To be able to understand the diagrams it is necessary to understand how the pneumatic symbols "work". For those students unfamiliar with them, the following explanation should help.
A page of basic pneumatic symbols can be found here http://www.rosscontrols.com/symbols2.htm
With reference to the diagram above: The isolating valve from the air receiver is open. Air at 30 bar flows to the automatic valve (4) which is closed. Air also flows to control valve (2) which is also closed. As long as the turning gear is out, air can also flow to the start control valve (1)
When a start signal is applied to the control valve (1) an air signal operates control valve (2). The air can now flow to the pilot valves, to control valve (3) which vents the automatic valve (4) which now opens allowing air to flow to the air start valves mounted in the cylinder head. Air will also flow to the governor booster servo, pulling the fuel pump control linkage to the zero position so that no fuel is injected whilst start air is admitted to the cylinders. When the pilot valve cam is in the correct position (i.e. as the piston comes over TDC on the power stroke), air flows through the pilot valve to the operating piston of the air start valve opening the valve and allowing the compressed air into the cylinder forcing the piston down and turning the crankshaft. Just before the exhaust valve opens, the pilot valve will vent, and the air start valve will close. By this time another air start valve will be open, allowing the engine to accelerate to firing speed, at which valve (1) will vent, control valve (2) will vent, the automatic valve (4) will shut and the governor will operate the fuel racks to allow fuel to be injected into the cylinders.
The emergency stop shown is independent of the governor stop signal. It can be operated by hand or from the engine shut down system (overspeed etc). If operated, an air signal opens a valve allowing compressed air to servo cylinders at the back of the fuel pumps which zero the fuel racks.
THE AIR START PILOT VALVE