Fuel economy and emissions evaluation of BMW Hydrogen 7 Mono-Fuel demonstration vehicles

international journal of hydrogen energy 33 (2008) 7607–7618

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Fuel economy and emissions evaluation of BMW Hydrogen 7 Mono-Fuel demonstration vehicles Thomas Wallnera,*, Hennning Lohse-Buscha, Stephen Gurskia, Mike Duobaa, Wolfgang Thielb, Dieter Martinb, Thomas Kornc a

Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439, USA BMW Group, 80788 Munich, Germany c BMW Group, 5900 Arcturus Avenue, Oxnard, CA 93033, USA b

article info

abstract

Article history:

This article summarizes the testing of two BMW Hydrogen 7 Mono-Fuel demonstration

Received 8 July 2008

vehicles at Argonne National Laboratory’s Advanced Powertrain Research Facility (APRF).

Received in revised form

The BMW Hydrogen 7 Mono-Fuel demonstration vehicles are derived from the BMW

27 August 2008

Hydrogen 7 bi-fuel vehicles and based on a BMW 760iL. The mono-fuel as well as the bi-fuel

Accepted 29 August 2008

vehicle(s) is equipped with cryogenic hydrogen on-board storage and a gaseous hydrogen

Available online 7 November 2008

port fuel injection system. The BMW Hydrogen 7 Mono-Fuel demonstration vehicles were tested for fuel economy as

Keywords:

well as emissions on the Federal Test Procedure FTP-75 cold-start test as well as the

BMW Hydrogen 7

highway test. The results show that these vehicles achieve emissions levels that are only

Efficiency

a fraction of the Super Ultra Low Emissions Vehicle (SULEV) standard for nitric oxide (NOx)

Emissions

and carbon monoxide (CO) emissions. For non-methane hydrocarbon (NMHC) emissions the cycle-averaged emissions are actually 0 g/mile, which require the car to actively reduce emissions compared to the ambient concentration. The fuel economy numbers on the FTP-75 test were 3.7 kg of hydrogen per 100 km, which, on an energy basis, is equivalent to a gasoline fuel consumption of 17 miles per gallon (mpg). Fuel economy numbers for the highway cycle were determined to be 2.1 kg of hydrogen per 100 km or 30 miles per gallon of gasoline equivalent (GGE). In addition to cycle-averaged emissions and fuel economy numbers, time-resolved (modal) emissions as well as air/fuel ratio data is analyzed to further investigate the root causes of the remaining emissions traces. The BMW Hydrogen 7 vehicles employ a switching strategy with lean engine operation at low engine loads and stoichiometric operation at high engine loads that avoids the NOx emissions critical operating regime with relative air/ fuel ratios between 1 < l < 2. The switching between these operating modes was found to be a major source of the remaining NOx emissions. The emissions results collected during this period lead to the conclusion that the BMW Hydrogen 7 Mono-Fuel demonstration vehicles are likely the cleanest combustion engine vehicles ever tested at Argonne’s APRF. ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.

* Corresponding author. Tel.: þ1 630 252 3003; fax: þ1 630 252 3443. E-mail address: [email protected] (T. Wallner). 0360-3199/$ – see front matter ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2008.08.067

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Nomenclature APRF BMS CH4 CO CO2 CVS DC DOE ECU EURO NCAP FID FTP GGE H2 H2O

1.

Advanced Powertrain Research Facility boil-off management system methane carbon monoxide carbon dioxide Constant Volume Sampling direct current Department of Energy engine control unit European New Car Assessment Program Flame Ionization Detector Federal Test Procedure gallon of gasoline equivalent hydrogen water

Introduction

Argonne National Laboratory’s Center for Transportation Research was approached by BMW to benchmark their BMW Hydrogen 7 vehicles. The complexity of the vehicle, the anticipated low level of emissions and the novel nature of the hydrogen on-board storage system required extensive preparation prior to performing valid fuel economy and emissions tests. A first phase entitled ‘Development of test procedures for hydrogen vehicles’ was completed in fall of 2007. During this phase up to 10 BMW hydrogen 7 bi-fuel vehicles were stationed at Argonne for a period of one month. In order to operate the vehicles, a liquid hydrogen refueling station was also set up at Argonne. The test activities allowed the identification of required upgrades to exhaust gas analyzers and emissions sampling design to allow accurate measurement at the anticipated emissions levels. In addition a novel approach to measure the fuel consumption on hydrogen vehicles similar to a carbon balance used for conventional vehicles using exhaust gas composition as well as intake and dilution air humidity was implemented. This was necessary because the complexity of the hydrogen fuel system does not allow tapping into the fuel lines for direct hydrogen consumption measurement. A second test phase with two BMW Hydrogen Mono-Fuel demonstration vehicles was completed in spring of 2008. The second test phase included testing of the vehicles for their emissions characteristics and fuel economy on the FTP-75 cold-start as well as the highway drive cycle.

