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Title:
ENGINE CONTROL STRATEGY
Document Type and Number:
WIPO Patent Application WO/2007/022602
Kind Code:
A1
Abstract:
An internal combustion engine (10) has at least one combustion chamber (15) defined by a piston (13) accommodated in a cylinder (11), a fuel injection system (40) for delivering a metered quantity of fuel entrained in a body of air, and an exhaust means (30). The engine (10) may also have an inlet means (20) for introducing a charge of air into the combustion chamber (15) to supplement the air delivered by the fuel injection system (40). The method of operating the engine (10) comprises opening the exhaust means (30) as the combustion chamber (15) expands to permit exhaust gas to discharge from the combustion chamber (15), closing the exhaust means (30) to interrupt discharge of exhaust gas from the combustion chamber (15), actuating the injection system (40) to deliver a metered quantity of fuel entrained in gas into the combustion chamber (15), and initiating combustion of the fuel.

Inventors:
CATHCART, Geoffrey, Paul (95 Rookwood Street, Mount Lawley, Western Australia 6050, AU)
ZUBKO, Alexander (33 Beaumarks Court, Mindarie, Western Australia 6030, AU)
Application Number:
AU2006/001249
Publication Date:
March 01, 2007
Filing Date:
August 28, 2006
Export Citation:
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Assignee:
ORBITAL AUSTRALIA PTY LTD (4 Whipple Street, Balcatta, Western Australia 6021, AU)
CATHCART, Geoffrey, Paul (95 Rookwood Street, Mount Lawley, Western Australia 6050, AU)
ZUBKO, Alexander (33 Beaumarks Court, Mindarie, Western Australia 6030, AU)
International Classes:
F02M67/02; F02B69/06; F02D21/00; F02D41/38; F02M57/00; F02M63/00
Domestic Patent References:
WO2002097255A12002-12-05
Foreign References:
US4944255A1990-07-31
US20040182359A12004-09-23
US20040089249A12004-05-13
US6619241B22003-09-16
US6564770B12003-05-20
USRE36768E2000-07-11
US5131354A1992-07-21
Attorney, Agent or Firm:
WRAY & ASSOCIATES (Level 4, The Quadrant 1 Williams Stree, Perth Western Australia 6000, AU)
Download PDF:
Claims:
The Claims Defining the Invention are as Follows:

1. A method of operating an internal combustion engine in a combustion cycle, the engine having at least one combustion chamber adapted to undergo expansion and contraction between maximum and minimum volume conditions, a fuel injection system for delivering a metered quantity of fuel to the combustion chamber entrained in a body of gas comprising an oxidant, and an exhaust means for discharging fluid from the combustion chamber, the method comprising opening the exhaust means as the combustion chamber expands to permit fluid to discharge from the combustion chamber, closing the exhaust means to interrupt discharge of fluid from the combustion chamber, actuating the injection system to deliver a metered quantity of fuel entrained in gas into the combustion chamber, and initiating combustion of the fuel in the combustion chamber, wherein the gas provides oxidant for the combustion process.

2. A method according to claim 1 wherein the majority of the oxidant for combustion is supplied by the body of gas.

3. A method according to claim 2 wherein the gas comprises air and the oxidant comprises oxygen in the air.

4. A method according to claim 1 , 2 or 3 wherein the gas comprises a pressurised gas to which the fuel is exposed for delivery of a gas-fuel mixture to the combustion chamber.

5. A method according to any one of the preceding claims wherein the exhaust means is closed during volume contraction of the combustion chamber.

6. A method according to any one of the preceding claims wherein opening and closing of the exhaust means is substantially symmetrical about the maximum volume condition of the combustion chamber.

7. A method according to any one of claims 1 to 4 wherein the exhaust means is closed during volume expansion of the combustion chamber.

8. A method according to any one of the preceding claims wherein the metered quantity of fuel entrained in gas is delivered into the combustion chamber near to or after closing of the exhaust valve and during volume contraction of the combustion chamber.

9. A method according to any one of the preceding claims wherein a stoichiometric ratio of oxidant to fuel exists in the combustion chamber at the time of combustion.

10. A method according to any one of the preceding claims wherein exhaust fluid discharging from the combustion chamber is subjected to a 3-way exhaust gas catalyst for the simultaneous treatment of NOx and HCs .

