12.10.12. Explain the basic differences between compression ignition (diesel) engines and conventional ignition engines.
In a diesel engine the fuel is injected directly into the cylinder during the compression stroke and ignites with the heat created by compression. There are no spark plugs.
In a conventional ignition engine fuel air mix enters via the inlet valve and is ignited by a spark plug during the compression stroke.
12.10.10. Explain the need for valve timing (i.e. valve lead, lag and overlap).
Because of the inertia of the fuel/ air mix travelling through the manifold to enter the cylinder, it has been found that better performance is obtained if the inlet valve is opened before top dead centre on the exhaust stroke ( valve lead). The exhaust valve is still open and only closes after the intake stroke has begun (valve lag). This lead on the inlet valve and overlap with both valves open allows the departing exhaust gases to help the inlet charge to start moving.The inlet valve closes after the start of the compression stroke (valve lag) because the inertia of the incoming gases means that they are still entering the cylinder for a time after the intake stroke is complete. Both valves are closed for the rest of the compression stroke and the power stroke, but the exhaust valve opens just before the end of the power stroke (valve lead).
12.10.8. Describe the correlation between engine rpm and power output.
Engine RPM – the higher the RPM the greater the power output
12.10.6. Explain the basic principle of operation of a four-stroke internal combustion engine.
The four stroke engine operates on what is known as the Otto Cycle.
1. Intake – inlet valve open
2. Compression – both valves closed
3. Power – both valves closed – mixture ignited
4. Exhaust – exhaust valve open
12.10.4(j) . injectors.
12.10.4(i) . spark plugs;
12.10.4(h) . camshaft;
12.10.4(g) . valve operating mechanism;
12.10.4(f) . valves;
12.10.4(e) . crankshaft;