GB2539906A - Fuel injection system for internal combustion engines - Google Patents

Abstract

A fuel injection 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 24. A method of supplying the secondary fuel includes the steps of determining the primary fuel flow rate for an engine combustion cycle, processing engine or primary fuel injection signals to obtain timing, determining the engine speed, measuring the secondary fuel pressure 26 and temperature 27 at the fuel inlet, computing a quantity of the secondary fuel, providing an injector signal for a proportional valve 22 to supply said calculated quantity of secondary fuel and injecting said secondary fuel either as a single point injection at an air intake into an inlet manifold or sequentially at close proximity to an inlet valve for each chamber of the engine.

Description

DESCRIPTION

FUEL INJECTION SYSTEM FOR INTERNAL COMBUSTION ENGINES

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 longer the 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; (ill) 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 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 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.

The majority of fuel injectors used in injection engines use ON/OFF to control the fuel injector and the fuelling amount is determined primarily by the fuel pressure and the time the injector is in OPEN’ state. For engines that have been converted to run using a sequential injection system alternate fuel such as LPG/CNG and where the primary fuel injectors have not been replaced with alternate fuel injectors there exists a known problem of the timing of the fuel into the combustion chamber since the alternate injectors are positioned in a different physical space. This is often exacerbated by positioning alternate fuel injectors away from the inlet manifold. This causes fueiling errors - resulting in an engine to idle poorly or even stall. In modern vehicles an engine stall is serious - loss of power steering, loss of braking power and thus presents a major safety issue. The problem is mainly identified when the engine load is changed, for example when the engine is idling and an additional load is required from an auxiliary appliance such as air conditioning/audio system/power steering causing the engine to stall due to insufficient fuel because of the extra time lag from the alternate fuel injection. One of the main issues to convert an engine using a multipoint sequential injection system is the amount of work and difficulty required to fit each injector into the inlet manifold.

In a simpler fumigation system the use of standard ON/OFF type fuel injectors for injecting additive fuel results in pulses of additive fuel into the air flow. Ideally each pulse would need to coincide with an induction stroke of a piston as in a sequential system. The exception to this is where more than one injector is used and where one injector closes another opens; but this will only occur at one engine load/speed. At other engine speeds/loads there will be aliasing between the injectors and further aliasing with the induction strokes dependant on the frequency of the injector firing and speed of the engine. 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 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 will cause non-linearity of gas delivery due to ‘water hammer’ effect in the fluid. The aliasing results in varying amounts of fuel being delivered to each chamber of the engine. To reduce this effect the fuel injectors would need to operate at much higher frequencies than each induction stroke and to achieve this would require multi injectors operating at high frequencies.

This invention discloses an electronically controlled proportional valve injection system which provides a continuous flow and controlled amount of additive fuel injected as a proportion of primary fuel as determined by a Injection Control Unit (ICU). The direct effect of adding small percentage of additive high octane fuel (typically in the range 3 - 15% by mass) causes a delay in the peak exhaust pressure with respect to crank angle and for an engine fitted with a knock sensor allows an ‘Atkinson Cycle effect’ which also improves combustion efficiency, reduce particulate matter and NOX. An advantage of the method is that it is fitted and installed without modifications to the top end of an engine.

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 graphical schematic which shows 5%, 10% and 15% by mass of the additive fuel injection with reduction for higher engine loads;

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

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

Figure 5 is a schematic diagram using a Proportional Gas Valve for injection.

Figure 6 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 (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 forthe 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 example in 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 a suitable envelope 15 for secondary fuel injection for smaller diesel engines that are used typically in modern motor cars and vans. The injection system for these engines offers more control for the primary fuel delivery and uses ‘multiburst’ (separate rapid firing of injectors) combined with a greater range for the common rail injection pressure. Thus the secondary fuel percentage 16,17 is made proportional to the primary fuel mass over the full load range of the engine. 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 10,19 is used for further fuel cost savings without detriment in engine emissions, for example when the engine is at idle or under part-load such as when ‘power take off is used.

