GB2539905A - A method of controlling secondary fuelling in an internal combustion engine using engine exhaust measurement - Google Patents

Abstract

A secondary fuelling system is connected electrically and hydraulically to improve the combustion efficiency and emissions of an internal combustion engine configured to be fuelled with a primary lower octane fuel and a controlled quantity of a secondary higher octane fuel. A method of supplying the secondary fuel includes the steps of measuring the peak exhaust pressure or the peak differential pressure of exhaust gases in the exhaust system for an engine cycle and using this information to calculate the peak exhaust pressure amplitude and timing and the fuel flow rate, determining the engine speed, measuring the secondary fuel gas pressure and temperature at the fuel inlet, computing a quantity of the secondary fuel and providing an injector signal to supply said calculated quantity of secondary fuel in the same engine cycle or the following engine cycle.

Description

DESCRIPTION

A METHOD OF CONTROLLING SECONDARY FUELLING IN AN INTERNAL COMBUSTION ENGINE USING ENGINE EXHAUST MEASUREMENT

This invention relates to a secondary fuelling system for an internal combustion engine, and to a method of controlling secondary fuelling of an internal combustion engine to improve the combustion efficiency and emissions.

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.

There is a known practice of retro-fitting internal combustion engines made to run on conventional oil-based fuels - in particular petrol (gasoline) or diesel - with a secondary fuelling system to supply a different fuel in addition, or sometimes as an alternative, to the primary fuel. Typically such “dual fuelling” systems belonging to the prior art have sought to maximize the quantity of the secondary fuel. For example petrol engines are converted to be fuelled using 100% gas. Diesel engines are converted to be fuelled typically in the range 20-99% gas. Gas is a less expensive form of fuel, so maximising its usage minimises fuel cost. A different approach is to introduce a relatively small quantity of a secondary fuel in order to improve overall fuel efficiency of the engine. It is known that the efficiency of a conventional internal combustion engine running on a primary fuel such as diesel or petrol (gasoline) is improved by supplying to the engine’s combustion chamber(s) a controlled quantity of a secondary fuel. Typically the secondary fuel is of higher octane value than the primary fuel. The reasons why such secondary fuelling systems offer efficiency benefits are complex.

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). Hydrocarbon fuels such as diesel have a molecular structure 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.

Three factors that affect the proportion of available fuel that is burnt include: (i) The molecular chain length of the fuel itself-the longerthe chain length, the less likely it is that complete combustion will occur in a given time frame; (ii) 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; (iii) The speed of the engine and piston velocity.

The amounts of fuel and time available for combustion are limited according to engine load and piston speed (RPM). Within the enclosed cylinder there are typically ‘hot’ and ‘cold’ regions, particularly around the cooled cylinder walls. In regions where there is no hydrocarbon for the oxygen to react, the oxygen reacts with nitrogen to produce NOx. In areas deficient of oxygen incomplete combustion will occur resulting in CO. In ‘cool’ areas where the chain combustion reaction has stopped there will be un-combusted fuel and air entering the exhaust phase. This also applies to rotary engines such as Wankel engines.

While the present invention is not reliant on any particular explanation of the effects of secondary fuelling, it is suggested that in suitable secondary fuel systems a small amount of the secondary fuel serves to improve the combustion of the primary fuel. This relates to ignition/injection timing and engine knock. It is well known that advancing ignition of the fuel in the combustion chamber offers potential efficiency benefits. However excessive advance of fuel ignition will cause harmful and deleterious engine knock and limits the degree to which fuel ignition is advanced. Secondary fuel in principle serves as a knock inhibitor, allowing combustion to take place earlier in the engine cycle without knocking and so improving energy efficiency. The combined fuel has an increased Research Octane Number (RON) compared to the primary fuel alone. The flame front velocity is higher for shorter length molecules of the secondary fuel and increases the flame front velocity for the secondary fuel overall, thus accelerating the combustion of the combined fuels. The combustion Is faster, allowing more time for combustion even in the ‘cold’ regions during the power stroke. Fuel which would, in a conventional engine, remain un-burnt and pass to the exhaust is, in a suitable secondary-fuelled engine, be burnt during the power stroke and contribute to engine output power. Engine efficiency is increased and the HC and particulate matter emissions are significantly reduced because more of the available fuel is combusted.

