Cost and robustness are key factors in the design of diesel engines for low power density applications. Although compression ignition engines can produce very high power density output with turbocharging, naturally-aspirated (NA) engines have advantages in terms of reduced cost and avoidance of system complexity. This work explores the use of direct injection (DI) and exhaust gas recirculation (EGR) in NA engines using experimental data from a single-cylinder research diesel engine. The engine was operated with a fixed atmospheric intake manifold pressure over a map of speed, air to fuel ratio, EGR, fuel injection pressure and injection timing. Conventional gaseous engine-out emissions were measured along with high speed cylinder pressure data to show the load limits and resulting emissions of the NA-DI engine studied. Well known reductions in NO_x with increasing levels of EGR were confirmed with a corresponding loss in peak power output. However, advancing fuel injection timing is shown to partially offset this disadvantage at the cost of higher engine noise. Partially premixed low temperature combustion (LTC) can be used over a portion of the operating range, further lowering the engine-out emissions of soot and NO_x. Since aftertreatment catalysts to reduce tailpipe NO_x and soot add cost and complexity to diesel engine-powered vehicles, the extensive use of EGR may have advantages for low power density NA diesel engines designed for low cost vehicle markets or for electrical power generation in range-extended electric vehicles.
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