b) A powertrain designer uses an aftertreatment system to reduce the emissions of a 4-cylinder SI engine. The conversion performance of the aftertreatment systems is an ideal on-off operation, as in Figure Q3-2, where the catalyst is totally inactive at temperatures less than 250°C. Once the catalyst temperature reaches this threshold value, it becomes active and converts all input mass of emission species into less harmful products. 100 NOx Conversion Efficiency Conversion efficiency [%] 50 40 80 20 10 0 100 150 200 250 Temperature (degC] 350 400 Figure Q3-2 /Question continued over 6 H3051 ENGINE TECHNOLOGY The time require to warm-up the catalyst from ambient temperature to the threshold value of 250°C (called light-off time) is empirically calculated using the following equation: Tlightoff = 550 (250 +273) Tamb] m(Texhgas - Tamb) where Tlightoff is the light-off time (s), Tamb is the ambient temperature (K), m is the exhaust mass flow rate (g/s) and Texhgas is the exhaust gas temperature from the engine (K). A test setup has been created to evaluate the effect of idle speed on the light-off time (Tlightoff) by keeping the exhaust gas temperature, air-fuel ratio, ambient temperature and volumetric efficiency unchanged for different idle speeds. Estimate the improvement in light-off time (Tlightoff) you expect to observe from the test setup by increasing the idle speed from 800 rpm to 1000 rpm. [8 marks] c) Modifying the ignition timing of the engine to increase the exhaust gas temperature (Texhgas) is another technique to reduce the light-off time (Tlightoff). To choose the most effective one, compare the following three engine-warm-up strategies by calculating their cumulative tailpipe NOx emissions at idle for 2 minutes, assuming unchanged ambient temperature (i.e., 25°C), air-fuel ratio and volumetric efficiency. The measured NOx emissions of the strategies are respectively 0.0100%, 0.0105% and 0.0110% of mass of the engine exhaust gas. Also, the exhaust mass flow rate at 800 rpm is 5 (g/s). a. 800 rpm and exhaust gas temperature of 750 K; b. 1000 rpm and exhaust gas temperature of 750 K; c. 1000 rpm with modified ignition timing to increase exhaust gas temperature by 50°C. Note: The exhaust mass flow rate of the engine changes by varying the engine speed, and therefore needs to be calculated for each strategy.