MULTIPLE-CYLINDER ENGINES

The discussion so far has been on a single cylinder engine. A single cylinder provides one power impulse every two crankshaft revolutions in a four-stroke-cycle engine and is delivering power only one fourth of the time. To provide for a more continuous flow of power, modem engines use four, six, eight, or more cylinders.

The same series of cycles discussed previously take place in each cylinder.

In a four-stroke cycle, six-cylinder engine, for example, the throws on the crankshaft are set 120 degrees apart, the throws for cylinders 1 and 6, 2 and 5, 3 and 4 being in line with each other (fig. 2-9). The cylinders fire or deliver power strokes in the following order: l-5-3-6-2-4. The power strokes follow each other so closely that there is a fairly continuous and even delivery of power to the crankshaft.

Even so, additional leveling off of the power impulses is desirable, so the engine runs more smoothly. A flywheel (fig. 2-9) is used to achieve this result.

To understand how the flywheel functions, let’s consider a single cylinder engine. It is delivering power only one fourth of the time during the power stroke.

During the other three strokes, it is absorbing power to push out the exhaust gas, to pull in a fresh charge, and to compress the charge. The flywheel makes the engine run without varying much of the speed during each revolution. It is a heavy steel wheel, attached to the end of the crankshaft. When it is rotating, considerable effort is required to slow it down or stop it. Although the wheel does slow down somewhat as it delivers power to the engine during the exhaust, intake, and compression strokes, the wheel speed increases during the power stroke. In effect, the flywheel absorbs some of the engine power during the power stroke and then provides it back to the engine during the other three strokes.

In a multi-cylinder engine, the flywheel functions in a similar manner. It absorbs power when the engine tends to speed up during the power stroke, and it provides power to the engine when the engine tends to slow down during intervals when little power is being delivered by the engine.

In addition to the engine itself, which is the power producer, there must be accessory systems to provide the engine with other requirements necessary to operate it. These systems are the fuel system, the lubrication system, the electrical system, the cooling system, and the exhaust system