Factors influencing the effectiveness of exhaust port air injection in oxidizing the hydrocarbons and carbon monoxide in engine exhaust gas have been investigated in order to establish guidelines for the engineering of vehicle emission control systems. Single-cylinder engine and vehicle studies have demonstrated that the temperature, composition, and residence time of the exhaust gas-air mixture are basic factors determining both the effectiveness of air injection and the type of oxidation process which occurs in the exhaust system. Both luminous and nonluminous oxidation have been observed. These basic factors are affected by such variables as: engine spark timing and air-fuel ratio, insulation and size of exhaust manifolds, injection air temperature and airflow rate, and the warmup characteristics of the air injection system. The warmup characteristics can be influenced particularly by spark timing and exhaust manifold design.
By optimizing the operating and design variables, it has been possible to greatly enhance the effectiveness of air injection in reducing hydrocarbon emissions. For example, with an experimental system, a composite hydrocarbon emission of 27 ppm has been measured on a vehicle tested in accordance with the California Motor Vehicle Exhaust Emission Test Procedure. On the other hand, for the same system, the carbon monoxide emission was 0.76%. Increasing the effectiveness of air injection in reducing carbon monoxide emission has proved to be most difficult.
Although these experimental systems may not be amenable to product engineering, they do illustrate the potential of air injection for reducing exhaust hydrocarbon and carbon monoxide emissions.