Common rail

With common - rail injection, which is also called memory injection is injection systems for internal combustion engines in which a high pressure pump brings the fuel to a high pressure level. The pressurized fuel fills a pipeline system, which is constantly under pressure during engine operation.

• 3.1 Derived Systems

• 4.1 Memory
• 4.2 Achievable pressures
• 4.3 Purpose and Benefits
• 4.4 injection performance 4.4.1 Illustrative example for calculating the hydraulic performance of injection processes

Term origin

The term originates from the English common-rail and is available for joint bar. It describes the use of a common high-pressure fuel accumulator, generally in the form of a tube on which the injection nozzles ( injectors) are connected to supply the cylinder with fuel.

Scope

Injection in the common rail method

The basic idea is the complete separation of the pressure generation from the actual injection process. Thus, a controlled exclusively by maps injection is possible. Injection timing and the injection quantity are controlled by an electronic engine control. These electrically actuated valve per cylinder ( cylinder individually ), the so-called injector, which replaces the conventional injectors classical diesel engines.

The injection process to the end of the compression stroke is divided into three groups:

• The pilot; for smooth engine running up to two pilot injections are possible
• The main injection, which can be further divided into first and second main injection
• And the post-injection, which can be used to reduce the in-cylinder soot formation for a low NOx value in SCR catalysts or for burning in diesel particulate filters.

Differences from the classical injection

Motors with series or distributor injection pump have its own high pressure line between the injection pump and injector for each cylinder. These high pressure pipes are not connected. The injection at the nozzle into a cylinder is triggered directly by an associated pumping operation of the injection pump.

In classical injection pump ( in-line pump, multiple plunger pump, which is a pump element for each cylinder ), the injection quantity and duration, that is the height of the effective piston stroke of the injection pump is not the crank angle dependent because the accelerator pedal position, the pistons are rotated, and receive a different effective stroke by using a circumferential chamfer ( = control edge ) on the piston with little actuated accelerator pedal, the high-pressure pumping later or end sooner leaves. This means that less fuel is pumped and injected in low torque requirements. The construction principle of the row and distributor pumps allows only one injection per cycle; Beginning and end of injection are determined by the control edge geometry and can be moved when required by a timing device.

The situation is different in the common-rail technology: Here injection amount and duration are independent of the crank angle can be electronically controlled and therefore pre-, main and post-injections possible; prior to 2012 to eight separate partial injections can be realized per working cycle of the engine. The pilot is mainly used for reduction of combustion noise, the post- injections ( particulate filter ) used for internal engine particle reduction or increase of the exhaust gas temperatures in the free-burn cycles at too high a pressure loss of particulate filters in the exhaust system.

Shortly before the triumph of the common- rail injection and distributor injection ( BOSCH VP44 radial piston pump and VP30 and VP37 - axial piston pump ) were also provided with high pressure solenoid valves for quantitative metering. This technique makes it possible to affect the coupled directly to the crank angle, the injection sequence during the compression phase of the fuel through the valve and to cause a pulsation of the fuel column between the pump piston and the injection valve during one piston stroke. This made it possible to realize even in the distributor pump technology up to three injection processes per operating cycle. The possible degrees of freedom of a common rail system, so that were not achieved.

History

Common rail systems have been developed for the direct injection diesel engines.

The principle is developed also from research at ETH Zurich in the years 1976 to 1992, but has there never been used on a vehicle. By continuous pumping of diesel fuel into a central discharge pipe, a high injection pressure of 1000 bar is produced. This common manifold (common rail) acting as a reservoir for all injection valves.

A development department of establishments injection devices work Aken, STC diesel engines Roßlau and SKL Magdeburg worked in the 1970s to an electronically controlled diesel injection system ( EDES ) for stationary diesel engines. Since the rail pressure does not change much over the duration of an injection, initially the term was used constant-pressure injection. At the Leipzig trade fair 1981, the SKL 6 VDS 26/20 ALE -2 as a common rail system after successful endurance testing over several thousand hours to the public before the full-scale engine. This document shows that at one had the engine with a slight heavy oil (36 cSt) operated. Fuel consumption has been reduced by 9 g / kWh and smoke opacity to 60%. The electronic control system worked with remarkable speed and precision.

1985, a modified W50 truck on the road - continuous operation, the development has been successfully tested in the GDR in 1987 but canceled due to lack of capacity to production implementation. The engine prototype can be visited in Chemnitz Museum of Industry today.

