bosch Maquinas

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THIESEL 2002 Conference on Thermo- and Fluid-Dynamic Processes in Diesel Engines Future and Potential of Diesel Injection Systems B. Mahr 1 1 Robert Bosch GmbH, Diesel Systems, DS-NF/SBN, Postfach 30 02 20, D-70469 Stuttgart, Germany. E-mail: [email protected] Telephone: +(49) 711 811 311 27 Fax: +(49) 711 811 453 34 Abstract. Heavy duty diesel engines are in conflict between the goals of emission reduction and optimization of fuel consumption.To fulfill future more stringent exhaust gas limits further developments on diesel engine tech- nology are necessary. The diesel injection system assists this development and becomes the decisive factor to reach the emission targets. In the last 30 years a trend to high pressure fuel injection systems with an increase of maximum injection pressure from 800 up to 2000 bar is visible. In future very flexible high pressure fuel injec- tion systems are necessary with multiple injection and rate shaping capabilities and a maximum injection pres- sure beyond 2000 bar. Very important is a high efficiency of the fuel injection system itself to reach low fuel consumption. New product engineering like new nozzle design (k-factor, vario nozzle,..) or new developed ac- tuators are key factors for the fuel injection development. With a flexible diesel injection system in each point of the engine map the optimum rate shaping, injection timing and multiple injection is possible to get the best com- promise between emission trade off and fuel consumption. For example with exhaust gas recirculation (EGR) a rectangular type main injection with high injection pressures at full load is recommended. On the other hand without EGR in this point of the engine map a boot or ramp shape injection leads to the best emission results at constant or improved fuel consumption. With a coupled post injection the soot emission could be reduced. The late post injection is assisting the exhaust gas aftertreatment systems. The regeneration of the diesel particulate filter is for example because of too low exhaust gas temperature without the support of the injection system not under all circumstances possible. The exhaust gas temperature management by late post injection is a measure to improve the efficiency of catalyst systems at low exhaust gas temperatures. Very important in future is the capa- bility of the electronic control unit (ECU) of the diesel injection system to control air management, exhaust gas emission management, tolerance reduction, diagnosis, vehicle functions and combustion process by the fuel in- jection system. 1. Introduction Diesel engines are in a conflict between emission reduction and optimized fuel consumption. Especially the fuel consumption of heavy duty diesel engines has a big impact on the overall costs of the haulage business. Since end of the 80’s we have a dramatic reduction of HC, CO, NO x and particulate mass emissions to fulfill the exhaust gas legislation, fig.1. NO x -emission was reduced about 72 percent from 1985 up to now. In the same period of time the CO-emission was reduced about 85 percent and the HC-emission about 81 percent. Addition- ally the diesel engine manufacturers reached a 86 percent reduction in the particulate mass emission since 1990. This was only possible with consequent optimization of the diesel engine technology. Examples of improve- ments are turbocharging, intercooling, four valve technology, EGR, combustion chamber design and high injec- tion pressures. In future furthermore exhaust gas aftertreatment systems, low sulphur diesel fuel and further improvements on diesel engine technology are necessary to fulfill future stringent exhaust gas legislation. The change in the on- highway market from inline pump systems to high pressure fuel injection systems (VP44, UIS, UPS, CRS) is in figure 2 presented. This market is mainly driven by emissions. In the off-highway diesel engine market this in- jection system is also gradually replaced by the high pressure fuel injection systems.

Transcript of bosch Maquinas

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THIESEL 2002 Conference on Thermo- and Fluid-Dynamic Processes in Diesel Engines

Future and Potential of Diesel Injection Systems

B. Mahr1

1Robert Bosch GmbH, Diesel Systems, DS-NF/SBN, Postfach 30 02 20, D-70469 Stuttgart, Germany.