The objective of the present invention is to convert an engine fordual 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 more time to combust the long chain molecules (‘Atkinson Cycle’ effect) 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. In engines equipped with suitable engine management systems the required adjustment of engine timing takes place automatically following installation and activation of a secondary fuel supply system, without replacement or adjustment of the factory fitted engine management. Modern diesel engines are typically fitted with a knock sensor and have an ECU which is will suitably adjust and optimise fuel injection timing provided that the secondary fuel is supplied in a manner sufficiently consistent for such adjustment to take place. To this end it is necessary that the properties of the combined fuel should be consistent from one engine cycle to the next.

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 are used. Modern diesel engines have injection technology using a Pilot injection pulse that is often advanced more than 35 degrees from 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 is a cause of engine knock. Thus the level of high octane fuel that is injected as an additive for diesel engines is limited to a maximum fraction (typically 15%) because the diesel or primary fuel injection timing is so advanced that knock would occur at these higher concentrations by the additive fuel itself.

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 is by injection into the combustion chamber, during the induction stroke, compression stroke or combustion stroke of the engine. The amount injected 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.

There are various challenges and problems involved in creating a practical secondary fuelling system capable of providing worthwhile fuel efficiency improvements. These include - how to optimise the efficiency gains provided by dual fuelling - provision of a practical means of controlling the quantity of secondary fuel supplied to the engine - in a secondary fuelling system, controlling the level of secondary fuelling suitably to permit the engine’s controller to adjust its own performance in a manner which provides efficiency gains - provision of a practical means of monitoring engine performance as a basis for control of secondary fuelling in a retro-fit secondary fuelling system, without digital communication with the engine controller.

Solution or alleviation of one or more problems associated with secondary fuelling is an object of the present invention.

In accordance with a first aspect of the present invention there is a secondary fuelling system connected electrically and hydraulically to improve combustion efficiency and emissions of an internal combustion engine, the internal combustion engine being configured to be fuelled with a lower octane primary fuel and the secondary fuelling system being configured to supply a controlled quantity of a higher octane secondary fuel, in addition to the primary fuel, to a combustion chamber of the engine, comprising steps of measuring the peak exhaust pressure and/or peak differential pressure of exhaust gases in the exhaust system for an engine cycle, processing said signal to obtain peak exhaust pressure amplitude and timing; determine engine speed; compute a first fuel flowrate using amplitude and timing data provided by peak exhaust pressure measurement, measure high octane gas pressure and temperature at inlet to compute simultaneously a proportional quantity of high octane additive fuel from said first fuel flowrate signal, provide an injector signal to supply said additive fuel for the current engine cycle or following engine cycle in an appropriate timing pattern such that a proportional amount of high octane additive charge is present in the cylinder for each primary fuel injection and limited to a maximum fraction of the diesel or primary fuel beyond which engine knock would begin to occur due to the high concentration of additive or secondary fuel.

The system may be implemented by means of microprocessor technology. It may be implemented using analogue circuitry. The calculation of the required quantity of the secondary fuel may accordingly be implemented using analogue electronics.

Preferred embodiments of the invention shall now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 is a graphical schematic which outlines the envelope forthe additive fuel injection with reduction for higher engine loads;

Figure 2 is a graphicai schematic which shows 5%, 10% and 15% by mass of the additive fuei injection with reduction for higher engine loads;

Figure 3 is a graphicai schematic which outlines the envelope for additive fuel injection;

Figure 4 is a graphicai schematic which shows 5%, 10% and 15% by mass of the additive fuei injection;

Figure 5 is an exampie timing diagram using two digital potentiometers to provide the gas injector signais to activate two gas injector soienoids.

Figure 6 is a schematic eiectronic circuit that provides the gas injector signals to activate two gas injector soienoids.

Figure 7 is a schematic diagram for a exhaust flow measurement system.

Figure 8 is a graphicai schematic timing of the Exhaust Pressure signal.

Figure 9 is a schematic diagram using a Proportional Gas Valve for injection.

Figure 10 is a schematic eiectronic circuit for a microprocessor controlled Proportional Gas Valve injection system.

