SEM Coffee Break Puzzle

FWIW, some model FM OP engines use a helically splined shaft to connect the crankshafts, by moving the shaft axially the phasing of the cranks to on another, can be altered to enable reversing of the engine.

Tom

"Tom" wrote

Ah, like the 'variator' used by Fiat on their variable timing camshaft drive. Anyway, I was totally wrong about how the thing works. Jim Cahill replied as follows:-

Dear Nick, Thanks for your comments. The phase of the two crankshafts is fixed. The smaller one runs at half the speed of the large one. At the start and end of the cycle, the two pistons are up against one another. Apart from a small amount of space which has to be left for the spark plug and valves, the volume inside the cylinder is near enough zero. This gives complete scavenging, and improves induction because there is very little spent gas left in the cylinder at the end of each cycle. Thus the induction stroke achieves higher vacuum, giving improved volumetric efficiency. The incoming charge is also cooler since there is less residual gas to warm it up. Combustion should also be cleaner because of the reduced contamination from spent gases.

As the cycle proceeds, the small piston moves outwards, reaching its maximum volume as the large piston completes one revolution and reaches the top of the compression stroke. So for say a 10:1 compression ratio, the small cylinder will be approximately 1/10th of the size of the large one. There will always be some dead space, so the small piston would be smaller by a certain amount.

Variable compression ratio can be achieved by varying the stroke of the small piston during operation. Engines have been constructed which vary the compression ratio by altering piston stroke. However, in an engine of otherwise standard configuration, variation of stroke is being made on components which have a lot of mass. The forces involved limit performance. This design has the advantage that the stroke would be varied on a small, lightweight piston running at half the engine speed. Assuming the reciprocating masses are one tenth of those of the main piston, the forces involved would be 1/40th of those on the main piston, making it much easier to vary the stroke.

You can apply the Miller approach to achieving the Atkinson cycle in the same manner as to any other 4 stroke. I will be putting a fuller description of the method of operation on my website, but if you are interested, there is an item on Stirling engines there.

Yours sincerely

Jim Cahill

The most successful VCR engines have been made by Continental for US Army Tanks using a piston within a piston. The outer piston being actuated by oil pressure supplied via the con rod. Later engines were producing 1500hp from 1360 ci. Brit idea patented by a Bill Mansfield of the BICERI, US3200798

Tom

A fuller explanation now available at

Is there not an attempt at variable compression ratio (tried by Saab?) where the cylinder block hinges along one side, so the block can be lifted or lowered slightly in relation to the crankshaft.

Or Bamford ;-)