Evaluating the Effects of an Electrically Assisted Turbocharger on Scavenging Control for an Opposed Piston Two Stroke (OP2S ) Compression Ignition Engine

Opposed piston two-stroke (OP2S) diesel engines have demonstrated a reduction in engine-out emissions and increased efficiency compared to conventional four-stroke diesel engines. Due to the higher stroke-to-bore ratio and the absence of a cylinder head, the heat transfer loss to the coolant is lower near ‘Top Dead Center.’ The selection and design of the air path is critical to realizing the benefits of the OP2S engine architecture. Like any two-stroke diesel engine, the scavenging process and the composition of the internal residuals are predominantly governed by the pressure differential between the intake and the exhaust ports. Without dedicated pumping strokes, the two-stroke engine architecture requires external devices to breathe. In the unique OP2S engine architecture studied in this work, the external pumping devices present in the air path include an electrically assisted turbocharger (EAT), an electrified EGR pump, and a back-pressure valve (BPv) located downstream of the turbocharger.In this work, various sweeps were experimentally recorded for these actuators to understand their effects on airflow rate, port pressure, and pressure differential. The objective of these experiments was to identify regions with high scavenging efficiencies while evaluating the effect of electrical power consumption as pumping losses on brake efficiency. The results indicated a higher scavenging efficiency was achievable at low engine speeds as less of the fresh intake charge is short-circuited to the exhaust during the blow down process. Also, with the combination of these actuators, an inherent challenge of decoupling airflow control from intake port pressure became apparent, and thus optimizing scavenging at a given port pressure became difficult. However, the EAT provides the flexibility of changing the air flow rate at a constant load but any increase in the brake thermal efficiency is negated by the electrical energy consumed from pumping more air through the compressor.