In this type of indicator, the cylinder gases enter past a valve into a chamber which leads to a gage. When the pressure above the valve equals that of the cylinder, the valve seats and traps the gas above the valve at its highest pressure, then this pressure is read on the gage. There are several other types of indicators. The one pictured in figure 3-2 is used to take compression readings

Figure 3-2.Trapped Pressure Indicator (small boat).

only on engines installed on small boats. Engines like the GM-6-71 do not have indicator cocks installed.

When taking compression readings on a 6-71 engine, you will perform the following steps:

1. Check the manufacturers technical manual for the minimum compression pressure required for the engine.

2. Start the engine and run it at approximately one-half the rated load until normal operating temperatures are reached.

3. Stop the engine and remove the fuel pipes from the injector and the fuel connectors on the cylinder to be tested.

4. Remove the injector and install the indicator adapter, with pressure gage attached, and use the crab nut to hold the adapter in place.

5. Use a space fuel pipe to fabricate a jumper connection between the fuel inlet and the return manifold connectors to by-pass fuel to and from the injector.

6. Start the engine again and run it at approx-imately 600 rpm.

7. Observe and record the compression pressure as indicated on the gage.

Another type of trapped pressure indicator is the Kiene indicator (figure 3-3). This indicator is basically a Bourdon gage connected to a cylindrical pressure chamber. The pressure chamber contains a check valve which allows the gas to

Figure 3-3.Trapped Pressure Indicator.

flow from the engine into the chamber until the pressures are equalized. This gage is attached to the chamber and the pressure is read directly. The check valve is an inverted piston seating on a seat piece. The valve moves up and down in a guide. A stop nut is used to adjust the travel of the check valve.

Most of you should become familiar with this indicator since it is widely used to check both the compression and firing pressures on main diesel engines and emergency generator diesel engines. Review figure 3-4A and B. It is a PMS situation requirement to be performed when the engine operating conditions indicate problems.