Exhaustive research REVives engine efficiency
Sun, 27th Jul 2008
Download as mp3
from the show Naked Science Questions and Answers
US scientists have found a way to get cars going 10% further on their fuel.
Writing in this week's Science, Ohio State University researchers Joseph Heremans, Vladimir Jovovik and colleagues have developed a thermoelectric compound that can turn the waste heat in the average exhaust into electricity. This can be tapped off and stored in a battery, along similar lines to how hybrid cars work at the moment. The new material is a compound of lead and tellurium with a small amount of thallium added. When heat is applied it generates a voltage.
"You can think of it like a bucket containing some water," explains Heremans. "If one side of the bucket is made hot and the other side cold, the water evaporates from the hot side and condenses on the cold side. In this material electrons behave in a similar way." The idea would be to place the material in contact with the hot exhaust gases close to where they emerge from the engine, whilst cooling the lower surface of the compound. Electrons could then be collected from the hot surface, passed through an external battery, and then returned to the cold surface, completing the circuit.
For many years scientists have been trying to achieve this effect at an efficient rate and with materials that can be mass produced, but without success. The new material has tackled the problem in an entirely new way. Previously scientists have tried to use carefully crafted nano-structures to control the flow of heat through the compound, since it's the temperature difference across the material that drives the generation of electricity.
But instead the approach taken by Heremans has been to try to capture more electrical energy from the heat that is already flowing through the material. They achieved this by taking the existing lead telluride (PbTe) compounds and adding thallium.
"About one in every 50 atoms in the material is thallium, and this increases the number of electrons that become available at certain energies," says Heremans. The result is a compound that is very easy and relatively inexpensive to mass produce, and could make a significant difference to vehicle efficiency. "Most cars are at best 25% efficient," explains Heremans, "so only a fraction of the energy in the fuel actually ends up in the wheels - a large amount goes straight out of the exhaust pipe. This system enables us, immediately, to improve that efficiency by at least 7%."