GB2462494A

GB2462494A - A variable compression ratio engine using a secondary piston

Info

Publication number: GB2462494A
Application number: GB0814973A
Authority: GB
Inventors: Edward Timothy Bishop
Original assignee: Connaught Motor Co Ltd
Current assignee: Connaught Motor Co Ltd
Priority date: 2008-08-15
Filing date: 2008-08-15
Publication date: 2010-02-17
Also published as: GB0814973D0

Abstract

A combustion engine comprising a combustion chamber 100, a drive piston and a volume adjusting means 160 moveable within the combustion chamber for adjusting the volume of the combustion chamber where the volume adjusting means is driven by the output of the engine. The volume adjusting means may be a piston actuated by rocker 220 comprising two arms 220a, 220b rotatable around a central fulcrum 230 which is synchronised with the drive piston to compress the contents of the combustion chamber during the compression stroke. Preferably the rocker is actuated by a pair of cams 240, 340 connected to a camshaft 260, each cam acting on a respective arm, where the camshaft and rocker are moveable longitudinally to vary the leverage of the first rocker arm on the volume adjusting means. The combustion chamber may comprise a fuel mixture inlet valve 120 which acts as an effective throttle and is actuated by a second flared arm of the rocker. The engine may also comprise a valve train that drives the volume adjusting piston and the engine may be a two-stroke engine.

Description

Variable compression ratio, valve throttling and the potential for desmodromic operation The present invention relates an engine with a variable compression ratio.

A conventional four stroke engine goes through a combustion cycle in four stages, the induction stroke, compression stroke, explosion stroke and exhaust stroke. The cycle starts with the piston at the bottom of the combustion chamber which is in the form of a cylinder. At this point, a fuel/air mix has already been introduced to the combustion chamber. The piston moves up the cylinder to compress this fuel/air mixture. This is called the compression stroke. When the piston reaches the top of its stroke, a spark plug emits a spark to ignite the fuel. The fuel charge in the cylinder explodes which drives the piston down. This is called the explosion stroke. Once the piston hits the bottom of its stroke, an exhaust valve allows the exhaust to leave the cylinder through a tail pipe. Then, as the piston moves back up the cylinder, the exhaust is pushed out of the cylinder through exhaust valve. This is called the exhaust stroke. Finally, when the piston reaches the top of its stroke, another valve opens to let more fuel mix enter the combustion chamber. As the piston moves back down the cylinder, the chamber is filled with the fuel mix. This is called the induction stroke.

A two-stroke engine condenses the length of the cycle from four stages into two. This is usually achieved by exhausting the combusted fuel mix through ports at the bottom of the cylinder at the end of the explosion stroke and also injecting the fuel mix into the cylinder at the same point in the cycle. However, this can be less efficient as some fuel mixture may be lost through the exhaust ports before it can be burnt. It is increasingly common to see mixes of valves and ports in different configurations as the drive for efficiency increases. This invention is designed to enhance both two and four stroke designs in any configuration of ports or valves.

The compression ratio of an engine is the ratio between the volume of a combustion chamber and cylinder with the piston at bottom dead centre and the volume when the piston is at the top of its stroke. The higher the compression ratio, the more mechanical energy an engine can squeeze from its air-fuel mixture. The compression ratio is usually set to allow the engine to operate at maximum efficiency at full load, wide open throttle. At any part-load condition, a conventional petrol engine is strangled by the throttle and the compression ratio is not optimal, leading to reduced efficiency. What is needed is a method of adjusting the compression ratio of the engine at part load to allow greater efficiency and giving levels of fuel economy closer to a diesel engine.

Cylinder bore diameter, piston stroke length, and combustion chamber volume are constant within a conventional engine. Therefore, the theoretical compression ratio for a conventional engine is mathematically constant, regardless of throttle settings. As the engine is usually designed such that the compression ratio is optimal for full throttle, the compression ratio is less than optimal at any part-throttle setting due to strangulation of the incoming gasses by the throttle.

A variable compression engine uses techniques to dynamically alter the volume of the combustion chamber, which results in a change to the compression ratio.

