You will find the figures mentioned in this article in the German issue of MTZ 06I2007 beginning on page 000. SGS-04-22
Der bivalente V12-Motor des BMW Hydrogen 7
The Bi-fuel V12 Engine of the new BMW Hydrogen 7
The V12 engine of world‘s first hydrogen-powered luxury saloon car, which was created in a series development process, represents a further landmark along the road to the hydrogen age. Thanks to se- quential hydrogen injection, combined with Valvetronic and a com- plex operating strategy, the 6.0 l engine develops 390 Nm of torque and a power output of 260 bhp. At the same time it clearly undercuts the toughest emissions regulation worldwide.
1 Introduction Due to the lack of a supply infrastruc- ture network for mono-fuel hydrogen vehi- Hydrogen generated by renewable means, cles, the BMW Group decided in favour of a as a carbon-free energy carrier, offers great- bi-fuel power train. est potential for securing the individual mobility of future generations. The devel- opment of the BMW Hydrogen 7 represents 2 Design Features an important step towards this goal. This was the first time in the world that a pre- 2.1 Basic Engine mium hydrogen vehicle underwent the en- The engine is based on the 6.0 l V12 petrol tire series development process, where the engine with Valvetronic and petrol direct identical standards as for all other produc- injection from the BMW 760i [3], Table. tion models of the BMW Group applied. To keep irregular combustion phenom- Authors: The BMW Hydrogen 7 will be built on a ena (knocking, self-ignition, backfiring) un- Wolfram Enke, Manfred Gruber, small scale of 100 units, which are destined der control, the compression ratio was low- Ludwig Hecht and Bernhard Staar for use worldwide by decision-makers in ered to ε = 9.5. This involved shortening the the fields of politics, business and research. connecting rod by 1.5 mm as well as adapt- One aim is to gain their support for the es- ing the geometry of the piston head. tablishment of the infrastructures and the Slits were made in the aluminium cylin- pioneering of the technologies needed to der block at the gussets between the cylin- smooth the way for hydrogen mobility. der liners in order to achieve more consist-
MTZ 06I2007 Volume 68 Alternative Drives DEVELOPMENT
ent heat dissipation around TDC (top dead Engine Data Petrol Bi-Fuel centre) by increasing the coolant flow in Type V-12 / 60° this area. A specially developed four-layer Firing Order 1-7-5-11-3-9-6-12-2-8-4-10 metal gasket establishes the sealing with Displacement 5972 cm3 the cylinder head. The particularities of the stoichiometric Bore / Stroke 89 / 80 mm combustion of hydrogen at full load, such Cylinder Spacing 98 mm as the higher flame propagation speed, the Number of Valves per Cylinder 4 smaller flame quenching distance and the Intake Valve Diameter 35 mm more intensive heat transfer rate than with Exhaust Valve Diameter 29 mm petrol, imply that the piston and the piston Intake Valve Lift 0.3 - 9.85 mm rings are subjected to quite higher thermal and mechanical stress. Exhaust Valve Lift 10.3 mm The new designed piston was therefore Main Bearing Diameter 70 mm configured for higher peak pressures (up to Big-end Bearing Diameter 54 mm 170 bar), Figure 1. Moreover, the thermal Compression Ratio 11.3 : 1 9.5 : 1 stress on the ring groove area has been re- Connecting Rod Length 140 mm 138.5 mm lieved trough the inclusion of a cooling Fuel Frade RON 98 H or RON 98 duct. 2
