The Four Stages of the Internal Combustion Engine

Suck, squeeze, bang, blow. Sounds dirty. When I was in school to become an automotive technician, a friend of mine had a shirt with these words on it. It is actually a very good way to describe the four stages of the internal combustion engine: intake, compression, power, exhaust. You may be thinking, “Why do I need to know about this?” Because it has everything to do with your engine running properly. If you have engine problems, they likely have something to do with at least one of these stages. So, it is helpful to be familiar with how the engine works, especially when diagnosing problems yourself.

Main Parts of the Engine

First, let’s take a look at the main parts of an engine. I am going to be incredibly simplistic here. Engines are complicated devices where everything has to sync. At the heart of the engine, there are four basic pieces that must work together to make the engine run. These are the cylinder, the piston, the valves, and the spark plug.

The piston moves up and down inside the cylinder. If you have ever had push-up ice cream (Google it, and then have one, push-ups are delicious), the cylinder is like the push-up cylinder, and the piston is like the stick and plastic disk that pushes the ice cream goodness to the top.

Intake

As the piston goes down, the valves open to let in air and gasoline. The air has passed through an air filter, and the gasoline is mixed with the air. The vacuum created by the piston “sucks” in the air (which contains oxygen, nitrogen, argon, carbon dioxide, methane, etc.). That is the first stage – intake.

Compression

The second stage is compression (or “squeeze”). When the piston reaches the bottom of the stroke, it starts going back up. The valves close and the air and fuel mixture in the cylinder compresses. Compression is important because it is like compressing a spring. It crams the air and fuel mixture in the cylinder to get it ready for the next stage.

Power

The next stage is power (sometimes called ignition or “bang”). At just the right time, the spark plug fires a pretty powerful spark.  This ignites the air/fuel mixture. It truly is an explosion. This is also known as the power stroke.  The explosion forces the piston back down the cylinder with tremendous force.

Exhaust

The cylinder is now filled with burned up fuel and any other gases. It has to go somewhere. This is the exhaust gas. So, the next step is the exhaust stage, or “blow.” Another valve opens up. The spent exhaust gases are pushed out of the cylinder as the piston is forced up. The exhaust gases are directed out to the exhaust system of the car, through the catalytic converter and muffler.

So How Does this Help?

So now that you know the basics, it is pretty easy to think about what might cause an engine to fail. For instance, if the air filter is blocked, not enough combustible gases will be sucked into the engine. When it is compressed with the fuel and ignited, the explosion will not be as great as it could be. So, you lose power.

If the spark plugs are bad, or if the timing of all of these parts is off, the whole system gets thrown out of whack. The issue of too little gasoline (blocked fuel filter) or too little compression (worn piston rings) also causes problems.  It all makes much more sense when you know how the engine is supposed to operate. And that’s why this is so important.

Written by Gary Pradel