The amount of power which an engine can produce is limited by how much fuel it can burn, and the amount of fuel it can burn is limited by the amount of oxygen in the cylinder. The amount of oxygen in the cylinder is limited by the amount of air in the cylinder. So, if more power is wanted then more air is needed, how do we achieve this? Well a large engine has more air, so it can produce more power, or we can compress the air before it gets to the cylinders.

   This is what a super-charger or turbo-charger does; they are a simple air compressor. A super-charger is a mechanically driven compressor run by a belt off the crankshaft. A turbo-charger is made to spin by blowing exhaust gasses over a turbine on one end of a shaft and compresses air with a turbine at the other end of the shaft. The diesel injection pump has a pressure sensor so that when the turbo-charger is winding up, extra fuel can be injected. There is one problem with compressing air, it gets hot and hot gas is less dense and therefore has less oxygen in it, so you lose a little bit of the advantage you would expect, unless you have an intercooler.

   An intercooler is a simply a heat exchanger mounted between the turbo-charger, or super-charger and the inlet side of the engine. Cold air is blown through it to cool down the hot, compressed air inside; this makes it denser and gives maybe another 10% more power.

   A diesel engine has no spark plugs. Because the air is so compressed and hot at the top of the stroke, the fuel ignites right away when it is injected. This is why a diesel engine is called a "Compression Ignition" engine and gasoline engines are called "Spark Ignition" engine. This also means that there are no ignition breakers, ignition coils, distributors and ignition wires to go bad. This allows diesel engines to start no matter if it is dry or rainy and wet.

   The key to fuel efficiency is to squeeze as much energy as possible out of a given amount of fuel by burning it as completely as possible. Diesel fuel itself has the potential to produce more power than a like quantity of gasoline. Furthermore, high compression ratios help diesels to burn their fuel more completely than gasoline engines. And because diesels always intake a full charge of air while varying the amount of fuel, the air-fuel mixture tends to be a very lean mixture at lower power outputs, which helps burn the fuel more completely.

   Some modern diesel engines are starting to come out with electronically controlled injection pumps, but the vast majority of them out there have purely mechanical pumps. No electricity is required to make a diesel engine run, except for a simple fuel cut off solenoid so that you can hut the engine down. I once drove from Madison, WI to Rochester, MN with a dead alternator on my diesel at night. Even with the lights on I made it home because the engine used no battery power.

   Turbo-charging a diesel is easier than turbo-charging a gasoline engine. One problem for a gasoline engine is that if the compression ratio is too high, and the pressure in the cylinder gets too high during the inlet stroke, then the fuel/air mixture can start to burn too soon, while the piston is still on the way up. A turbo increases the pressure in the cylinder making this problem worse. With a diesel engine, there is no fuel in the cylinder during the compression stroke, so a turbo can be used to pack as much air in there as desired without causing problems.

   The process of starting a diesel engine is less affected by temperature. When a gasoline engine is started from cold it needs a lot of fuel to make it run properly. If you do short trips all the time then you'll get far below the fuel economy that you'd normally get. Since emissions are proportional to the amount of fuel consumed, you'll be producing higher pollution levels. Diesel engines are ideal for short journeys because their efficiency is almost as high cold as when they are hot.

   Diesel engines last a lot longer than gasoline engines. This is because gasoline destroys lubrication and diesel fuel doesn't. Cold start-ups are a real killer for gasoline engines because of all that extra gasoline needed to start a cold engine.

   A compression ratio of 19:1 to 25:1 gives significant engine braking, but it also makes the engine hard to start. You'll need a good battery, one of very high amperage, and starter motor.

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