抄録
The reduction of total hydrocarbon (THC) emissions during cold start conditions is very important in order to achieve a tight control over exhaust emissions. The key factors to reduce these emissions include the spray characteristics, mixture formation, ignition, and injection control. In order to understand cold start combustion in a cylinder, the air-to-fuel ratio (A/F) near the spark plug was measured using a micro Cassegrain system (MCS). The local A/F and flame propagation speed were detected under the cold start conditions. This optical system consisted of a micro Cassegrain sensor (MC sensor), a spectroscopic unit with optical filters, photo-multipliers, and associated software. The A/F was determined from the intensity ratio of the radicals measured using their flame chemiluminescence. A practical engine and an M14 spark plug MC sensor were used for the measurements. The A/F and the THC concentration at the exhaust pipe were also measured by a conventional oxygen sensor and a flame ionization detector (FID). The A/F near the spark plug was compared to the A/F and THC concentration at the exhaust pipe. The A/F cyclic variation, which can barely be measured by an O_2 sensor, could be measured by the MCS just after starting the engine. The A/F near the spark plug was around 10 immediately after starting the engine, which is considered rich. It became leaner and converged after 50 cycles. The initial flame propagation became slower as the A/F near the spark plug became richer because the ignition delay and initial flame propagation were slower. The THC concentration was higher when the A/F was rich just after starting the engine. The MCS could evaluate the mixture formation of different injectors under cold start conditions and is therefore useful for achieving a reduction in the THC emissions.
