(A split-cycle engine has a pair of cylinders : one runs combustion & exhaust, the other pulls in air, pre-compressing it to feed the first cylinder when both pistons are at TDC.
A compounding engine takes high pressure exhaust from one cylinder into a larger cylinder to continue expansion, before
Both have theoretical advantages over regular engines, though neither have achieved widespread use. But, why not do both at the same time ?)
This Split-Cycle Compounding Engine combines both of them : the hot-cylinder is the same as usual, but the auxiliary cylinder is dual-acting (a dual-acting cylinder has an enclosed bottom through which runs the piston rod, giving it two chambers) : on the upstroke compression happens on top of the piston while exhaust transfer happens underneath . The downstroke is predictable and boring : fresh air intake on top, final exhaust expulsion on the bottom.
That - theoretically at least - is how to combine the two pre-existing designs efficiently. To make it actually work in practice, better than before, we can do a couple of things that really should have been done originally, anyways...
- The aux-cylinder sits half on top of the hot-cylinder, so the transfer is direct without a heat/pressure-wasting pipe. This also means that they're closer together on the crank-journal than if they were directly side-by-side. Which means a smaller-footprint engine.
- To waste as little pressure as possible from the aux-cylinder, there is an insulated staging tank between the top of the aux-cylinder and the top of the hot-cylinder, which holds a number of gulps of air for the hot-cylinder, at the required pressure. (more on that in a minute.)
- if the engine's application doesn't need immediate response to throttle changes (say a generator), the aforementioned tank can also act as a mixing chamber, which gives fuel and air loads of time to mix thoroughly.
So thar ya go.
For those thinking "well why not a 2cyl 4-stroke", a couple of more things...
- as opposed to a hot-cylinder, the aux-cylinder is under much less pressure at any given time so, despite being larger, it doesn't need to be as heavy and the seals don't need to be as tight, ie: it's lighter and less friction-y, requires less oiling, etc.
- the muffler is lighter since what hits it is cooler and less noisy. this is an addition to fuel efficiency.
Viable Pneumatic Regen/Launch - all the cylinder chambers are zero or near-zero displacement at TDC so, unlike with a regular engine, an air-storage tank can be used at very high pressure.
HCCI - the mixing tank natively creates an Homogenous Charge however, if the tank is at the auto-ignition pressure then it's a pipe bomb. Easily solved by staging the compression : the aux-cylinder pumps up the tank to say 15:1, and the hot-piston then provides a further 4:1 in the normal-engine manner, during the last bit of the upstroke.
" 'Atkinson' whether you want it or not" - eventually somebody will notice that - since all pistons go right to the top of their cylinders and the auxiliary cylinder is larger than the hot-cylinder, there has to be some wasteage, somewhere. Yup, easily solved by fixing the fresh-air intake valve to close prematurely during the intake stroke.