Prompted by the aggressive moves of Tesla Motors, the news is full of full-line car manufacturers announcing electric vehicles in the near future. One could easily conclude that the electric future is already written and that the internal combustion engine (ICE) will soon be put out to pasture. Even so, companies are still labouring to produce more efficient ICEs that will tilt the economics back to the use of gasoline a little longer.
Achates Power is developing an opposed piston, two-stroke engine that they claim offers substantial advantages over the now common configuration of engines (one piston per cylinder, valve train and head).
The most striking aspect of Achates’ design is the opposed piston layout. This is distinct from boxer engines found in cars like Subarus which do have opposed pistons, but in opposed cylinders.
Instead, Achates positions two pistons in the same cylinder both with cranks at either end connected by idler wheels on the outside of the block. This arrangement has the pistons nearly meet to create a combustion chamber between them.
This increased symmetry (Principle 4 – Asymmetry) improves the balance of the engine. Removing the head (Principle 2 – Taking Out) reduces heat loss.
The vast majority of vehicle engines use a four stroke cycle (intake, compress, power stroke, exhaust). Achates’ design uses a two stroke cycle (compress, power stroke) which can make double the power in the same number of rotations.
The intake and exhaust take place through ports in the cylinder – one piston uncovers the exhaust and the other the intake, at the bottom of their stroke. This eliminates the need for two more strokes and also eliminates complicated, costly, heavy and power robbing valve train components (Principle 2 – Taking Out).
Typical engines have one injector per cylinder, as they occupy an area already congested with valve train components. Opposed piston engines can have an arbitrary number of injectors (Principle 1- Segmentation) and achieve better fuel mixing for more efficiency, power and reduced emissions.
Opposed piston engine designs have been around since the 30s. I suspect one of the reasons for it being a viable option now is direct injection, a relatively new technology, particularly for gas engines. With the carefully timed injection (Principle 19 – Periodic Action) this design can now avoid knock and scavenging unburned fuel.