What Is a Piston and How Does It Work? - Enji


What Is a Piston and How Does It Work?

August 4, 2022

A car engine is a huge mechanism with typically around 200 moving parts that need maintaining. If you are having a problem with your engine, it could be one of these components rather than the whole engine.

In an internal combustion engine, the fuel and air mixture needs to compress in the combustion chamber so that it ignites the spark plug and keeps your engine running. This process repeats constantly as you drive your car. But what is a piston in this sequence?

A piston is a steel or aluminium component that moves inside the cylinder wall of the engine, with piston rings to keep the seal tight. There is one piston for every cylinder in your engine. On one end is the piston head, and on the other is a connecting rod which attaches to the crankshaft via a gudgeon pin. Without a piston, there would be no compression in the cylinder wall to help the air fuel mixture to explode and ignite.

How Do Pistons Work & What Do Pistons Do? 

Now we have answered your question about what is a piston, let's break down the next; how do pistons work?

The purpose of the piston is to transfer force from the expanding gas to the crankshaft so that the back and forth movement of the piston is transformed into a circular motion which can power the wheels. Most internal combustion engines work in a sequence of 'four strokes' of the piston - intake, compression, combustion and exhaust. When you p

Intake - The piston moves down inside the cylinder wall to draw in the fuel and air mixture into the combustion chamber.

Compression - As the piston moves back up it compresses the air fuel mixture causing high temperatures.

Combustion - The spark plug sits at the top of the cylinder head and the heat causes it to ignite the air fuel mixture. The fuel burns and expands which drives the piston and connecting rod downwards, moving the crankshaft into a circular motion.

Exhaust - The piston moves back up in the cylinder and pushes the gases out through the open exhaust valve.

This sequence repeats constantly as you drive your car. 

Symptoms Of A Bad Piston Ring Or Piston 

As you can see, the piston plays a very important part in the running of your car and the piston rings are vital in keeping the seal tight between the piston and cylinder walls. Engine oil is used to lubricate the moving parts, so if the seal is broken and engine oil gets into the cylinder and mixes with the fuel, it can cause problems. As the 'four stroke' sequence is repeated thousands of times, there is a lot of potential for wear and tear. But don't worry, there are many symptoms of a bad piston which will alert you to book in for an Engine Diagnostic with Enji.

Slow to Accelerate

Does your car feel sluggish when accelerating? This could be a sign that your piston rings have worn out and as a result, there is insufficient compression in the combustion chamber, causing low power output. So if you are putting your foot on the gas pedal heavier than usual and your car is still slow to accelerate, it is recommended that you book into a mechanic to run a compression test.

Excessive Oil Consumption

If your 'check engine' light is on, it is likely that your piston rings require attention. Worn piston rings can cause oil to leak into the combustion chamber, which makes your car consume more oil. So if you find yourself adding more fuel than usual, book a local mechanic to take a look at the piston rings.

Leaking Oil

If the piston rings fail, they can cause high pressure in the crankshaft and cause the engine seal to break and start leaking oil. You can check the pressure of the crankshaft yourself by turning off the engine and slowly opening the engine oil cap. What you should feel is no pressure but a small vacuum instead. If, however, the cap pops off itself followed by a lot of smoke, then you may have bad piston rings. If you don't feel confident enough to run this test yourself, book a mechanic and they can check for you and fix it.

Extra Exhaust Smoke

If the piston rings are wearing out, the engine oil can seep into the pistons and burn along with the fuel. You will know if this has happened as there will be blue smoke from your exhaust pipe. This is one of the more major symptoms and you should book a local mechanic immediately. Burning oil can cause a lot of damage to many components of the car engine and increase your replacement cost.

Dirty Air-Filters

The piston pulls in the air mixture via air-filters, which is necessary as the oxygen helps the fuel to explode and ignite. If you have bad piston rings, then the exhaust gases can leak into the piston and crankshaft, causing a blow-by into the air filters. If the air filters are oily, then it will decrease the amount of air intake and cause excessive fuel consumption. It is usually recommended that you change your air filter every 19,000km, but if you have a worn-out piston ring you may need to book in earlier.

Piston Ring Replacement Cost 

The piston rings replacement cost is relatively low at only $30 to $150 each. However, as the piston rings are located deep in your engine, the labour cost is quite high.

Your mechanic will need to remove the engine and take off the cylinder head which is an involved process. They will then need to check the cylinder walls and piston grooves for wear before replacing the piston rings, thoroughly cleaning, testing and reassembling the engine. This whole process can take your mechanic up to 16 hours to complete.

The average labour rate in Australia can be anywhere from $150 to $280 per hour, which means the piston rings replacement cost can be anywhere from $2,400 to $4,480 plus the cost of the rings themselves.

If left, a bad piston ring can affect many other moving components in the engine and your replacement cost will be a lot higher.

Piston Replacement Cost

There are many reasons why the whole piston will need to be replaced. If the bad piston rings have been allowing oil to leak or affecting the air fuel mixture, then the increased pressure and high temperatures can cause the whole piston to burn and sometimes melt. This can then affect the connecting rod and gudgeon pin, causing them to become loose and break away.

The piston replacement cost can be anywhere between $3,100 and $7,400 but easily twice that for a diesel engine. If there is more than one piston to replace, this could mean an entire engine rebuild. It may be more cost-effective for you to replace the whole car engine.

The longer you leave a bad piston ring, the more damage it can cause to the whole engine. Enji can connect you to a local mechanic before these issues become major symptoms. Book into a mechanic with Enji to ensure your piston is fixed.