The Investigation of Hydrogen-Based Vehicle Propulsion Systems at FKFS and IFS

Abstract

The defossilisation of transport is of central importance for meeting global climate targets. In addition to introducing battery-electric vehicles that today are mainly intended for individual and local passenger transport， fuels based on renewable resources offer the possibility of fulfilling energy-intensive transport tasks with low global greenhouse gas emissions. Hydrogen produced from renewable energy is gaining particular importance here， as its use in mobile applications ensures not only overall low global warming potential but also the lowest tailpipe greenhouse gas emissions.In this paper， the various aspects of the use of hydrogen in vehicles are briefly presented and the work at FKFS and IFS is illustrated using the development of an H₂-DI combustion process for ICE applications as an example.The presented FVV research project investigates a high-pressure direct injection concept on a single-cylinder passenger car engine test bench. It aims to operate a spark-ignited engine near stoichiometric conditions to produce a significant power output with low boost pressure demand. However， for a hydrogen engine， a stoichiometric mixture leads to an increased knocking tendency towards higher loads. To avoid pre-reactions in the end gas， the injection starts shortly before TDCF and the hydrogen jet is ignited by the spark plug. The injection duration and therefore the maximum hydrogen mass flow through the injector nozzle influence the combustion duration. Challenges of the investigated hydrogen combustion process are， among others， the higher NO_X emission level compared to the lean operation and the hydrogen slip into the exhaust system.

Keywords

hydrogen; internal combustion engine; direct injection; stoichiometric combustion

1 Hydrogen Powered Vehicles

As part of the efforts to avoid CO₂ emissions in road traffic， battery-electric vehicles are currently in the foreground in the sector of passenger cars and light duty vehicles. In the area of heavy commercial vehicles， battery-electric drives will initially not be the preferred solution due to the high battery capacity required for this （>1 MWh）， the associated increase in load （>45 t）， the high charging power required （>1 MW）， the nevertheless limited range （<650 km） and the charging times needed that will be eliminated in the context of the introduction of automated driving.

The electrical supply of heavy-duty electrical vehicles （HD-BEV） during travel requires the installation of an overhead line infrastructure at least on long-distance routes. Since Catenary-Trucks have to use two parallel overhead lines the supply of electrical power is limited （voltage） which makes the expansion of long motorway sections necessary and is associated with high costs.

So， from today's point of view， the use of hydrogen from renewable sources as an energy carrier represents an important possibility to realize high ranges and short filling times without local CO₂-emissions. Due to the high efficiencies that can be achieved 63%~65%， operation with fuel cells （PEM） as energy converters is envisaged in the medium to long term. However， the efficiencies of currently available PEM fuel cell systems drop sharply at high continuous power levels （<45%） and large amounts of heat have to be dissipated via the vehicle cooling system. With a 15~20 K lower temperature level of the coolant circuit compared to combustion engines， this poses great challenges for the thermal management of the vehicles， e.g. when driving up steep gradients over a longer period of time （e.g. crossing the Alps）. In addition， an increase in the annual mileage of heavy commercial vehicles， as is to be expected due to the increasing establishment of automated driving in long-distance traffic， will lead to significantly higher demands on the cycle stability of fuel cell systems.

For this reason， the use of hydrogen in combustion engines for heavy commercial vehicles is becoming increasingly important. Here， engines are used which are usually based on diesel engines or on natural gas engines derived from diesel engines. If not already done， these are converted to petrol-engine combustion processes. This usually includes the application of an ignition system， the reduction of the compression ratio， the adaptation of the piston geometry and the application of a mixture formation system for gaseous fuels. Today， external mixture formation systems （PFI） are often used， as they are easier to adapt and allow a high degree of homogenization. The associated effects such as lower mixture heating values， higher knock tendency as well as the danger of backfiring and pre-ignition make the development of systems for internal mixture formation （H₂-DI） necessary. Current developments concentrate on systems that operate at pressures between 15 and 30 bar. One reason for this is that gaseous fuels such as hydrogen are preferably stored in pressure accumulators in the vehicle at pressures between 300 and 700 bar， and the use of low rail pressures ensures a long vehicle range. Due to high losses （approx 7% for H2 @ 1-200 bar） during the compression of compressible media， recompression of the gas in the vehicle has not been attempted up to now.

2 Hydrogen Combustion Process

So， the current research and development of hydrogen combustion processes mainly focuses on lean direct injection operation to minimize engine-out NO_X emissions^［1-2］. Lean combustion comes along with a significant additional boost pressure demand to reach high engine loads. Therefore， the herewith presented research project investigates near-stoichiometric hydrogen combustion with direct injection^［3］. However，Ref.［4］ and ［5］ report a higher risk of pre-ignition or knocking for premixed engine operation at low air-to-fuel equivalence ratios and higher loads. Late compression stroke injection offers the possibility to avoid premixing and， thereby， pre-reactions. In particular， the project strives for a jet-guided combustion process using a spark-ignited hydrogen jet. The experimental investigations are carried out at a hydrogen engine test bench at the FKFS.

A passenger car gasoline engine （Mercedes-Benz M 254） is the basis for the single-cylinder research engine with a displacement of 500 cm³. The adaption to hydrogen includes only minimum changes without mechanical modifications. The relative positioning of the injector and spark plug is optimized for a jet-guided gasoline combustion process. The injector is mounted vertically near the central position of the combustion chamber， and the spark plug is positioned between the intake and exhaust valves^［6］. The outward-opening Bosch HDEV4 piezo-controlled gasoline injector is adequate for fundamental investigations with high-pressure hydrogen （gaseous） direct injection. However， for short hydrogen injection durations of about 10°CA in non-premixed jet-guided mode， the cross-section of this injector is merely adequate for low engine speeds and loads.

