Fuel system in jet engines
FUEL SYSTEM
Apart from providing fuel to the engine, the fuel system is also used to
control propeller speeds, compressor airflow and cool lubrication oil. Fuel is
usually introduced by an atomized spray, the amount of which is controlled
automatically depending on the rate of airflow.
So the sequence of events for increasing thrust is, the throttle opens and
fuel spray pressure is increased, increasing the amount of fuel being burned.
This means that exhaust gases are hotter and so are ejected at higher
acceleration, which means they exert higher forces and therefore increase the
engine thrust directly. It also increases the energy extracted by the turbine
which drives the compressor even faster and so there is an increase in air
flowing into the engine as well.
Obviously, it is the rate of the mass of the airflow that matters since it
is the change in momentum (mass x velocity) that produces the force. However,
density varies with altitude and hence inflow of mass will also vary with
altitude, temperature etc. which means that throttle values will vary according
to all these parameters without changing them manually.
This is why fuel flow is controlled automatically. Usually there are 2
systems, one to control the pressure and the other to control the flow. The
inputs are usually from pressure and temperature probes from the intake and at
various points through the engine. Also throttle inputs, engine speed etc. are
required. These affect the high pressure fuel pump.
diagram
FUEL CONTROL UNIT (FCU)
This element is something like a mechanical computer. It determines the
output of the fuel pump by a system of valves which can change the pressure
used to cause the pump stroke, thereby varying the amount of flow.
Take the possibility of increased altitude where there will be reduced air
intake pressure. In this case, the chamber within the FCU will expand which
causes the spill valve to bleed more fuel. This causes the pump to deliver less
fuel until the opposing chamber pressure is equivalent to the air pressure and
the spill valve goes back to its position.
When the throttle is opened, it releases i.e. lessens the pressure which
lets the throttle valve fall. The pressure is transmitted (because of a
back-pressure valve i.e. no air gaps in fuel flow) which closes the FCU spill
valves (as they are commonly called) which then increases the pressure and
causes a higher flow rate.
The engine speed governor is used to prevent the engine from over-speeding.
It has the capability of disregarding the FCU control. It does this by use of a
diaphragm which senses the engine speed in terms of the centrifugal pressure
caused by the rotating rotor of the pump. At a critical value, this diaphragm
causes another spill valve to open and bleed away the fuel flow
BASIC PHENOMENA
OTHER WAYS:
There are other ways of controlling fuel flow for example with the dash-pot
throttle lever. The throttle has a gear which meshes with the control valve
(like a rack and pinion) causing it to slide along a cylinder which has ports
at various positions. Moving the throttle and hence sliding the valve along the
cylinder, opens and closes these ports as designed. There are actually 2 valves
viz. the throttle and the control valve. The control valve is used to control
pressure on one side of the throttle valve such that it gives the right
opposition to the throttle control pressure. It does this by controlling the
fuel outlet from within the cylinder.
This patent is very helpful
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