|BMW Hydrogen 7.|
BMW today announced the market introduction of its 7 Series dual-fuel hydrogen combustion engine luxury sedan, the BMW Hydrogen 7. (Earlier post.) BMW will build the car in a limited series and deploy it to selected users in the US and other countries in 2007.
Fueled by liquid hydrogen, the BMW Hydrogen 7 is equipped with a 260 hp (191 kW) 12-cylinder engine and accelerates from 0-62 mph in 9.5 seconds. Top speed is limited electronically to 143 mph (230 kph). The BMW Hydrogen 7 features a dual-mode power unit—controlled at the touch of a button—that can switch from hydrogen to conventional premium gasoline. Engine power and torque remain the same regardless of mode.
The BMW Hydrogen 7 has successfully completed the entire Product Development Process (PDP) obligatory for all new BMWs. In this process, all components of the new technology were integrated into the overall vehicle according to the same criteria applied to standard production cars. The new hydrogen model is built at BMW’s Dingolfing Plant parallel to the other models in the BMW 7, 6 and 5 Series, with the drive unit in BMW Hydrogen 7 coming like all BMW twelve-cylinders from the engine production plant in Munich.
|Current status in terms of weight, volume and cost of various hydrogen storage technologies. Targets are those set by DOE. Click to enlarge. Source: DOE|
Fuel and storage. BMW Group has given preference to the use of liquid hydrogen as the appropriate source of energy for the automobile. Compared with gaseous compressed hydrogen, liquid cryogenic hydrogen offers higher gravimetric and volumetric densities (see chart at right).
Compared to gaseous hydrogen compressed to 700 bar in a tank of the same size, liquid hydrogen offers 75% more energy—and therefore a longer cruising range. Liquid hydrogen storage also offers a lower cost per unit of energy stored.
If one of the two types of fuel is fully consumed, the system automatically switches over to the other type of fuel. The cruising range in the hydrogen mode is more than 125 miles (200 km), with another 300 miles (500 km) available in the gasoline mode.
The Hydrogen 7 comes with both a conventional 74-liter (19.5-gallon US) gasoline tank and an additional fuel tank for liquid hydrogen that holds approximately 8 kilos or 17.6 lb of liquid hydrogen.
|Cutaway view of the LH tank, with the insulating layers visible between the two walls.|
The hydrogen tank is made up of a double-wall tank structure consisting of two-millimeter-thick stainless steel plates and featuring a 30-millimeter-thick vacuum super-insulation layer between the inner and outer tank. This configuration reduces heat transfer to a minimum, the interim layer offering the same insulating effect as approximately 17 metres or 56 feet of styropor. The connection pieces between the inner and outer tanks are made of carbon-fibre bands minimizing the conduction of heat.
BMW uses an analogy to convey the temperature consistency: if a tank of this kind were filled, with boiling coffee, the coffee would remain hot for more than 80 days before cooling down to a temperature suitable for drinking.
The tank insulation keeps liquid hydrogen at a pressure of 3-5 bar and at a consistent temperature of approximately -250° C over a long period. Boil-off management limits the inner pressure within the tank caused by any increase in temperature, and ensures a controlled purge of evaporated hydrogen.
Gaseous hydrogen able to escape in this way is diluted in a venturi pipe and oxidated in a catalyst to form vapor. The period in which a half-full hydrogen tank will be emptied completely in a controlled process is about 9 days, and even then the car is still able to cover approximately 12 miles (20 kilometers) in the hydrogen mode with the fuel remaining in the tank.
To fuel the engine, liquid hydrogen is evaporated in a specific, controlled process within the tank, building up a gas “cushion” under defined pressure. Gaseous hydrogen extracted from the tank uses heat from the engine’s coolant circuit to be warmed up for the subsequent fuel mixing process. This heat is generated by a system of two interacting heat exchangers. The heat exchanger in the secondary system capsule (SSC) receives its heat from the engine’s cooling circuit and delivers this heat, first, via the second heat exchanger to the hydrogen tank and, second, to the hydrogen itself warmed up for the subsequent fuel mixing process.
Engine and Emissions. The 6.0-liter Hydrogen 7 engine is based on the 12-cylinder unit carried over from the 760i. Maximum torque is 390 Nm (287 lb-ft) at an engine speed of 4,300 rpm.
The hydrogen engine uses fully variable VALVETRONIC valve management and variable double-VANOS camshaft control. Gasoline is supplied through direct injection and hydrogen is delivered with a hydrogen supply pipe integrated in the engine’s intake system.
Under full load the power unit in BMW Hydrogen 7 runs under stochiometric conditions: a complete balance of oxygen and hydrogen (lambda = 1). This mixture ratio also provides the highest level of performance and output on low emissions in the hydrogen mode.
Although unlike fossil fuels, the combustion of hydrogen generates neither hydrocarbons (HC) nor carbon monoxide (CO), it does produce NOx at high combustion temperatures. To reduce NOx the Hydrogen 7’s engine runs with a lean burn under partial load (lambda > 2). The lean burn keeps temperatures in the combustion process are relatively low, keeping NOX emissions to a minimum.
Such a lean burn mode can be maintained throughout a particularly wide range of operation in the engine control map. And since hydrogen offers particularly broad ignition limits and burns at a fast rate, only a small amount of fuel is required in the mixture to generate a high level of efficiency, according to BMW.
As the engine moves to a richer burn to boost engine output (reaching a max of lambda = 1), the engine management system helps to reduce the engine-out NOx. Remaining NOx is handled by a regular three-way catalyst.