The Combustion Process: Gas vs. Diesel Engines Explained

Although electric cars are edging their way into the market, most vehicles on the road today remain powered by gas or diesel engines. Both designs use fuel to convert chemical energy into mechanical energy, and both follow the basic four-stroke combustion process … so what’s the difference?

A Quick Glance

An external comparison of gas and diesel engines side by side reveals a lot of similarities with one important difference: spark plugs and their related components. In gas engines, spark plugs are usually screwed into the engine cylinder head, their electrode buried in the combustion chamber to deliver that essential ignition-inducing spark that makes the combustion process, well, combust. Managed by the ECU (engine control unit), an ignition coil sends electricity to a distributor, which is responsible for timing each cylinder’s spark at the exact right moment to produce power efficiently. A diesel engine uses the rules of physics to induce ignition in a different way, without spark.

Four Strokes, Two Methods

The steps are outwardly the same: intake, compression, combustion and exhaust. The difference is in the details. On the intake, gas engines pull in air and fuel, while diesel takes only air. During compression, the piston moves up, compressing the chamber contents (air and fuel in gas, air only in diesel). The thing to note here is that diesel compression ratios are significantly higher than gas — the air is more highly pressurized and thus gets a lot hotter. In the combustion phase on a gas engine, spark is sent to the compressed air and fuel, igniting the mixture and sending the piston down for the power stroke. In a diesel engine, however, the compressed air in the chamber is already so hot that no spark is necessary. Diesel engines inject a fine mist of fuel during this step; it ignites instantly, and the piston is forced down. The exhaust stroke is basically the same for both engines, emptying out the remaining burnt gases from the chamber.

A Clear Winner?

The main difference in operation is the timing of fuel injection and the means by which it is ignited. The nice thing about diesel is that it’s technically more efficient; the higher pressure and heat ignites more of the fuel per stroke, thus cutting emissions and waste. The reason gas engines don’t work the same way is that the higher the temperature in the chamber, the more likely you are to get engine knock, which is an untimed ignition. Engine knock is highly inefficient for fuel consumption, ruins spark plugs and gunks up your cylinder walls and valves, leading to bigger problems. The other consideration with diesel engines is cold starts. How do you provide heat to get things moving? Many designs use glow plugs to remedy this, warming the chamber before the process begins, but newer technology is able to delay the timing of direct fuel injection, allowing for even higher compression ratios to heat things up.

For a long time, diesel had a rather unfair bad rap. These days, technology has driven huge improvements in efficiency for the use of diesel in everyday driving. But gas-driven designs have made gains, too, especially with the move toward hybrids. In the end, it’s likely that both will be replaced by other technologies. For now and the near future, however, it’s nice to know what you’re driving with.

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Photo courtesy of Flickr.