CHAPTER TWO

FUEL CONTROL UNIT ON ALLISON BENDIX 250

2.1 INTRODUCTION TO ALLISON BENDIX 250.

The Allison 250 series turbo shaft engine consists of a compressor assembly, combustion

assembly, turbine assembly, and accessory gearbox assembly. The 250 series turbo shaft

engine, being an internal combustion engine, requires intake, compression, combustion,

and exhaust as does a reciprocating engine. This type of engine is mainly used in

helicopters.

As mentioned in the paragraph above, there are some assembly parts that form this

engine. Each of these assemblies has its own functions.

The compressor assembly has the following major components:

1. Compressor Front Support

2. Compressor Rotor

3. Compressor Case

4. Compressor Diffuser

5. Compressor Air Bleed Control Valve

6. Compressor Discharge Air Tubes

6

The combustion assembly consists of a combination of a combination outer case and a

combustion liner. The combustion outer case delivers most of the pressurized air into the

combustion liner.

The major components of the turbine assembly are as follows: Gas Producer Turbine

Support, Gas Producer Turbine Rotor, Power Turbine Support, Power Turbine Rotor, and

Exhaust Collector Support.

The gas producer turbine rotor absorbs energy from the exhausting gases and provides

the power necessary to drive the compressor rotor and gas producer gear train.

The figure of the Allison 250 series turbo shaft engine is shown in Figure 2.1 below.

Figure 2.1 Allison 250 series turboshaft engine.

The Allison 250 mainly gains its thrust from the rotating shaft. The intake section,

provided by the helicopter manufacturer, delivers air into the compressor assembly which

7

contains a rotor assembly that pressurizes the air. This pressurized air is delivered to the

combustion assembly by two compressor air discharge tubes. Fuel, sprayed into the

combustion linier via the fuel nozzle, is mixed with some of the pressurized air and

burned. The exhausting gases expand through the turbine rotors and into the exhaust

collector support which exhausts the gases through two ports into the helicopter-

furnished tail pipe assembly. See Figure 2.2 for a look on the engine cut out.

Figure 2.2 Engine cut out.

8

2.2 ENGINE CYCLE

There are five basic engine cycle operations : Starting, Acceleration, Ground idle,

Stabilize on air and Shutdown. Each of this condition operates on different input

parameters to create the desired fuel flow on each cycle.

At the starting N1 RPM (fuel control RPM) 15% (from the maximum 50970 RPM), the

metering valve is at minimum position to provide reduced fuel flow during the initial

starting cycle. As Pc increases, the bellows expands to complete the start derichment

action and allows the metering valve to return to the governor bellows starting

acceleration schedule.

When the twist grip is at ground idle and is moved to full open position, with N1 RPM is

between approximately 26.500 and 32.000(52% and 62.6%), the metering valve opens at

greater rate per N1RPM increase to provide enrichment for faster acceleration to ground

idle RPM. The cycle of increased fuel, increased N1 RPM, increased Pc, and increased

fuel continues until N1 RPM is slightly less than stabilized ground idle RPM.

At ground idle N1 RPM, the metering valve is moved to a slightly smaller opening. This

reduces the fuel flow to stop the acceleration and the engine stabilizes at the ground idle

RPM. With the twist grip at ground idle, these followings would occur :

1. Metering valve opening and closing forces in the bellows assembly are equal.

2. N1 RPM will be approximately 62.6 %

3. N2 RPM(power turbine governor RPM) will be less than 100%.

9

While the twist grip is moved from ground idle to full open position, the metering valve

is moved to a larger opening and increased fuel flow. As the N1 RPM increases from

ground idle RPM to approximately 62.6% to an approximate of 78% to 82%, fuel flow

shifts from the starting acceleration flow schedule to the enrichment flow schedule. As

the fuel flow increases, N1 RPM increases and N2 RPM increases. When N2 RPM reaches

approximately 100% and N1 RPM 103% , the metering valve moves to a slightly smaller

opening to reduce fuel flow for stabilized operation with the twist grip in full open.

Operation Input Output

Starting N1 RPM=15 %, Pc increased Fuel flow reduced

Acceleration N1 RPM=52%-62.6%, Pc

increased, twist grip=ground idle.