In the 1980s began, based on the results of the ETH, the preparation of Unijet common-rail system. The system was developed by Magneti Marelli, Centro Ricerche Fiat and the Elasis to 1993. Problems with the tolerances of the injectors first, however, prevented a planned series production. Bosch bought end of 1993, patents and developed the common rail system continues to series production. Thus came ten years after the first car with direct injection ( Fiat Croma TD id) in October 1997 the first vehicle with common rail injection on the market: the Alfa Romeo 156 JTD.

Followed in 1998 by Mercedes -Benz as the first German manufacturer with the C220 CDI. Even BMW has since worked exclusively with the common- rail injection. In the same year, Citroën started the development and led to the C6 a private system.

The PSA Peugeot -Citroen Group brought in collaboration with Siemens, the first piezo injectors on the market. The short reaction time of the piezoelectric art, the injection timing can be controlled more accurately and faster. Up to eight injections per combustion process are possible. Wherein the combustion process as well as the acoustic properties can be further promoted run, the motor reaches lower emissions and a lower consumption for the same performance.

The main providers of today's common-rail systems are Bosch, L'Orange, Delphi, Denso, Magneti Marelli and Continental.

For large diesel engines, common-rail injection has been produced since 1996 by the company L' Orange on the MTU Series 4000 engines.

As described above, the common rail is also used in large diesel engines. The main field of application is shipping, in which even highly viscous fuels - also known as heavy fuel oil ( HFO Heavy Fuel Oil = with viscosities up to 700 cSt at 50 ° C) - are burned. L'Orange and Bosch Austria are the only companies that offer the common-rail technology for heavy oil engines. Wärtsilä and Caterpillar (for the MaK brand ) collaborate with L'Orange, MAN Augsburg developed in conjunction with Bosch.

Derived systems

The Mitsubishi Carisma GDI ( gasoline direct injection ) was the first production car with stratified direct fuel injection in 1997 on the market.

In passenger cars, with spark-ignition engine is under other VW ( with the supplier Bosch) a representative of the direct injection. Here also provides a fuel rail (common rail) under the inlet channel arranged in the cylinder head, the electrically operated high-pressure injectors. However, the fuel pressure is relatively low at 200 bar (20 MPa) as compared with the diesel injection. In recent years, a large part of the engine range of VW / Audi FSI or TFSI -mentioned technique was changed.

The diesel common rail system,

The diesel common rail injection system is referred to as memory. A high-pressure pump ensures lasting for maintaining the fuel pressure in the manifold. It is usually coupled mechanically to the motor. The power of the high -pressure pump is designed so that at any time and in any operating condition of the fuel can be promoted more than the engine needs. So that the pump for normal operation is over-dimensioned. To control the pressure in uncontrolled pumps, a pressure control valve is used, and feeds back the amount of fuel not required from the manifold to ambient pressure relaxes into the fuel tank. Thus, the fuel at the pressure control valve can be heated to 140 ° C or more, which can damage or destroy fuel- carrying parts and require the use of a fuel cooler. Main disadvantage of this system with a non -pressure pump and the control valve, the high power requirement of the pump which always supports the maximum amount of fuel.

An improvement provide high-pressure pump with element shutdown dar. this individual pump elements of the high-pressure pump are switched off, as long as the remaining active pump elements can meet the fuel requirement of the engine. In this system the excess power of the pump is partially reduced, any remaining surplus must continue to be ramped through a pressure regulating valve.

Without pressure control valve systems can be operated with so-called suction throttle control. With this principle only as much fuel the high pressure pump is supplied as is necessary to maintain the desired pressure in the rail. The energy required for the generation of high pressure, making it as low as possible, and by eliminating the pressure control valve and the resulting heat there, the use of a fuel cooler to be avoided.

The combination of the suction throttle control with a high-pressure side pressure control valve allows efficient operation, a rapid pressure reduction in push mode and can diverted via the pressure control valve Quantity use for fuel heating ( eg in winter).

Memory

Diesel fuel is compressible. This compressibility uses the common rail system to dampen pressure surges from the hub of each pump piston. A larger storage capacity allows for a more uniform pressure with lower pressure peaks. However, it leads to a slower- system, since the pump requires more time in order to set a different required pressure. Prevents the smoothing of the delivery pressure, that of an injector during its injection phase, a pressure wave peak is effective and hence more fuel is injected as set by the map at a different injector, however, a Druckwellental is effective and that, therefore injects less. A fast system is required to inject the optimum amount of fuel even during the change of load and operating conditions.

