Double Compression Expansion Engine Concepts: Efficiency Analysis over a Load Range

Double Compression Expansion Engine (DCEE) concepts are split-cycle concepts where the main target is to improve brake efficiency. Previous simulations work [1] suggests these concepts has a potential to significantly improve brake efficiency relative to contemporary engines. However, a high peak efficiency alone might be of limited value. This is because a vehicle must be able to operate in different conditions where the engine load requirements changes significantly. An engine’s ability to deliver high efficiency at the most frequently used load conditions is more important than peak efficiency in a rarely used load condition. The simulations done in this paper studies the efficiency at low, mid and full load for a DCEE concept proposal. Two load control strategies have been used, lambda and Miller (late intake valve closing) strategies. Also, effects from charge air cooling has also been studied. The Miller load control strategy reduces the overall peak cylinder pressure (PCP) due to a reduced compression ratio. A lower peak pressure has the advantage of reducing friction loss, since this loss is correlated with peak cylinder pressure. However, the less diluted air/fuel-mixture leads to higher combustion temperature which will increase overall heat loss and reduce thermodynamic efficiency. The studies also show that the charge air cooler (CAC) should not be used at low engine loads, because of the high heat loss it creates. However, at high loads the CAC is very useful because it both improves thermal efficiency and increases the maximum load by 12%, given the limitation on air/fuel-ratio (λ should be kept above 1.2).