2.

BMW Hydrogen 7 vehicle

2.1.

Background

The BMW Hydrogen 7 vehicle has successfully completed the process of series development, meaning that the vehicle and all components have gone through the same design,

HC HP kg km kW l MJ mpg N2 NDIR Nm NMHC NOx ppm RPM SULEV THC

hydrocarbon horsepower kilogram kilometer kilowatt relative air/fuel ratio (lambda) megajoule miles per gallon nitrogen Non-Dispersive Infrared Newton meters non-methane hydrocarbons nitric oxides parts per million revolutions per minute Super Ultra Low Emissions Vehicle total hydrocarbons

manufacturing and quality control process as any other BMW vehicle. The new hydrogen model is built at BMW’s Dingolfing Plant (Germany) parallel to the other models in the BMW 7, 6 and 5 Series, with the drive unit in BMW Hydrogen 7 coming like all BMW twelve-cylinder engines from the Company’s engine production plant in Munich (Germany). ¨ V South Germany Technical Teaming up with the TU Inspection Authority, the BMW Group has successfully tested BMW Hydrogen 7 in a large series of the most demanding trials and test procedures focusing in particular on the car’s hydrogen components and going through all the homologation requirements made of a regular production vehicle [1]. The BMW Group has also put BMW Hydrogen 7 through a complete program of crash tests going beyond the usual European and US legal requirements. These crash tests include frontal offset collisions in accordance with EURO NCAP at an impact speed of 64 km/h or 40 mph, rear-end collisions with 100 and 40 percent overlap, as well as side-on collisions at the car’s most sensitive point directly on the fuel filler pipe. The BMW Hydrogen 7 Mono-Fuel demonstration vehicle was built based on the BMW Hydrogen 7 bi-fuel to showcase the emissions reduction potential of a dedicated hydrogen vehicle. Details on the BMW Hydrogen 7 bi-fuel vehicle can be found in [2].

2.2.

Overview of BMW Hydrogen 7 vehicle specifications

The specifications of the BMW Hydrogen 7 are summarized in Table 1. The vehicle is based on a BMW 760Li with a 6.0 L V12 engine. The hydrogen tank is a vacuum insulated cryogenic tank placed behind the rear seats. With a volume of 170 L it can store approximately 8 kg of hydrogen which is equivalent to about 8 gallons of gasoline (GGE) in terms of energy content. The power and torque curves of the BMW Hydrogen 7 are shown in Fig. 1 [3]. The engine achieves a peak torque of 390 Nm at 4300 RPM and a maximum power of 192 kW (w260 HP) at 5100 RPM.

international journal of hydrogen energy 33 (2008) 7607–7618

Table 1 – Specifications of BMW Hydrogen 7 vehicles Base vehicle Number of vehicles Seat positions Engine Transmission Drive Curb weight (est.) Wheelbasis Fuel injection system ECU modifications Engine configuration Engine operation strategy Catalytic converter

2.3.

BMW 760Li w100 4 6.0 L V12 6-speed automatic 2-wheel 2460 kg (5420 lb) 3124 mm (123 in) Gaseous port injection Series development hydrogen vehicle Naturally aspirated Lean burn/stoichiometric 3-way catalyst

On-board hydrogen storage system

As a result of research and development for several years, the BMW Hydrogen 7 is the first costumer car with a liquid hydrogen storage system [4,5]. To ensure optimum safety in even more extreme accident scenarios, the hydrogen tank was tested under exceptional conditions such as exposure to flames, firearm shots, massive mechanical damage, as well as the reaction of the fuel tank and safety equipment to a loss in insulating vacuum [6]. In an additional series of tests, tanks filled with hydrogen were fully encompassed by flames at a temperature of more than 1000  C (1830  F) for up to 70 min. Even under such conditions, tank behavior did not present any problems, with the hydrogen in the tanks escaping slowly and almost imperceptibly through the safety valves.

