Effect of Engine Operating Altitude Towards Traction Force on Wheels

Many modern day compact cars running on downsized engines with direct drive transmissions are having trouble meeting the practical demands of vehicle usage especially under the varying ambient conditions. The vehicle cannot launch from stationary on inclined roads at high altitudes due to inadequate traction force on wheels. This is due to reduced air density which directly reduces the apparent volumetric efficiency of a normally aspirated spark ignition engine hence reducing the torque output of the engine. The significant reduction in torque output compared to that obtained on the engine dynamometer based on ideal volumetric efficiency simply means the first gear ratio selected initially is no longer adequate to provide the necessary traction force on wheels. This paper details the necessary compensation required for first gear ratio selection to prevent this problem from occurring. It is found that traction force on wheels can drop by as much as 38.2% at high altitudes (1500 m) above sea level.