What could a Toyota Prius possibly have in common with a Porsche Cayenne, a Chevrolet Volt or a Ford Fusion? Each is available with a hybrid powertrain, a propulsion system made famous by the Prius.
While consumers benefit from the improved fuel economy, hybrids give automakers a way to meet ever-tightening Corporate Average Fuel Economy (CAFE) requirements and proposed limits on carbon dioxide (CO2) emissions. Hybrids have been in the U.S. market since late 1999 and there's a growing used hybrid market that's attracting buyers in search of fuel-efficient cars without a higher price tag.
But what exactly is a hybrid, and how does it improve fuel economy? Are all hybrids laid out essentially the same under the hood? Do they all provide similar fuel savings? Read on.
Defining Hybrids
A vehicle is a hybrid if it utilizes more than one form of onboard energy to achieve propulsion. In practice, that means a hybrid will have a traditional internal-combustion engine and a fuel tank, as well as one or more electric motors and a battery pack.
Hybrid cars are sometimes mistakenly confused with electric vehicles. Hybrids are most often gasoline-burning machines that utilize their electric bits to collect and reuse energy that normally goes to waste in standard cars. Theoretically, diesel-electric hybrids would be even more fuel-efficient, but hybrid systems and diesel engines both represent extra cost. So far, installing both in the same vehicle has proven to be prohibitively expensive.
Hybrid Car Glossary
Below are the terms most often used when referring to hybrid vehicles.
Motor-generator: The more accurate term for the electric motor. It provides supplemental acceleration "oomph" when operating as a motor by drawing electricity from the battery. Several hybrids have two, and a few models employ three.
Stop-start: Present on all hybrids, the engine's traditional starter motor is absent because the motor-generator takes on that function, too. Hybrid-control software shuts the engine off while stopped at traffic signals and automatically restarts it again with the electric motor when the driver releases the brake pedal. Eliminating the fuel waste of an idling gas engine causes overall mpg to climb significantly and tailpipe emissions to drop, especially in town.
Regenerative braking: An important function of the motor-generator is to generate electricity to recharge the battery as it absorbs a portion of the vehicle's momentum when slowing or coasting downhill. Normal cars waste all of their excess momentum as heat in the brakes. Regenerative braking is insufficient to stop a car quickly, so conventional hydraulic brakes are still necessary.
Electric drive: Operating the vehicle on electric power alone is possible if the hybrid system has enough electrical capacity. The maximum speed and distance over which electric-only operation can be sustained varies from essentially zero to a handful of miles, and has everything to do with the weight and aerodynamics of the vehicle, the strength of the motor-generator and, more than anything else, the capacity of the battery.
Hybrid Layouts
Not all hybrids possess these attributes in equal measure, nor do they operate the same way. It all begins with the layout of the system.
Series hybrids: This is the oldest hybrid type. Diesel-electric locomotives and ships using this layout appeared in the last century. In a series hybrid car, electric motors alone turn the drive wheels, so the motors must be large and powerful. But a series hybrid is not a "pure" electric vehicle. It has a dedicated engine that burns fuel and expels emissions. The engine powers a generator to produce the electricity onboard the vehicle.
The 2014 BMW i3 and the out-of-production 2012 Fisker Karma use series hybrid systems. But these cars from BMW and Fisker are also plug-in hybrids. More about that flavor of hybrid a little later.
Nearly every carmaker also has a series hybrid demonstration vehicle that uses a hydrogen-powered fuel cell instead of a gasoline engine to generate the electricity. Those cars, usually called fuel-cell electric vehicles (FCEVs), are expected to start entering the retail market in small numbers by 2015. from Honda, regenerative braking is the sole source of recharging power for the battery.
Series-parallel hybrids: As the name implies, these cars contain elements of both types. Conceptually, the engine and the electric motor feed into the transmission via separate paths, enabling fully independent propulsion via the engine or electricity. In parallel fashion, the motor-generator can either bolster the engine's output or provide battery charging via regenerative braking. Series-parallel motor-generators are sizable, so electric-only operation (at low speeds for a couple miles) is a standard feature. The engine can still power the car, but it can also be reassigned to battery charging duty while the electric motor drives the vehicle: the classic series operation.
In a series-parallel hybrid vehicle, a computer monitors driving conditions and the state of the battery to decide which mode is most efficient at any given moment. The seamless blending of these modes is then carried out by a unique continuously variable transmission (CVT) that uses a planetary gearset as opposed to a system of variable pulleys and belts. Series-parallel hardware is more expensive, but the payoff in efficiency is huge: To date, these hybrids offer the largest gains in mpg, the highest electric-only speeds and the longest electric-only run times. Ford's C-Max and Fusion hybrids and all Toyota and Lexus hybrids use series-parallel systems.
