Thursday, July 8, 2010

Misleading Metric: Max EV Speed for Conventional Hybrids

Porsche Cayenne Hybrid

The Porsche Cayenne Hybrid will go on sale later this year.

Last week, the U.K.�"s Advertising Standards Authority (ASA) stopped Porsche from running an ad claiming that the new Cayenne S Hybrid �Scan even travel to a speed of 37 mph without using its V6 petrol engine." The agency believes the 37 mph claim is misleading because the vehicle travels only for a limited time and distance using power exclusively from the electric motor and batteries. The ASA is right. Hopefully, the ruling will raise awareness that the ability for conventional hybrids to travel in �Sall-electric mode⬝ at high speeds is a cool trick�but relatively insignificant.

Porsche is not the only hybrid maker trumpeting the all-EV capability of its conventional hybrids. Ford promoted the Ford Fusion Hybrid�"s ability to travel up to 47 mph without using gasoline. The 2010 Toyota Prius�"s �SEV⬝ button allows drivers, under some conditions, to force the engine off, allowing the car to run up to 25 miles per hour from battery power alone for about one mile.

As any driver of a full hybrid�or a plug-in car�will tell you, it�"s a magical feeling when a vehicle moves down the road without gasoline in near silence. But in the case of a conventional hybrid�rather than the plug-in kind�the net result of maximizing the EV mode is to quickly drain a battery that was not designed for high demand all-electric driving. Just like when you need to catch your breath after a long sprint, the hybrid�"s gasoline engine is more likely to come on stronger and longer to refill that battery to an optimal level. In the end, that undermines the hybrid�"s core efficiency strategy of juggling different aspects of the drive cycle with the best mix of gasoline and electric power. The mixing and matching of electricity and gasoline is how a vehicle like the Toyota Prius can achieve a remarkable 50 miles to the gallon.

Efficiency vs. Electricity

So, it�"s a nice claim to say that a hybrid can travel X miles per hour all-electric, but the likely net result of high-speed EV mode is reduced MPG. This once again proves that the name of the game is efficiency, not electricity.

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