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Environment | CO2-Reduction
Reducing CO2 Emissions Innovation Through Intelligent Energy Management Mercedes-Benz no longer focuses just on reducing the CO2 exhaust emissions of its vehicles; with “TrueBlue” Solutions, a programme has been created that aims to continually lower the environmental impact of all of its products and associated manufacturing processes. With new engines and drive technologies and global implementation of BlueEFFICIENCY measures, the new E-Class is not only more fuel efficient and environmentally friendly that its predecessor, but also maintains the typical Mercedes-Benz virtues of safety, comfort and quality.
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1 Introduction Public discussion on the expulsion of greenhouse gas carbon dioxide (CO2) is very much linked to the operation of vehicles, although only 17 % of CO2 emissions can be directly attributed to road traffic, Figure 1. This is why the MercedesBenz TrueBlue Solutions programme accounts for the environmental performance of vehicles throughout their lifecycle. This includes all emissions related to acquiring raw materials, the production process, recycling and disposal. MercedesBenz is the only manufacturer in the world to have TÜV environmental certificates for four vehicle model series. This certificate, which will also be awarded for the new E-Class, assesses overall environmental performance and verifies that the current C-Class impacts the environment with 18 % less CO2 throughout its lifecycle, for example. The V-engine factory in Bad Cannstatt (Germany) also received the 2008 Energy Efficiency Award for reducing CO2 output by 70,806 t annually, thanks to the use of state-of-theart production technologies (including heat recovery and intelligent plant energy management). This equates to the annual CO2 emissions of approximately 34,000 E 200 CDI models driven 15,000 km per year each. On-board technologies have likewise adopted a holistic environmental approach. The highly-efficient BlueTEC exhaust cleaning system, for example, makes the fuel-efficient diesel-powered E-Class just as clean as its gasoline-powered equivalent. In addition to state-ofthe-art combustion engines featuring high compression and direct injection, BlueEFFICIENCY measures introduced in the C-Class are now also being applied across the range. The result is nothing less than impressive: The CO2 emissions of the new E-Class have been reduced by 31 g CO2/km or over 1.2 l/100 km compared to the predecessor model in mixed fleet operation. Another important step forward is the proof of everyday practicability of fuel economy technologies for customers. To this end, everyday journeys that, together, amount to almost two million kilometres, have been recorded and saved in a comprehensive database. This field test was conducted over the course
of one year so that driving profiles at different times of the day and year could be analysed across different vehicle classes. The journeys saved in the database enable economy-related variables to be described statistically in the form of collective occurrences. The basis for optimising energy management technologies is a synthesised realistic speed profile created from this data. The so-called MercedesBenz Fuel Economy Test accounts for grade and curvature influences, different season-specific temperature profiles, and distinguishes travel on city, country, and freeway roads in addition to customer-relevant driving manoeuvres. This allows the conflict between engine heat management and the heating rate of the passenger compartment (even at low outside temperatures) to be optimised very early in the development process. Take the radiator shutter used for the first time in the new E-Class, for example. As long as the engine is being operated at partial throttle only and requires minimal cooling, and the air conditioner is used at moderate outside temperatures, the radiator grille can be completely closed. This active aerodynamic measure improves the vehicle’s drag coefficient by 0.013. When tested in accordance with the mandatory New European Driving Cycle (NEDC), about 0.05 l could be saved per 100 km. This comparably small effect is due to the relatively low average speed of 33 km/h. In reality, however, average customer driving speed is 55 km/h, which enhances the shutter’s fuel-saving effect
The Authors Dr. Raimund Siegert is head of the department for simulation and analysis of integrated energy.
Frank Küster analyses the performance and fuel economy of the E-Class.
Manfred Nebel is responsible for the aerodynamic development of the E-Class.
Dr. Alexander Wäschle is responsible for the computational aero dynamics of the E-Class.
Corinna Diecke is responsible for realisation of fuel economy measures of the C-, and E-Class.
Figure 1: Breakdown of global CO2 emissions by industrial sector ATZextra I January 2009
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by approximately 0.13 l/100 km. Typically used as a business sedan, the E-Class also frequently travels long distances on freeways at a cruising speed of 130 km/h, the radiator shutter achieves even greater savings of 0.25 l/100 km.
