I've run several stories lately on hybrid vehicles -- comparing their costs against traditional gasoline engines and giving tips on how get the best fuel economy when driving them - and it occurred to me that it might be helpful to explain what makes these powertrains so different.
An article I found at the Toyota press site helped quite a bit to explain how their hybrid vehicles combine gas engines and electric motors to optimize fuel economy by enhancing the strengths and minimizing the shortcomings of each. Because of this article, many of my examples will be made using Toyota's Hybrid Synergy Drive system found in the Prius.
Today's popular hybrid powertrains utilize an internal combustion engine, combined with an electrical system made up of a motor, generator, and battery. Depending upon the vehicle's system, the gasoline engine may be able to drive the vehicle by itself, or it may drive the electrical system only (which in turn will actually drive the vehicle). Alternatively, the electrical system might be able to drive the vehicle by itself, or both systems may be able to work together to varying degrees. While the vehicles currently on sale utilize gasoline engines, some manufacturers are also developing similar hybrid systems that can be powered by Hydrogen.
traditional automotive internal combustion piston engine delivers high
horsepower and meets emissions and fuel economy requirements while also
providing smoothness, quietness, and reliability at reasobnable cost.
Unfortunately, because of our desire for performance, almost every
vehicle on the road happens an engine that is, most of the time, larger
than it needs to be.
In Toyota's example, a typical four-door sedan may have an engine rated at, say, 200 horsepower. That vehicle requires the engine's full 200 horsepower very little of the time, except only for quick passing maneuvers or while climbing steep hills. The vast majority of the time the engine is operating at a small fraction of its fully rated output. Once the sedan is accelerated up to freeway speed, as little as 20 or 30 horsepower may be needed to keep it moving. In fact, many drivers may seldom, if ever, call upon the full power output of the engines under their cars' hoods. What people really need is 200 horsepower every once in a while, maybe 100 horsepower from time to time, and about 30 or 40 horsepower most of the time.
Could an electric car do that? The pure electric vehicle is quiet and smooth without creating smog-forming emissions. However, after more than a century of research, the pure electric car has the same handicap it had 100 years ago: limited range. Exacerbating its limited range are a couple of other significant concerns: While a car with a gasoline engine can be completely refueled in a few minutes, literally hours are required to charge up an electric car's batteries. And while the gasoline vehicle runs just as well on the last drop of fuel as on the first, the further an electric car goes, the more its performance drops -- because the battery is discharging -- so as it reaches the last of its "range," it becomes increasingly sluggish.
In simple terms, the electric car doesn't have enough power when it's needed; the conventional gasoline car has too much when it's not needed. The hybrid helps solve both those issues.
The typical vehicle, because it has to deal with the widely varying speeds and conditions of traffic, has a more difficult duty cycle. Starts, stops, short trips, family vacations, stuck in traffic jams -- all these create different fuel economy and emissions. To deal with this, the typical automotive hybrid system utilizes both a relatively small gasoline engine as well as an electric motor. And depending on the vehicle's setup, the engine, the motor, or both together will drive the wheels. A battery pack supplies the electric motor, and a generator makes the electrical power to recharge the battery. Sophisticated electronic controls ensure the whole package works in harmony. Another way most hybrids save fuel is by shutting down the engine when the vehicle is stopped. This prevents wasting fuel from unnecessary idling when stuck in traffic or at stop signs. Also, during braking and other types of deceleration, kinetic energy normally lost is converted into electrical energy through the brakes, and then stored in the battery. The gasoline engine is linked to the drive wheels and a generator directly via a specialized transmission and, whenever it's running, it can also drive a generator that helps keep the battery charged. The generator supplies electrical power to the electric motor or charges the battery, as needed.
When accelerating from rest at a normal pace and up to mid-range speeds, the Prius is powered by its electric motor, which is fed by the battery. As the battery charge is depleted, the gasoline engine responds by powering the electric generator, which recharges the battery. Once up to speed and driving under normal conditions, the engine runs with its power divided between the generator (which in turn supplies the electric motor) and the wheels. The distribution of these two power streams from the engine is continuously controlled to maintain the most efficient equilibrium. If the need arises for sudden acceleration -- such as a highway passing maneuver or a quicker start from rest -- then both the gasoline engine and the electric motor work together to drive the wheels. Other hybrid vehicles may balance this equilibrium differently, but the premise is the same: gasoline and electric work together to provide clean, efficient horsepower.
The Toyota Prius is a popular hybrid example because it was designed from its very origins to be the most efficient vehicle sold today, with EPA mileage estimates as high as 46 mpg. Its body is as aerodynamic as possible, and was also constructed to be as light as possible. It's powered by a small 1.5-liter/76-horsepower, four-cylinder gasoline engine, but when assisted by its electric motor, the Prius has a combined horsepower of 110. Other hybrid vehicles utilize similar systems: Honda's hybrid Civic combines a 1.3-liter engine with a 20-horsepower electric motor for an ultimate output of 110 horsepower with economy of 40 city and 45 highway. Meanwhile larger vehicles do the same thing with bigger combinations: The Toyota Highlander's 3.3-liter gas engine is boosted by its electric motor for a combined output of 270 peak horsepower and economy figures of 27 city and 25 highway, and the Chevy Tahoe Hybrid combines a 6-liter engine with an electric engine promising 332 horsepower with economy of 21 city and 22 highway. (Of course, don't forget the old adage: "Your mileage may vary.")
The result is a vehicle that uses a smaller, more efficient gasoline engine to drive the wheels, or the generator which supplies (either directly or through the battery) the electric motor as well. The engine in these hybrid vehicles runs exclusively on gasoline (or in the future, other fuels such as diesel, bio fuels, or hydrogen), while the electrical portion of the power system never needs to be plugged in for a charge. There's no cord and no waiting, and for now, you can fill up at any normal gas station anywhere.