5.4.1. General information
If in the petrol engine fuel-air mix which ignites from a spark plug spark subsequently moves, then in the diesel engine it occurs under the influence of high temperature of air. At the end of a compression step air pressure in the cylinder reaches 28 bars, and temperature — about +700 °C. Fuel ignites under the influence of hot air, and pressure in the combustion chamber reaches 145 bars (D, TD) or 1350 bars at CDI. In the diesel the sprayed fuel instantly burns down. When and how many fuels should be injected into the camera, is defined by adjustment and installation of injection.
System of injection with the vortex camera
Fig. 4.60. A section of a head of cylinders with the vortex camera: 1 – nozzle; 2 – glow plug; 3 – the vortex camera with the inlet channel; 4 – cylinder
At diesels with vortex system (fig. 4.60) the combustion chamber is divided. The vortex camera is connected to nadporshnevy space, and when approaching the piston to the top point in it the vortex stream which effectively mixes fuel with air is created. Mix, having ignited, passes in the main combustion chamber. Engines with the vortex camera well proved, especially on big turns (more than 5000 min.-1).
System of injection with forkamery (precamera)
Fig. 4.61. A section of a head of cylinders from forkamery: 1 – nozzle; 2 – glow plug; 3 – a forkamer with the inlet channel; 4 cylinder
At forkamerny system (fig. 4.61) which is applied on diesel Mercedes engines working cameras are divided, as well as at vortex system. Forkamera is from above in a head of the block of cylinders. Ignition of fuel in the combustion chamber happens through the thin nozzles going from forkamer.
At direct injection fuel moves in the combustion chamber and burns down instantly.
This system has high profitability, but it has the shortcomings: high noise level of the engine, especially at start-up and sharp increase in turns.
Common Rail system
Fig. 4.62. Direct injection in the Common Rail system: 1 – nozzle; 2 – inlet branch pipe; 3 – the piston with special bore
Fig. 4.63. Scheme of the Common Rail system: 1 – general fuel stage; 2 – pressure sensor; 3 – pump; 4 – nozzle; 5 – adjusting valve of pressure
The Common Rail system (fig. 4.62, 4.63, 4.68) not only provides profitability and the minimum emission of ecologically harmful gases, but also surpasses modern diesel engines with forkamerny injection in comfort and noise level of operation of the engine. Therefore diesel engines CDI took the main place in development of engine building of Mercedes-Benz.
"Common Rail" is meant by the "General highway". If in systems with direct injection fuel under pressure moved on each nozzle separately, then in the Common Rail system fuel irrespective of sequence of injection is in the general fuel stage, the so-called accumulator.
Electronic control regulates injection pressure depending on speed and loading of the motor. The touch sensors accepting data on an operating mode of distributive and cranked shaft issue commands for optimum injection according to power setting. And supply of fuel and injection are independent from each other.
Fig. 4.64. Basic elements of the Common Rail system: 1 – pump of high pressure; 2 – delivery pump; 3 – nozzle; 4 – the magnetic valve of the dosing system; 5 – return fuel-supply line; 6 – pipeline of high pressure; 7 – fuel pressure sensor; 8 – pressure accumulator; 9 – valve of adjustment of pressure of fuel
Feature of this development is the special store (accumulator) of 8 (fig. 4.64) in which pressure to 1350 bars always remains. It is necessary in order that in the highway which connects the pump to nozzles there was always a fuel under the necessary pressure, ready to injection.
The highway connects to nozzles. On each nozzle there is a magnetic valve which regulates pressure and the number of supply of fuel. The microcomputer operates operation of the valve proceeding from an operating mode and loading of the engine. This system significantly increased profitability of operation of the engine and promoted considerable reduction of emission in the atmosphere of harmful gases.
Fuel Pumps of High Pressure (FPHP)
TNVD serve for supply of diesel fuel in nozzles under the high pressure (about 120 bars). 4-and 5-cylinder engines are equipped with distributive TNVD. The 6-cylinder engine has line TNVD. In total TNVD are located at the left on the engine and are given by a chain from a bent shaft. At the same time the frequency of rotation of a shaft of TNVD is twice less than the frequency of rotation of a bent shaft. TNVD have electronic control.
TNVD of distributive type
Engines E 220 D and E 290 TD are supplied with TNVD of distributive type (
fig. 4.64, 4.65). TNVD has the built-in toplivopodkachivayushchy pump and the sensor of temperature by means of which the signal is given for fuel interruption in supply. Electromagnetic valves, one for an engine stop, by the second for fuel supply are outside located. Carries out supply of TNVD fuel via thin channels in the corresponding cylinder.
