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Fundamental Difference in between Petrol Engine and Diesel Engine

Nepal Auto Trader

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The diesel and petrol engines that are used in most cars are highly similar. In essence, they are internal combustion engines that work using a two or four-stroke cycle. In an internal combustion engine, the power cycle is made up of four phases: intake, compression, power and exhaust.

In the intake phase, air is drawn into cylinder through the opening intake valve. In the compression phase, the intake valve closes and air 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 downwards and drive the crankshaft to produce motion. This is the power phase. The final phase is exhaust where the spent air-fuel mixture is expelled out of the cylinder through the opening exhaust valve so that a new cycle may begin.

The main difference between diesel and petrol engines is that petrol engines use spark plugs to ignite the air-fuel mixture, while diesel 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.

Therefore in diesel engines, air in cylinder would be very heavily compressed, typically to around 14 to 23 times its original volume. In petrol engines, the compression ratio is generally much lower, because they rely more on the spark plug to begin the power phase. The compression ratio is petrol engines is typically around only around 7 to 10, with high performance vehicles having higher 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 than petrol engines. In fact, diesel engines have the highest thermal efficiency of any internal combustion engine.

Pros of Diesel Engine

  • Not only are diesel engines more efficient, diesel fuel is also cheaper to purchase. This means diesel vehicles will be cheaper to run, which also explains why buses and most taxis have diesel engines.
  • Because diesel engines are so efficient, cars can get incredible mileage out of them. It is not uncommon for passenger cars with modest 50 liter fuel tanks to be able to travel over 1000km on a single tank. This means more time spent traveling and less time spent refueling.
  • 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. They can also go longer between maintenance.
  • 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.

Cons of Diesel Engine

  • 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. This depends heavily on the manufacturer.
  • 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. It makes diesel engines sound less refined and noisier.
  • Diesel engines are heavier and less eager to rev than petrol engines, which makes them undesirable in sports cars. This also makes diesel powered cars less peppy and engaging to drive.


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