Anexo 2 – Estados financieros auditados

The high-pressure fuel lines must withstand the system’s maximum pressure as well as pressure variations that can attain very high fluctuations. The lines are seamless precision- made steel tubing in killed cast steel which has a particularly consistent microstructure. Dimensions vary according to pump size (Table 1, next page).

All high-pressure delivery lines are routed to avoid sharp bends. The bend radius should not be less than 50 mm.

High-pressure lines High-pressure connection fittings, high-pressure delivery lines 125

Fig. 3 1 Nozzle holder 2 Sealing cone 3 High-pressure fitting 4 Seal 5 Edge-type filter 6 Union nut 7 High-pressure delivery line 8 Screw connections 9 Cylinder head Fig. 4 11 Expansion bolt 12 Perpendicular fitting 13 Molded seal 14 Edge-type filter 15 Nozzle holder 16 Cylinder head

17 Fuel return line (leakage-fuel line) 18 Union nut 19 High-pressure delivery line 10 Clamp 1 2 3 4 5 6 7 8 9 10

Sample of a perpendicular fitting

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Sample or a high-pressure fitting

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Length, diameter and wall depth of the high- pressure lines all affect the injection process. To cite some examples: Line length influences speed-sensitive the rate of discharge, while internal diameter is related to throttling loss and compression effects, which will be re- flected in the injected-fuel quantity. These considerations lead to prescribed line di- mensions that must be strictly observed. Tubing of other dimensions should never be installed during service and repairs. Defec- tive high-pressure tubing should always be replaced by OEM lines. During servicing or maintenance, it is also important to observe precautions against fouling entering the sys- tem. This applies in any case to all service work on fuel-injection systems.

A general priority in the development of fuel-injection systems is to minimize the length of high-pressure lines. Shorter lines produce better injection-system performance. Injection is accompanied by the formation of pressure waves. These are pulses that propagate at the speed of sound before fi- nally being reflected on impact at the ends. This phenomenon increases in intensity as engine speed rises. Engineers exploit it to raise injection pressure. The engineering process entails defining line lengths that are precisely matched to the engine and the fuel-injection system.

All cylinders are fed by high-pressure deliv- ery lines of a single, uniform length. More or less angled bends in the lines compensate for the different distances between the outlets from the fuel-injection pump or rail, and the individual engine cylinders.

The primary factor determining the high- pressure line’s compression-pulsating fatigue strength is the surface quality of the inner walls of the lines, as defined by material and peak-to-valley height. Especially demanding performance requirements are satisfied by prestressed high-pressure delivery lines (for applications of 1,400 bar and over). Before installation on the engine, these customized lines are subjected to extremely high pressures (up to 3,800 bar). Then pressure is suddenly relieved. The process compresses the material on the inner walls of the lines to provide increased internal strength.

The high-pressure delivery lines for vehicle engines are normally mounted with clamp brackets located at specific intervals. This means that transfer of external vibration to the lines is either minimal or nonexistent.

The dimensions of high-pressure lines for test benches are subject to more precise tol- erance specifications.

126 High-pressure connections High-pressure delivery lines

Table 1

d Outer line diameter d1 Inner line diameter Wall thicknesses indicated in bold should be selected when possible.

Dimensions for high- pressure lines are usually indicated as follows: d x s x l

l Line length

Main dimensions of major high-pressure delivery lines in mm

1 d1 d 1.4 1.5 1.6 1.8 2.0 2.2 2.5 2.8 3.0 3.6 4.0 4.5 5.0 6.0 7.0 8.0 9.0 Wall thickness s 4 1.3 1.25 1.2 5 1.8 1.75 1.7 1.6 6 2.25 2.2 2.1 2 1.9 1.75 1.6 1.5 8 3 2.9 2.75 2.6 2.5 2.2 2 10 3.75 3.6 3.5 3.2 3 2.75 2.5 12 4.5 4.2 4 3.75 3.5 14 5 4.75 4.5 4 3 17 6 5.5 5 4.5 19 5 22 7

