BMW Research and Technology Showcases Hydrogen Fuel Cell Hybrid, On-Board Reformer

. An output of 82 kW and a high torque that engages right from a standing start deliver the sporty handling typical of BMW.

The supercapacitor pack instead of the gearbox and conventional drive train is mounted in the central tunnel. The force of the 88 kW gasoline engine acts on the front wheels. A reduced gasoline tank leaves space for the hydrogen tank. This configuration means that no constraints are placed on the interior space of the research vehicle by contrast with the series model. The standard five seats are provided. Moreover, the weight of the fuel-cell hybrid vehicle is only just above the value for a corresponding series model.

Although currently better known for its efforts with hydrogen-fueled internal combustion engines, BMW has researched low-temperature PEM (Polymer Electrolyte Membrane) fuel-cell technology has been researched and developed at BMW since 1997. From the beginning, the research concentrated on application of fuel cell technology as an APU with comparatively compact dimensions and a maximally low weight. (The hydrogen concept vehicle BMW 750hL presented as early as 2000 had a fuel cell as a source of on-board electricity.) The researchers believed that using the small fuel cell for the supply of on-board electricity was the most cost-effective scenario in conjunction with the internal combustion engine for launching the technology.

BMW Forschung und Technik GmbH is bringing the fourth generation of APU units on stream. Apart from the increase in service life under automobile load cycles to the current level of 5,000 hours, the complexity of the system has gradually been reduced and a robust fuel-cell unit has been created. This system effectively operates at ambient pressure and achieves an efficiency of 58% for the system over a broad performance spectrum. The special design enables rapid changes in load, such as those occurring during the APU application, from virtually idle to full loading within the space of five milliseconds.

One of the most important issues relating to introduction of the low-temperature PEM fuel cell is the capability to start under cold conditions after a long period without use at temperatures below freezing. The cell design enables the external moistening of the gases to be omitted so that the system is in a position to supply the vehicle with energy after a period of only 30 seconds. Intensive tests, in some cases on extremely steep gradients, demonstrated that there was no degradation after several hundred frost starts. This confirms that the technology is ready for use in vehicles on the road, according to BMW.

The APU supplies the energy necessary for the on-board power supply in the fuel-cell hybrid vehicle, as was already the case in the BMW 750hL. All the units consuming electricity can be supplied in this manner, without having to tap the power of the internal combustion engine. This application, however adds traction support. The combination of the APU (providing a small but continuous output) and the back-up storage in the supercaps (delivering high outputs over a short period) achieves an efficient yet marketable drive system, specially designed for city trips, according to BMW.

Reformer technology: fewer emissions during cold start

The reformer unit. Click to enlarge.

For the second concept, BMW integrated hydrogen reformer technology in an otherwise standard 5-door BMW 1 Series. The initial seconds after cold starts present a particularly difficult scenario for meeting increasingly stringent emissions regulations because catalytic converters only achieve maximum effect after they have been heated to a specific temperature. The reformer technology intervenes precisely at this point and significantly reduces the level of engine-out emissions that are generated during a cold start.

The on-board reformer system. Click to enlarge.

Reformer technology can be used in gasoline and diesel engines. The system comprises a mixing zone, an injection valve, a spark plug and a special catalytic converter where fuel is partially oxidized with a limited supply of oxygen. The catalytic process that is initiated in this way selectively splits hydrocarbon chains (C_xH_y) to generate a synthesis gas with a proportion of approximately 21% hydrogen and approximately 24% carbon monoxide. This synthesis gas is then supplied to the conventional intake manifold of the engine by selectively delivering it into the cylinders via air assisted injection valves.

This mixture can completely replace conventional fuel while the engine and catalyst system is warming up. Since the gas mixture burns with virtually no residues, the critical phase for emission behaviour immediately after a cold start is transformed by reformer technology into a particularly clean operating mode.

The lower efficiency of the reforming process is the only disadvantage of using the synthesis gas generated from fuel compared with continuous operation of the internal combustion engine. When combustion of the gas occurs, 15 to 20% of the original fuel-energy content is converted into heat. However, this side effect is particularly useful during the warming-up phase. The heat generated in the reformer heats up the engine faster than during operation with gasoline or diesel fuel for the increase in engine efficiency as a result of a reduction in frictional losses and complete and very stable combustion.

The reformer system presented in the research vehicle is currently equivalent to the status of a prototype. However, series development is still precluded by the need to reduce the size of the components and the weight of currently some five kilograms to a maximum of one and a half kilos, while optimizing the characteristic properties of the system at the same time. Once these requirements have been met, the system could also be used to replace the auxiliary heater currently used in diesel engines or for regenerating the particle filter and for selective catalytic reduction (SCR catalysis) of nitrogen oxides (NO_x). The expense for the exhaust treatment used to date is then reduced accordingly.