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Hybrids 101

Guide to gasoline-electric hybrid cars and powertrains

Last updated: April 2014

Most people know by now that hybrids use electricity to achieve good gas mileage, but how their advanced powertrains work and whether they help your pocketbook is a bit more complicated. Here we'll take a look at the basic technology and its variations. In another article we discuss the cost, and potential savings, of owning a hybrid.

The theory behind hybrid cars

What sets hybrids apart from regular cars is that they essentially use two powertrains, an electric motor and a gasoline engine. The electric motor draws its power from a large battery pack that is recharged by the gas engine and by energy recouped from the brakes.

The electrically powered car has been a vision for the future for decades. But when its development stalled in the mid-1990s, hybrids emerged, promising a compromise between the benefits and limitations of both electric and gasoline powertrains.

Electric motors are very efficient at accelerating, and unlike their internal-combustion counterparts, produce their maximum power from a dead stop. But batteries with enough energy to drive long distances are bulky, heavy, and expensive. Recent progress in battery development has brought some new electric cars to market. But they are still niche products, mostly with limited range. For the time being, hybrids offer the best of both gasoline and electric cars.

By using electric motors for acceleration and hill-climbing, automakers can use smaller, more-efficient gasoline engines for everyday driving and long-distance cruising. By combining the two systems, the battery packs can be relatively small. The downside of a hybrid is that the dual drivetrains can be significantly more expensive than a traditional gas engine alone, and the battery packs take up space and add weight.

Hybrid technologies

Hybrids blend the power of a gas engine with an electric motor and batteries. But the two powertrains can be combined in a variety of ways--some more efficient than others. The hybrids with the best gas mileage tend to be full hybrids. Newer, plug-in hybrids allow you to burn even less gas by running exclusively as electric cars for a short distance. There are several significant variations that we'll explain in detail.  

Parallel vs. series hybrids
Most full hybrids use a parallel design in which either the gas engine or the electric motor alone can drive the wheels, or they can work in unison. Hybrids can also have a series configuration, in which only the electric motor drives the wheels, and the gas engine works mainly as a generator to provide electricity once the battery is depleted as with the Chevrolet Volt and the Honda Accord Hybrid.

Full hybrids vs. mild hybrids
Full hybrids can run for a limited time on electricity alone, and they use the gas engine to travel longer distances and/or at higher speeds. Examples include Toyota and Ford systems. Mild hybrids are the opposite of series hybrids: Only the gas engine turns the wheels, and the electric motor only provides a boost to the gas engine, augmenting the power to improve fuel economy. Only full hybrids can be designed to plug in and act as full electric cars.

Plug-in hybrid
Plug-in hybrids can (and should) be charged from the wall to work as electric cars some of the time. They normally use their electric range of between 10 and 35 miles first, then switch to normal hybrid operation. They can be either full or series hybrids, and some such parallel hybrids act as series hybrids under certain conditions when it's advantageous.

Plug-in hybrids allow you to recharge the batteries to maximize electricity use, running solely on electricity until the batteries need assistance. (Chevrolet would like us to call the Volt an "extended-range electric vehicle." That's accurate enough, but what it means is that the Volt fits in the category of a plug-in series hybrid, along with the Honda Accord Plug-in Hybrid.) Should your trips, or commute, be within the electric-only range, plug-ins can provide the benefits of a pure electric car, while having the engine available for longer trips without worry about getting stranded.

Full electric
A new wave of pure electric cars have taken to the streets for the first time since the 1910s, including the Nissan Leaf, the  Ford Focus EV, Mitsubishi i, Honda Fit EV, Smart ED (electric drive), and electric models of the Toyota RAV4 and Tesla Model S sedan. Finite battery storage and long recharge times limit the appeal for EVs for many. But studies show that most drivers travel less than 40 miles a day, making even a short-range electric car capable enough for most suburbanites or families in need of a second car. On the high end of the range, the $90,000 Tesla Model S will have a claimed 300-mile range. Even battery charge times, electric cars' other Achilles heel, are coming down, though the quickest full electrics still need about 3-1/2 hours to recharge fully.

Hybrid powertrains

Engines and fuel economy
Most hybrids use small, four-cylinder gas engines that are more efficient than larger V6s and V8s. Hybrid SUVs, however, come with either four-cylinder, V6, or V8 engines, depending on the size. Some SUV or luxury-car hybrids also offer V6 or V8 engines. The V6-powered hybrids are smooth, quiet, and quick. And big, V8-powered hybrid SUVs and pickups are responsive and offer large towing capacities. Four-cylinder models run the gamut in terms of refinement, but most four-cylinder hybrids are quieter and feel less strained than equivalent four-cylinder gas-only models.

Most hybrids come with some sort of continuously variable transmission (CVT). Some hybrids use conventional belt-type CVTs. More use planetary gear sets with infinitely variable ratios in a virtually wear-free design.

Drive wheels
Hybrid SUVs can be front, rear, or all-wheel drive. Most small-car hybrids are front-wheel drive, while luxury-car hybrids are rear-wheel drive. All-wheel-drive versions of the Toyota Highlander and Lexus RX 450h power the rear wheels via an extra electric motor on the rear axle of a front-wheel-drive design. It's light, simple, and efficient, and we've found it works well enough. Unlike traditional AWD vehicles that compromise fuel economy, hybrid AWD vehicles can often avoid such a penalty by using electricity to drive those extra wheels.

Batteries
Conventional hybrid batteries of nickel-metal hydride are quickly being replaced by lithium-ion batteries, which are smaller and lighter, for better efficiency. Lithium-ion batteries also come in several varieties: Some variations are more stable and have less power. Others have more power, but require more robust external systems to guard against fire and prolong their life. So far, the question of which systems work best in cars has not played out.

Nickel-metal hydride hybrid batteries typically have very good reliability in our Annual Auto Survey of our subscribers. Lithium-ion batteries are too new to have a proven track record. Automakers are required to warranty the batteries on any hybrid as an emissions control part for eight years and 80,000 miles in most states. In about 10 states, they're required to warranty them for 10 years or 150,000 miles, so the automakers have a vested interest in making them durable. Outside the warranty period, new nickel-metal hydride battery replacements can run as much as $3,000, but replacements have been relatively rare. And used batteries are available for much less. Batteries in some older Ford and Honda Hybrids have been more problematic than those in more popular Toyotas. On the other hand, expensive transmission replacements that are not uncommon in other cars are almost unheard of in hybrids.

Hybrid car maintenance

We checked with Honda and Toyota, the two best-selling hybrid manufacturers, about maintenance and reviewed the service schedules of two of the most popular hybrids and found that neither requires any special maintenance beyond what a regular car needs. Coolant changes on the Prius and other Toyota hybrids might be somewhat more complicated and expensive than in regular cars, but they don't have to be done any more often. (In the Lexus RX450h and Toyota Highlander Hybrid, there are battery-cooling vents in the rear footwells that must not be blocked.)

Both cars, including their battery packs, have been very reliable in our annual surveys ofConsumer Reports subscribers. Toyota reports that some Priuses have more than 200,000 miles on their original batteries. Under California state law, which has guided similar regulations in Massachusetts, Maine, New York, Rhode Island, and Vermont, hybrid manufacturers are required to provide a warranty on the batteries up to 150,000 miles. To find out whether the hybrids holds up, see our test of a 200,000 mile Toyota Prius.

   

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