Following these most demanding tests and examinations, ¨ V South Germany and the fire brigade specialists both TU acting as consultants arrived at the conclusion that the hydrogen car in terms of accident safety is at least as safe as a conventional gasoline car. Should the hydrogen supply dwindle, the dual-mode power unit featured in the bi-fuel version of the BMW Hydrogen 7 switches over to conventional premium gasoline. BMW Hydrogen 7 bi-fuel comes with both a conventional 74-L (16.3 gal) gasoline tank and an additional fuel tank taking up approximately 8 kg or 17.6 lb of liquid hydrogen. The cruising range of the car in the hydrogen mode is more than 200 km (125 miles), with another 500 km (300 miles) in the gasoline mode. An overview of hydrogen bearing parts in the BMW Hydrogen 7 vehicle is shown in Fig. 2 [7]. Major specifications of the on-board liquid hydrogen system are summarized in Table 2. The maximum tank pressure of 5.1 barg (74 PSIG) is controlled by opening a boil-off valve. The released hydrogen is fed to a catalyst which oxidizes the hydrogen to water vapor (referred to as boil-off management system or BMS). The openings of the intake and exhaust lines of the BMS are in lower section of rear bumper.

2.4. Differences between mono-fuel demonstration vehicle and bi-fuel vehicle Several changes were made to the BMW Hydrogen 7 bi-fuel vehicle for the mono-fuel demonstration application. These changes include hardware adaptations as well as software and calibration adjustments. On the hardware side the most significant changes are the removal of gasoline fuel system including fuel injectors, fuel lines, charcoal filters for tank ventilation and fuel rail. The two high-pressure fuel pumps were also removed, which reduces the parasitic losses on the engine. For stability reasons the gasoline fuel tank remains in the vehicle because it is a structural element. The vehicles are equipped with improved catalysts. A schematic of the catalyst setup is shown in Fig. 3 [8]. The catalyst setup consists of two monoliths; the first one covers the stoichiometric operating regime whereas the second one reduces NOx peaks that occur when switching from lean to stoichiometric operation. Also, one of the two mono-fuel vehicles tested at Argonne used non-petroleum based engine oil (however the results do not show a measureable difference). Changes to the software include an adapted calibration of the engine control unit (ECU) as well as the shift characteristics.

2.5.

Fig. 1 – Power and torque curves of a BMW Hydrogen 7 vehicle [3].

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Engine operating strategy

Hydrogen combustion engines are capable of operating at extremely low emissions levels [9]. The main product of complete combustion of hydrogen with air is water vapor. However traces of carbon based emissions (CO, CO2, HC) can be found in the raw exhaust. Their origin is generally attributed to the combustion of lubricating oil [10]. At the low emissions levels of hydrogen fueled combustion engine vehicles the ambient concentration of emissions in the intake air does have a measureable impact on the overall results. These components can be burned during the combustion or oxidized if the vehicle is equipped with a catalytic converter.

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LH2-Tank

1) Side view

12 V + Cable Feed Line (GH2)

Hydrogen Vent Opening

Gasoline Tank

Batteries

Boil-Off-Management-System (exhaust)

2) Top view

Hydrogen Vent Opening

Engine Cover / Hydrogen Rail 3) Isometric view

Fig. 2 – Location of hydrogen bearing parts [7].

Nitric oxide (NOx) emissions occur in combustion engines due to the high temperatures in combination with the presence of nitrogen (N2) from air. NOx emissions are the only critical emissions component for hydrogen combustion engines. For homogeneous operation the amount of nitric oxide emissions

Table 2 – Specifications of the on-board hydrogen storage system On-board storage concept Tank type Number of tanks Total volume Maximum tank pressure Storage temperature Hydrogen storage capacity Fuel nozzle Heat input Average boil-off rate Maximum boil-off rate

Liquid hydrogen Vacuum insulated cryogenic tank 1 170 L 5.1 bar (74 psi) 20 K 8 kg (8 GGE) Generation II cryo nozzle 1.5–3 W 16 g/h 33 g/h

created during combustion of hydrogen is exponentially dependent on the composition of the fuel air mixture. Lean mixtures with a relative air/fuel ratio (l) in excess of l ¼ 2 can be burned without generating NOx emissions. Below this relative air/fuel ratio of l ¼ 2 the nitric oxide emissions increase exponentially with a maximum around l ¼ 1.3. When approaching stoichiometric mixtures the raw NOx emissions decrease.

Monolith 1

Monolith 2

600 cells 120 – 150 g/ft

400 cells 150 g/ft

Fraction (Pt)=max Fraction (Pd)=max Fraction (Rh)=60

Fraction (Pt)=12 Fraction (Pd)=2 Fraction (Rh)=1

‘3-way-concept’ for general emissions reduction

‘NOx storage concept’ for reduction of NOx peaks

Fig. 3 – Schematic of the catalyst setup of the BMW Hydrogen 7 Mono-Fuel demonstration vehicles [8].

international journal of hydrogen energy 33 (2008) 7607–7618

A schematic of the engine operating strategy implemented in the BMW Hydrogen 7 vehicles is shown in Fig. 4 [11]. For low engine loads the engine is operated in the lean burn region. Load adjustment is accomplished by solely adjusting the relative air/fuel ratio, which also minimizes losses usually caused by use of a throttle. For higher load demands the engine switches from lean operating to stoichiometric operation and thereby avoids operation at the NOx critical air/fuel ratio regime. Stoichiometric operation allows using a 3-way catalyst effectively in order to reduce nitric oxide emissions using unburned hydrogen as a reducing agent.

