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I recently posted an article about the workings of two-stroke engines. Today’s post will explain how the four-stroke internal combustion engine works. It is important to understand how each of these engines works, because each type of engine has different applications and requires different care and maintenance.

Four-stroke engines are used in a variety of applications from cars and trucks, to motorcycles and jet skis, and from generators to water pumps. As their technology increases, four-stroke engines are finding their way on to more power equipment products such as trimmers and cultivators. The four-stoke engine offers three distinct advantages over two-stroke engines:

1. Four-stroke engines are more fuel efficient. The use of valves and an exhaust stroke allow the four-stroke engine to mitigate intake and exhaust gasses more accurately with less loss through scavaging.
2. Four-stroke egnines have better emissions. The valves help prevent fresh fuel from escaping through the exhaust port, and having a dedicated lubrication system means the four-stroke engine doesn’t have to mix oil with its fuel.
3. Four-stroke engines last longer. By nature of having a dedicated lubrication system, the four-stroke engine can run cooler than its two-stroke counter part and lubricate more consistantly since two-stroke engines require the measuring and mixing of lubricants and fuel.

Where it came from.

While he did not create the first internal combustion engine, Nikolaus Otto designed the first practical internal combustion engine in 1868. He was then joined by two other engineers, Gottlieb Daimler and Wilhelm Maybach, in 1872 and together they developed the four-stroke cycle or Otto cycle engine that is still used today.

The Otto cycle uses four strokes: intake, compression, combustion, and exhaust. This means that the spark plug only fires every other time the piston reaches Top Dead Center (TDC) as opposed to firing every time on a two-stroke engine. The four-stroke engnine is used in almost every industry, but it does have its disadvantages which is why the power equipment industry has been slow to utilize them. You’ll understand those disadvantages after we take a look at how the four-stroke engine operates.

The Four-stroke Cycle

If we start the process on the intake stroke, the piston will just be reaching TDC and the intake valve will open. The piston will move down drawing in a cylinder full of air/fuel vapor. A relatively small amount of fuel is used since it isn’t inhibited by lubricants.

Once the piston has traveled completely down and filled the cylinder, the piston will travel upward and begin to compress the air/fuel mixture in the cylinder. This is called the compression stroke and it makes the combustion process more efficient by ensuring more of the fuel ignites.

Just before the piston reaches TDC, the spark plug fires igniting the compressed air and fuel. The resulting explosion (combustion) forces the piston downward and is called the combustion stroke. The piston travels all the way downward and begins its return up.

Once the piston has reached the bottom of its stroke, the exhaust valve opens and the piston forces the exhaust gasses out of the opening. This is known as the exhaust stroke. Once the piston has reached TDC, it begins the cycle again as the intake valve opens.

The Disadvantages of Four-stroke Engines

As you can see, the four-stroke enigne has many more moving parts and requires even more precise manufacturing and tuning of those parts; valve timing and valve clearance are non-existent on a two-stroke engine. By having more moving parts and twice as many strokes, the four-stroke engine will not produce as many revolutions per minute (rpm) as a two-stroke engine. The four-stroke engine will, however, produce torque across a wider rpm range than a two-stroke will. That is, the four-stroke engine will produce usable power from 1,000 to 8,000 rpms where a two-stroke engine will be most effective between 10,000 and 15,000 rpms.

Another drawback to standard four-stroke engines is that, for the most part, they must be stationary. That is, the engine orientation must remain in relatively the same “upright” direction otherwise oil and fuel may seep into parts of the crankcase or back into the carburetor or even out through the muffler where it can impede engine operation. Some manufacturers have been working on one-way check-valves for oil systems that allow a four-stroke engine to be rotated during operation, but this further complicates the engine and increases the parts and weight of the engine.

Emissions regulations are putting pressure on manufacturers to utilize four-stroke engines more regularly, but there are still aplications where very high rpms are required that four-stroke engines just can’t produce. Equipment such as chain saws and leaf blowers, where engines typically exceed 15,000 rpms, will not be able to utilize current four-stroke engines.

Manufacturers are taking great steps to increase four-stroke aplicability and reduce the weight of the engines. Technology is also helping two-stroke engines increase their economy and ecology. While their differences remain, the gap between them is narrowing and they may soon be interchangeable.