Question # 65

How are engine-starting air valves operated? What attention do they require?

How are engine-starting air valves operated? What attention do they require?

By: Dimitros
on Sept. 30, 2016, 10:55 a.m.

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Answer(s):

The body of a starting air valve is a casting, the upper part of which is consider-ably larger than the lower part which fits into the engine cover. This larger part forms an air cylinder, and the piston working in this cylinder is fitted to the starting air valve spindle. The cover of the air cylinder on the starting valve is
connected through small-bore piping to the distributor valves. When a distri¬butor valve opens, compressed air flows through the piping to the upper side of the piston in the starting air valve, forcing the piston downwards and opening the starting air valve, which is connected to the starting air rail. A spring fitted within the valve under the piston closes the valve when the distributor valve closes.
The underside of the actuating piston and cylinder in the starting air valve is open to the atmosphere so that any air leakage from the actuating piston leaks off to atmosphere; air pressure cannot build up under the piston and cause sluggish action of the valve. The upper part of the valve spindle is sealed with small rings similar to piston rings and any air leakage from this part also goes direct to atmosphere.
It is good practice to lubricate the actuating piston in a starting air valve prior to manoeuvring the engine and at the time of engine shut-down, but care must be exercised to avoid over-lubrication. The lubricants used may be either grease or lubricating oil, as specified by the engine builder. After certain periods of service starting air valves are changed and orverhauled. Care must be taken to ensure that piston rings are free in their grooves. Should it be necessary to fit new rings, the end clearances of the rings must be carefully checked because they are usually made of brass which has a larger coefficient of expansion than the other parts of the valve. The valve and valve seat are ground with grinding paste and finished to a fine surface with lapping paste. It is essential to ensure- that all parts of the valve are scrupulously clean before reassembly.
After overhaul the tightness of the seat must be tested. In valves that are removable from the cylinder cover it is usual to connect the valve to an air supply and immerse the end of the valve in paraffin oil or clean water; valve leakage is then shown by a flow of air bubbles through the liquid. When the starting air valve parts are integral with the engine cylinder heads and the heads are off the engine, the valve seat tightness (when under air pressure) can be checked by the application of a solution of soapy water.
A modern type of starting air valve is shown in Fig. 11.1. This valve is opened by air supplied from a pilot valve. Instead of having a single piston as in the valves described above, the valve shown has three pistons. The air from the pilot valve passes down through the axial hole in the valve spindle. The air then passes through the radial holes shown and creates a force which acts on the three pistons and opens the valve. The spring situated round the lower part of the spindle causes the valve to close when the automatic valve shuts off air supply to the pilot valves. The engine starting air after leaving the automatic valve passes through the ports and across the valve seat into the cylinder when the valve opens. The starting air entering the engine cylinders exerts a force on the pistons and causes the pistons to move and set the engine in motion.
By: Dimitros
on Sept. 30, 2016, 10:55 a.m.

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