Injection process
Injection processes are the various options for providing the air-fuel mixture for internal combustion in heat engines by injecting fluids (fuels) under pressure. Conventional fuels are gasoline and diesel , increasingly also LPG at LPI. Injection processes and mixture formation are usually directly related because a certain mixture formation usually requires a specific injection process.
Processes that use gases under pressure to introduce the fuel (LPG except LPI, natural gas , hydrogen ) are called injection processes .
Differentiation and application
Injection processes are differentiated according to where the mixture is formed:
- Injection process for external mixture formation (outside the combustion chamber : typical intake manifold injection for gasoline engines )
- Injection process for internal mixture formation (within the combustion chamber: typical for diesel engines and hot-head engines )
Injection processes are used in various heat engines:
- Two-stroke engines and four-stroke engines
- Gas turbines (jet and turbofan engines)
- ...
External mixture formation
Injection methods for external mixture formation are divided into single-point injection (short SPI for single point injection) and multi-point injection (short MPI), depending on the location of the injection. With SPI, all cylinders of an engine are supplied with mixture from a central injection system. In the case of MPI, there are several injectors close to the cylinder, usually one injector per cylinder.
Furthermore, the external mixture formation through the control of the injection valves is differentiated into continuous injection processes (for example Bosch K-Jetronic ) and intermittent processes (for example Bosch L-Jetronic or Motronic ). In some cases, fuel is injected onto the closed inlet valve, which is called “upstream injection”. The intake duct and inlet valve are cooled better, and the fuel has more time to evaporate and thus be available for a good homogeneous mixture formation. When it evaporates, however, the fuel takes up a much larger volume and displaces pure intake air, which worsens the degree of delivery of the engine. At the same time, the evaporation process lowers the temperature, which leads to a higher density of the sucked in mixture and thus improves the delivery rate. All in all, it leads to an only partially compensated deterioration in the degree of delivery.
Internal mixture formation
The injection processes for internal mixture formation are basically similar, but the mixture formation variants are very different. A distinction is made here between direct and indirect injection processes. With direct injection processes, the fuel is injected directly into the (main) combustion chamber; this process is used for gasoline direct injection (BDE for short) as well as for diesel direct injection. The indirect injection process injects the fuel into a secondary chamber that is partially separated from the main combustion chamber; the pre-chamber and vortex chamber injection are known here in older diesel engine technology.
In the case of direct injection processes, a further distinction is made based on the type of fuel preparation. In diesel engines, a distinction is made between wall-distributing and air-distributing processes. In the wall-distributing process, part of the fuel is sprayed onto a body surface. Since this surface becomes hot during operation, the fuel evaporates from the surface. A typical procedure is the M procedure . With air-distributing injection, the diesel fuel is injected finely atomized through a multi-hole nozzle at high injection pressure. The droplet size of the injection jet is so small that the fuel can evaporate and burn in the hot compression air.
Wall-distributing processes basically also have an air-distributing component, through which ignition is initiated. In the case of air-distributing processes, the spray jet is not intended to strike a solid surface. If the injection nozzle is modified over its service life or through improper chip tuning, the longer-reaching spray jet can thermally overload the piston or cylinder head and lead to destruction.
With gasoline engines, a distinction is made between homogeneous and shift operation. In homogeneous operation, injection takes place in the intake stroke. Since the inlet valve is still open, the degree of delivery deteriorates as described for the external mixture formation.
The shift operation takes place in the compression cycle and is divided into jet-guided, wall-guided or air-guided processes. The entire cylinder is not filled with an ignitable mixture, but only a mixture cloud is generated and directed to the spark plug. In wall-guided and air-guided processes, the injection valve is located on the side of the cylinder head. The cloud of the ignitable mixture is either guided through the specially shaped piston crown (wall-guided) to the spark plug or through an air cushion (air-guided). In the jet-guided process, the injection valve is centrally located near the spark plug, and the ignitable mixture reaches the spark plug without being deflected.
In the case of internal mixture formation, there is less time available for injection, or injection must be carried out against a higher pressure in the combustion chamber. This is why a significantly higher injection pressure is used for the internal mixture formation in order to still achieve the required injection rate and spray jet quality. Particularly in the jet-guided process, in which injection is carried out directly in front of the spark plug, the requirements for the accuracy of the injection time and injection quantity are very high, so that this process can only be achieved through the use of piezoelectric components that are already known from the advanced common rail diesel direct injection . Injectors ( injection valve ) became feasible.
literature
- Peter Gerigk, Detlev Bruhn, Dietmar Danner: Automotive engineering. 3rd edition, Westermann Schulbuchverlag, 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 .
- Richard van Basshuysen, Fred Schäfer: Handbook Internal Combustion Engine Basics, Components, Systems, Perspectives. 3rd edition, Friedrich Vieweg & Sohn Verlag / GWV Fachverlage, Wiesbaden, 2005, ISBN 3-528-23933-6 .
- Grohe, Heinz: Otto and diesel engines . Vogel-Verlag, Würzburg, ISBN 3-8023-1559-6 .
- Robert Bosch GmbH: Handbook Automotive Technology . 25th edition, October 2003, Vieweg-Verlag.
Individual evidence
- ↑ Siemens-VDO shows spray-guided direct injection for gasoline engines, atzonline, accessed on May 7, 2004 ( memento of the original from January 25, 2010 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
