GB2530003A - Internal combustion engine fuel injection system - Google Patents

Abstract

A method for improving the combustion efficiency of an internal combustion engine using an add-on fuel system by adding a quantity of higher octane fuel to a lower octane fuel comprises the steps of determining the maximum proportion of higher octane fuel calibrated by the onset of engine knock for a given engine, measuring the quantity of the lower octane fuel for each engine cycle, computing a proportional and consistent quantity of the higher octane fuel according to the calibrated proportion value and providing an injector signal to supply the higher octane fuel in the correct proportion for the current engine cycle or the following engine cycle. The lower octane fuel may be diesel and the higher octane fuel may be petrol, liquefied petroleum gas (LPG), compressed natural gas (CNG), liquid natural gas (LNG), methane or hydrogen.

Description

DESCRIPTION

INTERNAL COMBUSTION ENGINE FUEL INJECTION SYSTEM

This invention relates to improvements in and relating to fuel injection of combustion engines, and in particular, to improvements in the combustion efficiency of petrol and diesel fuelled internal combustion engines.

Conventional internal combustion engines comprise a piston that reciprocates within a cylinder and a crank mechanism for converting the reciprocating movement of the piston into a rotational output for useful work. The operation and efficiency of an internal combustion engine depends on a great number of factors, including the type and mixture of fuel used, the compression ratio, the dimensions of the piston/cylinder, etc. The speed and completeness of the burn is one of the main factors that determine the overall efficiency of the engine. Combustion occurs during the power' stroke of the piston which has a fixed length and profile. For 100% combustion efficiency, all the fuel would be combusted homogenously producing only water and carbon dioxide. In practice this is a chain' reaction initiated by a spark (spark ignition engine) or heat generated with compression (compression ignition engine) which may require the aid of a glowplug'.

Hydrocarbon fuels such as diesel have a molecular structuie which is long, complex and slow to combust. These long hydrocarbons have a tendency to tangle and/or accumulate together, preventing efficient mixing of the fuel with the air or oxygen. The diesel fuel, being burnt in an enclosed chamber that is externally cooled, will ignite towards the centre of the chamber and the burn will progress outward at a slow rate. In the case of diesel engines this is largely responsible for smoke and particulate matter issuing from the exhaust system of the engine.

Two factors that affect the proportion of available fuel that can be burnt include: (i) The molecular chain length of the fuel itself -the longer the chain length, the less likely it is that complete combustion will occur in a given time frame; (U) The dimensions of the cylinder -the larger the volume, the longer it will take for the "flame front" to reach the boundaries of the cylinder, which for large dimensions or slow "flame fronts" may never occur.

The amount of fuel and time for combustion is limited according to engine load and piston speed (RPM) within an enclosed cylinder resulting in hot' and cold' regions for the chain reaction to progress, particularly around the cooled cylinder walls. In regions where there is no hydrocarbon for the oxygen to react, the oxygen may react with nitrogen to produce NOX. In areas deficient of oxygen incomplete combustion will occur resulting in CO. In cool' areas where the chain reaction has stopped there will be un-combusted fuel and air entering the exhaust phase. Similarly this also applies to rotary engines such as Wankel' and turbine engines. For a turbine the combustion time is limited between fuel ignition at the first expander stage and the time for the fluids to exit the last expander stage.

The best case for combustion efficiency would be for example, if pure hydrogen were combusted with oxygen alone. The worst case is where the fuel has long chain molecules combusted in air which is predominantly composed of nitrogen.

The objective of the present invention is to convert an engine for dual fuel operation in order to ensure a more complete burn of hydrocarbon fuel by introducing a precise amount of a secondary higher octane fuel as an additive to act as an accelerant for the combustion.

The anti-knock properties of the fuel additive allow the engine to adjust the injection timing which gives an Atkinson cycle' effect for the power stroke and thus more time to combust the long chain molecules of the primary lower octane fuel. This results in a reduction in fuel consumption while at the same time improving the emissions standard of the engine.

LPG and Petrol are two such readily available high octane fuels that can be used as an additive or secondary fuel(s) in order to improve the combustion of a lower octane primary fuel such as diesel. Universally prior art has sought to maximize the quantity of LPG/CNG gas introduced for example: petrol engines are typically converted to be fuelled using 100% gas; diesel engines are converted to be fuelled typically in the range 20-99% gas. This might be done because gas is frequently a less expensive form of fuel. In diesel engines the diesel is injected in a series of pulses often described as Pilot Pulse, Main Pulse and End pulse -often three or more pulses may be used. However improvements in diesel injection technology use a Pilot injection pulse that is often advanced more than 20 degrees TDC (top dead centre) in order to improve the efficiency and emissions of the diesel engine.

When replacing the diesel with significant amount of high octane fuels such as LPG/CNG/LNG/H and where combustion is initiated at these timings of TDC advance this will in itself cause the engine to knock unless the additive fuel is also injected as a multi-burst sequence as per the diesel fuel. For example in a gas or petrol engines the ignition spark is timed typically between 10 -20 degrees TDC advance; increasing this advance can cause engine knock. Thus the level of high octane fuel that can be injected as an additive gas for diesel engines limited to less than 15% because the pilot pulse injection timing is so advanced that knock would occur.

With the present invention precise quantities of a secondary high octane-rated fuel are introduced as a fuel additive using an Injector Control Unit (ICU), whereby the quantity of primary fuel injected into a combustion chamber of the engine during a combustion cycle is measured, and a consistent proportional quantity of a second or additive fuel or fuels having a higher octane rating is supplied to the combustion chamber prior to combustion.

Providing the two fuels are delivered into the engine as a single combined fuel with consistent fuel properties for each engine cycle, the engine may adapt to these new fuel properties. For engines fitted with a knock' sensor the Engine Control Unit (ECU) may modify the injection timing to improve the combustion because the combined fuel properties have an increased Research Octane Number (RON). Such additive fuels include, but are not limited to: liquefied petroleum gas (LPG), natural gas in CNG or LNG form, Browns Gas, methane, methanol, ethanol, hydrogen; and may comprise two or more different fuels of different molecularstructures. In particular, although this invention relates to gas injection of the additive fuel, a liquid injection system may also be used.