11. A method according to any one of the preceding claims wherein the engine has an inlet means for admitting an intake fluid comprising oxidant into the combustion chamber and wherein the method further comprises actuating the inlet means to admit intake fluid into the combustion chamber.

12. A method according to claim 11 wherein the inlet means is opened to admit the intake fluid into the combustion chamber after opening of the exhaust means and while the combustion chamber continues to expand.

13. A method according to claim 11 or 12wherein the inlet means is closed to interrupt admission of the intake fluid into the combustion chamber after the combustion chamber has reached its maximum volume condition and during subsequent contraction thereof.

14. A method according to claim 11 , 12 or 13 when dependent upon claim 4 wherein the pressure of the pressurised gas at exposure to the fuel is above the maximum air pressure available at the inlet means.

15. A method according to any one of the preceding claims wherein fuel is delivered to the combustion chamber shortly before the exhaust means is closed.

16. A method according to any one of claims 1 to 14 wherein fuel is delivered to the combustion chamber after the exhaust means is closed.

17. A method according to any one of the preceding claims wherein the engine is operated in a two-stroke combustion cycle and wherein the method further comprises switching the combustion cycle of the engine between the two- stroke cycle and a four-stroke cycle.

18. A method according to any one of the preceding claims further comprising operating the engine with spark ignition.

19. A method according to any one of claims 1 to 17 further comprising operating the engine with homogenous charged compression ignition.

20. A method according to any one of claims 1 to 17 further comprising switching operation of the engine selectively between spark ignition and homogenous charged compression ignition .

21. An engine operating in accordance with a method according to any one of the preceding claims.

22. A control system for an internal combustion engine having at least one combustion chamber adapted to undergo expansion and contraction between maximum and minimum volume conditions, a fuel injection system for delivering a metered quantity of fuel to the combustion chamber entrained in a body of gas comprising an oxidant, and an exhaust means for discharging fluid from the combustion chamber the control system being adapted to operate the engine with a combustion cycle comprising opening the exhaust means as the combustion chamber expands to permit fluid to discharge from the combustion chamber, closing the exhaust means to

interrupt discharge of fluid from the combustion chamber, actuating the injection system to delivering a metered quantity of fuel entrained in gas into the combustion chamber, and initiating combustion of the fuel in the combustion chamber, wherein the gas provides oxidant for the combustion process.

23. A control system according to claim 22 wherein the control system is adapted to switch operation of the engine between a two-stroke combustion cycle and a four-stroke combustion cycle.

24. A control system according to claim 23 wherein the switch between the two- stroke and four-stroke combustions cycles is achieved by varying valve timing and duration of valve opening.

25. An engine comprising at least one combustion chamber adapted to undergo expansion and contraction between maximum and minimum volume conditions, a fuel injection system for delivering a metered quantity of fuel to the combustion chamber entrained in a body of gas comprising an oxidant, an exhaust means for discharging fluid from the combustion chamber, and a control system, the control system being adapted to operate the engine with a combustion cycle comprising opening the exhaust means as the combustion chamber expands to permit fluid to discharge from the combustion chamber, closing the exhaust means to interrupt discharge of fluid from the combustion chamber, and actuating the injection system to delivering a metered quantity of fuel entrained in gas into the combustion chamber, and initiating combustion of the fuel in the combustion chamber, wherein the gas provides oxidant for the combustion process.

26. An engine according to claim 25 further comprising a cylinder within which the combustion chamber is defined and which accommodates a reciprocating piston, the maximum volume condition of the combustion chamber corresponding to the bottom-dead-centre position of the piston and the minimum volume condition corresponding to the top-dead-centre position of the piston.

27. An engine according to claim 25 or 26 wherein outlet means comprises at least one exhaust port and an exhaust valve for opening and closing the at least one exhaust port.

28. An engine according to claim 25, 26 or 27 further comprising an inlet means for admitting an intake fluid comprising oxidant into the combustion chamber

29. An engine according to claim 28 wherein the inlet means comprises at least one inlet port and an inlet valve for opening and closing the at least one inlet port.

