Diesel engines are very similar to gasoline engines. They are designated as internal combustion (the fuel burns inside the engine—in the combustion chamber), and they have much of the same functional hardware (pistons, crank shaft, valve train, and cam shaft). Even the fuel delivery systems are pretty comparable (fuel tank, fuel pump and fuel injectors). OK, so what’s the difference? It’s how the fuel is ignited. That’s what makes all the difference.
Make it Burn
Diesel engines rely on a principal known as compression ignition. Whereas gasoline engines ignite the fuel mixture with an electrical spark generated by their spark plugs (spark ignition), diesel engines utilize heat and pressure generated in the combustion chamber to kindle the fuel as it is injected into the cylinder. Diesel engines have no spark plugs—really, they don’t need them.
Gasoline vs. Diesel Fuel vs. Biodiesel
It is this fundamental difference that necessitates and allows for the use of two completely different types of fuel. Gasoline is a complex, highly volatile and explosive fuel that will combust easily under relatively low compression with a strong spark.
Diesel fuel is a much heavier, low volatility fuel (it is actually classified as a light oil and is sometimes referred to as fuel oil) that does not readily ignite from a spark. It is the intense heat from high compression that allows the fuel to burn. Biodiesel is very much a contemporary of petroleum diesel fuel. With very comparable physical characteristics (both are light oils) and similar chemical characteristics (both are carbon based), biodiesel readily burns in an unmodified diesel engine, albeit with vastly reduced emissions.
Diesel Engines: Goliaths of Iron
Diesel engines truly are behemoths built with a stoutness that begets their brute strength and durability. To wit, that very bulk is quite necessary for the high compression ratios that allow the whole process to work. With compression ratios of about 25:1 (compared to about 9:1 for gasoline engines), the cylinder pressure is immense and the stress placed on all the reciprocating mass inside the engine is tremendous.
The benefit? An extraordinary amount of torque is generated at relatively low RPMs (revolutions per minute), giving rise to diesel engines’ hallmark fuel efficiency. With that much force, a vehicle is pushed along a little farther with each stroke of the piston than it would be with a comparable gasoline counterpart. Pound for pound, it’s hard to beat the “bang for your buck” you get with a diesel.