In configuring the new piston rings, one Table 1: V12 engine’s key data for petrol (basis) and bi-fuel (H2 and petrol) had to keep the blow-by gases as low as pos- sible to avoid the adverse effects arising from a high hydrogen and water content in the crankcase. The 1.2 mm thick compres- needed. The amount of required heat for large cross-sectional areas within a very sion ring represents a compromise between evaporation is drawn from the engine cool- short period. good shape adaptability and mechanical ant and regulated by means of an electric robustness. pump. 2.3 Ignition System The blow-by gases are lead back into Due to its molecular weight, hydrogen The ignition system with solid-state igni- the combustion chamber through the in- handling demands high leak tightness in tion distribution has been adopted from take manifold. In order to exclude the oc- the fuel system. All detachable connec- the petrol engine. A racing surface igni- currence of backfiring in the crankcase, tions, as well as O-ring seals and sealing tion spark plug without protrusive ground an additional shutoff valve has been fitted surfaces on the pressure regulator and on electrode avoids self-ignition in the hydro- in the supply line of the crankcase ventila- the feed line are therefore of a double-wall gen mode. To reduce the hotspot effect, tion system. design. Eventual leakage occurring is iden- the spark plug calorific value has been se- The engine oil quality selection had to tified by a central hydrogen gas sensor. lected to be lower than for a petrol-only be optimised for the hydrogen operating The gaseous hydrogen reaches the rail engine. mode. of the intake manifold (3), Figure 2, trough Like for all gaseous-fuel engines, it was an electromagnetic pressure regulating 2.4 Thermal Management necessary to thoroughly tune the valves valve (1) and the partly flexible stainless One particular feature of the hydrogen op- with their seat rings due to the absence of steel feed line (2). The injection valves (4) erating mode is the broad variation in heat additives. A specially developed wear-opti- supply then the hydrogen sequentially to input between stoichiometric combustion mised alloy was adopted for the seat rings. the intake air. at full load and lean combustion at part At operating conditions close to the The rail pressure regulation is per- load. That fact, together with the high wa- knock limit, knock control requires retard- formed by means of pressure and tempera- ter content in the blow-by gas, implies that ed ignition timing. This implies higher ex- ture sensors (5). This simplifies map-con- the engine thermal management has to be haust gas temperatures, which consequent- trolled gas metering. of a high standard. It must ensure trough ly leads to higher thermal stress on the ex- The aluminium intake manifold with the oil conditioning optimum tribological haust valves. For that reason, these valves the integrated hydrogen collector is manu- conditions at all times. are made of a high thermal resistance Ni- factured by precision sand casting. The The hydrogen heat exchanger of the LH2 monic alloy. Both, the intake and the ex- high standard of gas-tightness required (liquid hydrogen) tank is connected to the haust valves have also an additional rein- means that the casting has to be of a su- engine coolant circuit, Figure 4. This addi- forcement in order to minimise wear. premely high quality. tional circuit is a self-regulating system The entire hydrogen supply system is de- that uses engine‘s heat to increase the hy- 2.2 Hydrogen Supply and signed to remain intact in the event of a drogen pressure inside the tank. A control Mixture Formation crash. thermostat maintains the temperature in In contrast to the petrol mode, hydrogen- The metering precision of the hydrogen the circuit at about 50 °C. The hydrogen mixture formation is based on a cylinder- injection valves, Figure 3, directly influenc- tank and safety control unit (see chapter selective intake manifold injection with a es the idling quality, the load control, the 4.3), which also manages the system diag- relatively low overpressure of 1 bar. This emissions performance and engine‘s ten- nosis (freeze-protection), activates the aux- pressure is generated exclusively by evapo- dency to backfire. The valve design with iliary water pump in the small circuit de- ration of cryogenic hydrogen in the tank, radial gas admission and axial discharge pending on the heat requirements of the and therefore no hydrogen feed pump is provides an optimum basis for handling hydrogen heat exchanger.