High-pressure lines Cavitation in the high-pressure system 127 Fig. 1 1 Cavitation Fig. 2 a A vapor bubble is formed b The vapor bubble

collapses c The collapsed

sections form a sharp edge with extremely high energy

d The imploding vapor bubble leaves a recess on the surface 1 Vapor bubble 2 Wall 3 Recess

Cavitation in the high-pressure system



Cavitation can damage fuel-injection systems (Fig. 1). The process takes place as follows:

Local pressure variations occur at restrictions and in bends when a fluid enter an enclosed area at extremely high speeds (for instance, in a pump housing or in a high-pressure line). If the flow characteristics are less than optimum, low- pressure sectors can form at these locations for limited periods of time, in turn promoting the formation of vapor bubbles.

These gas bubbles implode in the subsequent high-pressure phase. If a wall is located imme- diately adjacent to the affected sector, the con- centrated high energy can create a cavity in the surface over time (erosion effect). This is called cavitation damage.

As the vapor bubbles are transported by the fluid’s flow, cavitation damage will not neces- sarily occur at the location where the bubble forms. Indeed, cavitation damage is frequently found in eddy zones.

The causes behind these temporary localized low-pressure areas are numerous and varied. Typical factors include:

 discharge processes  closing valves

 pumping between moving gaps, and  vacuum waves in passages and lines Attempts to deal with cavitation problems by improving material quality and surface-harden- ing processes cannot produce anything other than very modest gains. The ultimate objective is and remains to prevent the vapor bubbles from forming, and, should complete prevention prove impossible, to improve flow behavior to limit the negative impacts of the bubbles.

d b c a 2 3 2 1 2 1 2 1

Implosion of a cavitation bubble

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Cavitation damage in the distributor head of a VE pump 1

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Electronic control of a diesel engine enables precise and differentiated modulation of fuel-injection parameters. This is the only means by which a modern diesel engine is able to satisfy the many demands placed upon it. The EDC (Electronic Diesel Con- trol) system is subdivided into three areas, “Sensors and desired-value generators”, “Control unit” and “Actuators”.

Requirements

The lowering of fuel consumption and harm- ful exhaust-gas emissions (NOX, CO, HC,

particulates) combined with simultaneous improvement of engine power output and torque are the guiding principles of current development work on diesel engine design. In recent years, this has led to an increase in the popularity of the direct-injection (DI) diesel engine which uses much higher fuel- injection pressures than indirect-injection (IDI) engines with whirl or prechamber sys- tems. Because of the more efficient mixture formation and the absence of flow-related losses between the whirl chamber/precham- ber and the main combustion chamber, the fuel consumption of direct-injection en- gines is 10 ... 15 % lower than that achieved by indirect-injection designs.

In addition, diesel engine development has been influenced by the high levels of com- fort and convenience demanded in modern cars. Noise levels, too, are subject to more and more demanding demands.

As a result, the performance demanded of the fuel-injection and engine management systems has also increased, specifically with regard to

 high fuel-injection pressures  rate-of-discharge curve variability  pre-injection and, where applicable,

post-injection

 variation of injected fuel quantity, charge- air pressure and start of delivery to suit operating conditions

 temperature-dependent excess fuel quan- tity for starting

 control of idle speed independently of en- gine load

 controlled exhaust-gas recirculation (cars)  cruise control, and

 tight tolerances for start of delivery and quantity, and maintenance of high preci- sion over the service life of the system (long-term performance)

Conventional mechanical governing of engine speed uses a number of adjusting mechanisms to adapt to different engine operating condi- tions and ensures a high mixture formation quality. Nevertheless, it is restricted to a simple engine-based control loop and there are a number of important influencing variables that it cannot take account of or cannot re- spond quickly enough to.

As demands have increased, what was origi- nally a straightforward system using electric actuator shafts has developed into the present- day EDC, a complex electronic control system capable of processing large amounts of data in real time. It can form part of an overall elec- tronic vehicle control system (“drive-by-wire”). And as a result of the increasing integration of electronic components, the control-system circuitry can be accommodated in a very small space.