Plug-in hybrids: These are not really a fourth type of hybrid because a plug-in could conceivably be based on any of the above layouts. Plug-in hybrids (also called PHEVs) began appearing in the market with the Chevrolet Volt at the end of 2010. Their distinguishing characteristic is a significantly enlarged battery that permits the electric driving range to swell beyond the mile or two possible with regular hybrids. It also provides a way to plug the battery into an electrical outlet for recharging while parked. The benefit of the plug-in hybrid is its ability to travel in all-electric mode for most short trips, reserving the gasoline engine for longer drives. That's a feature that can boost fuel economy into 100 MPGe territory. In addition to the Volt, plug-in hybrids include the Ford Fusion Energi, Toyota Prius and a version of the Honda Accord.
Hybrids, by Degree
Some hybrids have more power than others. Automakers have developed terms to describe their various levels of electric oomph.
"Mini" hybrids: This is a class that adds a modicum of electric assist to the stop-start system. Because these cars don't offer full hybrid capabilities, they can be built using very small and relatively inexpensive nickel-metal hydride or lithium-ion batteries to help keep costs down. General Motors pioneered the system, which it calls eAssist, and first offered it in the four-cylinder version of the 2013 Buick LaCrosse. Buick intentionally avoided using the term "hybrid" with this system because it believes, as do most other automakers, that consumers have greater expectations for anything labeled "hybrid."
Mild hybrids: These usually are parallel hybrids without sufficient power to propel the vehicle in all-electric mode more than a handful of yards. The gasoline engine essentially operates all of the time and is augmented by the electric motor when more power is needed for accelerating or climbing hills, for instance. The 2013 Honda Civic Hybrid is an example.
Strong hybrids: These can be parallel, series or series-parallel vehicles. They have large enough electric motors and powerful enough batteries to provide some degree of all-electric mode, along with stop-start, regenerative braking and gas engine assist. The Toyota Prius was the first strong hybrid in the market and remains the best-selling of all the hybrids.
Vehicles with stop-start systems: This is another class of hybrid-inspired vehicles coming into the market, making use of the stop-start systems that were perfected for hybrids. They are not hybrids because they don't have two power systems (although some in the industry call them micro-hybrids). Instead, they use a beefed-up starter motor and battery or capacitor to provide the stop-start function but have no electric assist for acceleration and no other aspects of a conventional hybrid.
Still, stop-start systems can offer 3-10 percent increases in fuel efficiency and are expected to become almost universal as automakers strive to achieve rapid improvement in their fleets' overall fuel efficiency. The 2013 Ram 1500 HFE (high fuel-efficiency) pickup from Chrysler has a stop-start system. Ford offers auto stop-start as a stand-alone optional feature of the 2013 Fusion SE.
All About Batteries
Hybrids employ two battery types. Nickel-metal hydride batteries are used in almost all current hybrids, but they are not sufficiently efficient and compact for plug-in use.
Lithium-ion batteries that are durable enough for automotive use are the battery of choice for plug-ins and, increasingly, for newer conventional hybrid models. They are lighter and more energy-dense than nickel-metal hydride batteries. Battery engineers continue to seek the next-generation hybrid or electric vehicle battery that will offer even lighter weight, lower cost and greater range.
Whatever the type, hybrid batteries are considered part of the emissions system, and as such are covered under terms of the mandatory emissions warranty: eight years or 100,000 miles in federal emissions states, and 10 years or 150,000 miles in states that adhere to California emissions standards.
Other Hybrid Considerations
Because of its very different powertrain, automakers had to make changes to the basic accessory equipment of the hybrid car to allow the engine to shut off when cruising on electricity or waiting at stoplights.
Traditional hydraulic power steering needs a running engine to drive the required pump, so hybrid engineers employed electronic power steering (EPS) instead. Adopting EPS is easy because it's rapidly becoming the norm on regular family cars anyway. It's possible to retain hydraulic steering by using a remote electric pump, but so far only Porsche has experimented with this approach in the interest of maintaining excellent steering feel.
Air-conditioning systems also use an engine-driven compressor, so hybrids with enough battery capacity use an electrically driven compressor instead. Some lower-cost hybrids retain their engine-driven compressors, but their drivers must engage an "Eco A/C" mode to limit compressor function when stopped. If they forget, or choose not to, the engine can't shut down at stoplights and they'll miss out on much of the hybrid fuel savings.
Then there's the engine itself. Hybrids that employ a CVT can take advantage of the more efficient Atkinson cycle, a variation of the traditional four-stroke engine cycle that significantly modifies when air is let into the combustion chamber. The Atkinson cycle requires engine speed (rpm) to be relatively constant, and a CVT keeps the engine's rpm in the Atkinson "sweet spot" via its ever-changing stepless gear ratios.
