COMPARISON OF PETROL ENGINES AND DIESEL ENGINES

1.      In petrol engine, a mixture of air and petrol is drawn into the cylinder and compressed, whereas in case of diesel engine, air alone is compressed.

2.      The compression ratios used in petrol engines are limited by to the range 9:1 to 11:1, whereas automotive diesel engines usually employ compression ratios from 15:1 to 24:1. This result in higher thermal efficiency of the diesel engine. Thus a saving of fuel occurs; in diesel engines fuel combustion per unit horse power output on this account is less by 30 to 40% as compared to petrol engines. In latest designs of diesel engines, the fuel combustion is claimed to be only half as in case of petrol engines of comparable size.

3.      There being no throttle valve in the air supply passage of diesel engine, it operates at a higher volumetric efficiency, which results in fuel economy. However, with the use of fuel injection in Morden petrol engines, this disadvantage no longer exists.

4.      Diesel fuel used in C.I. engines is cheaper and less volatile. This results in considerable saving in the running cost. The risk of fire is also comparatively less.

5.      Important components of a petrol engine, like carbonator, spark plug and the complete ignition system are not required in a diesel engine. Instead a fuel injection system, consisting of mainly an injection pump and injector is employed. Experience has shown the injection system of C.I. engines to be more reliable then the carburettor and the ignition system is used in S.I. system engines. However, lately fuel injection system has also been employed in petrol engines, making the carburettors obsolete.

6.      Because of comparatively lesser speeds in case of diesel engines and their more robust construction, there is reduced wear and tear and the intervals between the overhauls are increased.

7.      Higher combustion efficiency of diesel engines means less carbon monoxide and hydrocarbons; so ordinary there is no need of special emission control devices for these, but till recently NO_x emission was a problem in diesel engines in petrol engines. However, with rapid technical advances, NO_x and particulate have also been reduced significantly. Particulate filters have also been developed to limit the particulate matter in the engine exhaust. Moreover, due to better fuel efficiency, light -duty diesel engines now use 30-40% less fuel than petrol engines of similar power under similar circumstances.

8.      With its smooth combustion a diesel engine offers a high and more stable torque over a wide rangse for easier operation. In Morden common rail direct injection diesel engines, more torque is available even at low engine speeds then the petrol engine of similar size and similar circumstances.

9.      The initial cost of diesel engine is high on account of mainly two factors: firstly; because the engine has to be made from robust owing to the higher pressure involved compare to a petrol engine, and secondly due to high precision involved in the manufacture of fuel injection pump. However, this higher initial cost is fairly compensated by reducing running costs; the breakeven point is reached after a running of about one lake kilometres or even less.

As in case of diesel engines only fresh air is drawn and very small time is available for the fuel injected to form a uniform mixture, about 20% excess air at full load than required for chemically correct mixture has to be inducted so that less carbon die oxide gas is expelled with the exhaust. Besides CO and HC emission is also less. However, they do tend to generate higher NO_x and PM (particular Matter) than petrol engines. Higher NO_x emissions are caused by higher temperature and excess oxygen of the lean combustion process in diesel engines. Higher PM results due to incomplete combustion caused by low oxygen levels around fuel droplets. This occurs in spite of the excess air since diesel vaporizes less readily than petrol resulting in the incomplete air/fuel mixture in the injection system. For the same reason, for the same power output under similar conditions, a diesel engine will have to have a cylinder capacity at least 20% greater than the equivalent petrol engine.

Another disadvantage of diesel engine was that it ran less smoothly and was more noisily than the petrol engine. However now with the use of high pressure common rail direct injection, the noise level in diesel engines has vastly decreased, even acceptable for luxury class automobiles.

Earlier, a governor in a diesel engine was a must to control critical speeds like idling and maximum B.P. However, electronic control has now made this reluctant.

In diesel engines sometimes, especially during a cold start in winter, the cranking speed may not be sufficient to raise the temperature of compressed air to the self-ignition value. For this reason heater plugs or other starting aids have to be employed.