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All hybrid propulsion systems are run by computers. The hybrid's computer decides which source of power to use. The switching between gas and electric is usually fairly seamless, but can be noticeable on some models. So, be sure to take the car you’re interested in for a test drive, and see if the transition between electric motor and gas engine bothers you. (And remember: it’s only going to get worse as the car ages.)
The precise systems vary between manufacturers. Toyota uses a hybrid design in which the car is powered solely by the electric motor in stop-and-go traffic. If you accelerate quickly and suddenly need a lot of power when you're in stop-and-go driving, the gas engine will come on to assist the electric motor. At higher speeds, the gasoline engine provides the primary power for the Prius, and the electric motor assists when extra acceleration is needed, such as passing.
Honda, on the other hand, has traditionally used something called the Integrated Motor Assist. The Civic Hybrid is a "mild hybrid" design, which relies on its gas-powered engine augmented by the electric motor when a boost is needed to climb a hill or accelerate. The Civic can run solely on its electric motor, but only in certain situations, when cruising at around 40 mph.
Honda is retaining the IMA system for its cheaper hybrids, but reflecting a wider range of gas-electric cars in showrooms -- from va-voom performance models to economboxes -- it will now offer a total of three different hybrid types. IMA is the "one-motor" hybrid, but there will also be two-motor versions (including on the new Accord plug-in hybrid, which has 10 to 15 miles of all-electric range) and the more complex three-motor type for the NSX and others. The latter has two motors on the rear axle, which can operate independently to aid in handling. That's sophisticated!
 Honda's IMA system tends to deliver better highway mileage. Around town, Toyota's design gets the green thumbs-up because it relies more heavily on purely electric power.
Ford engineers debuted a new hybrid design featured in the 2010 Ford Fusion. The Fusion tries to balance fuel economy and performance. It can reach up to 47 mph without using the gasoline engine, and Ford claims that 94 percent of the braking energy is recovered, which allows accessories like air conditioning to run on electric power alone. It's got a nickel-metal hydride battery that's lighter and more than 20 percent more efficient than the batteries found in the earlier Escape hybrid. With EPA ratings 41 MPG in the city and 36 on the highway, the Fusion is truly impressive. It's a substantial, mid-sized family car that gets superior mileage.
Plug-In Electric Cars and Series Hybrids
A number of automakers are experimenting with hybrids that use the gas engine solely to recharge the batteries. That means there's no direct connection between the engine and the wheels -- only the electric motors are used to move the car. The batteries can be recharged "on-board," as with the Chevy Volt, at home, or at public-charging stations. Some more advanced concept hybrids may even recharge the batteries through regenerative braking only or via a fuel cell.
 The Chevy Volt represents an interesting development in hybrid design. It's called an "extended-range electric vehicle." Here's the basic idea: Before going to bed, you plug in your car. When you wake up in the morning, you've got a charge that will give you 40 miles of gasoline-free driving. If you commute 20 miles or less each way, you never use anything but that charge.
But if you go more than 40 miles, a gas engine kicks in and runs a generator that provides electricity to run the car. Under normal driving conditions, the gasoline engine in the Volt never directly powers the wheels or fully charges the batteries. It simply generates enough electricity to keep the car moving until you plug it in and recharge it again.
Other plug-in hybrids on the market now or coming soon include the Ford C-MAX, the Fisker Karma, the Honda Accord and a version of the Toyota Prius. Volvo is also has a diesel plug-in hybrid for Europe, and a probable gas version for America. That's one ultra-safe Volvo that will see the family fighting for the keys.
Taking things one step further, BMW is now testing a fully electric version of the Series 1 Couple called the ActiveE. They'd previously leased some 500 electric Mini Es, and will introduce the firm's first-ever all-electric production car, the i3 "city car," in 2013. Following that in 2014 is the i8 performance car, a plug-in hybrid somewhat akin to the Fisker Karma and reportedly with a similar high price.
 No matter which system you use, a computer takes care of divvying up the work between the gasoline engine and the electric motor. The end result is that the experience of driving a hybrid is pretty much like any other car. If it weren't for the multi-colored lights on the dashboard telling you what was going on, you wouldn't know anything special was happening.
Full Hybrids
So-called "full" hybrids like the Prius and Insight are the cars that originally made the pocket-protector-wearing crowd swoon. They were the first of the hybrids to reach the market. These vehicles can run on either the gas engine, or the electric motor. Or both. The power of the electric motor augments a small gasoline engine, giving both good gas mileage and reasonable performance. The end result for the Prius is 50 MPG in city, 49 MPG on the highway. The Prius gets its best mileage in the city because its power plant is off a lot of the time, while the car is either stopped or running on battery power alone. During city driving, the Prius is also able to generate additional electricity from frequent braking.
The Ford C-MAX offers a great deal of utility in either hybrid or plug-in hybrid versions. If you want a gas version, head for Europe -- where it also comes with seven seats.
Nissan has released the Leaf, a plug-in fully electric hatchback that can go up to 138 miles on a charge, under ideal conditions. The automaker says a full charge takes four to eight hours, but an 80 percent charge takes only 26 minutes, making it much easier to "refuel" for a trip home. The Leaf has struggled in the cold and low temperatures seem to drain the battery more quickly -- something to think about for drivers in the Great White North. For some owners in Arizona, hot weather has also been a problem, but Nissan is rushing to correct that battery issue.
Mild Hybrids
 Mild hybrids use electric power to help out the gas engine on hills and under acceleration. But they can't drive on the electric motor alone. Additional fuel savings kick in when the engine turns off during braking, coasting or at idle. Like full hybrids, mild hybrids incorporate beefy combination motor/generators in place of traditional starter motors, so the engine can quickly restart when the traffic light turns green.
The end result for most mild hybrids is that they generally get about 10 to 20 percent better fuel economy and increased power, compared to a similarly sized conventional engine. Current examples of mild hybrids include the Chevrolet Malibu and Buick Lacrosse with what's called eAssist.
Misguided Hybrids
 A number of automakers are using the mild- or even the full-hybrid design to add power to their vehicles, instead of putting the increased efficiency towards better fuel economy. In other words, they're using the hybrid halo to "green-wash" their overpowered models. Frankly, we're disappointed with this approach to using hybrid technology -- we think of these cars as misguided hybrids. By taking this route, these automakers are continuing to cater to the power mongers among us, instead of doing their part to increase the fuel economy of our woefully inefficient fleet of cars on the road today.
One of the most egregious new offerings is slow-selling BMW's X6 Hybrid, which was killed in 2011. It earned a place on our "cars not to buy" list. It sported a 400-horsepower, V8 hybrid engine, which offers precisely four miles more per gallon in the city -- 19 MPG, as opposed to 15 MPG for the conventionally powered X6. Other examples of what we consider misguided hybrids include the GMC Yukon/Chevy Tahoe SUV, in which the manufacturer added full-hybrid technology, without downsizing the gasoline engine. That's a missed opportunity to increase efficiency, and that's too bad for all of us.
Bogus Hybrids
Our favorite example in this category was the Chevy Silverado hybrid, a vehicle that like the BMW X6 has since been discontinued. It used the electricity it generated to power plug-in appliances -- no kidding! We're sure it served its purpose at tailgate parties, but we can't give it much credit in the hybrid department.
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