A couple of weeks ago, I piloted the Mercedes E300 Hybrid most of the way from Brighton to London in the RAC Future Car Challenge. By the time I reached Exhibition Road in central London, the trip computer said that I had averaged 65.7mpg. Which is right on the car’s brochure promise of 109g/km. Trouble is, the driving technique needed to extract that kind of fuel economy verged on the extreme.
The route up from Crawley required the fuel-saving entrants to scale a steep bit of the North Downs and as well a section of the fast-flowing M25 and a long stretch of busy urban roads from Putney Hill to Kensington.
All these parts of the route worked against the various electric and hybrid vehicles because climbing, accelerating and shunting along in heavy traffic are the most energy inefficient. Indeed, by the time I had cleared the North Downs the trip computer was indicating just 48mpg.
I’d had the opportunity to practise with the E300 for a few days on my normal commuting run and discovered that the best way of driving up the economy was just to coast downhill wherever possible. The E300 has the ability to shut off its engine and go into what the Germans call ‘sailing mode’. Whether this is true ‘sailing mode’ (where the transmission also goes into neutral, leaving the car to roll freely) I couldn’t find out, but it is amazingly effective.
When I rolled onto the M25, it was clear enough for me to get away with a 60mph cruise and then it turned into a long, gentle, downhill. So dramatic is the energy saved by coasting that, after less than 15 mins on the M25, I exited with a 55mpg average on the dial.
The A3’s 50mph speed limit was ideal for minimal fuel use and I further cheated by slipstreaming a solo Vauxhall Ampera. By the time I exited for Putney, the computer read 67mpg and I was surprised to drop just a couple of mpg by the time I shunted six urban miles to Kensington. But for all Mercedes’ complex Hybrid drivetrain, in terms of fuel saving, nothing cames close to coasting at every opportunity.
In the summer I drove a prototype Audi A6 with ‘intelligent sailing’. It used a 3D sat-nav system (so it knew when downhill stretches of road were approaching) and combined it with embedded information about its own drag and rolling resistance. This allowed the A6 to calculate when it could shut off its engine and just coast along on kinetic energy. Driven gently, it was surprising just how often the A6's engine would fall silent.
My experience of the prototype A6 and the production Mercedes has made me increasingly sure that the future lies not in complex petrol-electric drivetrains, but downsized normal engines (possibly, like the A6, combined with tiny, electrically-driven, turbochargers), coupled to a true ‘sailing’ mode transmission.
Of course, for the enthusiastic driver, this points to a bleak future. Cars with enough autonomy and intelligence to decide when to kill the engine and coast, in the pursuit of ultimate economy, suggests the driver will only be along for the ride.
But while the sheer simplicity and cheapness of active sailing convinces me it's a no-brainer, we’ll have to hope that carmakers include an off-switch.