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DR.-ING. NILS LINDENKAMP was Research Associate at the Institute of Internal Combustion Engines of the Technical University of Braunschweig and is now an Eemployee of the Gasoline Engine Pre-development of Volkswagen AG (Germany).
REDUCING OF EXHAUST EMISSIONS FROM DIESEL HYBRID VEHICLES The combination of Diesel engine and electric motor in hybrid electric vehicles promises the lowest fuel consumption. NOx and particulate raw emissions in diesel hybrid electric vehicles are high and expensive exhaust aftertreatment is therefore required to reduce them. At the Institute of Internal Combustion Engines at TU Braunschweig various strategies have been developed for an engine-internal reduction of exhaust emissions in diesel hybrid electric vehicles.
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PROF. DR.-ING. PETER EILTS is Director of the Institute for Internal Combustion Engines at the Technical University of Braunschweig (Germany).
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DIPL.-ING. BENJAMIN TILCH is Senior Engineer at the Institute for Internal Combustion Engines at the Technical University of Braunschweig (Germany).
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a Diesel engine, ❶. This article will describe the phlegmatisation of the internal combustion engine as well as the use of adjusted engine hardware using the turbocharging system as an example.
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CHALLENGES
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BACKGROUND
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ANALYSIS ME THOD
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PHLEGMATISATION
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OP TIMISED TURBO CHARGING SYSTEM
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TEST RESULTS
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SUMMARY AND OUTLO OK
3 ANALYSIS METHOD
Vehicle development does not only focus on the reduction of pollutant emissions, but also on reducing CO2 emissions and primary energy consumption. Hybrid vehicle concepts to reduce fuel consumption considerably are already available today. The combination of Diesel engine and electric motor promises the lowest CO2 emissions. But compared to gasoline hybrid electric vehicles preferred by the industry, there are two significant disadvantages. The costs for the system are very high due to the expensive Diesel engine plus the NOx and particulate raw emissions from the diesel hybrid electric vehicles optimised for low consumption are comparatively high, the latter requiring complex multistage exhaust aftertreatment. The high system costs would have to be reduced or at least put into perspective in order to make diesel hybrid electric vehicles attractive for the customers. One option would be to use hybridisation of Diesel engines not only to reduce CO2 emissions, but also for other, usually expensive, purposes. Hybridisation of Diesel engines, for example, could be used to reduce NOx and soot emissions significantly in order to meet tighter emission limits in the future without requiring expensive exhaust aftertreatment.
To evaluate the potential of reducing NOx and soot emissions in a full diesel hybrid electric vehicle, the untreated exhaust emissions measured in the NEDC, FTP75 and US06 cycles are compared to those from a reference vehicle with conventional drivetrain. For this purpose, a vehicle model of a car with parallel hybrid propulsion system based on Matlab/Simulink has been developed at the Institute of Internal Combustion Engines. It can be used to determine engine operating points, battery state of charge (SOC) and fuel consumption of the diesel hybrid electric vehicles in the respective driving cycle. Furthermore, the time-based torque/speed vector (T/rpm) of the internal combustion engine (ICE) obtained in the simulation was transferred to the automation system of an engine test rig to measure the exhaust emissions from the engine in the driving cycle at operating temperature. A dSpace system has been used as automation system to manage the complex pre-controlled torque control. Based on the obtained data, the strategies to reduce exhaust raw emissions that are to be analysed can be further optimised in the simulation. ❷ shows the basic approach. In order to run the tests as described, a suitable test engine is put on a high-dynamic engine test rig including exhaust measurement equipment. A small-volume, turbocharged three-cylinder Diesel engine with variable turbine geometry (VTG) turbocharger was used as test engine, ❸. The exhaust emissions were measured using a Sesam-FTIR from AVL, the soot emissions with a Smokemeter 415S as well as an Opacimeter 439S, also both from AVL, and the NOX emissions with a NOX sensor from Siemens.