3.

Test facilities and emissions equipment

3.1.

Vehicle dynamometer

Argonne National Laboratory’s Advanced Powertrain Research Facility performs hybrid vehicle testing and alternative fuel testing. The heart of the facility is a 4-wheel drive chassis dynamometer test cell. The 48 in rolls have a variable wheel base to accommodate different size vehicles. The dynamometer can absorb 250 HP per axle. The test cell also has a high voltage DC bus available for electric vehicles. The data acquisition is a custom design which allows great flexibility and rapid implementation of vehicle instrumentation. A schematic of the facility layout is shown in Fig. 5 [12]. The emissions measurement system has Super Ultra Low Emissions Vehicle (SULEV) capability. The facility is also equipped with fast NOx and hydrocarbon analyzers which were not used during these tests. In addition to the standard emissions equipment a hydrogen analyzer as well as a water analyzer was used to determine unburned fuel and humidity content in the diluted exhaust for hydrogen vehicle testing. The safety system is designed to handle advanced hybrid or alternative fuel vehicles. Gas detectors, as well as heat and smoke detectors are linked to a sprinkler system, air handling unit and the fire department. An emergency stop system with manual trigger switches and sensor triggers is integrated into the facility.

3.2.

Facility instrumentation

The main facility signals recorded are dynamometer speed, dynamometer force, test cell conditions, and the standard emissions from the bench (including NOx). The emissions bench provided time-resolved (modal) data based on diluted exhaust analysis and total emissions based on bag data. In addition vehicle information, e.g. time-resolved relative air/ fuel ratio, was collected during the tests and provided by BMW for further analysis.

3.3.

Test cell ventilation and gas warning system

The wide flammability limits of hydrogen require additional safety measures to effectively avoid ignitable fuel air mixtures in the event of a hydrogen release. In the event of an unintended hydrogen release the test cell ventilation system has to be capable of diluting the released amount of hydrogen below the lower ignition limit. In order to accomplish that the test cell is equipped with hydrogen sensors on the ceiling. The standard ventilation of the test cell results in approximately 0.5 complete air exchanges of the entire test cell per minute. This rate is more than sufficient to dilute any small undetected hydrogen leakages. If any hydrogen concentration is detected by one of the hydrogen sensors the test cell ventilation automatically switches to purge mode. This purge mode allows approximately 2 complete air exchanges per minute which should dilute even a larger amount of released hydrogen and purge it out of the building. As an additional safety measure an alarm on any of the hydrogen sensors will automatically trigger an alarm with the Argonne Fire Department. As mentioned earlier the BMW Hydrogen 7 cars are equipped with a vacuum insulated cryogenic tank holding up to 8 kg of liquid hydrogen at a temperature of 20 K (423.67  F). A potential failure mode of the tank is vacuum failure resulting in increasing temperature and hence evaporation of the cryogenic hydrogen. Should this occur the BMW Hydrogen 7 will release the hydrogen stored on-board through vent lines in the back part of the vehicle roof. Therefore it is a requirement that the BMW Hydrogen 7 vehicle is connected to a purge line when parked inside a building. This purge line must be open to the outside of the building and would allow releasing the hydrogen safely in case of a tank failure. A vacuum failure has never occurred on a BMW Hydrogen 7 vehicle. Nevertheless this measure is required as part of the operating instructions for these vehicles due to the lack of long-term experience with the cryogenic on-board storage. Due to the shape of the ventilation lines they are internally called ‘deer antlers’. In order to allow testing both BMW Hydrogen 7 mono-fuel vehicles once a day, deer antlers were installed in the vehicle test cell as well as the soak area. Fig. 6 shows the BMW Hydrogen 7 vehicle in the soak area connected to these ventilation lines.

3.4.

Fig. 4 – Schematic of engine operating strategy implemented in the BMW Hydrogen 7 vehicles [11].

