I’ve mentioned it in other posts, but I love mowing the lawn. While pushing my hardy mower up and down the grass to create those lovely stripes I’ve not really considered it before, but what are most lawn mower engines these days – 2 or 4 stroke? And what’s the difference?

As 2 cycle mowers have been phased out across the US, most lawn mowers are now 4 cycle. In a 2 stroke (or 2 cycle) engine, the gasoline and oil must be mixed. For a 4 stroke (or 4 cycle) engine, the oil and the gasoline are separated, in two different tanks.

Let’s look at this all in further detail, and discuss the differences between the two engines – and what this means for their use…

What is The Difference Between a 2 Stroke and a 4 Stroke Lawn Mower?

In a 2 stroke (or 2 cycle) engine, the gasoline and oil must be mixed. For a 4 stroke (or 4 cycle) engine, the oil and the gasoline are separated, in two different tanks.

2 stroke engines require fewer steps and have fewer moving parts. They are easier to maintain. Generally, a 2 stroke engine is much smaller, lighter weight, louder, smokier, and cheaper. 

A 4 stroke engine is more fuel-efficient and produces more torque. 4 stroke engines are also easier to start, more reliable, more powerful, and more expensive.

Both engines are examples of internal combustion engines. For a 4 stroke internal combustion engine, fuel and air are mixed inside a chamber – on the intake stroke. The mixture is put under pressure – the compression stroke. Then, a spark plug ignites the mixture and causes an explosion. 

The explosion causes a piston to move – called a combustion (or power) stroke. The pistons are attached to the crankshaft, which revolves. Finally, the exhaust is expelled – called the exhaust stroke.

The difference between a 2 stroke engine and a 4 stroke engine is how quickly this combustion cycle occurs and the number of times the piston moves up and down in each cycle.

In a 4 stroke engine, the piston completes one compressive stroke and one exhaust stroke for each revolution. The spark plugs fire only every other revolution – or once every four strokes.

In a 2 stroke engine, the combustion cycle is completed in one piston stroke. Spark plugs fire once for every revolution.

Here’s an easy to read reference on the differences (with videos of engines).

How Do I Know if My Mower is 2 or 4 Stroke? (Can You Tell Just By Looking at it?)

Check your manual first. Check to see if there are other ways to tell the difference – like stickers saying No Fuel Mixing – which means it’s a 4 stroke mower.

Then, check the fuel cap. If your mower has a 2 stroke engine, and it still has its original fuel cap, then the cap will read the mix ratio (like 32:1). If you look for a dipstick or separate fuel ports for fuel and oil on a 2 stroke engine, you won’t find them. Only the one.

If your mower is 4 stroke, then you should find a dipstick pretty easily. 4 stroke engines have separate fill ports for oil and gasoline, and they will be clearly marked. 

Why Are 2 Stroke Engines Banned in The US?

Banned is a strong word. “Phased out” would be better. From July 2019, new 2 strokes engines (unless they are fuel-injection ones) are being phased out. However, you can still use your old 2 stroke outboard motors or dirt bikes – these are not “banned”. 

The reality is that 2 stroke engines don’t meet current emission standards, so you won’t find any brand-new ones, unless they are fuel-injection ones. New 2 stroke engines are not allowed to be imported and sold in the US, Europe, Australia, and a few other countries.

The problem is that 2 stroke engines do not efficiently burn the oil/gas mixture, so much of it gets emitted into the air or water. This inefficient engine design has continued, unchanged, since the 1940s. 

According to the EPA, as much as 30% of the oil/gas mix you put into the engine comes out unburned into the air or waterways – that’s also 30% of your money wasted. 

The EPA estimated that old 2 stroke engines have spilled more oil into our waterways than the Exxon Valdez did in 1989. Yikes!

2 stroke engines are not allowed to be imported and sold in the US. If you have an old 2 stroke engine (outboard motor, dirt bike), then you can still use it. 

But when it dies and you have to replace it, you won’t find any new 2 stroke engines to replace it. Used 2 stroke engines are widely available privately. Check out Craigslist.

2 stroke engines are still manufactured and sold in many countries. New 2 stroke engines can be bought in South America, Africa, and Asia. 

Some 2 stroke engine manufacturers claim to have improved their 2 stroke engine emissions – like Evinrude, Suzuki, Honda, and Yamaha. These 2 stroke engines are direct fuel-injection.

What Happens When You Pour 2 Stroke Oil in a 4 Stroke Mower?

If you mess up and use 2 stroke oil in a 4 stroke engine, that’s OK – once. 

Oils made for 2 stroke engines have a solvent added to help it mix with the gasoline. So 2 stroke oil is slightly thinner. It also won’t have the additives usually found in 4 stroke oil.

Experts say it might smoke a little more (one person said “like a freight train”) and your 4 stroke engine might run a little hotter. Others say you may have shaved a year off the life of the 4 stroke engine. 

Check out your manual and the warranty-voiding fine print.

Here’s a video of a guy testing 2 stroke oil in a 4 stroke engine.

What Fuel Does a 4 Stroke Mower Take? (As Opposed To a 2 Cycle?)

Always check your owner’s manual first. 4 stroke engines should have gasoline with an octane level of 87 or higher – chainsaws need an octane of 89 or higher. Gasoline should have an ethanol level of between 0% and 10% or up to 15% MTBE (methyl tertiary butyl ether). 

Ethanol and small 4 stroke engines are not a good combination. Try to avoid E85 gas, which is 15% ethanol. Ethanol in a small engine creates water, so your engine may not start easily. Also, ethanol is corrosive and burns hot, which can cause your engine to overheat.

For 2 stroke engines, gasoline and oil needs to be mixed. Check your instructions, but most 2 stroke engines run at 40:1 (gas to oil), 32:1 or 50:1 ratio. 

Fun fuel fact: gasoline deteriorates in as little as 30 days, so make sure your fuel is fresh. If you know you’re going to have to store gasoline for longer, consider adding a fuel stabilizer to the can.

Will Mixed Fuel Ruin a Four Stroke Lawn Mower Engine?

If you accidentally pour a 2 stroke oil/gas mix into a 4 stroke engine’s gas tank AND you realize it right away, then drain out the gas tank – no harm, no foul. The small amount of oil will burn off.

If you pour 2 stroke oil/gas mix into the gasoline tank of a 4 stroke engine, and then run the engine, you will get lots of smoke. Your engine may run rough, and you may end up with fouled spark plugs. If the engine is fuel-injected, then the jets may get fouled. If the oil clogs the fuel line, then it may have to be replaced.

If you pour 2 stroke oil/gas into the oil tank of a 4 stroke engine, then drain the oil tank. If tipping the lawn mower doesn’t entirely drain it, you can remove the oil tank. You may need to wash out the oil tank with some mild dishwashing soap. After washing it, rinse it thoroughly. Let the inside air dry or dry it with some compressed air.

Check your warranty. Sometimes putting a mixed fuel in a 4 stroke engine will void the warranty.

Final Words

If you go into a store to buy a new lawn mower, the engine in it will be four stroke, as two stroke engines have been phased out across the US.

This doesn’t mean you can’t still use your old two stroke – as they haven’t been banned altogether.

What it does mean though, is that the most current answer to the question ‘are lawn mowers 2 or 4 cycle?’ – would be that the majority are now 4 stroke. Hope this helps.