In a known method to alter the volume of the combustion chamber, a variable compression engine has the capability to "lower" the cylinder head closer to the pistons and the crankshaft. This is achieved by replacing the typical one-part engine block with a two-part block, with the crankshaft in the lower block, and the cylinders in the upper portion. The two blocks are hinged together at one side (or perhaps slide together). By pivoting the upper block around the hinge point, the variable compression engine can be modified. In practice, the engine adjusts the upper block through a small range of motion, using a hydraulic actuator. The compression ratio can be varied and adjusted to suit the properties of the fuel, so that the engine will always run at the compression ratio best suited to the fuel and the desired throttle opening.

However, this type of engine is expensive and requires extra space. The extra modifications required also mean that it is not very cost effective and is often not durable due to the stresses and mechanical machinery required.

The present invention is directed to a system that is able to vary a compression ratio without the aforementioned disadvantages.

According to a first aspect of the present invention there is provided a combustion engine comprising; a combustion chamber, a drive piston, which drives the output of the engine, and a volume adjusting means moveable within the combustion chamber for adjusting the volume of the combustion chamber, the volume adjusting means being driven by the output of the engine.

The present invention will now be described by way of example with reference to the accompanying drawings: Figure 1 illustrates a view of an engine in the full load configuration during the induction stroke in accordance with the present invention; Figure 2 illustrates a view of an engine in the full load configuration during the compression stroke in accordance with the present invention; Figure 3 illustrates a view of an engine in the light load configuration during the induction stroke in accordance with the present invention; and Figure 4 illustrates a view of an engine in the light load configuration during the compression stroke in accordance with the present invention.

Figure 1 shows an embodiment of the present invention in which the combustion engine is being used at full load. Figure 1 shows the engine during the induction stroke. Rocker 220 has arms 220a and 220b and pivots around fulcrum 230. Arm 220a acts upon inlet valve 120 and arm 220b acts upon volume piston 160. This ensures that the movement of the inlet valve is always approximately a half cycle out of phase to that of the volume piston (depending upon the required grind of the camshaft). Correspondingly, cam 240 acts upon arm 220a and cam 340 acts upon arm 22Db. Cam shaft 260, to which cams 240 and 340 are fixed, can be moved sideways along the length of the engine as an entity with the rocker. The requirement for the actuation of the rocker instead of actuating the inlet valve and piston cylinder with the cams directly is that the rocker allows the position of the cams to be changed longitudinally relative to the inlet valve and volume piston and still provide the necessary force to actuate the inlet valve and volume piston.

A further requirement of the rocker is that the leverage on the inlet valve/volume piston varies depending on the position of the respective cam/rocker arrangement. Therefore, the rocker can be actuated to have greater or smaller reciprocal action on the inlet valve/volume piston depending on the position of the camshaft/rocker assembly relative to the inlet valve/volume piston.

The embodiment of figure 1 has a combustion chamber 100 with an inlet valve 120 and inlet port 140. A spring 300 biases the inlet valve into a closed position. Volume piston 160 is also shown to be biased into a retracted position from the combustion chamber 100 by spring 310. The camshaft 260 rotates which causes the cam 240 to actuate rocker arm 220a causing inlet valve 120 to move into an open position and allow fuel to enter via the inlet port 140. Simultaneously, rocker arm 220b moves away from volume piston 160, and biasing spring 310 pushes the volume piston into a retracted position, which increases the volume of chamber 100. In this figure, piston is in the retracted position.

In an alternative embodiment, a mechanical link is used to attach valve 120 and rocker arm 220a. Another mechanical link attaches piston 160 and rocker arm 220b. This allows the springs to be replaced or enhanced and causes valve 120 and piston 160 to have true desmodromic actuation i.e. positively closed by the actuation of the respective rocker rather through the action of a spring. This allows advantages such as higher rpm and horsepower and the use of wilder cam grinds for better performance.

