Challenges for Spark Ignition Engines in Heavy Duty Application: a Review 2018-01-0907
Spark Ignition (SI) engines operating on stoichiometric mixtures can employ a simple three-way catalyst as after-treatment to achieve low tailpipe emissions unlike diesel engines. This makes heavy duty (HD) SI engines an attractive proposition for low capital cost and potentially low noise engines, if the power density and efficiency requirement could be met. Specific torque at low speeds is limited in SI engines due to knock. In HD engines, the higher flame travel distances associated with higher bore diameters exacerbates knock due to increased residence time of the end gas. This report reviews the challenges in developing HD SI engines to meet current diesel power density. It also focuses on methods to mitigate them in order to achieve high thermal efficiency while running on stoichiometric condition. High octane renewable fuels are seen as a key enabler to achieve the performance level required in such applications. Apart from higher octane rating, the effect of higher latent heat of vaporization in liquid alcohol fuels was found to be beneficial in all operating conditions as it tended to reduce in-cylinder temperature and associated heat loss of the engine. Exhaust gas recirculation (EGR) was seen to be beneficial both at full load in limiting knock and part load conditions to decrease pumping losses. Increased in-cylinder turbulence was also seen to be beneficial in limiting knock as it reduces residence time of the end gas. Results and trends of combinations of these factors are discussed with respect to increasing engine specific torque and efficiency. The effect on emissions and part load conditions is included where results are available and gaps in knowledge are presented.