Vapor Trail

Do we need another reason to feel superior behind the wheel of a BMW?

It's not enough that the uber-brand's propeller-inspired logo is the universal symbol for envy. Now comes the possibility that someday we'll have a Beemer that jumps like a jackrabbit, corners like a snake in a rat hole and boasts zero emissions, so all God's creatures can breathe easy.

BMW AG offers a glimpse of this blue-sky future in the Hydrogen 7, scheduled for real-world testing this year when the auto maker rolls out a fleet of 100 cars for hand-picked “users” on three continents.

Based on the long-wheelbase 760i, the H7 features a 260-hp 6.0L V-12 that alternately burns gasoline or, with the push of the “H2” button on its steering wheel, liquid hydrogen.

To avoid the usual burst of emissions that accompanies a cold start, the H7 fires up using hydrogen. A bar-graph display at the base of the speedometer indicates the rate at which both fuels are consumed.

Running on gasoline, the H7 has a range of 311 miles (700 km). Switching to hydrogen adds another 124 miles (200 km). And this transition is as transparent as hydrogen itself.

Going from hydrogen to gasoline, or vice-versa, results in a benign, barely perceptible twitch. The sensation is not unlike the shift bump of an automatic transmission — the H7's silky 6-speed excluded — but decidedly more subtle than the change from gas engine to electric motor that occurs in a hybrid-electric vehicle.

The engine's performance is unchanged by the driver's fuel choice, but its character differs slightly. Under hydrogen power, there is a muted clatter that BMW attributes to the diesel-like energy density of liquid hydrogen.

Still, this noise is less pronounced than even a state-of-the-art spark-less mill.

Meanwhile, the H7 seems to have two voices. Powered by hydrogen, its exhaust note takes on a more hollow tone than the usually mellow hum of this smooth-running V-12.

Apart from the chrome-trimmed fuel-filler flap on its right hind quarter, a quick glance renders the H7 virtually indistinguishable from a standard 7-Series.

Closer inspection reveals a slightly taller powerdome, a design feature made necessary by the addition of larger injector valves. Meant to accommodate gaseous hydrogen, their increased size is in direct correlation to the demand for wider variation in flow rates and volumes, as well as system pressure, BMW says.

The H7's engine benefits from direct injection when burning gasoline. But when liquid hydrogen is coursing through its system, air meets fuel in the intake manifold.

Larger-than-normal valves accommodate wider flow-rate fluctuations and the broader pressure range required to handle hydrogen's volatility.

Hydrogen burns up to 10 times faster than gasoline, BMW notes, but that is something the auto maker's Valvetronic valve management system and trademark double-VANOS camshaft adjustment technology can accommodate.

A morning of moderately aggressive driving through 62 miles (100 km) of winding blacktop near Berlin burned 10 lbs. (4.46 kg) of hydrogen from the H7's 18-lb. (8-kg) capacity double-walled, vacuum-insulated stainless steel tank. Topping up on hydrogen set us back about €31 ($40), but the experience was guilt-free, having left only a trail of water vapor.

The miracle of hydrogen combustion is a predictable benefit of driving the H7. But refueling the world's first hydrogen-powered, premium-segment production car is not a thrill.

Barring unforeseen advancements in the delivery system, which requires the tank be purged before filling, the process takes about 10 minutes and involves wrestling with a hose and coupling that more closely resemble firefighting apparatus.

Simply pondering the construction of the tank (achieved with the help of Magna Steyr Fahrzeugtechnik AG & Co. KG) could drive one to distraction.

BMW opted for liquid hydrogen because, despite the obvious challenge of maintaining a temperature of -418 degrees F (-250 degrees C) to avoid boil-off, it is more energy dense in this form.

In terms of energy potential, gaseous hydrogen begins to produce diminishing returns when compressed beyond 10,000 psi. (690 bar). General Motors Corp.'s fuel cell-powered Sequel prototype uses gaseous hydrogen stored at that pressure.

Unfortunately, it is difficult to maintain the temperature of supercooled hydrogen. Therefore, the H7 features an elaborate “boil-off” system designed to vent tank pressure as the liquid slowly warms, turns into a gas and expands.

A half-full tank can be vented dry in just nine days, requiring another trip to the filling station.

Instead of selling the H7 models, BMW is loaning them to opinion leaders in a bid to generate buzz. California Gov. Arnold Schwarzenegger is in negotiations to obtain one.

The availability of hydrogen filling stations will help determine which markets in the U.S., Europe and Asia will see the car. California and Washington, D.C., are two.

Here in Berlin, the remnants of the Berlin Wall symbolize the H7's potential to foment change. Little is left of the infamous barrier, but its longest remaining span, along the river Spree, culminates at a filling station bearing the logo of Aral/BP — a partner with BMW in Germany's alternative-fuel initiative.