FIELD OF INVENTION

The present invention is directed to an internal combustion engine and in particular to the exhaust valve opening timing and lift strategy for 2-stroke internal combustion engine.

BACKGROUND OF THE INVENTION

Currently, the most commonly used combustion systems available in the market are either spark ignition (SI) or compression ignition (CI). SI engine normally involves the use of gasoline fuel whereas compression ignition involves the use of diesel fuel.

Both SI and CI engines can be operated using 4-stroke or 2-stroke. With the increasingly stringent tailpipe emission regulations, the market share of the 2-stroke engines has dropped significantly. Some countries have even banned the use of 2- stroke engines in order to reduce the emissions of hydrocarbon (HC) and carbon monoxide (CO) from motor vehicle.

However, 2-stroke engines are known to have higher power output, lighter engine weight and simpler overall engine construction. Such advantages have motivated many researchers around the world to continue developing the 2-stroke engine with the main objective of reducing the HC and CO emissions to below the current and future emission limits.

There is therefore a need for an improved internal combustion engine that is lighter, and/or has a higher power output, and/or has lower emissions.

Thus according to the present invention there is provided a 2-stroke internal combustion engine comprising: a cylinder; a piston located in the cylinder and connected to a crankshaft for reciprocal motion with respect to the cylinder, and defining a combustion chamber with the cylinder; a fuel injector to selectively inject fuel into the combustion chamber; at least one inlet valve to selectively deliver pressurised oxidising agent into the combustion chamber; at least one exhaust valve to selectively open and allow exhaust gases to be expelled from the combustion chamber and means for varying the exhaust valve operating characteristic.

Such an arrangement makes it possible for the effective compression ratio and residual gas content to be optimally varied for maximum efficiency across various engine rpm and loads covering start up, idle, part load, and full load engine operations. Furthermore, such strategy makes it possible for the cylinder temperature (combustion chamber temperature) as the piston nears TDC to be accurately controlled thereby allowing various fuels and combustion modes to be used using the same engine.

In one embodiment, in varying the exhaust valve timing, the effective compression ratio of the engine can be changed throughout the engine operation, hi advancing the points where the exhaust valve will open and close, the points where exhaust stroke will start and end can be optimally adjusted. Once the exhaust valve is fully closed, compression stroke starts and the charge in the cylinder will be compressed causing the charge temperature to rise significantly. Advancing the exhaust valve closing point will raise the charge temperature further whereas retarding the exhaust valve closing point will reduce the charge temperature.

Varying the effective compression ratio benefits the engine because the cylinder temperature can be accurately determined as the piston approaches TDC.

It is not possible for conventional engines to accurately control the cylinder temperate

^■ because the cylinder temperature normally gets higher than it should be at higher engine rpm and load. Moreover, with 4-stroke engine operation normally found in conventional engines, varying the exhaust valve timing to control the cylinder temperature often caused the charge in the cylinder to be compressed twice, once at the end of the exhaust stroke and another one during the main compression stroke. Furthermore, by closing the exhaust valve early during the exhaust stroke, significant amount of exhaust gas remains in the cylinder and this hinders the induction of air in the induction cycle.

Li another embodiment the effect of exhaust valve timing variation can be further enhanced by combining the timing variation with variation of exhaust valve lift. By having relatively lower exhaust valve lift during the exhaust stroke (when compared to other conventional 4-stroke engines), significant amount of exhaust gas can be retained. In retaining the exhaust gas, significant amount of heat can be kept for the purpose of raising the cylinder temperature for fuel auto-ignition purposes. Indirectly, lesser amount of compression work is needed to raise the cylinder temperature above the fuel auto-ignition temperature. This means, the exhaust valve closing can be further retarded resulting in very minimum compression work.

Experiments show that the small cross section area of opening during low valve lift operation does not result in any significant resistance against the piston moving upward.

In another embodiment the low valve lift can also be combined with exhaust valve deactivation in which 1 out of 2 exhaust valves can be deactivated. This feature is useful for a high performance engines that require large exhaust valve cross section area opening for high speed engine operation but at the same time requiring much smaller opening for idle or part load operation.

With cylinder temperature accurately controlled near TDC, various gaseous and liquid fuels can be ignited using the same engine. For example, an engine that is originally designed and optimized for diesel fuel can easily be adapted to run on fuels with much higher auto-ignition temperatures like natural gas and methanol. It is possible to do so by advancing the exhaust valve closing point together with relatively lower exhaust valve lift. The additional compression work and additional exhaust gas mass retained in the cylinder will increase the cylinder temperature near the TDC. The combination of exhaust valve timing and exhaust valve lift adjustments opens up the possibility for the engine to have broad cylinder temperature options near TDC. This provides an opportunity for automakers to offer vehicles that are capable of running on various liquid and gaseous fuels without significant drop in engine performance, efficiency and emissions.

