Happy Earth Day: How hybrids work

As we move into the terrible twos here at AutoblogGreen, it's time again to look at how some of the technology we talk about so much works. Hybrids are certainly one of the most popular vehicle types hereabouts but how many of you actually know how they work? The concept of the internal combustion-electric hybrid dates back over a century and was pioneered by the original Ferdinand Porsche.

While various companies and researchers have dabbled in various hybrid systems over the years, it wasn't until the early '90s that the technology took off in earnest. All three of the U.S.-based carmakers worked on hybrid technology at that time as part of the Partnership for a New Generation of Vehicles program that was funded in part by the U.S. Government. However, it was the Japanese who were first to bring the technology to the market. The first series production hybrid car was the Toyota Prius that launched in Japan in 1997, followed in 1999 by the Honda Insight. The Insight was actually the first to go on sale in the U.S. in December 1999, beating the Prius here by about six months. That's the short history. Read on after the jump to learn more about how these systems work.



In the simplest terms, a hybrid drivetrain is one that uses two or more power sources for propulsion. Since the birth of the automobile in 1886, cars have generally used a single power source. The vast majority have been internal combustion engines running on either gasoline or diesel fuel with power being transferred through some kind of transmission. The modern hybrid car has added some sort of electric drive into the mix along with an electro-chemical battery for energy storage. There are of course other combinations, some of which use hydraulic drives instead of electrical ones or hydrogen fuel cells instead of internal combustion engines.

Of the currently available hybrids there are two main varieties, the so-called Mild and Strong types. The primary ones currently available are the strong hybrids and this group includes all the Toyotas, Ford's Escape/Mariner, Nissan Altima and GM's Two-mode hybrids. A number of new mild hybrids will be coming to market in the next couple of years as a more cost-effective alternative. The strong hybrids have the ability to run on electricity, internal combustion or a blend of the two. The mild hybrids have smaller, less powerful electric motors which typically aren't sufficient to drive the vehicle on their own. What they can do is automatically start and stop the engine when the vehicle stops, provide some electrical power boost (so a smaller engine can be used) and recapture some kinetic energy during braking.

All current production hybrids use nickel metal hydride batteries for energy storage. Most manufacturers are currently testing lithium ion batteries which have greater energy and power density but are also a lot more expensive. There also concerns with the long-term durability and safety of the lithium batteries but these are rapidly being overcome and lithium batteries will start appearing in mass-produced hybrids in 2009.

Electric motors have some very cool characteristics such as the ability to produce torque from zero rpm. More important is their generating capability. If electric current is applied to a motor, the motor will turn. If you mechanically drive a motor it becomes a generator and produces current. When the driver lifts off the accelerator, the wheels drive the motor which then generates electricity to charge the battery.



Strong hybrids typically use a planetary gear continuously variable transmission. The electric motor is incorporated into the transmission and the EVT provides the ability to blend drive torque from the motor and engine. During acceleration energy from the battery flows to the motor to help drive the vehicle. If the driver's demand is low enough and there is enough juice in the battery, the vehicle can be driven a short distance on electricity alone. As the speed increases or if the acceleration demand is too great, the engine will start.

The engines in hybrids are typically configured to operate on an Atkinson cycle. This differs from the traditional Otto cycle in that the intake valve closes late after the piston starts to go back up on the compression stroke. The result is a longer power stroke than compression stroke, resulting in increased efficiency. The problem is reduced torque output. The immediate and continuous torque output of the hybrids electric motor can be applied at any time to supplement the lost torque of the Atkinson engine without reducing efficiency.

The true strength of the hybrid-electric power concept is the ability to use a smaller engine and provide torque on demand and then recover some of the kinetic energy that is normally converted to heat by the brakes. Because current hybrids have limited battery capacity (typically no more than 1.5-2-5kWh) they provide the greatest benefit when the driving cycle includes a lot of braking and accelerating (something not everyone understands). When a hybrid car is cruising at steady state on the highway, the ability of the hybrid system to provide assistance is limited by the battery capacity and limited regenerative braking.

Regenerative braking is not as easy as it might seem. You can't simply reverse the flow of energy through the motor. Batteries have a limited capacity to absorb power both in terms of rate of absorption, and the total energy. If you overcharge a battery it gets too hot and the lifespan of the battery is reduced (sometimes in a catastrophic thermal event). As a result regen braking is limited and traditional friction braking is still required.

One of the most important factors that engineers have to take into consideration when developing a new vehicle is consistency of performance. When a driver gets used to a particular vehicle, they expect it to behave in a consistent manner based on particular inputs. Inconsistent behavior can result in accidents when a car doesn't react as expected. That means the engineers must have a predictable model of the torque output of both the engine and electric motor under all conditions. The engine torque models are fairly well understood and the motors are pretty straightforward if you know how much current is going in. To understand that, you need to understand the battery state of charge which is also a complex problem. All of this data is required in order to seamlessly blend internal combustion and electric torque.

Similarly regenerative braking has to be blended with friction braking. Typically the powertrain control electronics will determine how much regenerative braking can be accepted at any point in time. When the vehicle is decelerating the braking system will look at the decel requested by the driver through the brake pedal and limit the hydraulic brake pressure so that the sum of the available regen and the applied friction braking equals the driver request.

This all sounds pretty simple right? Wrong!! By far the hardest part of developing a viable hybrid system is the control software. Getting all of these pieces to work together robustly under all driving conditions is enormously difficult. The battery, in particular, is enormously sensitive to temperature and nickel batteries tend to perform poorly at temperatures below the mid-thirties. The control software has to take into account ambient conditions. When driving, the state of the battery has to be constantly monitored so that the input from charging by the engine or the regen can be balanced by the outflow to the motors and wheels. In addition, the driver's actions have to be followed closely to avoid hiccups in performance.



Mild hybrids also have to deal with blending of the engine and motor torque but the control is simpler because the motors don't have enough power to drive the car on their own. The smaller motors typically provide automatic start stop capability, but a mild hybrid can provide the engineers with the capability to provide electrical power to sub-systems like power steering, climate controls, lighting, and entertainment. The electrical energy from regenerative braking can be channeled to these systems allowing parasitic losses from driving an alternator to be reduced.

Looking ahead, plug-in hybrids will use lithium batteries to provide additional electrical storage capability. The first units that are converted from existing hybrid models will have limited ability to run on electricity alone due to restricted electric motor power. As dedicated plug-in hybrids are developed with more powerful motors and smaller internal combustion engines, true electric drive capability will be enhanced.

Meanwhile current hybrid vehicles can provide a big boost in fuel efficiency, provided you live in warm climates, and do a lot of urban driving. If you live in areas that get long cold winters, you might want to just opt for something with a smaller gas engine or diesel instead. Same thing goes if you do a lot of highway driving.