Figure 5 shows an embodiment of the invention whereby standard ‘ON/OFF gas injectors 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 20 and compressed or ‘charged’ by charger 21 before entering the engine intake manifold. A proportion gas valve 22 is used to inject a flow of gas through nozzle 23 controlled using an electrical input. Typically this electrical input uses PWM current regulation to control the valve. The gaseous fuel 24 enters filter 25 at a regulated pressure measured by pressure senor 26)and temperature measured by sensor 27. 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 it provides a continuous flow and a controlled amount of gas additive fuel is delivered. In this preferred embodiment the gas valve is positioned close to the engine intake manifold post turbo. Where the length of intake pipe between the turbo and intake manifold the proportional valve is fitted pre-turbo at the air intake. This allows the action of the turbo to mix the gas with the air and to ensure that each cylinder receives the same amount of additional fuel for each induction stroke. Preferably a further gas shut off valve is installed prior to the proportional valve to provide additional safety along with shut offgas valves on the fuel tanks.

Figure 6 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 countersAimers. Engine data input 29 is signal conditioned 30 and input to processor 31. The processor computes the primary fuel flow, engine speed and the amount of high octane gas to inject. The gas pressure 32, gas temperature 33 is measured and the gas flowrate calculated as a proportion of primary fuel flow. The gas flowrate output signal 34 is a PWM signal used to actuate the proportional gas valve 22 through driver 35.

In this embodiment a LPG tank fuel level signal 36, signal conditioned through amplifier 37 and input to processor 31 is shown. Processor 31 is used to output on a display 38 the operation of the gas injection system for the driver of the vehicle. Such display parameters include the fuel level, engine speed, engine firing, instantaneous fuel flow, gas pressure/temperature, average fuel flow. The display will have a control input to adjust the ratio of injected gas/injected diesel. The displayed parameters enable diagnostics and provide feedback to the driver that the additive fuel is injecting and also to aid the driver to drive efficiently.

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 circuit is powered from the vehicle battery via the ignition circuit and protected with fuse. For vehicles 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 is 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 vaive is closed as the intake valve is opened. An alternate embodiment is where an electronically controlled proportional valve is 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 the injection period is determined according to engine speed, and the amount of fuel proportionally controlled using the valve. 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 primary fuel flowrate and the proportional amount of second additive fuel or alternative fuel supplied to the combustion chamber during the same combustion cycle in real time. A 10 volt regulator is used to supply the preferred voltage for the MOSFET driver. The gas injection signals are displayed using LED’s and a further LED used to indicate correct operation of the gas injection system via a ‘watchdog’ circuit and are 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.

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 fuels 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 (5)

1. A fuel injection 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 amount and continuous flow of a higher octane secondary fuel, in addition to the primary fuel, to a combustion chamber of the engine, comprising steps of determining the primary fuel flowrate for an engine combustion cycle for the current engine cycle or following engine cycle; processing engine or primary fuel injection signals to obtain timing; determine engine speed from said timing signals; measure high octane fuel pressure and temperature at engine inlet to compute simultaneously a proportional quantity of high octane additive or alternative fuel from said first fuel flowrate signal(s), provide injector signal(s) for a proportional valve(s) to supply said additive fuel; inject high octane additive fuel either as a single point injection at the air intake into the inlet manifold, or sequentially at close proximity to an inlet valve for each chamber, such that a proportional amount of high octane additive or alternative fuel charge is present in the cylinder for combustion.

2. A method according to Claim 1 whereby the proportional amount of secondary high octane fuel is limited to a maximum fraction of the primary low octane fuel beyond which engine knock begins to occur due to the high concentration of additive or secondary fuel.

3. A method according to Claims 1-2 whereby the instantaneous fuel flowrate, additive fuel injection, engine speed, temperatures/pressures are displayed and the proportion of additive fuel is adjusted on an instrumentation panel.

4. A method according to any preceding claim wherein the second additive fuel or alternative fuel is injected sequentially using a proportional valve 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.

5. A method according to any one of Claims 1-4, wherein the second additive fuel or alternative fuel is petrol(gasoline), liquefied petroleum gas (LPG), compressed natural gas (CNG), liquid natural gas (LNG), methane, hydrogen, (Browns gas) is injected using a proportional valve.