For three decades now, the second commandment of every automotive engineer – right behind ‘reduce cost’ – has been ‘reduce fuel consumption.’ The drive to use less fossil fuel has dictated the design of engines, transmissions and control systems for decades. Now, it is pushing the development of completely different technologies for generating power. 

In spite of all these truly marvelous improvements, the energy efficiency of the most modern production car is still less than 20 percent. Most of the energy used to move the vehicle at any speed over any distance is literally thrown away as heat. About half of that wasted energy goes through the brakes. Today, almost every manufacturer is developing ways to recover a significant portion of that wasted energy with regenerative braking.

The Civic Hybrid uses the standard transmission- behind-the-engine configuration. Because the Integrated Starter/Alternator (ISG) provides regenerative braking, the transmission handles both positive and negative power delivery. The ultrathin, brushless DC motor is installed between the engine (below, in red) and the transmission. Honda figured out how to make the motor smaller by winding the poles asymmetrically, achieving 27 percent greater space efficiency. The closer poles not only reduce the size of the motor, but also improve torque.

It’s elementary
While the technology itself is often complex, the concept is quite simple: Brakes slow and stop a car by converting kinetic energy – the energy of motion – into heat energy, which is then dissipated to the air. We burn fuel to make heat and put kinetic energy into the car, then we throw that energy away as waste heat. The goal of regenerative braking is to recover some of that energy, store it and then use it to put the car into motion again. It is estimated that regenerative braking can eventually be developed to recover about half the energy we now waste as braking heat. Depending on the type of vehicle, this would reduce fuel consumption by 10 to 25 percent below current levels.

Regenerative braking can be extremely powerful. According to Craig Van Batenburg, who teaches Honda and Toyota hybrid service at Automotive Career Development Center in Worcester, MA, no more than 17 percent of its capability is used in these cars to ‘avoid putting people into the windshield.’ Even at that low level of use, in a typical mixture of highway and around-town driving, regenerative braking can recover about 20 percent of the energy normally wasted as brake heat. This reduces the drawdown of the battery charge, extends the overall life of the battery pack and reduces fuel consumption.

Right now, the Honda Insight, Toyota Prius and Honda Civic hybrid are the only production cars that use regenerative braking. However, regenerative braking has been used in trains, elevators and other industrial equipment for almost a century, and it will likely be used on many more cars and light trucks in the next decade. The technologies for recovering kinetic energy vary greatly, and some ideas are more promising than others. Here’s a look at what’s being seriously developed for automotive use.

Charging batteries
Kinetic energy can be converted into electrical energy with a generator, and it can be returned to a larger grid system or stored for later use. Electric regenerative braking has its roots in the ‘Dynamic Brakes’ used on electric trolley cars in the early 20th century. 

The driver’s control handle had a position that cut power to the traction motors and supplied a small, finely controlled excitation current to the motors’ field windings. This turned the motor into a generator that was driven by the motion of the car. Increasing the magnetic field current increased the generating load, which slowed the car, and the current being generated was routed to a set of huge resistors on top of the car. These resistors converted the current to heat, which was dissipated through cooling fins. By the 1920s, techniques had been developed for returning that current to the power grid, making it available to all the other cars in the system and so reducing the load on the streetcar system’s main generator by as much as 20 percent. Regenerative braking systems like this are still being used in cities around the world.

It’s just as easy to feed the current generated from braking into an on-board storage system, either batteries or a very powerful capacitor. The trick is to make those components small enough to be practical, but still have enough storage capacity to be useful. A big breakthrough came with the development of the electronically controlled permanent-magnet DC motor.

Motors work by activating electromagnets in just the right position and sequence. A conventional DC motor has groups of wire windings on the armature that act as electromagnets. The current flows through each winding on the armature only when the brushes touch its contacts. Surround the armature with a magnetic field, apply current to just the windings that are in the right position, and the resulting magnetic attraction causes the armature to rotate. The brushes lose contact with that set of windings just as the next set comes into the right position. Together, the brushes and rotation of the armature act like a mechanical switch to turn on each electromagnet at just the right position.

There’s another way to make a DC motor: Instead of using electromagnets on the armature, attach permanent magnets to it. Because it’s impossible to switch the polarity of permanent magnets, switch the polarity of the field windings surrounding them. This is a brushless, permanent-magnet DC motor. Of course, it’s only possible with the help of electronic controls that can switch the current in the field windings fast enough. The computer-controlled, brushless, permanent-magnet DC motor is ideal for use in electric vehicles. When connected to nickel-metal hydride (NiMH) batteries that can charge and discharge very quickly, the package is complete.

By the way, any motor becomes a generator when the armature is driven mechanically while surrounded by a magnetic field. While it’s quite a feat of computer programming, the same control unit that operates the permanent-magnet DC motor handles the regenerative braking chores, too.

The Prius’ engine compartment has the engine on the left (right side of vehicle) and the hybrid powertrain on the other side. The motor, generator and electronic controls have their own radiator, electric coolant pump and cooling control  system. A screen in the center of the 2004 Prius dashboard (below) shows power flow at any given moment, including when the motor is charging the batteries during regenerative braking.

Launch-assist
Another idea that’s not in production but that has some real possibilities is electric launch-assist, in which some of the recovered energy is stored in a package of ultracapacitors instead of the battery pack. A capacitor is an electrostatic device that stores electrical energy. An automotive-type battery can store 10 times more energy than a capacitor, but it uses a chemical reaction to generate electricity. Its discharge rate is slower, and it operates within a very narrow voltage range. A capacitor can accept and store a wide range of voltages, up to the limit of its design. It also can return that voltage in varying amounts and rates of discharge (current draw), but its charge is depleted quickly. If you think of a battery as something that generates electricity, then think of a capacitor as an accumulator.