2 BACKGROUND
4 PHLEGMATISATION
Exhaust emissions from turbocharged Diesel engines generally increase with acceleration from low speed and load ranges since both control dynamics and control quality of the air path (boost pressure and EGR control) are limited due to the slow control system behaviour [2, 3]. During application it is necessary to optimise the trade-off between highly dynamic response and low transient exhaust raw emissions. Furthermore, the exhaust emissions increase significantly with higher engine loads. The NOx emissions increase due to the comparatively high peak temperatures and lack of exhaust gas recirculation, soot emissions due to low combustion air ratios. Diesel hybrid electric vehicles with conventional control strategy promise a considerable reduction of exhaust emissions since electric driving and start-stop operation do not generate emissions in the driving cycle. But the higher weight and the load point shifting which is sometimes necessary to compensate for low battery charge conditions lead to an increased engine load spectrum in steady-state and transient operation. The increase overcompensates the positive effect of emission-free driving, which in turn will not lead to reduced exhaust emissions as compared to conventional diesel hybrid electric vehicles. NOx and soot emissions, however, can be considerably reduced through a consequent improvement of the hybrid control strategies, especially during dynamic operation of the engine. In the following, some promising approaches are presented to demonstrate how the electrification of the drivetrain can reduce the exhaust emissions in
Most of the exhaust emissions from modern Diesel engines measured in the driving cycle occur during dynamic operation of the engine. The strategy of phlegmatisation therefore aims at reducing the requirements on the Diesel engine dynamics through torque boost, for which a powerful electric motor is required. The air path therefore still can offer the optimum mixture composition (in the cylinder) despite the quick change in the torque desired by the driver. The primary goal of the phlegmatisation strategy is the significant reduction of soot emissions in the driving cycle [2, 3]. Preliminary tests concerning different possibilities of phlegmatisation showed that limiting the internal combustion engine dynam-
1 CHALLENGES
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Smoothing of transient engine load Limitation of operating maps Alternative combustion process
High speed engine start Potentials to reduce exhaust gas emissions
Lowering compression ratio Thermal management Adaption of engine hardware
Electrical driving
❶ Measures to reduce the untreated exhaust emissions from a diesel hybrid electric vehicles [1]
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ICE TYPE
Three cylinder diesel engine with direct injection
SUPERCHARGING SYSTEM
VTG turbocharger
DISPLACEMENT [cm³]
1422
COMPRESSION RATIO [-]
18.5
EXHAUST GAS AFTERTREATMENT SYSTEM
Oxidation catalytic converter, external exhaust recirculation
EMISSION LEVEL
Euro 4
❸ Technical data of the test engine
❷ Basic approach [1]
5 OPTIMISED TURBOCHARGING SYSTEM
ics through linear torque increase is the best compromise between using as little electrical energy as possible and maximum soot reduction potential [1]. The maximum torque gradient (MTG) characterises the torque increase that is used to limit the dynamic engine operation. ❹ illustrates a load step with different limiting torque gradients. The given desired torque is achieved later with increasing phlegmatisation. The air path is therefore allowed more and more time to provide the desired fresh air mass flow during the load step. In addition, the injection mass required to provide the torque is reduced due to the decreasing load step. Thus, with increasing phlegmatisation, the combustion air ratio decreases less and less during the load step. This results in a considerable reduction of soot emissions already with a maximum torque gradient of MTG = 100 Nm/s as compared to an unlimited full load step, ④.
