Why are diesel engines used in heavy vehicles?


Heavy vehicles like trucks, buses, railways, SUV’s, tractors, mobile cranes, tank trucks and many others have become such an essential part of the transportation system and other economic activities that they have become a lifeline for these industries. Be it carrying tons of goods or taking hundreds of passengers from one place to other, heavy vehicles depend on certain performance parameters that must be met in order to achieve a high amount of load-carrying capacity.

We have all seen heavy vehicles can carrying tons of load from one place to another quite easily but many of us still wonder why are diesel engines used in heavy vehicles so often and not petrol engines?

Diesel engines provide higher torques at very lower speeds which is essential to push the heavy loads carried by heavy vehicles. Diesel engines have a higher compression ratio, greater efficiency, lower fuel consumption rate and high durability, these are the reasons why are diesel engines used in heavy vehicles.

Let us see in detail why these factors are so important for diesel engines which power heavy vehicles.

5 reasons why are diesel engines used in heavy vehicles rather than petrol engines?

Torque

torque
Torque | Image source- allaboutcircuits.com

Torque is a rotational force or moment of force that tends to cause rotation about an axis. The point from which the body rotates is called the ‘axis of rotation’.

As diesel engines use diesel fuel for the combustion process the compression ratio required for burning diesel is greater than that of petrol/gasoline. To achieve a higher compression ratio the diesel engine uses longer crankshaft and large cylinders so that more air is compressed in order to auto-ignite the fuel. This makes it possible for the diesel engines to produce more torque than petrol engines even at lower RPM (revolutions per minute).

Compression ratio

One of the major factors that contribute to achieving higher torques at lower RPM is the compression ratio.

The compression ratio is defined as the ratio of the total volume of the cylinder to clearance volume.

Why are diesel engines used in heavy vehicles compression ratio
Petrol vs Diesel compression ratios | Image source- stillrunningstrong.com

Compression ratio of diesel engines is 18-24 which is higher than the compression ratio for petrol engines which is 8-12. The difference in the compression ratio’s are due to the autoignition temperatures and volatility of the fuel. Diesel ignites at a temperature of 210 degrees C and has less volatility while petrol has a high ignition temperature of 260 degrees C and high volatility. This is the reason why spark plugs are used in petrol engines.

Higher the compression ratio higher is the torque achieved and lower is the fuel consumption. As heavy vehicles have to travel long distances, lower fuel economy comes a major advantage for the companies economically in the long run.

Engine size

Engine size
Diesel engine | Image source – macktrucks.com

To achieve higher compression ratio the length of the piston stroke needs to be more to be able to compress more air for igniting the fuel in the combustion chamber which naturally increases the size of the engine. For this purpose diesel engines require rugged structural design. As the size of the heavy vehicles is large they can easily accommodate these large engines and sustain the vibrations produced by the engine. As higher speeds are not a priority for heavy vehicles, diesel engines offer the required torque due to slower combustion of fuel. 

Due to high fuel storage capacity thanks to the large fuel tank and piston cylinders and slower fuel combustion rate the diesel engine provides greater fuel efficiency than petrol engines. This is a huge time and money-saving advantage as the driver does not require to refill the tank often and can travel longer distances within time.

Greater efficiency

Petrol and diesel are two different fuels with different properties. Diesel is less volatile than petrol but has a higher flash point temperature of 52 deg C.

The flash point of a liquid is the lowest temperature at which its vapour would ignite if provided an ignition source.

As the flash point of diesel is higher than that of petrol, diesel burns more steadily than petrol providing higher torques even at lower RPM. Petrol has the property of vaporizing and burning quickly than diesel, as a result of this property petrol engines do provide higher speeds quickly but cannot generate the torque required to move heavy loads. So for a litre of fuel, diesel engines generate more force than petrol engines.

Engine durability

The higher torque and efficiency requirements of heavy vehicles make the size of diesel engines large and heavy to use. To meet the high power requirements, diesel engines have a rugged structural design constructed with extremely high quality and hard materials which can withstand the high vibrations produced inside the combustion chambers.

This drastically improves the durability of the engine but also makes it very expensive as compared to petrol engines. This high price is justified by the durability and power transmitting capacity of the diesel engines making it more efficient and feasible to use it for a longer duration of time economically.

The bottom line

Factors like high torque, compression ratio, engine size and engine durability are the major reasons why are diesel engines are used in heavy vehicles. The design of diesel engines have evolved greatly in recent times and they have become more compact and lightweight to be able to adapt to light vehicles.

Many automobile companies provide options to buy diesel engine powered cars as they are more durable, have lower maintenance costs and also diesel is cheaper than petrol in many countries. This gives a major leverage to the automobile companies to profit from a particular model of a car across various countries. So big or small, diesel engines will continue to power heavy vehicles with high torque and greater efficiency.

Also read: 8 Types of CNC machines and Their Application


Aditya Bhosale is a mechanical engineer by qualification and works as a freelance content writer and web developer. As a film enthusiast, booklover and science freak, quickbinge.com is an effort by my team to share our insights about ideas and stuff worth binging.
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