Thermodynamically, why are diesel engines used in heavy vehicles

Rahul Patil

115w ago

Heavy vehicles like trucks and buses have been used with petrol engines in the past, in the USA.

However, petrol engines tend to have less low rpm torque than diesels, which was especially true in naturally aspirated engines. Petrol engines are also less efficient than diesel , the latter using upto 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.

Diesel engines are bigger, heavier built due to the higher compression ratio and higher vibration levels. That also makes them more fit for heavier vehicles where the weight and noise/vibration of the engine is less of a problem, because the bigger vehicle has more mass to damp/spread these vibrations.

Private car owners preferred petrol engines for the refinement and power, so diesel engines saw use mainly for commercial vehicles. It was not until the 1990s-2000s that diesel engine technology matured to an extent that many mainstream car owners were happy enough with the diesel engine's NVH (noise-vibration-harshness) levels, to be acceptable in passenger cars.

For really big applications - like railway locomotives and ships, petrol engines need to be so big that designing powerful and effective spark ignition is problematic. Here the slow burning diesel with its more constant ignition temperature is more effective. Ships and locomotives do not have as much engine size concerns, they aren't intended to do the kind of speeds like cars, so engine size and weight is less of a problem - but fuel efficiency was, considering how much they travel. So locomotives and ships have really huge engines - the size of a small house, displacing dozens of liters for locomotives, and hundreds of liters in ships. A ship diesel engines have as many as 24 cylinders, with each piston 3 feet or more, across but running only a few hundred rpm at peak. 100-150rpm is not uncommon. Imaging that engine running 24 hrs a day, several days at a stretch. That sort of fuel requirement in petrol vs diesel alone means more fuel carriage to complete the journey.

Aditya Pathak

Asst. Manager (Mech.) at Tata Consulting Engineers (2015-present)

147w ago

There are quite a few reasons I could think of:

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

ii. Diesel engines employ higher compression ratios; so the peak pressures and temperatures reached are higher as compared to petrol engines, which in turn requires the engines to be heavy and robust.

iii. Also, for the same maximum pressure and temperature, the efficiency of a theoretical Diesel cycle is higher as compared to the Otto cycle.

iv. If we try to use petrol in bigger-sized engines, it might lead to too much of knocking (because of their pre-ignition).

Akshay Vaishnav

Design engineer and Powertrain lead @GT Motorsports, Formula Student Team, GTU

115w ago

The answer somewhat lies in the question. Particularly heavy vehicles are meant to carry heavy load and transfer this load from one place to another place. Now for this you need a vehicle with an engine which substantially helps you in economical way of course. So diesel engines with high compression ratio are used over gasoline engines.

When you are carrying such a heavy load (for instance a train or a truck or a bus), you are relying more on power rather than the torque. Because you need to carry such a huge amount of load and for that you require more power, not more torque.

Light and medium vehicles (like a bike or a car) don't possess huge amount of load, they need to move faster in shorter time, so they need more torque(comparatively).

Also due robustness of diesel engines and high compression ration they are preffered over gasoline engines. The injection pressure in some cases is ~200 bars, which is directly affected by power and efficiency of the engine.

Cost of the fuel might be the other reason. As we know, in India, cost of diesel is lower than gasoline, so for mass transportation of goods or passengers, the cost of diesel is low.

Thanks
Akshay.

Vivek Vetal

A Chemical Engineering student

77w ago

1. Diesel and petrol have almost same calorific value at room temperature. But the density of diesel is greater than petrol thus gives 15% more energy per unit volume on combustion.

2. Diesel engines produce much more torque (you can say the pushing force) than petrol ones, which is more required for heavy vehicles.

3. Diesel engines are more fuel efficient than petrol engines.

4. Diesel is an oil where petrol is a spirit. Diesel is less volatile as well as less inflamable. Thus safer to carry in large quantity.

5. In my country, India, government gives a large subsidy in diesel, it is almost ₹20/litre ($1.1/gallon) cheaper.

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