Pump-nozzle system
The unit injector ( English Unit Injector System ) is an injection system for internal combustion engines. The main characteristic is the separate injection pump for each cylinder in a common housing with the injection nozzle . The pressure lines between the pump and nozzle, which hinder the rapid pressure build-up, are thus avoided.
history
Rudolf Diesel already started thinking about combining the fuel pump and injection nozzle. In 1905 Carl Weidmann received a patent for a pump-nozzle system, and in 1911 Frederick Lamplough. At that time, the fuel in diesel engines was not injected into the cylinder, but rather blown into it with compressed air ( air injection ). The first commercially successful unit injector systems were designed by CD Salisbury for Winton . The engines equipped with it ran in locomotives and submarines. In 1934 Arthur Fielden had the pump-nozzle system used by the GM Diesel Division in two-stroke diesel engines from 1938 onwards. It was used in ship and truck diesel engines from the 1950s. In these engines, the pumps are mechanically driven by the camshaft below via tappets, push rods and rocker arms. The son Prosper L'Oranges has also undertaken developments in this direction.
Volvo introduced the first engine with electronically controlled pump-nozzle systems in its FH 12 truck in 1993. Its 12 liter D12A engine with four valves per cylinder was equipped with an overhead camshaft. The unit injectors, which Volvo calls unit nozzle holders, are mounted directly above the combustion chamber and are driven by the overhead camshaft and controlled electrically by a control unit.
Because of the legally required reduction of exhaust emissions which until the 1990s usual in car diesel fuel injection systems (distributor and in-line injection pump) were mainly due to the relatively long high-pressure lines and the associated limit the pressure rises not viable.
Therefore, the idea of the pump-nozzle system from Bosch for Volkswagen was taken up, further developed and used in passenger car diesel engines of the Volkswagen Group from 1998. The engine designation is VW EA188 . The first vehicle with pump-nozzle technology was the VW Passat B5 with a 1.9-liter engine that developed 85 kW.
At the same time, Magneti Marelli developed a common rail injection system for Fiat with a common high-pressure line for all cylinders. With it, the time, duration and number of injection processes can be varied better. That is why engines with common rail comply with lower emission limits and run more quietly. Common rail systems are cheaper to manufacture than pump-nozzle systems and the injection pressures that can be achieved are now equal. From 2008, Volkswagen also gradually equipped its diesel engines with common rail injection.
principle
As with distributor and in-line injection pumps, but different from the common rail system, the injection pressure in the pump-nozzle system is generated separately for each cylinder. This is done in a plunger pump with a piston that is actuated by its own cam on the camshaft .
In order to obtain a pressure curve that is favorable for the injection process, a steep pressure increase is necessary over time. To do this, the piston must be accelerated strongly. This is achieved purely mechanically through the oval cam shape in conjunction with the rocker arm or tappet.
The pressure buildup in the space below the piston, the plunger assembly may be any one by opening and closing solenoid valve or by a piezo - actuator -operated valve is controlled. When the valve is closed, the piston builds up pressure and the fuel is injected through the injection valve . By opening the control valve, the injection process is aborted, whereby a pressure drop as quickly as possible and a sudden abortion of the injection process are necessary for good combustion. Piezo actuators work up to three times faster than magnetic actuators. For example, the VW Passat 2.0 TDI built in 2005 (125 kW / 170 PS) had a pump-nozzle injection system with piezo-actuated valves that reached a peak pressure of 220 MPa .
advantages
- Since the pressure in the pump-nozzle unit (PDE) - and thus the injection pressure - is generated by the cams of the camshaft , the drive energy required for this can only be generated in the area relevant for the injection.
- Pump-nozzle systems are less vulnerable than common rail injections (no high-pressure pump, no rail). The failure of a unit injector does not necessarily lead to a standstill of the engine.
- The high pressure favors a fine misting of the fuel introduced by the injection valves. Smaller droplets mean a smaller volume to surface ratio, which can result in less soot formation.
- The non-sealing plunger principle of the PDE enables the use of almost all fuels (gasoline, alcohol, ethanol, LPG, biofuels, etc.).
- If the injection valve fails (jamming of the nozzle needle or contamination of the nozzle), fuel cannot flow permanently into the combustion chamber.
- With a pump-nozzle system, high specific outputs of more than 60 kW / l can be achieved.
disadvantage
- The maximum pressure at the nozzle depends on the cam shape. The injection can therefore only be triggered (in terms of time, that is, viewed over the angle of rotation) as long as the cam pushes the pump piston in. This means that the range of possible injection times is restricted to a certain range around top dead center .
- A lot of effort, as a separate pump is required for each cylinder.
- Since the point in time and quantity of injection cannot be changed in fine steps and pre- and post-injection are only possible to a limited extent, running the engine is considered to be less sophisticated. Furthermore, the exhaust gas temperature cannot be varied quickly enough. This is necessary in order to achieve emission standards above EURO 4.
- Because the pressure build-up in the pump-nozzle unit (PDE) should take place as suddenly as possible, the drive energy required for this can only be applied in the area relevant for the injection. The associated high dynamic load due to the changing pressure build-up in the individual PDE requires appropriate dimensioning of the camshaft and its drive design. A wide toothed belt or a spur gear drive is therefore necessary for the camshaft drive . Because of their high tensile stiffness and low damping capacity, chains cannot transmit the high load peaks and they break.
literature
- Peter Gerigk, Detlev Bruhn, Dietmar Danner: Automotive engineering. 3rd edition, Westermann Schulbuchverlag GmbH, Braunschweig 2000, ISBN 3-14-221500-X .
- Max Bohner, Richard Fischer, Rolf Gscheidle: Expertise in automotive technology. 27th edition, Verlag Europa-Lehrmittel, Haan-Gruiten 2001, ISBN 3-8085-2067-1 .
- Karl-Heinz Dietsche, Thomas Jäger, Robert Bosch GmbH: Automotive pocket book. 25th edition, Friedr. Vieweg & Sohn Verlag, Wiesbaden 2003, ISBN 3-528-23876-3 .
Web links
Individual evidence
- ↑ Patent US1981913 : pumping fuel. Filed May 5, 1933 , published November 27, 1934 , Applicant: General Motors Corp., Inventor: Arthur Fielden.
- ^ A b c Hans-Hermann Braess, Ulrich Seiffert: Vieweg Handbook Motor Vehicle Technology. Vieweg + Teubner. Wiesbaden. 2012. ISBN 9783834882981 . P. 240
- ↑ Frank DeLuca: History of fuel injection (PDF): www.disa.it . Retrieved November 28, 2009.