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Emissions equipment and upgrades

Argonne National Laboratory’s Advanced Powertrain Research Facility (APRF) is equipped with a full-dilution Constant Volume Sampling (CVS) exhaust dilution system

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H2 metering and safety panel

H2 safety and emergency system

H2 exhaust content sensor

H2 delivery hose 250 psi

Outside safety and conditioning system

Hydrogen line

Air handling unit H2 12 pack cylinders at 2000 psi / ~7.3 kg

Test cell

Calibration gas room

Conditioning room

Control room

Outside

Emissions bench Data acquisition system

Fig. 5 – Schematic of Argonne’s Advanced Powertrain Research Facility (APRF) [12].

combined with an AVL AMA 4000 emissions bench. The system is rated for testing cars at the Super Ultra Low Emissions Vehicle (SULEV) level. Preliminary tests of a BMW Hydrogen 7 bi-fuel vehicle performed in fall of 2007 allowed the identification of required upgrades to the emissions sampling and measurement system to allow accurate resolution at the low emissions levels (a fraction of SULEV) expected from the vehicles. The major upgrades performed were

The first four measures are required to allow accurate determination of the water content in the exhaust. A water analyzer was integrated into the emissions sampling system to allow for that measurement. In order to stay below the maximum range of the water analyzer (5 Vol-%) a high dilution rate was necessary. This required all tests to be run at CVS flow rates of approximately 24 Nm3/min. These high flow rates in combination with heated lines all the way from the tailpipe to the sampling point reduce the chance of water condensation in the exhaust stream. The effectiveness of these measures can be seen in Fig. 7 showing the speed trace as well as the trace of water concentration of the diluted

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Trace time [s] Fig. 6 – Deer antlers connected to BMW Hydrogen 7 in vehicle soak area.

Fig. 7 – Typical humidity levels of the diluted exhaust during a FTP-75 test.

Diluted H2O [Vol-%]

Vehicle speed [mph]

- Adapt piping to increase CVS flow rates for reduced humidity levels of diluted exhaust - Integrate bag-in-bag setup for ambient bags to mitigate water diffusion - Apply heating to all exhaust piping from tailpipe to measuring point to avoid condensation of water - Integrate water and hydrogen analyzer to allow fuel consumption measurement using ‘water balance’ method

- Integrate a new Non-Dispersive Infrared (NDIR) Analyzer to allow for the required stability to accurately determine hydrocarbon (CO) emissions at the low emissions levels - Identify cross-sensitivity of emissions analyzers to water content

international journal of hydrogen energy 33 (2008) 7607–7618

exhaust versus test time for an FTP-75 cycle. The maximum concentration of water does not exceed 4 Vol-% during an FTP-75 test. The increased water content and the expected low emissions levels also required identifying the cross-sensitivity of the emissions analyzers to changes in water content. This is necessary because the emissions results are calculated as a difference between ambient bag readings and sample bags readings for the different test phases. Due to the high water content of hydrogen engine vehicles the sample bags have considerably higher water content than the ambient bags. Emissions analyzers are known for being cross-sensitive to water resulting in an emissions concentration reading that is higher than the actual emissions concentration. To identify the cross-sensitivity of the analyzers the system was fed with zero air and a known concentration of water (using a Hovacal humidity generator). A sample result of these tests is shown in Fig. 8 indicating that the CO analyzer shows about 0.5 ppm of concentration per percent of water. The cross-sensitivity results are used to correct the emissions bag readings for CO and CO2 based on the measured water level during postprocessing (the modal results presented are not corrected for cross-sensitivity). The FID also shows slight impact of increased humidity levels on the actual HC reading. For the FID it is suspected that the offset is not caused by crosssensitivity of the analyzer itself but rather due to detuning of the mass flow to the burner of the FID unit. The effect could possibly be determined by using humidified span gas. However, the HC results shown for this study are not corrected for humidity effects.

4. Fuel consumption measurement and validation The BMW Hydrogen 7 vehicles have a fairly complex fuel system that does not allow tapping into it for direct fuel consumption measurement. For that reason the preliminary tests in fall of 2007 were used to implement an advanced approach for fuel consumption measurement developed for hydrogen vehicles. This approach requires measurement of

H2O [Vol-%]

3 2

intake air and dilution air humidity as well as water and hydrogen content in the diluted exhaust. Based on these numbers the fuel consumption can be back-calculated applying a water balance using calculations similar to those used for carbon balance with conventional fuels. Details on the method, the equations as well as the results compared to other consumption measurement methods can be found in [13–15]. Fig. 9 shows a schematic of the setup used to verify the accuracy of the water balance. For this approach the fuel consumption of a hydrogen powered vehicle (GMC Silverado converted to hydrogen operation by eTec and Roush; for more details on the vehicle see Ref. [16]) was measured directly using a coriolis mass flow meter and comparing it to the results of the water balance. A sample result for several steady-state operating points is shown in Fig. 10. The hydrogen fuel flow determined with both methods, the direct measurement as well as the water balance and the relative difference are plotted versus test time. During the steady-state phases at various fuel flow rates the differences between direct measurement and calculated results based on water balance are very small. It is apparent that during the load changes the differences become more pronounced. Due to the applied concept the water balance is not designed to measure highly dynamic fuel consumption. However the overall fuel consumption during a drive cycle is expected to match closely even with deviations during highly dynamic operation. This was confirmed by running a Japan 10–15 test cycle. The overall fuel consumption of the direct fuel measurement using a coriolis meter deviated less than 2 % from the water balance measurement confirming that the water balance provides accurate hydrogen fuel consumption values.

