2022-08-10 04:08

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Chapter 8 Fuel injection

第八章 燃油喷射

The emphasis in this chapter is on medium speed engines with camshaft actuated individual jerk pumps for each cylinder. Higher speed engines are also covered, including those which use camshaft pumps (or block pumps): those in which all the jerk pump elements are grouped into one or more complete units, each equipped with a common camshaft. This section does not apply (except in a general way) to low speed two-stroke engines whose systems are described later under individual makes.




The essence of a diesel engine is the introduction of finely atomized fuel into the air compressed in the cylinder during the pistonrsquo;s inward stroke. It is, of course, the heat generated by this compression, which is normally nearly adiabatic, that is crucial in achieving ignition. Although the pressure in the cylinder at this point is likely to be anything up to 200 bar, the fuel pressure at the atomizer will be of the order of 1300–1800 bar.

柴油机的要点是在活塞向内行程时,将雾化良好的燃料引入气缸内压缩的空气中。当然,由压缩过程产生的热量,通常是接近绝热的,这对实现点火十分重要。虽然此时气缸内的压力可能高达200 bar,但雾化器处的燃油压力大概有1300-1800bar。

There is a body of evidence to suggest that high injection pressure at full load confers advantages in terms of fuel economy, and also in the ability to digest inferior fuel. Most modern medium speed engines attain 1200–1800 bar in the injection high-pressure pipe. Some recent engine designs achieve as much as 2300 bar when pumping heavy fuel. For reasons of available technology, the earliest diesel engines had to use compressed air to achieve atomization of the fuel as it entered the cylinder (air blast injection), and while airless (or solid) injection delivered a significant reduction in parasitic loads it also presented considerable problems in the need for high precision manufacture, and the containment of very high and complex stresses.


The very high standard of reliability and lifetime now attained by modern fuel injection systems, notwithstanding their basic simplicity, reflects a considerable achievement in Ramp;D by fuel injection equipment manufacturers.


In the early days of airless injection many ingenious varieties of combustion chamber were used, sometimes mainly to reduce noise or smoke, or to ease starting; but often in part to reduce, or to use modest, injection and combustion pressures. A growing emphasis on economy and specific output, coupled with materials development and advances in calculation methods allowing greater loads to be carried safely, has left the direct injection principle dominant in modern medium speed and high speed engine practice.


Direct injection is what it says it is: the fuel is delivered directly into a single combustion chamber formed in the cylinder space (Figure 8.1), atomization being achieved as the fuel issues from small drillings in the nozzle tip.


For complete combustion of the fuel to take place, every droplet of fuel must be exposed to the correct proportion of air to achieve complete oxidation, or to an excess of air. In the direct injection engine the fuel/air mixing is achieved by the energy in the fuel spray propelling the droplets into the hot, dense air. Additional mixing may be achieved by the orderly movement of the air in the combustion chamber, which is called lsquo;air swirlrsquo;. Naturally aspirated engines usually have a degree of swirl and an injection pressure of around 800 bar. Highly turbocharged engines with four-valve heads have virtually no swirl, but have an injection pressure of 1200–1800 bar to provide the mixing energy. Where indirect injection is exploited, some high speed engines retain a pre-chamber in the cylinder head into which fuel is injected as a relatively coarse spray at low pressure, sometimes using a single hole. Combustion is initiated in the pre-chamber, the burning gases issuing through the throat of the chamber to act on the piston (Figure 8.2).


Fuel/air mixing is achieved by a very high air velocity in the chamber, the air movement scouring the walls of the chamber and promoting good heat transfer. Thus the wall can be very hot-requiring heat resistant materials—but it can also absorb too much heat from the air in the initial compression strokes during starting and prevent ignition. It is these heat losses that lead to poor starting and in



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