Navigant Research Blog

Improving the Performance of Hybrids

— 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.


Flywheels Offer Hybrids a Mechanical Advantage

— April 4, 2014

It is often assumed that all hybrid vehicles must use a battery for energy storage.  But the essence of a hybrid powertrain is not necessarily engine-off operation, but to provide more efficient transportation over a stop/start journey drive cycle.  The key factor in this mode is to be able to recapture large amounts of energy very quickly and then reuse it, which requires high power density.  While batteries typically have a high energy density, they often do not respond well to high charge rates and may not be able to capture all the available energy from regenerative braking.  Larger vehicles, in particular, have a lot of kinetic energy to store when slowing down.

So the focus for hybrid vehicles is often high power density rather than high energy density.  It is this factor (as well as the lower cost) that has led some manufacturers, particularly Toyota, to continue installing nickel-metal hydride batteries when the rest of the industry has shifted to the higher energy density of the lithium ion battery.  But there are other options for higher power density, if total energy capacity is not an issue.  Ultracapacitors are one alternative and Navigant Research has produced a report on another option: Hydraulic Hybrid Vehicles.  However, a new alternative technology based on the flywheel is now in testing.

Powerful and Economical

Volvo Car Group has recently been conducting testing in the United Kingdom of a flywheel developed by Flybrid Automotive (now part of Torotrak) to determine the potential for fuel savings.  Initial results show a performance boost of 80 hp while improving fuel economy by up to 25%.  The testing uses real-world driving data from public roads and test tracks in both Sweden and the United Kingdom.  Volvo has installed the flywheel system on the rear axle of a front-wheel drive passenger car.  Under braking, the vehicle kinetic energy is used to spin a 6 kg carbon fiber flywheel at up to 60,000 rpm.  When the driver wants to accelerate again, the energy from the spinning flywheel drives the rear wheels directly via a specially designed transmission.

The benefits for the driver are that the engine can be switched off during some braking and accelerating maneuvers, plus there is extra power available when needed to supplement the internal combustion engine.  The Volvo test vehicle is about 1.5 seconds quicker than the standard vehicle going from 0 to 60 mph.

Mechanics of Storage

The Flybrid system uses the mechanical motion directly to power the transmission, so there are no energy losses transferring from one format to another.  Another type of flywheel system, developed for motor racing by Williams Hybrid Power (and since April 1, part of GKN), uses a flywheel driven by an electric motor.  Instead of storing energy chemically as in a battery, the energy is stored mechanically in the spinning flywheel and then converted back to electricity to be used by the electric drive motor.

Both systems use the same mechanical energy storage format and have to address the same issues.  Safety and reliability are important, as is longevity.  Cost is also important, and at present, the flywheel is a lot cheaper than a battery.  It’s good to see some alternative solutions being adopted by larger companies, and this topic will be covered in much more detail in our upcoming report on vehicle efficiency.


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