2 Stroke vs 4 Stroke Engines – What’s The Difference?

Stihl Chainsaw

Since many lawn and garden power tools are offered in both 4-stroke and 2-stroke models, you may be wondering which you should choose and why.

This article gets a little technical but we hope we’ve made it as easy as possible to understand some of the differences and given you the tools to make an informed choice.

Note: The terms stroke and cycle are often used interchangeably, and you might see them both here. However, technically, they are not the same.

A Little History

You may be surprised to learn that the four-stroke engine actually came about before the two-stroke engine. In 1879, Karl Benz developed the two-stroke engine in an attempt to simplify the existing four stroke internal combustion engine that had been around since about the 1850s. While the two-stroke is indeed simpler, it has its own drawbacks.

Two stroke engines are simpler while four stroke engines are more complex. Each type has its own pros and cons.

How They Work

First, let’s define what we mean by four strokes or two strokes.

Four Stroke Model –

Here is a breakdown of the four “strokes” of a four stroke internal combustion engine.

4 stroke engine
4 stroke Internal Combustion Engine https://commons.wikimedia.org/w/index.php?curid=12597090

This picture graphically demonstrates the process described below.

  1. The Intake Stroke: While the intake valve is open and the exhaust valve is closed, the air/fuel mixture is drawn into the combustion chamber.
  2. The Compression Stroke: While both valves are closed, the air/fuel mixture is compressed.
  3. The Power Stroke: While both valves are closed, the air/fuel mixture is ignited and the resulting explosion forces the piston out creating power.
  4. The Exhaust Stroke: While the intake valve is closed and the exhaust valve is open, the burned gases are pushed out by the piston.

Two Stroke Model –

2 stroke engine
2 Stroke Internal Combustion Engine https://commons.wikimedia.org/wiki/File:2-Stroke_Engine_ani.gif

This graphic demonstrates the operation of a 2-stroke engine:

In a two stroke engine, the power and exhaust strokes are combined (Stroke 1) and the intake and compression strokes are combined (Stroke 2)

So what does all this mean for you?

When choosing your small engine equipment, you might wonder which type of engine is better and why you should choose one type over the other. We wish it were simple to say one is definitely better than the other, but that’s too simplistic of an answer. For various reasons, one type may suit you better for certain purposes and another type for other purposes.

Let’s dive in a little deeper and look at some of characteristics of these engines and how they compare.

Weight

Two stroke engines are generally lighter than comparable four stroke engines. The heavier four stroke engines tend to weigh about 50% more because there are more parts required for the more complex design.

Power

Four stroke engines are more powerful than comparable two stroke engines due to their greater efficiency. That said, the lighter weight two stroke engines can have a greater power-to-weight ratio which means you get more power per pound of engine weight with a two stroke engine.

Additionally, with the greater consistency of power in a 2-cycle engine you can have a practical engine with only one cylinder. Conversely, a 4-stroke engine requires multiple cylinders to get consistent power generation.

Picture this: The four stroke engine produces power on 1 in every 4 strokes, whereas the 2-stroke engine produces power on every other stroke or 1 in every 2 strokes.

Fuel Efficiency and Pollution

Four stroke engines are more fuel efficient, hands down. However, with the greater use of fuel injection in two stroke engines, the efficiency difference can be reduced. The greater complexity of the four stroke engines allows for more controlled air-fuel intake and exhaust cycles which leads to greater fuel efficiency and cleaner operation.

With a two-cycle, some of the fuel is wasted since it escapes during the combined intake and compression stroke. Conversely, with a four-cycle, there is no fuel wasted during intake of the air/fuel mixture since the fuel is drawn into a closed system.

The use of oil/gas mixtures in two stroke engines also decreases the efficiency of the combustion and increases the harmful emissions (burned oil). With a two-cycle engine, the oil/gas mixture burns less efficiently than gas alone. When you are burning the oil that’s in the gas, the oil creates a dirtier combustion product.

Lubrication

The 4 stroke has the dedicated lubrication system so engine oil is separate from the fuel. For two-cycles, the lubricating oil must be pre-mixed with the fuel and goes into the same system.

In a 4 stroke engine, oil is injected from a separate reservoir and the engine will get better lubrication from oil alone than from the diluted lubrication mixture (oil diluted with gas) in a 2 stroke engine.

For two-cycles, an oil/gas mixture does not lubricate as well as oil alone, detracting from the engine’s lifespan. However, because the lubricating oil is circulating with the fuel, it allows a 2 stroke engine to automatically function in any orientation, making it particularly useful for hand held power equipment.

Some modern 4 stroke engines now tout lubrication systems that will work at any angle.

Maintenance and Durability

Two stroke engines are simpler and have fewer moving parts making them easier to repair. However, since four stroke engines run much cleaner and have a more effective dedicated lubrication system, they will incur less wear and will last longer.

Since two cycle engines burn oil as well as fuel, they create a dirty combustion product. The dirtier combustion puts more wear and tear on the engine’s system because of the burned oil in the chamber. This reduces the overall life of the engine and makes a two-cycle generally less durable.

Additionally, with 2 stroke engines, you need to mix fuel with 2 cycle engine oil and maintain a separate container for the mixture. Oil/fuel mixture should be used within 30 days of mixing. This is an extra maintenance hassle that is avoided with 4 stroke engines which generally run on standard automotive gas.

Cost

Two stroke engines are less expensive than comparable four stroke engines. However, the cost per hour of use of the two stroke engines will be higher due to the added cost of oil to mix with the gas and the lower fuel efficiency of the two stroke engines. Also, while two stroke engines are less expensive to purchase they do not last as long so they will need to be replaced more often, possibly increasing long-term costs.

Other Considerations

Two-cycle engines tend to have fewer overheating concerns since they draw in outside air more frequently.

Due to their less efficient nature, two cycle engines are typically limited to small watercraft, motorbikes and lawn and garden equipment, where the light weight and simplicity is particularly beneficial. Where weight is less of a concern, a four-cycle engine is more commonly preferred.

Analysis

2 Stroke Engines

Advantages:

  • Less expensive to purchase
  • Lighter weight
  • High power-to-weight ratio
  • Simpler / Fewer moving parts / Easier to repair
  • Inherently lubricates well at all angles

Disadvantages:

  • Less fuel efficient / More polluting
  • Must mix oil and gas
  • Wear out quicker

4 Stroke Engines

Advantages:

  • More Efficient / Cleaner
  • More Powerful
  • Easier and Less Expensive Fueling
  • Lasts Longer

Disadvantages:

  • More Expensive to Purchase
  • Heavier
  • More Moving Parts
  • More Complex / More Moving Parts / Harder to Repair
  • Does not Typically Lubricate Well at all Angles

So which type should you choose?

Well, as you can see, there’s no single answer to this question. It depends on what type of equipment you are speaking of. It also depends on how you would like to use that piece of equipment.

If weight is a serious consideration, then you might appreciate the relatively lighter weight of a two-cycle engine. If you want maximum efficiency, you might choose a 4 stroke engine even if it means it is heavier.

If you are less likely to be able to clean and maintain the dirtier burning two stroke engine, you might want to choose a four cycle model. But you might choose a two cycle for it’s less complex construction so that you can tinker with it yourself or it will cost less to have repaired.

At LawnLifestyles, we have reviewed products with both two and four cycle engines and compared their relative benefits. If you are in the market for a string trimmer, we encourage you to see our reviews here:

We hope you have enjoyed this article on the advantages and disadvantages of the 2 cycle and 4 cycle engine. If you have your own thoughts or experiences on these engines, please comment below. We’d love to include you in the conversation.

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