LIGHT-LOAD BURN RATE ANALYSIS IN AN AIR-COOLED UTILITY ENGINE
Brossman, John Richard
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In order to maximize the power-to-weight ratio, small, air-cooled utility engines are designed for WOT performance. Low-speed, light-load conditions suffer due to high residual gas retention, poor volumetric efficiency and an unfavorable in-cylinder flow field that can lead to heat release past EVO. Conditions with significant heat release past EVO have been found to contribute as a dominant HC emissions source at low-speed and lightload. The effects of a dual spark configuration, an alternate spark location, and bulk flow improvement through swirl enhancement were investigated on a single-cylinder, air-cooled utility engine. A steady flow analysis of various swirl enhancement techniques was conducted on a steady flow bench with a swirl adaptor to obtain a global view of the intake flow angular momentum. Through in-cylinder pressure recording and exhaust gas sampling, a single-zone heat release and ring-pack model were utilized to analyze combustion and emissions performance. A similar operating condition was defined and explored to determine the light-load performance of various engine designs to isolate characteristics leading to improved performance. It was found that a dual spark configuration advances combustion leading to a reduction in burn duration, but increased burning rate only occurs when the flame fronts are held independent. With a retarded combustion phasing strategy, HC and NOX emissions were reduced independent of speed and load due to lower ring pack loading and a favorable post-oxidation environment. Conditions with significant heat release past EVO through retarded combustion phasing were found to not increase HC emissions; an oxidation reaction was still in effect. A port blockage that induced a tangential flow was found to significantly improve the part-throttle steady flow swirl coefficient leading to a greater global swirl ratio. Engine combustion tests using this blockage indicated a significant advance of combustion leading to decreased burn duration and reduced COV independent of speed and load. Due to the improved burn rate, lean operating conditions were explored and resulted in a significant reduction in HC emissions, but increased NOX leading to a negligible change in the HC+NOX parameter from the rich condition. A similarity condition was defined based on mean piston speed, IMEP and combustion phasing with engines of various size and geometry. This first order assessment provided similar thermodynamic conditions at CA50, with the intake and combustion chamber geometry produced in-cylinder flow field variations impacting combustion duration. Engines with centralized spark positions and pent-roof combustion chambers were found to improve the CAIgn-10 and CA10-50 duration over the side spark, bathtub configurations. Significant differences of the flame termination period, CA50-90, indicate favorable flow fields late in the cycle and may indicate a dependence on the cooling package associated with the engine.