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 presents unique benefits in a hybrid architecture such as its lower heat transfer due to a favorable surface area to volume ratio and lack of a cylinder head, as well as the thermodynamic benefits of two stroke operation with uniflow scavenging. A particular focus of this effort is the work extraction efficiency of two design concepts. The first design concept 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 peak torque required to for the electric machine. However, the friction losses caused by the geartrain limit 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. Furthermore, feedback control algorithms are required to maintain the relative phasing between the two crankshafts and thus requires larger torque capability from the electric machines. A parametric study of motor and engine characteristics was conducted to project the efficiency entitlement of each hybrid architecture based on models of the electric motor/generator which have been validated by experimental results. Elimination of the geartrain increases the ceiling for the system efficiency. However, in practice the larger amplitude torque pulsations from the electric motor required to track the desired exhaust crankshaft lead may offset the benefits in friction reduction.