I've heard that a "Flat Rated Engine" is an engine where the power is reduced to keep the internal temperature within certain limits when the ambient temperature is above a certain point. Is that correct? How does it work?
Yes, a flat rated engine (my familiarity is with gas turbines, but the concept may apply to other engine types), is an engine for which the control system is designed to achieve the same level of thrust (hence the "flat" term), up to a certain OAT, then for higher ambient temperatures, the level of thrust decreases as a function of OAT, normally in a pretty linear fashion. This control system operation gives identical performance over the flat portion of the OAT range even while the engine degrades (until it degrades to the point it hits the EGT limit), making the plane performance predictable. But for higher OATs, it stops the EGT continuing to increase. The temperature at which the thrust starts to decrease is typically 25 or 30 deg C, and can be found in the EASA or FAA engine type certificates (e.g. see section 6 of this EASA one for the CFM56-5). See the graph from this CFM56 document below. So, your understanding was correct.
Flat rating is a concept by which an engine, as stated above is restricted, in terms of power or thrust output by the fuel control unit to produce a uniform power or thrust output as the aircraft climbs or descends until the ambient air conditions will not allow the engine to operate at or beyond the maximum thermodynamic output of the engine. Like any other heat engine, the power or thrust produced by an aviation gas turbine (the gas core) depends on the available mass flow rate of working fluid (air in this case) as well as the operating temperature limits of the engine's hot section. The maximum available power or thrust a gas core can produce at sea level at STP is referred to as the core's thermodynamic rating. As an example, the Pratt & Whitney Canada PT6-66D turboshaft engine has a thermodynamic rating of 1,850 horsepower, but the FCU flat rates this to a constant maximum power output from the driveshaft to 850 ESHP, which remains available all the way up to approx FL260, thereafter decreasing with an increase in altitude.
The Flat Rating Concept
The rating of a jet engine is the thrust performance that is guaranteed by the manufacturer for a new engine under specific operating conditions such as, take off, maximum continuous, climb, cruise.
Some non Prat and Whitney (P&W) engines are rated to a constant compressor speed (RPM). P&W rates there engines to a constant exhaust gas temperature (EGT). This is referred to as the flat rating concept. To get your head around this concept you must understand the following principle.
The temperature and density of the ambient air vary inversely.
Lower temperature = > Higher Density
Higher Temperature = >Lower Density
The amount of airflow (lbs/sec) through the engine is a function of compressor speed and air density. It is greater when the compressor speed and density are high.
The compressor speed is a function of the energy available to the compressors turbine. That energy comes from the combustion or air and fuel, so the turbine turns faster when the fuel flow rate is greater. The compressor speed is also a function of the airflow through the compressor. Higher rates of air flow reduce the speed of the compressor.
The compressors rotational speed and the amount of airflow through the compressor are independent, they affect each other though.
The turbine inlet temperature is proportional to the energy available to turn the turbine. The exhaust temperature is proportional to the turbine inlet temperature. So a higher EGT corresponds to a larger amount of energy to the turbine so it can turn the compressor faster.
When EGT is held constant, or lowered the result is a prolonged hot section life and at the same time provides the thrust to meet the certification requirements.
When an engine is Flat rated it means that an engine of high Horsepower rating is constrained to a lower horsepower rating. The engine output in this case will always remain the same, but when atmospheric conditions such as high temperatures and high altitude (Hot and High) reduce the power output of the engine it has more headroom before it falls below the limited maximum output.