RESIDUAL GAS EFFECTS ON COMBUSTION IN AN AIR-COOLED UTILITY ENGINE

Abstract

Air-cooled utility engines are often equipped with cams designed to ensure maximum charging efficiency, but the resulting valve lift and timing often correlates to poor engine performance at low speeds and light loads, which manifests itself in poor combustion stability and elevated pollutant emissions. The poor performance at low-speeds and lightloads can be attributed to the presence of high levels of exhaust gas residuals (EGR) trapped in the cylinder. In this project the combustion stability and pollutant emissions of an air-cooled utility engine were investigated at low speeds and light loads with high residual fractions. An incylinder sampling valve was used to measure the amount of residual fraction in the charge. Varying the cam phasing, opening and closing times of both cams, effected a change in the amount of residual gas retained. A Fast Flame Ionization Detector (FFID) was used to measure the cycle-resolved hydrocarbon emissions from the combustion process, and a standard five-gas analytical emissions bench measured steady-state emissions. In addition, spark timing and different fuel mixture preparation techniques were tested to investigate their influence on combustion progress. The different fuel preparation systems included a stock carburetor, a carburetor-mounted liquid fuel injection (CMLFI) system, and a homogeneous mixing system (HMS). The residual fraction was found to be relatively insensitive to the fuel mixture preparation system, but was to a moderate degree sensitive to the ignition timing. The residual fraction was found to be strongly affected by the amount of valve overlap and engine speed. The measured residual fraction was compared to three models used for EGR prediction in order to quantify their applicability to utility engines. The three models used for comparative analysis include: Yun and Mirsky [16], Fox, Cheng and Heywood [12], and an ideal gas model. None of the three EGR models investigated was able to accurately predict the residual fraction over the entire range of conditions tested. Cylinder pressure-derived parameters were used to evaluate combustion performance parameters for cycle-to-cycle analysis of the investigated operating conditions. The CMLFI showed a slight improvement in performance relative to the other fuel preparation systems. Advancing ignition improved the cyclic variability of the tested conditions, with an exception at very light load operation at low speeds. The combustion stability was found to be sensitive to increasing amounts of overlap. The carburetor produced higher steady-state emissions, unburned hydrocarbons (uHC) and oxides of nitrogen (NOx), than the other fuel preparation systems. Advancing ignition and decreasing engine speed increased the measured uHC concentrations. The resulting uHC increased with increasing valve overlap. A strong prior cycle effect was found in the cycleresolved hydrocarbon concentrations at high residual fraction.