The brief included completing a whole lap of the Nürburgring at race pace using purely hydrogen. A Rapide S was chosen because it offered the most packaging space for the four hydrogen tanks and because it would be the model most likely to be fitted with the technology should a road car application follow.
The Rapide S race car is essentially a road-going version with a stripped-out interior, stiffened suspension and an added rollcage.
The hardware changes necessitated by the hydrogen adaptation add about 80kg to the Rapide S’s weight. But King points out that this is considerably less than the weight of batteries that would be needed to complete a flat-out lap of the ’Ring on electric power, the other way of achieving zero tailpipe CO2 emissions. The overall weight of the car is about 1600kg.
King describes those hardware changes as being “relatively straightforward” alterations. A whole re-engineering of the Rapide S was certainly not needed.
Four hydrogen fuel tanks have been added: two over the rear axle and two where the passenger seat would be. These hold 3.3kg of hydrogen at 350bar, enough for about a lap and a half of the 16.1-mile Nürburging circuit (Nordschleife and grand prix circuit combined), and the equivalent of about 15 litres of petrol.
Hydrogen fuel lines take the fuel to Aston Martin’s familiar 6.0-litre V12 engine, which King said “needed no new hardware” to be able to run on hydrogen as well as petrol, or a combination of both.
“The biggest challenge was to equalise the power delivery,” said King, hydrogen having a much lower power density than petrol. The solution was to fit a pair of turbochargers to the V12 engine for the first time.
Hydrogen injectors were added next to the petrol ones to inject into the manifold, with other changes including a reduced compression ratio, a revised intake manifold and a revised cylinder head gasket.
“The clever bit really comes in the electronics of the engine management,” said King, adding that changes were made here to allow the engine to adapt to the different properties to burn hydrogen instead of petrol.
The driver is able to select between running on pure hydrogen, pure petrol or a blend of both. But Aston wanted to limit the running on the blend mode to fully test the capabilities of running on pure hydrogen and demonstrate that it could be done.
The rules of the Nürburgring 24 Hours limit power outputs to 550bhp for entrants, so Aston was unable to gain any power benefits from the turbochargers when running in petrol mode, although mid-range torque is said to have improved substantially.
When running on pure hydrogen, power is down by about 50-60bhp, according to King. In clear traffic, lap times of 10min 15sec were expected in petrol mode, and 10min 30sec on hydrogen. Aston started stints by running on pure hydrogen and then ran the tanks until empty after a lap and a half before petrol power automatically took over.
To refuel, a special hydrogen filling station was at the entrance to the pits, where the Rapide S’s four tanks were filled with hydrogen (supplied by Linde Gas, headed by former Premier Automotive Group chief Wolfgang Reitzle) in just over a minute before heading down the pits to its normal stop for petrol, tyres and a driver change. Being able to run an extra lap between stops more than compensated for the minute lost in the pits while taking on hydrogen.
Any road car application for the technology is five years away at the earliest, said King, adding that there were no plans at all to do so at the moment. “The idea of the car is as a credible alternative to a battery hybrid vehicle,” he said. “You can travel at start-up, in the city, and in traffic on hydrogen, and get all the benefits of petrol performance out of the city.
“Converting internal combustion engines to run on hydrogen is a good bridging technology to hydrogen fuel-cell vehicles, which the industry is fully behind. It can help expand the network, bring down the cost and improve the technology.
"We’re at least five to seven years away from seeing it in a road car, but running it successfully in a race car can only increase interest and awareness.”