Fuel cell auxiliary power units for ICE-Hybrid vehicles expand the electric driving range and increase fuel economy in urban driving cycles.
Reduction of greenhouse gas emissions brings the story of fuel economy back to the public focus. Even if less than 20 percent of the CO2 emissions are coming from traffic, the automotive industry is requested to play a significant part to reduce the CO2 emissions. For this reason, all vehicle manufacturers are working on advanced internal combustion engines and hybrid vehicles, which should combine the pollutant emissions of Spark Ignition (SI) engines with the fuel economy of diesel engines.
The introduction of hybrid powertrains opens the door for electric driving, without reduction in comfort, acceleration and top speed. In hybrids, the reduction in fuel consumption comes mainly from engine stop instead of idling, brake energy recovery and electric driving or shift of operation point during low load phases. Electric driving makes the automobile completely free of local emissions. But although the battery technology has made significant improvements with regard to capacity, weight, volume and durability, the range of pure electric driving is limited to acceleration phases and few minutes of low load cruising. This article shows a way to extend electric driving with efficiency, which internal combustion engines achieve only at maximum load, and accommodate simultaneously the increasing electric power consumption of modern passenger vehicles.
The public considers hybrid vehicles a promising solution to the CO2challenge. Especially for inner-city driving, hybrid vehicles offer a big potential for fuel consumption reduction. The fuel consumption benefits result from different hybrid features (Figure 1):
Further, boosting by the electric machines offers the potential for a consequent engine downsizing without affecting in drive-away torque. To fully exploit the benefit of these hybrid functions, each powertrain component has to be designed with regard to maximum efficiency of the total powertrain. Due to the complexity of hybrid powertrains an optimisation of the total system cannot be achieved with conventional methods. Hence, a combination of simulation and numerical optimisation must be used to optimise the hybrid system. In context of the research project “Optimised Layout of Gasoline Engines in Hybrid Powertrains” supported by the “Forschungsvereinigung Verbren-nungsmotoren”, simulation models of two parallel hybrid powertrains and one power-split concept have been built up. A parametric approach for the description of the internal combustion engine, the powertrain components and the operation strategy was chosen which allows for a variation of these main hybrid parameters in the simulation model.
With this approach simulations with various combinations of internal combustion engine, powertrain components and operation strategy can be conducted. The optimisation can be made for minimum fuel consumption under consideration of performance target values.
Figure 2 displays the fuel consumption results for a parallel hybrid in three different vehicle classes and the fuel consumption of the standard engine with variable valve timing in conventional powertrains. Compared to the standard engine with VVT in the parallel hybrid the fuel consumption benefit is about 22 percent for medium and SUV class vehicles. For compact class vehicles the benefit decreases to 16 percent. The higher benefits for big vehicles comes from the higher vehicle weight which increases the recoverable brake energy and the higher fuel consumption benefit when the engine is stopped during vehicle standstill. Moreover, the substitution of low engine loads with electric driving results in a bigger fuel consumption benefit with increasing engine displacement.
Fuel Cells systems are able to produce electric energy with extremely high efficiencies of more than 50 percent and only steam as exhaust gas when they are fueled with hydrogen. A hydrogen fuel cell system mainly consists of a fuel cell stack, an air supply unit and a hydrogen supply system. To operate a vehicle with such a hydrogen fuel cell system, power electronics and an electric motor is necessary. In most exhibited demonstrators, an additional battery permits brake energy recovery and reduces dynamic operation of the fuel cell. Even if the efficiency of the fuel cell system is extremely high the driving range is limited by the storage volume of hydrogen. High pressurised hydrogen up to 700 bars provides 30 percent of the energy density of petrol or diesel. Therefore, the challenges of fuel cell vehicles are hydrogen production, on board storage and reduction of costs.
Fuel cells bring with them benefits for the environment and the user by producing electric energy on board for electric auxiliaries of the vehicle which normally is produced with a generator driven by the internal combustion engine. During low load driving the efficiency of this power generation is less than 25 percent. With a fuel cell auxiliary power unit (APU) with on-board fuel processing from conventional fuel, the efficiency of power generation will increase to more than 30 percent.