2 Energy Management as the Key to Reducing CO2 Energy management views the vehicle as an overall system whose components interact with each other. Three main objectives were pursued to reduce CO2 emissions: – minimise the loss of energy – recover energy – control energy on demand. Behind this development philosophy was a network of engineers from all areas of design in search of avoidable energy loss and driving resistance. State-of-the-art simulation and testing methods revealed superfluous friction in the engine, what could be done to reduce tyre rolling resistance, and where excess weight could be saved.
2.1 Avoiding Loss by Minimising Driving Resistance The most effective element in reducing CO2 emissions is avoiding unnecessary loss. Here, a distinction must be made between internal loss in the driveline and external loss caused by drag and rolling resistance. As the new engines were optimised, a great deal of attention was paid to improving efficiency by minimising friction, for example. Heat management prevents coolant from being pumped through the engine block while the engine is cold. This allows the combustion chambers to quickly reach operating temperature and coolant temperatures to be controlled with respect to driving style and ambient conditions. The port injection system with mechanical compressor used in the old four-cylinder engine was replaced by a turbocharged direct injection system. While the turbocharger greatly reduces friction (no engine power is wasted to mechanically drive the compressor), the homogeneous direct injection system further improves efficiency. The vaporised fuel in the combustion chamber now lowers the operating temperature, reduces 198
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Figure 2: Regenerative braking via intelligent alternator management
knocking, and allows compression ratio to be increased. The transmissions available also play an important role in minimising friction. An optimised torque converter used with four-cylinder engines reduces the hydraulic loss due to relative slip. The six-cylinder diesel engine automatically disengages from the transmission to reduce engine load and thus, fuel consumption, while the vehicle is idling (that is at stoplights or in traffic jams). Here, the automatic transmission shifts from “D” to “N” and back again as required. The systematic reduction of external loss by using tyres with improved low rolling resistance, lowering weight by increasing the use of lightweight construction, and optimising aerodynamics also greatly contribute to improving fuel economy. Here, too, environmental friendliness must be achieved without sacrificing safety or comfort. As low-resistance tyres were being developed, for example, it was critical that good handling and braking performance were retained. The tyre belt, which consists of multiple layers of wrapped high-strength steel, minimises deformation and further reduces weight. The chemical composition of the tyre, however, is what really made a difference: The rubber blend used for the contact surface and the sidewall were chosen in a way to reduce rolling resistance by up to 17 %. Weight is another important factor. Typically, every 10 kg weight saving in a
decrease in fuel consumption of approximately 0.02 l/100 km or half a gramme CO2/km in the NEDC test. In real-world conditions, however, this effect can multiply to 0.03 l/100 km. Therefore the lightweight design package used in the new E-Class helps to achieve important CO2 emissions targets.
2.2 Recovering Energy In a normal car, kinetic energy is wasted in the form of heat every time the driver applies the brakes. This is not the case in the new E-Class, which features an innovative alternator management system. Every deceleration and braking manoeuvre results in an increase in powernet voltage, which charges the battery. The increased load placed on the alternator assists the driver during braking and allows some of the braking energy used to be recovered. This temporarily stored power makes intelligent energy management, Figure 2, whereby the alternator operates load-free during periods of acceleration. The electric functions and comfort features within the powernet are supported by the fully-charged battery during this time. The customer does not notice this switchover, since the typical comfort of the E-Class is not compromised here, either. Alternator management reduces fuel consumption by approximately 0.1 l/100 km and carbon dioxide emissions by 2.5 g/km during NEDC testing. The Mercedes-Benz Fuel Economy Test, which more accurately re-
critical element that must be taken into account when specifying components for the climate control and brake systems, for example, so that comfort and safety functions can continue to be available without having to start the engine prematurely.
3 Aerodynamic Optimisation in Detail 3.1 The Potential of Aerodynamics
Figure 3: Discussion of aerodynamic simulation results on the powerwall
flects real driving conditions, reveals that acceleration and braking from higher speeds occurs more frequently than as simulated in the NEDC test, especially during travel in urban areas. This increases the fuel economy improvement.
