Downloaded from https://www.everyspec.com FM 1-506 the raw materials and the great number of man-hours involved, complex designs drive up the cost. The most satisfactory design is generally the simplest that will meet requirements. to the engine. Some significant properties of aviation fuels are discussed below. Heat Energy Content or Net Heating Value. The energy content or heating value of a fuel is expressed in heat units (British thermal units [BTUs]). A fuel satisfac- tory for aircraft engines must have a high heat energy content per unit weight. A high heat energy content causes the weight of fuel carried to be lower than a low heat energy content.
Then more of the load-carrying capacity is available for the payload Aviation gasoline and JP fuels are very desirable from this standpoint. The heat energy content for aviation gasoline is about 18,700 BTUs/pound, and for JP fuels about 18,200 BTUs/pound. The various alcohols, which have maxi- mum energy content of about 12,000 BTUs/pound, do possess some other desirable characteristics as an inter- nal combustion engine fuel. High Thermal Efficiency Thermal efficiency is a measure of the losses suffered in converting heat energy in the fuel into mechanical work; it is the ratio of the heat developed into useful work to the heating value of the fuel.
High thermal effciency, therefore, means high fuel economy-something of great importance in aircraft engines. The less fuel re- quired for a military mission, the greater the military load that can be carried and the lower the fuel cost. Freedom From Vibration A power plant that is free from vibration is important in the light, somewhat flexible aircraft structure since severe engine vibration will in some cases reduce the life of certain structural parts. The need for freedom from vibration is met usually by using a large number of cylinders to offset the vibration torque delivered by the individual cylinders. Counterweights are installed on crankshafts to balance rotating masses.
These are usually hinged to provide dynamic damping of vibration which results from power impulses and to counteract un- desirable torsional or twisting vibration. Also, flexible engine mount isolators are used to permit certain move- ments of the power plant that are harmful to aircraft structures. Ease of Maintenance Volatility. A volatile liquid is one capable of readily changing from a liquid to a vapor when heated or when contacting a gas into which it can evaporate. Since liquid fuels must be in a vaporous state to burn volatility is an important property to consider when choosing a suitable fuel for an aircraft engine.
Volatility determines the start- ing accelerating vapor-locking and distribution charac- teristics of the fuel. Gasoline and JP fuels are very satisfactory because they can be blended during the refin- ing process to give the desired characteristics. Because of the nature of constant pressure combustion in gas turbine engines a highly volatile fuel is not necessary. JP fuels are of rather low volatility while aviation gasoline is highly volatile. Comparing a highly volatile fuel like aviation gasoline to a less volatile one like JP fuel the following effects become apparent.
The highly volatile fuel – The requirement of ease of maintenance is espe- cially important to military operations in the field. Simplicity of design and use of standard parts, when possible, assist in keeping maintenance at a low level. Starts easier in cold temperatures. Has a slightly better combustion efficiency. Leaves less deposit in the combustion chamber and on the turbine blades. Is a greater fire hazard. Creates a greater danger of vapor lock of the fuel system. Has high evaporation losses through the breather of the fuel tank at high altitudes.
Operating Flexibility Flexibility is the ability of a power plant to run smoothly and provide the desired performance at all speeds from idling to maximum power output. The wide range of operating requirements demanded of aircraft engines presents difficulties rarely encountered in other power plant fields. In addition to the requirement of unfailing reliability, the engine must operate in widely varying positions, altitudes, and atmospheric conditions. NOTE: The last two difficulties are practi- cally nonexistent with fuels having low volatility. FUELS Requirements Stability. The fuels used in aircraft engines must be stable.
Because aviation fuels are sometimes stored for long periods, they must not deposit sediment. The gums An engine fuel must be tailored to an engine and vice versa since there must be enough quantities of fuel available 1-2