, and a hand valve 14 connects with a fuel supply pipeline 16 which is arranged to convey the fuel in a liquid state to a fuel injector nozzle device 18 via a vacuum operated lock-off valve 20 combined with a fuel filter unit. This valve 20 is conveniently arranged to be opened under the control of a vacuum pipeline 22 connected to the engine inlet manifold indicated at 24. The fuel injector nozzle device 18 incorporates a solenoid operated needle valve operable to regulate fuel flow through the aperture of a nozzle 25 which opens into an elongate mixing chamber portion 26 of an inlet duct structure 28 which leads from the air intake to the combustion chambers of the engine via the inlet manifold 24. In this embodiment the inlet duct structure 28, shown in greater detail in Figure 2, is conveniently provided by adapting a conventional type of tubular air filter and has an outer cylindrical casing 30 with a series of air inlet apertures 32 along its length. These form the air intake. The casing is lined with a standard cylindrical paper filter element 34 defining a central hollow cylindrical cavity forming in this case the elongate mixing chamber portion 26. The casing 30 is closed at one end by a cap 36 providing an end wall and it is at this position that the fuel supply nozzle device 18 is placed, as shown. It will be noted that before connection with the fuel injector nozzle device 18 the supply pipeline 16 is shown as entering the mixing chamber and has a coil 17 in the path of vapourising fuel from the nozzle. This provides some pre-cooling which can be useful to ensure that all the fuel is in a liquid state when it reaches the nozzle aperture.

At the opposite end, the casing 30 terminates in a short tubular extension 38 of reduced diameter which is connected to the engine inlet manifold 24, conveniently through a connecting section of conduit 40 which may represent part of an existing air intake system of a conventional carburettor and which includes a butterfly type valve 42 forming variable throttle valve means under the direct control of the operator or of a governor for regulating the engine intake and speed. Upstream of the mixing chamber, adjacent the air intake, there is provided a temperature sensor 44 responsive to the temperature of the incoming air, and a second temperature sensor, indicated at 46, is placed downstream of the mixing chamber in a position, conveniently adjacent the open end of the casing, where the fuel will be fully vapourised and mixed with the air.

It will be seen that with this structure, there will be a region of relatively still air at the closed end of the mixing chamber where the fuel is injected. This helps to ensure that the fuel will vapourise and mix with the air in the shortest possible length of mixing chamber, additional air entering at various places distributed along the length of the mixing chamber after at least most of the fuel has vapourised so that there is a progressive mixing.

Adjacent the throttle valve 42 on the engine side the conduit 40 is provided with a short side limb forming a housing 52 communicating with atmosphere through a small air vent 54. The housing 52 is partitioned by the diaphragm of a pressure transducer 50 which, in response to sudden variations in pressure within the conduit 40 following rapid opening movements of the throttle, provides a transient electrical signal output which is fed to the mixture control system hereinafter described wherein it can be utilised for causing the mixture strength to be enriched under these conditions before full control is reestablished in an equilibrium state by the temperature sensors.

The temperature sensors 44 and 46 are conveniently electrical transducers, either- thermocouples or thermistors, and they are incorporated in a mixturφ control system wherein their outputs are combined (at 56 in Figure 1) to provide a signal representative of difference between the temperatures sensed by each. This temperature difference signal is then compared

(at 58 in Figure 1) with a preset reference signal to derive an error signal providing the control output of the mixture control system which is applied to control operation of the valve of the fuel injector device 18 so as to regulate the fuel supply as required, in accordance with the invention.

The basic control system is shown in more detail in Figures 3 to 5 and is designed (a) to compare the temperature of the inducted air against that of the vapourised fuel/air mixture, (b) to compare the temperature difference sensed (ΔT) against a preset reference signal level to give an error signal proportional to the error in the fuel/air ratio, and (c) to convert the error signal into a fuel injector control signal (the mixture control output signal) to correct the fuel/air ratio and maintain it at a constant value under all engine loads and speeds. Assuming the engine is installed in a vehicle, the control system is powered off the vehicle battery via a filtering and voltage stabilising circuit, as shown in Figure 3a, supplying a stable "centre-tapped" voltage to the control circuits.

The temperature sensors 44 and 46 in the control system shown in Figures 3 and 4 are negative temperature co-efficient (N.T.C.) thermistors, TH1 and TH2, which are preferably of the fast response glass encapsulated bead type. The output signals from these, however, need linearising in a signal processing section of the circuit before comparison. Such linearisation is conveniently achieved by "curve fitting" the output signals to an inverse exponential function derived from the charging curve of a capacitor fed via a resistor from a fixed voltage. When suitably scaled this gives a linear output of temperature difference over a wide temperature range.