Another approach to reduce soot emissions is to improve the slow transfer behaviour of the air path. An efficient means for this purpose is the optimisation of the turbocharging system, aiming at a significantly improved response of the air path. During acceleration, the dead volume of the charge air duct can be filled more quickly and the EGR mass in the inlet manifold can be removed at an early stage. The associated fast increase of the fresh air mass flow results in a minor decrease of the combustion air ratio in the cylinder if the torque curve of the load step is constant. With regard to acceleration from low speed and load ranges, this in turn results in a significant reduction of soot emissions. To analyse the potential of the turbocharging system optimised for hybrid electric vehicles, a feasible solution had to be determined
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Without phlegmatisation ohne Phlegmatisierung MMG 100 Nm/s MTG == 100 MMG m/s MTG == 50 50 Nm/s MMG MTG == 25 25Nm/s Nm/s
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❹ Potential of different maximum torque gradients (MTG) to reduce soot emissions with a load step from 10 Nm to 200 Nm at 2000 rpm
❺ Comparison of both steady-state full load torque, transient response of the simulated turbocharging systems and VTG turbocharger (basic charging system) [5]
in comprehensive preliminary tests. For this purpose, a one-dimensional model to simulate the gas exchange of the test engine was generated and successfully validated based on a basic measurement. Conventional as well as electrically assisted turbocharging could then be simulated using the engine model [5]. The preliminary tests with the simulation model showed that a series connection consisting of electrical compressor (eBooster) and VTG turbocharger is the best compromise between improved response, high low-end torque and low fuel consumption, ❺. Another positive aspect is the fact that the eBooster can provide the Diesel engine with appropriate boost pressure any time since it does not depend on the enthalpy of the exhaust gas. To simulate the combination of eBooster and VTG turbocharger in the test engine a variable intake path has been developed. The standard inlet air and exhaust gas path, consisting of VTG turbocharger, charge air intercooler, EGR path and exhaust gas path, has not been changed. The variable intake system, which simulates the eBooster of the combined turbocharging system, is directly connected to the compressor inlet of the VTG turbocharger, ❻. To reduce soot emissions as efficiently as possible by using an eBooster, the boosting strategy has to be defined clearly in advance. Depending on engine speed, desired engine torque and increased torque requirements, the optimum level and time response of the boost pressure as well as beginning and end of the boosting process have to be determined. ❼ shows an example of the boosting strategy, with optimised boost pressure at 1500 rpm. The diagram shows that
the soot emissions (see opacity) in transient operation can be reduced to the steady-state emission level if the engine speed is 1500 rpm and the boost pressure 1.3 bar upstream of VTG compressor. Since the response of the internal combustion engine can be improved significantly, much less additional torque of the electric motor is required when the hybrid electric vehicles is accelerated quickly in order to comply with the dynamic speed input. For the following tests, the full hybrid electric motor with 25 kW was replaced by a less powerful one with 15 kW in the vehicle simulation model. Due to the low power of the electric motor, it is possible not only to replace the motor, but the hybrid electric vehicles inverter can also be smaller. This results in reduced costs and, additionally, reduced vehicle mass. 6 TEST RESULTS In the following, test results to show the potential of the introduced strategies to reduce exhaust emissions from a diesel hybrid electric vehicle in the US06 cycle are presented. The exhaust emissions from a reference vehicle (road resistances VW Golf V, vehicle weight 1300 kg) with conventional drivetrain and a diesel hybrid electric vehicles (vehicle weight 1500 kg) with conventional control strategy (that is to say start-stop system, brake energy recovery, electric driving, electric boosting, load point shifting) was measured in the driving cycle to quantify the reduction potential exactly, ❽.
