Figure 2-8. - Comparison of two-stroke and four-stroke cycles.

the diesel engine, many parts are somewhat heavier and stronger, so they can withstand higher temperatures and pressures that the engine generates. The engines differ also in the type of fuel used and how the air-fuel mixture is ignited. In a gasoline engine, the air and fuel are mixed together in a carburetor or fuel injection system. After this mixture is compressed in the cylinders, it is ignited by an electrical spark from the spark plugs.

A diesel engine has no carburetor. Air alone enters the cylinder where ii is compressed and reaches a high temperature due to compression. The heat of compression ignites the fuel injected into the cylinder and causes the air-fuel mixture to burn. A diesel engine requires no spark plugs; the contact of diesel fuel with hot air in the cylinders causes ignition. In a gasoline engine, the heat from compression is not enough to ignite the air-fuel mixture, so spark plugs are required.

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.

Figure 2-9. - Crankshaft for a sixcylinder engine.