All hybrid propulsion systems are run by computers. The hybrid's computer decides which source of power to use. It's all quite seamless. You rarely notice it switching back and forth between gas and electric. 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, uses 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 and Toyota's systems each excel in their own way. Honda's 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 have a new hybrid design featured in their 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.

Finally, BMW, Chrysler, Daimler and GM have just announced the development of a "Two Mode" hybrid power-train system. In this design, two electric motors power the vehicle all the time, and the gas engine is used when needed for additional power.

Plug-In Electric Cars & 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. 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. Expect to see the Chevy Volt at dealerships by the end of 2010, if the project stays on its current time line.

Taking things one step further, BMW is now testing a fully electric version of the Mini. They're currently leasing 500 Mini Es, a battery-powered electric car, which they hope to have in consumer production within a few years. The Mini E has to be plugged in overnight to recharge the lithium ion batteries. The German automaker also plans to come out with the X6 crossover, a gas powered hybrid that uses an electric motor assist for towing heavy loads.

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 it spends more of its time operating on battery power alone.

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 beefed up 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 Lexus GS450H, the RX400h and BMW's new X6.

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 BMW's X6 Hybrid, which has earned a place on our "cars not to buy" list. It sports a 400-horsepower, V8 hybrid engine, which offers precisely 2 miles more per gallon in the city — 14 MPG, as opposed to 12 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

Finally, there are some vehicles being peddled as hybrids that really aren't hybrids at all. Examples include the Saturn Aura Green Line, the Saturn Vue Green Line and the Chevy Malibu Hybrid. The engines in these cars do shut off at idle, but that's about it. That modification does improve the fuel efficiency a small bit, but it's not exactly save-the-planet material, and we think it's misleading to call these cars hybrids. How do Chevy and GM get away with calling them hybrids? Good question!

GM did have plans to turn the Green Line series into full hybrids within a year — but, unfortunately, those plans disappeared with the closing of Saturn.

Our favorite example in this category was the Chevy Silverado hybrid, a vehicle that's 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.