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3 Functional Features In the range of maximum power output, compensate the torque leap, the ignition with a mixture slightly richer than stoichi- timing has also to be simultaneously ad-
3.1 Full Load ometric, the NOX can be reduced in the justed to the λ value. The throttle valve, The maximum torque and power output three-way catalytic converter using the which responds more slowly to adjust- level will be achieved at stoichiometric hy- slight hydrogen surplus (approximately ments than the mixture and the ignition drogen operation with very low emissions 1%). The high reactivity of hydrogen allows timing, must be tracked. While the throttle and high efficiency. conversion rates of more than 99.9 % and valve is moving to the target position, the
One characteristic effect of intake mani- therefore very low tailpipe NOX concentra- torque is adjusted by the ignition timing. fold injection of gaseous hydrogen is, how- tions. A modified catalytic coating im- Once the throttle valve reaches the posi- ever, a charge loss, since at stoichiometric proved the rate of conversion even further. tion, the ignition timing returns back to operation the hydrogen displaces approxi- At λ values of approximately 0.97, the re- the optimum combustion range. mately 30 % of the aspirated air. sidual concentration of hydrogen after the Moreover, the lower weight and higher catalytic converter is likewise lower than 3.4 Bi-Fuel Operation sonic speed of hydrogen also affect mani- 0.1%, Figure 6. A simultaneous operation of the engine on fold dynamics, worsening the degree of At λ values between 1 and 1.8 the three- hydrogen and petrol must be absolutely ex- charging. This can, nevertheless, be partly way catalytic converter is ineffective due to cluded. The fuel switch itself represents a compensated by optimisation measures, the lack of reaction partners. No efficient particular challenge, and it will be required and in the case of the V12 hydrogen engine exhaust after-treatment is possible for this under the following circumstances: a compromise between petrol and hydro- range without additionally added substanc- – manually switching by the driver gen operation had to be found. es. Due to the very high NOX emissions and – automatic switching if one of the two The characteristic of hydrogen combus- restricted temperature limits at this λ fuel tanks is detected empty tion strongly depends on the mixture com- range, the use of NOX-storage converters – automatic switching to petrol mode in position. At a λ (relative air/fuel ratio) value proved to be not successful. the event of a defect in the hydrogen sys- close to 1, the ignition energy and the igni- The engine always starts on hydrogen. tem (backup solution). tion advance demand decrease rapidly, This avoids consumption-intensive heating- Before performing the switching, the sys- while the combustion speed and the gradi- up strategies for the catalytic converter. tem checks whether the tank level of the ent of the combustion pressure increase 3.3 Operating Strategy target fuel and its pressure are adequate. If steeply. At a stoichiometric mixture of hy- The operating strategy is divided into three so, the engine management switches over drogen and air, the pressure build-up gradi- ranges, Figure 7: the cylinders of one cylinder bank to the ent and the combustion speed are signifi- – Range 1: in the upper load range, the selected fuel in the same sequence as the cantly higher than for that of petrol and engine is operated at λ = 0.97. Exhaust firing order. The bank is then synchronised air. The ignition timing for optimum com- after-treatment is performed by the by torque alignment. Afterwards, the same bustion is approximately 1 CA (crank angle) three-way catalytic converter, which us- is performed on the second cylinder bank. before TDC. es unburned hydrogen to reduce NOX At stoichiometric operation, the V12 hy- raw emissions. 3.5 Avoiding Irregular Combustion drogen engine achieves nevertheless a pow- – Range 2: at low loads, the engine is oper- The position of the injection valves in the er output of 191.2 kW (260 bhp) and a peak ated at λ > 1.8. The extremely low NOX intake manifold is a decisive factor to man- torque of 390 Nm, Figure 5. raw emissions do not require exhaust age engine‘s operating behaviour. To pre- after-treatment. vent backfiring due to hot residual gas, it is 3.2 Emissions – Range 3: Operation at the λ range be- important to ensure that when it opens
