On April 2020, India will move from Bharat Stage IV to Bharat Stage VI where the combined emission limit of Total Hydrocarbons (THC) and Nitrogen oxides (NOx) of 0.79g/km will independently reduce to 0.1g/km and 0.06g/km respectively. This reduction in emission limit however may prove to be challenging for small engines (below 200 cc) with the existing generation of engines predominantly in cold operating conditions. When the vehicle is started after soaking (engine turned off for few hours), considerable amount of THC emission is generated which can be attributed to poor fuel vaporization and incomplete combustion due to flame quenching in the combustion chamber. Also, the catalyst is inactive to chemical reactions until the accumulated heat energy from the hot exhaust mass flow elevates the catalyst temperature to facilitate efficient conversion of THC, CO and NOx to H2O, CO2 and N2. This temperature point is termed as catalyst light off temperature. Hence, most of the tail pipe emissions vented out to the atmosphere in cold phase is without after-treatment. In case of two wheelers on a typical drive cycle, around 60% (fig. 3) of the total tail pipe THC emitted is before the catalyst attains light off temperature. Thus, any form of exhaust gas treatment that can lead to reduction of pre-catalyst emissions and faster catalyst light off, will prove to be beneficial in overall emission reduction. In this context, Secondary Air injection (SAI) is explored as an effective exhaust treatment method in tackling cold phase emissions.
In the present study, a series of tests were conducted on a single cylinder 200cc engine fitted with a Mechanical throttle body and electronic fuel injection system. From these tests, various aspects of cold phase emissions were characterized. This paper explores in detail the impact of SAI on pre-catalytic oxidation of THC, catalyst temperatures and catalytic reactions. Present study also gives an insight into the operation of SAI such that it does not compromise the functionality of the three way catalyst (TWC). It was also observed that it is beneficial to have different modes of SAI operation such as open loop mode without oxygen sensor feedback, closed loop mode with oxygen sensor feedback and engine load specific operation of SAI. Upon application of best optimal configuration of SAI a reduction of 25% in THC and 4% of NOx was observed.