marinediesels.co.uk - Members Section

Engine Assessment

Balancing An Engine Using Peak Pressures

 

 

Indicator power cards cannot be taken using mechanical equipment on a medium speed engine because the engine is running at too fast and vibrations set up in the drive will give an erratic irregular diagram. If a computer system is not fitted, then a degree of engine balancing can be achieved by looking at the compression pressures and the maximum cylinder pressure.

 

It should be understood that power developed in the cylinder is directly related to the Mean Indicated Pressure (MIP), and not the peak pressure, which is related not only to the quantity of fuel burnt but the injection timing. However if the timing of the fuel injection is correct, and combustion conditions correct within the cylinder, then balancing of the engine can be successfully carried out.

 

It may be possible to take a set of draw cards to ensure that injection timing and cylinder combustion is correct. However if this is not possible, then an assumption has to be made that conditions are correct and that injection equipment is in good condition. Obviously visible signs of poor or incorrect combustion such as smoke at the funnel or excessively variable exhaust temperatures should be investigated before a set of readings are taken. The turbocharger should be clean and operating efficiently, with the inlet air temperature and pressure at the normal parameters.

 

To measure pressure in the cylinder, a peak pressure indicator is used. This can be a gauge type indicator as shown below or a more sophisticated digital gauge which will measure max peak pressure, minimum peak pressure and average peak pressure over a number of cycles It will also measure the rate of pressure rise, which if too high can damage bearings. It is also possible to use a traditional mechanical indicator to trace a vertical line up the paper, the height of which is proportional to the max cylinder pressure.

 

 

The indicator is installed on the indicator valve using the wing nut & plug (1). When the indicator valve is opened, the check valve (4) will rise and fall with the engine cylinder pressure, trapping the maximum pressure in the pressure chamber (2), and this average maximum pressure can be read directly on the dual scale pressure gauge (3). The gauge may be conveniently positioned for ease of reading by loosening the gauge bolt wing nut (5). The pressure in the pressure chamber (2) and on the pressure gauge (3) is removed by loosening the wing nut (5).

 

 

Shown opposite: A Kistler Digital Peak Pressure Meter

 

Measuring functions:

pmax

maximum peak pressure

 

pmin

minimum peak pressure

 

pav

average peak pressure

 

Sdev

standard deviation of the peak pressure

 

dp/ca

maximum gradient of the pressure trace

 

r/min

speed

 

ppeak

instantaneous peak pressure of continuous measuring function

 

 

Before taking the maximum cylinder pressures, the compression pressures should be recorded. This is done by measuring the cylinder pressure with the fuel taken off the unit. To do this, the engine should be run at reduced load to avoid stressing the crankshaft. Poor compression will indicate worn rings or liner or a valve not seating correctly. It is not possible to balance an engine successfully if it has these faults.

 

Once the compression pressure on all the cylinders has been obtained, the engine load should be increased to at least 85%, although full load is preferable. ( A generator, for safety reasons should not be loaded above 85% in case it trips off). The engine load should be steady whilst the engine peak pressures are recorded, so in the case of a main propulsion engine, liase with the OOW to ensure there will be no change of course for the next 30 minutes or so.

 

Take the peak pressures. At the same time record the cylinders fuel pump rack setting and the exhaust temperature. Also recorded are the inlet manifold or scavenge air pressure and temperature.

 

When attempting to balance the engine attention must be paid to the difference between maximum pressure and compression pressure for each cylinder, together with the exhaust temperature and fuel pump setting. It is not advisable just to increase the fuel to cylinders with lower maximum pressures. Some cylinders may need  an increase in fuel, others a decrease.

 

For example: If a particular cylinder has a low peak pressure, but good compression pressure, and  a low exhaust temperature, with the fuel rack a couple of points below the average, it is perfectly acceptable to increase the fuel rack on that cylinder.

 

However if a cylinder has average peak and compression pressures and high exhaust temperatures with an average rack setting, then either it should be decreased a small amount or left alone to see the changes when other units have been adjusted. In all cases any adjustment should be small and it must be recorded in case there is a need to return the engine to its original settings. After each adjustment, wait and see the effects, taking further peak pressures if necessary, before continuing.

 

It is not acceptable to balance the engine alone on exhaust temperatures, fuel rack settings or peak pressures.

 

 

 

 

DHTML Menu / JavaScript Menu Powered By OpenCube