BMW is taking a different approach to hydrogen than most other auto makers. Rather than developing a fuel cell-powered electric drive train, the company has adapted the existing combustion engine of its 7-series sedan to be able to run on compressed hydrogen and gasoline. The result is a dual-fuel car that can run solely on hydrogen. We took it for a ride this week among the rush-hour traffic of downtown San Francisco to get a taste of the hydrogen highway of the future.
The Hydrogen 7 is based on BMW's 760Li model, and uses a modified version of the flagship luxury sedan's six-liter V-12 engine. The Hydrogen 7 is built at BMW's Dingolfing plant alongside the 5- 6- and 7-series production models.
The Hydrogen 7 has a cruising range of 125 miles under hydrogen power and 310 miles on gasoline. The car runs in hydrogen mode by default and automatically switches to gasoline when the hydrogen runs out. An H2 symbol in the center of the speedometer display lets the driver know when the car is running in hydrogen mode.
Refueling the Hydrogen 7 takes slightly longer than filling up with gas, and requires a Formula One-style semiautomatic fueling system with a special tank connection. Hydrogen sensors are placed at five points around the car to alert the driver to any leakages or deficiencies with any components exposed to hydrogen. If the car is parked for more than 17 hours, the hydrogen begins to boil off and is released into the air as water via a controlled process.
One of the first things that drivers of the Hydrogen 7 notice is the H2 button on the right of the steering wheel, which enables drivers to switch between hydrogen and gasoline. When changing between the two sources, we noticed a slight blip in the throttle, but otherwise, the transition is seamless. The modified V-12 engine produces a maximum of 260 horsepower in the Hydrogen 7, which is about 350 pounds heavier than a regular 760Li. Unsurprisingly, the ride is more sedate than that in the gasoline-only 7-series models.
BMW opts for liquid--rather than gaseous--hydrogen storage due to the improved energy density of the fuel in liquid form. BMW estimates that liquid hydrogen can store 75 percent more energy than gaseous hydrogen compressed at 700 bar. When the car is in hydrogen mode, cryogenic hydrogen moves from the the tank into a heat exchanger, where it is warmed to the correct temperature to enable it to be fed into the engine's hydrogen intake manifold.
The Hydrogen 7 has been through the same front-, side-, and rear-impact crash tests as regular 7-series BMWs, as well as specific crash and fire testing for the car's hydrogen components. The Hydrogen 7's tank features a boil-off management system that allows evaporated hydrogen gas to escape in order to prevent excessive pressure build-up.
The Hydrogen 7 is being produced in small numbers and will not be sold on the open market due to its prohibitively high price. Instead, BMW is initially leasing 25 cars to high-profile customers in the United States. Comedian Will Ferrell was the first recipient of the car last month, and other lessees include Sharon Stone, Brad Pitt, and Davis Guggenheim.
The Hydrogen 7 has two fuel tanks situated behind and beneath the rear seats, one for hydrogen and one for gasoline. The double-walled hydrogen tank comprises inner and outer walls of stainless steel, which are separated by a high-level vacuum to prevent heat transfer. The tank, which holds 17.5 pounds of liquid hydrogen at -418 degrees Fahrenheit, is wrapped with aluminum reflection films and glass-fiber layers for extra insulation.