According to a first aspect of the present invention: The amount by mass of the additive fuel injected is typically a proportion lying between a lower threshold value corresponding to a relative proportion of the second fuel that is just sufficient to significantly improve the flame front speed, and an upper threshold value corresponding to a relative proportion whereby the engine begins to knock or oxygen available for combustion of the first fuel is depleted and decreases the overall efficiency of the engine. Only a small amount of additive is typically needed to improve the combustion of the primary fuel. The flame front velocity is higher for shorter length molecules and increases the overall flame front velocity forthe dual fuel overall thus accelerating the combustion process of the combined fuels. The combustion is faster, allowing more time for combustion even in the cold' regions during the power stroke. Much of the fuel which would normally remain un-burnt and pass to the exhaust without gas injection would be burnt during the power stroke with gas injection. The HC and particulate matter emissions are significantly reduced. Accordingly, the combustion efficiency of the engine is improved because more of the available fuel for combustion is combusted during the power stroke.

According to a second aspect of the present invention where a controlled proportional quantity of a second fuel or fuels is injected prior to and/or with the primary fuel injection: A common property for the short chained molecules including hydrogen is they have a high Research Octane Number. Although hydrogen (RON> 130) does not fit well into the normal definitions of octane number, it has low knock resistance in practice due to its low ignition energy (primarily due to its low dissociation energy) and extremely high flame speed. These traits are highly desirable in rocket engines, but undesirable in Otto cycle engines. However, as a second fuel hydrogen raises overall knock resistance as do the other secondary fuels for example; methane (RON 120), methanol/ethanol (RON 109), ethane (RON 108), propane/butane (RON 112). The anti-knock properties of the combined fuel are increased.

The higher octane number of the combined fuel, the more compression this fuel can withstand before detonating. Accordingly, the engine efficiency is improved because fuels with a higher octane rating can be used in high-compression Otto cycle engines that generally have better performance both in terms of power and economy.

According to a third aspect of the present invention where a controlled proportional quantityofa second fuelorfuels is injected priorto and/orwith the primaryfuel injection: For Otto cycle engines equipped with a knock sensor the ECU will retard the ignition timing when detonation is detected. Retarding the ignition timing reduces the tendency of the fuel-air mixture to detonate, but also reduces power output and fuel efficiency because the power stroke is effectively shortened. However the anti-knock properties of the combined fuel are increased and allow the ignition timing to be further advanced thus allowing more time for combustion and effectively extending the power stroke (Atkinson cycle). Accordingly, the engine efficiency is improved because when fuels with a higher octane rating are used the ignition timing can be further advanced providing better performance both in terms of power and economy.

According to a forth aspect of the present invention where a controlled proportional quantity of a second fuel or fuels is injected prior to and/or with the primary fuel injection: Diesel has a very low octane (RON 20) and is more typically described by the Cetane number which reflects the fuels ability to auto-ignite. A diesel engine uses a high compression ratio to allow auto-ignition of the diesel fuel and is therefore not designed to combust lighter LPG or other small molecule fuels alone. For Diesel cycle engines equipped with a knock sensor the ECU will retard the diesel injection timing when detonation is detected. The ignition is controlled by an engine ECU using a high pressure injection system where the primary fuel is directly injected into the cylinder towards the end of the compression stroke. The injection typically is in a series of short pulses timed to minimise or eliminate engine knock and to maximise the combustion time for the power stroke. The anti-knock properties of the combined fuel are significantly increased and allow the injection timing to be significantly advanced thus allowing more time for combustion and effectively extending the power stroke (Atkinson cycle). The initial auto-ignition of the primary fuel into the chamber containing a non-stoichometric amount of compressed secondary fuel and air is not affected.

Combustion of the gas uses much of the available oxygen which is already diminished by introducing the gas into the combustion chamber. By increasing the quantity of gas injected beyond a upper threshold value, the TDC advance of the Pilot pulse either may cause knock from the introduced additive fuel and/or the oxygen available for burning the diesel being diminished. Therefore the improved efficiency peaks at a relatively low level of introduced gas. This peak will depend on the engine design, aspiration and load/speed. Accordingly, the engine efficiency is improved because when fuels with a higher octane rating are used the injection timing of the Main pulse(s) can be further advanced providing better performance both in terms of power and economy.

The improved efficiency is seen by the management system ECU as a reduced load on the engine and will adjust the amount of diesel injected. Introduction of the additive fuel may be by injection into the combustion chamber, during the induction stroke, compression stroke or combustion stroke of the engine. The amount injected may be determined on the basis of the amount of first fuel injected in a combustion cycle of the engine, ideally a current or immediately preceding cycle. In order for the ECU to adapt to the new fuelling the ratio of added fuel/primary fuel must be constant even when the engine is accelerated/decelerated; therefore a near instantaneous measurement of primary fuel flow is required or at least a measurement made before each engine cycle. For these reasons the additive fuel must be injected directly or close to the inlet valve. The primary fuel flow can be measured using engine inputs (air/fuel) and/or engine outputs (exhaust). Two methods (12) are provided to determine the additive fuel amount to inject: Method (1) Fuel Flow Measurement -Primari Fuel The amount of fuel injected is a function of the fuel pressure and the time the injector is open or injecting: Flow = Time X./Pressure For engines where the fuel pressure is fixed either by a constant pressure fuel pump or by a mechanical arrangement such as cam acting on a spring pressurizing an injector reservoir then the fuel flowrate is directly proportional to the injector open time.

For engines where the fuel pressure is varied such as in a common rail injection systems the fuel flow is primarily govemed by the fuel pressure where the injector is operating at optimum timing. As the engine is load is increased or decreased the fuel pressure from the pump does not respond instantaneously so the difference or error is compensated varying the open time of the injector by the ECU; i.e. the injector open time varies as the load is changed and returns to the optimum timing. Therefore to calculate fuel flow which responds to changes in engine load both the injector open time and the fuel pressure need to be measured.

The use of multiple injection events has been adopted by many manufacturers of common rail engines and as a consequence the fuel injection events are considerably shorter than was the case with engines equipped with older distributor pump technology. Thus it has become problematic to use a digital processor (microprocessor) with sufficient processing capacity to be used as a second ECU in order to be able to intercept the injector signal and process this signal in real time.