Fuel flow increased

Ground idle N1 RPM approximately 62%, N2

RPM less than 100%, twist grip=

ground idle.

Fuel flow reduced

Enrichment N1 RPM=62.6%-74.3% or 82%,

N2 RPM=less than 100%, Pc= 31

PSIA, twist grip=ground idle to

fully open

Fuel flow increased

Stabilized on air N1 RPM=103%, N2 RPM=

approximately 100%, Pc

increased, twist grip=fully open

Fuel flow reduced

and stabilized.

Table 2.1 Engine Operation Parameters.

10

2.3 ACCESSORIES GEARBOX

The accessory gearbox is the main structural member of the engine which is used to

provide support and/or mounting for the other assemblies. The Gas Producer Fuel

Control is located on the Accessories Gearbox. Accessories required for the operation of

the 250 series turbo shaft engine can be classified as driven or non driven .It incorporates

two separate gear trains: the gas producer gear train and the power turbine gear train.

Each gear train drives the accessories in a certain RPM ratio. The RPM ratio for power

turbine and gas producer gear train are independent from one another. See figure 2.2 for a

closer look on the gear trains.

Accessories driven by the Gas Producer Gear Train:

1. Gas Producer Tachometer Generator.

2. Single Element Fuel Pump Assembly.

3. Starter-Generator.

4. Gas Producer Fuel Control.

5. Oil Pump Assembly.

Accessories driven by the Power Turbine Gear Train :

1. Power Turbine Tachometer Generator.

2. Power Turbine Governor.

11

Figure 2.3 Accessory Gear Train

Each gears rotate in a certain gear ratio respectively to Gas Producer and Power Turbine

gear trains. See table 2.2 and 2.3.

Name Of Gear Gear Ratio RPM at 100% N1

Spur Adapter Gearshaft

Spur Fuel Control Gearshaft

Spur Gas Producer Train Idler Gearshaft

Spur Gas Producer Train Idler Gear

1 to 1

0.0824 to 1

0.1507 to 1

0.0728 to 1

50.970

4.200

7.700

3.711 Table 2.2 Gas Producer Gear Train RPM Ratio.

12

Name Of Gear Gear Ratio RPM at 100% N2

Helical Power Train Drive(Pinion) Gear

Helical Torquemeter Gearshaft

Spur Power Train Idler Gear

Spur Power Governor Gearshaft

1 to 1

0.3012 to 1

0.1518 to 1

0.1518 to 1

33.290

10.027

5.053

5.053 Table 2.3 Power Turbine Gear Train RPM Ratio.

2.4 FUEL SYSTEM ON ALLISON BENDIX 250

The Fuel system on Allison Bendix 250 consisted these following parts:

1. Fuel Pump Assembly: Mounted on rear of the accessory gearbox beneath the rear

power output pad.

2. Gas Producer Fuel Control : Mounted on the right rear side of accessory gearbox.

3. Power Turbine Governor : Mounted on the left side of accessory gearbox.

4. Fuel Nozzle : Threads into center rear side of combustion outer case.

The fuel system meters fuel during all cycle of the engine operations to make sure the

right amount of fuel was supplied to engine in respect to the appropriate cycle. The

followings are some of the criteria which the fuel system must be able to fulfill:

1. Have the capability of starting the engine on the ground or in flight.

2. Meter fuel as a function of N1 RPM during starting and ground idle operation-twist

grip in the ground idle position.

3. Meter fuel as a function of N2 RPM when twist grip is in the full open position.

4. Meter fuel as a function N1 and N2 during the time that the twist grip is moved from

ground idle to full open position.

13

5. Provide overspeed protection for the gas producer rotor system.

6. Provide a means of stopping fuel flow to the engine.

To have a better illustration on how the fuel system works, see Figure 2.3. Fuel is

supplied by the helicopter fuel system to the inlet of the engine driven fuel pump

assembly. The single element fuel pump assembly consists of one spur gear type pump,

filter, filter bypass valve, and regulator valve. Fuel enters the engine fuel system at the

inlet port of the pump and passes through the filter before entering the gear element. The

filter bypass valve allows fuel to bypass the filter element if it becomes clogged. The

bypass return flow from the fuel control is passed back to the inlet of the gear element

through a pressure regulating valve which maintains the bypass flow pressure above inlet

pressure. By means of passages leading to auxiliary filling ports on the periphery of the

gear element, a portion of the bypass flow is used to fill the gear teeth when vapor-liquid

conditions exist at the gear element inlet.

