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More and more airplanes in the general aviation fleet have fuel
injected engines. One of the lessons pilots need to learn about
starting fuel injected engines is how to start them when they are
hot -- how to prevail in the dreaded "hot start".
Note: a "hot start" in turbine engines is a
completely different situation, with a far more critical outcome.
Today's we'll stick to our fuel-injected piston topic.
Why hot starts are an issue
Fuel injected engines depend on liquid fuel in the lines all the
way to fuel injector nozzles, which combine air and fuel just
before it enters the cylinders for combustion. Any vapor in the
fuel lines interrupts flow, and may form a "vapor lock" that
prevents fuel delivery altogether. Fuel vapor results from
undissipated heat, usually in an engine-driven fuel pump but
sometimes elsewhere in the fuel injection system.
The procedure to perform a hot start differs based on the
engine/airframe combination and, significantly, the type of fuel
injection system involved, but all hot start procedures have one
thing in common: replacing fuel vapor with liquid fuel. This can be
accomplished in several ways:
- Running a boost pump while the mixture control is in the idle
- Attempting start with the mixture in idle cutoff, and advancing
the mixture when the engine fires (note; this is the normal start
procedure in some fuel-injected engines).
- Purposely flooding the engine (by running the a boost pump with
the mixture set to rich), then performing the Flooded Start
- Performing the normal start with the throttle set more open
than normal, hoping the engine-driven fuel pump pushes vapor out of
Put 20 pilots in a room and you'll probably come up with about
18 different ways to hot-start a fuel injected engine. Any may work
on any given day, but generally there's one technique that works
best for a specific engine/airframe combination. Some options (like
intentionally flooding) may result in a damaging backfire or, in
extreme cases, an engine-warping liquid lock in the cylinders.
Aero-tip of the day: Ask engine experts (user
groups, manufacturers, etc.) to learn a safe, dependable way to hot
start the fuel injected engines you fly.