Aviation Engine Dynamics
Unlike automobile engines, aircraft engines run at high power
settings for extended periods of time. In general, the engine runs at
maximum power for a few minutes during taking off, then power is
slightly reduced for climb, and then spends the majority of its time at
a cruise setting—typically 65% to 75% of full power. In contrast, a car
engine might spend 20% of its time at 65% power accelerating, followed
by 80% of its time at 20% power while cruising.
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The power of an internal combustion reciprocating or turbine aircraft engine is rated in units of power delivered to the propeller (typically horsepower) which is torque multiplied by crankshaft revolutions per minute (RPM). The propeller converts the engine power to thrust horsepower or thp in which the thrust is a function of the blade pitch of the propeller relative to the velocity of the aircraft. Jet engines are rated in terms of thrust, usually the maximum amount achieved during takeoff.
The design of aircraft engines tends to favor reliability over
performance. Long engine operation times and high power settings,
combined with the requirement for high-reliability means that engines
must be constructed to support this type of operation with ease.
Aircraft engines tend to use the simplest parts possible and include
two sets of anything needed for reliability. Redundancy via independence of function
lessens the likelihood of a single malfunction causing an entire engine
to fail. For example, reciprocating engines have two independent magneto ignition systems, and in modern aircraft the engine's mechanical engine-driven fuel pump is always backed-up by an electric pump.
Aircraft spend the vast majority of their time travelling at high
speed. This allows an aircraft engine to be air cooled, as opposed to
requiring a radiator.
In the absence of a radiator, aircraft engines can boast lower weight
and less complexity. The amount of air flow an engine receives is
usually carefully designed according to expected speed and altitude of
the aircraft in order to maintain the engine at the optimal temperature.
Aircraft operate at higher altitudes where the air is less dense than at ground level. As engines need oxygen to burn fuel, a forced induction system such as turbocharger or supercharger
is especially appropriate for aircraft use. This does bring along the
usual drawbacks of additional cost, weight and complexity.
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