Engine Variations in Layout
The simplest form of 4-stroke engine has a single cylinder, but this is not acceptable in a car because of the uneven torque (turning effort) which results from only one power stroke at every second revolution of the crankshaft, and because of the vibration that would be set up.
Uneven torque can be improved by the stored energy of a heavy flywheel, but this is insufficient in a 4-stroke engine to give smooth running at low speeds. There is no simple way of counterbalancing the up- and-down movement of the single piston.
To give reasonable smoothness, the minimum number of cylinders is two, providing one power stroke for each crank shaft revolution. This greatly improves the balance, compared with that of a single- cylinder engine, but the torque remains noticeably irregular at low speeds.
Nearly all cars have at least four cylinders, and they provide one power stroke for each half revolution of the crankshaft. The cylinders can be arranged in-line, in V fashion, or horizontally opposed—that is, set sideways in two lines on either side of the crankshaft.
Torque and balance
An in-line 4-cylinder engine has evenly spaced power strokes and so gives reasonably smooth torque. It tends to produce some vibration, but this is largely suppressed by the rubber mountings on which the engine is fitted.
The torque of the compact V-4 can be as good as that of an in-line 4-cylinder, but this layout is fundamentally not as well balanced, whatever the angle between the banks of cylinders.
Hence the V-4 suffers from vibration which can be kept to an acceptable level only by incorporating an extra shaft which is counter-weighted to cancel out vibration of the crankshaft assembly.
The horizontally opposed 4-cylinder is more compact and better balanced than the in-line. These advantages are offset in some makes of car by the difficulty of access for working on the engine. Six cylinders give the best balance of in-line engines used in modern cars.
The V-6 is theoretically less smooth than the in-line, but both have an even torque delivery. So has the horizontally opposed ’six’, which runs very smoothly but is costly to build.
Of the 8-cylinder variants, the V-8 is mainly used in modern cars. It is a compact, well-balanced engine with even torque. Some earlier cars had ’straight eights’ (eight cylinders in-line), but their long crankshafts were prone to torsional vibration at high speeds.
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