In one embodiment, cam shaft 260 is part of the engine valvetrain. The valvetrain is under the action of the output of the engine, and comprises the mechanisms and parts used to control the valves of the engine. In this embodiment, inlet valve 120 and volume piston 160 may be actuated using the power generated by the engine.

In figure 1, the sideways movable camshaft and rocker is in the maximum rightward position, also known as the full load position. As the cam/rocker assembly acts upon the valve further from the fulcrum than the point at which the volume piston connects with arm 220b, the leverage effect results in the cam 340 driving the volume piston to reciprocate over a shorter stroke. The inverse leverage action is occurring on the inlet valve from rocker arm 220a.

Therefore, the inlet valve will open a maximal amount.

However, the flared shaping of arm 220a, in which the arm thickness increases relatively to the distance from the fulcrum, allows cam 240 to act upon arm 220a such that the inlet valve 120 is always brought back to the same closed' position, no matter how far open the inlet valve was when in the open position.

In figure 2, the movable camshaft and rocker assembly is still in the maximum right position. Figure 2 shows the engine during the compression stroke.

However, cam shaft 240 has rotated 180 degrees, allowing the inlet valve to be returned to the closed position by spring 300, preventing further fuel from entering chamber 100 and preventing the escape of any fuel mix from chamber 100. Furthermore, cam 340, via arm 220b, has pressed volume piston 160 down into the chamber 100, which decreases the volume of chamber 100. At the same time, piston 280 has moved upwardly in the combustion chamber 100, compressing the fuel/air mixture. As the volume piston 160 has been pressed into the chamber, the fuel/air mixture is further compressed. When the piston 280 is at its maximum height within the chamber 100, a spark plug (not illustrated) creates a spark that causes combustion of the fuel and associated energy release. Therefore, in this mode, the motion of volume piston 160, and therefore its effect on compression, is minimized, whilst the valve opening and actuation is maximized for performance.

From figures 1 and 2, it is clear how the volume of chamber 100 depends on the positioning of volume piston 160. Volume piston 160 moves in time with piston 280 so that when the piston 280 has reached the topmost position, maximally compressing the fuel mix in the chamber, the volume piston is fully forward, maximally reducing the volume of chamber 100. In this way, the compression ratio of the combustion chamber is further increased by the motion of piston 160.

Figure 3 shows the present invention at in the same point in the cycle as in figure 1. However, in this instance, the movable camshaft and rocker assembly is in the maximum leftward position, also known as the light load position. As cam 340 and rocker arm 220b are at a similar point relative to the fulcrum as that at which the volume piston connects with arm 220b, the leverage effect results in the cam 340 driving the volume piston to reciprocate over a longer stroke than in the configuration of figure 1. The resultant change in leverage on in let valve 120 by cam 240 is the opposite to that of the volume piston and the inlet valve now only opens a small amount. In this way, the amount of fuel mixture allowed into the engine is reduced and the engine is effectively throttled down. The inlet valve still returns to the same closed position as that of figure 1 due to the flared shape of arm 220a.

The increased movement of volume piston 160 in this configuration results in a higher compression ratio for the combustion chamber. This, therefore, allows greater efficiency for the engine at light load, when this would normally be a problem.

Finally, figure 4 shows the present invention in the light load configuration at the same point of the cycle as that of figure 2. Volume piston 160 is fully extended, minimizing the volume of chamber 100.

A further advantage of the present invention is that the force on the volume piston in the compression phase can be used to actuate the camshaft during the retraction of the volume piston. This would allow the engine to have an improved efficiency.

If a conventional camshaft and valve design were to be used on a two stroke engine, the engine revs are severely limited by valve train accelerations as the cam is asked to rotate twice as fast as in a four stroke. Selection of rocker ratio and spacing in the design proposed herein can effectively limit accelerations on the camshaft by multiplying the lift afterwards, liberating the two-stroke cycle from its conventional ported and oil-mix design The volume piston (160) need not be actuated by a cam. It could, for example, be actuated hydraulically or electromagnetically by a suitable control unit. The piston could be actuated via a mechanical linkage.

