In this article I'm going to discuss the properties of the diesel and gas engines, and compare them to each other. I'm specifically referring to their use in automobiles.

Looking at them from a distance both the gas and the diesel engine work in about the same way. A fuel made from refined crude oil is burned inside a cylinder and the hot expanding gasses forces the piston to move. The movement of the piston is then transferred to the wheels through crankshaft, gearbox and transmission.

It's when we look at the engines a little closer that we start to notice that there are differences. Differences in gas mileage, smell, vibrations, price and expected age to name a few.

Ignition System

The gas engine has an ignition system with high voltages, wiring and spark plugs. The diesel does not need this, it compresses the fuel/air mixture until it's so hot it ignites all by itself. In the gas engine it's the spark plug that ignites the fuel-air mixture through a high voltage spark at the right time.

Another thing that separates the diesel from the gas engine is that gas engines adds the fuel vapor to the air in the intake manifold either through a carburetor or through fuel injection nozzles. The air/fuel mixture is then sucked into the cylinder to be burnt the next time the piston strikes.

The diesel on the other hand sucks clean air into the cylinder and then waits for the piston to compress the air as much as it will go. At this time when the air is compressed and hot enough to ignite the fuel, diesel is injected directly into the cylinder (or a small space connected to the cylinder) through high pressure nozzles. The fuel ignites instantly due to the high temperature in the cylinder.

The differences is fuel injection and ignition results in the diesel burning the fuel more efficiently than the gas engine. That's why a diesel has a better gas mileage than it's gas powered counterpart.

This is not to say that a diesel is better. It's different, it has drawbacks too. Diesel engines usually is harder to start in cold and freezing weather. If you remember it was compressed hot air that ignited the fuel. When it's too cold outside the air will never become hot enough to ignite the fuel, and the engine won't run.

Cold Starting a Diesel

To handle this diesels have something called glow plugs for handling cold start situations. Electrical power from the accumulator is used to preheat the glow plugs inside the cylinders before the engine is started. This way the hot glow plugs ignite the fuel when the air is not hot enough to do it. Then when the engine starts it only takes a few seconds for the cylinders to get hot enough to ignite the fuel without help and the glow plugs are not needed again until the next cold start.

This is one of the nuisances of having a diesel in your car. The glowing takes from a couple of second to half a minute and can easily lead to stress when in a hurry. You just have to wait until it's ready or the car won't start. On the other hand if the engine is well done the glowing does not take long and you soon get accustomed to it.

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