A common question when it comes to vehicle maintenance is “what’s the difference between a spark plug and a glow plug?” The answer is simple. Spark plugs are part of the ignition process in gasoline engines and glow plugs help start diesel-powered rides. While the basic difference is easy to explain, the specifics of how do glow plugs work on diesel engines and how spark plugs work in gas engines is a bit more complex. Read on if you’re interested in digging deeper into the spark plug vs glow plug discussion.
How Spark Plugs Work
All combustion engines need three things – fuel, air and heat or an ignition source. A spark plug is the ignition source for a gas-powered engine. When you turn over your engine, the cylinders compress the air/fuel mixture making it hotter. The compression ratio for a gas engine is 8:1 – 10:1, not enough to cause ignition of the mixture. Therefore, an additional ignition source – a spark plug – is needed. While a glow plug is only needed upon ignition in a diesel engine, the spark plugs in gas engines are used with every power stroke.
How Does a Glow Plug Work?
While a spark plug is used for ignition in a gas engine, a diesel engine’s ignition is caused by compression. The compression ratio of a diesel engine is much higher than gas, anywhere from 17:1 to 25:1. The typical diesel engine compresses the air/fuel mixture into a much smaller space with temperatures reaching up to 2,000 degrees, plenty hot to cause ignition. So, what are glow plugs used for? Glow plugs are necessary for only one reason – cold starts. For instance, if your engine’s cold from sitting overnight, the additional heat from a glow plug is needed to heat air in the cylinder and aid in ignition. Typical glow plugs heat up in 5-15 seconds when you turn over your engine to help it start. Like mentioned before, a glow plug shuts off after ignition once the vehicle has already been started.
Choose the Right Plug for Your Ride
Now that you know the simple difference between a spark plug and a glow plug, you can make a more informed decision when shopping for your ignition system. But it doesn't stop there. For even more information, including heat range and setting the spark plug gap, check out this helpful article that goes deeper into everything you need to know about spark plugs.