An on-board fuel processor consists of a reformer catalyst to convert the hydrocarbons into hydrogen, CO2 and CO and several gas cleaning catalysts. The gas cleaning is necessary because of the limited CO tolerance of the anode electrode of the polymer electrolyte fuel cell (PEFC). The gas cleaning process can be handled in two water gas shift steps, where the CO is converted with steam to hydrogen and CO2. Such a gas processor has an overall efficiency of up to 80 percent using gasoline as fuel. The complexity of an on-board fuel processor is comparable to modern diesel after treatment systems with particulate filter and NOx-trap. Beneath the energy losses for on-board hydrogen production there are other peripheral consumers inside the fuel cell system like cooling pumps and air compressor. The energy losses of peripherals from current fuel cell systems are in a range of 20 percent of the produced electric power. Also the hydrogen utilization of the fuel cell stack has a great impact on the overall efficiency of the APU-system. Optimisation of the membrane electrode assembly (MEA) for usage of reformats with a hydrogen amount of less than 50 percent can reduce the required fuel excess of currently 20 percent.
In combination with hybrid vehicles the APU can significantly extend the electric driving range in urban cycles. The fuel cell provides the power during low power driving. During acceleration, additional power is provided by the battery which is charged by braking and by the fuel cell system. For the simulation the optimised parallel hybrid is used. Additionally to this hybrid a fuel cell system with an output power of 15kw was implemented into the hybrid powertrain (Figure 3).
The vehicle data for the parallel hybrid with and without fuel cell are given below:
Primary goal of the battery management is to control the battery state of charge (SOC). For the parallel hybrid with fuel cell range extender battery recharging is performed in three priority steps:
Depending on the battery SOC the fuel cell or the battery can provide the demanded power for electric driving. As the efficiency for the electric energy supply can be improved by the fuel cell system, an extension of the electric driving range results in higher powertrain efficiency. In figure 3 it can be seen that the electric driving limit for the parallel hybrid with fuel cell range extender was increased significantly resulting in a shorter operation time of the combustion engine. Figure 4 shows the simulated fuel consumption values for a conventional vehicle, the parallel hybrid and the parallel hybrid with fuel cell range extender.
Hybridisation offers a big potential to reduce the fuel consumption especially in the ECE (urban part of the NEDC cycle) as all hybrid features – start/stop, electric driving and energy recovery – can be fully used. With the parallel hybrid a fuel consumption reduction of 32 percent in the ECE can be achieved compared to the conventional vehicle. A further reduction of 6 percent can be achieved with the parallel hybrid with fuel cell range extender. The advantages of the hybridisation which have their greatest impact in urban cycles can be improved by the fuel cell range extender. For the fuel cell system with on-board fuel processor and a maximum system efficiency of 34 percent was used. A further fuel consumption decrease can be achieved with an improved fuel cell system with a maximum system efficiency of 37 percent. The significant fuel consumption reduction of 10 percent in the ECE compared to the parallel hybrid without fuel cell shows that an improvement of the fuel cell system has to be one main target for the parallel hybrid with fuel cell system. Additional improvements to electric power generation for vehicle electrics and engine auxiliaries are not taken into account in the simulation model and should result in a further fuel consumption reduction, especially for customer use driving.
Hybrid vehicles can substantially reduce fuel consumption by energy recovery and electric driving in urban cycles. A further improvement of powertrain efficiency can be achieved by implementation of a fuel cell system into the hybrid powertrain. The limited battery capacity for electric driving can be compensated by a fuel cell system, which provides electric power with higher efficiency than the internal combustion engine and the conventional generator. With a fuel cell auxiliary power unit (APU) with on board fuel processing from conventional fuel, the efficiency of power generation will increase to more than 30 percent.
With small fuel cell systems and an on-board fuel processing, a fuel reduction of 36 percent compared to conventional vehicles and 6 percent compared to parallel hybrids in the ECE cycle is possible. For the NEDC, a fuel consumption reduction of 22 percent compared to conventional powertrains and 2 percent compared to parallel hybrid powertrains is possible. Further optimisation potential is given by improvement of fuel cell system efficiency and weight. Higher fuel cell system efficiency can be achieved by reducing peripheral losses and increasing conductivity of MEAs for reformat, which leads to a reduction of fuel consumption. Simulations with such an improved fuel cell system, efficiency results in a fuel consumption reduction of 10 percent in the ECE.
Acknowledgements
Thanks to the “Forschungsvereinigung Verbrennungskraftmaschinen e.V.” (FVV) and the “Bundesministerium für Wirtschaft”, for funding parts of the research projects, presented in this article.