30. An engine according to any one of claims 25 to 29 wherein the fuel injection system comprises a fuel metering injector and a mixture delivery injector, the mixture delivery injector having a holding chamber into which the fuel metering injector delivers the fuel, the holding chamber being exposed to pressurised gas for delivery of an gas-fuel mixture to the combustion chamber.

31. An engine according to claim 30 further comprising a compressor for supplying pressurised gas to the fuel injection system.

32. An engine according to any one of claims 25 to 31 further comprising a three-way exhaust gas catalyst for the simultaneous treatment of NOx and HCs.

33. An engine according to any one of claims 25 to 32 wherein the control system is adapted to operate the engine in a two-stroke combustion cycle.

34. An engine according to any one of claims 25 to 32 wherein the control system is adapted to operate the engine in a four-stroke combustion cycle.

35. An engine according to any one of claims 25 to 32 wherein the control system is adapted to switch operation of the engine between a two-stroke combustion cycle and a four-stroke combustion cycle.

36. An engine according to claim 35 further comprising means for varying valve timing and duration of valve opening to switch between the two-stroke and four-stroke combustions cycles.

37. An engine according to any one of claims 25 to 36 wherein the engine is adapted to operate with spark ignition.

38. An engine according to any one of claims 25 to 36 wherein the engine is adapted to operate with homogenous charged compression ignition.

39. An engine according to any one of claims 25 to 36 wherein the engine is adapted to operate with either spark ignition or homogenous charged compression ignition and to switch therebetween.

40. An engine according to any one of claims 25 to 39 wherein the majority of the oxidant for combustion is supplied by the body of gas.

41. An engine according to claim 40 wherein the gas comprises air and the oxidant comprises oxygen in the air.

42. An engine according to any one of claims 25 to 41 wherein the exhaust means is closed during volume contraction of the combustion chamber.

43. An engine according to any one of claims 25 to 42 wherein opening and closing of the exhaust means is substantially symmetrical about the maximum volume condition of the combustion chamber.

44. An engine according to any one of claims 25 to 41 wherein the exhaust means is closed during volume expansion of the combustion chamber.

45. An engine according to any one of claims 25 to 44 wherein the metered quantity of fuel entrained in gas is delivered into the combustion chamber near to or after closing of the exhaust means and during volume contraction of the combustion chamber.

46. An engine according to any one of claims 25 to 45 wherein a stoichiometric ratio of oxidant to fuel exists in the combustion chamber at the time of combustion.

47. An engine according to any one of claims 28 to 46 wherein the inlet means is opened to admit the intake fluid into the combustion chamber after opening of the exhaust means and while the combustion chamber continues to expand.

48. An engine according to any one of claims 28 to 47 wherein the intake means is closed to interrupt admission of the intake fluid into the combustion chamber after the combustion chamber has reached its maximum volume condition and during subsequent contraction thereof.

49. An engine according to any one of claims 25 to 48 wherein fuel is delivered to the combustion chamber shortly before the exhaust valve is closed.

50. An engine according to any one of claims 25 to 48 wherein fuel is delivered to the combustion chamber after the exhaust valve is closed.

51. A method of operating an internal combustion engine substantially as herein described.

52. A control system for an internal combustion engine substantially as herein described.

53. An internal combustion engine substantially as herein described with reference to the accompanying drawings.

Description:

"Engine Control Strategy"

Field of the Invention

This invention relates to engine control strategies for internal combustion engines. In certain applications, the invention may be applicable to an engine system capable of operating in either two-stroke or four-stroke combustion cycles and also switching between the two-stroke and four-stroke combustion cycles.

Background Art

A reciprocating internal combustion engine operating with a two-stroke combustion cycle may be piston ported or alternatively may have valves controlling the induction and exhaust processes. In the case of piston ported designs, port operation is typically symmetrical about the bottom-dead-centre (BDC) position of the piston; that is, the timing of opening of the inlet port before BDC position is approximately equal to the timing of closing of the inlet port after BDC, and the timing of opening of the exhaust port before BDC is also approximately equal to the timing of opening of the exhaust port after BDC. Typically, there is some overlap whereby the inlet and exhaust ports are open simultaneously so that the inducted air can assist in clearing the combustion chamber of exhaust gas.

