The following generalization may be made on the use of diesel versus petrol engines in profitable vehicles and, more recently, cars:
1] The diesel engine has superior fuel economy than the petrol engine. This is because its thermal efficiency is 30–36 per cent, compare with the 22–25 per cent of a petrol engine. thermal efficiency is the ratio of functional work performed by the engine to the internal energy it receives from its fuel.
2] The diesel engine has normally proved to be more reliable, to need fewer maintenance and also to have a longer life than an equivalent petrol engine. These advantages derive mostly from its sturdier construction and cooler running characteristics.
3] Although a petrol engine develops its highest power at higher rotational speeds than an equivalent diesel engine, the latter can provide improved pulling power. This is because the utmost turning effort or torque exerted by the crankshaft of the diesel engine is greater and also improved maintained over a wider range of engine speeds.
4] A disadvantage of the diesel engine is that it tends to be intense and bulky in relation to its power output. This is explained by the greater operating pressures and loads that have to be catered for in the structure of the diesel engine.
5] The noise and vibration level of the diesel engine, particularly under idling and low-speed operation, compares unfavorably with the petrol engine. Again, this is primarily a function of the much higher cylinder pressures in the diesel engine. More current diesel engine motor cars have nevertheless been praised for their low level of noise at motorway cruising speeds.
6] The diesel engine is sometimes criticized for having smoky exhaust of unpleasant odor, though it is the invisible products of combustion in the exhaust gases of a petrol engine that are more injurious to the environment. Avoidance of a smoky exhaust with a diesel engine is mostly a question of good driving technique, regular maintenance and proper adjustments.
7] A safety concern is that the fuel oil used in motor vehicle diesel engines is far less dangerously flammable than petrol, thus dropping the fire risk in the event of an accident. For taxation purposes, the fuel oil used in automotive diesel engines is referred to as DERV (an abbreviation of Diesel Engine Road Vehicle).
8] Finally, the basic cost of the diesel engine, together with its associated fuel injection equipment, is normally higher than that of an equivalent petrol engine.