Wren 44 Kerostart Supplement
Copyright Wren Turbines Ltd 2012. All rights reserved.
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How does it work?
The kerostart system is basically an alternative form of heating
from the propane system used in gas start engines.
Heating is achieved by igniting and burning a small amount of
fuel using a small plug called the Burner. This is a small ceramic
element enclosed in a stainless steel housing with a very small
clearance around its tip. The housing fits the normal glowplug
hole and uses the same glowplug cable.
The element is heating by passing electrical current through it
and its tip glows bright orange (voltage set in (“Glowplug
Power”). Kerosene fuel is then pumped through the narrow gap
next to the glowing element and it bursts into flame. This flame
then provides enough heating in the combustion chamber to
allow fuel to vapourize when passed down the vapourizer tubes
in the normal way.
The amount of fuel used for the burner is very small and the fuel
pump is unable to run this slowly in a reliable way, so the
solenoid valve supplying the burner switches on and off rapidly
to help regulate the flow. The longer the valve stays open the
more fuel passes and this is how the ECU controls the flow.
The setting for this is important as too little flow and there will
only be a small flame, but too much and the element may be
cooled so much the fuel fails to ignite into flame. Also, if the
engine is spinning too fast while the small flame is going it may
blow the flame out so rpm must be carefully controlled (“Starter
Power at Ignition” and “RPM Ignition Kero”).
This stage is called
Ignition
.
In the ECU, the amount of fuel fed to the burner is controlled by
the function “
Pump Power Ignition Kero
” in the START menu
and the engine speed after the burner has ignited, is controlled
by “RPM Ignition Kero”.
Once the ECU has seen enough temperature rise from the small
flame it will begin pulsing the main fuel valve to the combustion
chamber in addition to the burner - a very small amount initially
so as to establish combustion properly. This stage is called
Preheat
. Functions used at this stage are “RPM Preheat Kero”
(sets the rotor speed at this point) and “Engine Preheat Fuel”
(amount of fuel allowed into chamber).
Once the ECU has seen enough temperature rise (set in “EGT
End Preheat”) it will gradually increase the starter speed and
main fuel valve opening time to increase the fuel flow to the
main chamber and conversely reduce fuel flow to the burner
until it is shut off completely. This stage is called
Switchover
.
At
switchover
the fuel pump is driving at a new rate set by a
number -
“Pump Start Point”
and the rate of increase of the
starting is set by “Pump Start Ramp” and the initial %flow
through the fuel valve is set by “Engine Min Flow”. This %flow
and increasing rpm continues increasing as the engines
accelerates up to the point where the fuel valve is 100% open –
display shows “FuelRamp”. The fuel pump power keeps rising
until the starter is no longer needed (“RPM Off Starter”) and
continues until the engine reaches the idle speed set.
Note we have highlighted
“Pump Power Ignition Kero”
and
“
Pump Start Point”
in red as these are the main adjustments
needed to “tune” a new fuel pump or adjust for a worn one.