The invention discloses an Injection Control Unit (ICU) which measures the injector on' time and fuel pressure (if required), uses an analogue computer electronic circuit which calculates the primary fuel flow and an electronic circuit to provide signals to actuate gas or liquid additive fuel injector(s) in an appropriate timing pattern. The ICU system can work on all electronic controlled fuel injected engines. An advantage of the method is that it can accommodate multi-burst firing of the injector control signals.

Method (2 Fuel Flow Measurement -Exhaust Gas The exhaust gases comprise of air and combustion products such as H20/C02. Other combustion products such as particulate matter (PM)/NOXICO may also be present and are an indication of inefficient combustion. However the total mass of exhaust products is equal to the total mass of input matter (air and fuel). Thus an approximation can be made for the amount of diesel injected by measuring the mass flow of the engine exhaust. In order for the ECU to adapt to the new engine fuelling, the amount of gas additive needs to be determined for each engine cycle according to a primary fuel flowrate measurement. This can be accomplished on a cycle by cycle basis providing the measurement transducer has a fast response and is situated close to the exhaust ports, preferably on the exhaust manifold of the engine. Preferably a pressure transducer is used to provide an exhaust flow measurement using the differential pressure principle across the exhaust system from exhaust manifold to the outlet which is at atmospheric pressure. This peak signal provides the primary fuel flow measurement to calculate the amount of high octane additive fuel to inject for each cycle.

For an engine fitted with a diesel particulate filter (DPF) it may be possible to use a DPF pressure measurement directly to provide the peak signal from an exhaust cycle of a cylinder.

This peak requires measurement before an induction stroke begins as may be the case for low speed engines. For high speed engines the pressure transducer must be located closerto the exhaust port to ensure the peak is measured before an induction cycle begins. The peak pressure measurement assumes a constant pneumatic resistance across the exhaust system. The DPF pressure differential builds up over time as the pneumatic resistance increases due to the trapped PM's. Therefore the differential DPF signal can be subtracted from the absolute exhaust pressure measurement to compensate. A similar compensation may be required for systems using Exhaust Gas Re-circulation (EGR), selective reduction catalyst (3CR) or in systems where the turbo charger is fitted with variable vane technology.

Preferred embodiments of the invention shall now be described, by way of example only, with reference to the accompanying drawings in which: (i.) Figure lisa graphical schematic which outlines the envelope forthe additive fuel injection with reduction for higher engine loads; (ii.) Figure 2 is a graphical schematic which shows 5%, 10% and 15% by mass of the additive fuel injection with reduction for higher engine loads; (iii.) Figure 3 is a graphical schematic which outlines the envelope for additive fuel injection; (iv.) Figure 4 is a graphical schematic which shows 5%, 10% and 15% by mass of the additive fuel injection; (v.) Figure 5 is a typical Pulse Width Modulated (PWM) Injector signal; (vi.) Figure 6 is a schematic circuit diagram for injector signal conditioning.

(vii.) Figure 7 is a typical injector signal after appropriate signal conditioning.

(vih.) Figure 8 is a signal conditioned PWM injector signal that shows a digital potentiometer output for a 3 msec PWM is on time.

(ix.) Figure 9 is a signal conditioned PWM injector signal that shows a digital potentiometer output for a 4.5 msec PWM is on' time.

(x.) Figure 10 is a schematic electronic circuit for a single cylinder engine injector on' period measurement.

(xi.) Figure 11 is an example timing diagram for twin or multi cylinder engines.

(xii.) Figure 12 is a schematic electronic circuit for twin or multi cylinder engine injector on' period measurement.

(xiii.) Figure 13 is a four stage differential amplifier circuit which combines the injector on' period signal with a fuel pressure signal.

(xiv.) Figure 14 is an example timing diagram using two digital potentiometers to provide the gas injector signals to activate two gas injector solenoids.

(xv.) Figure 15 is a schematic electronic circuit that provides the gas injector signals to activate two gas injector solenoids.

(xvi.) Figure 16 is a schematic diagram for a exhaust flow measurement system.

(xvii.) Figure 17 is a graphical schematic timing of the Exhaust Pressure signal.

(xviii.) Figure 18 is a schematic diagram using a Proportional Gas Valve for injection.

(xix.) Figure 19 is a schematic electronic circuit for a microprocessor controlled Proportional Gas Valve injection system.

Figure 1 defines the envelope (1) for the secondary fuel injection for engines that are slow speed (RPM) and high torque as used typically in heavy goods vehicles (HG\'5 busses/coaches and rail. The X axis is the mass of primary fuel expressed as a percentage of full load. The Y axis is the percentage of secondary fuel injected as a percentage of the primary fuel i.e. the engine load or power. The minimum edge [Minimum Fraction] of the envelope (2) is where significant improvement of combustion begins, typically at 5% by mass secondary fuel injection for LPG. The maximum edge [Maximum Fraction] of the envelope (3), typically 15% by mass, is where the inefficiency of combusting a secondary fuel (for an engine designed to combust primary fuel) significantly counters the Atkinson cycle' effect therefore resulting in no gain in the overall combustion efficiency. Point (10) represents the fuel flows at engine idle speed. At higher engine loads (4) the secondary fuel percentage may be reduced because (i) there is less oxygen available and (ii) the engine may be required to perform within the design engine load limits for the high load conditions. For example, Figure 1 shows a decrease in percentage mass secondary fuel injection beyond 60% load of the engine, i.e. the 15% profile (3)is reduced to 13.8% at point (5), 9.4% at point (6), 6.7% at point (7) and 4.75% at point (8). For engines where there is no excess air available at full load then the profile for the amount of gas injected may need to be reduced to zero. The solid lines (11, 12) in Figure 2 show a more typical profile where maximum combustion efficiency occurs to produce the minimum of harmful emissions. Profile (11) shows a decreasing percentage injection above 60% (4) full load and profile (12) at 80% (9) load. The point at which the decrease begins is determined by the duty cycle of the engine; in practice, for a HGV, this would be typically set at just above the engine load for a cruising fully laden vehicle. Dashed line (13) and dotted line (14) are the 15% and 5% secondary fuel injection percentages respectively.

