Will Mixed Gas Hurt a 4-Stroke Engine?

If you have gas-powered equipment and power tools at home, you know that there are some peculiarities that you need to be very careful about.

Like, how a two-stroke engine should always take mixed gas. Doing otherwise is a big no-no. That also means that you need to mix it before you pour it into the fuel tank.

And to make matters worse, mixed gas should not be left to sit unattended for long periods of time. So what happens if you just end up with too much mixed-gas on your hands?

Can you use the mixed gas with your tools that have a four-stroke engine?

Will mixed gas hurt a four-stroke engine? Adding mixed gas to a four-stroke engine will not damage it when it is done in small quantities. Adding large amounts of mixed gas to a four-stroke engine may potentially gum up the spark plug and damage it and can also result in lowered performance; thus, it should be avoided.

Fortunately, things are not that complicated.

Below I get into more detail about what you need to know about mixed gas and how good of an idea it is to use it with a four-stroke engine.

What Is the Difference Between a Four-Stroke Engine and a Two-Stroke Engine?

There are two main types of engines: a two-stroke and a four-stroke engine.

Two-stroke engines take premixed oil. You can easily distinguish them by the fact that they have just one fuel tank, which is usually marked that it takes fuel and oil on the cap. The fuel and oil need to be mixed before they are poured into the fuel tank. There are also specific oil to fuel ratios that have to be followed.

On a four-stroke engine, the gas and oil do not premix. There are two separate tanks for fuel and oil; each marked accordingly.

Before we get into that, we need to go through how the two types of engines actually work.

How Does a Two-Stroke Engine Work?

Two-stroke engines are primarily used for small applications, although there are some exceptions.

A two-stroke engine goes through two strokes.

  1. Intake and Exhaust Stroke: When the piston is in the top position (Top Dead Center or TDC), the air and the mixed fuel enter the crankcase through the intake port. As the piston moves down, it compresses the air and fuel mixture. When the piston reaches the bottom part of its movement (Bottom Dead Center or BDC), the compressed air and fuel mixture enters the cylinder. Simultaneously as the fuel enters the cylinder, the exhaust gas is pushed through the exhaust port.
  2. Compression and Power Stroke: As the piston starts moving upwards again, it compresses the air and fuel mixture that is located in the cylinder. When it reaches the TDC, the air-fuel mixture is going to be ignited by the spark plug. At the same time, more fuel and air mixture enters the crankcase.

It is important to note that a two-stroke engine fires once for every two movements of the piston or every one revolution of the crankshaft.

How Does a Four-Stroke Engine Work?

A four-stroke engine is a bit more complicated. It goes through four different strokes or cycles.

  • Intake Stroke: Starting from the TDC as the piston moves down, it draws air into the cylinder.
  • Compressing Stroke: When it reaches the BDC and starts moving upwards, it compresses the air. Simultaneously fuel is sprayed inside the cylinder, and a controlled explosion is created.
  • Power Stroke: The explosion is going to force the piston to move downwards, reaching the BDC again.
  • Exhaust Stroke: As the piston goes upwards, it forces the exhaust gas out.

Unlike the two-stroke engine, a four-stroke engine combusts once every four movements of the piston or for every two revolutions of the crankshaft.

Why Do Two-Stroke Engines Need Mixed Gas?

The unique thing about two-stroke engines is that you will have the fuel and air inside the crankcase where the crankshaft is located.

Because of that, you need to put oil in the fuel to lubricate the moving parts of the crankshaft. Failing to do so, you are risking damaging the piston and the crankshaft.

Why Does a Four-Stroke Engine Not Need Mixed Gas?

A four-stroke engine does have a crankcase and a crankshaft as well, but why does it not need mixed gas?

The difference here is that with a four-stroke engine, the crankcase is closed off – the crankcase is separated from the cylinder and the fuel.

What you will find inside the crankcase of a four-stroke engine is only oil and no fuel. The fuel is sprayed directly into the cylinder.

This provides for better lubrication of the crankshaft, removing any need for oil to be present in the fuel.

Can Mixed Gas Damage a Four-Stroke Engine?

If you add mixed fuel to your four-stroke engine, it will start to smoke a lot.

It could potentially damage your four-stroke engine if you use mixed fuel in large quantities. The chances of a problem occurring are significantly lowered if you use small amounts of the mixed gas and mix it in with the standard gas. That way, it will be more diluted.

The problem stems from the fact that the cylinder of a four-stroke engine is not designed to have any oil in it. This may not necessarily damage them but the oil can leave residue and foul the plug if there is oil in large quantities.

Conversely, many car racers actually add clean two-stroke oil to their fuel for extra lubrication so that the engine runs as smoothly as possible, lowering the friction. Some people also believe that it improves their MPG.

Additionally, engines are not designed to work only with the best and most perfect fuel. That is simply not the case as there are different types of fuel and fuel quality.

So overall mixed gas and even oil in small quantities will not necessarily damage a four-stroke engine in any way. Things are however, very different when it comes to adding four-stroke fuel to a two-stroke engine.

If you try using clean fuel with a two-stroke engine, you are risking damaging it beyond repair. This can happen in the span of a few minutes, so it should be avoided at all costs.

What to Do with Your Leftover Mixed Gas?

If you have a weed eater or some other equipment and tools that take mixed gas, you may end up having large amounts of mixed gas that you will not be able to utilize.

Letting the fuel sit is actually going to degrade it, rendering it unsuitable for later use.

One of the first things, especially if you don’t want to risk it, is to add fuel stabilizer to your mixed fuel before adding it to the fuel tank. That way, you will get rid of any problems associated with old fuel.

And depending on how often you will be using your mixed gas, you might not run into any problems at all.

On the other hand, a four-stroke engine can take and use two-stroke fuel, but it needs to be in very low quantities compared to the rest of the fuel.

It is not recommended to use more than half of a gallon of mixed fuel per ten gallons of standard fuel.

Do All Lawn Equipment Take Mixed Gas?

The lawn equipment (like tillers, trimmers, etc.) have smaller engines, which are typically two-stroke.

For years lawnmowers came with two-stroke engines as well. And although there are some advantages to them, they also have some disadvantages. One of which is that two-stroke engines are producing high amounts of pollution and gas emission.

Some data even suggests that running a lawnmower for one hour can be as harmful to the environment as 40 cars.

As a result of that, there has been a strong push from the authorities to move on to cleaner alternatives, which is the electric and battery-powered equipment.

Even a four-stroke engine is a bit cleaner and produces fewer emissions compared to a two-stroke engine.

And there is more to it. Two-stroke engines can become very expensive in the long run. Oil is not cheap, and mixing it with your fuel every time you are going to be using your tools ends up being very costly.

As a result, two-stroke engines are being gradually phased out (at least where it makes sense), and they are being replaced by cleaner and cheaper solutions.


Peter Toth

Hi! I'm Peter, the owner of BackyardGadget. Working around the house has always been a big part of my life. I've created this site to share my experience, and to help people choose the right tools for the job. Thank you for stopping by!

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