Diesel Vs Petrol Engines. Why are diesel engine more noisy and fuel efficient ?

Diesel vs Petrol, most pertinent and relevant today with ever increasing petrol prices, making Indian car buyers skew their new car buying decisions in favour of diesel cars. We would love if car experts could contribute towards this diesel Vs petrol debate.

I have Two Questions:

1). Why Diesel Engine Produce More Noise than petrol Engine?

2) Why Diesel Engine gives more mileage than petrol?

Diesel Vs Petrol Engines. Why are diesel engine more noisy and fuel efficient ?

Diesel engine noise is referred to as a Clatter or Knock. Higher noise levels in a diesel engine could be;

1) Combustion noise

2) Mechanical noise

Combustion Noise
is due to rapid build up of pressure inside a diesel engine during combustion stroke. In a diesel engine, the fuel ignites spontaneously shortly after injection begins. During this ignition delay, the fuel is vaporizing and mixing with the air in the combustion chamber. Combustion causes a rapid heat release and a rapid rise of combustion chamber pressure. The rapid pressure rise is responsible for the "diesel knock or clatter" which is very audible.

Combustion Noise can be reduced by increasing the Cetane number of any diesel fuel. This in turn decreases the amount of knock by shortening the ignition delay time considerable. Indirect injection systems limit the pressure rise by throttling of the combustion gasses through the pre-chamber throat.  Advanced direct injection engines use pilot injection to start combustion off more slowly. Injection timing, rate of injection, compression ratio, turbo boost, etc all effect how much diesel clatter one hearsMechanical Noise could be because of pump noise, gear rattle as big diesel engines  require super strength front end drives, such gears often rattle. Loose fitting pistons which skirts against the cylinder wall also can cause diesel clatter.

Diesel engines are more fuel efficient than petrol engines because

Diesel engines work on much higher compression ratio which packs in that much more energy in the mixture resulting in much better thermodynamic efficiency. Thanks to higher compression ratio diesel engine can work on much leaner air fuel mixture thus adding to its fuel efficiency. Lower pumping loses as the throttle is always wide open.