METHOD OF OPERATING AN INTERNAL COMBUSTION ENGINE

Abstract: A method of operating an internal combustion engine under low and medium load conditions comprises supplying to the engine a fuel and air charge that is not spark ignitable and exposing the charge to a series of sparks during the early part of the compression stroke to create within the charge high energy gas pockets. The sparks promote subsequent auto-ignition of the gases in the high energy pockets towards the end of the compression stroke or during the early part of the power stroke.

Description: METHOD OF OPERATING AN INTERNAL COMBUSTION ENGINE Field of the invention The present invention relates to a method of operating a spark ignition internal combustion engine under low and medium load conditions which comprises supplying to the engine a fuel and air charge and raising the temperature an pressure of the charge to cause at least part of the charge to ignite spontaneously. Background of the invention Spontaneous combustion in spar, ignited internal combustion engines is known and r.as generally been regardεc as undesirable as it normally manifests itself as knock wnen the engine is operating under hign load and as such is a cause of severe engine damage. Spontaneous combustion differs from normal combustion m that, m the place of a controlled burning of the charσe by flame propagation from a spark, there is a detonation wnen the cnarge effectively explodes and generates a very hign coπ iustion pressure. More recently, it nas been realised that tnough sucn spontaneous ignition should indeed be avoided under high load operating conditions, it has advantages if it can be induced at lower engine loads m that it produces cleaner emission gases (much lower NOx m particular) and allows leaner mixtures to be burnt completely. However, such combustion is difficult to control and various systems have been proposed to achieve controlled spontaneous combustion, also termed auto-ignition . Of course auto-ignition occurs all the time m compression ignition engines, sucn as diesel engine. In s cn engmes , auto-ignition is controlled by not introducing fuel into the air charge until after the cnarge has been compressed and the mixture is ignited spontaneously as soon as the fuel is injected under high pressure. The present invention is not concerned with such high compression engines, where the compression ratio is typically around 20:1, but with spark ignition engines, such as petrol engines, where the charge is premixed and the compression ratio is typically around only 10:1. The engine of the invention is spark ignited at high load and controlled auto- lgnition is relied upon only under lower engine loads to improve emissions and lean burn capability. For this reason, increasing the compression ratio to assist auto-ignition is not an available option as the engine would not be able to operate at high load. Earlier proposals for promoting controlled auto- lgnition m a premixed charge engine have included heating the intake air, using high levels of internal EGR (where the gases are naturally still hot), or using a lower octane fuel. Obiect of the invention The present invention seeks to provide a method of operating a spark ignition engine under low and medium load conditions to promote auto- ignition while allowing normal spark ignition of the charge under h gh load operating conditions . Summary of the invention In accordance with the present invention, there is provided a method of operating an internal combustion engine under low and medium load conditions which comprises supplying to the engine a fuel and air charge that is not spark ignitable and exposing the charge to a series of sparks during the early part of the compression stroke to create within the charge high energy gas pockets, the sparks promoting subsequent spontaneous combustion (auto-ignition) of the gases m the high energy pockets towards the end of the compression stroke or during the early part of the power stroke . US 4,846,129 discloses a lean burn engine m which the lean burn limit is extended by using a series of sparks instead of a single spark to ignite the mixture. Tne sparks are not used to promote auto-ignition but to ignite the mixture. On the contrary, steps are taken to ensure that auto-ignition is avoided at all times. In the present invention, the pre-mixed fuel/air main charge is ignitable neither by a single spark nor by multiple sparks on account of the high dilution ratio, that is to say a high air/fuel ratio or a high proportion of recirculated exhaust gases . Instead the sparks serve only to ionise and heat the charge locally, producing a series of high energy gas pockets that are carried along by m- cylinder charge motion and distributed widely throughout the mam charge. When these high energy gas pockets are further compressed and heated, they reach the auto-ignition temperature and spontaneously ignite. The