Thus, referring to Figures 3 and 4, the thermistors TH1 and TH2 are fed from preset voltages (preset to allow adjusting out any manufacturing tolerance variations) and the current output signals are sensed by low value resistors, amplified by amplifiers DA1 and DA3, and are then comparedwith a scaled down curve of the charging voltage curve of a capacitor C1 fed through resistor R1 from the stabilised voltage supply Vs. Switching element DA2 switches when a sample of the scaled down capacitor voltage (from a potentiometer P which sets the scale factor) reaches the level of the output of the thermistor amplifier DA1 and passes a pulse which triggers a monostable element M of which the output, acting through transistor TE, discharges and resets the capacitor C1. Switching element DH4 triggers when the capacitor charging voltage curve passes the output level of thermistor amplifier DA3 and resets when DA2 triggers the capacitor reset monostable M.

The initial temperature difference signal, provided by the resulting output from DA4 is in the form of a variable width pulse having a duration or pulse width "t" linearly proportional to the temperature difference between the two thermistors, ΔT°C. This pulse signal is then converted into a dc voltage V. of which the amplitude is a linear function of the pulse width and thus of the sensor temperature difference ΔT°C. This is carried out in the pulse converter element of the circuit which includes transistors TE1 and TE2 controlling the charging of a high value capacitor C2.

The voltage output signal V_t is then passed to a comparator element wherein it is compared with a preset voltage reference signal V_r selected by a potentiometer P which sets the required temperature difference. This comparator element comprises a unity gain amplifier DA5 associated with preset voltage controls providing adjustable lead and lag characteristics to enable the loop transfer function to be controlled and adapted for response and stability.

The resultant output is an error voltage signal forming an output control signal which is applied to a proportional comparator (proportional control amplifier) DA6. DA6 is also fed with a triangular or "sawtooth" waveform from a waveform generator and produces a square wave pulse output which has a fixed repetition rate and which provides the electrical drive signal for operating, via a switching circuit, the solenoid controlled needle valve of the fuel injector nozzle device 18.

The effect of the error signal is to vary, in proportion to its value, the width or length of the pulses produced, the pulse width being variable from zero to full maximum duration over a narrow control band or narrow range of error signal.

The output drive signal operates repetitively to open and close the fuel regulating needle valve, and is in effect modulated by the error signal to vary the pulse width and thereby the ratio of opening time to closing time of the valve in each cycle of operation, so regulating the fuel flow as required. It will be noted that the injector needle valve solenoid is powered directly by the battery voltage V_B via a ballast resistor R_B, thereby safeguarding the stabilised voltage supply lines from switching transients. As indicated in Figure 3b, instead of the thermistors TH1 and TH2, thermocouples TC1 and TC2 can be used as the temperature sensors in a differential pair configuration. In this case, a pre-amplifier (head amplifier) situated close to the mixing chamber should be used, the output of this then being further amplified by a "buffer amplifier" to provide the voltage output signal V_t which is compared with the voltage reference signal V_r. Such thermocouple sensors must have a high value of the ratio of sensing area to thermal mass for ensuring a fast response.

The output of the pressure transducer 50 after a "snap" opening of the throttle valve can be applied, as a transient additional control voltage, to temporarily enrich the mixture by causing it temporarily to alter appropriately the pulse width of the output drive signal, either by arranging for it to temporarily to reset the reference voltage level V_r or by arranging for it to change the amplitude of the "sawtooth" waveform fed to the proportional comparator.

If, for speeding up the effective response of the system to rapid throttle operation, there is provided, as an alternative to the. pressure transducer 50, a throttle position sensor associated with means for differentiating the output to derive a signal representative of the rate of change of throttle position, this signal, at least when it exceeds a predetermined value, may also be applied to control either the reference voltage level V_r or the "sawtooth" waveform amplitude thereby temporarily to vary the pulse width of the output drive signal. The response of the control system can similarly be speeded up for sudden changes in other engine operating parameters by feeding additional control signals representative of such parameters to the waveform generator to vary the amplitude of the "sawtooth" waveform, hence varying directly the pulse width of the output drive signal and thus augmenting the effect of the error signal.

The reference voltage V can also be controlled or modified, if required, by other variable engine parameters or control functions providing appropriate additional control signals, thereby to control or modify the particular value of temperature difference maintained between the temperature sensors and the overall operating characteristics of the system. In the specific embodiment described, such additional control signals could for example be arranged to drive a reversible electric motor forming a reference control controlling the setting of the potentiometer P_r.