Because the exhaust port closes after BDC, some pumping work is performed on the exhaust gas to cause it to discharge from the combustion chamber. A significant problem with an engine operating under the two-stroke cycle is the short-circuiting of fresh charge from the intake port to the exhaust port which increases fuel consumption and emissions of unburned hydrocarbons.

Gasoline engines for automotive applications almost exclusively use four-stroke combustion cycles due to the favourable combination of good engine efficiency over a wide range of operating conditions, low emissions, high levels of refinement (noise and vibration) and high reliability. The two-stroke combustion cycle can offer some advantages in specific areas of operation compared to a

four-stroke combustion cycle, including low engine speed performance, reduced vibration at low engine speeds and, when coupled with control over the inlet and exhaust phases, potential for reduced fuel consumption for an engine that can switch between two-stroke and four-stroke operation.

The challenge for most of the world's automobile manufacturers, faced predominantly in Europe, is to reduce the vehicle fuel consumption, not only during the legislated test procedure, but also for real world customer driving. Many of the current technologies address only one of these areas (that is either legislated test procedure or real world driving).

Gasoline engine lean burn technology is one area where much development has occurred in the last 10 years, and offers the largest reduction in fuel consumption for a given engine displacement. Lean burn engines, however, have not penetrated successfully into the market place, and in fact output volumes are reducing, with many manufacturers replacing their current lean-burn engines with engines that operate at stoichiometric air-fuel ratio. This is primarily due to the very high cost associated with the after-treatment of the emissions during lean operation. As well, lean operation has been found to be limited, leading to reduced or even no benefit achieved for many driving conditions, this offering little advantage over the stoichiometric combustion systems currently in production.

Currently more focus is being placed on stoichiometric operation combustion systems, due to simplicity and low cost of the after-treatment systems. New techniques for fuel consumption reduction are being investigated including downsizing and turbo-charging (adopting a smaller engine with higher specific output) in order to maintain a stoichiometric combustion system, and increase the engine operating efficiency. As well, adopting a combustion system which utilises existing three-way catalyst technologies (application for stoichiometric operation), the technology is suitable for all of the advanced vehicle markets, including USA, Japan and China. In order to further improve the engine efficiency of a stoichiometric combustion system for greater reduction in fuel consumption, the inlet pumping work (work required to draw the inlet air into the cylinder past a throttle) at part load operation needs to be reduced. Currently, the introduction of

EGR (exhaust gas residuals) is used to increase the charge dilution at part load operation in order to increase the volume of cylinder charge, and therefore reducing inlet pumping losses. This technique typically leads to reductions in the order of 2 to 5% in fuel consumption for typical vehicle driving conditions.

It is against this background that the present invention has been developed.

Disclosure of the Invention

According to a first aspect of the present invention there is provided a method of operating an internal combustion engine in a combustion cycle, the engine having at least one combustion chamber adapted to undergo expansion and contraction between maximum and minimum volume conditions, a fuel injection system for delivering a metered quantity of fuel to the combustion chamber entrained in a body of gas comprising an oxidant, and an exhaust means for discharging fluid from the combustion chamber, the method comprising opening the exhaust means as the combustion chamber expands to permit fluid to discharge from the combustion chamber, closing the exhaust means to interrupt discharge of fluid from the combustion chamber, actuating the injection system to deliver a metered quantity of fuel entrained in gas into the combustion chamber, and initiating combustion of the fuel in the combustion chamber, wherein the gas provides oxidant for the combustion process.

Preferably, the majority of the oxidant for combustion is supplied by the body of gas.

Preferably, the gas comprises air, in which case the oxidant comprises oxygen in the air.

Preferably, the fuel injection system comprises a fuel metering injector and a mixture delivery injector, the mixture delivery injector having a holding chamber into which the fuel metering injector delivers the fuel, the holding chamber being exposed to pressurised gas for delivery of a gas-fuel mixture to the combustion chamber.

- A -

Preferably, the exhaust means is closed during volume contraction of the combustion chamber. Alternatively, the exhaust means may be closed during volume expansion of the combustion chamber.

Preferably, opening and closing of the exhaust means is substantially symmetrical about the maximum volume condition of the combustion chamber.

Preferably, the metered quantity of fuel entrained in gas is delivered into the combustion chamber near to or after closing of the exhaust means and during volume contraction of the combustion chamber.