No primary CO2, CO or HC emissions occur tween 0.97 and 1.8 in a hydrogen inter- there is only air, and no combustible mix- in the hydrogen operating mode. Minimal nal combustion engine is excluded, as ture, at the intake valve. The hydrogen amounts of HC occur solely as a result of no effective exhaust after-treatment is must be injected into the intake manifold engine‘s oil consumption, but these are possible. in such a way that it is drawn completely then oxidised in the three-way catalytic Thanks to this strategy, the BMW Hydrogen into the combustion chamber by the charge converter. The residual concentration at 7 undercuts the currently toughest emis- cycle. the tailpipe is negligible. sion regulation world-wide – SULEV – by Mixtures of undefined composition can For hydrogen engines the only relevant approximately 70 %. result in irregular combustion behaviour. emissions are NOX (nitrogen oxides), which The driver must not notice the automat- Injection therefore must be always carried occur as a result of the very high process ic switch between the operating ranges. out at an over-critical pressure ratio in or- temperatures achieved during stoichiomet- However, since as much as 25 % of the der to optimise metering precision. ric combustion of hydrogen. torque has to be compensated in a switch However, since hydrogen engines can be process, high demands are placed on the 3.6 Knock Control operated homogeneously over a broad λ engine management. The basic functions of knock control and range, the wide ignition limits of hydro- Even before the switch threshold is sensing technology originate from the pet- gen/air mixtures (4 to 76 vol.-% in air) open reached, the throttle valves and the Valvet- rol engine. Due to the higher demands of up entirely new perspectives for the avoid- ronic are pre-positioned. As soon as a switch the hydrogen operation, however, the func- ance of NOX, inclusively without charge becomes necessary, the engine manage- tionalities needed to be extended and stratification. While operating the engine ment changes to the other mixture be- adapted. with very lean mixtures (λ > 1.8) NOX emis- tween two combustion cycles. Instead of The same applies to the filter frequen- sions are minimal due to the low process passing through the excluded range, a sud- cies of the engine management, since the temperature. den changeover takes place. In order to noise signature of knocking hydrogen com-
MTZ 06I2007 Volume 68 Alternative Drives DEVELOPMENT
bustion differs from the known variables range of unthrottled load control. The vehi- 4.3 Hydrogen Tank and for petrol operation. As the engine is oper- cle mileage in the hydrogen operating Safety Control Unit ated across a broad λ range, the knock de- mode is around 200 kilometres. A further control unit was developed in or- tection parameters such as filter frequency, The BMW Hydrogen 7 with 6-speed auto- der to cover all hydrogen supply functions knock measuring window and knock in- matic transmission accelerates from 0 to and vehicle’s hydrogen safety monitoring. tensity vary considerably. 100 km/h in 9.5 seconds. The top speed is The software was developed on the basis of In view of the very rapid combustion electronically governed at 230 km/h. the International Electrochemical Com- and the associated high pressure build-up There’s room for further substantial mission’s norm IEC61508 SIL3 (safety integ- gradient in stoichiometric operation, very consumption improvements through the rity level 3) to comply with the highest pos- fast detection and response times are re- development of a hydrogen mono-fuel en- sible safety standards. quired in an event of knocking. gine design. The main functions of this control unit include the management of the operating 3.7 Catalytic Converter Protection mode (petrol / hydrogen), of the refuelling Function 4 Electronic Control Units procedure, the supply of hydrogen to the In case of high exhaust-gas temperatures engine, the pressure regulation in the tank exceeding the maximum permissible ad- 4.1 Engine Management’s during hydrogen withdrawal, calculation mission temperature of the catalytic con- Software Development of the mileage and also other safety, moni- verter, a function to protect it must actu- Existing developments and functions toring, diagnostic and service functions. ate. As mixture enrichment has no effect served as the basis for the new software. in gaseous-fuel engines, the following These were taken as the starting point in strategy is applied in the hydrogen mode: developing the functions for hydrogen and 5 Summary when the critical exhaust-gas temperature bi-fuel operation with load-neutral switch is reached, one cylinder per bank is between petrol and hydrogen operating The BMW Hydrogen 7 has completed all switched, cyclically in the firing order, to modes. the necessary and customary procedures in the leaner mixture NOX negligible range. If Innovations in the project: a series development process. The special a higher reduction in the temperature is – general functions for operating the hy- features of