The gas producer fuel control delivers metered fuel to the fuel nozzle. The fuel nozzle

atomizes the fuel and sprays it into the combustion liner where it is mixed with air and

burned.

14

 

Fig 2.4 Fuel System Schematic

15  

2.5 FUEL CONTROL BENDIX SYSTEM

The gas producer fuel control and power turbine governor provide speed governing of the

power turbine rotor and overspeed protection for the gas producer rotor system. The fuel

control system is pneumatic-mechanical and sense N1 and N2 speeds, compressor

discharge air pressure(Pc), and twist grip position to regulate and maintain fuel flow

within established limits.

There are several parameters involved in the fuel control system :

• N1 RPM : Gas Producer Turbine

Input.

• N2 RPM : Power Turbine Input.

• P0 : Bypass Fuel

• P1 : Pump Discharge Fuel.

• PA : Ambient Pressure.

• Pc : Compressor Discharge

Pressure.

• PX : Acceleration Bellows

Pressure.

Table 2.4 Parameters Involved in fuel control system.

Fuel flow is a function of Pc as sensed in the gas producer fuel control. Variations in fuel

flow schedules are obtained by modulating the Pc pressure to Px and Py pressure in the

gas producer fuel control through the bleed down circuit actuated by the governors

sensing N1 RPM and N2 RPM. The design of the fuel control system is based upon

controlling the engine power output by controlling N1 RPM. With the twist grip in full

open and N2 RPM at the setting of the power turbine governor, N1 is established by

power turbine governor action upon the gas producer fuel control. See Figure 2.5 for fuel

control Bendix system schematic.

16

 

Fig 2.5 Fuel Control System Schematic

17  

The gas producer fuel control, mounted on the accessory gearbox, is the basic component

of the engine fuel control system.

Figure 2.6 Gas Producer Fuel Control.

This unit is a hydro-mechanical device which schedules the fuel flow delivered to the

engine to provide proper engine operation during all starting and load operating

18

conditions. It is driven by the gas producer gear train at a speed proportional to gas

producer (N1) RPM. The gas producer fuel control has a quadrant and a pointer which are

used to indicate the position of the twist grip. The quadrant has the following markings:0,

5, 30, and 90. When the pointer is between 0 and 5, the cutoff valve is against the valve

seat, and the cutoff valve is closed. When the twist grip is moved from the cutoff position

to the ground idle position, the cutoff valve opens and the pointer will indicate 30. The

cutoff valve is fully open at all twist grip positions between 30 and 90.

Additional sensing parameters required by gas producer fuel control to properly schedule

fuel flow are :

1. Gas Producer Fuel Control Lever Position(controlled by twist grip)

2. Pump Discharge Pressure(P1) (supplied by fuel pump).

3. Compressor Discharge Air Pressure(Pc)(sensed off diffuser scroll).

4. Regulated Air Pressure(Pr)(supplied by power turbine governor).

5. Governor Reset Pressure(Pg)(supplied by power turbine governor).

The power turbine governor is mounted on the accessory gearbox, supplements the gas

producer fuel control to provide a complete engine fuel control system. See figure 2.7 for

a schematic of Power Turbine Governor.

The power turbine governor, via the gas producer fuel control governor reset assembly,

alters the fuel schedule determined by the gas producer fuel control to maintain desired

power turbine speed under all load conditions. The power turbine governor is driven by

19

the power turbine gear train at a speed proportional to power turbine(N2) speed.

Additional sensing parameters required to accomplish it’s governing function are:

1. Power Turbine Governor Lever Position.

2. Compressor Discharge Air Pressure(Pc) sensed at diffuser scroll.

Figure 2.7 Power Turbine Governor.

20