How Hybrids Work

Automakers have been experimenting with hybrid vehicles for decades, thanks in large part to a pair of oil crises in the 1970s and 1980s that seemed to portend a need for more fuel-efficient vehicles in the years ahead. However, once gasoline shortages eased and long lines at the pump faded from memory, so did the desire for more fuel-efficient vehicles.

Even so, work on hybrids continued sporadically over the years, with interest in battery-electric and hybrid vehicles kicking into high gear with the advent of California's zero-emissions vehicle mandate in the early 1990s. During this time, three distinct types of hybrid configurations were explored — the parallel hybrid, the series hybrid and a combination of both.

Parallel Hybrids

2005 Honda Civic Hybrid

Honda's Integrated Motor Assist (IMA) system used in the Accord Hybrid, 2003-2005 Civic Hybrid and the now-discontinued Insight typifies a parallel hybrid configuration, in which an internal combustion engine provides primary power and an electric motor supplies additional power during times of high demand. The electric motor never powers the car on its own. The 2007 Saturn Aura Green Line and 2007 Saturn Vue Green Line are also parallel hybrids.

Series Hybrids

Chevrolet Volt concept read more >

A series hybrid uses an internal combustion engine — or some other source like a fuel cell — as a sort of generator to create electricity for powering drive motors. The combustion engine does not provide vehicle propulsion, only the electric drive motors do. This configuration isn't used on its own in any production hybrid vehicle today, although it is being explored at GM through its Chevrolet Volt concept car. Click here to read more about the Volt.

The Hybrid Combo

2007 Nissan Altima Hybrid view gallery >

Most hybrids — including those from Toyota and Ford, as well as the coming Altima Hybrid from Nissan — use a combination of parallel- and series-hybrid power so that they operate on internal combustion power most of the time, solely on electric power under certain conditions, and under both internal combustion and electric power in high-load conditions, such as intense acceleration or when climbing hills.

Most non-hybrid vehicles are equipped with engines that offer much more power — and consequently consume much more fuel — than is needed most of the time in order to enable intense bursts of acceleration necessary for passing another vehicle on the highway or climbing a hill, for instance. Hybrids change this dynamic by using a small, fuel-efficient combustion engine coupled with an electric motor or motors to boost output when extra power is needed.

Unlike the IMA system in previous Honda Civic Hybrids, the fourth-generation system that was introduced in the 2006 Civic Hybrid operates on either internal combustion or electric power, depending on driving circumstances. It does so differently than Toyota and Ford models by operating on electric power only under low-load conditions at cruising speed, rather than at low speeds. So the Honda Civic will always run its gasoline engine when accelerating from a stop, but shut it down once it's cruising and the conditions don't require more energy than the electric motor can provide. Whereas Ford and Toyota hybrid vehicles can run on electricity alone from 0 mph to about 35 mph under light acceleration.

2007 Saturn Aura Green Line view gallery >

Both hybrid configurations offer similar fuel-economy and emissions advantages, with hybrids achieving either Super Ultra Low Emission Vehicle (SULEV) or Advanced Technology Partial Zero Emission Vehicle (AT-PZEV) emissions levels in California and certain Northeast states. These designations are specific to California and the Northeast states that have adopted its stringent emissions regulations. Federal designations do not use identifiers such as SULEV or PZEV, but rather classifications such as Tier 2/Bin 2, which is how a PZEV is identified on the federal level.

Lest you think these confusing acronyms are simply governmental bureaucracy, consider that an SULEV runs 90 percent cleaner than the average new vehicle today. Efficiency gets even better when adding zero evaporative emissions and a 150,000-mile emissions system warranty, the requisite upgrades to achieving AT-PZEV status.

Automatic Engine Shut-Off

One advanced feature that all hybrids currently on the market have is the ability to automatically shut their internal combustion engine off while stopped or coasting under certain conditions to save fuel and cut exhaust emissions (Honda's hybrids only shut their engines off when stopped). To put the value of this single feature into perspective, consider that the Texas Transportation Institute's 2005 Urban Mobility Study found that an estimated 2.3 billion gallons of fuel were wasted by engines idling in traffic jams in 2003.

Regenerative Braking

Hybrids optimize combustion-engine power through an ingenious mechanism called regenerative braking that captures energy normally dissipated from the combustion engine and brakes as the vehicle coasts or decelerates. This way, hybrids can generate their own electric power with drive motors reverting to generators that feed this wasted energy as electricity back to the batteries. An alternator charges a hybrid's batteries during normal operation if the battery pack's charge is low.

Other Ingenious Technology

2007 Honda Accord Hybrid view gallery >

Honda's Variable Cylinder Management (VCM) used on its Accord Hybrid further boosts fuel economy. While the vehicle is cruising and the engine isn't being too heavily taxed, VCM turns the Accord Hybrid's gasoline-powered V6 into a fuel-sipping three-cylinder by shutting down one bank of cylinders. The engine changes over from six to three and back to six cylinders seamlessly without a driver ever knowing. Chrysler and General Motors both employ similar technology in V8 combustion engines, so we expect that coming full-size hybrid trucks from these manufacturers will also have it.

Another example of advanced technology that increases hybrid vehicles' efficiency is electrically assisted steering and braking systems. They're lighter and don't tax the combustion engine, thus reducing fuel consumption. They also allow a hybrid vehicle to drive normally when the engine is shut down during coasting: Traditional hydraulic steering and braking systems stop working as soon as the gasoline engine shuts down. Initially, hybrid powerplants combined the high-torque characteristics of an electric motor with a smaller-than-normal internal combustion engine to provide adequate power and super-high fuel economy.