Page 67 - Offshore Electrical Engineering Manual

54 CHAPTER 4 Prime Mover Selection Criteria

FUEL GAS PRESSURE

As with a conventional petrol engine, the fuel–air ratio is controlled by means of a
fuel injection engine management system, with the fuel gas pressure of a few pounds
per square inch being accurately controlled.

FUEL GAS TEMPERATURE AND COMBUSTION KNOCK
Natural gas is predominantly methane which has a high octane rating and is therefore
quite resistant to combustion knock (‘pinking’). However, factors which increase
charge temperature will also increase the likelihood of knock occurring. Such factors
are

1. compression ratio
2. fuel gas temperature
3. high ambient air temperature
4. charge air temperature output from the turbocharger intercooler (where this is
fitted).

There are other factors associated with the design of the engine which would
affect how prone the engine is to combustion knock, and it is important to ensure that
a manufacturer’s warranty is provided, which states that the engine will run at the
rated power output without any shortening of operational life, for the fuel and condi-
tions expected on the platform. It would be prudent to provide the manufacturer with
a recent platform fuel gas analysis if this is available.

SPARK DURATION AND VOLTAGE
As with most spark-ignition engines, it is important that the sparks are of sufficient
voltage and duration to ensure good combustion. If the ignition engine generator
sets poor sparking, the consequent poor combustion will give erratic speed control,
with the effect being noticeably different from the regular hunting associated with
governor control problems in that it appears as a random speed wandering over a few
tens of revolutions per minute. The problem will tend to be worse at lower loadings
and can make it difficult to obtain satisfactory operation from autosynchronising
equipment.

FUEL GAS DISTURBANCES
Fuel gas is tapped from the platform process separators at often very high tempera-
tures and pressures, and these must be reduced to values within the operating enve-
lope of the prime movers being supplied. Instrumentation will be required to ensure