Hybrid Gut Check

As a relatively new — and complex — form of automotive technology, hybrid-electric vehicles (HEVs) have created several uncertainties and misconceptions among the car-buying public.

In an effort to separate hybrid perception from reality, Toyota Motor Corp., the epicenter of the technology with five Toyota- and Lexus-brand HEVs currently on sale, is looking to unravel the mysteries and set the facts straight regarding one of the industry's most scrutinized developments.

Dave Hermance, executive engineer-environmental engineering for the Toyota Technical Center, defines HEVs as “vehicles with two or more energy storage systems, both of which must provide propulsion power.”

Most HEVs currently feature either a diesel or gasoline internal combustion engine (ICE) mated to a powerful electrical system. Excess energy is stored in nickel-metal-hydride battery packs and is used to supplement the power of the ICE when appropriate.

It is this electrical system that is the source of some of the most pressing concerns regarding HEVs, most notably, how long the batteries will last.

Toyota says the cost of the battery packs in its current Hybrid Synergy Drive (HSD) system has been reduced 36% since their introduction but still cost at least $3,000 to replace. However, they operate in a much less stressful manner than batteries in other consumer electronics applications, and thus have a much longer life expectancy.

Toyota believes most HEV battery packs will last the life of the vehicle. To date, the auto maker says it has not had any service replacements or warranty claims for under-performing batteries in any of its HEVs, including the first Prius models that debuted in Japan in late 1997.

Toyota's internal durability testing, which simulates extensive driving on mountain roads in temperatures exceeding 100° F (38° C), uncovered little if any degradation in battery performance over 180,000 miles (289,682 km), well beyond the auto maker's 8-year/100,000-mile (160,934-km) warranty on HEV batteries and components, Hermance says.

This extended performance comes from the limited state-of-charge (SOC) in which the batteries operate.

Batteries in consumer electronics applications such as cellular phones and laptop computers tend to be fully charged and then fully discharged. That causes the batteries to operate frequently at their minimum and maximum 20% of capacity, which is when most damage occurs, Hermance says.

Conversely, Toyota's HSD uses aggressive battery performance and temperature management controls to keep its packs between a 50%-70% SOC. This significantly reduces the stress loads on the batteries and greatly increases service life.

Hermance says utilizing more of the battery's capacity can enhance the range and emissions performance of HEVs, but that comes at the expense of durability. This is of particular relevance to plug-in HEV proponents, some of whom reconfigure their HEV's operating system so that it can be recharged via a wall outlet, allowing for much greater use of electric-only power.

In addition to durability issues, plug-in HEVs require larger electric motors for sufficient acceleration, which, coupled with the larger battery packs, increases weight and cost and reduces performance.

Overall, Toyota says plug-in HEVs won't be commercially or technologically feasible to build until battery technology improves.

Smaller, more powerful lithium-ion batteries, which should begin to appear in HEVs by the end of the decade, may have the capability to make plug-in HEVs a reality on a larger scale. However, Hermance says Toyota still is evaluating the technology.

The price of electricity and how it is produced in different areas plays a role in the practicality of plug-in HEVs, he says. The additional use of dirty, coal-fired power plants for recharging may offset the environmental gains from running in electric-only mode. Expensive electric bills in energy-strapped areas may outweigh gains realized at the gas pump.

The mass of electronics in these new vehicles also has been cited as a potential hazard to emergency personnel who may have to extricate a driver trapped in an HEV following a crash.

Hermance says this is a non-issue with Toyota HEVs because their electrical components and orange, color-coded power cables are shielded and tucked away from an emergency response team's likely work area. The high-voltage battery pack also is automatically disconnected from the vehicle when the ignition key is in the off position; airbags have been deployed or the vehicle has rolled over.

In addition, the auto maker provides rescue personnel with free emergency-response guides, located on a dedicated Toyota HEV website.

To date, there have not been any reports of injuries related to HEV electrical systems, Toyota says.

On the other side of the hybrid perception spectrum is the concern HEVs don't deliver their promised U.S. Environmental Protection Agency fuel economy ratings, and, in fact, exhibit a much greater shortfall than conventional vehicles.

Hermance counters that higher-efficiency vehicles, in general, tend to fall short of their EPA ratings by a higher margin than the average car or truck. However, on a fuel-consumed basis, nearly all vehicles incur about the same shortfall in real-world driving vs. the EPA tests.

The problem, he says, is the current measuring procedure for fuel economy is non-linear and provides misleading information.

In addition, Hermance cites a study by Consumers Union, a nonprofit testing and information organization, in which a Toyota Prius' observed fuel economy was 20% less than its EPA combined city/highway rating of 55 mpg (4.3 L/100 km). The same test showed a 4-cyl. Toyota Camry's rating to be only 11% less than its EPA rating of 27 mpg (8.7 L/100 km).

However, on a gallons-per-year basis over 15,000 miles (24,140 km), the Camry used an additional 71 gallons (269 L) of fuel vs. only 68 gallons (257 L) in the Prius.

Toyota suggests the EPA should correct this discrepancy by labeling new vehicles on a fuel-consumed-per-year basis, similar to how home appliances are rated for their efficiency and energy consumption, in addition to the current fuel-economy rating.

The current EPA fuel economy rating test procedures were last updated in 1985. Proposals are under way to revise the standard using more vehicle-specific data, including high speed/rapid acceleration, use of air conditioning and cold temperature operation. Initial changes are set to take affect for the '08 model year, with another adjustment queued for '11, the EPA says.

When they take effect, the agency says the new procedures will alter the weighting of the city/highway average and reduce mileage ratings for most vehicles by 10%-20% to better represent modern, real-world driving conditions.

However, fuel-economy values for most HEVs reportedly are expected to fall nearly 30%.

Hermance takes issue with this speculation, noting that, on a gallons-per-year basis, fuel consumption for most of Toyota's HEVs should be affected only marginally by the revisions.

“Hybrids will continue to give more to the driver and take less from the world,” he says.