Document discussing generator de-rating for extreme ambient temperatures, and operating on JP8 fuel.

1. De Rating of Generators for use in Afghanistan
Elevation and Temperature
STANDARD CONDITIONs for all engine ratings are 59°F and 29.92”Hg. These conditions are related to sea
level at some northern latitude. Any change in temperature or elevation is going to impact the available air. A
Relative Air Density gauge would be useful in determining how many molecules of air are available to make
horsepower, or to transfer heat away from the radiator.
The standard accepted loss for elevation is 3% per 1000 feet, of increase above sea level. There is a standard
accepted loss for increase in temperature also. Starting at 59°F, each 11°F increase in temperature, calls for a
1% loss in power making potential.
So in Jalalabad at roughly 2000 feet above sea level, on a 120°F day, there will be a loss, of the ability to make
horsepower, of 6% for elevation and 5.5% for temperature. Starting from the 80% associated with prime usage
(24/7), we are now down to 68%. This is only related to the ability of the engine to make power.
While at Khogyani, which is approximately 4000 feet, the loss is now 12% for elevation, and again the same
5.5% for temperature. Starting with the same 80% we are now down to 62%, of the ability to make power (to
do work - as in, spin an AC alternator).
Radiator Efficiency
There is also some documentation that indicates ambient temperatures above 90°F reduce the ability of air to
transfer heat away from the radiator. The effect of altitude on heat transfer, from a heat exchanger, is around
1% per 1000 feet. However that only applies to the ability to remove heat from the engine, and the actual
impact on horsepower is probably not significant. Accordingly there will be over heating as the upper limit of
the de rated power output, is approached.
JP8 used as Fuel
The use of JP8 as fuel also has a negative impact on the generators ability to make power. A low performance
engine (light carts) is affected less. An N/A engine (normally aspirated – non turbo) with mechanical fuel
injection will see a 3% reduction in power. While a modern sophisticated engine with an electronic engine
management system can see a loss as high as 25%. Any FGW generator above a P200 uses electronics, and
will therefore be more adversely affected by the use of JP8.
50Hz versus 60Hz
Additional consideration needs to be placed on re-strapped units. If a generator was built to operate at 1500
RPM and we change it to 1800 RPM the fan and water pump may no longer function properly. If the fan blade
and/or coolant pump impeller speeds are increased beyond their designed capabilities the fan will stall (air will
swirl around the blade tip rather than flow thru the radiator), and the coolant pump will cavitate. The coolant
may also move to quickly thru the engine to remove the required amount of heat. A 1500 RPM engine might
have a smaller fan pulley than an 1800 RPM unit. This way the fan speed is operating within the designed
parameters. Another technique is to change the curve, or pitch, of the fan blade, to compensate for the change
in RPM. The diameter of the fan blade may also be smaller on a higher RPM engine.