Why are diesel engines not used in small cars?

Diesel and gasoline are both byproducts of crude oil refining, but they have quite different combustion characteristics. This makes each fuel type have its own advantages and disadvantages.

And it can be seen that the use of diesel engines is not really common in small cars and it often appears as an option for a few popular car models. In fact, it can be seen that diesel engines appear more often in trucks and heavy vehicles , why is this so?

What advantages does diesel engine have?

Diesel engines are ignited by compression, as opposed to the "spark" in gasoline engines. A higher compression ratio ensures higher thermal efficiency and complete fuel combustion. Diesel vehicles are known for their higher mileage, making refueling less frequent. This is especially necessary for heavy vehicles because they often travel for long periods of time and through areas where refueling is difficult to access.

Despite having a comparable calorific value to gasoline, diesel engines are more energy dense, resulting in higher torque.

Moving around while carrying a lot of weight will consume a lot of energy. The diesel engine's slow combustion process ensures that this energy is supplied as torque to the vehicle, even at low speeds. This makes diesel fuel beneficial for large vehicles that frequently have to travel with heavy loads and sometimes rough terrain.

Diesel engines are designed to take advantage of the above three characteristics. They have longer bores and strokes and are often turbocharged to maximize the torque delivered to the vehicle.

Diesel engines have a higher compression ratio than gasoline engines. Diesel engines are often bulkier and heavier than gasoline engines of the same capacity due to their more complex structure and need for a higher compression ratio. This makes them unsuitable for small cars that emphasize agility and fuel economy in the city.

According to the above argument, it is clear that diesel is a very powerful fuel. However, diesel engines also have some disadvantages that make them unfavorable for use in smaller cars.

NVH (Noise, Vibration, Harshness - noise, vibration and shock) levels are higher

Due to the higher compression ratio, diesel engines produce stronger sounds and vibrations than gasoline engines. While this does not have a significant impact on heavy vehicles due to their large size, it can become uncomfortable and even dangerous for smaller vehicles. Therefore, many manufacturers have tried to improve diesel engines for use in some of their car models, such as jeeps and SUVs of some car brands that all have diesel engine options.

Diesel engines are often noisier and vibrate more than gasoline engines , especially when starting and accelerating. This affects the comfortable driving experience, especially for those who use cars in the city. The cost of manufacturing a diesel engine is higher than that of a gasoline engine due to its more complex structure and use of higher quality materials. This leads to higher vehicle costs, affecting consumer accessibility.

Maintenance costs are higher than gasoline engines

Machinery subjected to high vibrations will wear out faster and may fail prematurely . As for diesel engines, their components must be designed to withstand higher levels of NVH. This increases maintenance costs significantly when compared to gasoline engines. It should be noted that although diesel engines require less frequent maintenance, any benefit gained from this is offset by the cost of each maintenance need.

This makes diesel engines less popular among small car owners whose goal is economical transportation.

Issues related to emissions

Despite being more efficient than gasoline engines, diesel engines still emit harmful nitrogen oxides, which cause respiratory failure. 

Older diesel engines are associated with releasing even more NOx into the atmosphere than usual. Therefore, many countries around the world have taken measures to ban diesel engines that are more than 10 years old. Therefore, to avoid emissions-related penalties and taxes, diesel engines are often not preferred in small cars.

Old diesel engines can emit more NOx and fine dust particles than gasoline engines, causing environmental pollution. Small cars are often used mainly in cities for short travel distances. Gasoline engines with fast acceleration and fuel economy in heavy traffic conditions are more suitable for this need. Meanwhile, diesel engines are more efficient when operating long distances with high loads, suitable for trucks, SUVs and pickup trucks.

However, it should be noted that diesel engine technology is continuously improving, with a focus on reducing size, weight, noise and emissions. Therefore, in the future, there will likely be more small cars using diesel engines, especially those that focus on performance and fuel economy.

In addition, a number of other factors can also affect the use of diesel engines in small cars, such as fuel prices, tax policies and emissions standards of each country .