Navigant Research Blog

Improving the Performance of Hybrids

David Alexander — April 23, 2014

The fundamental goal of a hybrid powertrain is to improve fuel efficiency by recovering kinetic energy from the vehicle as it slows down and storing it to reuse later.  Traditional vehicles convert that energy to heat in the brakes and then let it disperse into the atmosphere.  Hybrid electric vehicles (HEVs) generate electricity via an electric motor and store it in batteries or ultracapacitors.  The resistance of the motor slows the vehicle.

An understanding of how the hybrid powertrain works is helpful for drivers who wish to maximize their efficiency on the road.  Being careful not to accelerate too hard and slowing down steadily without using the foundation brakes are techniques that have been used since HEVs first went into production in the late 1990s.  As they became more popular, some original equipment manufacturers had to deal with complaints from drivers who didn’t achieve the promised fuel economy.  Some of the deficit was due to driving technique – if you hit the brakes hard when driving, then the regeneration cannot take place and energy will not be saved for reuse.

Hills Ahead

The basic hybrid system will deliver more fuel economy benefits to drivers who understand how to get the most out of it.  But there are limits.  Once the battery is fully charged, no more energy can be stored and the vehicle is then no more efficient than its conventional counterpart.  It’s very difficult for a driver to work out how to get around this limitation.

However, Mercedes-Benz has done just that with its latest Intelligent HYBRID operating system, which was introduced in the S-Class in summer 2013 and is now available in the new 2014 C-Class.  These cars use data from their navigation systems to look ahead for hills.  When the vehicle detects a downhill stretch coming up, it knows that the hill presents a good opportunity to capture energy, so it activates the electric motor to start draining the traction battery.  Taking more of the power from the electric drive means that the engine uses less liquid fuel, improving fuel economy.

This advanced technology is a first for the consumer market.  Another application that uses the latest digital map data is Audi’s adaptive headlight system, which can anticipate upcoming curves to better illuminate the road ahead without dazzling oncoming vehicles.  Continental’s eHorizon system optimizes gear shifting to best handle the upcoming terrain.  Mercedes is the first to offer map-based efficiency technology in consumer vehicles.

These innovations will undoubtedly benefit vehicle owners by delivering improved fuel economy in real-world usage.  However, the standardized tests used by government agencies to help consumers compare vehicles are unlikely to register an improvement because they do not include going up and down hills and are typically conducted on a chassis dynamometer, or rolling road.  Another reason to reevaluate how comparison testing is done.

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