Fig.1 shows a schematic view of the experimental setup. The external charging unit delivers a pressure of up to 3 bar absolute， and the conditioning system controls the intake air temperature. The intake plenum and throttle flap reduce pulsations in the air path. An exhaust flap allows for an adjustment of the resulting exhaust back pressure. 300 bar bottle bundles provide hydrogen that the pressure regulator reduces to a maximum rail pressure of 200 bar. In addition to the measurement of temperatures and both static and dynamic pressures， a Horiba exhaust gas analyzer and an H₂ mass spectrometer are installed. For safety reasons， an active crankcase ventilation system provided by Hengst reduces accumulated H₂ below the piston.

Fig.1  Schematic SCE experimental setup^[3]

The air-to-fuel equivalence ratio is calculated from the measured oxygen content of the exhaust gas using Eq.（1）^［7］. This equation assumes complete fuel conversion， generating inaccurate results for incomplete stoichiometric combustion. Therefore， the first experiments are performed with a slightly over-stoichiometric air-to-fuel equivalence ratio of 1.1.

$${\lambda }_{{\mathrm{o}}_{\mathrm{2}}}=\frac{[{\mathrm{O}}_{\mathrm{2}}]-\mathrm{1}}{\mathrm{4.7733}[{\mathrm{O}}_{\mathrm{2}}]-\mathrm{1}}$$

For future tests， a recently published modified equation considering the products of incomplete combustion is a promising option to control a stoichiometric operating point accurately^［1］.

Fig.2　illustrates the intended injection and ignition strategy. For an ignition timing variation， a definition of the ignition timing relative to the injection duration is beneficial.

Fig.2  Schematic explanation of injection timing and ignition timing relative to the overall injection duration

Fig.3 shows the cycle-averaged cylinder pressure signal and calculated heat release rate for a variation of the ignition timing for a 1 500 min^－1 and 5 bar IMEP operation point at an air-to-fuel equivalence ratio of 1.1. The maximum rail pressure of 198 bar for this specific mass flow enables the minimum injection duration for the given injector and operating point. For this study， MFB 50 is kept constant at 8° aTDCF， which leads to an adjustment of the injection timing between the presented cases. The colored bars in the upper graph illustrate the injection phases and the arrowheads the spark timings.

Fig.3  Cylinder pressure and heat release rate (average of 200 working cycles) for a variation of the relative ignition timing (1 500 min^－1, 5 bar IMEP, λ =1.1, 8 °CA aTDCF MFB50)

For the earlier ignition timings， a fast conversion of the already injected fuel results in a first peak in the HRR， but the rate drops to a lower level for the remaining injection time. Ignition timing at EOI leads to a fast fuel conversion with the highest peak in HRR due to rich conditions at the spark plug.

Fig.4 presents significant correlations for the variation of the relative ignition timing. Early ignition favors combustion instability， as can be seen from an increased coefficient of variation for IMEP， p_max and MFB50， combined with a long combustion duration and a low maximum pressure gradient. This enables an advantage in NO_X emissions but reduces indicated efficiency simultaneously. For relative ignition timings of 50% to 70 %， the H₂ slip into the exhaust system decreases encouraged by accelerated flame propagation， but pressure gradients rise to an unacceptable level. Further investigations， like a variation of the MFB50 timing， will be performed for the different cases to find a compromise between cylinder pressure gradient， H₂ slip into the exhaust manifold and indicated efficiency. A combustion control with multiple injection pulses similar to Ref.［9］ could be a promising option to reduce the cylinder pressure gradient. The study shows the feasibility of a jet-guided combustion process with near stoichiometric conditions for the given operating point. For this research engine and operating point， the NO_X engine-out emissions are an order of magnitude smaller compared to the homogeneous operation at an air-to-fuel equivalence ratio of 1.1. Nevertheless， an exhaust gas aftertreatment system， which is not part of this project， is inevitable for NO_X tailpipe emission reduction.Ref.［4］ and ［8］ discuss NO_Xreduction with H₂ as a reducing agent and the challenge to mitigate NH₃ and N₂O slip.

Fig.4  Significant measurements for a variation of the relative ignition timing (1 500 min^-1, 5 bar IMEP, λ= 1.1， 8 °CA aTDCF MFB50)

3 Conclusion

In this paper the concept of high-pressure direct injection for a hydrogen-fueled ICE is introduced. First results show the feasibility of a jet guided combustion process with near stoichiometric conditions.

Acknowledgements

The presented single-cylinder hydrogen engine study forms part of an international research project （FVV project No. 1446） performed by the Institute of Automotive Engineering （IFS） at University of Stuttgart under the direction of Prof. André Casal Kulzer and by the Automotive Powertrain Technologies Group of Dr. Patrik Soltic at Swiss Federal Laboratories for Materials Science and Technology （EMPA）. Based on a decision taken by the German Bundestag， it was supported by the Federal Ministry for Economic Affairs and Climate Action （BMWK） and the AIF （German Federation of Industrial Research Associations e.V.） within the framework of the collective research networking （CORNET） program （IGF/CORNET-No. 307 EN）. Furthermore， it was financially supported by Swiss Federal Office of Energy （SI/502205-01） and FVV e.V. （funding No. 6014462）. The project was conducted by a working group led by Dr. Stephan Liebsch （IAV GmbH）. The authors gratefully acknowledge the support received from the funding organizations， from the FVV e.V. and from all those involved in the project.

Abbreviations：

Symbol	——	Definition
aTDCF—After top dead center firing
°CA—Degree crank angle
DI—Direct Injection
EOI—End of injection
HRR—Heat release rate
IMEP—Indicated mean effective pressure
MFB50—50 % mass fraction burned
SOI—Start of injection