Torque Fluctuation due to Combustion


The vibrating force due to the combustion pressure causes torque fluctuation in the crankshaft, which is transmitted to the drive train.
It also acts upon the cylinder block as a reaction force and causes an engine vibration. The combustion pressure fluctuates less when the engine rpm is high and when an engine has a larger number of cylinders. The torque fluctuation becomes larger when the throttle valve opening angle is wide (when the load to the engine is large), because the combustion pressure increases.Due to the engine structure, torque fluctuation is inevitable. If the cause of the vibration or noise is the torque fluctuation, repair can be very difficult.

Vehicle side solutions
The fluctuation of the combustion pressure causes an elastic deformation in the crankshaft, which creates torsional vibration. The torsional vibration increases around the edge of the crank shaft, so the pulley is designed to have a torsional damper.
A flywheel retains the engine torque fluctuations as inertia, and decreases the torque fluctuations for smoother engine revolution. A heavier flywheel may decrease the torque fluctuations by increasing the inertial efficiency, but gives poor engine response.The flywheel must therefore be designed with appropriate weight, bearing both of these characteristics in mind.
Some engines are provided with flywheels with built-in torsional dampers to further reduce the torsional vibrations. A flywheel with torsional damper is divided into two sections. The damping mechanism comprised of compression springs are located between these sections, and decrease torsional vibrations and torque fluctuations.

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