Although a diesel operates on a four-stroke cycle just like a gas engine, there are major differences in each stroke:
• Intake Stroke: Diesel engines have an open intake manifold, so a full charge of air can be drawn into the cylinders on every intake stroke. Gas engines have a throttle valve that regulates how much air-which is mixed with fuel in proportion to the air-is drawn into the cylinders. In its simplest terms, an engine is an air pump; more air moving through the engine results in more power.
The key difference between gasoline and diesel engine designs is that gasoline powerplants intake a gas/air mixture, then ignite it with a spark. Diesel engines take in air, compress it, squirt fuel directly into the cylinder, and-kaboom! Combustion is triggered by the heat from the compressed air.
• Compression Stroke: Gas engines typically run a compression ratio of 8:1 up to 10:1. Diesel engines can run over 20:1. Higher compression facilitates efficient burning of fuel, hence more power and better economy. Gas engines cannot run high compression because the heat would cause the air-fuel mixture to self-ignite prematurely causing a power loss.
• Ignition and Power Stroke: Gas engines ignite the air-fuel mixture with a spark plug. Diesel engines inject a calibrated amount of fuel into the compressed air, where heat from the internal combustion chamber ignites the fuel. With a gas engine, the combustion is more controlled, but the burning process is limited. With a diesel, the instant combustion is more violent, lasts longer (depending on the length of the fuel-injection pulse), and literally pounds the cylinder walls and pistons, resulting in more noise. Diesel engines are also designed to have a longer crankshaft stroke than gas engines. The geometric principles are complicated, but a longer stroke combined with the longer burning results in considerably more torque.
• Exhaust Stroke: Both engines are similar in that the piston forces hot gasses into the exhaust manifold. But most diesel engines are equipped with turbochargers that use exhaust gasses to spin a turbine to pump large amounts of fresh air back into the intake manifold. Today's factory-installed turbochargers on diesels can provide 30 pounds per square inch of boost. Gas engines can hardly handle much more than 15 psi, unless they're modified.