GB2226098A

GB2226098A - Piston and connecting rod

Info

Publication number: GB2226098A
Application number: GB8926260A
Authority: GB
Inventors: Anthony Edgar Blackburn
Original assignee: Individual
Current assignee: Individual
Priority date: 1988-12-03
Filing date: 1989-11-21
Publication date: 1990-06-20
Also published as: GB8926260D0; GB8828274D0

Abstract

A four-stroke piston engine comprises main pistons (18) each having a secondary piston (24) reciprocally movable in a chamber therein, each main piston being connected to its connecting rod (26) via its secondary piston, the secondary pistons each having a lateral extension or flange (29) retained in a wider part of the chamber to provide substantially sealed pneumatic spaces (34, 35 respectively) above and below the lateral extension and means (45) for equalising pressure in the upper and lower spaces, whereby the pistons have a smaller clearance from the cylinder head at the end of their exhaust stroke than at the end of their compression stroke. The connecting rod (26) has a forked little end (28) to fit outside of the secondary piston to which it is pivotally attached. The ratio of the diameter of the secondary piston body relative to the flange is less then 1:2. The secondary piston body is hollow and has vents (47) communicating both with the crank case and the lower pneumatic space and the flange is formed as a dish-shape having a chamferred upper edge (37) and its underside (39) recessed. <IMAGE>

Description

Title: Piston Engines DESCRIPTION This invention concerns piston engines.

In piston engines, particularly 4-stroke petrol engines, the exhaust residue is hot and it preheats the fresh fuel/air mixture. This preheating causes the mixture in the cyinder to be relatively hot at the start of combustion. However, because the mixture is relatively hot, the heat added by the combustion of fuel produces a smaller increase in pressure and less power.

If the amount of the exhaust residue could be reduced then the temperature of the fuel/air mixture in the cylinder would be reduced. That would mean that the same heat and fuel would produce a greater pressure and more power. As the amount of the exhaust residue was reduced so the proportion of fresh fuel/air mixture would be increased, which would further increase the combustion pressure and power output for a given quantity of fuel.

Furthermore, it would be desirable to have a clearance at the end of compression that was related to the desired pressure of fuel mixture in the cylinder for optimum economy.

In my earlier patent application No. WO 86/04955 I proposed a four-stroke engine wherein piston clearance is dependent on the pressure within the cylinder. Specifically, I proposed a four-stroke piston engine comprising main pistons each having a secondary piston reciprocally movable in a chamber therein, each main piston being connected to its connecting rod via its secondary piston, the secondary pistons each having a lateral extension or flange retained in a wider part of the chamber characterised by substantially sealed pneumatic spaces above and below the lateral extension and means for equalising pressure in said upper and lower spaces whereby the pistons have a smaller clearance from the cylinder head at the end of their exhaust stroke than at the end of their compression stroke.

The object of this invention is to provide improvements to the above-mentioned proposals for four-stroke engines.

Of particular concern are weight and material savings and ease of manufacture, as well as improved efficiency.

According to a first aspect of the present invention there is provided a four-stroke piston engine comprising main pistons each having a secondary piston reciprocally movable in a chamber therein, each main piston being connected to its connecting rod via its secondary piston, the secondary pistons each having a lateral extension or flange retained in a wider part of the chamber to provide substantially sealed pneumatic spaces above and below the lateral extension and a means for equalising pressure in said upper and lower spaces whereby the pistons have a smaller clearance from the cylinder head at the end of their exhaust stroke than at the end of their compression stroke, wherein the connecting rod has a forked little end to fit outside of the secondary piston to which it is pivotally attached.

According to a second aspect of the present invention there is provided a four-stroke piston engine comprising main pistons each having a secondary piston reciprocally movable in a chamber therein, each main piston being connected to its connecting rod via its secondary piston, the secondary pistons each having a lateral extension or flange retained in a wider part of the chamber to provide substantially sealed pneumatic spaces above and below the lateral extension and a means for equalising pressure in said upper and lower spaces whereby the pistons have a smaller clearance from the cylinder head at the end of their exhaust stroke than at the end of their compression stroke, wherein the ratio of the diameter of the secondary piston body to the flange is less than 1:2.

According to a third aspect of the invention there is provided a four-stroke piston engine comprising main pistons each having a secondary piston reciprocally movable in a chamber therein, each main piston being connected to its connecting rod via its secondary piston, the secondary pistons each having a lateral extension or flange retained in a wider part of the chamber to provide substantially sealed pneumatic spaces above and below the lateral extension and means for equalising pressure in said upper and lower spaces, whereby the pistons have a smaller clearance from the cylinder head at the end of their exhaust stroke than at the end of their compression stroke, wherein the secondary piston body is hollow and has vents communicating both with the crank case and the lower pneumatic space.

The flange preferably has a chamfered upper edge and preferably has its underside recessed.

The forked connecting rod little end allows the secondary piston to have a relatively small diameter main body, so saving material, reducing weight and providing a larger surface area under the flange. The flange part can have a relatively large diameter but special shaping may also reduce its weight.

Other preferred features and advantages of the present invention will now be further described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a section through a cylinder of a four-stroke engine at the end of its compression stroke; and Figure 2 is a section through a cylinder of a four-stroke engine at an intermediate point in its cycle, the cylinder of this drawing showing variations to the cylinder of Figure 1.

