Engineering students at Duke University designed and built the world's most fuel-efficient vehicle, a fuel cell EV that can travel more than eight miles on a single gram of pure hydrogen. According to Duke Engineering and Guinness World Records, that's the gasoline equivalent of 14,573 miles per gallon or 0.01614 liters per 100 km. (I'll verify the math on that later.)
The team named the car "Maxwell," after the scientist who developed electromagnetic theory, the principle behind electric motors, generators, radios and almost every modern electronic device. During the record-breaking run, Maxwell's speed averaged over 15mph (24 km/hr).
Specifications
When it comes to fuel efficiency, the power-to-weight ratio is a dominating factor. To minimize the mass, Maxwell sports a honeycomb-structured carbon-fiber monocoque (integrated body and frame) that weighs a mere 24 kg (53 lbs). Its sleek design gives Maxwell a drag coefficient of 0.10. By comparison, a Tesla Model S has a drag coefficient of 0.24.
Maxwell's powertrain features a 250 W brushless DC motor, originally designed for model aircraft, with a single-speed chain-drive reduction gear. The custom control circuitry includes a Teensy board, an embedded controller based on the popular ARM series of 32-bit microcontrollers.
Hybrid Electric Energy Storage System
One problem with any vehicle is that its powertrain needs to be strong enough to handle quick acceleration, which only happens for a fraction of the time it's running, resulting in a car that has more power than it usually needs. The Duke team realized that this would require a pretty heavy fuel cell. (Although hydrogen is the lightest element in the universe, a fuel cell, which combines hydrogen and oxygen to generate electricity, is made of some very dense materials.) Rather than making a fuel cell that can deliver huge bursts of power, the engineering students chose a hybrid design: a small fuel cell with a bank of supercapacitors.
Fuel cells, like other batteries, are high in energy density, which gives the vehicle its distance capabilities. Supercapacitors don't hold a lot of energy, but they have a high power density, which is the rate at which they can charge and discharge. In effect, a fuel cell is like a marathon runner and a supercapacitor resembles a sprinter.
Maxwell's 100W proton exchange membrane fuel cell provides the power to maintain the car's cruising speed, which, thanks to inertia and good aerodynamic design, doesn't require a lot of juice. Speeding up, on the other hand, is a power-thirsty process. To deliver the short bursts of extra power needed for acceleration, the supercapacitors kick in.
Fuel Efficiency: The Math
Maxwell traveled 8.5 miles on one gram of hydrogen. How does that equate to 14,573 miles per gallon of gasoline? Well, it's a combination of energy density and efficiency. Neither Duke Engineering nor Guinness showed the calculations, so naturally, I did the math with some ballpark figures.
One gram of hydrogen contains around 130 kJ of energy. A fuel cell is about 60 percent efficient and a good DC electric motor can achieve efficiencies of 90% or better. Together, that makes the fuel cell EV 54 percent efficient at converting hydrogen into motion.
If a gram of hydrogen contains 130 kJ of energy and the system is 54 percent efficient, then just 70 kJ is used to move the car; the rest of the energy is dissipated as heat. Since 70 kJ moves the car 8.5 miles, it takes 8.24 kJ of energy to travel one mile. This is the actual energy required to move the vehicle one mile.
To determine the equivalent miles per gallon of a regular car, we start with the energy density of gasoline, about 45 kJ per gram. If it takes 8.24 kJ to travel one mile, then all other things being equal, 45 kJ (one gram of gasoline) should move the car 5.4 miles. (Since we're calculating energy per mile and equivalent mileage, we don't account for the efficiency of the internal combustion engine (ICE), but if you're keeping score at home, an actual ICE is only about 25 percent efficient.)
One gallon of gasoline weighs 2,800 grams. At 5.4 miles per gram, an equivalent ICE car would need to travel about 15,000 miles on a gallon of gas to achieve the same fuel-efficiency, which is pretty close to the estimates by Duke and Guinness.
It's a Prototype … and a Learning Experience
Maxwell is not a production model, obviously, but a proof-of-concept and a laboratory-on-wheels. Even better, the engineering students share their design details on the team blog, so others can learn the basics of EVs and automotive design. They'll even answer questions for readers. I still don't anticipate hydrogen cars being a major factor any time in the near future, largely due to the difficulty in obtaining pure hydrogen and the lack of infrastructure to support it, but as an engineering project, I have to admit that Maxwell is pretty cool.
Education and clean technology are two of my passions, so it's nice to see the pair combined into one project. Way to go, Duke Engineering!
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