Airflow and Fuelflow through a Four-Stroke Diesel Engine
Air flowing into a four-stroke diesel engine gets cleaned as it passes through the air filter. It then flows through tubing until it is compressed in the spinning blades of the turbo. As a result, the air becomes denser and hotter, so it gets cooled down in an intercooler. The intercooler is connected to the air intake of the engine with hoses. As the piston slides to the bottom of its stroke, the combustion chamber fills with air due to the opened intake valve. This is known as the intake stroke. The intake valve(s) close and the piston pushes the air up toward the cylinder head. During this phase, known as the compression stroke, the air takes up approximately 1/16 the space it did before.
A pump (either electrical or mechanical, located in the tank or on the framerail) sends fuel at a low pressure to the injection pump. The injection pump boosts the pressure significantly up to 17,000-30,000 psi. Fuel is then injected into the combustion chamber (filled with superheated air) at a tremendous amount of pressure just before top dead center. The resulting combustion pushes the piston back down. This is called the power stroke. The final cycle happens when the exhaust valve(s) opens and the piston pushes the exhaust out. The exhausted air still has plenty of energy to push the exhaust side of the turbo. Next, the air enters into the exhaust and exits out the tailpipe.
Combustion Ignition
Combustion ignition is a key characteristic of a diesel engine, and the easiest way to explain it is with a fire piston. These ancient fire-starting devices consisted of a piston with a recessed tip, and an air-tight cylinder. When the two are quickly pushed together, the temperature of the air in the cylinder rises high enough to combust a piece of tinder put on the end of the piston. The diesel engine uses the same principle as the fire piston, only on a much larger and more complex scale.
If you have a head for figures, the equation PV=nRT is very useful. This equation defines the relationship between pressures (P), volume (V), amount of gas present measured in moles (n), the universal gas constant (R), and temperature (T). As pressure increases in the cylinder, so does temperature. So when the piston compresses the air inside the cylinder to 1/16 of its original volume, the temperature inside the cylinder exceeds 400 degrees. This is enough heat and pressure to make diesel fuel ignite without the use of spark plugs.
A Closer Look at Diesel Combustion
One of the main differences between a gasoline engine and a diesel engine is the type of combustion performed. A diesel's burn is very complex and uses the same principle as a candle, where fuel and air mix as a result of the combustion. Convection currents and turbulence play a large role in how the unmixed (heterogeneous) fuel is burned. A gasoline engine, on the other hand, mixes the fuel and air completely (homogenous) well before its comparatively simple combustion. One of the downsides to injection gasoline engines is that when the piston compresses the fuel air mixture, some of it gets stuck in the imperfections of the cylinder walls. That is why gasoline engines have higher carbon monoxide (CO) and hydrocarbon emissions compared to diesel engines.