The compression ratio at which a 4 stroke petrol engine can operate is limited by the quality of the fuel available. In order to prevent detonation occurring at high compression ratios, a high octane fuel is required. The cost and other disadvantages of high octane fuel limits its use, and, in general, lower quality fuel is employed in conjunction with reduced compression ratios.

The swept volume of an engine is determined by the bore and stroke of the engine. The compression ratio is adjusted by altering the unswept volume. The chosen compression ratio is often achieved by using a suitable piston crown and cylinder head profile.

While the unswept volume serves to limit the compression ratio and prevent detonation of the air/fuel mix, it reduces the pumping effectiveness of the engine. The degree of suction available during the induction stroke is reduced by gases remaining in the unswept volume from the previous cycle. Similarly, exhaust scavenging is incomplete. Hot gases remaining in the cylinder reduce the density and quality of the fresh air/fuel charge, further reducing mass flow and hence engine power. The hot gases remaining in the cylinder also increase the thermal stresses on engine components. Apart from possible resonance effects, as the engine speed increases, the pumping performance deteriorates.

Valve overlap is employed in order to compensate for the mechanical inertia of the gases within the engine. The overlap seeks to improve scavenging and induction. This arrangement suffers from the disadvantage that a certain amount of fresh charge may be drawn directly into the exhaust.

The effectiveness of the arrangement also varies according to the engine speed and resonances within the induction and exhaust system. An object of this invention is to provide a construction which improves the breathing and performance of the four stroke internal combustion engine.

Accordingly this invention describes a construction, in which the unswept volume is reduced to the minimum. A secondary piston or pistons which may be driven by a crank and connecting rod arrangement operating at a speed related to that of the main engine crank are incorporated. Said piston or pistons operate in conjunction with the main piston to influence the volume enclosed within the combustion space in a prescribed cyclic manner.

A preferred embodiment of the invention will now be described with reference to the accompanying drawings in which: FIGURE 1 Shows a sectional schematic of one embodiment of the engine.

FIGURE 2 Is a graph illustrating the manner in which the volume within the engine varies throughout one 4 stroke cycle for an embodiment of the type shown in Figure 1.

FIGURE 3 Is a graph illustrating the manner in which the volume within the engine might be required to change to implement a cycle in which the expansion stroke is greater than the compression stroke.

With reference to Figure 1, in one form, the present invention uses two pistons oscillating in cylinders which are connected together. The swept volumes of the pistons are chosen according to the particular cyclic variation in volume desired. Figure 1. shows a schematic representation of the arrangement. In the figure, piston 1 is driven by crank 2 and piston 3 is driven by crank 4.

The physical disposition of the cylinders and pistons may be chosen at will for any particular application.

Normal provision is made for inlet and exhaust valves of which only one is shown at 5. The spark plug, injector, or other means of initiating combustion is also provided in the usual way. For clarity it is not illustrated in the sectional view of Figure 1.

At the start of the cycle, the cranks are aligned to arrange for the combustion chamber volume to be a minimum. Crank 4 rotates at half the speed of crank 2.

The relative directions of rotation may be chosen at will. The secondary piston 3 executes its cycle at half the engine speed, and consequently it will have traversed half of its stroke by the point at which the main piston 1 has reached the end of its induction stroke. One advantage of said arrangement is that induction is improved over the present form of four stroke engine not only by the minimisation of the dead space but by the additional induction caused by the secondary piston 3. Furthermore, since said secondary piston 3 is moving with its greatest velocity at this point in the cycle and the main piston 1 is stationary, the induction volume continues to increase and the induction period extends slightly into that period during which the main piston 1 commences the compression stroke. The exhaust period is likewise lengthened with both the compression and expansion strokes correspondingly shortened in angular terms.

One advantage of said arrangement is an improvement to induction and scavenging of the engine. A second advantage is that valve overlap can be reduced. Yet another advantage is that the mean temperature of the fresh charge is lowered resulting in improved thermal efficiency and greater mass flow. A further advantage of the construction is that improved quality of the fresh charge will lead to cleaner combustion. Note that a proportion of the power developed by the engine is extracted through crank 4.

With reference to Figure 2, The graph shown is for an arrangement of the type illustrated in Figure 1 in which the swept volumes of the main cylinder and the secondary cylinder are in the ratio 10: 1. The unswept volume is zero in the illustration. Fig 2. shows the typical behaviour of the enclosed volume throughout two revolutions of crank 2 of Figure 1 which results in one revolution of crank 4 of Figure 1.