As memory used directly in the high pressure pump integrated accumulator volume, the rail itself, as well as between different manufacturers, an accumulator for each injector, which is located as close to the injector.

Achievable pressures

The rail pressure (ie the pressure in the accumulator ) of the time up to 300 MPa ( 3000 bar ) can be used for very high injection pressures.

Some manufacturers are also working on a pressure-boosted common-rail system. In this case, the injection pressure is increased by means of a lower pressure in the pressure accumulator during the injection phase to pressures up to 250 MPa is currently (2500 bar) at the nozzle. The pressure ratio is performed by an integrated in the injector hydraulic intensifier with control functions. The principle is also referred to as Amplified pressure common rail system ( APCRS ). It is advantageous to lower the compressive stress of the high pressure pump, which has to provide only the lower supply pressure in the rail. This means that the pressure-dependent leakage losses in the pump and injector are lower. Disadvantages are the required higher flow rate of the high pressure pump and additional hydraulic losses caused by the pressure ratio, and the higher costs by more complex injectors. Such a system by Bosch for commercial vehicle engines of the company Daimler Trucks already in series production.

Purpose and Benefits

• A common rail injection optimizes the combustion process and the engine performance and reduces particulate emissions. Due to the very high pressure of the fuel is atomized very finely. Small droplets of fuel have, in relation to the volume on a large surface. The one hand favors the speed of the combustion process and on the other hand a low particulate matter in the emissions. As a drawback, the percentage of small particles is greater, which contributes to particulate matter problem.
• Driven by the internal combustion engine high-pressure pump brings the presupply (for current systems, in the car usually an electric feed pump in trucks is generally a mechanical pump ) from the tank supplied fuel at the required, predetermined by the control unit injection pressure in the pressure accumulator. The injectors ( injectors ) are connected to the common high -pressure manifold (fuel busbar) and inject the fuel directly into the combustion chamber.
• A common rail injection has structural advantages. Firstly, the decoupling between pressure generation and injection control compared to injection by the injection pump or pump -nozzle system: The injection timing can be freely selected in a CR system. Second, it must for the pressure generator unit less regardless of the position of the existing PTOs ( timing belt, timing chain, etc.) are taken.

Injection capacity

To permanently maintaining the high rail pressure a certain pump power must be provided by the motor based on consumption.

Illustrative example for calculating the hydraulic performance of injection processes

Consider:

• Average fuel consumption: 8 dm ³ per 100 km at 160 km / h
• Pressure Common Rail: 1800-2200 bar
• Limit is 1600 to 2500 bar. Higher or low values ​​can usually be traced back to: Engine cold or too warm
• Under load or idle
• Very dirty injectors
• Injection volume is not compatible with the injection control unit

The required injection rate resulting from the injection volume and the increase in pressure.

For the injection rate applies:

In:

• Flow rate, volume flow rate per unit time
• Delivery height in meters
• Driving power of the pump in watts
• Density of the pumping medium weight force per unit volume
• Efficiency, in practice always be less than 1

Under the approach of an equivalent pressure head according to the formula:

Results in:

Used:

The average required power of 710 W (equivalent to about 1 hp) is the increase in efficiency through the direct injection ( see there) compensated by far. It should be noted that internal leakage and recirculation quantities not taken into account and a larger injection volume and consequently more power be required when accelerating. In contrast, no pumping power is necessary for linear operation.

Disadvantages

• At various common-rail systems more amount of fuel is pressurized in different operating points or even in the entire useful engine characteristic, as is required for injection, control and leakage. The excess amount is deactivated via a pressure retaining valve and returned to the fuel tank, resulting in a high strain point is formed. The total motor efficiency is reduced by the Absteuermenge; the temperature of the Absteuermenge makes a stronger temperature fuel system and in some cases, a fuel cooling required. As countermeasures high pressure pumps are used with cylinder shut-off or demand high-pressure delivery with the use of a Saugdrosselmagnetventils. This can be usually omitted because of the low return levels in the tank to a fuel cooling.
• Through the permanent upcoming high pressure it can in case of malfunction of the injection valve ( jamming or fouling of the nozzle or control valves ) come to continuous injection at which the injector creates a constantly burning flame during all cycles - equal to a burner - and cylinder head, valves and exhaust system thermal overload with the risk of serious engine damage or even a fire in the engine. In traditional systems, or pump-nozzle systems, this danger is a result of only temporarily applied high pressure considerably lower. In large engines of this incident are hedged by Flow limitation valves that prevent a continuous injection and thus a destruction of the engine and allow the remaining cylinders continued operation of the engine.