2.3 Controlling Energy on Demand Another important part of energy management is controlling energy according to situational requirements. The power steering pump which, in conventional hydraulic steering systems, provides maximum assistance (regardless of the current steering needs of the driver), exemplifies the potential savings that can be achieved. By implementing an additional valve, assistance can now be increased or decreased depending on the driving situation. This leads to an additional reduction in fuel consumption of approximately 0.15 l/100 km in the NEDC test. The E-Class also features new adaptive fuel pumps for both gasoline and diesel engines. Here, the engine control unit only allows the pump to run at full capacity during maximum acceleration, which saves another 0.15 l/100 km or 3.5 g CO2 in the NEDC test. The new advanced stop-start system is also an important milestone and will be offered in the E 200 for the first time. In order to maximise savings potential at idle (for example, when the car is at a stoplight) the direct-injection four-cylinder gasoline engine with six-speed transmission incorporates direct-start technology. Fuel is injected directly into the cyl-
inders during the compression stroke and immediately ignited. As the stopstart strategy was being developed, it was important that the efficiency of brief stops be maximised. Simulations have revealed that an engine at operating temperature should not be switched off the first 5 s after the vehicle has come to a stop. The number of stops under 5 s (such as when rolling through right-of-way intersections) is over 40 % during real-world driving. As a result, the stop-start strategy tries to avoid these inefficient stops. The way in which the customer experiences the engine stop is equally as important as the increased efficiency achieved. Here, too, the Mercedes-Benz Fuel Economy Test provided the average stop-duration to cover 95 % of real-world applications. This minimal bridging time is a
Aerodynamic drag always increases with the square of speed. The air resistance of a vehicle thus exceeds rolling resistance from 80 km/h and is one of the main drivers of fuel consumption and CO2 emissions at high speeds. In other words, a vehicle with a coefficient of drag of 0.27 consumes more than 2 l/100 km just to overcome air resistance. It therefore makes sense to maximise a vehicle’s “slipperiness”. Lift force, which works against the weight of a vehicle and thus reduces the contact force of the wheels on the road, also increases with the square of speed. Rear-axle lift is especially critical when braking at high speeds and must not exceed requirement specifications even when the vehicle is travelling at its Vmax.
3.2 Optimisation Begins Early The art of determining the aerodynamics of vehicle geometry from the early stages of development up to “design freeze” involves creating contours that not only minimise drag, but also keeping the driving dynamics affecting forces within their limits. Aerodynamics engi-
Figure 4: Coordination of rear air flow via integrated spoilers in the taillamp covers ATZextra I January 2009
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DOI: 10.1365/s40111-009-0153-7
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neers thus have to work within these parameters; the dimensional concept, technical feasibility, and design requirements usually greatly limit the options avail able for meeting requirements. This is why it is important to be able to make initial clarifying statements on target drag values very early on when vehicle proportions are being established. It was in this phase and in subsequent draft planning that 1:4- and 1:1-scale wind tunnel measurements and computer simulations were used to determine pivotal values and synergies to point the vehicle in the right direction, Figure 3. Final design and aerodynamics work for vehicle geometry was then performed during the 1:1-scale model phase, which is very important because this is when required aerodynamic measures can be incorporated into the overall design without incurring additional cost. Such optimisation takes place primarily in the wind tunnel, where Computational Fluid Dynamics (CFD) results and the vast quantity of information they provide are readily available and can be used to solve problems and exhaust potential opportunities in a way that is difficult to achieve with static measurements only. The tail end of the E-Class is a typical example of overall optimisation that was achieved in this phase of development: The height, spoiler lip, and minimal curvature of the trailing edge of the trunk lid at its centre point are important parameters that help maintain target rearaxle lift. In order to optimise the way in which air is directed at the rear or encourage uniform air flow across the ve hicle’s rear contour to minimise resistance, lateral break-away edges were required, which were subtly integrated into the lens covers of the rear taillamps, Figure 4. This not only reduced air resistance considerably, but also further minimised uplift. As soon as the vehicle geometry was defined, an aerodynamic hard model was built using a current chassis and prefabricated and current major assemblies with drivetrain. This model is then used to optimise add-on parts such as the radiator grille, wheel spoilers, underfloor panelling, and taillamps and to enhance the water management characteristics of the outside mirrors, side windows, and rear window. 200
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Figure 5: Optimal design of outside mirrors with respect to air resistance, aero-acoustics, and dirt deflection
Figure 6: Closed fan louver improves aerodynamics of radiator grille (right)
3.3 Aerodynamic Highlights The outside mirrors are relatively large from an aerodynamic perspective and are a very striking extension of vehicle geometry, Figure 5. The mirrors are also affected by a highly accelerated air flow around the A-pillar, which is another reason why it is advantageous to optimise the aerodynamic contours of the outside mirror housing. Taking the rear view, wind noise, aerodynamic force, dirt deflection, and design into account, a shape that optimally met all requirements could be found by way of a sophisticated optimisation process. The new outside mirrors now account for a mere 3 % of overall resistance, which is up to 70 % less than the mirrors of comparable vehicles.
