PRINCIPLES OF NAVAL ENGINEERING 



fuel injection system is to deliver fuel to the 

 cylinders, under specified conditions. The con- 

 ditions must be in accordance with the power 

 requirements of the engine. 



The first condition to be met is that of the 

 injection equipment. The quantity of fuel in- 

 jected determines the amount of energy avail- 

 able, through combustion, to the engine. Smooth 

 engine operation and even distribution of the 

 load between the cylinders depend upon the 

 same volume of fuel being admitted to a particu- 

 lar cylinder each time it fires, and upon equal 

 volumes of fuel being delivered to all cylinders 

 of the engine. The measuring device of a fuel 

 injection system must also be designed to vary 

 the amount of fuel being delivered as changes 

 in load and speed vary. 



In addition to measuring the amount of fuel 

 injected, the system must properly time injec- 

 tion to ensure efficient combustion, so maxi- 

 mum energy can be obtained from the fuel. 

 Early injection tends to develop excessive cyl- 

 inder pressures; and extremely early injection 

 will cause knocking. Late injection tends to 

 decrease power output; and, if extremely late, 

 it will cause incomplete combustion. In many 

 engines, fuel injection equipment is designed to 

 vary the time of injection, as speed or load 

 varies. 



A fuel system must also control the rate of 

 injection. The rate at which fuel is injected de- 

 termines the rate of combustion. The rate of 

 injection at the start should be low enough that 

 excessive fuel does not accumulate in the cylin- 

 der during the initial ignitiondelay (before com- 

 bustion begins). Injection should proceed at 

 such a rate that the rise in combustion pressure 

 is not excessive, yet the rate of injection must 

 be such that fuel is introduced as rapidly as is 

 permissible in order to obtain complete com- 

 bustion. An incorrect rate of injection will 

 affect engine operation in the same way as 

 improper timing. If the rate of injection is too 

 high, the results will be similar to those caused 

 by an excessively early injection; if the rate 

 is too low, the results will be similar to those 

 caused by an excessively late injection. 



A fuel injection system must increase the 

 pressure of the fuel sufficiently to overcome 

 compression pressures and to ensure proper 

 distribution of the fuel injected into the com- 

 bustion space. Proper distribution is essential 

 if the fuel is to mix thoroughly with the air and 

 burn efficiently. While pressure is a prime 

 contributing factor, the distribution of the fuel 



is influenced in part, by "atomization" and 

 "penetration" of the fuel. As used in connec- 

 tion with fuel injection, atomization means the 

 breaking up of the fuel, as it enters the cylinder, 

 into small particles which form a mist-like 

 spray. Penetration is the distance through which 

 the fuel particles are carried by the kinetic 

 energy imparted to them as they leave the in- 

 jector or nozzle. 



Atomization is obtained when the liquid fuel, 

 under high pressure, passes through the small 

 opening or openings in the injector or nozzle. 

 As the fuel enters the combustion space, high 

 velocity is developed because the pressure in 

 the cylinder is lower than the fuel pressure. 

 The friction created as the fuel passes through 

 the air at high velocity causes the fuel to break 

 up into small particles. Penetration of the fuel 

 particles depends chiefly upon the viscosity of 

 the fuel, the fuel-injection pressure, and the 

 size of the opening through which the fuel enters 

 the cylinder. 



Fuel must be atomized into particles suf- 

 ficiently small so as to produce a satisfactory 

 ignition delay period. However, if the atomiza- 

 tion process reduces the size of the fuel particles 

 too much, they will lackpenetration; the smaller 

 the particles the less the penetration. Lack of 

 sufficient penetration results in the small par- 

 ticles of fuel igniting before they have been 

 properly distributed. Since penetration and 

 atomization tend to oppose each other, a com- 

 promise in the degree of each is necessary in 

 the design of fuel injection equipment if uniform 

 fuel distribution is to be obtained. The pres- 

 sure required for efficient injection, and, in 

 turn, proper distribution, is dependent upon the 

 compression pressure in the cylinder, the size 

 of the opening through which the fuel enters the 

 combustion space, the shape of the combustion 

 space, and the amount of turbulence created in 

 the combustion space. 



The fuel system of a gasoline engine is basic- 

 ally similar to that of a diesel engine, except 

 that a carburetor is used instead of injection 

 equipment. While injection equipment handles 

 fuel only, the carburetor handles both air and 

 fuel. The carburetor must meet requirements 

 similar to those of an injection system except 

 that in the carburetor air is also involved. In 

 brief, the carburetor must accurately meter 

 fuel and air, and in varying percentages, ac- 

 cording to engine requirements. The carburetor 

 also functions to vaporize the fuel charge and 

 then mix it with the air, in the proper ratio. 



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