Strategies for Low Engine Speed Torque Enhancement of Natural Gas Engine: Valve Overlap and Compression Ratio

Abstract

For a long time diesel engines are used as prime movers for commercial vehicles. However, since last two decades,  countries are promoting natural gas vehicles to improve air quality. Since then, natural gas based commercial vehicles and their powertrains have witnessed fast development, principally driven by need to meet emission standards.

A widely used CNG commercial vehicle is passenger mass transport bus. In some cities, authorities impose maximum speed limit for such buses considering safety. This limit may be as low as 40km/hr. Thus, with low vehicle speeds, frequent stops and starts, and traffic conditions, such vehicles demand high torque output at slow engine speed. In here, various approaches for improving the torque of engine are briefly summarized. The objective is to observe effect of valve overlap and compression ratio for improving torque at low engine speeds as very limited literature exists that focuses on low engine speed zone.

Use of turbocharger, direct injection, electronic wastegate control, VVT, etc. have potential for torque improvement. The extent of their impact on low speeds is not clearly evaluated. Naturally aspirated six cylinders engine is simulated and virtual output is verified against experimental data from test bed at full throttle, to verify effective representation of model, so that numerical simulation of different technologies can be performed before experimental activity. Further testing is done with a few different compression ratios and valve overlaps. It is seen that there is scope for optimizing torque and power at low engine speed zone.