Figures 1 to 4 relate to the manner in which quantity of secondary fuel varies as a function of quantity of primary fuel supplied to the engine, in certain embodiments of the present invention.

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 (HGV) 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. The minimum edge 2 [Minimum Fraction] of the envelope is where significant improvement of combustion begins, typically at 3% by mass secondary fuel injection for secondary fuel in the form of LPG. The maximum edge 3 [Maximum Fraction] of the envelope, 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 because the larger amounts of secondary fuel become a cause of knock thus resulting in no gain in the overall combustion efficiency. Point 10 represents the fuel flows at engine idle speed. At higher engine loads to the right of line 4 the secondary fuel percentage is reduced because (i) there is less oxygen available and (ii) the engine is 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 will need to be reduced to zero.

The three lines in Figure 2 represent actual fuelling profiles. In each case the quantity of secondary fuel is substantially proportional to the quantity of the primary fuel in a low load region, up to approximately 60-80% of maximum fuelling. The three lines respectively represent secondary fuelling of 5%, 10% and 15% of primary fuelling, by mass, in this load load regime. This level of secondary fuelling is reduced at higher loads for the reasons just discussed. Solid lines 11,12 in Figure 2 show a typical profile where maximum combustion efficiency occurs to produce the minimum of harmful emissions. Profile 11 shows a decreasing percentage injection above 60% of full load (point 4) and profile 12 at 80% load (point 9). The point at which the decrease begins is determined by the duty cycle of the engine; in practice, for exampie in a HGV, this wouid be typicaiiy set at just above the engine load fora cruising fuiiy iaden vehicie. Dashed iine 13 and dotted iine 14 are the 15% and 5% secondary fuei injection percentages respectiveiy.

Figure 3 defines a suitabie enveiope 15 for secondary fuei injection for smaiier diesei engines that are used typicaiiy in modern motor cars and vans. The injection system for these engines offers more controi for the primary fuei deiivery and uses ‘muitiburst’ (separate rapid firing of injectors) combined with a greater range for the common raii injection pressure. Thus the secondary fuei percentage 16,17 is made proportionai to the primary fuei mass over the fuii ioad range of the engine. The soiid iine 18 in Figure 4 shows a more typicai profiie where maximum combustion efficiency occurs to produce the minimum of harmfui emissions. The optimum secondary fuei percentage depends on the engine design - bore/stroke, shape of piston/cyiinder head, vaive configuration, aspiration system, fuei grade, engine ioad and speed. For iow engine ioads a higher percentage of secondary fuei 10,19 is used for further fuei cost savings without detriment in engine emissions, for exampie when the engine is at idie or under part-ioad such as when ‘power take off is used. in secondary fueiiing systems described herein, the amount of secondary fuel to be supplied to an internal combustion engine is determined based on the quantity of the primary fuel being supplied.

The exhaust gases comprise air and combustion products such as H2O/CO2. Other combustion products such as particulate matter (PM)/NOx/CO 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 based on the mass flow of the engine exhaust. Exhaust gas pressure can be used as a measure of this mass flow. In order for the ECU to adapt to the secondary 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 provided that the measurement transducer (e.g. a pressure sensor) has a sufficiently fast response and is situated close to the exhaust ports, preferably on the exhaust manifold of the engine. A pressure transducer can be used to provide an exhaust flow measurement using the differential pressure principle across the exhaust system from exhaust manifold outlet, to a point further down the exhaust piping or to 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.

Some engines have factory fitted exhaust pressure sensors whose outputs may be employed for the present purpose. Specifically, in 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 closer to 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 PMs. The differential DPF signal can be subtracted from an absolute exhaust pressure measurement to compensate. A similar compensation may be required for systems using Exhaust Gas Re-circulation (EGR), selective reduction catalyst (SCR) or in systems where the turbo charger is fitted with variable vane technology.