E-mail: [email protected]: +(49) 711 811 311 27Fax: +(49) 711 811 453 34

Abstract. Heavy duty diesel engines are in conflict between the goals of emission reduction and optimization offuel consumption.To fulfill future more stringent exhaust gas limits further developments on diesel engine tech-nology are necessary. The diesel injection system assists this development and becomes the decisive factor toreach the emission targets. In the last 30 years a trend to high pressure fuel injection systems with an increase ofmaximum injection pressure from 800 up to 2000 bar is visible. In future very flexible high pressure fuel injec-tion systems are necessary with multiple injection and rate shaping capabilities and a maximum injection pres-sure beyond 2000 bar. Very important is a high efficiency of the fuel injection system itself to reach low fuelconsumption. New product engineering like new nozzle design (k-factor, vario nozzle,..) or new developed ac-tuators are key factors for the fuel injection development. With a flexible diesel injection system in each point ofthe engine map the optimum rate shaping, injection timing and multiple injection is possible to get the best com-promise between emission trade off and fuel consumption. For example with exhaust gas recirculation (EGR) arectangular type main injection with high injection pressures at full load is recommended. On the other handwithout EGR in this point of the engine map a boot or ramp shape injection leads to the best emission results atconstant or improved fuel consumption. With a coupled post injection the soot emission could be reduced. Thelate post injection is assisting the exhaust gas aftertreatment systems. The regeneration of the diesel particulatefilter is for example because of too low exhaust gas temperature without the support of the injection system notunder all circumstances possible. The exhaust gas temperature management by late post injection is a measure toimprove the efficiency of catalyst systems at low exhaust gas temperatures. Very important in future is the capa-bility of the electronic control unit (ECU) of the diesel injection system to control air management, exhaust gasemission management, tolerance reduction, diagnosis, vehicle functions and combustion process by the fuel in-jection system.

1. Introduction

Diesel engines are in a conflict between emission reduction and optimized fuel consumption. Especially the fuelconsumption of heavy duty diesel engines has a big impact on the overall costs of the haulage business.

Since end of the 80’s we have a dramatic reduction of HC, CO, NOx and particulate mass emissions to fulfillthe exhaust gas legislation, fig.1. NOx-emission was reduced about 72 percent from 1985 up to now. In the sameperiod of time the CO-emission was reduced about 85 percent and the HC-emission about 81 percent. Addition-ally the diesel engine manufacturers reached a 86 percent reduction in the particulate mass emission since 1990.This was only possible with consequent optimization of the diesel engine technology. Examples of improve-ments are turbocharging, intercooling, four valve technology, EGR, combustion chamber design and high injec-tion pressures.

In future furthermore exhaust gas aftertreatment systems, low sulphur diesel fuel and further improvements ondiesel engine technology are necessary to fulfill future stringent exhaust gas legislation. The change in the on-highway market from inline pump systems to high pressure fuel injection systems (VP44, UIS, UPS, CRS) is infigure 2 presented. This market is mainly driven by emissions. In the off-highway diesel engine market this in-jection system is also gradually replaced by the high pressure fuel injection systems.

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Fig. 1. History of European emission standards

Fig. 2. Change to high pressure fuel injection systems (Krieger and Maier 2001)

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Future and Potential of Diesel Injection Systems 7

Fig. 3. One of the first applications of the inline pump in 1927

The inline pump system is in production at BOSCH since 1927. Figure 3 demonstrates one of the first applica-tions of the inline pump. This fuel injection system was continuous updated and optimized and is since 75 yearsstill in production.

The development of the maximum injection pressure of heavy duty engines over the last 30 years is presentedin figure 4. In average the pressure was increased from 800 (inline pump) to 2000 bar (high pressure fuel injec-tion systems) in only three decades (Projahn 2002).

Fig. 4. Development of injection pressure of HD engine

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2. Diesel injection system specification

The specification for all diesel engines are restricted more and more regarding exhaust gas limits, fuel consump-tion, noise and costs and this with increasing demands on driveability, lifetime, power output, service and diag-nosis, Figure 5. Applying EGR or increased rated speed to a heavy duty diesel engine higher maximum injectionpressures are essential . To achieve future more stringent exhaust gas limits additionally low sulphur diesel fuelis necessary. The flexible diesel fuel injection system is assisting this development with the measures multipleinjection, rate shaping of the main injection and an increased maximum injection pressure. Furthermore high ef-ficiency of the injection system itself is important. New developments on actuators (new solenoid and piezotechnique), nozzle design and ECU control strategies also for exhaust gas aftertreatment are key factors for fu-ture diesel engine technology. Mixing of oil with the fuel must be reduced for more stringent exhaust gas limitsdue to the influence of sulphur content in the lubrication oil on the overall soot emissions with a downstreamoxidation catalyst (Jacob et al. 2001).

Fig. 5. Key factors for diesel fuel injection system development

3. Optimum pressure and needle lift curve

In several engine tests with pressure controlled and needle lift controlled development tools the system require-ments for a future flexible diesel fuel injection systems were investigated on single cylinder diesel engine andmulti cylinder engines. As a summary of the results the optimum needle lift and pressure curve in figure 6 wasinvestigated. The upper curve in figure 6 shows the pressure at nozzle needle seat and the curve below the corre-sponding needle lift curve with multiple injections.