sparks thus induce spontaneous ignition of the high energy gas pockets at a later stage m the engine cycle and the combustion system may be regarded as being a spark induced compression ignition (SICI) system. The entire charge does not however spontaneously ignite but burns by flame propagation between the high energy gas pockets. Nevertheless, because of the numerous high energy pockets and the shorter flame propagation distances involved, the combustion of the charge will be completed m a shorter time than m a conventional spark ignition engine. This combustion mechanism is to be contrasted with the known systems m which controlled auto-ignition is achieved by heating the intake air to increase the average charge temperature at the beginning of the compression stroke. In this prior art proposal, when the charge is compressed, its temperature will rise sufficiently towards the end of the compression stroke to exceed the auto- ignition temperature, whereupon the whole charge will auto- ignite. In this invention, the high energy gas pockets are produced m the main charge during the early part of the compression stroke and will be hotter than the main charge. Even allowing for some cooling because of thermal conduction and fluid mixing, these gas pockets when compressed along with the main charge will reach a higher temperature than the mam charge towards the end of the compression stroke, sufficiently to reach the auto-ignition temperature earlier, whereupon the high energy gas pockets will auto-ignite and initiate combustion across the mam charge. The strong tendency for the high energy gas pockets to auto-ignite is on account of the fact that these gas pockets are not only hot but also highly ionised. By varying the intensity, frequency and timing of the sparks occurring at the beginning of the compression stroke, it is possible to control the instant at which auto-ignition of the high energy gas pockets will occur later m the cycle. As a failsafe against the possibility that there may not be sufficient energy m the gas pockets to induce auto- lgmtion towards the end of the compression stroke, it is possible to avoid a non-firing cycle by applying a stronger ma ignition spark and, if desired, additionally introducing a pilot direct injection of rich fuel towards the spark plug at a last predetermined instant later than the expected instant of auto-ignition to initiate normal ignition. Such a failsafe mam ignition spark will normally prove redundant as the charge will already be burning; it will nevertheless do not harm if combustion has already commenced. However, it will guarantee proper firing if for any reason, such as variation in fuel quality, the charge in the high energy pockets should fail to ignite spontaneously. Even when the charge combustion is initiated by a conventionally timed ignition spark, the flame propagating outwards from the spark ignited kernel will compress the charge ahead of it and will further increase the pressure and temperature of the high energy gas pockets within the charge. The further temperature and pressure increase could eventually be sufficient to cause the gas pockets to auto- ignite. In this case the timing of auto-ignition would follow the timing of the main ignition spark, and is variable and controllable by controlling the timing of the main ignition spark. Immediately after auto-ignition of the pre-mixed charge, more fuel may be injected into the hot charge and burnt as a diffusion flame at the rate it is injected. This could be used to consume more of the oxygen present in the charge and achieve higher loads without knock. This makes it possible to run a diesel engine by SICI without relying on a high compression ratio. It will be appreciated that the invention may be applied to two-stroke and four-stroke internal combustion engines burning all kinds of fuels, such as gasoline, diesel, alcohol and gaseous fuels. When applied to a two- stroke engine, reference to "compression stroke" should be construed to mean the part of the up-stroke of the piston after the ports has been covered by the piston, during which period the charge is being compressed. Brief description of the drawings The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of the engine and its management system, and Figure 2 is a graph showing the variation of the pressure withm the combustion chamber with time during the compression and power periods of the engine cycle. Detailed description of the preferred embodiments The block diagram of Figure 1 is generally conventional and shows schematically an engine 10 having a management system 12 which applies output signals to a fuel metering system 16 and a spark ignition system 18 m dependence upon various input signals, including one from a demand pedal 14. The ignition system 18 needs to be capable of producing sparks rapidly so that numerous sparks are generated m the early part of the