The Four-stroke Engine Cycle

Historic car enthusiast Ian Sumner checks the engine of a Jaguar D-Type replica classic car which is displayed at the annual RNAS Yeovilton Air Day in Yeovil, England.

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Benefits: More fuel-efficient, less polluting

Drawbacks: More complicated, more expensive to manufacture

Remember that Benz Patent Motorwagen we talked about? In addition to having a single piston, or cylinder, it was a two-stroke engine, like many early motors. Stroke refers to the movement of the piston in the engine.

Four-stroke engines were one of the earliest improvements made to internal combustion engines in the late 1800s. On a four-stroke engine, there are four steps the engine takes as it burns gasoline: intake, compression, power, and exhaust [source: CompGoParts.com]. These steps all occur when as piston moves up and down two times.

Earlier, simpler two-stroke engines accomplish the same task -- burning gasoline to create mechanical motion -- but they do it in two steps. Today, two-stroke engines are found on small equipment like lawnmowers, small motorcycles, and large, industrial engines. Nearly all cars use the four-stroke cycle.

Four-stroke engines carry several benefits, including improved fuel economy, more durability, more power and torque, and cleaner emissions. However, compared to two-stroke engines, they are more complicated and expensive to make, and require the use of valves for the intake and exhaust of gases.

In spite of this, four-stroke engines have become the industry standard for cars, and they likely aren't going away any time soon. We'll learn more about the role of valves and how they've been improved upon later in this article.

Next, we'll learn about forced induction, and how it made its way from airplanes onto everyday cars.