The key to exhaust heat recirculation is a device called an exhaust gas recirculation (EGR) valve. The EGR valve opens when it encounters back pressure from the car's exhaust and channels it back into the combustion chamber. You might wonder what good this does, since the air in the chamber is mixed with gasoline to make it combustible. Well, one thing it does is to make the fuel warmer. Warm fuel heats up more efficiently and therefore produces more miles per gallon. Once the EGR valve senses that the engine is warm enough, it redirects the exhaust elsewhere to prevent the engine from overheating.
Warming the coolant and the fuel not only helps the engine reach its optimal temperature faster when the engine is first started, but it also has a specific benefit for hybrids. Most hybrids are designed so that the internal combustion engine turns off when the vehicle is stopped. If it remains off for too long, the engine can get cold. EGR helps keep the engine from cooling down too quickly.
How does exhaust gas recirculation reduce pollution? The emissions targeted by EGR come from nitrous oxides that are produced at very high temperatures. By mixing the car's exhaust with the intake air, the amount of oxygen in the mixture is reduced and its combustibility is also decreased, which causes the fuel to burn at a lower temperature. In most EGR systems, the exhaust is also cooled before it's mixed with the gas. Therefore, fuel mixed with exhaust burns cooler and less likely to produce N2O. The lower temperatures also help fuel economy. With fuel less prone to detonation, the programmers who write the software timing routines for modern engines have more control over the precision of the engine's timing. The lower temperatures also help to avoid heat transfer energy losses, meaning that more of the car's energy goes into providing power for its wheels.
As we've seen, recirculating exhaust can both increase fuel efficiency and reduce pollution. But did you know that it may also be able to produce electricity? We'll explore that concept on the next page.