What is an Interference Engine and Why Can They be Problematic?

Many vehicles use engines with a specific configuration and valve placement. These motors are known as interference engines.

To understand what an interference engine is you first must understand what is going on within your engine. Inside all engines are valves that allow air and/or fuel into and out of the combustion chambers, camshafts open the valves, your timing chain rotates your camshafts, and your crankshaft rotates your timing chain. The crankshaft rotates due to the up-and-down movement of the pistons. 

An interference engine has a design where the internal valves can extend into the area of travel of the pistons. If the pistons and valves can be in the space area at the same time that means they can hit each other; and interfere with each other (hence the name). The valves under normal conditions are never able to be in the area of the piston at the same time. The timing chain is responsible for controlling the movement of the valves and the timing chain spins due to the movement of the pistons.

Since the pistons are responsible for the valve movement, we can design the entire system to control itself and not allow interference. This is one reason why timing is important in an interference engine. As long as the timing chain and timing system are functioning properly the engine has no issue. An interference engine only has a problem when the timing is off.

What Causes Faulty Timing?

What causes an engine’s timing to be off? If the timing chain jumps or breaks you can then have the pistons strike the valve. Let’s start with what happens if the timing chain “jumps”. This means the chain becomes disconnected from the crankshaft sprocket and the sprocket keeps turning. Once the timing chain becomes connected again it is now out of synch because either the crankshaft or the camshaft was able to spin freely while the other did not. Once everything is connected again the shafts will be out of phase. Now the valves will open early or late. Either way, it’s not a good situation. Now the piston is moving up and down while the valves are opening and closing. If the piston is moving up and the valves are still open they will both want to occupy the same space. 

Timing chain

What happens if the piston hits the valves? The pistons have so much momentum behind them that they will bend if not break the valves once they come in contact. The piston is very heavy and cycles at the RPMs of the engine. Typical RPMS are in the 1,000s. So the pistons are moving very fast with a lot of weight. Now the engine has a serious problem. A bent or broken valve won’t seal. This means air can enter and exit the combustion chamber freely. This will make the engine run rough and, in most cases, cause such a loss of power, that the vehicle is not drivable. Almost always a check engine light will be triggered. 

Close up of engine valves bent from timing failure
Maz2331, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons

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