- Explain the purpose of a ‘Heat Exchanger’ and list two (2) common types of engine water cooling systems.
A heat exchanger allows the internal fresh water circulating through the engine to be cooled by the sea water which is passed through a system of rods (tube nest) in the heat exchanger. In most engines raw water enters through a sea cock (intake) and strainer, through the raw water pump to the heat exchanger and then overboard: Tube Nest Cooling. Another method is where the fresh water passes through a number of pipes along the keel of the vessel. This fresh water is cooked by sea water as it passes through the pipes.
- List 6 advantages Diesel Engines have over other alternative power sources such as LPG or Petrol.
- Proven performance and reliability under working conditions.
- Lower fuel consumption in relation to output.
- Greater torque for longer periods.
- Diesel engines generally burn fuel more efficiently.
- Diesel fuel is less flammable than petrol ( although diesel fuel will still ignite and burn)
- Fumes from diesel engines exhaust are less toxic and polluting than those emitted from petrol engines.
- Explain a complete cycle of a 2 stroke engine:
- Exhaust and inlet - as the piston approaches the bottom of its stroke ( BDC Bottom Dead Centre) the exhaust valve is open and exhaust gases escape. As the piston moves to the BDC position, inlet air drives in from uncovered ports forcing out any remaining exhaust gases and filling the cylinder with clear air.
- Compression and power - as the piston passes the BDC position, exhaust valves and inlet ports are sealed with the cylinder full of fresh air. As the piston moves upward to the top of the stroke (TDC Top Dead Centre) the air becomes compressed, igniting fuel which is injected into the top of the cylinder. The exhaust gases then expand rapidly forcing the piston back down the cylinder.
- Explain a complete cycle of a 4 stroke cycle engine:
- Induction - The exhaust valve is closed and the inlet valve is opened the first downward movement of the piston sucks fresh air into the cylinder.
- Compression- Both inlet and exhaust valves are closed. On the first upward movement of the piston the air is compressed. This heats the air. The fuel is injected just before the piston reaches the TDC ( Top Dead Centre)
- Power- Both inlet and exhaust valves remain closed. The piston is forced down when the fuel ignited in the cylinder.
- Exhaust- The inlet valve is shut and the exhaust valve is opened. The final stroke occurs when the piston expels the exhaust gases out of the cylinder.
- What is the purpose of lubrication?
- Prevent contact between metal surfaces.
- Reduce friction wear and heat.
- Remove foreign agents ( metal shavings etc) which might otherwise damage parts.
- In regards to fuel pumps, what regulated the amount of fuel that is injected into each cylinder?
The amount of fuel injected into the cylinder is determined by the movement of the fuel rack on the fuel pump, which is attached to each injector. As the fuel rack is rotated, more or less fuel is pumped into the cylinder.
The fuel rack is also attached to the governor, which reacts to changes in the engine speed and moves the fuel to rack to compensate the flow of fuel. The fuel rack is also used to close off the fuel supply and as a result will stop the engine.
The pump can incorporate a governor to maintain a set speed, with fluctuations in load, by metering the amount of fuel.
- In what condition is the fuel when it enters the cylinder from the fuel injector? What is the purpose of this?
The fuel enters the cylinder in an atomised form so it will mix completely with the hot compressed air and ignition can take place with efficient combustion.
- Explain the purpose of a turbo charger.
A turbo charger causes a larger mass of air into the cylinder to that of a same cubic capacity normally aspired engine. This allows for a portion increase in the amount of fuel that can be injected and burnt in the cylinder thereby providing an increase in the power output of the engine.
- Explain the purpose of a Heat Exchanger and list two common types of engine water cooling systems.
A heat exchanger allows the internal fresh water circulating through the engine to be cooled by the sea water which is passed through a system of rods ( tube nest) in the heat exchanger. In most, raw water enters through the sea cock ( intake) and strainer, through the raw water pump to the heat exchanger and then overboard. Another method is keel cooling where the fresh water passes through a number of pipes along the keel of the vessel. The fresh water is cooled by sea water as it passes through the pipes.
- In a tube nest cooling system, 2 pumps are used to circulate water. Explain the purpose of each pump.
The first pump circulates fresh water through the internal area of the engine. The second pump circulates sea water to cool the fresh water.
