SI Engine Operation with Retarded Ignition: Part 2 -HC Emissions and Oxidation 1999-01-3507
Spark retard is desirable for decreasing cold start hydrocarbon emissions and lighting off the catalyst more rapidly. The focus of this work is to better understand the nature of the HC emissions as spark is retarded and investigate the location of the oxidation (in-cylinder or in the exhaust port and manifold). Fast FID measurements were taken in the exhaust port of a single cylinder research engine during cold, retarded spark engine operation (1200 rpm, 2.5 bar IMEP, 20 °C fluids).
At moderate spark retard both Fast FID (exhaust port) and exhaust plenum HC levels decreased due to reduced crevice volume fraction at the end of burn, and increased in-cylinder burn up. In contrast, at large spark retard the port HC's increased dramatically while the exhaust plenum levels continued to fall to near zero. This is thought to be due to the onset of incomplete in-cylinder combustion along with increased exhaust port and manifold after-burning caused by the increasing exhaust gas temperatures.
Operation with a fully warmed up engine (90 °C coolant) produced much lower HC concentrations in the exhaust port illustrating the impact of fuel evaporation.
Heat release analyses of the cylinder pressure traces indicate that the combustion was 25-40% incomplete at EVO with retarded spark timing.
Visual observations through a light pipe in the exhaust port at very retarded spark timings suggest the presence of a pre-mixed flame due to incomplete combustion at EVO (blue light) as well as droplet and/or liquid fuel burning in the exhaust port (yellow/orange speckles from diffusion flames).