No Spark Needed! Why Diesel Engines Don’t Have Spark Plugs
Internal combustion engines don’t all function in the same way. Case in point is the way a petrol powered engine operates compared to a diesel powered one. Despite both having the same four stroke formula of operation: Intake, compression, combustion, exhaust, they differ in the way they ignite their fuel.
Combustion of Petrol And Diesel
See, petrol or gasoline has a much lower flash point compared to diesel, which means it can be ignited at a much lower temperature. Flash point is defined as the lowest liquid temperature at which said liquid gives off enough vapour to be ignited.
Petrol has a flash point of -43 degrees Celsius, which means it can easily be ignited by a spark. Diesel on the other hand, has a flashpoint of 52 degrees Celsius or higher (depends on type of diesel), which means a spark will not have the same effect on it. As such, its temperature will need to be raised high enough to induce spontaneous combustion/autoignition.
How Does A Petrol Engine Work?
Modern ICE combustion engines have a 4-step cycle based on the movement of each piston. First, a specific ratio of fuel and air mixed and injected into the cylinder (this depends on direct or port injection) as the piston moves downward.
Once it moves up again, the cylinder compresses this air and fuel mixture, causing the temperature to rise and forcing the molecules closer together. This is done to create an optimum condition for efficient combustion to occur.
Once ignited, the combustion and expansion of gases from that process, forces the piston downwards. Once the piston comes back up, the exhaust valves open as well, expelling the gases from the cylinder. That’s why they are known as four stroke engine: Intake, compression, combustion, and exhaust.
The Pistons are connected to the crankshaft, and the latter’s rotation governs the up and down movement of these pistons. However, the opening and closing of the intake and exhaust valves are controlled by the rotation of the camshaft. The crankshaft is connected to the camshaft via a timing belt/chain. The former controls the rotation speed of the latter.
How Does A Diesel Engine Work?
A diesel engine still uses the four stroke process as a petrol engine. However, there’s a slight difference to how it creates an ideal environment for combustion. When the piston rises after the intake process, it is allowed to compress the air first. This is done to dramatically increase its temperature to facilitate the ideal temperature to combust diesel.
Then, diesel is injected into the chamber, which mixes with the compressed air. At this point, the diesel ignites, and the combustion and expansion of gases pushes the piston downward. Once the piston comes back up again, the gases are pushed out into the exhaust. They may use glow plugs, which help heat the air up especially during a cold start.
Diesel engines produce a considerable amount of torque at low rpm (engine speed), which makes them ideal for pulling heavy loads. This is why lorries, buses and excavators use diesel engines to power them. They are also more efficient and robust compared to petrol ones, which is why diesel is still an ideal fuel source for heavy goods vehicles.