With an engine capable of accurately controlling the cylinder temperature near TDC, various combustion modes ranging from spark ignition, controlled auto-ignition, diffusion combustion, premixed combustion and Homogeneous Charge Controlled Ignition (HCCI) can be used. In particular, the problem faced by engine researchers to auto-ignite fuel that is homogenously distributed in the cylinder can be effectively solved, hi the past, the homogenous fuel charge ignites either too early or too late with changing engine rpm and load. Fuel ignition that is too early will cause the cylinder pressure to build up, resisting piston from moving upward. This phenomenon normally happens at higher engine load where the cylinder temperature is relatively high. On the other hand, fuel ignition that is too late is likely to reduce thermal efficiency as the cylinder pressure build up happens too late to efficiently push the piston downward. This phenomenon normally happens at lower engine load where the cylinder temperature is relatively low.

This is where the combination of exhaust valve timing strategy and 2-stroke operation provides a solution to the problem of uncontrollable auto-ignition, hi general, any widely used fuel has its own auto-ignition temperature, therefore both exhaust valve timing and lift can be optimized to enable specific auto-ignition temperature to be reached at about 15 to 0 degree before TDC depending on engine speed and load.

Thus various homogenous charge combustion modes like HCCI and Controlled Auto Ignition (CAI) can be adopted to lower the engine raw emissions without compromising fuel economy. It is also possible for engines according to the present invention to switch between stratified charge auto-ignition and homogenous charge auto-ignition mode. Furthermore, engines that depend on spark plug to ignite the fuel charge in one mode can also switch to CAI mode in which the operation range involving CAI mode can be extended to broader engine rpm and load.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention are now described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 shows a schematic view of an internal combustion engine according to the present invention, Figure 2 is a diagram of the exhaust valve opening and closing during engine start up operation,

Figure 3 is a diagram of exhaust valve opening and closing during engine idle operation,

Figure 4 is a diagram of exhaust valve opening and closing during engine full load operation, and

Figure 5 shows the graph of the exhaust valve lift vs. crank angle.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Figure 1 shows an internal combustion engine 10 having a cylinder 12, a piston 14, a connecting rod 16 and a crank shaft 18. The piston reciprocates within the cylinder to drive the crank shaft via the connecting rod in a manner well known in the art. The internal combustion engine 10 further includes an exhaust valve 20 which can be selectively opened and closed to allow exhaust gases to pass from the combustion chamber 22 into the exhaust port 24. An actuation means 26 operates to open and close exhaust valves 20 as will be further described below. A fuel injector 28 is provided to selectively inject fuel into the combustion chamber 22.

An inlet valve 30 is operable to selectively deliver a pressurised oxidising agent into the combustion chamber.

The actuation means 26 allows the exhaust valve operating characteristics to be varied. Typically the actuation means 26 will allow a variation in one or more of the exhaust valve operating characteristics, such as a variation in one or more of the exhaust valve opening point, the exhaust valve closing point, the exhaust valve lift, the exhaust valve opening profile, the exhaust valve closing profile. The exhaust valve lift may be varied between a minimum and a maximum. The minimum exhaust valve lift can be zero. The exhaust valve opening profile equates to the speed at which the valve opens relative to the axial position of the piston in the cylinder, and where the valve is opened via a cam, the exhaust valve opening profile equates to the opening ramp profile of the exhaust cam. The exhaust valve closing profile equates to the speed at which the valve closes relative to the axial position of the piston in the cylinder and, where the exhaust valve is cam operated, it is the equivalent of the closing ramp profile of the cam.

Figure 2 shows the valve opening and closing strategy used during start up operation. The exhaust valve opening (EVO) is advanced as close as possible to TDC in this case 45° after TDC. This enables the exhaust valve to function as an inlet valve during engine start up. The vacuum created when the piston moves down will make it possible for ambient air in the exhaust pipe to be sucked into the cylinder, hi making sure that there will no foreign materials being sucked into the cylinder through the exhaust valve, a catalytic converter, when fitted, will function as air filter during start up.

hi this case the inlet valve involves the use of electronically control gas injector or solenoid control poppet valve to control the delivery of pressurised oxidising agent or air into the combustion chamber, and as such an ECU (electronic control unit) controlling the inlet valve can be arranged to ensure the inlet valve stays shut until after the first revolution of successful engine fire up has occurred. Alternatively, in a further embodiment the inlet valve may be operated during start up to selectively deliver the pressurised oxidising agent into the combustion chamber to increase the charge mass.

