If you drive a petrol-powered vehicle, then spark plugs are used to get the engine into motion and the vehicle moving.
These small components deliver an arc of electric charge that ignites the fuel and air that is mixed in the cylinder head. The resultant explosion sets the piston in motion within the cylinder. The moving piston then moves the crankshaft, and that motion is transferred on to the transmission and then drive shaft, causing the vehicle to move.
A diesel-powered vehicle gets going in almost the same way. The only difference is that it doesn’t need a spark plug to kick things off.
Why? Well, it’s all got to do with compression.
First, a bit of physics. Air heats up as it is compressed. Squeeze enough of it quickly enough within an engine cylinder and the temperature and environment will be just right for it to ignite fuel without the need of a spark. This is what happens in a diesel engine. Air is compressed, diesel is injected into the ignition chamber and instant combustion occurs.
Incidentally, compression is also partly the reason diesels are so much more fuel-efficient (up to 30 per cent more) than petrol vehicles. All vehicles – petrol or diesel – have a set compression ratio. This ratio is the volume (or capacity) of an engine cylinder when the piston is at the bottom of the cylinder versus the volume of the combustion chamber (the bit at the top of the cylinder where the fuel/air mixture is ignited) when the piston is at its uppermost reach in the cylinder. In a petrol car, this ratio will be somewhere around 8:1 to 10:1. In a diesel it can be around 16:1 to more than 20:1. The higher the ratio, the more air is compressed, and more of the injected fuel is ignited during combustion. The result is a more powerful explosion and better performance.
While compression ignition means that diesels don’t require spark plugs, they can come fitted with components called glow plugs.
While glow plugs don’t ignite the fuel, they are useful when an engine, or the environment the vehicle is in, is very cold. They are, essentially, small heaters that warm the compressed air in the cylinder, aiding the heating of compression and helping ignition when a cold engine starts for the first time.
Want to learn more?
MTA Queensland’s Vehicle Maintenance course is designed for those who are interested in gaining valuable information and learning good practices for everyday vehicle use. The result: a safer journey with fewer breakdowns and repairs. If you have ever wanted to learn how to carry out preventative maintenance, then this course is for you.
22 April 2020
Please note the information contained on this page is general in nature and does not take into account your specific needs or circumstances. You should consider whether the information is appropriate to your needs, and where appropriate, seek professional advice.