Monday, January 23, 2012

Fuel system - INTRODUCTION

1. The functions of the fuel system are to provide the engine with fuel in a form suitable for combustion  and to control the flow to the required quantity necessary for easy starting, acceleration and stable running, at all engine operating conditions. To do this, one or more fuel pumps are used to deliver the fuel to the fuel spray nozzles, which inject it into the combustion system (Part 4) in the form of an atomized spray. Because the flow rate must vary according to the amount of air passing through the engine to maintain a constant selected engine speed or pressure ratio, the controlling devices are fully automatic with the exception of engine power selection, which is achieved by a manual throttle orpower lever. A fuel shut-off valve (cock) control lever is also used to stop the engine, although in some instances these two manual controls are combined for single-lever operation.
2. It is also necessary to have automatic safety controls that prevent the engine gas temperature, compressor delivery pressure, and the rotating assembly speed, from exceeding their maximum limitations.
3. With the turbo-propeller engine, changes in propeller speed and pitch have to be taken into account due to their effect on the power output of the engine. Thus, it is usual to interconnect the throttle lever and propeller controller unit, for by so doing the correct relationship between fuel flow and airflow is maintained at all engine speeds and the pilot is given single-lever control of the engine. Although the maximum speed of the engine is normally determined by the propeller speed controller, over-speeding is ultimately prevented by a governor in the
fuel system.
4. The fuel system often provides for ancillary functions, such as oil cooling (Part 8) and the hydraulic control of various engine control systems; for example, compressor airflow control (Part 3).


5. The control of power or thrust of the gas turbine engine is effected by regulating the quantity of fuel injected into the combustion system. When a higher thrust is required, the throttle is opened and the pressure to the fuel spray nozzles increases due to the greater fuel flow. This has the effect of increasing the gas temperature, which in turn increases the acceleration of the gases through the turbine to give a higher engine speed and a correspondingly greater airflow, consequently producing an increase in engine thrust.
6. This relationship between the airflow induced through the engine and the fuel supplied is, however, complicated by changes in altitude, air temperature and aircraft speed. These variables change the density of the air at the engine intake and conse- quently the mass of air induced through the engine. A typical change of airflow with altitude is shown in fig. 10-1. To meet this change in airflow a similar change in fuel flow (fig. 10-2) must occur, otherwise the ratio of airflow to fuel flow will change and will increase or decrease the engine speed from that originally selected by the throttle lever position.

7. Described in this Part are five representative systems of automatic fuel control; these are the pressure control and flow control systems, which are 

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