The question, “why do they use diesel in large vehicles rather than petrol?” is a good question. It is a good question because it implies another question, “why don’t they use petrol in large vehicles rather than diesel?” After all, petrol pollutes less than diesel. Right? Wrong, actually. Diesel engines produce far fewer emissions than comparably-sized gasoline engines. Well, at least gasoline engines are more fuel efficient than diesel engines. correct? No. It is the reverse. Diesel engines are at least  33 percent more fuel efficient than comparably sized gasoline engines, and often, more than 33 percent. So, that means for every six (6) miles a gasoline engine travels on a gallon of gas, a diesel engine of comparable-size travels nine (9) miles on a gallon of diesel.

However, the fact that diesel engines are less pollution and get better fuel efficiency are not the reasons why. Neither is the reason that answers the question, “why do they use diesel in large vehicles rather than petrol?” 

Torque is the reason why. Diesel engines have much greater low-end torque than gasoline powered engines.

Low-End Torque and How It is Different from Horsepower?

Torque is the amount of force a rotating mechanism generates. The amount of force an engine crankshaft produces is torque, for example. Low-end torque is the amount of torque an engine generates through a crankshaft at low engine speeds — a.k.a., low revolutions per minute (rpm). The speed at which the crankshaft rotates while producing that force is irrelevant to the measure of torque. If the shaft produces a one-foot pound of force — whether the shaft is spinning at one revolution per minute or 15,000 rpm, — the amount of torque being generated is still one-foot pound. 

Horsepower, on the other hand, does account for the revolution speed of a rotating mechanism. And, horsepower also accounts for the torque of the rotating mechanism. That is because torque is a factor in determining horsepower. The formula for horsepower is torque multiplied by rpm. 

The reason they use diesel in large vehicles, rather than petrol, is because diesel engines have generated more torque than gasoline-powered engines. 

At a certain rpm, a gasoline-powered engine begins generating more horsepower than the comparably-sized diesel engine. That is because the rpm maximum of diesel engines is about half that of a gasoline engine. But, until a diesel engine reaches its rpm maximum, it will generate more horsepower than a comparably sized gasoline engine. 

Low-end torque is critical in the trucking business. 

A diesel engine’s capacity to generate torque and horsepower at low rpm is necessary for pulling heavy loads. “Whether the truck is driving in the mountains or on flat roads for hours at a time, they require engines and fuels that produce a high amount of torque,” TruckFreighter.com explains, “Diesel engines and fuel produces the number of torque semi-trucks need, which also allows the engine to have a higher power to weight ratio.”

Why Diesel Engines Produce More Torque than Gasoline Engines

The reason that diesel engines produce more torque than gasoline engines is that diesel fuel has greater compression resistance than gasoline. Because of the stability of diesel fuel — its compression resistance, — mechanical engineers can develop compression engines for diesel fuel. Because diesel engines are compression engines and gasoline engines are spark-fired engines, diesel engines have more torque.

Compression Engines versus Spark-Fired Engines

Gasoline engines are spark-fired combustion engines. That means a spark — from a spark plug — ignites the fuel inside the engine cylinders. Diesel engines ignite diesel fuel by compressing it until it auto-ignites. At a certain point, all fossil fuels auto ignite when compressed. It is at what point that a fuel auto-ignites that determines what kind of combustion engine that fuel can power. More specifically, it is the amount of pressure that a fuel can withstand that determines whether or not the fuel is appropriate for a compression engine. 

Thermal Efficiency and Combustion Engines

The autoignition engine of a fossil fuel produces much higher thermal efficiency ratings than spark ignition can. The reason being, thermal efficiency is a measure of the difference in temperature between the fuel that enters an engine in relation to the temperature change it creates when combusted in an engine. The greater the difference, the more thermally efficient the engine. 

Thermal efficiency determines everything from fuel efficiency to power and torque.

Again, the measure of thermal efficiency is the difference between the temperature of the energy going in an engine — fuel — and the temperature as it exits — exhaust. But, the definition of thermal efficiency is different than the formula that measures it. The definition of thermal efficiency is the amount of energy — fuel — put into an engine that is converted into mechanical energy, a.k.a., power. A simple layman’s definition of thermal efficiency is the amount of energy that goes into an engine that is converted into work.

What determines the thermal efficiency of an engine is the engine’s compression ratio. 

Compression Ratios of Gasoline and Diesel Engines Answers “Why do they use diesel in large vehicles rather than petrol?”

Again, diesel is very resistant to compression. Gasoline is not. Compression ratio is the difference between the space inside a cylinder when the piston is at bottom-dead-center and top-dead-center. Bottom-dead-center is where the piston sits when a cylinder fills with vaporized fuel. Once the cylinder is full, the piston rises. It is at top-dead-center that the fuel ignites. The compression ratio of a spark-fired gasoline engine is 8:1 and 12:1. If the compression ratio of a gasoline engine is any higher, the fuel in the engine will pre-ignite and the engine will throw a rod.

The compression ratio of a diesel engine is between 14:1 and 25:1 and sometimes it is even higher. In other words, it is possible to compress diesel almost twice as much as gasoline before it auto-ignites. A compression engine with a ratio of between 8:1 and 12:1 — because the energy density of gasoline is so low — makes gasoline engines inefficient to the point of being useless. 

In other words, gasoline engines will likely never have considerable torque in relation to diesel engines. The reason being, gasoline will probably always be spark-fired.  

The reason why they use diesel in large vehicles rather than petrol is torque. Diesel has greater compression resistance than gasoline. Because diesel fuel has more compressive resistance than gasoline, diesel engines can be compression-fired engines. Gasoline engines can not. Because diesel engines can be compression-fired engines, they have greater thermal efficiency than gasoline engines. The reason they have greater thermal efficiency is that they have higher compression ratios. The higher the compression ratio, the more energy is released. The more energy released, the greater the torque. 

So, simply, they use diesel in large vehicles rather than petrol because diesel has greater compressive resistance.