Figure 3 defines the envelope (15) for the secondary fuel injection for smaller engines that are used typically in modern motor cars and vans. The injection system for these engines offer more control for the primary fuel delivery and use multiburst' (separate rapid firing of injectors) combined with a greater range for the common rail injection pressure. Thus the secondary fuel percentage may be made proportional to the primary fuel mass over the full load range of the engine (16, 17). The solid line (18) in Figure 4 shows a more typical profile where maximum combustion efficiency occurs to produce the minimum of harmful emissions.

The optimum secondary fuel percentage depends on the engine design; bore/stroke, shape of piston/cylinder head, valve configuration, aspiration system, fuel grade engine load and speed. For low engine loads a higher percentage of secondary fuel may be used for further fuel cost savings without detriment in engine emissions (10, 19). For example, when the engine is at idle or under part-load such as when power take off is used.

Figures 5-13 disclose the method for measuring the injector open or on' time. Figure S is an illustration for the voltage across a typical fuel injector when PWM is used to control the current passing through a solenoid injector. The average current is determined by the duty cycle (pulse width) of the signal. Typically the frequency of the PWM is approximately 24 KHz in modern injector driver systems. However it is not uncommon to see much higher voltages across an engine fuel injector -negative and positive voltages may be observed as high as several hundred volts in part due to the back electromotive force from the solenoid. Initially a high (peak) current is required to crack' open the solenoid (20) for a short duration typically 0.2-2 msecs duration (20-21) and may involve high voltages to achieve this. If this current were to be maintained the solenoid would overheat and fail; the PWM technique is used to supply just enough current to hold' the injector open (22). It therefore can be deduced that the injector is fully open for the hold' current period (22) and fuel is flowing through the injector until it is closed (23).

Referring to Figure 6 a combination of signal diodes, zener diodes, resistors and capacitors may be used to provide a high impedance input to the circuit from each engine injector. In this way there is minimal interference of the engine injector signals. Any negative voltage and/or high frequency spikes are removed and the positive voltage is limited to just below 5 volts. The injector signals are passed on to a differential amplifier (24). Both common ground and push-pull engine injector signals can be accommodated using the differential amplifier (24) which has a single output for each injector. For engine injector signals that use a common ground then one of the differential amplifier inputs for each injector is also grounded.

The signal is further cleaned by using a Schmitt trigger gate such as a NAND gate (25) which in Figure 6 is used for two engine injectors providing outputs (26, 27). Outputs (26, 27) are also used to activate and time the gas injection, i.e. the gas is only activated when the engine injectors are operating. An additional input (28) is shown which can de-activate the gas injection; for example if there is no gas available (gas tank empty). For engines injectors that do not use the PWM technique to control current (such as some solenoid systems and piezo injectors) an additional input (29) can be used to provide a pseudo PWM signal at 24 KHz when the injector is on'. This pseudo signal would emulate a PWM injector signal including the initial peak' period where the injector is opening. In this way the circuit can provide outputs for all types of engine fuel injector.

Figure 7 shows a typical the injector signal after appropriate signal conditioning. The PWM pulses (26) are fed to the clock input of a digital potentiometer which increments a wiper' for every positive or negative edge transition of the engine fuel injector signal. The digital potentiometer effectively counts the number of PWM pulses of the engine injector signal providing an analogue voltage output from the wiper'. Figure 8 shows the output (30) of the digital potentiometer wiper for an injector open for 2 msecs. In this example the voltage output is 2.1 volts which correspond to twenty one counts or increments made by the digital potentiometer. Since the frequency of the PWM is constant the number of counts and corresponding output voltage (30) is directly proportional to the hold time of the injector.

Figure 9 shows the output (31) of the digital potentiometer wiper for an injector open for 3.5 msecs (32). In this example the voltage output is 3.5 volts which correspond to thirty five counts or increments made by the digital potentiometer. Figures 8 and 9 are for clarity to illustrate the principle for using a digital potentiometer to measure the open time of a fuel injector. In a typical diesel engine the injector open times vary between 0.2 -2.5 msecs and for petrol engines injectors 1 -25 msecs. A 128 step digital potentiometer providing 0-5 volt output is preferred in the following description of the present invention; however digital potentiometers having a greater number of step increments may be used. Thus it is possible to measure the open time of a typical diesel fuel injector to within a few microseconds when a PWM signal is used to drive the injector.

Figure 10 is schematic circuit for a single cylinder engine comprising of one fuel injector where a single digital potentiometer (34) can be used in conjunction with a sample/hold operational amplifier (35) and pulse generator (36) to provide an analogue voltage proportional to the open time of the fuel injector. A digital potentiometer has an up/down or increment/decrement input (37). It is possible to decrement the wiper output (38) from the maximum value to zero (Figure 8, 39) within the peak current time period as typical clock speeds of modern devices range up to 1 MHz. For example to count down 128 steps using a 1 MHz clock is 0.128 msec, which is within the peak current time period used by most fuel injectors. When an injector signal arrives (33) a dual monostable (36) generates a chip select' pulse (40) just larger than the maximum injector signal period including multi-burst firing (typically <15 msecs) from the first transition of the injector signal. Dual monostable (36) generates a further up/down pulse (37) which is just smaller than the peak current period of the injector (typically less than 1 msec) from a transition of the chip select' signal. The chip select signal (40) is used to set the hold condition of the sample/hold amplifier (35). Electronic switch (41) switches either a high frequency clock (400 KHz -1 MHz) or the injector signal (33) to the clock input of the digital potentiometer (34) depending on the logic state of the up/down signal (37). Thus on arrival of an injector signal: chip select (40) sets sample/hold (35) to hold, enable digital potentiometer (34), up/down signal (37) set to down', switch (34) connects high frequency clock to the clock input (42) of digital potentiometer (34) and therefore the wiper output (38) decrements to zero. After the wiper has reset', the up/down signal (37) is returns to up', switch (34) connects injector signal (33) to clock input (42) of digital potentiometer (34) and the wiper output (38) increments according to the number of PWM pulses of the injector signal (33). The chip select' signal returns and the sample/hold amplifier (35) then can sample and output the new wiper voltage. In this way an accurate measurement of the injector open' time is made which automatically takes into account the time taken for the injector to open.

