Basic engine design considerations, Other Engineering


Effect of engine size:

The two parameters involved are the diameter, d and stroke, L of the engine. The effect of doubling each of this is discussed below:

1.      The piston head area π (d2)/4. Therefore the doubling cylinder diameter would increase the piston head area (or the cylinder area) four times which means that for a given piston speed and mean effective gas pressure, doubling the cylinder diameter would increase the power four times.

2.      Doubling the stroke length for the same cylinder diameter would double the power.

3.      If, for a given crankshaft speed, the stroke is doubled, the piston speed would also be doubled. Or in other words if the piston speed is to remain constant even after doubling the stroke. The crankshaft speed needs to be reduced to half.

4.      Doubling the cylinder diameter increases the piston thrust four times, while doubling the stroke doubles the crank-throw. Since torque is proportional to the piston thrust and the crankshaft speed need to be doubling both the cylinder diameter and the piston stroke would increase the torque 4*2, times.


In a four stroke engine, energy is supplied to crankshaft only during the power stroke. During the remaining three strokes, some of this energy is absorbed to overcome the frictional and pumping losses, besides inertia loads due to reversing of the direction of motion of the piston assembly. Due to this three would be substantial fluctuation in the crankshaft speed in the one cycle. This flywheel on the crankshaft absorbs the excess energy during the power stroke and transfers back some of this stored energy to the crankshaft during the remaining strokes. Thus flywheel reduces the fluctuation of crankshaft speed during one cycle. However, it never completely eliminates this fluctuation. A cycle torque curve for a single cylinder engine shows that the mean torque may be defined as the steady torque which would provide torque load equivalent to the actual fluctuating torque load during one cycle. Thus the area of torque diagram above the mean torque line represents the excess energy stored in the flywheel resulting in the increase of its speed, while the area of torque diagram below the mean torque curve represents the energy drawn from the flywheel, causing reduction in the crankshaft speed. As the size of the flywheel determines the energy absorbed or released by it, it would determine the amount of speed variation during a cycle. A large flywheel would reduce the speed fluctuation to minimum, but this advantage would be accompanied by increase of flywheel inertia due to which the acceleration or declaration of the engine would also be slow, which in other words means sluggish response. Thus in case of automobiles, where quick response is a necessity, there is a limit to which flywheel size may be increased to obtain smooth engine operation.

Posted Date: 7/21/2012 8:50:01 AM | Location : United States

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