Why Are 4 Stroke Engines Not Reversible?

The four-stroke engine is the most common engine type in the world. It was first developed in 1876 by Nikolaus Otto and completes four separate processes – intake, compression, power, and exhaust in two revolutions of the crankshaft. But while 4-stroke engines are incredibly efficient and reliable, they have one major drawback – they are not reversible.

4-stroke engines are not reversible because their valvetrains, crankshaft, intake, and exhaust systems operate in a specific order. When one tries to run the engine in reverse, these systems no longer line up, and the engine will not run properly. Instead, the water and oil pumps may get damaged.

The rest of this article will explore why 4-stroke engines are not reversible and the potential consequences of trying to run one in reverse. Read on for more insights into these and how these engines work (source).

Can You Make a 4-Stroke Engine Run Backwards?

It’s not possible to make a 4-stroke engine run backwards. This is because the order in which the valves open and close and the crankshaft rotation are explicitly designed for forwarding motion. So, if you try to run the engine backward, the order of operation will reverse, and it won’t fire.

To put this into perspective, let’s revisit how a four-stroke engine works:

In a nutshell, a 4-stroke engine works by sucking in air and fuel (compressed together in the cylinders), then igniting the fuel to create a powerful explosion that pushes the piston down. Next, the piston is connected to the crankshaft, which rotates and activates the gears that turn the wheels.

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To make a 4-stroke engine run backward, you would need to change the direction of the camshaft, interchange the carburetor and muffler, and reverse the motion of the crankshaft. 

And even if you could do all that, it’s likely that the engine would not run very well – or at all.

Suppose you decide to reverse the camshaft. In that case, the intake and exhaust valves would open and close in the opposite order, and the fuel/air mixture would not be adequately ignited. 

This would cause the engine to run inefficiently, leading to timing issues (source).

In addition, reversing the camshaft would also put unnecessary stress on the water pump and oil pump. These components are not designed to run in reverse and could easily be damaged.

You could interchange the carburetor and muffler. However, this would likely reduce the engine’s power due to restricted intake flow. In addition, the carburetor and muffler valves are different-sized, so the intake flow is not smooth when switched. 

This could lead to a loss of power and damage the engine.

What Happens When You Run a 4-Stroke in Reverse?

Now that we have seen why it is impossible to make a 4-stroke engine run backward let’s look at what happens when you try.

When you try to run a 4-stroke engine in reverse, the water and oil pump may get damaged, along with the gears. Also, the intake and exhaust valves won’t work correctly, and the engine will lose power.

Here’s a rundown of these consequences in more detail:

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The Water and Oil Pumps May Get Damaged

The water pump circulates coolant through the engine, while the oil pump lubricates the internal parts. 

When you run a 4-stroke engine in reverse, these pumps will be working against the direction they’re designed to move in.

As a result, the pumps may wear out prematurely or even break. Therefore, it is crucial to always run a 4-stroke engine in the correct direction (source).

The Gears May Get Damaged

Just as the water and oil pumps can be damaged when a 4-stroke engine is run in reverse, the gears can also be affected. 

The gears are responsible for transmitting power from the crankshaft to the wheels, so the engine will not run correctly when they are damaged.

Note: Damaged gears can cause the engine to make a lot of noise. Additionally, the engine may lose power.

The Intake and Exhaust Valves Won’t Work Correctly

Typically, the intake valve opens to let in air and fuel, while the exhaust valve opens to let out exhaust gas. 

When you try to run a 4-stroke engine in reverse, you reverse the order of operation. This means that the intake and exhaust valves will open at the wrong time and will not work correctly.

Pro Tip: To take care of your engine’s intake and exhaust valves, I recommend cleaning them regularly. For instance, this CRC Throttle Body Cleaner (link to Amazon) may come in handy when you need to clean your engine’s intake valves. It’s designed for gasoline engines and will help dissolve gum and varnish deposits, ensuring your valves work correctly.

The Engine Will Lose Power

Running a 4-stroke engine in reverse will cause it to lose power because of the problems mentioned above. As a result, not only will the machine be less efficient, but it may also stall or even break down.

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Can a Diesel Engine Run Backward?

A two-stroke diesel engine can run backward, but a four-stroke diesel engine cannot. Notably, two-stroke engines don’t rely on rigid intake and exhaust valves. Instead, they have pistons that can run in either direction, making them reversible. 

If you need to reverse a diesel engine, it’s best to check whether it is a two-stroke or four-stroke engine. If it’s a four-stroke, you’re out of luck – it cannot run in reverse. 

Why Do Diesel Engines Run Backward?

Diesel engines may run backward due to poor lubrication, a clogged filter, or low oil pressure. In such a case, the best course of action is to take the engine to a mechanic to inspect it. To avoid this, always ensure the engine is lubricated correctly and has enough oil pressure.

Note: This only applies to two-stroke diesel engines. As I’ve highlighted throughout this article, four-stroke diesel engines cannot run in reverse.

The Bottom Line

There are a few reasons why you cannot reverse four-stroke engines. One reason is that the direction of the camshaft would have to be changed, which would cause timing issues. Another reason is that the carburetor and muffler would have to be interchanged, reducing the engine’s power due to restricted intake flow.

Finally, running a four-stroke engine in reverse can damage the water and oil pump, as well as the gears. As a result, it is generally not recommended to run a four-stroke engine backward.

Recommended Reading:

  • 4 Stroke Kick Start Stuck: 8 Causes and Fixes
  • 2 Stroke vs. 4 Stroke: Which One Is Best for You?


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About the Author: Tung Chi