The Art of the Helicopter John Watkinson - Karatunov.net

Engines and transmissions 211
a hot part of the engine where icing cannot occur. A fuel injected helicopter needs
no hot air system and full power is always available with a corresponding increase in
safety. Throttle response is instant. The flow measuring venturi restricts airflow much
less than a carburettor, and maximum power is increased.
There are two types of fuel injection system. In mechanical systems, the fuel delivery
is continuous and is determined by the fuel pressure in the fuel manifold (also known
as the fuel distributor or flow divider). Fuel manifold pressure is displayed on a gauge
in the cockpit, often combined with the inlet manifold pressure gauge. In electronic
systems, the fuel delivery is controlled by a solenoid valve, one incorporated in each
nozzle. In this case the fuel pressure is held constant relative to manifold pressure and
the amount of fuel delivered is determined by the period of the pulses that operate the
solenoids.
The engine power is controlled as usual by a throttle disc in the inlet duct. In a
mechanical system, a small venturi is created in the duct and the suction created is a
function of inlet airflow. The fuel pressure is made proportional to the venturi suction
in the fuel control unit shown in Figure 6.10(a). The pilot’s mixture control changes
the balance in the fuel control unit and the idle cut-off function stops all fuel flow.
The only real drawback of mechanical fuel injected engines is that hot starting can
be difficult because residual engine heat vaporizes the fuel in the pipes leading to the
nozzles. The solution is to purge the pipes and cool them with fresh fuel by running the
boost pump for a few seconds with the throttle ‘cracked’ (partially open). The surplus
fuel will flow into the inlet manifold and must be purged by operating the starter with
the ignition off and the throttle wide. The throttle is then returned to idle, ignition is
switched on and the starter operated again. Once the engine is running the boost pump
is switched on again as a flight backup for the mechanical pump.
In electronic systems, a signal processor computes the correct amount of fuel to
inject. It does this based on the mass flow of air into the engine. Figure 6.10(b) shows
that engine RPM is measured by sensing ignition pulses or with a sensor adjacent to
a toothed wheel. In the inlet tract, the air mass flow may be measured by a hot-wire
sensor. Alternatively the manifold pressure may be measured. The air temperature is
also measured to compensate the computation. As a result the mixture is always correct
and no human intervention is required. In automotive applications, there is only one
processor. This represents a single point of failure not acceptable in aviation. However,
it is quite possible to provide one processor per cylinder each having its own sensors.
It is also necessary to provide electrical power to the fuel injection system even if the
main electrical power system fails. This is easily done using power-generation windings
in the magnetos as is the practice in motorcycles.
Whilst the fine nozzles of injection systems are more prone to blockage than a
carburettor, with proper filter and fuel management this is not an issue.
6.12 The turbocharger
The power a piston engine can develop is a function of the charge burned on each
power stroke. Clearly a greater charge can be obtained by using a larger engine, but
this will be heavier, negating some of the increase in power. A more efficient way of
increasing the power is to get more charge into a given engine.
In a naturally aspirated engine, when the piston goes down on the intake stroke,
charge is pushed into the cylinder by atmospheric pressure. The elements of the induction
system such as the air cleaner, the venturi and the valve stems all impede the flow.