The 7 Series vehicle will use a bi-fuel 6.0-liter V12 engine fueled by gasoline and liquid hydrogen (LH). Both the engine and the LH storage system were on display at the Hydrogen Expo. (Earlier post.)

The engine, which uses direct injection for gasoline but port injection for the hydrogen, is optimized for neither—in other words, it sits in the middle. Running on hydrogen, it can produce 170 kW (228 hp) of power and 337 Nm of torque.

A monofuel-fuel hydrogen version of this engine powered the record-setting H2R in 2004 (earlier post), highlighting that there is further power enhancement work that can be done.

With the 170-liter liquid hydrogen tank storing about 8 kg of hydrogen, the car has a hydrogen-fueled range of 200 to 300 kilometers (125 to 185 miles). On gasoline, the car has a range up to 500 kilometers (310 miles).

Running on hydrogen, the bi-fuel engine far exceeds SULEV II targets in the US:

• Less than 1% of the 0.01 g/mi NMOG
• Less than 1.0% of the 1.0 g/mi CO
• 30% of the 0.02 g/mi NO_x. The monofuel version delivers less than 10% of the NO_x target.

BMW combines engine management with catalytic aftertreatment to reduce the NO_x emissions. Through the mid range of the engine curve, BMW operates on a lean burn to reduce NO_x. Once a power threshold is crossed, is switches to a stoichiometric mix, with the aftertreatment handling the NO_x.

BMW continues to explore the utility of liquid hydrogen as a vehicle fuel (earlier post). With its higher volumetric density (0.070 kg/liter) than gaseous hydrogen (0.030 kg/liter @ 10,000 psi, liquid hydrogen offers BMW an way to satisfy what it sees as its customers needs better than a compressed hydrogen gas system. (Both BMW and DaimlerChrysler were in full agreement during the panel discussion that storage is the biggest obstacle automakers faces in delivering hydrogen vehicles acceptable to the market.)

Cutaway view of the LH tank, with the insulating layers visible between the two walls.

One of the downsides of liquid hydrogen is the energy required to compress it. The other is that as the liquid hydrogen inexorably warms from its fueling temperature of -253º C, it expands, increasing the pressure in the tank. Once internal pressure in the tank reaches 5 bar, excess pressure is relieved via a pressure check valve.

Boil-off management catalytically converts the hydrogen together with oxygen in the air to form water.

With current tank technology, the hydrogen takes about one day with the engine not running to reach a pressure of 5 bar.

BMW notes that it is “carefully observing” the ongoing technical development of gaseous fuel tanking systems, and will ultimately deliver the best solution for the customer.

In close cooperation with the South German Technical Inspection Authority (TÜV), BMW has simulated a variety of accident scenarios with the tank, which it is developing with Magna Steyr. (BMW earlier had worked with Linde on LH tanks. Linde is still working with BMW on standardizing liquid hydrogen fueling systems.)

In one series of tests, full tanks with blocked safety vales were destroyed under high pressure. Additional safety valves inside the tank ensure that the hydrogen discharges safely under such conditions.

Another series of test exposed filled tanks to fire on a test bed, surrounding the tanks with flames of almost 1,000 º C for up to 70 minutes.

A final series of test deformed full LH tanks by hard and solid objects causing “substantial damage” in the process. No tank exploded. TÜV concluded that running a car on liquid hydrogen can be just as safe as running a car on gasoline.

BMW is definitely in a minority among automakers with respect to its work with liquid hydrogen, especially given the additional energy required to compress the gas. In response, the company keeps coming back to the need to fit a technology to customer requirements and capabilities.

There are a number of reasons BMW sees it worthwhile continuing to work with LH—one is the range of the vehicle as mentioned before. The other is the geographic area available for refueling stations in Europe.

Liquid hydrogen storage requires a smaller footprint than on-site production, for example, and BMW argues that those restrictions would prove to be important obstacles in the widespread deployment of a hydrogen infrastructure.

With a road program looming, the company should soon be able to collect both quantitative data and subjective responses from its test customers on the LH bi-fuel vehicles, just as other automakers are collecting data on their fuel-cell prototypes, to enhance its work and validate—or disprove—its assumptions.