Longevity - think of an 18 wheeler capable of 1,000,000 miles of operation before major service)

Lower fuel costs (lower fuel consumption per kilowatt (kW) produced)

Lower maintenance costs - no spark system, more rugged and more reliable engine,

A gasoline engine intakes a mixture of gas and air, compresses it and ignites the mixture with a spark. A diesel engine takes in just air, compresses it and then injects fuel into the compressed air. The heat of the compressed air lights the fuel spontaneously.

A gasoline engine compresses at a ratio of 8:1 to 12:1, while a diesel engine compresses at a ratio of 14:1 to as high as 25:1. The higher compression ratio of the diesel engine leads to better efficiency.

Gasoline engines generally use either carburetion, in which the air and fuel is mixed long before the air enters the cylinder, or port fuel injection, in which the fuel is injected just prior to the intake stroke (outside the cylinder). Diesel engines use direct fuel injection -- the diesel fuel is injected directly into the cylinder.

Note that the diesel engine has no spark plug, that it intakes air and compresses it, and that it then injects the fuel directly into the combustion chamber (direct injection). It is the heat of the compressed air that lights the fuel in a diesel engine.

The injector on a diesel engine is its most complex component and has been the subject of a great deal of experimentation -- in any particular engine it may be located in a variety of places. The injector has to be able to withstand the temperature and pressure inside the cylinder and still deliver the fuel in a fine mist. Getting the mist circulated in the cylinder so that it is evenly distributed is also a problem, so some diesel engines employ special induction valves, pre-combustion chambers or other devices to swirl the air in the combustion chamber or otherwise improve the ignition and combustion process.

One big difference between a diesel engine and a gas engine is in the injection process. Most car engines use port injection or a carburetor rather than direct injection. In a car engine, therefore, all of the fuel is loaded into the cylinder during the intake stroke and then compressed. The compression of the fuel/air mixture limits the compression ratio of the engine -- if it compresses the air too much, the fuel/air mixture spontaneously ignites and causes knocking. A diesel compresses only air, so the compression ratio can be much higher. The higher the compression ratio, the more power is generated.

Some diesel engines contain a glow plug of some sort (not shown in this figure). When a diesel engine is cold, the compression process may not raise the air to a high enough temperature to ignite the fuel. The glow plug is an electrically heated wire (think of the hot wires you see in a toaster) that helps ignite the fuel when the engine is cold so that the engine can start.

Smaller engines and engines that do not have such advanced computer controls  use glow plugs to solve the cold-starting problem

Today’s modern diesels are quiet and normally require less maintenance than comparably sized gas (natural gas or propane) units.  Fuel costs per kW produced with diesels is normally thirty to fifty percent less than gas units. 

1800 rpm water cooled diesel units operate on average 12-30,000 hours before major maintenance is required.  1800 rpm water cooled gas units normally operate 6-10,000 hours because they are built on a lighter duty gasoline engine block.   

3600 rpm air-cooled gas units are normally replaced – not overhauled at 500 to 1500 hours. 

Because the gas units burn hotter (higher btu of the fuel) you will see significantly shorter lives than the diesel units.