To reduce noise and NOx-emissions one or two pilot injections at low pressure level should be possible. Tocontrol generating of NOx during the first phase of combustion a pressure controlled rate shaping of the maininjection with boot shape, variable boot length, triangular (ramp) or rectangular shape at completely opened nee-dle should be possible. A maximum injection pressure of at least 2000 bar is advantageous. The nozzle needleshould be closed rapidly at the end of the main injection. Additionally the possibility of a coupled post injectionunder high pressure is necessary to reduce soot emission. With a late post injection at moderate pressure it ispossible to manage exhaust gas temperature for regeneration of a diesel particulate filter or to provide hydrocar-bons for NOx adsorber catalyst.

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Future and Potential of Diesel Injection Systems 9

Fig. 6. Optimum pressure and needle lift curve for future heavy duty diesel engine FIE

4. Improvement of diesel injection system

With an analytic approach a new high pressure fuel injection system called APCRS was developed to fulfillthese requirements (Mahr, Dürnholz, Polach and Grieshaber, 2000). The hydraulic layout of the pressure ampli-fied development tool is apart from the amplifier piston modules similar to the standard Common Rail Systembut with a middle rail pressure. In each injector amplifier modules are integrated to generate high injection pres-sures determined by a stepped piston.

The amplifier is activated by a second solenoid valve in the injector. Without activating this solenoid the in-jection system acts as a standard common rail system because of the bypass path with non-return valve. Varyingenergizing timing of both solenoid valves flexible pressure curves from ramp to rectangular can be generated.Pilot injections and late post injections are possible without activating the amplifier piston at the intermediatepressure in the rail. With this reduced pressure of the late post injection the risk of oil dilution can be minimized.

To fulfill future exhaust gas limits especially with EGR high injection pressures (2000 bar and more) are re-quired. An increase of the system pressure with a standard common rail system for heavy duty engines leads tohigh effort on rail and high pressure pump design, lower efficiency and durability. High pressure pump and railof the development tool APCRS are designed for intermediate pressure level and higher capacity depending onthe ratio of amplifier piston. To avoid mixing of engine oil with fuel the high pressure pump for APCRS is fuellubricated. The APCRS is designed for multiple injection, rate shaping, high injection pressure and high hydrau-lic efficiency of the injection system to reduce emissions and fuel consumption.

4.1 Rate shaping

With a first prototype of the development tool APCRS, consisting of an amplifier piston module and a stan-dard common rail injector, several engine tests were carried out to investigate the impact of rate shaping of themain injection on soot-NOx-trade-off and fuel consumption. In figure 7 engine results of a single cylinder enginewith one liter displacement at 1400 rpm under full-load without EGR with variation of start of injection arecompared. With the boot shaped pressure curve an advantage in soot-NOx-trade-off and specific fuel consump-tion was figured out in comparison to the rectangular shape of a typical common rail injection. It’s possible toreach with rectangular injection shape as common rail type injection the same fuel consumption than with bootshape and the same injection duration (DoI), but higher soot and NOx-emission. With the boot shape injection anearlier start of injection is possible than with rectangular injection shape.

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Fig. 7. Comparison of boot shape injection with rectangular shape (CR) injection at a single cylinder engine with a displace-ment of approximately 1 l/cylinder

Without EGR at part load a common rail type pressure curve under moderate pressure or a ramp type injection ispossible. At full-load without EGR the boot type injection curve demonstrates the best results. With EGR thecommon rail type pressure curve with a high injection pressure leads in a wide area of the engine map to the bestemission results. Rate shaping of the pressure curve in this case is less important than the pressure level.

At full-load and low speed the torque normally is limited by the maximum allowed cylinder peak pressure,therefore with boot shape injection higher mean effective pressures are feasible without EGR. With a flexiblefuel injection system it’s possible to optimize the rate shaping of the pressure curve in each point of the enginemap with and without EGR to get the best compromise between emissions and fuel consumption.

To fulfill future exhaust gas limits exhaust gas aftertreatment systems are necessary. Euro 5 exhaust gas limitsare for example possible with a SCR catalyst system with urea as reductant. This system allows NOx-reduction-rates up to 90 percent. Applying a boot shape main injection in figure 8 lower NOx-emission in the Europeansteady state cycle than with square or ramp shape injector are feasible. Alternative a lower urea consumption atequal NOx-emission is possible. With a SCR catalyst system using low sulphur fuel and an oxidation catalyst up-stream the SCR system an additional soot reduction up to 40 percent is measured (Mahr, Polach and Ripper,2000).

