December 26, 2007 > TechKnow Talk
Hybrid Vehicles: When Two Engines are Better than One
Given the extravagant media coverage of global warming in recent years and current corporate efforts to become more environmentally friendly, it is not surprising that drivers have caught the spirit and are buying hybrid vehicles in record numbers. Of course, increasing gasoline prices have provided a less altruistic motive for consumers to "go green." How do hybrids work and why are they so efficient?
The vast majority of hybrid passenger vehicles currently on the road are gasoline-electric hybrids. Toyota Prius, the first mass-produced hybrid, was introduced in Japan in 1997 and in the U.S. in 2000. The 2-passenger Honda Insight was the first to be widely marketed in the U.S., in 1999. Since then, both Honda and Toyota have placed several additional hybrid models in U.S. showrooms. Spurred by the popularity of these imports, U.S. automakers have also fielded several hybrids.
Gasoline contains a tremendous amount of latent energy. For this reason it has been the fuel of choice for motor vehicles for 100 years. But gasoline is becoming expensive and its combustion creates greenhouse gases and other pollutants. Electricity on the other hand, is clean and relatively inexpensive. But electricity cannot deliver nearly as much energy as gasoline. As a result, electric vehicles tend to be underpowered for the type of driving most of us do, lacking the ability to provide high speed, long range, and rapid acceleration.
Most cars and trucks spend the majority of their operating life cruising at speeds between 25 and 70 mph. A relatively small percentage of the time they are accelerating or climbing a hill. Cruising at constant speed, even at freeway speeds, requires relatively little energy, about 20-25 horsepower (hp). Yet most conventional cars and trucks are equipped with engines capable of generating 200 hp or more. This extra power is needed only during acceleration or climbing; such a large engine is heavy and operates inefficiently at constant speed.
Enter the hybrid, which uses a much smaller, lighter, and more efficient gasoline engine, supplemented with a battery-powered electric motor or, perhaps more accurately, an electric motor supplemented with a gasoline engine. Most (not all) shut down the gasoline engine when stopped and are capable of accelerating gently from a standstill to about 30 mph using only electric power. More rapid acceleration and higher speeds require contributions from the gasoline engine.
In this way, hybrids marry the clean, cost-effective aspects of electricity with the power efficiencies of gasoline, reducing emissions while maximizing fuel economy. To accomplish this, several technologies have been developed. First, on-board computers determine when to start the gasoline engine and how much power to derive from it, based on the inputs of the driver via the accelerator pedal, how much power is required to provide the requested acceleration or speed, and the charge state of the batteries.
Second, hybrids use complex gearing mechanisms to deliver power to the transmission using only the electric motor or a combination of the electric motor and the gas engine. Some of these cleverly designed mechanisms operate very efficiently, maintaining rotational energy with constantly spinning components, and engaging the various drive gears as needed. Once power is delivered to the transmission, a conventional drive train is employed to turn the wheels.
Finally, hybrids rely on regenerative braking systems. As the vehicle decelerates, kinetic energy is recovered directly from the braking system or by transferring some engine braking to the electric motor, which in regenerative mode acts as a generator, returning energy to the batteries. This process, managed by the computers and enabled by the sophisticated electric motor/generator, allows the batteries to maintain a charged state, eliminating the need for recharging from an external source.
Some hybrids also maximize fuel efficiency and performance by aggressive weight reduction throughout the vehicle. In addition, some, such as the Insight, rely heavily on aerodynamic design to improve efficiency at highway speeds. The end result is a driving experience similar to that offered by comparable gasoline-only vehicles, but with greatly reduced emissions and lowered operating costs. Of course all this technology comes at a price, typically $3000-5000 more than a comparable gasoline-only vehicle.
Not all hybrids are created equal. Manufacturers have "tuned" these technologies to achieve differing results. For example, the Prius was designed to minimize emissions. The designers of the Insight sought the best possible gas mileage, though they also achieved excellent emissions results. The Insight model has been discontinued, having suffered from a limited market for a 2-seat hybrid now that full-size models are available. Many of the lessons learned in its pioneering design have been incorporated into other Honda hybrids.
Because many hybrids shut down the gasoline engine at low speeds, they excel at fuel efficiency under conditions that yield the worst results for conventional gasoline powered vehicles. Running on batteries only, these hybrids handle stop-and-go city driving and heavy commute traffic with little or no gasoline consumption.
There have been many criticisms of hybrids regarding safety but these have not held up to scrutiny. For example, crash test results for hybrids are no worse than those for conventional vehicles of similar class. Emergency response personnel initially expressed concern over potential exposure to battery materials, but this has not been a problem. Others fear electromagnetic radiation from the electrical components, but there is no scientific evidence that the levels of exposure experienced by passengers in a hybrid vehicle are dangerous.
Hybrids have also come under attack for being less environmentally friendly than manufacturers portray them. Specifically, a considerable quantity of fossil fuel is consumed in manufacturing and transporting the large batteries. In addition, there are questions regarding the best way to dispose of the batteries, though manufacturers claim they are non-toxic and fully recyclable when handled properly. These are valid concerns, and are likely to be more effectively addressed as the number of hybrids increases, driving economies of scale in the manufacturing and disposal processes.
Next month: Who buys hybrids, how economical are they, and what's next in hybrid technology?