Preferably, a stoichiometric ratio of oxidant to fuel exists in the combustion chamber at the time of combustion.

Preferably, the engine comprises a 3-way exhaust gas catalyst for the simultaneous treatment of NOx and HCs

An engine operating in accordance with this method may be particularly suitable for certain engine operation conditions, such as low speed and low load conditions.

The engine preferably also has an inlet means for admitting an intake fluid comprising oxidant into the combustion chamber. In such a case, there may be engine operating conditions where the method may also comprise actuating the inlet means to admit intake fluid into the combustion chamber.

Preferably, the pressure of the pressurised gas in the holding chamber is above the maximum air pressure available at the inlet means.

The inlet means is preferably opened to admit the intake fluid into the combustion chamber after opening of the exhaust means and while the combustion chamber continues to expand.

The intake means is preferably closed to interrupt admission of the intake fluid into the combustion chamber after the combustion chamber has reached its maximum volume condition and during subsequent contraction thereof.

Typically, the engine comprises a reciprocating engine comprising a cylinder within which the combustion chamber is defined and which accommodates a reciprocating piston, the maximum volume condition of the combustion chamber corresponding to the bottom-dead-centre (BDC) position of the piston and the minimum volume condition corresponding to the top-dead-centre (TDC) position of the piston.

The outlet means typically comprises at least one exhaust port and an exhaust valve for opening and closing the at least one exhaust port. Further, the inlet means typically comprises at least one inlet port and an inlet valve for opening and closing the at least one inlet port.

Preferably, the fuel injection system delivers fuel to the combustion chamber shortly before the exhaust valve is closed. This reduces the possibility of fuel and/or oxidant being lost though to the exhaust system. Alternatively, the fuel injection system delivers fuel to the combustion chamber after the exhaust valve is closed.

The valves may be controlled for movement between their respective open and closed conditions in any appropriate way. The valves may, for example, be operated mechanically, electro-mechanically, hydro-mechanically, electro- hydraulically or pneumatically.

Pressurised gas is typically supplied to the fuel injection system by a compressor. The compressor may be of a reciprocating piston type.

The engine may be operated in accordance with a two-stroke combustion cycle or a four-stroke combustion cycle.

The method according to the invention may further comprise switching the combustion cycle of the engine between the two-stroke cycle and the four-stroke cycle.

The method may comprise operating the engine with spark ignition. Alternatively, the method may comprise operating the engine with homogenous charged compression ignition (HCCI).

In addition to switching operation of the engine selectively between two and four- stroke operation, the method may involve switching operation of the engine selectively between spark ignition and HCCI.

Operating an engine by the method according to the invention may possibly address some of the difficulties associated with downsizing of engines in order to meet certain emission targets while providing acceptable driveability.

An engine operated by the method in accordance with the invention may also be suitable for integration in a hybrid system.

According to a second aspect of the invention there is provided a control system for an internal combustion engine having at least one combustion chamber adapted to undergo expansion and contraction between maximum and minimum volume conditions, a fuel injection system for delivering a metered quantity of fuel to the combustion chamber entrained in a body of gas comprising an oxidant, and an exhaust means for discharging fluid from the combustion chamber the control system being adapted to operate the engine with a combustion cycle comprising opening the exhaust means as the combustion chamber expands to permit fluid to discharge from the combustion chamber, closing the exhaust means to interrupt discharge of fluid from the combustion chamber, and actuating the injection system to delivering a metered quantity of fuel entrained in gas into the combustion chamber, and initiating combustion of the fuel in the combustion chamber, wherein the gas provides the oxidant from the combustion process.

Preferably, the control system is adapted to switch operation of the engine between a two-stroke combustion cycle to a four-stroke combustion cycle, and to selectively switch between the two-stroke and four-stroke combustion cycles.

The switch between the two-stroke and four-stroke combustions cycles may be achieved by varying valve timing and duration of valve opening.