With reference to Fig 2, note that at the start 6 and end 7 of the cycle the volume is a minimum. At the mid point 8 in the cycle, the volume reaches a minimum which is larger than at the start 6 and end 7 of the cycle. Furthermore, the volume reaches a maximum 9 before and 10 after the mid point of the cycle.

Consideration of Figure 2 confirms that the arrangement illustrated in Figure 1 implements the requirements of minimum volume at the start and end of the cycle, with a restricted degree of compression the extent of which can be determined at will.

Certain alternative constructions described offer the further advantage that the compression ratio can be altered readily even during operation of the engine.

In one construction, piston 3 and crank 4 may be replaced by a free piston which remains seated against a seating at the base of its chamber by the action of compressed gas in the region behind it. The opposite face of said free piston is acted upon by the gases within the combustion chamber in the same manner as piston 3 in Figure 1. The pressure of the compressed gas is such that exhaust gas back pressure is insufficient to raise the free piston. During the induction stroke, the free piston remains seated. On the compression stroke, said free piston remains seated until the pressure in the cylinder becomes sufficient to overcome the force of the compressed gas acting on the opposite face of the free piston. Thereafter the free piston moves under the action of the air/fuel mix in the cylinder. The movement of the free piston thereby regulates the extent to which the air/fuel mix is compressed.

It is advantageous to have the free piston constructed with the minimum mechanical inertia. The gas pressure in the space behind the free piston can be adjusted to suit engine operating conditions. The free piston may be spring loaded as an alternative or adjunct to gas pressure. The gas pressure and spring load may be arranged in tandem or in opposition to one another to alter the dynamic behaviour of the free piston arrangement.

Said construction is considered to be particularly advantageous for those engine applications where constant speed operation is desired.

The air/fuel mix is ignited in the usual manner. The high pressure generated holds the free piston away from its seating during the greater part of the power stroke. The free piston tends to return to its seating as the pressure in the cylinder falls below that which was required to raise it. Thus the energy given up in moving the free piston is recovered. At the end of the power stroke, the exhaust valve opens.

During the exhaust stroke, the free piston remains on its seating while the cylinder is scavenged.

In another construction, a flame arresting element is interposed between the secondary piston chamber and the main cylinder. The air/fuel mix in the secondary piston chamber will not ignite until it passes into the main cylinder.

Said construction renders combustion progressive. One advantage of said construction is a reduction in engine stresses and peak pressures while maintaining a high average pressure in the cylinder.

Another advantage of the said construction is that more controlled combustion can be expected to lead to a reduction in emissions with correspondingly improved fuel efficiency. An additional advantage expected will be a reduction in noise level. A further advantage is that the exhaust system may be simplified with the possible elimination of the requirement for, or at least a reduction in the demands upon a catalytic converter. The reduction in back pressure resulting from said simplifications is expected to further improve the breathing and performance of the engine.

The description given above makes reference to petrol engines. The principle may equally be applied with benefit to any 4 stroke cycle engine irrespective of fuel type or method of ignition.

In a further construction, a free piston operates in two stages. The first stage acts as already described. The second stage movement takes place during the power stroke and stores energy from the hot gases by movement against a higher back pressure or force. Said action reduces the ultimate pressure reached in the engine. In engines of known design, the point of highest gas pressure occurs near top dead centre. At said position, the engine is unable to extract significant useful work from the gas. By storing energy in the displacement of a free piston against a compressed gas or spring, the engine is able to accept greater air t fuel charges without sustaining damage. The energy stored in movement of the free piston is returned through the main piston when the crank is at a more advantageous position. The secondary action is arranged to occur only when cylinder pressures exceed those determined to be optimal for the power stroke of the engine. As with the action of the free piston during compression, the operating point of the secondary motion can be altered during operation to suit engine and load conditions.

In another adaptation, said second stage movement is implemented in a separate free piston and cylinder. In a further adaptation, a portion of the cylinder head itself may constitute the said second stage or even the first stage free piston. In the foregoing description, a gas has been described as providing the necessary back pressure or force against which the said free piston or pistons operate. As an alternative to gaseous back pressure, hydraulic, mechanical or other means may be employed to provide the necessary energy storage medium.

