Petrol vs Diesel Engine - What is the Difference?

When it comes to buying a car for yourself, an inevitable dilemma of choosing between a petrol and diesel engine arises. To add to the confusion, the age-old debate between petrol vs diesel engine users, about their benefits and shortcomings has been going on for years.

Petrol and Diesel – we hear about them in our daily life now and then. They are primarily used as fuel in automobiles and are also used for other purposes such as power generation and lubrication. These fuels exist in the form of fossil fuels or crude oil, which is extracted from the earth, refined and separated through fractional distillation, and is made available to us. The role that these fuels play in our day-to-day lives is of significant importance, and a world without them is unthinkable.

Petrol – also known as gasoline, is a mixture of combustible hydrocarbons, although its exact composition varies according to its grade or octane rating. Octane rating is what describes the quality of the fuel, and its value is based on the ratios of specifically two compounds that make it up – iso-octane and normal heptane. The quality of the fuel is directly proportional to the number in the octane rating – the larger the number in the octane rating, the higher the quality of the fuel.

Diesel is a type of energy-dense fuel, a mixture of hydrocarbons, used mainly to power heat engines, including cars, trucks, and generators. It can be obtained from crude oil or can be made from biomass. It is specifically designed to be used in diesel engines such as trucks and other heavy vehicles, where its less volatile and slow combustion nature can be optimally utilized.

Although both petrol and diesel are derived from mineral oil, the exact methods of refining them differ. Generally speaking, refining diesel is easier than petrol, but diesel contains a lot more pollutants that need to be extracted before it can reach the same emission levels as that of petrol. Diesel has a higher mass density than petrol, meaning that if we take the same volume of both, diesel has more energy.

Scientists have developed several engines, but almost every kind of automobile in the world runs on fossil fuels. The engines inside them utilize the fuel to convert it into energy which powers them, which in turn powers the vehicle. The two most common types of engines that can be found on the market are petrol engines and diesel engines. Although they both function similarly, there is a considerable amount of difference between petrol and diesel engine. They both work on the Otto cycle and the diesel cycle respectively.

Both engines have the same fundamental steps of how they work. They are:

Intake
Compression
Power
Exhaust

Petrol vs Diesel, there are differences between the two engines based on how the fuels burn. Petrol readily evaporates and efficiently mixes with the air. As a result, all it takes is just a spark to produce smooth combustion in a well, pre-mixed gasoline engine. On the flip side, diesel does not properly mix with air, but if atomized diesel is exposed to air of considerably high temperature, spontaneous combustion takes place.

Petrol Engine

Petrol engines are most commonly found and used in cars and scooters. It is an internal combustion engine that works on the principle of spark ignition. In such engines, the fuel is mixed with the air before compression. Earlier, the compression used to take place in a device called the carburettor, but today’s petrol engines use electronically controlled fuel injection technology, where the fuel is introduced into an internal combustion engine using an injector.

Petrol engines may run on the two-stroke cycle or the four-stroke cycle. A Wankel engine is another type of internal combustion engine that converts pressure into rotational motion.

Unlike the diesel engine, in a petrol engine, the fuel is compressed along with the air. One reason why it is done so can be because of the highly volatile nature of petrol. Once the fuel and the air are compressed, an electric spark plug is used for ignition. A spark plug provides the necessary spark that ignites the mixture and the process of combustion takes place, producing energy that in turn moves the pistons.

In any engine, cooling is of utmost importance. Without proper cooling, the engine will probably seize and stop working. In extreme cases, if the metal gets too hot, the pistons weld themselves into the cylinders, which is a clear indication that the engine is destroyed.

Petrol engines can be both air-cooled and liquid-cooled. Fins can be incorporated to increase the surface area on the cylinders and the cylinder head. In liquid cooling, water was primarily used as the coolant, but today it is a mixture of water with either ethylene glycol or propylene glycol. These compounds have a much lower freezing point and a higher boiling point when compared to water. They not only prevent corrosion but also contain lubricants and other supplements that safeguard water pump seals and bearings.

Speed and Efficiency

Petrol engines have lighter pistons, crankshaft, and connecting rods, which are partially responsible for giving them higher rotation speeds at which they run. Another reason for this can be because petrol burns quicker than diesel.
The pistons in petrol engines have much shorter strokes when compared to the piston strokes in diesel engines, which result in less time taken to finish a stroke. Even with that benefit given, when we talk about efficiency, petrol engines fall short in comparison to diesel engines.

Their thermal efficiency is typically somewhere around 20% (average), which is nearly half of the efficiency provided by diesel engines. This could be because of the lesser compression ratios of the petrol engines. However, some newer engines are said to clock in at better thermal efficiency (up to 38%).

Petrol engines are quieter and less vibrating compared to diesel engines. This is because, in a pre-mixed mixture, the combustion process taking place is much smoother and propagates well, whereas in a diesel engine, the combustion could begin anywhere in the combustion chamber and it turns out to be an uncontrolled process. To reduce the excessive vibration, diesel engines are forced to opt for a more rugged and heavy design, whereas petrol engines are always preferable for lightweight applications.