According to a third aspect of the invention there is provided an engine comprising at least one combustion chamber adapted to undergo expansion and contraction between maximum and minimum volume conditions, a fuel injection system for delivering a metered quantity of fuel to the combustion chamber entrained in a body of gas comprising an oxidant, and an exhaust means for discharging fluid from the combustion chamber, the control system being adapted to operate the engine with a combustion cycle comprising opening the exhaust means as the combustion chamber expands to permit fluid to discharge from the combustion chamber, closing the exhaust means to interrupt discharge of fluid from the combustion chamber, and actuating the injection system to delivering a metered quantity of fuel entrained in gas into the combustion chamber, and initiating combustion of the fuel in the combustion chamber, wherein the gas provides the oxidant from the combustion process.

The engine may operate in a two-stroke combustion cycle or a four-stroke combustion cycle. Further, the engine may be switched selectively between the two-stroke combustion cycle and the four-stroke combustion cycle.

Brief Description of the Drawings

The invention will be better understood by reference to the following description of one specific embodiment thereof as shown in the accompanying drawings in which:

Figure 1 is a schematic view of an engine operated in accordance with the embodiment, the engine being switchable between two-stroke and four- stroke combustion cycles;

Figure 2 is a comparative diagram showing exhaust valve lift during operation in one mode of a two-stroke cycle, as well as inlet and exhaust valve lift during operation of the engine in a four-stroke cycle;

Figure 3 is a cycle diagram illustrating the relative timing of operation of exhaust valve of the engine when operating in the two-stroke combustion cycle at low engine speed and load conditions with the inlet valve remaining closed; and

Figure 4 is a cycle diagram illustrating the relative timing of operation of inlet and exhaust valves of the engine when operating in a two-stroke combustion cycle with both inlet and exhaust valves being used.

Best Mode(s) for Carrying Out the Invention

The embodiment is directed to a reciprocating internal combustion engine 10 which is capable of operating in either two-stroke or four-stroke cycles of operation, with selective switching therebetween.

The engine 10 comprises a cylinder 11 and a piston 13 accommodated in the cylinder. The cylinder 11 and piston 13 cooperate to define a combustion chamber 15. The combustion chamber 15 undergoes volume expansion and contraction upon reciprocatory movement of the piston 13 within the cylinder 11 between top-dead-centre (TDC) and bottom-dead-centre (BDC) positions.

An inlet means 20 is provided for introducing an air charge into the combustion chamber and an outlet means 30 is provided for discharging exhaust gas from the combustion chamber.

The inlet means 20 comprises an inlet port 21 opening onto the combustion chamber 15 at the terminal end of a delivery duct 23, and an inlet valve 25 for opening and closing the inlet port 21. A control means 27 is provided for controlling operation of the inlet valve 25 to cause opening and closing thereof.

The exhaust means 30 comprises an exhaust port 31 opening onto the combustion chamber 15 at the entry end of a discharge duct 33 and exhaust valve 35 for opening and closing the exhaust port. A control means 37 is provided for controlling operation of the exhaust valve 35 to cause opening and closing thereof.

The control means 27, 37 for controlling opening and closing of the inlet and exhaust valves may take any appropriate form. The valve control means 27, 37 may, for example, comprise mechanical control mechanisms such as cams on a camshaft.

A fuel injection system 40 is provided for direct injection of fuel into the combustion chamber. In this embodiment the fuel injection system 40 comprises a dual fluid fuel injection system of the type disclosed in US Patent RE 36768 and International Application WO 99128621 , the contents of both of which are incorporated herein by way of reference. The injection system 40 delivers a metered quantity of fuel entrained in air into the combustion chamber 15. The injection process can establish a stratified yet stoichiometric charge which assists ignition while also enabling a conventional three-way catalytic converter to be employed for exhaust after-treatment without the need for a lean NOx trap.

An ignition device (not shown), such as a spark plug, is provided for igniting a combustion mixture charge in the combustion chamber 15 at appropriate timings.

The exhaust duct 33 communicates with an exhaust system (not shown) incorporating a catalytic converter in the form of a conventional three-way catalytic converter.

The engine can be operated in a two-stroke combustion cycle or a four-stroke combustion cycle, and can be switched therebetween according to the operational demand on the engine.