In a further construction, a free piston may be influenced in its operation by mechanical means. Said free piston may be arranged to operate at half of the engine speed in a 4 stroke cycle. A cam or similar drive mechanism may be arranged to hold said free piston against its seating during the exhaust stroke, thereby ensuring optimal scavenging. The free piston may then be permitted to rise under the action of a spring during the induction stroke, inducing the maximum amount of charge. The free piston may remain at its maximum displacement during the compression stroke, making space available for the compressed charge and limiting the compression ratio in order to prevent detonation as already described. The charge may be ignited as in a normal engine and the cam may then return the free piston to its seating as the expansion stroke progresses to completion.

One advantage of a cam is that a rapid response can be achieved by using a suitable profile. A movable stop may be arranged in order to limit the stroke of said free piston. Said arrangement permits the compression ratio to be altered while the engine is running. One advantage of such an arrangement is to provide the ability to alter the engine torque at constant speed. Another advantage is to compensate for variations in engine breathing under different operating conditions. A further advantage is to alter the compression according to the type of fuel available.

As an alternative, the free piston may be held against its seating by gas pressure or by a spring as already described. A cam may be arranged to assist the lifting of the free piston. One advantage of such a positive drive arrangement for said free piston is a reduction in possible problems arising through inertia effects which may cause said free piston to operate out of its correct phase relationship to the main piston. A double acting cam with or without gas or spring assistance may be employed to operate said free piston positively in both directions as an alternative to the above arrangement.

If a free piston arrangement is used without cams or other directly connected drive means, its movement is dependent upon several different factors. The inertia mass of the free piston and the friction in its cylinder must be overcome by the differential force across the free piston in order for it to move in its cylinder. As the speed of the engine increases, the force required increases in order to move the piston sufficiently rapidly to match the speed of the engine.

This may be facilitated by reducing the pressure of gas or other force which holds the secondary piston on its seating. Appropriate valves and ducting may be arranged to vary the gas pressure acting on the said free piston in a cyclic manner to ensure optimal operation. Said construction is considered to be particularly advantageous in application to engines constructed with banks of several cylinders of each size.

There is no restriction upon the number of pistons and cylinders which can be interconnected in a manner similar to that illustrated in Figure 1. Free pistons and kinematically linked pistons may be used in combination. The speeds at which said cranks rotate may be chosen at will along with the swept volumes of the pistons and cylinders to which they are connected. In said manner, the construction may be adapted to approximate any practical thermodynamic cycle.

With reference to Figure 3, in a further construction, the swept volume of the expansion and exhaust strokes can be arranged to be greater than the swept volume of the induction and compression strokes. Said arrangement is considered particularly advantageous, being capable of implementing a cycle in which the expansion of the hot gases is greater than the compression which was applied to the fresh charge. One advantage of said arrangement is that a greater amount of work is extracted from the hot gases prior to release. A further advantage is that the pressure of the gases at the point in the cycle when the exhaust valve opens will be lower, reducing exhaust silencing requirements.

With reference to Figure 3, the start volume 11 and end volume 12 of the cycle are equal to the minimum volume of the combustion chamber. The induction stroke takes place approximately between the start 11 and the maximum volume at 14. Compression occurs approximately between the maximum volume at 14 and the minimum at point 13. As already described, the minimum at point 13 is a greater volume than at the start 11 and end 12 of the cycle.

The compression ratio of the engine is determined by the volumes at the maximum 14 and minimum 13 only. The expansion or power stroke takes place approximately between the minimum 13 and the maximum 15. The exhaust stroke occurs approximately between the maximum 15 and the minimum at the end of the cycle 12.

According to one construction, the particular cycle illustrated in Figure 3 may be implemented using three kinematically linked pistons operating in their respective cylinders, influencing the volume of a single combustion space. Said pistons may be driven by separate cranks through connecting rods and operate at speeds in the ratios 1 : 2: 3. Said cranks may be aligned to reduce the volume of the combustion space to the minimum at the start of the cycle. The swept volume of said pistons may be in ratios close to the values 1: 1.8 : 0.26, respectively to implement an approximation to the cycle illustrated. The precise shape of the volumetric cycle may be adjusted to suit thermodynamic considerations by a suitable choice of swept volume for each of the pistons.

Additional pistons and cylinders may be added in order to refine the shape of the cycle. Pistons may be kinematically linked or may be of a free piston type with or without assistance in operation as already described. In common with engines of known design, valve timing in the present construction may be arranged to utilise inertia effects in the gases to influence the effective operational commencement and completion of strokes.