Advantages

Smoother combustion because of the pre-mixing of the fuel and the air.
Lesser noise; calm and quiet.
Lightweight structure and components
Lower cost

Diesel Engine

Another type of internal combustion engine is the diesel engine that works on the principle of compressing the air alone and not the fuel. In this type of engine, fuel is combusted by letting it interact with the air that has a highly elevated temperature because of being mechanically compressed in a cylinder. Because it follows this principle, it is also called a CI (compression-ignition) engine. This type of engine stands out from the rest, such as petrol engines and gas engines, because it does not incorporate a spark plug for ignition of the fuel.

When the engine compresses only the air in the cylinder, it raises the temperature to such a high degree that the ignition of atomized diesel fuel that is injected into the combustion chamber is spontaneous. The torque (rotational force) produced by a diesel engine is controlled by manipulating the air-fuel ratio, which means, instead of controlling the air intake, the diesel engine works by altering the amount of fuel that gets injected. This air-fuel ratio is usually high.

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Diesel engines can be designed in both two-stroke cycles and four-stroke cycles. Initially in the past, they were used as a replacement for steam engines. Later on, their usage was carried into submarines and ships. Further down the timeline, they were used in locomotives, trucks, and electricity generation plants. Then in the 1930s, they began to be used in automobiles, and since then, gradually, the utilization of diesel engines in larger vehicles has increased.

Efficiency

Diesel engines have a fairly high-efficiency rate owing to their high compression ratios. Also, the loss of charge exchange is low due to the lack of a throttle valve, which results in lesser fuel consumption in low and mid load situations. This alone renders the diesel engine a very economical option.
Although the theoretical efficiency of a diesel engine is 75%, in a real-world application, it is much lower. Modern-day car diesel engines may have a possible efficiency of up to 43%, whereas, in diesel engine trucks and buses, the efficiency rate of up to 45% seems achievable.

However, these are the peak efficiencies produced by diesel engines, while the average efficiency over a driving cycle is even lower. For example, for an engine with a peak efficiency of 44%, the average efficiency would be somewhere around 37%.

Paramount Advantages

Diesel engines have the highest effective efficiency when compared to all other combustion engines.
Diesel engines inject the fuel into the combustion chamber directly and have no air intake restrictions apart from the air filters and intake plumbing.
Since the diesel engine compresses only the air, it can achieve a good compression ratio without the risk of self-ignition. On the other hand, in a petrol engine, such a high compression ratio is not possible. Higher compression ratios mean better efficiency of the cycles, resulting in better fuel economy.
In India, diesel as fuel costs less compared to petrol. This is primarily because of the differential taxation structure at the Centre and the states. Also, it is believed to be mostly used by farmers and bus and truck fleets. Therefore, in India, diesel is priced lower than petrol to reduce the burden on the farmers and the bus and truck fleets.
Diesel’s chemical composition is made up of a long chain of hydrocarbons when compared to petrol. This means that diesel has a higher mass density than petrol, which means that if we take both diesel and petrol of the same volume, diesel has a higher energy density.
Diesel is not as volatile as petrol, and so, the atoms of diesel don’t form a flammable vapour around them, which results in them having lesser risk of catching fire.
They don’t incorporate the use of spark plugs for ignition, which makes them a viable option in humid regions,

Some differences between Petrol and Diesel Engine

In a petrol engine, the fuel is injected along with the air into the compression chamber so that it can mix well with the air, which results in smooth combustion. On the contrary, in a diesel engine, the fuel is injected very late – only when the piston reaches TDC (top dead centre). This is because the air is compressed to its maximum capability only at TDC, raising its temperature to the highest possible value. Injecting fuel only at this point will result in maximum and effective combustion.
Diesel engines don’t rev as high as petrol engines. One main factor behind this is because diesel engines require heavy, strong, and beefy internals since they experience a much higher degree of combustion pressure compared to petrol engines. These heavy components are harder to spin and it takes a toll on the RPM.
While petrol engines are definitely powerful and give a punchy feeling at lower RPMs, diesel engines win the game at higher RPMs. Also, the output provided by diesel engines at higher RPMs is constant.

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What should you choose, Petrol or Diesel Engine?

While choosing to buy a petrol engine or a diesel engine, you need to keep in mind certain factors. If you are someone who wants to drive to work or have an occasional outing, a petrol engine suits you the best. If you are someone who likes to drive around a lot and go on long trips more often, a diesel engine is a better choice.
If you plan to use your vehicle for a commercial purpose, such as a passenger taxi, or transportation, getting a diesel engine would be the best bang for your buck.
Another factor that you need to consider is your comfort. Petrol engines are quieter and calmer whereas diesel engines are noisy and vibrate a lot.
When it comes down to the cost of both engines, petrol engines are cheaper in comparison. The reason diesel engines are a bit on the pricey side is that they will be less expensive to run owing to the cheaper fuel rates, whereas petrol is expensive in contrast.

Bottom line is, both engines have their own set of pros and cons. It all boils down to what you prefer more over the other. Old beliefs such as diesel engines life expectancy are less than petrol engines is a myth. While there is a noticeable difference between petrol and diesel engine in terms of performance and longevity, it all comes to how you use and maintain the engine, whether or not you provide it with the necessary and timely servicing.