In a conventional marine diesel engine the Diesel Engine overhauling power is produced by hot compressed air igniting fuel sprayed under very high pressure into the cylinder head. A marine diesel engine does not use a carburetor to mix fuel and air or spark plugs to ignite the mixture. Instead it employs the pistons to compress the air to 3000 kPa which causes it to become extremely hot and the fuel is ignited as soon as it is injected into the cylinder.
Some marine diesel engines are fitted with a heater plug in the inlet manifold or a glow plug in the pre-combustion chamber of each cylinder to provide additional heat to the combustion air during starting.
Diesel engines are heavier and slower revving than petrol engines but they are also more reliable because they do not rely on external carburetion or an electrical spark for ignition.
Newer engines use an electronic fuel injection system whereby fuel and air are mixed more thoroughly in the pre-combustion chamber before entering the cylinder. This system maximizes power and fuel economy and is also less polluting.
Every boater should have an understanding of how their engine works so let’s start by explaining the mechanical cycles.
Most reciprocating piston internal combustion engines work on one of two mechanical cycles-either the four-stroke cycle or the two-stroke cycle. These cycles designate, in correct sequence, the mechanical actions by which the fuel and air gain access to the engine cylinder, the gas pressure – due to combustion – is converted to power and, finally, the burnt gas is expelled from the engine cylinder.
The Basic Four-Stroke Diesel Engine
From its name, it is obvious there are four strokes in one complete engine cycle. A stroke is the movement of the piston through the full length of the cylinder and – since one such movement causes the crankshaft to rotate half a turn – it follows that there are two crankshaft revolutions in one complete engine cycle.
The four strokes in the order they occur are:
1. Inlet stroke. With the inlet valve open and the exhaust valve closed, the piston moves from top dead center (TDC) to bottom dead center (BDC), creating a low-pressure area in the cylinder. Clean, filtered air rushes through the open inlet valve to relieve this low-pressure area, and the cylinder fills with air.
2. Compression stroke. With both valves closed, the piston moves from BDC to TDC, compressing the air. During this stroke the air becomes heated to a temperature sufficiently high to ignite the fuel.
3. Power stroke. At approximately TDC, the fuel is injected, or sprayed, into the hot, compressed air, where it ignites, burns and expands. Both valves remain closed, and the pressure acts on the piston crown, forcing it down the cylinder from TDC to BDC.
4. Exhaust stroke. At approximately BDC the exhaust valve opens and the piston starts to move from BDC to TDC, driving the burnt gas out of the cylinder through the open exhaust valve.
The Two-Stroke Diesel Engine
The two-stroke engine uses two piston strokes to complete one power stroke and, therefore, fire twice as often as a four-stroke engine. A two-stroke engine is smaller and simpler with fewer moving parts. A two-stroke engine has the potential to produce twice as much power as a four-stroke engine of the same size, however, because of the extra fitting required in a two-stroke diesel engine, for example blowers and governors, they become more expensive to produce. There has been a shift towards four stroke diesel engines which have become more efficient and smaller.
Protect Your Marine Diesel Engine
Protect your engine by avoiding long periods (more than 10 minutes) of idling in a “no-load” situation. This is often done to charge batteries or cool refrigeration but if done repeatedly it will glaze the bores of the engine and cause premature engine failure. If the vessel is in a berth the engine can be put in gear to create load at idle.
All boat owners should have an understanding of basic marine diesel engine maintenance to keep themselves and their families safe on the water.