Diesel engine parts are usually heavier or more rugged than those of similar output gasoline engines. Their engine blocks and cylinder blocks are usually made of cast iron.
In a diesel engine, just air enters the combustion chamber first. It is then highly compressed. Fuel is injected and ignites due to heat of the compressed air. That's why diesels are called compression-ignition engines.
In direct injection fuel is injected directly into the combustion chamber. In indirect injection fuel is sprayed into a smaller separate chamber in the cylinder head. A glow plug helps the combustion start.
Most valves in diesels are parallel to the center-line of the engine. Small 4-stroke engines usually have one inlet and one exhaust valve per cylinder. Larger 4-stroke diesels may have two of each per cylinder.
Valves in diesel engines are usually operated by a pushrod system. The camshaft is mounted in the engine block near the crankshaft. It keeps the valves working in sequence.
In uniflow scavenging air flows towards top of the cylinder. In crossflow scavenging air enters one side of cylinder, exhaust gases exit on the other. In loop scavenging air flows up in a loop across the top of the piston.
In a 4-stroke cycle, only one stroke in four delivers energy. In a 2-stroke, it is only one stroke in two. A flywheel stores this energy to help turn the crankshaft through the non-power strokes.
Clearance between the piston and its cylinder wall must be kept small. This is done by piston rings - expandable metal rings held in grooves in the side of the piston.
