A diesel engine ignites fuel with heat created by compression. It does not normally use spark plugs.
The four strokes
- Intake: the piston moves down and the intake valve opens, drawing in air.
- Compression: both valves close and the piston moves up. Compressing the air sharply raises its temperature.
- Power: near the top of the stroke, the injector sprays finely atomised diesel. It ignites in the hot air and expanding gas pushes the piston down.
- Exhaust: the exhaust valve opens and the piston pushes combustion gas out.
The connecting rod turns this reciprocating motion into crankshaft rotation. A four-stroke cylinder produces one power stroke for every two crankshaft revolutions. Multiple cylinders overlap their firing events, making the engine smoother.
Why a diesel suits a cruising boat
Small marine diesels deliver useful low-speed torque, have no high-voltage ignition system, and can run efficiently for long periods at a suitable load. Their reliability still depends on clean fuel, unrestricted air, correct temperature, steady lubrication and a propeller that allows the engine to reach its specified operating range.
Mechanical versus electronic control
An older mechanically governed engine meters fuel with a mechanical injection pump and injectors. A modern common-rail engine uses an electronically controlled high-pressure rail and precisely timed injectors. Common rail can improve control, emissions, noise and efficiency, but it adds sensors, wiring, control modules and injection pressures dangerous enough to penetrate skin. Never crack open a high-pressure line to “see if fuel is coming through”.
Continue with mechanical versus common-rail engines and horsepower, torque and rpm.