It is also to imagine that the wheels also greatly contribute to overall resistance such that further optimisation could be achieved here as well. By optimally designing the contour and crosssection of the spokes and by making detailed improvements to the shape of the rim flange, the air resistance generated by the wheels was reduced to a level that could otherwise only be achieved with hub caps. One of the main highlights of the aerodynamics package is the active radiator blind. Since the engine only requires a minimal amount of cooling air the majority of the time it is running and the additional loss in total pressure created by air moving through the radiator and the engine compartment account for up
The result of the optimised aerodyna mics of the new E-Class is a coefficient of drag of cd=0.25 for the E 200 CDI and the E 220 CDI. No other vehicle in its class is this aerodynamic, Figure 7. Compared to the previous E-Class, drag could be reduced by an average of Δcd=-0.02, which reduces fuel consumption by 0.08 l/100 km (2 g CO2/km) in the NEDC test. Customers who frequently drive long distances at an average cruising speed of 130 km/h can even save up to -0.4 l/100 km.
4 Helping the Driver Drive Efficiently Figure 7: Most aerodynamic car in its class (cd = 0.25)
Figure 8: Display of current fuel consumption and gearshift recommendations encourages efficient driving
to 10 % of total air resistance, it only makes sense to reduce the flow of cooling air when it is not required. The previous E-Class also used an air management control system for a number of engines offered. This system, however, only closed the inlets in the bumper. This was sufficient to reduce drag by up to Δcd=-0.008. Much greater potential, of course, can be tapped by temporarily sealing off the entire area in front of the radiator. This is where the louver comes in, a circular-shaped device mounted behind the fan to block the cooling module. This, however, can only work if the area in front of the radiator is well sealed to thus preventing cooling air from flowing through the radiator grille, Figure 6 (A small amount of residu-
Since an economic driving style can significantly improve fuel economy, the new E-Class combines an intelligent fuel economy display with a gearshift recommendation. Experience gained from the Mercedes-Benz eco-training programme has shown that drivers can improve fuel economy by 15 % simply by driving efficiently. This eco-training programme has more or less been “integrated” into the new E-Class in the form of an easy-to-read display that indicates current fuel consumption in the middle of the speedometer. The bar graph plots in real time when the driver upshifts or takes his or her foot off of the accelerator to activate overrun fuel cut-off, Figure 8, and thus helps the driver put the finishing touch on achieving maximum fuel economy. n
al air does seep through a few movable slats to provide minimal cooling, however.). This measure further reduced drag by up to Δcd=-0.013, depending on the engine type.
3.4 Overall Efficiency In addition to these very influential aerodynamic improvements, optimised individual components such as the wheel spoiler, seal profiles, and underfloor panelling often only reduce drag by an almost irrelevant amount (Δcd=-0.001). This is why obtaining approval for such minimal improvements and implementing them is usually a painstaking process. Even these measures are worth pursuing, however, since they can result in considerable overall savings. ATZextra I January 2009
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