Referring to the accompanying drawing, a four-stroke engine cylinder 10 comprises a chamber 12 closed at one end by cylinder head 14 and having a main piston lb reciprocally movable in the chamber. The cylinder head 14 has a pair of valves 18 (only one shown) one whereby exhaust gases are expelled and the other whereby a fuel/air mixture is introduced. The main piston 16 has piston rings 20 for sealing between the piston 16 and the cylinder wall.

Within the main piston 16, shown as being of two-part construction (16A, 16B) is a secondary or pneumatic piston 24 that is connected to connecting rod 26 which is in turn connected to a crankshaft (not shown). The connecting rod 26 has a forked little end 28, within which the lower end of the secondary piston 24 is located by means of a gudgeon pin 30 therethrough.

The two part construction of the main piston 16 has its joint at a low pressure region, so making it easier to provide effective sealing. In my earlier proposals the two part construction had one part fitting inside the other. Now the two parts are overlapped but provide together a continuous main piston outer surface. Assembly of the pistons of the invention may be facilitated by this construction since the piston rings can be compressed in the lower component 16B when the piston crown can be fitted much more easily.

The secondary piston 24 has a cylindrical part 27 and an upper flange like part 29 integral therewith.

The piston 24 is reciprocally movable within the main piston and is sealed by means of piston rings 32 around the edge of the flange 29 and 33 around the cylindrical part 27. Seal 33A (right hand side of Figure 1) is of synthetic rubber but may be of the piston ring type 33B (left hand side of Figure 1). Alternatively, it may be desirable to eliminate seals 33 if the clearance remains small (see Figure 2). The flange 29 forms two pneumatically sealed chambers 34 and 35 one above the flange and one below.

The flange 29 is actually shaped as a dish having a radially sloping upper surface 36 leading to a chamfered edge portion 37. Below the chamfered edge 37 the flange has an annular lip 38 surrounding an annular recess 39 on the underside of the flange about the cylindrical part 27 of the piston 24. The upper inner surface 40 of the chamber 34 is shaped to correspond with the upper surface shape of the flange 29 and the lower surface 41 of the chamber 35 is shaped to correspona with the underside surface shape of the flange 29.

Inner side wall 44 of the main piston includes one or more vents or notches 45, whereby pressure equalisation can take place between the pneumatic chambers 34 and 35 and lubricating oil can be transferred from one chamber to the other.

The cylindrical part 27 of the secondary piston 24 has a vent 47 open to the crankcase so that any loss of air from the pneumatic chambers can be restored. Oil transfer in the form of oil mist also can take place via the vent 47. Higher up the cylindrical part 27 of the piston 24 are ports 49A and B positioned to allow transfer of air and oil into the lower pneumatic chamber 35 when the piston 24 is near its highest point. As an alternative to vent 49A a groove 50 in the outer surface of the cylindrical part 27 can similarly allow the transfer of air and oil into the lower chamber 35 when the piston 24 is near its highest point relative to the piston 1b (see Figure 2).

In operation, the main piston is free to move upwards in the cylinder in response to the pressure therein. Thus, in the exhaust stroke the pressure is low so that the invention of the main piston causes it to move close to the cylinder head, thus expelling virtually all of the exhaust gases. However, in the compression stroke, the pressure in the cylinder is high so that the clearance between the piston and the cylinder heat at top-dead-centre is greater than for the exhaust stroke.

Furthermore, under different load conditions, the pressure in the chamber will be different and the main piston will move to give an appropriate clearance. This is due to the volume of the chambers 34, 35 being varied according to the pressure in the cylinder allowing the main piston to move relative to the connecting rod at top-dead-centre for both exhaust and compression strokes.

The pneumatic chambers 34, 35 act as springs, allowing the piston crown to move down to low compression at full throttle, to move up to high compression when cruising and to change position within one cycle. A higher piston crown height at the end of the exhaust stroke can reduce the exhaust residue in the cylinder head by as much as 90 and eliminate the need for valve overlap and exhaust and inlet tuning.

When cruising the charge purity can increase from about 30% fresh mixture to 90 fresh mixture, resulting in faster combustion and any increase in heat supplied.

Less heat will be rejected due to the cruising expansion ratio of between 20 and 40:1. (Thermal efficiency = 1 - Heat rejected/Heat supplied.) Reductions in fuel consumption and undesirable exhaust emissions may result.

Claims

1. A four-stroke piston engine comprising main pistons each having a secondary piston reciprocally movable in a chamber therein, each main piston being connected to its connecting rod via its secondary piston, the seconaary pistons each having a lateral extension or flange retained in a wider part of the chamber to provide substantially sealed pneumatic spaces above and below the lateral extension and a means for equalising pressure in said upper and lower spaces whereby the pistons have a smaller clearance from the cylinder head at the end of their exhaust stroke than at the end of their compression stroke, wherein the connecting rod has a forked little end to fit outside of the secondary piston to which it is pivotally attached.

2. A four-stroke engine as claimed in claim 1, wherein the ratio of the diameter of the secondary piston body to the flange is less than 1:2.

3. A four-stroke engine as claimed in claim 1 or claim 2, wherein the secondary piston body is hollow and has vents communicating both with the crank case and the lower pneumatic space.

4. A four-stroke engine as claimed in claim 1, claim 2 or claim 3, wherein the flange is formed as a dish shape.

5. A four-stroke engine as claimed in claim 4, wherein the flange has a chamfered upper edge.

6. A four-stroke engine as claimed in claim 4, wherein the flange has its underside recessed.

7. A four-stroke piston engine substantially as hereinbefore described with reference to and as illustrated in Figure 1 or Figure 2 of the accompanying drawings.