Thus, if required it can be arranged for the reference voltage to be controlled by a "cruise economy" function effective to adjust or reset its value to weaken the mixture under light load conditions. Or it can be controlled by quantities giving optimisation of mixture strength for different angles of ignition advance, throttle openings/engine speed combinations, engine temperature, or climate e.g. providing correction factors for changes in humidity and/or temperature of the incoming air, or for otherwise achieving optimum fuel/air ratios and performance under variable conditions of operation.

In particular, as has already been indicated, the functioning of the system to give the correct fuel/air ratio for optimum or near optimum performance under variable conditions can be checked by monitoring the water content of the exhaust gases or by monitoring the firing voltage characteristics in a spark ignition engine and by deliberately varying the fuel/air ratio periodically in a scanning sweep or search to determine, in a short duration sampling test period, whether or not these parameters have maximum and minimum values respectively. The results can then be utilised to select and adjust or reset the reference voltage when required.

By way of example, a manner in which such additional means of control may be incorporated in the embodiment described is illustrated in block diagram form in Figure 6. As shown, the arrangement relates to use of measurements of the firing voltage (or build-up times thereof) but it would be basically similar when designed to relate to the alternative use of measurements of percentage water content in the exhaust gases.

In the arrangement of Figure 6, to monitor the firing voltage characteristics a voltage receiver is provided which receives the voltage fed in the ignition system through the high tension lead to the distributor or to a spark plug. The voltage and current waveforms at least during the ignition voltage build-up to the point of discharge can be analysed and the signals produced representing the firing voltage or firing voltage build-up time are processed to provide data which is entered in an electronic memory in the signal processor, at least at the commencement and during each scanning sweep or search.

The scanning sweeps or searches providing intermittent sampling are controlled by the sampling programmer in which trigger signals initiate at intervals the generation of sweep voltage signals of alternating form which are fed as control signals to a reference control, for example a reversible electric motor as previously mentioned, which controls the element that provides the reference voltage, namely, the potentiometer

P_r which is denoted in the block diagram as

"Reference Store".

During each scanning sweep or search, the signal processor is activated by the sampling programmer so that it not only registers the successive changes in the firing voltage but also analyses the measurements to determine the minimum value thereof and compares this with the existing value at the commencement of the sweep or search. Any difference produces a corresponding reference error voltage signal output which is fed to the reference control which drives the potentiometer P and adjusts or resets the reference voltage to cause the mixture strength to be corrected accordingly. Each new setting of the reference voltage will then be held until the next sampling test period.

If additional control signals representing other parameters or operating characteristics are also fed to the "Reference Control", if these are for the purpose of causing the mixture strength to be maintained at a value which is near to but not at its optimum or stoichiometric value for maximum power output, as with a "cruise economy" function for example as previously referred to, means will of course be provided for cancelling or artificially negating such additional control signals, or their effects, during each sampling test period to enable the correct reference signal voltage necessary for minimum firing voltage (or maximum exhaust water content) to be established.

It will be appreciated that much of the circuitry involved in these further modifications and developments, representing refinements of the basic temperature control arrangement will utilise integrated circuits and may well be incorporated in a suitably designed microprocessor unit.

Industrial Applicability

The invention is particularly suitable for spark ignition internal combustion engines, including conventional spark-ignition petrol engines, Wankel rotary engines and gas turbines, which can all be arranged in accordance with the invention to utilise any of the liquefied gaseous fuels hereinbefore mentioned.

If required, such spark-ignition internal combustion engines adapted to utilise liquefied gaseous fuel in accordance with the invention can also be arranged to have a dual fuel supply where this is advisable, as for example in the case of generator engines performing a vital function or engines of vehicles which may be used in places where a gas fuel supply may not be available. In a typical arrangement of dual fuel supply system, an original or existing petrol carburettor of a spark-ignition petrol engine is fed via a solenoid operated valve in the petrol supply line and the usual butterfly valve in the carburettor is used to control the air flow when operating on the liquefied gaseous fuel. The mixture supply pipe downstream of the carburettor will provide the mixing region for the liquefied gaseous fuel, and when the liquefied gaseous fuel supply system is to be used the solenoid valve in the petrol supply line is closed, sufficient time being allowed for the float chamber of the carburettor to empty itself before the liquid gas fuel supply system is switched on. A similar solenoid operated valve is fitted in the liquid gas fuel supply line and a three- way control switch is preferably used which enables only one solenoid to be activated at any one time and which provides an "off" position to allow either system to empty itself before switching to the other.

For so-called diesel or compressionignition engines the invention will generally best be applied by replacing the fuel oil injectors with spark plugs and using spark ignition.