Prior art such as Patent GB2488814 discloses a method which measures the current passing across a fuel injector using a Hall Effect current transducer. In this system the total injector on' period is measured including the peak current period and this error would accumulate during a multi-burst injection event. The current sensor requires a calibration time in order to lock' onto the injector signal and is not immune to stray magnetic field interference.

The present invention offers significant advantages because voltage across the injector is measured and thus provides an interference free measurement of the injector open' time.

The method automatically takes into account the time taken for the injector to open and thus provide a better measurement (particularly for multi-burst injection events) of fuel flow almost ii instantaneously; i.e. the ICU can provide a measurement as soon as it is switched on and an engine injector signal is immediately processed. For multi -cylinder engines the sample/hold amplifier is not required since a previous injection signal output can be used until the current injection measurement has completed. Figure ii is an example timing diagram where two digital potentiometers are used with two injector signals (43, 44). A monostable outputs chip select signals (45, 46) on the first ve edge transition of the injector signals (43, 44). A further monostable outputs up/down' signals (47, 48) on the --ye transition of chip select signals (45, 46). Up/down signal (47) is used to enable an electronic switch to connect a high frequency clock input to a first digital potentiometer to down-count (49) and on disable connect the injector PWM signal to up-count for the injector on' period (50). The -ye transition of chip select signal (45) is used to connect the output voltage (Vi) of a first digital potentiometer via setting of a D-Type flip-flop (51) using pulse (52) which enables the output (Vi) to be switched to output signal (53). Similarly the -ye transition of chip select signal (46) is used to connect the output voltage (V2) of a second digital potentiometer via re-setting of a D-Type flip-flop (51) which enables the output (V2) to be switched to provide an updated output signal (53). In this example the injector on' period is reduced over four periods (50, 54, 55 and 56) resulting in the corresponding output voltage (53) to reduce (Vi, V2, V3 and V4).

Figure 12 is an embodiment of an electronic circuit that realises the timing diagram of Figure ii. Dual monostables (57, 58) output chip select' (45, 46) and up/down (47,48) signals for the digital potentiometers (59,60). Electronic switch (61) either connects the injector PWM signals (43, 44) or the high frequency down-count' signal (62) which in this embodiment is generated by an astable timer (63). A D-type flip-flop (64) and NAND function logic gates (65) provide the enable signals for electronic switch (66) to selects the appropriate digital potentiometer output signal (49, 50) to output (53). The function of resistor (67) and capacitor (68) is to remove high frequency noise and smooth' the output signal (53).

This arrangement can be used for two cylinder engines and may be sufficient for additive injection in four cylinder engines. In a four cylinder engine the injector signals used can be chamber 1/3 or chambers 2/4 or when the two injector signals are equally phased apart. Similarly three 3 injector signals can be used for straight six engines, four for V6N8 engines. However for sequential injection systems and where a fast update time is required a measurement can be made from each injector for every chamber.

Where the fuel pressure is not constant (for example in a common rail engine) signal (53) representing the injector on' period has to be combined with a fuel pressure' signal to determine the fuel flow. This signal is available on engines which vary the fuel pressure in order to control the amount of fuel delivered. Typically the pressure transducer would have a separate ground, regulated voltage supply (5 volts) and output a signal that typically varies between 0.5 -4.75 volts. For pressure transducers typically used in the automotive industry the output voltage is not linearly proportional to the fuel pressure. Since it is not possible to vary the fuel pressure instantaneously then the injector on' period is also adjusted in orderto control the amount of fuel delivered as the engine is accelerated/decelerated. The injector on' timing is optimised for efficient operation of the engine and therefore a change in this timing represents an error' caused by the lag' when adjusting the fuel pressure according to engine load demand. Thus the fuel flow can be approximated by: Flow = Gi(Ion + Oi) x Gp(P + Op) Where: Flow fuefflowrate Ion = Injector on' period signal Gi = Gain applied to Injector on' signal Oi = Offset applied to Injector n'period signal P = Fuel Pressure signal Gp =Gain applied to fuel pressure signal Op =Offsel applied to fuel pressure signal Ideally the above equation the term Gi(Ion + Oi) has a unity value when the injection timing is optimised and the fuel pressure has stabilised according to engine load. By careful selection of offset and gain the fuel pressure signal sufficiently approximates the fuel flowrate for an injector with a fixed on' period within the operational limits (figures 1 -4) of the gas injection system. Thus the fuel flow can be further approximated by: Plow = Gain(Gi(Ion + 01) + Gp(P + Op)) + Offset In the above equation the term Gi(Ion + Oi) has a zero value when the injection timing is optimised and the fuel pressure has stabilised according to engine load. A further Gain and Offset are applied to the combined signal to improve the approximation. Both of the above equations can be realised using electronic components such as a multiplier amplifier or summing operational amplifier.

Figure 13 shows an embodiment of a four stage differential amplifier circuit which combines the injector on' signal (53) with the fuel pressure signal (69, 70). A unity buffer amplifier (71) combines the pressure transducer signal ground (70) with an offset adjustment (72) using a summing amplifier (73) and the resulting signal passed to the first stage (74) of differential amplifier (74, 75, 76). Unity gain buffer amplifiers (77, 78) feed fuel pressure signal (69) and injector on' signal (52) to a resistor network where the gain ratio can be adjusted (79) before summing by amplifier (80). The output of the summing amplifier (80) is passed to the first stage (75) of differential amplifier (74, 75, 76). The overall gain of the combined signals can be adjusted (81) for the differential amplifier (76) which outputs a signal (82) representing the fuel flow of the injector. In this way for engines where there is no fuel pressure signal the input (69) can be grounded and only the injector on' signal (53) processed by the amplifier.