4.2 Coupled post injection

The coupled post injection under high injection pressure is a measure to reduce the soot emissions (Mahr et al2000). This is shown in left diagram of figure 9 at 1710 rpm and half-load at an single cylinder engine with twoliter displacement with EGR. The influence of the coupled post injection on the soot emission is presented at amaximum injection pressure of 1800 bar and 2000 bar with a rectangular shape main injection. With the highermaximum injection pressure of 2000 bar the NOx-emission could be decreased.

In the right diagram of figure 9 the specific fuel consumption for the different rail pressures with and withoutpost injection are shown. No significant change in fuel consumption was found with changes in the EGR-rate.The fuel consumption with the higher injection pressure of 2000 bar is slightly higher than with 1800 bar. Withthe coupled post injection the fuel consumption was slightly reduced.

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Future and Potential of Diesel Injection Systems 11

Fig. 8. Influence of rate shaping of main injection on NOx-emissions and fuel consumption with SCR catalyst application

Fig. 9. Engine results with rectangular shape injection with and without coupled post for a single cylinder diesel engine witha displacement of approximately 2 l/cylinder

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Beside the injection system the EGR-rate, the shape of the combustion chamber, the compression ratio, the airmotion and the air-fuel-ratio are important measures to improve the combustion to reach low raw emissions ofexhaust gas to meet the stringent exhaust gas limits of the future. The impact of a higher boost pressure togetherwith a higher possible EGR-rate on the soot emission is visible in the left graph in figure 9 at 1800 bar withoutpost injection.

With higher boost pressure soot emission can be reduced significantly. Suitable changes on the engine arehigher maximum combustion peak pressure, possibility of a higher EGR-rate and higher charge air pressures byVTG or even better with a two stage turbocharger systems. This requires a flexible high pressure fuel injectionsystem with a high average fuel injection pressure combined with an efficient electronic control strategy.

4.3 Late post injection

The regeneration of a diesel particulate filter (DPF) is not under all engine conditions possible because of toolow exhaust gas temperatures. High exhaust gas temperatures for the regeneration of a DPF or the desulphuriza-tion of lean NOx catalysts are feasible with a late post injection, fig. 10. The exhaust gas temperature manage-ment at low exhaust gas temperatures is an important feature to increase the temperature to get higher efficienyof the exhaust gas aftertreatment system. Furthermore the generation of hydrocarbons for the regeneration of aadsorber catalyst system is assisted by the late post injection of a fuel injection system if nessesary. To avoid oildilution a moderate injection pressure for the late post injection is recommended.

Fig. 10. Purpose of late post injection

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Future and Potential of Diesel Injection Systems 13

4.4 Nozzle design

The spray hole, as the precision geometry feature of the nozzle body, defines the spray characteristics andthus the emissions of the engine. The Spray characteristics of the nozzle are particulary influenced by the micro-geometry of the spray holes. The parameters of the spray-hole (diameter, length, inlet edge, geometry, form andmicro surface) work together for an optimal flow profile so that the injection spray requirements of the engineare met (Potz et al. 2000). With improved nozzle geometry the fuel injection system is adapted to the combustionchamber design to improve the combustion and to reduce emissions, fig. 11. The spray hole geometry has an im-pact on soot and NOx-emissions and the seat geometry on engine noise. With a reduced sac hole volume the hy-drocarbon emission is decreased. The spray hole diameter is defined by number of spray holes, injection durationand injected fuel quantity at full load. Therefore the diameter of the spray hole is at part load to large for an op-timum combustion with low soot formation. Vario nozzles or two phase nozzles are using the benefit of a re-duced spay hole orrifice at part load to reduce particulate emission.

With ks-nozzles and k-factor nozzles with conical spray hole, it’s possible to reduce cavitation effects in thespray hole and to increase the efficiency of fuel and air mixture. Addiditionally a reduced distribution of hydrau-lic flow at maximum needle lift is achieved, fig. 12.

The influence of conic spray holes in comparison to cylindric spray holes on engine emissions are demon-strated in figure 13. With the conical spray holes a benefit in soot-NOx-trade-off and in the specific fuel con-sumption was found at part load and at full load at this four cylinder diesel engine with approx. 4 l displacement.