compression stroke. An ignition system with this capability has already been proposed that relies on forming nanometre sized fibres on to the end of a piezoelectric crystal to make a high voltage transformer. By applying a primary a.c. voltage of between 100V and 150V, it is claimed by the manufacturers of the transformer that an output voltage of up to 20kV can be produced at up to 40 kHz to drive a spark plug (see European Automotive Design, March 1999, Page 14). The invention is concerned with the operation of an engine when under low and medium load and when supplied with a highly diluted fuel mixture. Such a mixture cannot support flame propagation because of the low concentration of the fuel in the charge and cannot for this reason be readily ignited, especially m the early part of the compression stroke when its temperature and pressure are both low. To promote clean combustion, the invention proposes firing a series of sparks as shown m Figure 2 near the commencement of the compression stroke. Such sparking may continue for as long as the charge temperature and pressure are too low for the mixture to be ignited by tne sparks. The effect of the sparks is not to ignite the mixture but to create pockets of high energy gases that are carried with the charge motion and distributed throughout the charge. When later the cycle, the compression of the charge increases the pressure and temperature these pockets, they undergo auto- ignition at the instant shown Figure 2 preceding TDC. Auto-ignition occurs at the pockets almost simultaneously and initiates combustion of the mam charge by flame propagation from numerous kernels so that the charge is consumed more rapidly than when it is initiated conventionally by a single ignition spark. This results m a higher and more sharply defined pressure peak as shown but localised high temperatures are avoided, thereby reducing the production of NOx. Because the engine at this time is not operated with a high load, the peak pressures are not so high as to risk damaging the engine. It is however important not to use SICI at high load as it could cause engine damage . Figure 2 shows that a ma spark is still fired before TDC m the conventional manner. However, this spark occurs after the expected timing of the auto-ignition and will normally serve no purpose. However, the spark does guarantee charge combustion the event that auto-ignition does not occur as predicted for any reason. If the mixture is so lean that it would not be ignitable even near TDC, then it is possible to inject additional fuel directly into the combustion chamber and such injection is preferably directed towards the spark plugs to create a cloud of high fuel concentration in the vicinity of the spark gap. When the engine has a facility for in-cylinder fuel injection, it is possible to inject additional fuel after auto-ignition has occurred, to consume any surplus oxygen and produce more power from the engine without running into knock. The timing of auto-ignition will depend on the energy stored in the high energy gas pockets and by varying the spark voltage, frequency and timing one can influence the instant at which auto-ignition occurs.

Claims: CLAIMS 1. A method of operating an internal combustion engine under low and medium load conditions which comprises supplying to the engine a fuel and air charge that is not spark ignitable and exposing tne charge to a series of sparks during the early part of the compression stroke to create within the charge high energy gas pockets, the sparks promoting subsequent auto-ignition of the gases m the high energy pockets towards the end of the compression stroke or during the early part of the power stroke. 2. A method as claimed in claim 1, wherein the fuel and air charge is rendered non- ignitable by the series of sparks by setting a high dilution ratio of air or EGR gases to f el . 3. A method as claimed m claim 1 or 2 , wherein the intensity and/or frequency and/or timing of the series of sparks is/are adjustable to vary the energy stored m the high energy gas pockets. 4. A method as claimed m any preceding claim, wherein a mam ignition spark is fired near top-dead-centre to ensure charge combustion in the event of failure of the high energy gas pockets to auto- ignite. 5. A method as claimed m claim 4, wherein a pilot direct injection of rich fuel is additionally introduced and directed towards the spark plug to ensure normal ignition of the charge . 6. A method as claimed m any preceding claim, wherein immediately after auto-ignition of the high energy gas pockets, additional fuel is injected into the engine cylinder to produce more power.

Inventors: MA THOMAS TSOI-HEI, MA, THOMAS, TSOI-HEI
Applicants: FORD GLOBAL TECH INC, MA THOMAS TSOI HEI, FORD GLOBAL TECHNOLOGIES, INC., MA, THOMAS, TSOI-HEI
Application year: 2000
Main classification: F02P9
IPC codes: F02B1, F02B3, F02B51, F02B75, F02D41, F02P9