5. Test program for BMW Hydrogen 7 vehicles The test program for the BMW Hydrogen 7 mono-fuel demonstration vehicles focused mainly on the Federal Test Procedure FTP-75 cycle. With the vehicle soak area as well as the dyno cell equipped with deer antlers each of the two BMW Hydrogen 7 Mono-Fuel demonstration vehicles could be tested on a cold-start test once per day. This was intended to generate robust and statistically proven results. In addition to using two vehicles the results of two drivers operating the

1

mFuel 1.5

0

1 0.5

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100

200

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CO, HC [ppm]

H2O CO HC

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Intake air Humidity

Dilution air Humidity

Sample point [H2O] [H2]

0 400

Time [sec] Fig. 8 – Influence of increased water content on CO and HC analyzer readings.

Fig. 9 – Schematic of setup for validation of water balance for fuel consumption measurement on hydrogen vehicles.

international journal of hydrogen energy 33 (2008) 7607–7618

Fig. 11. The different sections show overall test conditions, concentration readings for the different test phases, corrected emissions for the individual phases and the weighted cycle emissions and fuel economy numbers.

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Overall emissions results

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Time [sec] Fig. 10 – Correlation between fuel consumption measurement methods for steady-state operating points.

vehicles were generated to eliminate or at least reduce the driver influence. In addition to several FTP-75 cold-start tests on each vehicle the cars were also tested for fuel economy using the Highway cycle. The results presented for fuel economy and overall emissions of the BMW Hydrogen 7 Mono-Fuel demonstration vehicle are average numbers of multiple tests. For the FTP-75 cold-start tests five tests including both cars and two drivers were used for calculations. For the Highway cycles two tests, one on either car, were used.

6.

Results

6.1.

Fuel economy numbers

The fuel economy numbers for the BMW Hydrogen 7 MonoFuel demonstration vehicle are summarized in Table 3. The average fuel consumption during the FTP-75 cold-start test is 3.7 kg of hydrogen per 100 km. When assuming a lower heating value of gasoline of 41.5 MJ/kg and a density of 0.765 kg/l, 1 kg of hydrogen has the same energy content than 1 gallon of gasoline. Converted to the widely used miles per gallon (mpg) scale results in approximately 17 miles per gallon of gasoline equivalent (GGE) on the FTP-75 cycle. The equivalent numbers for the highway cycle are 2.1 kg/100 km and 30 miles/GGE. These numbers are fairly good considering the weight of the vehicle and the large size of the engine. A sample plot of a post-processing sheet used for determining the fuel economy and emissions numbers is shown in

Table 3 – Summary of BMW Hydrogen 7 fuel economy numbers FTP-75 [kg/100 km] FTP-75 [miles/GGE] Highway [kg/100 km] Highway [miles/GGE]

3.7 17 2.1 30

A summary of the emissions results for the BMW Hydrogen 7 Mono-fuel demonstration vehicles (average over five FTP-75 tests) can be found in Fig. 12. The results are shown in absolute numbers as well as percentage of the Super Ultra Low Emissions Vehicle (SULEV) standard. The results for nitric oxides, the most critical emissions component for hydrogen powered vehicles, are at an average of 0.0008 g/mi, which is equivalent to 3.9% of the SULEV limit. The non-methane hydrocarbon (NMHC) emissions are actually 0 g/mi (0 % of SULEV). Any negative values are set to zero. In order to achieve a zero-value, the vehicle actively has to reduce the concentration compared to ambient. Due to the fact that the BMW Hydrogen 7 vehicle mainly operates at lean air/fuel ratios and is equipped with a three-way catalyst, it actively oxidizes HC from the intake air during most phases of the test. Carbon monoxide emissions during the FTP-75 cycle are 0.003 g/mi (0.3% of SULEV), which is barely more than the ambient concentration and might result from instabilities or cross-sensitivities of the analyzer. The way the cars are tested and the results calculated, any concentration of emissions in the intake air is attributed to the car. Because of the low emissions levels of the BMW Hydrogen 7 vehicles these background emissions impact the results. Fig. 13 shows the theoretical results of a case, where any concentration of emissions in ambient air is just passed through an imaginary car without changing the concentration. Due to the emissions background of the intake air (NOx w 0.02 ppm, THC w 0.7 ppm, NMHC w 0.06 ppm, CO w 0.5 ppm) the results of the imaginary car are not zero but account for 0.5% of SULEV for NOx, 2% for NMHC and 0.12% for CO. This comparison clearly demonstrates that the BMW Hydrogen 7 cars actively reduce hydrocarbon emissions in order to achieve the 0 g/mi value. It also demonstrates that the CO values of the BMW are in the range of ambient air. The numbers shown are for one test case; they do vary with changes in the background concentration.