A typical use for a capacitor is the starter mounted on an air compressor motor. The motor itself may be designed to operate on 110 or 220 volts, but it’s not strong enough to start turning from a dead stop against the load of the compressor. The starter capacitor boosts the voltage to the motor for just a second, then recharges itself from the line voltage. In such short bursts, the motor can handle thousands of these over-voltage jump-starts without overheating.

A capacitor stores its charge on two layers of conductive material separated by an insulator. The size, shape and materials all play a role, but in general, the greater the surface area of the conductors, and the closer they are to each other without touching, the greater the charge that can be stored and returned. The amount of energy that can be stored is called its capacitance. 

An ultracapacitor has a surface area that is several orders of magnitude greater than conventional types, and the separation is less than 10 angstroms (one angstrom is one ten-billionth of a meter). It can hold a pretty big charge, and it’s voltage output and discharge rate can be controlled with external circuitry. For instance, large ultracapacitors are often used to provide hours of backup power for computer systems after a general power failure. That is a relatively long and slow discharge when compared to capacitors used to start a motor. However, ultracapactors also can be used to deliver very high voltage for a shorter period of time.

A group of electrical engineers at the Catholic University of Chile are testing a battery-powered research vehicle built on a small Chevrolet truck chassis. They installed a bank of ultracapacitors regulated to provide 40 kW of energy – about 53 horsepower –for up to 20 seconds. That’s enough power to accelerate the 4,200-pound truck from a dead stop without drawing any current from the batteries, but they are using it as launch-assist power rather than the only source of launch power. The idea is to charge the ultracapacitor with regenerative braking, then use the charge only for acceleration from a dead stop. This will reduce the depth of each battery charge/discharge cycle, which will greatly extend the life of the battery pack.

As in a hybrid vehicle, the ultracapacitor in this research vehicle is charged with regenerative braking. Voltages are as high in either direction as 300 Vdc and can flow at 100 amps continuously or 200 amps for two minutes. It can be charged and discharged quite rapidly even in urban drive cycles, and it can withstand thousands of charge/discharge cycles with no loss of performance. In addition to extending the charge state and overall life of the batteries, it also allows regenerative braking even when the batteries are fully charged. At this early point in its development, the ultracapacitor pack is large, heavy and requires its own cooling system. However the launch-assist idea has a lot of potential, and it’s not limited to electric vehicles.

Hydraulic storage
Ford Motor Co. is experimenting with a system they call Hydraulic Power Assist (HPA). This system converts kinetic energy to hydraulic pressure, then uses that pressure to help accelerate the vehicle. It’s currently being tested on a four-wheel drive (4WD) Lincoln Navigator with a 4.0L V8 engine in place of the standard 5.4L engine.

A variable-displacement hydraulic pump/motor is mounted on the transfer case and clutched to the output shaft that powers the front driveshaft. The HPA system will work with or without 4WD engaged. A valve block mounted on the pump contains solenoid valves to control the flow of hydraulic fluid. A 14-gallon, high-pressure accumulator is mounted behind the rear axle, with an almost identical low-pressure accumulator right behind it to store hydraulic fluid. The master cylinder has a ‘deadband,’ meaning the first few millimeters of travel do not pressurize the brake system. When the driver presses the brake pedal, a pedal movement sensor signals the control unit, which then operates solenoid valves to send hydraulic fluid from the low-pressure reservoir to the pump. The pumping action slows the vehicle, similar to engine compression braking, and the fluid is pumped into the high-pressure reservoir. Releasing the brake and pressing hard on the accelerator signals the control unit to send that high-pressure fluid back to the pump, which then acts as a hydraulic motor and adds torque to the driveline. The system can be used to launch the vehicle from a stop and/or to add torque for accelerating from any speed.

While the concept is elegantly simple, the system is actually very complicated. Additional components include pulse suppressors, filters and an electric circulator pump for cooling the main pump/motor. There are leakage problems with seals and valves, getting air out of the hydraulic fluid is difficult and the system is noisy. In its current state, the system demands different driving techniques. Still, this system was built just to prove the concept, and the engineers are confident that these problems can all be solved and that a control system can be developed that will make HPA transparent to the driver. In carefully controlled, but admittedly ‘quick and dirty’ tests, they measured a 23 percent improvement in fuel economy and even greater improvements in emissions reduction, all with the dyno set for a 7,000-pound vehicle. Results like these are very encouraging, especially considering the system can be added to existing powertrains. It proves that you don’t need an electric motor/generator and battery pack to take advantage of regenerative braking.

Energy management
There are some limitations that will always affect even the best regenerative braking systems. First of all, it only acts on the driven wheels, so it will always have to be coupled with ABS. So far its use is limited to electric or hybrid-electric vehicles, where its real contribution is to extend the life of the battery pack rather than save fuel. 

Ford’s HPA system could be developed for any type of vehicle with any type of drivetrain, and it would contribute directly to fuel savings. However, it adds weight and complexity, and it will probably be expensive. 

Still, the pressure to reduce emissions and fuel consumption is unrelenting. After years of searching for ways to put less energy into the vehicle and still get the job done, the industry is finally developing ways to stop throwing away energy. Since we’re now wasting about 80 percent of of that energy, it certainly seems like an idea worth pursuing.