Thus, the invention is well suited to the conversion of compression-ignition engines, as in motor lorries or heavy commercial vehicles for example, to run as a spark-ignition engine on liquefied gaseous fuel such as LPG type "bottled gas". Such a conversion involves placing, in the air supply pipe to the inlet manifold, the variable throttle valve linked to the driver's accelerator pedal, the temperature sensors and the nozzle device, connecting the nozzle device to a gas cylinder by means of the fuel supply pipe which should of course be connected to the bottom of the gas cylinder so that it receives liquid fuel, and converting to a spark-ignition engine by replacing the injectors by spark plugs connected to an ignition circuit. The spark plugs may be inserted in the tapped bores initially occupied by the injectors although if the spark plugs are not of the correct size initially, it may be necessary to re-tap these bores. No modification need be made to the compression ratio of the engine. It is noteworthy that in conventional diesel engines, the power output is usually limited by the amount of smoke in the exhaust gas becoming unacceptable. This stage is generally reached when only about 60% of the oxygen in the ingoing charge air is burnt with the fuel, and it has accordingly been proposed to use up this "spare" oxygen and obtain additional power by mixing propane or butane with the ingoing charge air. A modification of the engine in accordance with the present invention, however, may be used with greater effect to obtain increased efficiency and power output, in which respect the cooling effect on the ingoing charge air is especially beneficial.

As an alternative to converting a diesel engine into a spark ignition engine in order to adapt it to utilise a liquefied gaseous fuel in accordance with the invention, in some cases where a fuel such as liquefied natural gas or methane is to be used, mixed in approximately stoichiometric proportion with the ingoing charge air, the normal or part-modified existing diesel injection system may be retained if it is arranged to inject a small shot of diesel fuel as an ignition shot to initiate each power stroke. Since the temperature near the end of the compression stroke in this arrangement will be less than the self-ignition temperature of methane but above that of diesel fuel oil, in effect the combustion is controlled by the injection of the fuel oil. The engine can also be regarded as functioning as a dual fuel engine.

Claims

1. An internal combustion engine having an air and fuel supply system which includes inlet duct means wherein there is a mixing region in which liquefied gaseous fuel supplied from a storage tank or reservoir mixes with combustion air supplied from an air intake so as to provide a substantially homogeneous combustible fuel/air mixture which passes to the respective combustion chamber or chambers of the engine prior to ignition characterised in that there is provided in combination:

(a) a fuel supply line arranged to convey the liquefied gaseous fuel in a liquid state from said tank or reservoir to nozzle means through which, in use, the liquefied gaseous fuel is introduced directly into said mixing region where it is subjected to rapid vapourisation and mixing with the combustion air accompanied by cooling of such air; (b) sensing means arranged to sense the temperature difference between the incoming air upstream of said mixing region and the fuel/air mixture downstream of said mixing region so as thereby to monitor the cooling effect of the vapourisation and mixing of the liquefied gaseous fuel which cooling effect is related to the fuel/air ratio of the mixture, and

2. An internal combustion engine as claimed in Claim 1 further characterised in that the control means is adapted to relate or compare an output of the sensing means representative of the actual temperature difference sensed with a preset reference quantity representative of said particular value thereby to derive an error quantity representative of the extent to which said actual temperature difference differs from said particular value, and to utilise said error quantity through a feedback loop to provide a mixture control output effective to control operation of a metering control device which regulates the supply to said mixing region of one of the constituents of the fuel/air mixture thereby to vary the fuel/air ratio in a sense which causes said actual temperature difference to approach said particular value and so reduce or eliminate said error quantity.

3. An internal combustion engine as claimed in Claim 2 further characterised in that the metering control device comprises valve means operable to regulate the supply of the liquefied gaseous fuel through said nozzle means to the mixing region of said inlet duct means.

4. An internal combustion engine as claimed in Claim 3 further characterised in that said valve means comprises a needle valve incorporated in a fuel injector nozzle device which provides said nozzle means, said nozzle device having a nozzle aperture opening into said mixing region.

5. An internal combustion engine as claimed in Claim 3, further characterised in that said valve means is associated with an actuator and with means for generating a drive signal of pulse form effective to energise said actuator and cause the valve means to open and close intermittently in repetitive cycles of operation, and in order to control operation of the metering control device means are provided in the mixture control system for applying the error quantity to modify or modulate and control said drive signal so as to vary the ratio of opening time to closing time of the valve means to regulate the supply of the liquefied gaseous fuel as required.

6. An internal combustion engine as claimed in Claim 5, further characterised in that the actuator and valve means comprise a solenoid operated needle valve incorporated in a fuel injector nozzle device which provides said nozzle means, said nozzle device having a nozzle aperture which opens into the mixing region.

