Your car only uses 15 percent of the energy from its gas, but engineers are working on sucking every last drop from your gasoline.

This is the second part in an eight-part series on the future of transportation. New articles published every Monday.

If you had to invent an efficient way to move a person from one place to another, you could hardly do worse than the modern automobile. After more than a century of refinement, even the most over-engineered slab of German perfection wastes 85 percent of the energy in the fuel we put into it.

Most of that energy is squandered by the car's beating heart, the internal combustion engine, which alone wastes 62 percent of the energy that enters the gas tank, according to the EPA. Where does that energy go? It's radiated away into the atmosphere as heat.

By comparison, the rest of the average car is relatively efficient. The stereo, air conditioner, and power windows combined eat up only 2.2 percent of the car's energy intake. Ditto air resistance (2.6 percent); friction between the wheels, their bearings and the road (4.2 percent); braking (5.8 percent); and the "driveline," which includes the transmission and all the other parts of the car that transmit the force of the crankshaft to the wheels. Idling uses up the remaining 17 percent of the energy in a car, which explains why hybrid vehicles turn themselves off at stoplights.

That means that of the 130 million joules of chemical energy in the average gallon of gasoline, only 19.5 million are converted into the kind of kinetic energy that matters-forward motion of the car. The rest are literally disappearing into thin air.

Only a century of cheap oil could keep all that low-hanging energy out of the hands of innovators who want to recover as much of that wastage as possible. Now that most of the developed world has finally jumped on the conservation bandwagon, their dreams are becoming a reality.

There are three basic ways to recover wasted energy in a car: regenerative shocks, regenerative braking, and recovery of waste heat from engine exhaust. Other options, like the "wind-energy-capturing device for moving vehicles" described as a tiny wind turbine attached to the top of a truck-are pipe dreams, despite having been issued patents by a U.S. Patent office that apparently doesn't care whether an invention will actually work. (If this did work, pinwheels on big rigs would be the perpetual motion machines that could end our dependence on Mideast oil immediately.)

Regenerative Shock Absorbers:

Zack Anderson, the co-founder of Levant Power, a company tackling regenerative shock absorbers. When a car rolls over a bump, its hydraulic shocks absorb kinetic energy, which is then dissipated as heat. This isn't a big deal if you're driving a Camry, but if you're the world's single largest consumer of liquid fuels and your fleet of Humvees is rolling across the unpaved badlands of Afghanistan, the savings start to add up.

"Depending on the vehicle type and terrain, we're increasing fuel efficiency in the [U.S. military's vehicles] by up to six percent," says Anderson. That's 60 percent of the energy lost through the vehicle's suspension. Theoretically, says Anderson, they could recover 100 percent of the energy wasted by shocks, which would increase fuel efficiency in heavy vehicles on bumpy roads by up to 10 percent.

Installing the same system on a passenger vehicle rolling on smooth terrain would increase its fuel efficiency by only about 2 percent, but if Levant can make its shocks cheaply enough, they could show up on a car near you soon. That's because Anderson and his team have created shocks that are exactly the same dimensions as regular automobile shocks and that function in nearly the same way. "A standard shock is a piston," says Anderson. "When the car's wheels move up and down, that piston forces a thick fluid through small hole, and that causes heat. What we're doing instead is, we're using that piston movement to turn a hydraulic motor that spins an electric generator and that produces electricity."

Regenerative Brakes:

Regenerative braking is the best-known method for recovering wasted energy from a vehicle. The same electric motors that convert electric energy into the motion of the car's wheels can operate in reverse-as generators that turn unwanted kinetic energy into electricity. It's the same principle as a wind turbine, only instead of harvesting the breeze, the car is harvesting the momentum of the wheels as you stomp on the brakes. But to maximize the regenerative brakes, you have to drive a little differently.

"Folks tend to be on the throttle, then brake late and hard," says Dave Lee, a product communications specialist at Toyota whose job description unofficially includes coaching obsessives on how to squeeze every last mile out of their hybrids. Typical braking overwhelms the batteries of a hybrid, which are limited in terms of how much voltage they can handle without being damaged. That's why the average hybrid vehicle driver is only getting 10 miles per gallon better than drivers in comparable non-hybrid cars. By driving 50 or 60 yards ahead of yourself and coasting as much as possible, Lee says, a Prius owner can approach the theoretical maximum Toyota's engineers estimate you can get out of a hybrid's regenerative brakes, which translates to about 30 percent better fuel economy than a non-hybrid vehicle.

Heat Recovery:

The white whale of automobile waste recovery is the embarrassing inefficiency of the internal combustion engine, which reached its current form at the turn of the last century and has seen only incremental improvement since. The exhaust gases flying out of the back of your car are hot enough to melt lead, so it shouldn't be too hard to use them to produce energy. Engineers have been attempting it for 50 years. The trick is doing it simply and consistently.

Dan Coker, CEO of Amerigon, figures his engineers have it just about right. Their "thermoelectric" system uses solid-state electronics to convert heat directly into electricity. Amerigon's system, which is being tested by BMW and Ford, achieves more than 10 percent efficiency in converting heat into electricity, but the exact value is a trade secret.

"If we can get it to one thousand watts, we might even be at a level where you could consider eliminating the alternator," says Coker. The alternator provides all the electricity required by a conventional vehicle, and excommunicating it from a car would take a significant drag off the average automobile's engine. According to BMW, a 1000-watt thermoelectric converter could reduce fuel consumption by up to 10 percent. But getting 1,000 watts would require either a lot of very hot exhaust or close to 20 percent efficiency of conversion of heat to electricity, which is unheard of.

In a different approach, Honda's engineers have proposed a system that uses a system in water is vaporized by exhaust gases, and that steam is run through a tiny turbine that produces electricity They report that it could reduce fuel consumption by up to 32 percent. To date, published results put the actual fuel savings at 3.8 percent.

Taken together, simple addition suggests that an enterprising home mechanic with access to state-of-the-art technology could increase the fuel efficiency of his or her car by between 30 and 40 percent. It's hardly a revolutionary number, until you consider that the world consumes 85 billion barrels of oil a day, and auto manufacturers are already bumping up against the limits of what they shave off of a car in order to make it use less gas.

"Weight is huge," says Anderson, "and you have car companies that are desperate to take just one, two, maybe three pounds of weight off a vehicle." Making cars sleeker and lighter can only take us so far, and until we rid ourselves of the internal combustion engine, poor roads, and rush hour traffic, the 85 percent of the energy in a vehicle that's wasted remains a big, fat target for auto manufacturers the world over.

Illustrations by Keith Scharwath.