A primer on diesel engines

The longstanding petrol alternative.

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The longstanding petrol alternative.

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The humble diesel engine is really the workhorse behind modern society. Learn about how this deeply impressive engine works and the differences between it and petrol engines.

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The birth of diesel engines came about during the end of the 19th century when inventors started looking at ways to replace steam engines, which were deemed to be inefficient and bulky; and petrol engines, which were somewhat dangerous and also inefficient. Steam and petrol engines were only around 10% to 12% efficient, which made them expensive to run and could not be reliably used for certain applications. Back in the day, petrol engines would leak and because petrol is very volatile, the engines would pose a fire hazard especially if used in confined areas. The father of the modern day diesel engine was the German Rudolf Diesel. In his research, he found that air, if compressed heavily enough, would get so warm that it would ignite fuel. This is the basis of the diesel engine.


Both diesel and petrol engines are internal combustion engines that work using a four-stroke cycle. The four strokes or phases are intake, compression, power and exhaust. Air is first drawn into cylinder, and then it is compressed with fuel. At this point, the mixture of fuel and air is ignited to cause an explosion. It is this explosion that causes the piston to move downwards and drive the crankshaft to produce motion. Thefinal phase is exhaust where the spent air-fuel mixture is expelled out of the cylinder so that a new cycle may begin. The main difference between diesel and petrol engines is that the latter uses a spark plug to ignite the air-fuel mixture, while the former engines rely solely on heavily compressed air. As mentioned earlier, Rudolf Diesel found that the temperature of air could be made to increase high enough if it was heavily compressed. The temperature would rise so high to the point where it could cause the ignition of diesel fuel. In diesel engines, air in cylinder would be compressed typically around 14 to 23 times. In petrol engines, the compression ratio is generally much lower, typically around only around 7 to 10, with high performance vehicles having high compression ratios of up to 13. High compression ratios are desirable because it results in higher thermal efficiency. In other words, more energy can be extracted out of the air-fuel mixture. This also explains why diesel engines are considerably more efficient (by around 25% to 30%) than petrol engines. And thanks to the increased emphasis on efficiency, emissions and environmental-friendliness, improvements to diesel technology, diesel cars are in vogue. Almost every automaker today has a diesel version of their most popular model.

Weighing the pros and cons


1. Diesel engines are the most efficient type of internal combustion engine, capable of converting over 45% of the fuel energy into mechanical energy. Petrol engines, on the other hand, are typically only about 30% efficient.

2. Not only are diesel engines more efficient, diesel fuel is also cheaper to purchase. At the time of writing, diesel fuel is around 40% cheaper per liter than petrol. This means diesel vehicles will be cheaper to run, which also explains why buses and most taxis have diesel engines.

3. To withstand the high compression of gases within the cylinders, diesel engines are built to be extremely hardy and will generally last longer than their petrol counterparts.

4. Diesel engines can be made to run on alternative and renewable fuels like biodiesel with little or no modifications. Biodiesel generally refers to used vegetable oil that has been used for cooking, and is then recycled and treated so that it can be used to power diesel cars.


1. Diesel engines need to be built stronger to withstand the high compression of gases, as a result, they usually cost more to manufacture. Consequently, diesel cars can sometimes cost more than their petrol equivalent.

2. Diesel engines produce a distinctive knocking sound that is referred to as diesel clatter. This sound is the result of the sudden ignition of fuel which causes a pressure wave.

3. Diesel engines are heavier and less eager to rev than petrol engines, which makes them undesirable in sports cars.

4. Since diesel engines do not use spark plugs to start the ignition process, and because diesel fuels are less flammable, diesel engines can be difficult to start in cold weather.