The cross-sectional area and the stroke length of the volume piston (160) can be selected to provide the desired variation in compression ratio. The shape of the piston can be selected to allow it to fit conveniently into the cylinder, avoiding valves, spark plugs etc. Instead of a single small piston, multiple such pistons could be provided in each cylinder. In the complete engine, one or more such small pistons are preferably provided in like manner in each cylinder.

In one embodiment of the invention, the degree to which the inlet valve is opened can also be used as an effective throttle, by controlling the amount of fuel mixture that enters the combustion chamber. An advantage provided by this feature is that the throttle plate or slide can be excluded, enhancing the throttling efficiency.

The applicant hereby discloses in isolation each individual feature described herein and any combination of two or more such features, to the extent that such features or combinations are capable of being carried out based on the present specification as a whole in the light of the common general knowledge of a person skilled in the art, irrespective of whether such features or combinations of features solve any problems disclosed herein, and without limitation to the scope of the claims. The applicant indicates that aspects of the present invention may consist of any such individual feature or combination of features. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention.

Claims

Claims 1. A combustion engine comprising; a combustion chamber, a drive piston, which drives the output of the engine, and a volume adjusting means moveable within the combustion chamber for adjusting the volume of the combustion chamber, the volume adjusting means being driven by the output of the engine.
2. The combustion engine of claim 1, wherein the volume adjusting means is a volume adjusting piston in a piston shaft connected to the combustion chamber.
3. The combustion engine of any preceding claim, wherein the volume adjusting means is actuated by a volume adjusting actuation means such that it is synchronised with the drive piston to compress the contents of the combustion chamber during the compression stroke.
4. The combustion engine of claim 3, wherein the volume adjusting actuation means is a rocker comprises two arms rotatable around a central fulcrum, the first arm actuating the volume adjusting means.
5. The combustion engine of claim 4 wherein the rocker is actuated by a pair of cams connected to a camshaft, each cam acting on a respective arm of the rocker.
6. The combustion engine of claim 5, wherein the camshaft and rocker are moveable in a longitudinal direction relative to the volume adjusting means, and wherein the leverage of the first rocker arm on the volume adjusting means is dependent on the longtitudinal position of the rocker relative to the inlet valve.
7. The combustion engine of claim 6, wherein the volume adjusting actuation means is a volume adjusting cam, actuated by a camshaft. -10-
8. The combustion engine of any proceeding claim, further comprising a fuel mixture inlet valve, wherein the degree to which the fuel mixture inlet valve opens during the induction stroke of the combustion engine acts as an effective throttle.
9. The combustion engine of claim 8, wherein the inlet valve is actuated by an inlet valve actuation means such that it is synchronized with the drive piston to open during the induction stroke and close during the compression stroke.
10. The combustion engine of claim 4 and claim 9, wherein the inlet valve actuation means is the second arm of the rocker.
11. The combustion engine of claim 10, wherein the arm of the rocker used to actuate the inlet valve is flared such that the inlet valve is always returned to the same closed position regardless of the longtitudinal position of the rocker relative to the inlet valve.
12. The combustion engine of claim 9, wherein the inlet valve actuation means is an inlet valve cam, actuated by a camshaft.
13. The combustion engine of claim 1, further comprising a valve-train driven by the output of the engine, the volume adjusting means being driven by the valve-train.
14. The combustion engine of any preceding claim, wherein the engine is a two-stroke engine and further comprises an exhaust valve configured to allow exhaust gas to leave the combustion chamber, and wherein the compressing action of the volume adjustment means reduces the volume in the combustion chamber during the exhaust phase to facilitate the exhaust gases in escaping through the exhaust valve. -11 -
15. The two-stroke combustion engine of claim 14, wherein the movement of the exhaust valve is separate from the drive piston.
16. The combustion engine of claim 1, wherein the engine is a two-stroke engine and further comprises an exhaust valve configured to allow exhaust gas to leave the combustion chamber, and wherein the compressing action of the volume adjustment means reduces the volume in the combustion chamber after the exhaust valve has closed.
17. A combustion engine has herein described, with reference to theaccompanying specification and drawings.