(a) List FOUR safety interlocks that may be fitted to a direct air start medium speed diesel engine to prevent inadvertent starting during maintenance.(4)
(b) Explain, with the aid of a diagram, the opening period of an air start valve on a four stroke diesel engine in relation to the crank angle, inlet and exhaust valves.(6)
Safety Interlocks for Direct Air Start Medium Speed Diesel Engine
To prevent inadvertent starting during maintenance, several interlocks are typically fitted to a direct air start medium speed diesel engine:
Mechanical Interlocks
- Turning gear engagement: The engine cannot be started while the turning gear is engaged.
- Cylinder head covers: Removing cylinder head covers can trigger an interlock preventing engine start.
Electrical Interlocks
- Master switch: A dedicated switch must be in the “maintenance” position to disable the start sequence.
- Safety interlock switches: These can be located at various points around the engine, such as control panels or access points.
Pneumatic Interlocks
- Air supply isolation: The main air supply can be isolated using valves with interlock switches.
Additional Considerations
- Clear signage: Warning signs should be prominently displayed indicating that maintenance is in progress and the engine must not be started.
- Lockout/tagout procedures: Implementing a formal lockout/tagout system can further enhance safety.
By incorporating these interlocks, the risk of accidental engine start-up during maintenance is significantly reduced. It is essential to regularly test and maintain these safety devices to ensure their effectiveness.
Opening Period of an Air Start Valve
The opening period of an air start valve on a four-stroke diesel engine is typically around 10-15 degrees before Top Dead Center (BTDC) and closes around 120-150 degrees After Top Dead Center (ATDC).
Factors influencing the opening period:
- Engine size and type: Larger engines may require longer opening periods.
- Compression ratio: Higher compression ratios might necessitate earlier or later valve opening.
- Starting air pressure: Higher pressure can reduce the required opening period.
- Engine design: The specific characteristics of the engine’s combustion chamber and piston shape influence the optimal timing.
Relationship to Inlet and Exhaust Valves:
- Overlap: To ensure effective starting, there might be a slight overlap between the closing of the air start valve and the opening of the exhaust valve. This helps to clear the cylinder of residual exhaust gases.
- Inlet valve: The air start valve typically closes before the inlet valve opens to prevent mixing of starting air and fresh air.
It’s important to note that these are general guidelines, and the exact timing can vary depending on the engine manufacturer’s specifications.