Work Extraction Efficiency in a Series Hybrid Opposed Piston Engine

This work investigates the development of a novel series hybrid architecture utilizing a single cylinder opposed piston engine. The opposed piston engine’s natural balance presents unique benefits in a hybrid architecture due to its capability of achieving engine downsizing by reducing cylinder count instead of reducing bore diameter, which incurs the higher heat transfer losses. A particular focus of this effort is the work extraction efficiency of two design concepts. The first design concept considered utilizes a geartrain to couple the crankshafts of the engine in a conventional manner, providing a single power take-off for coupling to an electric motor/generator. In this design, the large inertia of the geartrain dampens the speed fluctuation of the single cylinder engine, reducing the speed and torque oscillations required to be absorbed by the electric machine. However, the friction losses caused by the geartrain limits the maximum work extraction efficiency. The second design concept eliminates the geartrain by coupling an electric machine directly to each crankshaft of the opposed piston engine. While elimination of the geartrain avoids the frictional losses of the first design, the large torque and velocity oscillations characteristic of a single cylinder engine are no longer damped and the necessity to maintain the relative phasing between the two crankshafts is placed solely on the electric machines. Utilizing experimental results coupled with electric motor/generator modeling, a parametric study of motor and engine parameters was conducted to project the efficiency entitlement of each hybrid architecture. Findings show that while the elimination of the geartrain increases the ceiling for the system efficiency, undamped torque pulsations from the engine can offset any benefits in friction reduction.