7. An internal combustion engine as claimed in Claim 3, further characterised in that the mixing region is provided by a portion of the inlet duct means which is in the form of an elongate mixing chamber having provision for the entry through filter means of the combustion air from the air intake at various places distributed along its length, one end of the mixing chamber being connected to the engine inlet manifold leading to the combustion chamber or chambers and the opposite end being closed by an end wall in or adjacent which said nozzle means is located whereby the liquefied gaseous fuel introduced through the latter mixes progressively with incoming combustion air along the length of said mixing chamber.

8. An internal combustion engine as claimed in Claim 3 in which the inlet duct means contains variable throttle valve means for controlling the supply of the combustion air or fuel/air mixture and thus the speed of the engine, further characterised in that there is provided means arranged to respond to rapid opening of said throttle valve means so as to produce a transient additional control signal effective temporarily to increase the supply of the liquefied gaseous fuel to the mixing region, before sufficient time elapses for control of the fuel/air ratio of the mixture in accordance with said temperature difference to be re-established.

9. An internal combustion engine as claimed in Claim 8, further characterised in that said valve means of the metering control device is associated with an actuator and with means for generating a drive signal of pulse form effective to energise said actuator and cause the valve means to open and close intermittently in repetitive cycles of operation, and in order to control operation of the metering control device means are provided in the mixture control system for applying the error quantity to modify or modulate and control said drive signal so as to vary the ratio of opening time to closing time of the valve means to regulate the supply of the liquefied gaseous fuel as required, and circuit connections are provided for applying said transient additional control signal either to the drive signal generating means so as to control the drive signal directly or to reference control means effective to vary the preset reference quantity so as to control the drive signal indirectly.

10. An internal combustion engine as claimed in Claim 2 further characterised in that the sensing means comprises: (a), a pair of temperature sensors in the form of electrical transducers disposed in the path of the incoming combustion air or fuel/air mixture respectively upstream and downstream of the mixing region and (b), means, associated with said transducers, for deriving from their outputs an electrical signal representative of the difference between the temperatures sensed by each, which signal, constituting the output of the sensing means, is fed to a comparator element of the control means wherein it is compared with an electrical reference signal to derive the error quantity in the form of an electrical error signal which is utilised to provide the control output controlling operation of the metering control device.

11. An internal combustion engine as claimed in any one of Claims 2 to 10, further characterised in that the mixture control system includes reference control means adapted to operate following an alteration in the humidity and/or temperature of the incoming combustion air from the air intake so as to adjust or reset the reference quantity to represent a new particular value of said temperature difference effective still to maintain or restore the fuel/air ratio and level of engine performance at substantially the same value as existed before said alteration in the humidity and/or temperature of the incoming combustion air.

12. An internal combustion engine as claimed in Claim 11, further characterised in that means are provided for monitoring the values of the humidity and/or temperature of the incoming combustion air and for comparing the measurements obtained with reference data contained in data storage means so as to derive an output which controls operation of the reference control means.

13. An internal combustion engine as claimed in any one of Claims 2 to 10, further characterised in that the mixture control system also includes reference control means for controlling the setting of the reference quantity and analyser means for monitoring and analysing, at least at intervals, a second operational parameter of the engine, otϊier than the cooling effect of the vapourisation and mixing of the liquefied gaseous fuel, which is dependent on the mixture strength and which also has a known characteristic value or quality when the fuel/air ratio is stoichiometric or at an optimum value, said analysis being effective to check whether said characteristic value or quality is present and, if not, to derive a corresponding reference error quantity which is applied to said reference control means so as to cause the reference quantity to be reset and corrected thereby to change the fuel/air ratio and bring or restore it to its stoichiometric or optimum value.

14. An internal combustion engine as claimed in Claim 13 further characterised in that the second operational parameter which the analyser means is arranged to monitor and analyse is a quantity having a value which varies with mixture strength and attains a maximum or minimum when the fuel/air ratio is stoichiometric or at an optimum value, and for ascertaining said maximum or minimum value in carrying out said analysis means are provided for deliberately varying the mixture strength in a scanning sweep or search performed during intermittent relatively short duration sampling test periods.

15. An internal combustion engine as claimed in Claim 14 further characterised in that the second operational parameter which the analyser means is arranged to monitor and analyse is the percentage amount of water in the exhaust gases.

16. An internal combustion engine as claimed in Claim 14 and provided with a spark ignition system, further characterised in that the second operational parameter which the analyser means is arranged to monitor and analyse is the firing voltage or build-up time thereof.