MPLE MECHANICS
Page 60
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by Preceptor
Internal combustion engine 2
All engines fitted to motor vehicles, except small ones used on motor cycles, have more than one cylinder.
Light vans, lorries and cars generally have four-cylinder engines while the larger vehicles are fitted with six, eight or even 12 cylinders. This is taken for granted, but often in examination, the question is asked: "Why is this so?" "Why is a larger single cylinder engine not used when more power is required?".
The torque on the crankshaft of a single-cylinder engine reaches its maximum value during the power stroke. On the other strokes, exhaust, induc tion and compression, effort has to be applied to the crankshaft to keep it turning. This effort comes from the energy stored in the flywheel during the power stroke. A heavy flywheel is required to smooth out these torque fluctuations and, even then, the driving effort, particularly at low speeds, is rather jerky.
The four-stroke single-cylinder engine has only one power stroke every two revolutions. If, however, four cylinders are employed, there will be a power stroke every half revolution. If six or more cylinders are used, the engine will be even smoother.
Incidentally, a useful formula to find the angle through which the crankshaft turns between firing strokes is 720/n where n equals the number of cylinders. For example, a straight eight engine has a firing impulse every 720/8 equals 90 degrees. For a two stroke engine, the formula must be changed to 360/n.
A single-cylinder engine is more difficult to cool than a multi-cylinder one of equal power. The larger the piston, the longer the heat path from the centre of the piston to the water-jacketed cylinder walls.
A single-cylinder engine is at a disadvantage when engine balance is considered, Each time a piston reaches top dead centre and bottom dead centre, when the piston has to change direction, it also has to stop momentarily; the larger the piston, the greater the force required to cause the change in direction. These forces cause severe vibrations that are difficult to eliminate, In multi-cylin der engines, it is possible to arrange the pistons so that they are moving in opposite directions: this neutralises the vibratory forces.
Large single-cylinder engines have a low power to weight ratio when compared to multi-cylin der types. The power of an engine varies as the square of its cylinder diameter, but the weight increases as to the cube of the linear dimensions. It follows, therfore, that the weight of a single cylinder engine increases much more rapidly than the power if the piston diameter is increased. If, for example, the diameter of the cylinder bore is doubled then the power developed by the engine would be four times as great, but the weight would be increased eightfold.
Engines fitted to commercial vehicles are generally of the
in-line or V-type layout, the number of cylinders varying from four to eight. The V-type engine has the advantage of being much shorter than the in-line type. It does not take up so much room when installed in a vehicle and a shorter stiffer crankshaft free from "whip" can be fitted.
Overhead valves closed by a spring are universal, but the method used to open the valves vary from engine to engine. The types can be broadly classified into those operated by push rods and rocker arms and those employing overhead camshafts. The simplest method of driving a camshaft is shown in Figure 1 which illustrates a chain drive employed on many private motor cars and light vans and cars.
Figure 2 is a more complex drive employed on a compression ignition engine where as well as driving the camshaft the chain provides the drive for the injection pump.
Figure 3 shows a similar drive but using gears.
Figure 4 shows a relatively new design, used increasingly on small engines for driving overhead camshafts. Here, a toothed belt is employed and this has the advantage of being very quiet in operation. It must be remembered in all these cases a 2:1 reduction must be employed, the camshaft running at half engine speed.