Figure 14 is an example timing diagram where two digital potentiometers are used with two injector signals (43, 44) to activate two gas injector solenoids. Here an alternative method is disclosed for measuring the injector on' period whereby a minimum of circuit components are used and where the peak current period for the gas injectors can be adjusted for different models of gas injector. A monostable outputs chip select' signals (45, 46) on the first transition edge of the injector signals (43, 44). Afurthermonostable outputs up/down' signals (83, 84) on a transition edge of chip select signals (45, 46). The signals (83, 84) are used to enable an electronic switch to either connect a high frequency clock input to digital potentiometers to down-count or connect the injector PWM signal to up-count for the injector on' period. The -ye transition of chip select signal (45) is used to connect the output voltage (Vi) of a first digital potentiometer via setting of a D-Type flip-flop (51) which enables the output (Vi) to be switched to output signal (53). Similarly the -ye transition of chip select signal (46) is used to connect the output voltage (V2) of a second digital potentiometer via re-setting of the D-Type flip-flop (51) which enables the output (V2) to be switched to output signal (53). In this example the injector on' period is reduced over four periods resulting in the corresponding output voltage (53) to reduce (Vi, V2, V3 and V4). This signal is combined with the fuel pressure signal if required as described above (Figure 13) and is used as the reference signal (82) in order to determine the gas injector on' times (Ti, T2, T3 and T4).

Alternatively microprocessor could be used to count the PWM pulses to provide a diesel injector on' period albeit not instantaneously' as in the circuit described above.

Further monostables output up/down' signals (87, 88) for the gas injector digital potentiometers. In this embodiment the trigger for these signals is taken from the +ve edge transition of chip select' signals (45, 46) and in doing so the gas injectors will operate simultaneously with the engine injector firing. Whilst this is not a problem for most diesel engines, petrol engines which utilise an Atkinson' cycle effect by varying the valve timing and phase during an engine cycle it may be required to adjust the gas injection timing. The gas may be injected as close to the inlet valve(s) as possible using an injector for each cylinder hereby referred to as sequential timed gas (or gas in liquid phase) injection. Ideally the gas is injected when the inlet valve is open and the exhaust valve is closed. This may result in a delay after the inlet valve is first opened during the induction stroke, until the exhaust valve is closed; at which point the gas can be injected without passing directly into the exhaust. Thus the gas injection cycle begins on an induction stroke where the inlet valve(s) is open and the exhaust valve(s) is closed and ends when both inlet and exhaust valves are closed. Incorrect gas injection timing is a major cause of engine malfunction and inefficient combustion for LPG converted petrol engines because the gas injectors cannot occupy the same position in the inlet manifold as the petrol injectors and therefore require different timing to the petrol injectors. Thus it may be required that the gas injection needs to be advanced or delayed for correct operation in a sequential system, and therefore other crankshaft position/velocity sensors may be used to trigger signals (87) and (88) to achieve this. Where a non sequential system can be used (as in non-Atkinson' cycle effect diesel engines) the gas can freely flow through the intake manifold and the gas can be injected asynchronously to the engine. In this instance the trigger for the gas injectors for example can be obtained from an astable running at an appropriate frequency; provided that there is always additive injected in the correct ratio for each induction stroke of the engine.

In this embodiment the period of signals (87, 88) is set to ensure the opening of the gas injector. The peak current time is for a gas injector to open typically ranges from 1 -3 msecs.

Signals (87, 88) are used to set a D-type Flip-Flop (89), and used as the up/down' input which resets a gas timing digital potentiometer by connecting a high frequency clock input which thus counts down or decrements the output to zero (90). The digital potentiometer is then set to increment using clock signals from an adjustable astable in the range 1 -S KHz (91). A comparator operational amplifier is used to compare the output voltage from the digital potentiometer with the reference signal representing the fuel flow (82). When these signals are equal (Dl, D2, D3 and D4) the comparator output changes and this transition resets the D-type flip-flop (92). Thus digital potentiometer signals (93, 95) and D-type Flip-Flop output (94, 96) represent the gas injector on' periods according to the measured fuel flowrate signal (82).). In this example the gas injector on' period is reduced according the reducing fuel flow measurements (Dl D2, D3 and D4) and corresponding gas injector on' time (TI,T2, T3 and T4) for two gas injector outputs (97,98). The gas injector outputs (97, 98) comprise of an adjustable peak' current time (87, 88) and a 24 Khz PWM signal where the duty cycle is also adjustable to provide an average current rating to maintain the open state of the gas injector.

Figure 15 is an embodiment of an electronic circuit that realises the timing diagram of Figure 14 where inputs (99, 100) are the triggers for two gas injectors. These signals can either be taken from ve or -ye edge transition of chip select' signals (45, 46) or other crankshaft position/velocity sensors that can provide an injector timing sequence for example when an advance/retard is required for the gas injectors. Monostable (101) outputs the gas injector peak' pulses (102, 103) which are also used as up/down signals for the digital potentiometers (104, 105) and as enable inputs for an electronic switch (106) which either connects a high frequency down-count' signal (107) or the adjustable up count' signal (108) to the clock inputs of digital potentiometers (104, 105). In this embodiment the up count, down count' and adjustable gas injector PWM signals are generated by astable timers (109, 110). On arrival of an engine firing pulse (99) monostable (101) generates a peak gas injector pulse (102) and the gas injector begins to open and during this peak current period the digital potentiometer decrements to zero. A D-type flip-flop (111) is set' and enables the peak signal to pass through NAND function logic gate (112) boosted through driver (113) and activate the gas injector MOSFET (114). When the gas injector has opened and the peak pulse period completed the NAND gate then allows a 24 KHz signal to pass in order to maintain the hold' condition for the gas injector. The digital potentiometer (104) is incremented using the up count' signal and comparator (115) compares the digital potentiometer output to the input fuel flowrate signal (82). When this output voltage is equal to the fuel flowrate signal (82) the output of the comparator (115) changes state causing the D-Type flip-flop (104) to reset and thus the gas injector is switched off (112, 113). On airival of an engine firing pulse (100) an identical process using a monostable (101), D-type flip-flop (111), digital potentiometer (105), electronic switch (106), astable timers (109, 110), comparator (115), NAND gate (112) and driver (113) are used to activate a second gas injector. A variable resistor (117) can be used to adjust the duty cycle of the PWM hold current to allow different specification gas injectors to be used. Variable resistor (118) is used to control the on' period of the gas injector for a given fuel flowrate (82) signal. The amount of gas delivered is thus a function of the gas pressure and resistor (118) value. Alternatively a microprocessor could be used to process the gas injector firing signals and provide a gas injector on' period albeit not instantaneously' as in the circuit described above.

