BASIC AIR START SYSTEM

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Receiver stop valve - manual isolation of the whole system. Usually a slow opening valve to prevent pressure surge and explosions.

Turning gear interlock - prevents admission of air to the control lines in the event that turning gear is engaged thus preventing attempted start.

Master/pilot valve - operated by start air lever to allow passage of control air to operate automatic valve and load up distributor slide valves.

Automatic valve - opens and closes automatically to charge air start manifold as starting is required. Reduces manual effort required and conserves air during start. Often incorporates a non return valve which prevents passage of high pressures from start manifold to the receiver. If not included in the auto valve then a separate N.R. valve must be fitted.

Distributor - times admission of pilot air to operate cylinder air start valves in correct sequence, so that engine starts in the desired direction.

Cylinder air start valves - admit starting air to individual cylinders when operated by distributor pilot air.

Air manifold relief valve - relieves excess pressure in manifold.

Flame trap - prevents passage of flame from cylinder to manifold in the event of a jammed cylinder air start valve.

Bursting cap - ruptures to relieve excess pressure in the event of a start air line explosion.

Operation of system

The receiver stop valve is opened and air passes to the automatic valve and to the master valve (assuming turning gear is disengaged). Both valves remain closed at this stage.

When the start lever is moved to the start position the master valve opens allowing control air to pass to the distributor slide valve loading chambers and to the automatic valve operating chamber.

The automatic valve is now open and air passes to each cylinder air start valve and to the distributor. Depending upon distributor position, pilot air will pass to either one or two of the cylinder air start valve operating chambers opening the valves and allowing start air to enter the cylinders.

As the engine rotates the distributor passes pilot air in the correct sequence to each cylinder air start valve until minimum cranking speed is reached. At this point start air admission is ended and the fuel is put on and the engine will run.

Overlap

To ensure that the engine will start in any position it is necessary to incorporate overlap in the start air valve operation. This means that before one valve closes the valve for the next cylinder in firing sequence must be open. Typically, the overlap should be about 15 degrees of crank rotation.

The potential air admission period (AAP) for each cylinder is from a point just after TDC to the point at which the exhaust is open. The required air admission period for a 2-stroke engine is given by:

AAP =           360             + Overlap

                                                        No. of Cylinders

For engines with four cylinders or less this can cause problems, depending on the exhaust timing, requiring the use of some form of starting assister.

Where there are a large number of cylinders involved such that the required AAP is much less than the potential AAP then it is common to find that some cylinders are not used for air start purposes. With V type engines this may mean that only one bank of cylinders is fitted with air start equipment.