❻ Structure of the variable intake system to simulate an eBooster 07- 0 8 I 2 012
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ing the vehicle weight by 50 kg (with eBooster) reduces the NOx emissions and fuel consumption by approximate 2 to 3 % only as compared to the conventional hybrid electric vehicles. The soot emissions can be significantly reduced when hybrid technology and eBooster are combined due to the highly dynamic US06 driving cycle profile. As opposed to the conventional hybrid electric vehicles, the soot emissions can be reduced by approximate 26 % as compared to the reference vehicle, ⑧. In contrast to the soot emissions, the hybrid electric vehicles with eBooster still emit more nitrogen oxide in the US06 cycle than the reference vehicle. In order to also reduce the NOx emissions, it seems promising to combine this strategy with the strategy of limiting the maximum engine torque, compare [4]. The latter strategy aims at reducing the full load torque of the ICE to avoid that part of the engine map where the NOx emissions are disproportionately high. In accordance with the goal of the strategy of limiting the maximum engine torque (in terms of optimum NOx and soot values), nitrogen
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-34 % Fuel cons. V e rbra uch NO NOx X R uß Soot Conventional HEV operating strategy
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Due to the additional vehicle weight and the correspondingly increased engine load spectrum, the NOX emissions from the hybrid electric vehicles are higher than those from the reference vehicle. In terms of fuel consumption, the positive effect of the start-stop system overcompensates the additional weight. The fuel consumption of the hybrid electric vehicles is only slightly below reference value. The potential of the phlegmatisation strategy to reduce soot emissions is high due to the highly dynamic US06 driving cycle profile. Compared to the reference vehicle, the particulate emissions can be reduced by up to 34 %. The nitrogen oxide emissions and the resulting fuel consumption are similar to those of a diesel hybrid electric vehicle with conventional control strategy. Comparing the combination of hybrid technology and eBooster to conventional hybrid electric vehicles reveals similar values for NOx emissions and fuel consumption, so the eBooster apparently hardly influences these parameters in spite of the changed air-fuel ratio and the improved response of the engine during acceleration. Reduc-
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Charge-air pressure [barA]
VTG position [%]
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Air ratio [%]
1.1
nICE = 1500 rpm TICE,target = 160 Nm
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Opacity [%]
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Boost pressure [barA]
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❼ Full load step at 1500 rpm and different boost pressures
eBooster + conv. operating strategy
Soot-optimised NOx-optimised eBooster + limit. TICE,max
❽ Potential of the analysed strategies to reduce NOX and soot emissions in the US06 cycle in comparison to the conventional reference vehicle
oxides could be reduced by 15 % and soot emissions by 42 % as compared to the reference vehicle, ⑧. 7 SUMMARY AND OUTLOOK NOx and soot emissions can be considerably reduced through a consequent improvement of the hybrid operating strategies, especially during dynamic operation of the engine. For this purpose, different strategies to reduce the exhaust emissions in a diesel hybrid electric vehicles were analysed at the Institute of Internal Combustion Engines as part of a PhD thesis [1]. The thesis focuses on the phlegmatisation strategy as well as on the optimisation of the turbocharging system to reduce soot emissions. An Engine-in-the-Loop test rig was set up at the Institute of Internal Combustion Engines for further tests [6]. As opposed to the presented development method where T/rpm vectors are used for the engine test rig, a real time co-simulation of the vehicle model is run on the Engine-in-the-Loop test rig. The co-simulation can take thermal influences on the operating behaviour of the internal combustion engine into account. The combination of simulation model and engine test rig is therefore an efficient tool for developing and evaluating further thermal control strategies to reduce fuel consumption and exhaust emissions. REFERENCES [1] Lindenkamp, N.: Strategien zur Reduzierung der Reduzierung der NO X- und Partikelemissionen eines Dieselhybridfahrzeugs. TU Braunschweig, Dissertation, 2010 [2] Predelli, O.; Bunar, F.; Manns, J.; Buchwald, R.; Sommer A.: Laying Out D iesel-Engine Control Systems In Passenger-Car Hybrid Drives. Hybrid Vehicles and Energy Management, Braunschweig, 2007 [3] Blumenröder, K.; Bunar, F.; Buschmann, G.; Nietschke, W.; Predelli, O.: Dieselmotor und Hybrid: Widerspruch oder sinnvolle Alternative? 28. Inter nationales Wiener Motorensymposium, 2007 [4] Lindenkamp, N.; Eilts, P.: Strategien zur Reduzierung der NO X- und Rußemissionen in einem Dieselhybridfahrzeug. Hybrid Vehicles and Energy Management, Braunschweig, 2009 [5] Lindenkamp, N.; Stöber-Schmidt, C.-P.; Eilts, P.: Strategien zur Reduzierung der NOX- und Rußemissionen in einem Dieselhybridfahrzeug. VDE-Kongress E-Mobility, Leipzig, 2010 [6] Tilch, B.; Reimers, T.; Eilts, P.: Using an Engine-in-the-Loop test bench for model based optimization of hybrid power trains. 21. ASIM Symposium Simulationstechnik, Winterthur, 2011
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