Figure 16 is an embodiment of a system whereby the cycle by cycle measurement of fuel flow is determined by measuring the exhaust pressure. Combustion products enter exhaust manifold (119) to the exhaust (120), pass pressure senor (121), turbocharger (122), oxidation catalyst (123), diesel particulate filter (124), selective catalytic reduction (125) and finally exit from silencer (126) where these components are fitted. The DPF filter may have two pressure transducers (127, 128) which are used to provide a differential signal across the DPF which is used by the ECU to determine DPF purge. Pressure sensor (121) is best positioned pre -turbo if fitted to obtain a signal which better correlates to exhaust mass flow and hence provide a fuel flow measurement.

The engine speed (RPM) is determined by a timer measurement using the diesel injector actuation signals from the ECU. A more precise measurement can be made using several injection pulse timing measurements from each cylinder. Thus it is possible to determine the crank angle by using a timer initiated on an injection pulse and the time an exhaust valve opens can be determined. As soon as the exhaust valve is opened a pressure wave travels through manifold (119) into exhaust pipe (120) and arrives at pressure sensor (121). This time interval is fixed and depends on the distance of pressure sensor (121) from the exhaust valve. A timer value which varies as a function of RPM can therefore be calculated which is used to start the ADC sampling of the exhaust pressure sensor (121).

Figure 17 shows a graphical representation of the exhaust pressure signal (129). After an injection pulse (130, 131) has been detected the sampling begins after crank angle (132) which is calculated using engine speed and loaded into a timer as a time interval. The exhaust pressure is sampled for timed window (133). During the window the ADC values are stored.

S These values may be processed digitally to remove spikes. A stacked average method could be used whereby when a new value is added to a stack, the last stack value discarded, the stack is then averaged to give a new stack average; this value is stored. When the window has completed the stored stack averages are compared and a peak value for the window obtained. The process is repeated for each diesel injection thus obtaining a peak value of exhaust pressure for each engine cycle. The magnitude of this pressure signal relates to the amount of primary fuel combusted and provides a signal that can be used to determine the amount of high octane fuel added for each engine cycle.

Figure 18 shows an embodiment of the invention whereby the standard gas injectors described above are replaced with a single proportional gas valve. This application requires a fast response proportional valve that can actuate within a few milliseconds. The engine air intake passes through air filter (135) and may be compressed or charged' by charger (137) before entering the engine intake manifold. A proportion gas valve (138) is used to inject a flow of gas through nozzle (139) controlled using an electrical input (140). Typically this electrical input uses PWM current regulation to control the valve. The gaseous fuel (141) enters filter (142) at a regulated pressure measured by pressure senor (142) and temperature measured by sensor (143). In this way temperature/pressure corrections can be made to ensure the correct amount of gas is injected. The main advantage for using a proportional valve is that a continuous amount of gas is delivered rather than being injected as pulses. In this preferred embodiment the gas valve is positioned close to the engine intake manifold post turbo.

Figure 19 is a schematic circuit diagram of the gas additive injection system using a proportional gas valve. A diesel injector signal (144) is signal conditioned (145,146) and input to processor (147). Several or all the engine diesel injectors may be used for inputs, however not all the diesel injector inputs are necessary as other injector inputs may be simulated by the processor. For example several measurements can be made from a single injector to determine if the engine is accelerating or decelerating and therefore a predictive technique used to simulate the firing other injectors. The injector on time, common rail diesel fuel pressure (148) is measured and engine speed (RPM) calculated to determine the primary fuel flowrate. The gas pressure (149), gas temperature (150) is measured and the gas flowrate calculated as a proportion of primary fuel flow. The gas flowrate output signal (151) is PWM signal used to actuate proportional gas valve (figure 18, 138) through driver (152).

This circuit requires minimal modification to use the exhaust method described above to calculate a primary fuel flow measurement. A diesel injector signal (144) is used to calculate the engine speed and exhaust gas pressure (figure 16, 121) measurement timings (figure 17, 132 and 133). The exhaust gas pressure measurement now replaces the common S rail input (148) signal and the processor determines a primary fuel flow measurement using the exhaust gas measurements. The gas valve is actuated as described above.

In this embodiment a tank fuel level signal (154), signal conditioned through amplifier (155) and input to processor (147) is shown. Processor (147) can be used to output on a display (156) the operation of the gas injection system for the driver of the vehicle. Such display parameters may include the fuel level, engine speed, injector firing, instantaneous fuel flow, gas pressure/temperature, fuel flow. The display may also have a control input to adjust the ratio of injected gas/injected diesel.

In this way the control function of an engine management system (EMS) is not affected and remains in full control of the engine; there are no software modifications required for the EMS, a modern EMS will adapt to the new engine fuelling automatically. For example, for engines fitted with a knock' sensor the EMS may advance the injection timing for the new fuelling because of the improved RON of the duel fuel. However, since the required signal data is hardwired within the ECU the ideal configuration would be to incorporate the gas injection electronic circuit directly from this unit.

Power to the gas injection system is via a fuse from the ignition circuit; in the event of a vehicle incident (for example if the air bags inflate) this circuit is immobilised and the gas system shut down.

The diesel injector signal is primarily used to determine when the gas injectors are active; for example when a vehicle is decelerating or going downhill, when there is no load demand, the EMS may de-activate the diesel injectors to conserve fuel; henceforth the gas injectors are also de-activated; i.e. gas is only injected when diesel is injected.

The circuit is powered from the vehicle battery via the ignition circuit and protected with fuse. For vehicles without automatic emergency engine shut off it may be preferable to interface the gas supply control to the electronic circuit to enable an automatic gas shut off in the event where the engine is not running for a period of time. In this event the relays for the gas tank solenoid and vaporiser solenoid may de-activateci; hence gas is supplied when the engine runs in normal conditions and can be shut off in an emergency automatically.

This example can be used for gas injection at the air intake for engines where there is no valve timing overlap between exhaust stroke and induction stroke of the piston; i.e. the exhaust valve is closed as the intake valve is opened. At least two or more injectors are required to ensure consistent gas delivery during the induction stroke of a piston and preferably the gas injection period at least twice the period for a piston stroke for each injector.