Operation of the engine in the four-stroke cycle can offer the favourable combination of good engine efficiency over a wide range of operating conditions,

low emissions, high level of refinement (noise and vibration) and high reliability. The two-stroke combustion cycle can offer some advantages in specific areas of operation compared to a four-stroke engine, including low speed engine performance, reduced vibration at low engine speeds, and when coupled with control over the inlet and exhaust phases, potential for reduced fuel consumption. In order to exploit the benefits of four-stroke and two-stroke combustion cycles, the engine is capable of switching between these two modes of operation.

The determining factor as to whether the engine 10 operates in the two-stroke cycle or the four-stroke cycle is the timing of operation of the intake and exhaust valves 25, 35, as well as the duration of opening of the valves.

The inlet and exhaust valves 25, 35 operate at double the frequency of actuation in the two-stroke cycle as compared to the four-stroke cycle. Specifically, the inlet and exhaust valves 25, 35 acuate once for each rotation of the engine crankshaft (i.e. each 360°) in the two-stroke cycle, whereas they actuate once for each two rotations (i.e. each 720°) of the engine crankshaft in the four-stroke cycle, as is evident from Figure 2 which is a comparative diagram showing inlet and exhaust valve lift. It is, of course, necessary for the valve control means 27, 37 to accommodate the cycle switching.

Further, it may be that the engine 10, when operating in a two-stroke combustion cycle, does not perform a combustion stoke on every engine cycle. The engine 10 may, for example, perform a combustion stroke at intervals (such as on alternate cycles) in order to afford improved fuel consumption in certain operating conditions. Similarly, it may be that the engine 10, when operating in a four-stroke combustion cycle, does not perform combustion stoke on every alternate engine cycle.

The engine can operate in various modes to perform two-stroke combustion cycles. One such mode involves delivering of combustion air into the combustion chamber 15 entirely by way of the injection system 40. In other words, the air in which fuel is entrained constitutes the entire quantity of air available for the combustion process, with no combustion air being delivered by the inlet means

20. Accordingly, there is no inlet pumping work when the engine is operating in this manner.

The control strategy for operating the engine in such a mode is depicted in the cycle diagram of Figure 3 and involves:

(1 ) opening the exhaust valve 35 during the power stroke (as indicated by point EVO on the cycle diagram);

(2) closing the exhaust valve 35 during the compression stroke after BDC (as indicated by point EVC); and

(3) injecting the fuel entrained in the combustion air near to or after closing of the exhaust valve.

With this control strategy, the inlet valve 25 remains closed.

Load control for the engine operating in accordance with this strategy is performed by way of the injection system 40.

The exhaust valve 35 operates at double the frequency of actuation in the two- stroke cycle as compared to the four-stroke cycle. Specifically, the exhaust valve

35 actuates once for each rotation of the engine crankshaft (i.e. each 360°) in the two-stroke cycle, whereas it actuates once for each two rotations (i.e. each 720°) of the engine crankshaft in the four-stroke cycle (as does the inlet valve 25), as is evident from Figure 2 which is a comparative diagram showing inlet and exhaust valve lift.

The engine can also be operated under other control strategies to perform two- stroke combustion cycles.

One such other mode involves use of the inlet valve 25, the control strategy being depicted in Figure 4 and comprising:

(1 ) opening the exhaust valve 35 during the power stroke;

(2) opening the inlet valve 25 while the exhaust valve 35 is open during the power stroke;

(3) closing the exhaust valve 35 during the power stroke prior to BDC;

(4) closing the inlet valve 25 during the subsequent compression stroke; and

(5) injecting fuel into the combustion chamber near to or after closing of the exhaust valve 35.

With this strategy, air delivered into the combustion chamber upon opening of the inlet valve 25 provides the primary source of combustion air, although it of course supplemented by the air in which the fuel is entrained.

The engine may also be operated in a further mode performing a conventional two-stroke combustion cycle, where opening and closing of the valves are generally symmetrical about BDC.

From the foregoing, it is evident that the embodiment provides a simple yet highly- effective control strategy for operating an engine in a two-stroke cycle at certain appropriate operating conditions (typically low engine speed and low load). The control strategy avoids inlet pumping losses and can enhance fuel consumption.

Further, symmetric timing of the opening and closing events of the exhaust valve about BDC can be beneficial in reducing losses associated with the compression and expansion strokes of the engine.

Modifications and improvements can be made without departing from the scope of the invention.

Throughout the specification, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.