( Our Engines )

What is a split-cycle engine?

Split-cycle engines separate the four strokes of intake, compression, power and exhaust into two separate but paired cylinders. The first cylinder is used for intake and compression. The compressed air is then transferred through a crossover passage from the compression cylinder into the second cylinder, where combustion and exhaust occur. A split-cycle engine is really an air compressor on one side with a combustion chamber on the other.

Split-cycle engines appeared as early as 1914. Many different split-cycle configurations have since been developed; however, none has matched the efficiency or performance of conventional engines. 

PPrevious split-cycle engines have had two major problems - poor breathing (volumetric efficiency) and low thermal efficiency.

Breathing (Volumetric Efficiency)

The breathing problem was caused by high-pressure gas trapped in the compression cylinder. This trapped high-pressure gas needed to re-expand before another charge of air could be drawn into the compression cylinder, effectively reducing the engine's capacity to pump air and resulting in poor volumetric efficiency.

Low Thermal Efficiency

The thermal efficiency of split-cycle engines has always been significantly worse than a conventional Otto cycle engine. The primary reason: They all tried to fire like a conventional engine - before top dead center (BTDC).

In order to fire BTDC in a split-cycle engine, the compressed air, trapped in the crossover passage, is allowed to expand into the power cylinder as the power piston is in its upward stroke. By releasing the pressure of the compressed air, the work done on the air in the compression cylinder is lost. The power piston then recompresses the air in order to fire BTDC.

By allowing the compressed gas in the transfer passage to expand into the power cylinder, the engine needs to perform the work of compression twice. In a conventional engine, the work of compression is done only once; consequently, it achieves much better thermal efficiency.