Typically the injection period will be in the range 3 -80 msecs. A single gas injector may be used providing there are at least sufficient injection periods within a stroke of the piston in order to deliver consistent quantities of gas for each engine cycle. However a single injector system hence would have a shorter working life and for high speed engines and the short injection period required may cause non-linearity of gas delivery due to water hammer' effect in the fluid. However the circuit can also be used for liquid injection systems and high pressure gas injection systems where the injection occurs at a precise time during the induction and/or compression stroke and correspondingly involve shorter gas injection periods. Given that the injection time of the primary fuel is shorter than the injection time of the secondary fuel it is possible to determine the current flowrate and the second additive fuel or alternative fuel can be supplied to the combustion chamber during the same combustion cycle using the current primary injector signal in real time.

A 10 volt regulator may be used to supply the preferred voltage for the MOSFET driver (113). The gas injection signals may be displayed using LED's and a further LED used to indicate correct operation of the gas injection system via a watchdog' circuit and may be used to signal trouble' codes such as for example, low gas pressure'. In this event the gas injectors can be automatically switched off. Ten segment bar LED's may be also used to indicate gas fuel level and also provide an instantaneous fuel flowrate display for the operator.

Typical results on a diesel engine are considered to provide a reduction in primary fuel consumption of around 10-20%, using only 5 -12% of additive high octane fuel to achieve this improvement. Emission reductions are typically 40% to 70% reduction in nitrogen oxides, 80% to 98% reduction in smoke and particulate matter, and a reduction of carbon dioxide and other emissions reflective of the reduction of overall fuel used and the efficiency of the engine.

Similarly for petrol engines because the high octane fuel additive improves the combustion.

These improvements are achieved in a non-invasive manner so that the engine life and/or periods between servicing will be extended due to improved combustion and the reduction in carbon deposits.

The present invention also offers a low cost solution for the conversion of diesel or petrol injection engines to be converted to run on lower cost fuels such as LPG or green fuels such as methane or bio-gas.

Claims (16)

1. CLAIMS1. An add-on fuel system connected electrically and hydraulically to improve the combustion efficiency of an internal combustion engine by adding a quantity of high octane fuel to a lower octane first fuel comprising the steps of: (i) Determining the maximum proportion of high octane additive fuel calibrated by the onset of engine knock for a given engine, typically less than 15% by mass for diesel engines; (ii) Measuring the quantity of a first fuel for each engine cycle; (iii) Computing a proportional and consistent quantity of high octane additive fuel according to the calibrated proportion value; (iv) Providing an injector signal to supply additive high octane fuel in correct proportion for the current engine cycle or following engine cycle.
2. 2. A method according to Claim 1 for measuring the quantity of a first fuel of an internal combustion engine comprising the steps of: (i) Counting the number of pulses supplied by a pulse width modulated signal to a primary fuel injector to maintain an injector open for an injection event; (ii) Computing an analogue first fuel flowrate signal using an analogue computer from said pulse count; (iii) Computing an analogue additive fuel flowrate signal using an analogue computer from said first fuel flowrate signal.
3. 3. A method according to Claims 1 and 2 for measuring the quantity of a first fuel of an internal combustion engine where the engine fuelling is primarily controlled by varying the fuel pressure comprising the steps of: (i) Measuring the fuel pressure signal from the engine fuel pressure sensor; (ii) Computing an analogue second or alternative fuel flowrate signal by using an analogue computer which combines said injector open time signal and said fuel pressure signal.
4. 4. When an injector current is not controlled using the pulse width modulation technique, a method according to claims 1-3 wherein measuring the quantity of a first fuel is determined by counting the number of pulses as supplied by an appropriate pulse generator for the injector open period or by timer measurement of the injector open time.
5. 5. A method according to Claim 1 for measuring the quantity of a first fuel of an internal combustion engine comprising the steps of: (i) Measuring the timing of the primary injector signals; (ii) Computing a timing window to measure exhaust gas pressure; (iii) Measuring the peak exhaust gas pressure or peak differential pressure of exhaust gases in the exhaust system for an engine cycle; (iv) Computing a first fuel flowrate using the data provided by exhaust pressure measurements; (v) Computing an additive fuel flowrate signal from said first fuel flowrate signal.
6. 6. A method according to Claims 1 -5 whereby a proportional valve is used to inject the additive fuel comprising the steps of: (i) Measuring the timing of the primary injector signals; (ii) Computing engine speed; (iii) Computing a continuous additive fuel flowrate to be used by a proportional valve injection system.
7. 7. A method according to claims 1-6 whereby the instantaneous fuel flowrate is displayed on an instrumentation panel.
8. 8. A method according to claims 1-6 whereby the injector firing and correct second or altemative fuel operation can be displayed on an instrumentation panel and the proportion of additive fuel adjusted.
9. 9. A kit as claimed in claims 1 -8 where a digital processor or microprocessor(s) is used to compute the second or alternative fuel flowrate and provide a second or alternative fuel injector signal(s).
10. 10. Given that the injection time of the primary fuel is shorter than the injection time of the secondary fuel: a method according to claims 1 -9, wherein the second additive fuel can be computed and supplied to the combustion chamber during the same combustion cycle.
11. 11. A method according to claims 1 -5 whereby the second or alternative fuel injection timing can be retarded or advanced with respect to the primary fuel injection timing.
12. 12. A method according to any preceding claim wherein the second additive fuel or alternative fuel can be injected directly and/or indirectly into the combustion chamber at a subsequent point in time coincident with an induction, compression or combustion stroke of the engine.
13. 13. A method according to any of claims I -5, wherein said injection of the second additive fuel or alternative fuel is into an inlet intake of the engine during the induction stroke of the chamber in question.
14. 14. A method according to claim 11 or 12, wherein injection into said inlet intake begins after all outlet valves of the chamber in question have closed in or following the preceding exhaust stroke of the engine.
15. 15. A method according to claim 11 -14, wherein injection into said inlet intake ceases before the inlet valve of the chamber in question closes in or following said induction stroke of the engine.
16. 16. A method according to any one of claims ito 15, wherein the second additive fuel or alternative fuel is petrol(gasoline), liquefied petroleum gas (LFG), compressed natural is gas (CNG), liquid natural gas (LNG), methane, hydrogen, (Browns gas).