Figure 5 is an example timing diagram where the pressure measurement from the exhaust pressure sensor signal has been processed (20) to identify peak signals 21,22 using a standard peak detector circuit. A typical peak detector circuit comprises of a comparator operational amplifier triggered when the input pressure signal voltage is greater than an adjustable reference voltage to produce output signal (20). In this embodiment signal 20 is divided into two separate signais 21, 22 using a D-type flip flop to actuate two additive fuei injectors. Further division is required for engines which have more cyiinders or operate at high engine speeds where the period of the peak signais exceeds the period for the additionai fuei injection period. A typicai peak detector amplitude circuit comprises of operationai ampiifiers where a capacitor is charged during the ‘peak’ period and peak and heid by a diode, the output voitage iatched using a switch to provide a stabie flowrate output 23 for each peak cycie. The ampiitude of the peak signais 23 is identified here as VI ,V2,V3 and V4 which show a decreasing peak ampiitude measurement. Two digitai potentiometers (24, 25) are used to activate and hoid open two gas injector soienoids (26, 27) where the hoid time is directly proportionai to the ampiitude of the measured peak exhaust pressure 28,29. In this embodiment the period of signais 30, 31 is set to ensure the opening of a gas injector. The peak current time is for a gas injector to open typicaiiy ranges from 1 - 3 ms. Signals 30, 31 are used to set a D-type Flip-Flop 32, 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 33. The digital potentiometer is then set to increment using clock signals from an adjustable astable in the range 1 - 5 KHz 34. A comparator operational amplifier is used to compare the output voltage from the digital potentiometer with the reference signal representing the fuel flow 23. When these signals are equal (D1, D2, D3 and D4) the comparator output changes and this transition resets the D-type flip-flop 35. Thus digital potentiometer signals 24,25 and D-type Flip-Flop output 26, 27 represent the gas injector ‘on’ periods according to the measured fuel flowrate signal 23. In this example the gas injector ‘on’ period is reduced according the reducing fuel flow measurements (D1, D2, D3 and D4) and corresponding gas injector ‘on’ time (T1,T2, T3 and T4) for two gas injector outputs 28,29. The gas injector outputs comprise of an adjustable ‘peak’ current time 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 6 is an embodiment of an electronic circuit that realises the timing diagram of Figure 5 where inputs 21,22) are the signals triggers for two gas injectors. These signals are either be taken from +ve or -ve edge transition of peak detector signals or other crankshaft position/velocity sensors that provide an injector timing sequence for example when an advance/retard is required forthe gas injectors. Monostable 36 outputs the gas injector ‘peak’ pulses 37, 38 which are also used as up/down signals forthe digital potentiometers 39,40 and as enable inputs for an electronic switch 41 which either connects a high frequency down-count’ signal 42 or the adjustable ‘up count’ signal 43 to the clock inputs of digital potentiometers 39, 40. In this embodiment the ‘up count, ‘down count’ and adjustable gas injector PWM signals are generated by astable timers 44, 45. On arrival of an engine firing pulse 21 monostable 36 generates a peak gas injector pulse 37 and the gas injector begins to open and during this peak current period the digital potentiometer decrements to zero. A D-type flip-flop 46 is ‘set’ and enables the peak signal to pass through NAND function logic gate 47 boosted through driver 48 and activate the gas injector MOSFET 49. 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 39 is incremented using the ‘up count’ signal and comparator 40 compares the digital potentiometer output to the input fuel flowrate signal 23. When this output voltage is equal to the fuel flowrate signal the output of the comparator 40 changes state causing the D-Type flip-flop 46 to reset and thus the gas injector is switched off 49, 50. On arrival of an engine firing pulse 22 an identical process using a monostable 44, D-type flip-flop 46, digital potentiometer 40, electronic switch 41, astable timers 44, 45, comparator 51, NAND gate 47 and driver 48 are used to activate a second gas injector. A variable resistor 52 is used to adjust the duty cycle of the PWM hold current to allow different specification gas injectors to be used. Variable resistor 53 is used to control the ‘on’ period of the gas injector for a given fuel flowrate signal. The amount of gas delivered is thus a function of the gas pressure and resistor 53 value. An fully sequential embodiment of the invention is possible when either an additional signal, for example a diesel injector, or signal from a pressure sensor fitted at the outlet for a given chamber is used which identifies which combustion chamber corresponds to a given peak signal from an exhaust pressure senor mounted in the exhaust pipe.

