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I am often asked what the difference is between two-stroke and four stroke engines and which one is better? Because these two types of engines have inherently different characteristics, they are used for different purposes. Those differences are getting narrower as technology increases, and eventually the engines may be interchangeable on equipment.

I should preface this article by saying that I am not an engineer nor have I ever manufactured an engine so be kind with your emails and posts. Now, there are always exceptions and specially manufactured items, but for the purposes of information this article will address ’standard’ two-stroke engines currently produced for the OPE market.

Two-stroke engines are used in many applications such as motorcycles, jet skis, chain saws, and trimmers. This type of engine has three distinct advantages over four-stroke engines.

1. Two-stroke engines do not have valves. This simplifies their construction and reduces weight.
2. Two-stroke engines fire once every revolution, while four-stroke engines fire once every other revolution. This gives two-stroke engines a significant power boost.
3. Two-stroke engines can work in any orientation, which can be important in something like a chainsaw. A standard four-stroke engine may have problems with oil flow unless it is upright, and solving this problem can add complexity to the engine.

These advantages make two-stroke engines lighter, simpler and less expensive to manufacture. Two-stroke engines also have the potential to pack about twice the power into the same space because there are twice as many power strokes per revolution. The combination of more power and less weight gives two-stroke engines a great power-to-weight ratio compared to many four-stroke engine designs.

You don’t normally see two-stroke engines in cars, however, espescially in the United States. That’s because two-stroke engines have a couple of significant disadvantages that will make more sense once we look at how it operates.

The Two-stroke Cycle

We’ll start the process at the spark plug firing. Fuel and air in the cylinder have been compressed, and when the spark plug fires the mixture ignites. The resulting explosion drives the piston downward. Note that as the piston moves downward, it is compressing the air/fuel mixture in the crankcase. As the piston approaches the bottom of its stroke, the exhaust port is uncovered. The pressure in the cylinder drives most of the exhaust gases out of cylinder.

Fuel Intake

As the piston reaches the bottom of its travel, the intake port is uncovered. The piston’s movement has pressurized the mixture in the crankcase, so it rushes into the cylinder, displacing the remaining exhaust gases and filling the cylinder with a fresh charge of fuel.

Note that in many two-stroke engines that use a cross-flow design, the piston is shaped so that the incoming fuel mixture doesn’t simply flow right over the top of the piston and out the exhaust port. Although some scavaging and escaping does occur, those are topics to be discussed later.

The Compression Stroke

Now the momentum in the crankshaft starts driving the piston back toward the spark plug for the compression stroke. As the air/fuel mixture in the piston is compressed, a vacuum is created in the crankcase. This vacuum opens the reed valve and sucks fresh air/fuel in from the carburetor.

Once the piston makes it to the end of the compression stroke, the spark plug fires again to repeat the cycle. This is the reason it’s called a two-stoke engine. There is a compression stroke and then a combustion stroke. In a four-stroke engine, there are separate intake, compression, combustion and exhaust strokes.

In a two-stroke engine, the piston is doing three different things:

1. On one side of the piston is the combustion chamber, where the piston is compressing the air/fuel mixture and capturing the energy released by the ignited fuel.
2. On the other side of the piston is the crankcase, where the piston is creating a vacuum to suck in air/fuel from the carburetor through the reed valve and then pressurizing the crankcase so that air/fuel is forced into the combustion chamber.
3. Meanwhile, the sides of the piston are acting like valves, covering and uncovering the intake and exhaust ports drilled into the side of the cylinder wall.

It’s amazing that this is all occurring at speeds upto 16,000 times (revolutions) per minute. That’s what makes two-stroke engines so simple and lightweight. Take a look at your car’s tachometer. It probably starts to redline at 8,000 rpms and tops out at 10,000 rpms. A two-stroke engine is just hitting it’s power curve at those levels.

Lubrication

If you have ever used a two-stroke engine, you know that you have to mix special two-stroke oil in with the gasoline. Now that you understand the two-stroke cycle you can see why. In a four-stroke engine, the crankcase is completely separate from the combustion chamber, so you can fill the crankcase with heavy oil to lubricate the crankshaft bearings, the bearings on either end of the piston’s connecting rod and the cylinder wall. In a two-stroke engine, on the other hand, the crankcase is serving as a pressurization chamber to force air/fuel into the cylinder, so it can’t hold a thick oil. Instead, you mix oil in with the gas to lubricate the crankshaft, connecting rod and cylinder walls. If you forget to mix in the oil, those same amazing rpms are going to seize the engine.

Disadvantages of the Two-stroke

Two-stroke engines have two important advantages over four-stroke engines: They are simpler and lighter, and they produce about twice as much power. So why do cars and trucks use four-stroke engines? There are four main reasons:

1. Two-stroke engines don’t last nearly as long as four-stroke engines. The lack of a dedicated lubrication system means that the parts of a two-stroke engine wear a lot faster.
2. Two-stroke oil is expensive, and you need about 2.6 (some engines may require as much as 4) ounces of it per gallon of gas. You would burn about a gallon of oil every 1,000 miles if you used a two-stroke engine in a car.
3. Two-stroke engines do not use fuel efficiently, so you would get fewer miles per gallon.
4. Two-stroke engines produce a lot of pollution.

 Emissions are a big concern for the two-stroke industry. The pollution comes from two sources. The first is the combustion of the oil. The oil makes all two-stroke engines smoky to some extent, and a badly worn two-stroke engine can emit huge clouds of oily smoke. The second reason is less obvious but can be seen each time a new charge of air/fuel is loaded into the combustion chamber, part of it leaks out through the exhaust port. That’s why you see a sheen of oil around any two-stroke boat motor. The leaking hydrocarbons from the fresh fuel combined with the leaking oil is a real mess for the environment.

It is because of these disadvantages that two-stroke engines are used for applications that require a fantastic power-to-weight ratio like a chainsaw and that aren’t required to run for long periods of time like a long-haul truck or commuter car.

In the meantime, manufacturers have been working to shrink and lighten four-stroke engines as well as increase the efficiency and ecology of two-stroke engines.