Once first firing is successful, the exhaust valve opening must be retarded as soon as possible so that the exhaust valve opening point is moved to just before the BDC (see figure 3). The use of motorized WT requires some time before the exhaust valve opening point can be returned to the normal position as it may to be several engine revolutions before the exhaust valve opening nears BDC.

Once start up is achieved, the inlet valve must start to be operated to allow the oxidizing agent or air into the cylinder.

Though it is desirable to run the engine at stochiometric air-to-fuel ratio at part load and full load, the start up operation requires lean air-to-fuel ratio right until the exhaust valve opening point is retarded close to BDC. Lean air-to-fuel ratio will continue to be used if the engine idle stability is still not up to the required standard, hi general, the lean air-to-fuel ratio is required to ensure that there will be more than enough oxygen available for. combustion to take place.

One way of achieving the exhaust valve opening points of figures 2 and 3 is using a wide range variable valve timing mechanism preferably a motorized variable valve timing (WT). With motorized WT, the exhaust valve can be opened about 45° to 100° after TDC during start up.

The use of camless valvetrain system involving solenoid, hydraulic or pneumatic operation will give greater flexibility during start up operation because the exhaust valve can be opened as early as 0° after TDC. At the same time, the exhaust valve opening can be returned back to the "normal" position near the BDC right after the first successful fire up.

Figure 2 shows the exhaust valve timing and lift strategy for engine idle operation. During engine idle, the exhaust valve closing point will be retarded relatively close to TDC. The exhaust valve closing point (EVC) is 60° before TDC but can range from 45°-70° before TDC depending on how many exhaust valves are used and how low the valve lift is.

At the same time, the exhaust valve opening point will also be retarded very close to the BDC. There is no need for the exhaust valve to open earlier as there will be a relatively low amount of exhaust gas to be discharged during part load thus the blowdown period can be significantly shortened. The exhaust valve opening point (EVO) is 1° before BDC but can range from l°-8° before BDC depending on how many exhaust valves are used and how low the valve lift is.

To avoid the oxidizing agent or air from being short circuited into the exhaust port during the compression stroke, the oxidiser agent or air will only be delivered into the cylinder after the exhaust valve is almost closed or fully closed. In this case the inlet valve opens (IVO) at 35° before TDC and closes at 25° before TDC.

As the exhaust valve opening is retarded during idle, the compression work may not be sufficient to increase the cylinder temperature to optimum temperature for fuel auto-ignition. Furthermore, exhaust gas at idle is known to have a much lower gas temperature than compared to the gas temperature at full load.

Thus in order to raise the combustion chamber temperature for optimum fuel auto- ignition, the exhaust valve lift can be lowered as low as lmm lift. The low exhaust valve lift is effective in retaining as much exhaust gas as possible once the exhaust valve is fully closed. The retained exhaust gas will provide both residual gas mass and heat to raise the cylinder temperature.

As described above, the duration of the exhaust valve opening is kept constant when the lift is being varied from minimum to maximum lift. Figure 3 shows the exhaust valve lift variation which is accompanied by constant exhaust valve opening duration.

In keeping the exhaust valve opening duration constant during the low exhaust valve lift mode at idle or part load, it is possible to prolong the time available for the piston to push the exhaust gas out of the cylinder without causing the cylinder pressure to rise. With no significant increase in cylinder pressure, the work required to overcome the resistance in pushing out the exhaust gas is also minimised.

Experimental work shows that, even with minimal compression work, it is still possible to raise the cylinder temperature to more than 150° Celsius above the fuel auto-ignition temperature at idle. Thus low exhaust valve lift is effective in retaining the exhaust gas which in turn will provide the mass at elevated temperature to increase the cylinder temperature.

Experimental work also shows that the cylinder pressure during the fuel auto-ignition at idle is much lower than cylinder pressure of conventional CI engines. Such a low cylinder pressure will be beneficial in lowering the boiling point of liquid fuels thus making the fuels easier to evaporate. Ability of fuel to evaporate faster right after injection will make it possible for the ignition delay to be minimized. Minimum ignition delay on the other hand minimizes premixed combustion where sudden release of heat often cause noise and erratic combustion.

Figure 4 shows the valve timing and lift strategy for full load operation. At full load operation, the exhaust valve lift is at its maximum lift. Maximum valve lift will enable maximum exhaust gas to be discharged into the exhaust port. The exhaust valve opens and closes earlier than at idle (figure 3). The exhaust valve closing point (EVC) is adjusted to ensure that the cylinder temperature is high enough for auto ignition.

Experiments show that it is possible to keep the cylinder temperature at a desired temperature (say 2000° Kelvin) even at full load operation. This is achievable by keeping the cylinder temperature high enough to get the fuel auto-ignited but not so high that additional heat from the fuel oxidation will bring the cylinder temperature significantly above 2000° Kelvin.