Alternatively a microprocessor could be used to process the gas injector firing signals and provide a gas injector ‘on’ period. Figure 7 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 54 to the exhaust 55, pass pressure sensor 56, turbocharger 57, oxidation catalyst 58, diesel particulate filter 59, selective catalytic reduction 60 and finally exit from silencer 61 where these components are fitted. The DPF filter has two pressure transducers 62, 63 which are used to provide a differential signal across the DPF which is used by the ECU to determine DPF purge. Pressure sensor 56 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 peak detector circuit or using the diesel injector actuation signals from the ECU. A more precise measurement are 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 is determined. As soon as the exhaust valve is opened a pressure wave travels through manifold 54 into exhaust pipe 55 and arrives at pressure sensor 56. This time interval is fixed and depends on the distance of pressure sensor 56 from the exhaust valve. A timer value which varies as a function of RPM is therefore calculated which is used to start the ADC sampling of the exhaust pressure sensor 56.

Figure 8 shows a graphical representation of the exhaust pressure signal 64. After an injection pulse 65, 66 has been detected the sampling begins after crank angle 67 which is calculated using engine speed and loaded into a timer as a time interval. Using a microprocessor and high speed ADC the exhaust pressure is sampled for timed window 68. During the window the ADC values are stored. These values are processed digitally to remove spikes. A stacked average method is 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 69, 70. The process is repeated for each combustion cycle thus obtaining a peak value of exhaust pressure for each cylinder firing. The timing of this peak is used to generate a peak pulse and used to determine the engine speed. The magnitude of the pressure signal, the peak signal 69, 70 relate to the amount of primary fuel combusted and provides a signal that is used to determine the amount of high octane fuel added for each engine cycle.

Figure 9 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 actuates within a few milliseconds. The engine air intake passes through air filter 71 and compressed or ‘charged’ by charger 72 before entering the engine intake manifold. A proportion gas valve 73 is used to inject a flow of gas through nozzle 74 controlled using an electrical input. Typically this electrical input uses PWM current regulation to control the valve. The gaseous fuel 75 enters filter 76)at a regulated pressure measured by pressure senor 77 and temperature measured by sensor 78. In this way temperature/pressure corrections are 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 10 is a schematic circuit diagram of the gas additive injection system using a proportional gas valve. In this embodiment a ‘single chip’ computer is used which has RAM, ROM (Flash memory), ADC channels and counters/timers. Exhaust pressure readings are continually digitised and the exhaust pressure peak amplitude and timing determined. This data is used to compute the primary fuel flow. The exhaust pressure signal 79 is signal conditioned 80 and input to processor 81 for analogue to digital conversion. The engine speed and amount of high octane gas is computed. The gas pressure 82, gas temperature 83) is measured and the gas flowrate calculated as a proportion of primary fuel flow. The gas flowrate output signal 84) is a PWM signal used to actuate proportional gas valve through driver 85. Alternatively a diesel injector signal is used to calculate the engine speed and exhaust gas pressure measurement timings.

In this embodiment a LPG tank fuel level signal 86, signal conditioned through amplifier 87 and input to processor 81 is shown. Processor 81 is used to output on a display 88 the operation of the gas injection system for the driver of the vehicle. Such display parameters include the fuel level, engine speed, engine firing, exhaust pressure, instantaneous fuel flow, gas pressure/temperature, average fuel flow. The display also has 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 will 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 exhaust pressure peak signal is 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 de-activates the diesel injectors to conserve fuel; there is no firing and 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 vehicies without automatic emergency engine shut off it is 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 are de-activated; hence gas is supplied when the engine runs in normal conditions and shut off in an emergency automatically.

This example is 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 one 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 ms. A single gas injector is used providing there are sufficient injection periods within a stroke of the piston in order to deliver consistent quantities of gas for each engine cycle for a non-sequential system. However a single injector system would have a shorter working life for high speed engines and the short injection period required causes non-linearity of gas delivery due to ‘water hammer’ effect in the fluid. However the circuit is 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 is supplied to the combustion chamber during the same combustion cycle using the current primary injector signal in real time. A 10 volt regulator is used to supply the preferred voltage for the MOSFET driver. The gas injection signals displayed using LED’s and a further LED used to indicate correct operation of the gas injection system via a ‘watchdog’ circuit and be used to signal ‘trouble’ codes such as for example, ‘low gas pressure’. In this event the gas injectors are automatically switched off. Ten segment bar LED’s are 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 - 25%, using 3 - 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 offers a low cost solution for the conversion of diesel or petrol engines to be converted to partially run on lower cost fuels such as LPG or green fuels such as methane or bio-gas and provide a major benefit both in terms of combustion efficiency and reduced harmful emissions. The invention also offers a useful diagnostic tool for engine performance since an exhaust pressure measurement would indicate issues relating to compression, piston ring wear/sealing and/or primary fuel injector performance.

