2-stroke engine is reversible and 4-stroke engine is irreversible generally. Propeller thrust must be reversible for manoeuvring. Manoeuvring is easy in 2-stroke engine, because 1 rotation of crankshaft makes 1 rotation of camshaft also.
Compressed air (for air starting engines), 4-stroke engines can be started by various methods
Cooling water (s.w/f.w)
Starting Period: minimum period to produce sufficient torque for starting the engine. Exhaust valve opening decides this period.
Overlap: Overlapping for air-starting is also quintessential.
It’s the period in which 2 or more cylinders receive compressed air.
To start the engine from any position of crank, there should overlap.
It depends on starting air period, exhaust timing and number of cylinders.
Up to 3 cylinders, there are no overlap between cylinders.
Simply, if we have 3 cylinders then for 2-stroke each cylinder covers (360/3) degree i.e. 120. It is also called firing interval.
For overlap, (Firing interval < Starting period), here firing interval (120 degree) is not less than starting period (105 degree).
That’s why it can’t be started by compressed air.
Hence, for air-starting there should be 4 or more cylinders.
Equipment: (Components in system)
Air bottle – for storage of compressed air
Air compressor – for generation of compressed air
Distributor – for control of air starting valve
Starting air valve – to supply air into engine
Safety items (safety v/v, relief v/v, fusible plug, non-return v/v, interlock etc.)
Starting period = 105 degree
Number of cylinder > 3
Pressure of compressed air = 30 bar
Control air pressure = 8 bar
Main air pressure = 30 bar
Starts should available for reversible = 12
Starts should available for irreversible = 6
Slow turning = 5 to 8 rpm
For new engine, sufficient pressure for auto-ignition = 12 bar
Mean piston speed (mps) – sufficient to move piston with certain desired speed
Compression ratio – sufficient for auto-ignition
All pumps related to engine and its auxiliaries should work
Valve (air starting v/v) must close before exhaust valve open
Compressed air stored in Air bottle with the help of Air compressor, is admitted in the multi-cylinder (>3) engine.
There are two air lines (pilot & distributor), one goes to cylinder head to open & close the air starting valve.
Pilot air which opens the valve is called as Control air and distributor air is also called main air.
Pilot air operate both the opening-closing of valve and distributor.
Another line (distributor) air goes engine to push the piston.
Movement of piston (reciprocating motion) produces torque for rotation of crankshaft.
This torque should be sufficient to rotate the crankshaft which finally builds sufficient compression for auto-ignition.
At sufficient compression, auto-ignition occurs and engine will start.
After starting, no use of starting air valve or compressed air.
Each cylinder head is equipped with air starting valve.
Valve opens according to firing sequence.
Air distributor is opened by a cam in accordance with engine firing sequence.