A parametric analysis of cold start emissions reduction with secondary air injection based post-oxidation actuation in exhaust manifold of a turbocharged GDI engine

Abstract

In this research, an experimental investigation of secondary air injection-based post-oxidation phenomena and its influence on emissions, particularly during cold start operations, was conducted. The main purpose of this research is to reduce the conversion load to save the light-off time of the catalyst with the help of post-oxidation actuation. A suitable chiller and secondary air injection (SAI) setup were integrated with a commercial 4-cylinder 1.6 L gasoline direct injection (GDI) engine which has a compression ratio of 10.5, to achieve the research objectives. The engine was coupled with a low inertial dynamometer to facilitate the experiments. The SAI was delivered at the exhaust port. The emissions were measured upstream (TC up) and downstream (TC down) of the turbocharger and downstream of the three-way catalyst (TWC down). It was observed that with early spark timing (SA) and a lower percentage of SAI, effective post-oxidation could not be triggered, preventing an increase in exhaust temperature. The SAI implementation led to a reduction in both in-cylinder and exhaust port temperature while also increasing emissions of carbon monoxide (CO) and total hydrocarbon (THC) emissions due to a shift in the in-cylinder combustion toward a rich mode. However, it was also noted that with a further retardation of SA (−15 deg. bTDCf), and increment in SAI, the post-oxidation possibility can be increased and hence can reduce the THC, CO, and hydrogen (H₂) emissions along with minimizing the catalyst light-off timing. It was noted that despite these benefits, further retarding SA causes to loss of brake power, and hence thermal efficiency decreased which needs to be optimized. With retarded SA, the H₂ concentration which is the key factor of post-oxidation actuation was also measured at TC down and TWC down and found lower than TC up. This was attributed due to H₂ consumed in the post-oxidation as adequate exhaust temperature attained in the exhaust manifold when SA retarded. The H₂ concentration was also increased at TC up and TC down as SAI was introduced due to in-cylinder combustion shifting to the rich mode. Moreover, the THC oxidation rate was determined to be higher in the cold state, as higher THC emission was emitted and favorable conditions such as sufficient temperature and oxygen (O₂) availability were achieved for the oxidation process. Due to the higher THC in a cold state, the time characteristic for the THC oxidation was noted higher compared to a warm state. However, characteristic time in chemical reaction decreased due to the post-oxidation improvement when spark timing was further retarded.