Effect of synchronized secondary air injection on exhaust hydrocarbon emission in a spark ignition engine

A synchronized secondary air injection (SAI) method has been proposed to reduce hydrocarbon (HC) emission by injecting secondary air intermittently into the exhaust port. The method has been tested in a single-cylinder spark ignition engine operating at cold-steady/cold-start conditions. HC concentration and exhaust gas temperature were monitored with a fast-response flame ionization detector and with thermocouples respectively. Effects of exhaust air-fuel ratio (A/F), engine A/F, spark timing and air injection timing have been investigated and the results are compared with a continuous SAI method and the baseline condition. Results show that HC reduction rate and exhaust gas temperature are sensitive to the timing of synchronized SAI. Two optimum SAI timings are observed which are at 100° crank angle (CA) after top dead centre (ATDC) and 230° CA ATDC depending on spark timing. At the cold-steady condition, the synchronized SAI is found to be more effective in HC reduction and also has much higher exhaust gas temperature than either the continuous SAI or the baseline condition. At the cold-start condition when a catalyst is not used, the accumulated HC emission during the first 120 s decreases by about 55 per cent with the synchronized SAI compared with that of the baseline condition, while the result with the continuous SAI is comparable with the baseline condition.