6.3.

Modal results

A better understanding of the cause for emissions can be found when analyzing time-resolved (modal) results. As mentioned earlier the BMW Hydrogen 7 Mono-Fuel demonstration vehicles use an operating strategy with lean burn and stoichiometric operation depending on the load demand. A trace of relative air/fuel ratio during a FTP-75 cold-start cycle is plotted in Fig. 14. The data was collected from the engine control unit. The top portion of the plot shows the vehicle speed versus test time, the bottom the relative air/fuel ratio. The average relative air/fuel ratio (l) during most of the cycle is between 2.5 < l < 3.5. The phases where lambda is less than 2 are startup as well as the fastest accelerations in the second hill of the 505 cycles. During the 10 min soak phase a l-value of 1 is shown, however, the engine is turned off during this period.

international journal of hydrogen energy 33 (2008) 7607–7618

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Fig. 11 – Sample result sheet for FTP-75 test on a BMW Hydrogen 7 vehicle. The impact of the changes in relative air/fuel ratio on the nitric oxide emissions is shown in Fig. 15. The trace shows the modal NOx readings of the diluted exhaust stream during a FTP-75 cold-start test. During most of the test cycle the emissions trace actually matches the zero-line. Four phases

during the entire test are relevant for the NOx emissions measurement. These are the two engine starts as well as the two fast accelerations during the second hill of the 505 cycles. The first engine start shows slightly higher spikes than the second one, which might be due to the cold catalyst during the

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3.9%

0%

0.3 %

NOx

NMHC

CO

0.0008 g/mi

0 g/mi

0.003 g/mi

0% SULEV

Fig. 12 – BMW Hydrogen 7 emissions results compared to SULEV limits.

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Fig. 14 – Relative air/fuel ratio during FTP-75 test.

during lean operation as well as stoichiometric operation. Therefore a sound explanation of the THC spikes during the switching phase cannot be directly derived from the available data. It could be speculated that the lack of oxygen during the switch-over process results in oxidation of unburned hydrogen (H2) rather than hydrocarbons (HC). Another explanation could be the cross-sensitivity of the Flame Ionization Detector (FID) used for THC measurement to water; during the switch-over process the water content increases significantly. Further explanations include the cross-sensitivity of the FID to a sudden increase in hydrogen in the exhaust or an origin of the hydrocarbon spikes from engine oil. The causes are speculations and need further, more detailed investigation. Overall the oxidation in the catalyst dominates over the spikes during switch-over and the increase during the first start-up and results in overall emissions lower than the ambient concentration. Fig. 17 shows the modal results of methane (CH4) emissions. As can be seen from the 10 min soak, the background concentration of methane is approximately 1.8 ppm. Similar to the total hydrocarbon emissions the methane traces during

Vehicle speed [MPH]

first start. The second critical phase during the fast acceleration shows a very distinct spike during the second 505 cycle and only a minimal blip during the first 505 cycle. The reason for the spikes during the acceleration is the switching from lean operation to stoichiometric operation (see Fig. 14). The engine switches every cylinder individually (possible with individual port fuel injection) from lean burn to stoichiometric operation. The average exhaust air fuel ratio during this transition is lean resulting in a low catalyst conversion ratio while some cylinders run stoichiometric and consequently exhaust high levels of NOx. These two peaks during the fastest accelerations occurred throughout the testing performed; the height of the spikes did vary significantly. The modal diluted total hydrocarbon (THC) emissions are plotted in Fig. 16. The horizontal section during the 10 min soak shows the background level of THC emissions in the dilution air. The BMW Hydrogen 7 vehicle is capable of actively reducing the THC concentration as can be seen by the fact that the average curve during engine operation is lower than the one during the soak phase. The most interesting phases during vehicle operation are again the engine starts as well as the switch-over from lean to stoichiometric operation. During the first engine start the THC emissions decrease from an increased level. This is likely due to the catalyst warm-up. The second engine start only shows a minor peak during that phase. Both switching phases from lean to stoichiometric operation show a distinct THC emissions peak. Unlike NOx emissions that can only be converted at stoichiometric operation, the oxidation of hydrocarbons in the catalyst occurs

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Fig. 13 – Emissions signature when sampling ambient air through an imaginary vehicle.