The shaft of the pump is connected to the channel of a head, and delivery pistons are separated from each other under pressure of fuel (about 8 bars) while ledges on a cam washer do not make effort to basic end faces. During turn of a shaft the delivery channel is closed and the channel of high pressure opens.
Are in a head of the distributive pump 4 (or 5 – for the 5-cylinder engine) delivery channels and according to them 4 (5) channels of high pressure. As soon as cams on a cam washer are combined with basic ledges, pistons begin to compress fuel, pressure increases, and at achievement about 120 bars occur injection.
The amount of injectable fuel is defined by length of the course of the delivery piston which is regulated by axial movement of basic ledges. Axial installation of a shaft of the distributive pump is regulated by two magnetic valves and a returnable spring of the regulator of amount of fuel.
The corner of an advancing of injection of fuel is regulated by the provision of a cam washer in relation to a head of the distributive pump. Against the direction of rotation earlier time of injection, respectively in the direction of rotation — is determined later by turn of a washer. The turn of a cam washer around an axis of a shaft is made by means of the adjusting valve and a returnable spring. At installation in situation "early" the adjusting valve will be open, pressure will increase, and the regulating piston will go to the left. When closing the valve pressure goes down, and the piston under the influence of a spring goes to the right.
On models E 300 D line TNVD (fig. 4.66) with electronic control — ERE (Elektronisch geregeltes Reien-Einspritzsystem) is established. Line TNVD have separate pump section for each cylinder which gives fuel to the corresponding nozzle on the steel pipeline of high pressure. The main knots of ERE system are line TNVD and the block of electronic control by system of injection.
Fig. 4.68. Line TNVD (ERE) in a section: 1 – a reception part for connection of the pipeline of high pressure with a nozzle; 2 – delivery valve; 3 – plunger; 4 – mechanism of adjustment of the speed (ERE); 5 – the operating lath; 6 – lever of installation of a plunger; 7 – socket; 8 – roller pusher; 9 – plunger spring; 10 – fuel pump; 11 – electrohydraulic regulator; 12 – camshaft
In the lower part of the pump the camshaft of 12 (fig. 4.68) is established. By means of its cams pump sections according to sequence of injection are put in action. The main parts of pump section are: delivery valve 2, cylinder and plunger 3, rotary plunger plug and spring 9.
When the plunger is in the lower situation, through an inlet opening the cavity over it is filled with fuel. The cam of a shaft of the pump moves a pusher up, the spring of a plunger contracts, the plunger blocks an inlet opening, pressure increases.
When pressure reaches 120 bars, the needle of a nozzle rises and fuel comes to the precamera. The injection is made until the plunger does not open the fuel supply adjustment outlet. At this moment pressure over a plunger sharply falls, the delivery valve is closed, previously having passed a small amount of fuel back in the cylinder. Pressure in a fuel tube and a nozzle sharply falls. The nozzle is closed.
The plunger has the spiral polished channel on a side surface, and depending on the provision of a plunger the exhaust outlet of adjustment of supply of fuel remains some time closed. The way which passes a plunger at the closed exhaust outlet is called the forcing course. The more the forcing course, the more fuel is injected into the engine cylinder.
Rotary plugs of plungers of all pump sections are connected via the short lever with the operating lath. At assembly of the pump rotary plugs are installed in such a way that all pump sections force identical amount of fuel.
The lath of adjustment of supply of fuel is an important part of TNVD by means of which there is a dispensing of the forced fuel in each cylinder. The operating lath connects to an accelerator pedal via the electronically controlled regulator of the speed (ERE).
The electronic regulator of speed is from the back party of the fuel pump and operates a lath. The regulator is operated rectangular pulse tension with frequency about 190 Hz. Depending on the mode force of action of an executive magnet changes, and it, overcoming effort of a spring, advances a lath in the direction of "Start" or, respectively, "Volllast" (full loading). Lath course length at the same time makes 19,5 mm.
Decrease in noise level of operation of the engine
In earlier applied TNVD with direct injection with a big pressure (to 145 bars), noise of operation of the engine was much higher, than at forkamerny models. The Common Rail system before the main portion of fuel injects small, so-called pilot, a fuel dose which provides "heating" of the combustion chamber. Thanks to it optimum conditions for ignition of the main fuel are created, it ignites much quicker as pressure and temperature rise smoothly, but is not spasmodic. It influences not only noise reduction, but also reduction of toxicity of the fulfilled gases.
Proceeding from it experts of Mercedes-Benz took additional measures for decrease in noise level of the engine. The special noise-attenuating casing (damper) of a head of cylinders and an inlet collector, strengthening of a case and cover of the camshaft concern to them.