The mechanical design of a diesel engine.

The mechanical design of internal combustion engines, particularly diesel engines is a challenging and interesting task. Since the combustion process in the diesel engine is never uniform and smooth, they are prone to more vibration and noise compared to petrol engines. Thus diesel engines require a rugged structural design.

Out of the four strokes, it is only during the power stroke that a tremendous amount of force is exerted on the piston. So a single cylinder engine will always have high force non-uniformity.

Similarly, the output power will also have a fluctuating nature. The variation of force and output power with the piston movement is seen on the figure.

As a result of high force non-uniformity a single cylinder engine will never have a smooth operation; rather it would have a noisy and vibrating operation.


Smooth operation with more number of cylinders

With more number of cylinders, one can overcome these problems. Consider a 4 cylinder engine; with 4 strokes occur at a time. The power stroke is always present in the engine and the total force and total power in 4 cylinder engine will have a better force and power uniformity. In short, the more cylinders an engine has the smoother it will operate. A 4 cylinder engine generally operates in the following firing order of 1-3-4-2. Such a firing order makes sure that the combustion force is balanced along the length of the engine.

A heavy flywheel acts as a power reservoir which further helps to smooth out any non-uniformity of power. When the engine supplies extra power the flywheel absorbs the power and during low power operating regimes, the flywheel releases the power of the engine. Thus the power output will have a better uniform nature.

The use of counter weights is necessary as a huge unbalance force arises in the form of dynamic unbalance due to the excessive mass at the connecting rod side. Such unbalanced force is balanced by providing counterweights on the crank side.

Controlling the opening and closing operation of values are achieved by a pair of cam shafts, which derives its motion from the engine. During a complete engine cycle, the crankshaft rotates twice, but the exhaust/intake valve open only once. This means that the camshaft needs to be activated only once in an engine cycle. Thus the cam shafts need to rotate at half the speed of crankshaft. This speed reduction is achieved by using a double sized wheel over at the camshaft side than in the crank side.