Continuously Varying Exhaust Outlet Diameter to Improve Efficiency and Emissions of a Small SI Natural Gas Two-Stroke Engine by Internal EGR 2018-01-0985

With continuously increasing concern for the emissions from two-stroke engines including regulated hydrocarbon (HC) and oxides of nitrogen (NO_x) emissions, non-road engines are implementing proven technologies from the on-road market. For example, four stroke diesel generators now include additional internal exhaust gas recirculation (EGR) via an intake/exhaust valve passage. EGR can offer benefits of reduced HC, NO_x, and may even improve combustion stability and fuel efficiency. In addition, there is particular interest in use of natural gas as fuel for home power generation. This paper examines exhaust throttling applied to the Helmholtz resonator of a two-stroke, port injected, natural gas engine. The 34 cc engine was air cooled and operated at wide-open throttle (WOT) conditions at an engine speed of 5400 RPM with fueling adjusted to achieve maximum brake torque.

Exhaust throttling served as a method to decrease the effective diameter of the outlet of the convergent cone. Throttling balanced energy and exergy flows by acting as a source of internal EGR and lowering combustion temperatures, which yielded lower heat transfer. Though exhaust throttling improved efficiency, it deteriorated power density by affecting both trapping and scavenging efficiencies. In-cylinder pressure was used to investigate combustion phasing, heat release rate, and trapped mass inside the cylinder. Exhaust throttling had a negative impact on the delivery ratio and positive impact on trapping efficiency by inducing backpressure on the exhaust port. NO_x and total hydrocarbon (THC) emissions decreased for throttled cases.