Suitable secondary fuels for use in embodiments of the present invention include, but are not limited to: liquefied petroleum gas (LPG), natural gas in CNG or LNG form. Browns Gas, methane, methanol, ethanol, hydrogen and petrol (gasoline), and may comprise two or more different fuels of different molecular structures. In particular, although embodiments described herein relate to gas injection of the additive fuel, a liquid injection system may also be used. A common property for the short chained molecules including hydrogen is that they have a high Research Octane Number. For the embodiment to have effect the octane number of the secondary fuel needs to be higher than that of the primary fuel. Thus for example petrol (gasoline) is used as a secondary fuel in a diesel engine. 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. However, as a secondary 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 the octane number of the combined fuel, the more compression the combined fuel withstands before detonating. Accordingly, the engine efficiency is improved because fueis with a higher octane rating are used in high-compression Otto cycle engines that generally have better performance both in terms of power and economy.

The anti-knock properties of adding secondary fuel, which delays the peak pressure in the combustion chamber, allow the engine to adjust the injection timing to give 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. This adjustment of the engine is also facilitated by the cycle-by-cycle adjustment of the quantity of the secondary fuel supplied.

Claims (7)

1. A secondary fuelling system connected electrically and hydraulically to improve combustion efficiency and emissions of an internal combustion engine, the internal combustion engine being configured to be fuelled with a lower octane primary fuel and the secondary fuelling system being configured to supply a controlled quantity of a higher octane secondary fuel, in addition to the primary fuel, to a combustion chamber of the engine, comprising steps of measuring the peak exhaust pressure and/or peak differential pressure of exhaust gases in the exhaust system for an engine cycle, processing said signal to obtain peak exhaust pressure amplitude and timing; determine engine speed; compute a first fuel flowrate using amplitude and timing data provided by peak exhaust pressure measurement, measure high octane gas pressure and temperature at inlet to compute simultaneously a proportional quantity of high octane additive fuel from said first fuel flowrate signal, provide an injector signal to supply said additive fuel for the current engine cycle or following engine cycle in an appropriate timing pattern such that a proportional amount of high octane additive charge is present in the cylinder for each primary fuel injection and limited to a maximum fraction of the diesel or primary fuel beyond which engine knock would begin to occur due to the high concentration of additive or secondary fuel.

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) Using a peak detector circuit to determine the peak exhaust pressure; (ii) Computing an analogue first fuel flowrate signal using an analogue computer from said peak exhaust pressure measurement: (iii) Computing an analogue additive fuel flowrate signal using an analogue computer from said first fuel flowrate signal; (iv) Using the peak detector circuit to time the high octane gas additive injection.

3. A method according to Claims 1 and 2 whereby a proportional valve is used to inject the additive fuel comprising the steps of: (i) Measuring the timing between the peak amplitude exhaust pressure signals; (ii) Computing engine speed; (iii) Computing a continuous additive fuel flowrate to be used by a proportional valve injection system.

4. A method according to Claims 1-3 whereby the instantaneous fuel flowrate, additive fuel injection, engine speed, temperatures/pressures are displayed and the proportion of additive fuel adjusted on an instrumentation panel.

5. A kit as claimed in Claims 1-4 where a digital processor or microprocessor(s) is used to process the exhaust gas pressure measurement, compute peak pressure, compute timing and determine engine speed, compute primary fuel flow rate, determine the second or additive fuel flowrate and provide a second or additive fuel injector signals).

6. A method according to any preceding claim wherein the second additive fuel or alternative fuel is 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; 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; 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; 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.

7. A method according to any one of Claims 1-15, wherein the second additive fuei or aiternative fuel is petrol(gasoline), liquefied petroleum gas (LPG), compressed natural gas (CNG), liquid natural gas (LNG), methane, hydrogen, (Browns gas) is injected sequentially.