Fig. 11. Influence of nozzle design on exhaust gas emissions

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Fig. 12. Conical spray hole

Fig. 13. Influence of conic sprayholes on engine emissions. (4 cylinder engine, 4 l displacement, Qhydr 600 cm3 at 100 bar)

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Future and Potential of Diesel Injection Systems 15

4.5 New and improved control strategies

Since more than 25 years the diesel engine is equipped with an electronic engine control unit. New sensors andnew developments at the engine itself (VTG, EGR, engine braking systems...) enable further improvement of thecomplete system with new engine control strategies in the ECU especially during transient engine conditions.

In figure 14 the NOx-emission and opacity results in hot test heavy duty transient cycle (HDTC) are comparedwith and without the function transient rail pressure increase. During acceleration of the engine the maximuminjected fuel quantity is reduced depending on the fuel-air-ratio to avoid soot formation. With a transient railpressure increase of a common rail system during acceleration the soot emission (opacity) was reduced with aneclectable disadvantage on NOx-emissions (Becher et al. 2002).

Another example for new control strategies is the transient, closed loop EGR correction. In figure 15 the hottest HDTC emission results are compared with and without this new control strategy. Without the transientclosed loop correction high peaks in opacity (soot formation) are found because of a too high EGR rate at lowfuel-air-ratios during acceleration. The control strategy is based on lambda sensor signal or on difference pres-sure measured with a venturi nozzle in the exhaust gas pipe.

With the ECU tolerance reduction is achieved to reduce emissions. New control functions are for examplezero fuel quantity calibration, fuel quality balancing control and single cylinder control for each cylinder.The potential for future diesel optimization is in the complete system control of the combustion process in thediesel engine, figure 16. Beside the control of the fuel injection system itself it’s more and more important tocontrol additionally air management and exhaust emission management with the engine ECU to get the bestcompromise in soot-NOx-trade-off and fuel consumption in each point of the engine map and during transientconditions.

Fig. 14. Influence of transient rail pressure increase on emission results in hot test HDTC (approx.. 1 l/cyl.)

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Fig. 15. Influence of transient, closed loop EGR correction on emissions results in hot test HDTC (approx. 1 l/cyl.)

Fig. 16. Potential for future diesel system optimization

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Future and Potential of Diesel Injection Systems 17

Conclusions

Beside the injection system the EGR-rate, the shape of the combustion chamber, the compression ratio, the airmotion and the air-fuel-ratio are important measures to improve the combustion to reach low exhaust gas rawemissions to achieve future stringent exhaust gas limits for heavy duty engines. With higher boost pressure asignificant reduction of soot emission is feasible. Suitable changes on the engine are higher maximum combus-tion peak pressure, higher EGR-rate and higher charge air pressures by VTG or even better with two stage turbo-charger systems. In addition to engine internal measures for emission reduction exhaust gas aftertreatment sys-tems are necessary to fulfill future exhaust gas limits. The exhaust gas aftertreatment system are assisted by theinjection system by temperature management. This requires a very flexible fuel injection system with a high av-erage injection pressure, rate shaping and multiple injection. Further development targets are reduced tolerancesand optimized nozzle design. Beside the control of the fuel injection system itself with new control strategies it’smore and more important to control additionally air management and exhaust emission management with the en-gine ECU to get the best compromise in soot-NOx-trade-off and fuel consumption in each point of the enginemap and during transient conditions.

References

Becher S, Forthmann S and Tichy B (2002) Abgasrückführregelung beim Nkw-Dieselmotor im dynamischen Betrieb,Tagung:Emission Control, Dresden

Jacob E, Gotre W, Rothe D, Rammer F and Richter K.(2001) The influence of lubricating oil on the emissions of diesel en-gines with exhaust gas aftertreatment. 22. Internationales Wiener Motorensymposium, VDI Reihe 12 Nr. 455, pp 286-301

Krieger K and Maier R (2002) Challenges of global market requirements on diesel FIE, AVL International Commercial Pow-ertrain Conference, Budapest.

Mahr B, Polach W and, Ripper W(2000) Dosing system for reducing agent of SCR catalysts, VDA Technical Congress 2000

Mahr B, Dürnholz M, Polach W and Grieshaber H (2000) Heavy Duty Diesel Engines - The potential of injection rate shap-ing for optimization of emissions and fuel consumption, 20. Internationales Motorensymposium, VDI Reihe 12 Nr. 376

Potz D, Christ W and Dittus B (2000) Diesel system – The determining interface between injection system and combustionchamber, Thermo- and Fluid-dynamic Processes in Diesel engines, Springer, pp 133-143

Projahn U (2002) Requirements on diesel fuel injection equipment and fuel cleanliness under consideration of global marketaspects, 5th International Filtration Conference