When it comes to the performance and efficiency of an opposed-piston, two-stroke engine, does the number of cylinders make a difference? If you guessed “yes”, you’re right.
Based on extensive analysis, Achates Power has determined that its three-cylinder, opposed-piston engine is the optimal design from a gas-exchange perspective, especially when compared to a two- or four-cylinder design. The reason: the gas exchange duration in a two-stroke engine is about 120 degrees crank angle. In a three-cylinder design, the scavenging events are aligned in a way that they have minimal interference with each other and still keep enough mass flow going over the cycle to provide adequate energy to the turbocharger so that it operates most efficiently to compress the intake air.

Cross Charging
The three-cylinder, opposed-piston, two-stroke engine configuration has slightly overlapping gas-exchange events, resulting in a phenomenon known as cross charging.
In a two-cylinder configuration, however, the gas-exchange events are too timely separated. This separation causes the turbocharger to lose energy over the cycle, which has a negative effect on the turbine’s efficiency—especially at lower loads and engine speeds. The loss of turbocharger energy has to be compensated by the crank-driven supercharger, which causes a reduction in brake thermal efficiency. Conversely, in a four-cylinder configuration, the gas-exchange events overlap too much. This causes cross charging to occur at a point in time when hot exhaust gases are leaving the cylinder. The interruption of exhaust gas flow causes an increase in residual gas content and, therefore, a lower scavenging efficiency—leading to a reduction in power. Even with a complex design of the exhaust manifold to separate the pulses, there will be communication over the twin scroll turbine housing. Separating the exhaust system into two turbochargers leads back to the two-cylinder problem with the energy flow leak over the cycle.
While two-, four- and five-cylinder options are all viable as part of a comprehensive family of engines, a three-cylinder, opposed-piston, two-stroke design is optimal. This three-cylinder design is just one of the many thermal efficiency benefits of the Achates Power A48 engine. Last month, we highlighted the importance of our powertrain’s stroke-to-bore ratio in a post called: Stroke-to-Bore Ratio: A Key to Engine Efficiency. And, in the coming months, we’ll cover other thermal efficiency strengths including the two-stroke architecture, the opposed-piston design, and the patented Achates Power combustion system.

Clean Diesel Engine Engine Design

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