Why trucks run on diesel

By Colin Windell

In the same way we accept a slice of buttered bread will fall butter side onto the ground, it is generally accepted trucks run on diesel fuel – but why do they?

The simple answer is torque. Big trucks need big torque to move big loads.

Torque is the rotational force transmitted to the crankshaft. In other words, higher rotational force helps in pulling heavy loads.

Diesel, as a fuel, has slower combustion characteristics compared to petrol. So, for a given power, it produces higher torque at low speeds, which is the primary requirement of a heavy engine.

Diesel burns gradually due to its fuel property of being non-volatile, unlike petrol, which is volatile. Gradual burning helps in transmitting better torque and get better fuel efficiency. The stroke length of diesel engines is also higher than in petrol engines and that also helps in producing better torque.

The initial gear ratios of heavy vehicles are around 5:1 to 6:1 which is one of the reasons why trucks and heavy vehicles are able to produce high torque.

In a diesel cycle the combustion takes place at constant pressure, unlike in a car, where the fuel is ignited by a spark plug.  Inside the combustion chamber each atomised drop of diesel needs to be in vaporised under pressure and temperature to make it ignite.

However, when the piston moves up to build pressure and the combustion process starts, not all the injected diesel gets burnt at once.The fine vapor ignites first and then flame propagates to other injected fuel.

Simultaneously the piston is also moving and, as soon as it reaches Top Dead Centre (TDC) it starts moving down,so pressure starts decreasing with increasing volume. But, as diesel is still left in the combustion chamber which ignites, this compensates with the loss in pressure towards end of the cycle making it almost a constant pressure combustion process.

A diesel draws in the maximum amount of air on every stroke. In a diesel the amount of fuel added is what controls the power and the throttle controls how much fuel is added. This means a diesel always runs lean so at idle the engine uses hardly any fuel. This lean mixture allows for the addition of large quantities of fuel even at low revs.

The only real draw back to this torque production is a limited rev range. This is compensated for by a gear box with lots and lots of gears.

However, petrol engines tend to have less low rpm torque than diesels, which is especially true in naturally aspirated engines. Petrol engines are also less efficient than diesel, the latter using up to 45% of its energy derived from combustion. In the long run, this leads to lot of savings on running cost for commercial vehicles, which need to run more as an income generating asset.

A ship diesel engine have as many as 24 cylinders, with each piston nearly a metre across but, running only a few hundred rpm at peak – 100 r/min – 150 r/min is not uncommon. Imaging that engine running 24 hours a day, several days at a stretch. That sort of fuel requirement in petrol versus diesel alone means more fuel carriage to complete the journey.

Modern diesel engines have overcome disadvantages of earlier models of higher noise and maintenance costs. They are now quiet and require less maintenance compared with petrol engines of similar size and are more rugged and reliable.

An 1 800 r/min water cooled diesel unit operates for 12 000 hours to 30 000 hours before any major maintenance is necessary. An 1800 r/min water cooled petrol engine usually operates for 6 000 hours to 10 000 hours before it needs servicing.

While heavy loads may need torque to get moving there is a side benefit. That benefit is mileage. Because the diesel operates at fairly low rpm it uses less fuel.

A fully loaded semi gets around 39 l/100 km of fuel. If you were to build a petrol engine to the same cubic capacity you might get 235 l/100 km or worse.