hydrogen as the fuel have been needed, a further cylinder is made leaner. drogen combustion engine taken into account. Basis framework condi- The resulting drop in performance is of a – functions for bi-fuel operation and com- tions had to be established, such as: similar magnitude of the protection func- pensation between the petrol and hy- – training the personnel working on the tion in the petrol mode. drogen operating modes project – special torque structure for hydrogen – qualifying and equipping the workshops 3.8 Fuel Consumption and – hydrogen rail pressure control – integration into the plants‘ production Driving Performance – calculation of the injection times from processes The hydrogen engine still offers considera- the relative fuel amount for hydrogen – setting up hydrogen engine and compo- ble potential for reduction of fuel consump- – realisation of torque-neutral operating nent test rigs tion due to the special characteristics of the mode switch (petrol/hydrogen) at all – creating the hydrogen infrastructure at hydrogen/air mixture. In stoichiometric op- load points the various test locations. eration, high thermodynamic efficiency lev- – hydrogen rich operation (λ ≅ 1) Altogether, it has been proved that the hy- els can be achieved thanks to the extremely – hydrogen lean operation (λ > 1.8) drogen-powered combustion engine vehi- rapid combustion. Load control is neverthe- – switch between lean/rich mixture opera- cle is ready for production on an industrial less susceptible to losses due to the quantita- tion scale. The same production facilities as for tive regulation, but the increased use of – exhaust temperature limiting for hydro- petrol engines are used. All quality and BMW’s Valvetronic should bring about fur- gen safety requirements are satisfied to an iden- ther improvements on that. – knock control for hydrogen tical standard. In lean operation (λ > 1.8) the engine runs – backfiring detection The full expertise for the development unthrottled and load control is performed – hydrogen safety concept according to and production of hydrogen-powered com- by means of qualitative regulation. The com- the German Association of the Automo- bustion engines is consequently available. bustion speed and therefore the thermody- tive Industry (VDA) The insignificantly higher weight (<10 namic efficiency fall sharply, though, as the – acceleration monitoring in hydrogen %) of the engine can be offset by a mono- mixture becomes leaner. From λ ≅ 4 on, one lean operation fuel concept. The production costs of a combustion cycle no longer suffices to fully – diagnosis for hydrogen operation. mono-fuel power train are on a par with complete the combustion. In order to those of a petrol engine provided the neces- achieve optimum consumption in the lean 4.2 Auxiliary Control Unit for Hydrogen sary production scale can be achieved. operation, a combination of qualitative The V12 petrol engine of the BMW 760i has Higher specific power outputs are pos- and quantitative control is required above two cylinder banks each with six high-pres- sible with supercharging and/or direct in- a certain air surplus level. sure petrol injectors, actuated by two auxil- jection. This has already been proved on The vehicle achieves a hydrogen con- iary control units, Figure 8. The bi-fuel V12 the test rig with real engines. sumption of 3.6 kg per 100 km. This equates engine has been equipped with six hydro- Even if the BMW Hydrogen 7 is not com- to approximately 13 l of petrol per 100 km. gen injection valves per bank and with sev- parable to the BMW 760i in terms of road The consumption is lower than in the pet- eral sensors. This required the development performance, it demonstrates that sustain- rol mode (13.9 l per 100 km) due to the of an auxiliary control unit for hydrogen able mobility and the proverbial sheer driv- thermodynamic advantages and the wider operation. ing pleasure are not mutually exclusive.
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References [1] Kiesgen, G.; Klüting, M.; Bock, C.; Fischer, H.: The new 12-cylinder hydrogen engine in the 7 series:
The H2 ICE age has begun. SAE 2006 [2] Klüting, M.; Kiesgen, G.; Berger, E.; Rottengruber, H.: Hydrogen powertrain development for powerful, efficient and clean vehicles. NHA 2006 [3] Jägerbauer, E.; Fröhlich, K.; Fischer, H.: Der neue 6,0-l-Zwölfzylindermotor von BMW. In: MTZ 64 (2003), Nr. 7-8, S. 546 ff.
Figure 1: Cooling duct piston Figure 2: Engine‘s hydrogen supply system Figure 3: Hydrogen injection valve Figure 4: Coolant circuit for the hydrogen heat ex- changer Figure 5: Full-load characteristic curves
Figure 6: NOX diagrams for λ = 0.97 Figure 7: Operating strategy with λ leap Figure 8: Auxiliary control unit for hydrogen - func- tional diagram
MTZ 06I2007 Volume 68