There are at least 3 fundamental types of fuel injection systems in
production, whether for automotive applications or aircraft. A
"Speed-Density" essentially multiplies engine speed times manifold air density
to get a desired fuel flow. A "Speed-Theta" system multiplies engine
speed times a function of throttle position. And finally a "Mass Air
Flow" system measure air flow directly. In present-day automotive
applications air-flow systems are most common, but there are a lot of
speed-density systems used, primarily by Honda and Chrysler. The
Continental system is a speed-theta system, also used by Harley Davidson and
some Italian cars. (hmm....) The speed function is provided
by the constant-displacement gear pump and the "theta" function is provided by
the barrel valve linked to the throttle. Note that there is nothing in
the system that is related in any way to air flow - the assumption is that an
engine of a given displacement running at a given throttle opening will want a
given amount of fuel. Maybe, but a lot of conditions affect it and that
is why there are so many final adjustments and tweaking required. And
since fuel flow is the result of a balance of flow through various orifices
operation of the boost pump can upset the system, since it essentially
bypasses the engine-driven pump. The engine-driven pump is not just a
fuel supply - it is an essential metering element in the system.
The "Bendix" Precision Airmotive system used by Lycoming is an air flow
system. It uses a venturi to measure air flow and controls fuel flow in
proportion. It does this by regulating fuel flow so that the pressure
drop across the metering orifice is the same as the venturi throat vacuum.
Idle air flow is too low to accurately measure so there is a separate
"theta" control system for idle and that is the only normal adjustment in the
system. The system is completely insensitive to supply pressure as long
as the pressure is within an acceptable range of typically 15 to 40 psi.
One big advantage of this type of system is that the system can be accurately
calibrated on the bench. If you order a replacement system you can just
bolt it on and go. Another advantage is that the orifice size of the
injectors doesn't affect the fuel flow (orifice balance does affect the fuel
distribution, though). These advantages are the main reason I went to
the trouble to put a Lycoming engine in my ES. The only other option in
my mind was to adapt a Precision Airmotive fuel system to a Continental
engine - probably less work. What are the disadvantages of the "Bendix"
system? One is that the venturi causes and unavoidable pressure drop in
the system and therefore the engine will produce slightly less power.
Another is that there is no return path that can separate vapor,
notwithstanding the "purge" valve that Air Flow Performance sells.
The comment about altitude compensation is partly true. Some
Continental systems have an aneroid bellows that adjusts for altitude, but I
have no idea as to whether it works well. A standard Precision system
has not altitude compensation, but there are units available that have
altitude compensation. Even without it the Precision system has an
advantage since the venturi system inherently compensates by the square root
of air density - it essentially takes out half of the altitude effect.
I've suggested to Various Lancair people the possibility of offering a
Lycoming option, but I don't think there is much interest in doing so.
Gary Casey
"The TCM System varies greatly
from its Lycoming
counterpart, the Bendix Fuel Servo. Perhaps
most noticeable is the absence of a venturi
to schedule fuel flow proportionate to air flow.
The Bendix
system regulates fuel flow by
means of a delta pressure across an air diaphragm. Comparatively,
the Continental System
has no way to sense airflow. It does not
compensate for altitude or density changes,
nor does it correct for MAP unless turbocharged.
"
http://www.kellyaerospace.com/articles/ContinuousFlow.pdf#search=%22io550%20fuel%20pump%22
Bob
Belshe