Experiments show that cylinder pressures just before the start of fuel auto-ignition is still relatively low when compared to conventional diesel engine. Whereas the conventional diesel engine cylinder pressure prior to the start of fuel auto-ignition can be as high as 100 Bar, the cylinder pressure for the exemplary embodiment is only 35

Bar. Maximum cylinder pressure of the exemplary embodiment is only about 80 Bar if compared to maximum cylinder pressure of conventional diesel engines that can be as high as 180 Bar. With the method to operate the engine established for start up, idle and full load operations, it is assumed that valve strategy for part load operation which lay in between idle and full load can be interpolated.

As mentioned above, the means for varying the exhaust of valve operating characteristic could be an ECU controlling a solenoid which opens the exhaust valve, an ECU controlling a hydraulically operated exhaust valve or an ECU controlling a pneumatically operated exhaust valve. Alternatively, the means for varying the exhaust valve operating characteristic could be a cam system such as is shown in EP1300551, or in US5636603 or any other known type of cam operated variable valve system.

With regard to valve lift, the invention can utilise variable valve lift cam systems or alternatively it can utilise valve lift systems where there are only two or more discreet valve lift settings.

As shown in figure 2, the inlet valve closes at approximately 10° before TDC and this is following approximately 170° of compression (the exhaust valve closes approximately 170° before the inlet valve closes) during engine start up conditions. As shown in figure 3 the inlet valve closes approximately 25° before TDC following a compression stroke of approximately 30° during idle operation. As shown in figure 4 the inlet valve closes approximately 25° before TDC following a compression stroke of approximately 45° during full load operation.

Clearly, under all three circumstances, the pressure in the combustion chamber will be above atmospheric pressure and will depend upon the design of the engine itself. In order for the inlet valve to selectively deliver a pressurised oxidising agent to the combustion chamber, the oxidising agent must be at a pressure above the pressure in the combustion chamber. The oxidising agent may be pressurised to 50 bar or more, alternatively 100 bar or more, alternatively 150 bar or more, alternatively 200 bar or more. The invention also provides a method of operating a 2-stroke internal combustion engine including steps of: a) providing a 2-stroke internal combustion engine as defined in any one of claims 1 to 9, b) running the engine.

Preferably the method includes the steps of: a) monitoring at least one engine parameter, b) varying the exhaust valve operating characteristics so as to create a combustion chamber temperature near TDC sufficient to auto ignite the fuel.

Typically, the parameter monitored will be a temperature, most typically an exhaust gas temperature. By knowing the exhaust gas temperature it is possible to modify the exhaust valve operating characteristic on the following engine cycle to provide a temperature in the combustion chamber which will auto ignite the fuel being used.

Typically other parameters will be measured including engine speed, engine power demand, the temperature and/or pressure of the oxidising agent, a cylinder temperature, a fuel temperature. Knowing various parameters, and providing the ECU with a suitable algorithm, will allow the engine to be run in an auto ignition mode.

The engine has been described in respect of a single cylinder. In further embodiments multiple cylinders can be used and where such multiple cylinders are used various engine configurations such as in line engines, V engines, W engines, radial engines etc.

As described above, an oxidising agent is delivered into the combustion chamber. This oxidising agent can be air. Alternatively it can be a gas comprised mainly of oxygen. In particular the gas can be 90% oxygen or above, alternatively 95% oxygen or above. As will be appreciated the fuel is injected directly into the combustion chamber. As will be appreciated the water is injected directly into the combustion chamber. As will be appreciated the oxidising agent is injected directly into the combustion chamber. As will be appreciated the injected oxidising agent is the sole source of oxidising agent after start up (e.g. the engine does not include traditional inlet valves).

In some embodiments a water temperature sensor can be provided to determine the temperature of the water just prior to injection.

hi some embodiments two fuel injectors can be provided.

A first fuel injector may inject a first fuel of a higher cetane value prior to a second fuel injector injecting a second fuel of a lower cetane value. In some embodiments both fuels are ignited by auto ignition. Injecting a first fuel of a higher cetane value requires a lower auto ignition temperature which allows more energy to be extracted from the previous power stroke. Once the first fuel has started to burn the temperature within the combustion chamber, increases, in particular to a temperature at or above the auto ignition temperature of the second fuel, which can then be injected and will auto ignite. Thus preferably the first fuel is injected when the combustion chamber conditions are such as to be below the auto ignition temperature of the second fuel. In this way power can be produced by using low grade fuels, e.g. fuels with a low cetane value. Such fuels tend to be cheaper than higher grade fuels. The first fuel may be a high grade of fuel such as a diesel fuel. The second fuel may be a fuel derived from plants, such as a biomass fuel. The second fuel may be pyrolysis oil.