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international journal of hydrogen energy 33 (2008) 7607–7618

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combustion or cracking of lubrication oil. The afore mentioned causes are pure speculation and further research is required to definitely determine the source of the hydrocarbon emissions spikes. The modal concentrations of diluted carbon monoxide (CO) together with vehicle speed versus test time during the FTP-75 cold-start test are shown in Fig. 19. The ambient concentration of approximately 0.5 ppm of CO can again be seen during the 10 min soak phase. However, the readings are not as stable as the ones for nitric oxides or total hydrocarbons. This could be due to the fact that the CO analyzer shows significant cross-sensitivity to changes in water concentration that also occur during the soak phases (compare Fig. 7 and Fig. 8). The analyzer’s resolution was found to be in the range of approximately 0.2 ppm, meaning that any difference in readings lower than this threshold could also result from measurement noise. The increased readings of CO during engine operation are very likely also only due to the water cross-sensitivity of the analyzer. As shown in Fig. 8 the CO analyzer shows about 0.5 ppm of carbon monoxide (CO) per percent of water in the

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engine operation are lower than the background concentration indicating that the vehicle actively reduces emissions. However, the CH4 trace also shows two significant spikes when the vehicle switches from lean operation to stoichiometric operation during the hard acceleration of the second hill in each 505 cycle. Both, the peaks in total hydrocarbons as well as methane are significantly higher than the ambient concentration of the respective concentration. Therefore the peaks cannot just be attributed to a lack of emissions conversion efficiency of the catalyst. The flame ionization detectors used for measuring hydrocarbon emissions are sensitive to changes in humidity. The sudden increase in humidity caused by switching from lean to stoichiometric operation could increase the hydrocarbon readings (see [17]). Flame ionization detectors use hydrogen as a burner gas. A sudden increase in hydrogen concentration as seen during the transition from lean to stoichiometric operation (see Fig. 18) could also cause increased hydrocarbon readings. There are several other possible causes for the HC peaks including for example actual

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diluted exhaust. This is corrected in the bag readings and therefore not reflected in the average results. However, the modal data is not corrected for cross-sensitivity and hence shows a significant dependence on water content.

LLC as well as BMW Group in Munich. The authors would like to express the gratitude to all the individuals from BMW involved in preparing, performing as well as analyzing these tests.

references

7.

Conclusions

Two BMW Hydrogen 7 Mono-Fuel demonstration vehicles were tested at Argonne National Laboratory’s Advanced Powertrain Research Facility (APRF). Due to the low anticipated emissions levels as well as the complex on-board hydrogen storage system, upgrades to emissions equipment and sampling system had to be performed to allow accurate determination of vehicle emissions and fuel consumption. A test phase with two BMW Hydrogen 7 Mono-Fuel demonstration vehicles was completed in spring of 2008. The two vehicles were tested on the FTP-75 cold-start as well as the highway drive cycle achieving fuel economy numbers of 3.7 kg of hydrogen per 100 km on the FTP-75 cycle and 2.1 kg of hydrogen per 100 km on the highway cycle. These values are equivalent to a gasoline fuel consumption of 17 miles per gallon (mpg) and 30 mpg respectively. The emissions results on the FTP-75 cycle showed emissions levels as low as 0.0008 g/mi of nitric oxide (NOx) emissions, 0 g/mi of nonmethane hydrocarbon (NMHC) emissions and 0.003 g/mi of carbon monoxide (CO) emissions. These emissions numbers are equivalent to 3.9% of Super Ultra Low Emissions Vehicle (SULEV) level for NOx, 0% for NMHC and 0.3% for CO likely making the BMW Hydrogen 7 Mono-fuel demonstration vehicle the cleanest combustion engine car tested at Argonne’s APRF.

Acknowledgements The submitted manuscript has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (‘‘Argonne’’). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC0206CH11357. The U.S. Government retains for itself, and others acting on its behalf, a paid-up non-exclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government. This research was funded by DOE’s FreedomCAR and Vehicle Technologies Program, Office of Energy Efficiency and Renewable Energy. The authors wish to thank Lee Slezak and Gurpreet Singh, program managers at DOE, for their continuing support of hydrogen activities. Testing of the BMW Hydrogen 7 vehicles was only possible with the extensive support through BMW of North America,

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