The engine converts fuel energy into rotating force, but its supporting systems keep it alive.
| System | Main parts | What failure usually looks like |
|---|---|---|
| Fuel | Tank, pickup, shut-off, primary filter, lift pump, secondary filter, injection system, return | Hard starting, surging, power loss, smoke |
| Air | Intake, filter or silencer, manifold, turbo and intercooler if fitted | Black smoke, low power, high exhaust temperature |
| Lubrication | Sump, pickup, oil pump, galleries, filter, cooler | Low pressure, wear, overheating, seizure |
| Closed cooling | Coolant pump, passages, thermostat, heat exchanger, expansion tank | Overheating, coolant loss, corrosion |
| Raw water | Seacock, strainer, pump, impeller, heat exchanger, exhaust injection elbow | Weak exhaust flow, overheating, hose damage |
| Exhaust | Mixing elbow, waterlock, hose, muffler, gooseneck, outlet | Fumes, water ingress, restriction, noise |
| Starting and charging | Battery, isolator, fuse, cables, starter, alternator, relays | No crank, slow crank, flat battery, charging alarm |
| Propulsion | Coupling, gearbox, shaft or saildrive, bearings, propeller | Vibration, slipping, poor thrust, leaks |
Inside the block are pistons, connecting rods, crankshaft, camshaft, valves and cylinder head. Compression raises air temperature enough for injected diesel to ignite. Bearings carry rotating loads on a film of oil. The cylinder head seals combustion pressure and controls intake and exhaust flow. The flywheel smooths the pulses and transfers torque to the transmission.
Most yacht auxiliaries use a heat exchanger. Coolant circulates inside the engine while seawater removes heat from the coolant and then usually enters the wet exhaust. The fluids should never mix. Oil in coolant, coolant in oil or seawater in a cylinder indicates a serious breach.
Learn more: How combustion works · Fuel systems · Cooling systems · Wet exhausts