What's a mild hybrid system?

The Saturn Green Vue Line Hybrid, which is technically a mild hybrid. What's a mild hybrid, and how is it different from a "regular" hybrid?
What's a mild hybrid, and how is it different from a "regular" hybrid?
AP Photo/Jerry Mendoza

Whenever a car company attaches the world "hybrid" onto a new car, it's easy to have a few expectations about what that car will be like and what it might offer. We can assume, for instance, that it'll be more fuel-efficient than a typical gas-powered car. And you can be pretty sure that the hybrid car in question will use some combination of an electric motor and a gasoline engine.

The popularity of hybrid cars, in part, comes from a lot of advertising and other writings about new hybrid car technology -- specifically, gas-electric hybrids. It's hard to shake this notion, especially because the majority of hybrids available commercially to the public are gas-electric hybrids. But what many people might not know is that there are several different types and classifications of hybrid vehicles, and there's no such thing as a "standard" hybrid. Technically, a hybrid vehicle is any form of transportation that uses two or more separate systems to propel the vehicle. They can either work together or separately to make the car go.


We can categorize hybrids by the nature of the power source. Of course, the most well-known type of hybrid is the gas-electric, since it offers a nice middle-ground between power (from the gas engine) and low emissions (from the electric motor). But there are also models that use different combinations of diesel, gasoline and electric power. Hybrids can also have a different drivetrain structure -- generally speaking, that's how the internal power systems work together. In the case of gas-electric hybrids, for instance, there are parallel hybrids, series hybrids or even combinations of the two.

We can also differentiate hybrids by the degree of hybridization, or how extensively the vehicle takes advantage of its systems. One type or degree of hybrid you may have heard about is called a "mild" hybrid. The name might sound a little odd -- especially since we typically expect big savings and a high mile-per-gallon average when we buy a hybrid car -- and a mild version implies less of that.

So, what exactly is a mild hybrid, anyway, and what does it have in common with the popular perception of hybrid cars? And what is it about the mild hybrid system that makes it not quite as hot-and-spicy as another hybrid? Find out on the next page.


Mild Hybrid Systems: Engine Assist

Electric motors in mild hybrids can't run the vehicle on their own, but they do help the gasoline-engine start and stop at the right times to save fuel. Above, the Connaught Engineering HYBRID+ system can retrofit cars to perform as mild hybrids.
Electric motors in mild hybrids can't run the vehicle on their own, but they do help the gasoline-engine start and stop at the right times to save fuel.
AP Photo/Kirsty Wigglesworth

In reality, mild hybrids don't have much in common with commercial hybrid cars at all, at least not the kind we generally think of. In fact, mild hybrids are much closer to conventional cars with gasoline engines rather than the ubiquitous gas-electric hybrid cars we hear so much about.

To define a mild hybrid, it helps to know the different degrees of hybridization. The most common hybrid vehicle is the full hybrid, or strong hybrid. These vehicles usually use a 30- to 70-kilowatt electric motor alongside a gasoline engine. The electric motor in a full hybrid will most likely be in use entire time the car is running and will use a large battery for power. Common examples of full hybrids are the Toyota Prius and the Ford Escape.


Mild hybrids, on the other hand, are on the other end of the spectrum. They're still classified as gasoline-electric vehicles, but the extent of how much -- or rather, how little -- the car uses the electric motor is what defines them as mild. The big difference between a mild hybrid and a full hybrid is that the electric motor in a mild hybrid cannot (and does not) actually propel the vehicle on its own. The gasoline engine in a mild hybrid is the piece of machinery doing all of the grunt work and the electric motor serves only to assist.

So, the electric motor in a mild hybrid is acting as a power booster. The real benefit of the mild hybrid system is that it saves fuel by shutting off the gasoline engine when the vehicle is stopped, braking or cruising. Also, the electric motor helps the gas engine restart with improved efficiency -- that is, much more efficient than say, a driver using the ignition to switch the engine on and off. Depending on the system, some mild hybrids can also capture mechanical energy during braking.

Although they don't have the same fuel efficiency as full hybrids, mild hybrids still offer increased savings at the pump when compared to a conventional gas-powered car. In fact, because they're not burning gas at certain points during your drive, a mild hybrid can improve fuel efficiency between 10 and 15 percent. Another positive side to mild hybrids is their low price tag: because they're not quite as sophisticated as a full hybrid, mild hybrids cost less to produce, and are therefore less expensive at the dealership. And although they're not as prevalent as full hybrids, several companies have produced mild hybrid cars, including BMW, Chevy, Honda and general Motors. So, mild hybrids are out there -- you just have to know where to look.

For more information about hybrid cars and other related topics, follow the links the next page.


Lots More Information

Related HowStuffWorks Articles

  • Green Car Congress. "Buick Mild Hybrid Business Concept Vehicle Unveiled in Shanghai." April 21, 2009. (April 24, 2009)http://www.greencarcongress.com/2009/04/buick-mpv-shanghai-20090421.html
  • HybridCars.com. "BMW's Mild-Hybrid Diesel Vision." Feb. 25, 2008. (April 24, 2009) http://www.hybridcars.com/news/bmws-mild-hybrid-diesel-vision.html
  • Olvera, Jennifer. "5 'Mild Hybrid' Facts." GreenCar.com. May 12, 2008. (April 17, 2009) http://www.greencar.com/articles/5-mild-hybrid-facts.php
  • Soultek.com. "Do mild hybrid vehicles make sense?" Feb. 14, 2008. (April 24, 2009) http://www.soultek.com/clean_energy/hybrid_cars/do_mild_hybrid_vehicles_make_sense.htm