RYA Diesel Engine Course

The four strokes

1. Induction (intake): The piston moves down the cylinder and the inlet valve opens. Air alone is drawn into the cylinder — there is no fuel mixed in at this stage.
2. Compression: Both valves close and the piston moves back up, compressing the air to roughly 20:1 (compared with about 10:1 in a petrol engine). This compression heats the air to approximately 500-700 degrees Celsius.
3. Power (combustion): Near the top of the compression stroke, the injector sprays a fine mist of diesel fuel into the superheated air. The fuel ignites instantly without any spark. The expanding gases force the piston down, turning the crankshaft and producing power.
4. Exhaust: The exhaust valve opens and the piston moves back up, pushing the burnt gases out through the exhaust manifold and into the exhaust system.

Fuel flow path (in order)

1. Fuel tank — stores the diesel. Usually fitted with a shut-off valve at the outlet. Tanks should have inspection hatches for cleaning.
2. Primary fuel filter / water separator — the first line of defence. This filter removes water and large particles from the fuel. Many have a transparent bowl so you can see if water has collected. Water is denser than diesel and sinks to the bottom of the bowl.
3. Lift pump (fuel feed pump) — a low-pressure mechanical or electric pump that draws fuel from the tank and pushes it to the injection pump. Some systems rely on the injection pump alone to draw fuel.
4. Secondary fuel filter (fine filter) — removes smaller particles that passed through the primary filter. Typically a paper element that must be replaced at service intervals.
5. Injection pump — a high-pressure precision pump that delivers exactly the right amount of fuel to each injector at exactly the right moment. This is a complex, expensive component — do not tamper with it.
6. Injectors — precision nozzles that spray an atomised mist of fuel directly into the combustion chamber at very high pressure (typically 150-200 bar). The spray pattern must be correct for efficient combustion.
7. Fuel return line — excess fuel not used by the injectors returns to the tank via a return line. This also helps cool the injectors.

Raw water (direct) cooling

• Seawater is drawn directly through the engine block and cylinder head to absorb heat, then expelled through the exhaust
• Simpler system with fewer components
• Found on older and smaller engines
• Disadvantage: salt water causes internal corrosion and scale build-up over time, reducing engine life
• The engine runs at whatever temperature the seawater allows — there is no thermostat control

Indirect (freshwater) cooling

• The engine block is cooled by a closed freshwater circuit (just like a car engine), containing a mixture of fresh water and antifreeze/corrosion inhibitor
• A heat exchanger (similar to a small radiator) transfers heat from the freshwater circuit to the raw water circuit
• The raw water passes through the heat exchanger, absorbs heat, and is then injected into the exhaust
• A thermostat in the freshwater circuit maintains optimal engine temperature (typically 75-85°C)
• This is the most common system on modern marine diesels — it protects the engine internals from salt water corrosion

Battery types found on boats

• Lead-acid (flooded/wet cell) — the traditional type. Affordable, well-understood, but requires maintenance (checking electrolyte levels, topping up with distilled water). Must be kept upright. Produces hydrogen gas when charging, so needs ventilation.
• AGM (Absorbed Glass Mat) — sealed, maintenance-free, spill-proof, can be mounted in any orientation. More expensive but longer-lasting and better suited to the marine environment. Handles deep discharge better than standard lead-acid.
• Gel — similar advantages to AGM but more sensitive to charging voltage. Less common on boats.
• Lithium (LiFePO4) — lightweight, very high energy density, excellent deep-cycle performance. Significantly more expensive and requires a battery management system (BMS). Increasingly popular on modern yachts.

Typical battery bank setup

• Starter (engine) battery — dedicated to starting the engine. Usually a high-cranking-capacity battery. Should never be used for domestic loads.
• Domestic (house) battery bank — powers lights, instruments, fridge, chartplotter, autopilot, etc. Often consists of two or more batteries wired in parallel for greater capacity.
• Battery switch — a rotary switch (1 / 2 / Both / Off) or individual isolator switches that allow you to select which bank is active. The 'Both' position parallels the banks for emergency starting if the starter battery is flat.

Pre-start checks (do these every time)

1. Engine oil level — check the dipstick with the engine cold and the boat level. Top up if below the minimum mark. Use the correct grade of oil specified in the engine manual.
2. Coolant level (indirect-cooled engines) — check the freshwater header tank. The level should be between the min and max marks when cold. If it has dropped significantly, investigate for leaks before just topping up.
3. Raw water seacock — confirm it is fully open.
4. Raw water strainer — visually inspect through the transparent lid. Clean if any debris is visible.
5. Drive belts — check tension and condition. Look for fraying, cracking, or glazing.
6. Visual inspection — look around the engine for oil or water leaks, loose hoses, chafed wires, or anything that looks different from normal.
7. Bilge — check for unusual amounts of oil or water under the engine. A small amount of drips is normal on some older engines, but a sudden increase indicates a problem.
8. Fuel level — check you have sufficient fuel for your passage with a reserve.