Injection control

The opening of the injectors ( " injector ") is not as manifold injection or Control sleeve series injection triggered by the fuel pressure, but by electrical activation, but the fuel pressure provides the major force for lifting the nozzle needle. About the length of time and the current strength of the Injector of injection rate can be influenced and extremely short opening times are achieved, or allow one or more pre- before the main injection, one or more injections after the main injection. Pilot injections are possible as a single process with electronically modifiable distributor pumps and the system pump nozzle. They heat the combustion chamber prior to a certain extent, thus leading to an overall smoother combustion process of the following main injection. Furthermore, with the help of this pilot, the formation of nitrogen oxides can be reduced since, inter alia, by the pre- the maximum combustion temperature is reduced. It also responds to the rise in temperature over time somewhat smaller, which protects the material and reduces noise emissions. Through a close to the main injection remote injection particulate emissions can be reduced inside the engine. Further downstream injections can be used for regeneration of the particulate filter.

The injection nozzles ( injectors) are electro- magnetically or piezo- electrically actuated, is controlled from the electronic engine control unit.

The control unit calculates from the signals of multiple temperature sensors (coolant, charge air and lubricating oil), air mass sensor, accelerator pedal position sensor, possibly oxygen sensor, speed and phase sensor and rail-pressure sensor the necessary injection rate or injection duration and actuates the injectors with the corresponding control pulses for injection onset and duration. Especially with the most modern systems you are working with multiple pilot injections.

Both the injection timings and the respective injection pressure, and sometimes also the time course can be almost freely defined. This facilitates the adaptation to the respective operating state of the internal combustion engine.

Meanwhile, one or more injections are provided to increase the energy content in the exhaust gases for the burn-off temporarily to burn off the soot particle filter.

Although a big jump has been achieved in terms of exhaust and especially run behavior of diesel engines with common-rail system, however, a much higher number of components, this requires, resulting in very high demands on reliability result and system complexity has increased.

Dissemination

Meanwhile, almost all car manufacturers, the common- rail system. Also, the Volkswagen Group, which sat a long time on the competing system pump nozzle (PD ), has largely converted to common rail. They promised themselves from the competition situation of the pump - nozzle ( kicked Bosch thereby Siemens VDO Automotive to; since 2008 Continental Automotive Systems ) by stimulating competition and tried mainly by the beginning in terms of the achievable injection pressures technically superior PD system, the exhaust emission limits without to achieve particulate filter. Pump -jet engines have especially against common rail engines with unregulated high pressure pump a slight reduction in fuel consumption because no high-pressure excess amount is created.

Since 2007, the storage pressure of common rail systems has drawn level with those of the PD system and the increasing dissemination let the system costs of the CR system decline. In addition, possible with the pump-nozzle element maximum of three closely spaced injections per cycle, while the piezo - injectors of the common rail system per engine cycle to be able to realize eight injections even further apart. An achievement of the Euro - 6 emissions standard (effective from 1 September 2014 ) is therefore not possible with the pump -nozzle system, a development with this constructive and financial costs for a PD engine in the passenger car sector was uneconomic.

Pump - line-nozzle systems are on the contrary even in heavy-duty engines of DAF and the four-cylinder engines for the Mercedes -Benz Atego.

Due to the use of passenger car diesel engines in light-duty vehicles also increases the proportion of common-rail systems.

In modern common rail heavy duty vehicles is now state of the art and also in large numbers in series production (example: MAN Truck & Bus).

The common-rail system can be used with much smaller system pressures also for gasoline direct injection. The vapor bubble formation plays an important role. Gasoline and diesel injection systems are structured differently also with respect to the lubricating properties of mutually moving components, such as plain bearings, pump elements, coatings. Therefore, a unification of the two systems is technically and economically possible only in a few sub-components.

In the market, several vendors are finding. Important quality characteristics in injection systems include speed of the pressure build-up, efficiency, injection quantity deviation, regulator quality, noise and durability.

Besides the use in motor vehicles ( high-speed engines), the common-rail injection is also used in large diesel engines use, ie at four-stroke medium-speed engines and two-stroke slow-speed, for example, as the marine diesel use.

In addition, the common-rail technology is also used in aircraft engines, for example, the Thielert Centurion 1.7.

Vehicles with Common Rail engines are offered by many manufacturers. Everyone has their own shortcuts.