How Exhaust Heat Recovery and Recirculation Works

Exhaust pipe emitting engine exhaust.
This may surprise you, but there are many ways to use the exhaust from this vehicle to increase fuel efficiency and reduce its harmful effects on the environment.
Joe Raedle/Getty Images News

With worldwide reserves of fossil fuels gradually diminishing and air pollution increasing, automotive engineers are constantly on the lookout for ways to make cars more fuel efficient and to reduce their carbon emissions. One of the most surprising places they've found wasted energy is in the car's exhaust. Actually, automotive designers have been tapping the hidden power of automobile exhaust since the early 1970s. Because this technology recycles the exhaust before it can exit the vehicle, it also helps reduce the emissions produced by a car and helps fight air pollution.

Technologies made to maximize the efficiency of a vehicle's exhaust are known collectively as exhaust heat recovery and recirculation. There are several ways to use a vehicle's exhaust to increase its fuel efficiency and make it run with fewer emissions. For example, the heat of the car's exhaust can be used to warm the engine coolant to keep the engine running warm, even when the motor has been turned off for a significant length of time. The interior of the car can also be warmed using exhaust heat, even in very cold weather. The amount of nitrous oxide (N2O) emissions can be reduced, and a vehicle's exhaust can actually be used to generate electricity. The term exhaust heat recovery is used for the process by which the exhaust's heat energy is recycled through both the car and the engine, so it's a part of all of these technologies.


Although these technologies can be used in any car, truck or SUV with an internal combustion engine, they're particularly important to hybrid vehicles, which need to produce maximum fuel efficiency and minimal emissions. Some of the most advanced implementations of this technology can be found in the 2010 Toyota Prius. In the next few pages, we'll look at how automotive engineers have made this technology possible.

Nitrous Oxide Emissions

Exhaust pipe.
Would you believe that his tiny little thing could produce so much waste?

The internal combustion engines in cars, trucks and other vehicles produce several kinds of pollution. One of the most common is carbon dioxide emissions, which play a significant role in global warming. Reducing carbon emissions has become one of the most important goals facing automotive engineers. However, automobile engines produce other emissions as well. One of the major components of smog is N2O -- nitrous oxide -- and these emissions are also produced by internal combustion engines.

Like carbon dioxide, nitrous oxide is a greenhouse gas. This means that it traps the heat in solar radiation -- sunlight -- within our atmosphere and uses it to heat the Earth's surface. Without the heat trapped by greenhouse gases, the surface of the earth would be too cold to support life. However, the right balance is important. While too little would turn the earth into a frozen snowball, too much would turn it into a sweltering jungle or desert. Human beings and our technology have evolved to require a certain climate. Anything that changes that climate may affect the way we live, dramatically altering agricultural patterns and melting the polar icecaps.


It's clear that reducing nitrous oxide emissions from cars is just as important as reducing carbon emissions, but how can the emissions be reduced? Nitrous oxide is produced at very high temperatures, so anything that lowers the operating temperature of an internal combustion engine would reduce N2O emissions. That's where exhaust heat recirculation comes in. We'll talk about this in more detail on the next page.

Exhaust Heat Recirculation

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.


Electricity from Exhaust

Toyota Prius Plug-In Hybrid.
Thermoelectric devices would help boost a hybrid car's range by filling in for the batteries that run its electric motor.
Bryan Mitchell/Getty Images News

Thermoelectric materials, as the name implies, can produce heat from electricity. These materials were discovered in 1821 by the German physicist Thomas Seebeck. They've generally been too expensive and inefficient to be of any use to automotive engineers, but this has started to change: The U.S. Department of Energy has expressed interest in funding the development of a practical thermoelectric system that could be used in cars.

There are many sources of wasted heat in cars, including the radiator and the engine, but the biggest source is probably the exhaust. Given that most cars already recirculate exhaust in an EGR loop and that this technology will be even more important in the future, this provides an ideal opportunity to trap this otherwise wasted heat and use thermoelectric devices to convert it into electricity. This electricity could be used to power the car's electrical systems, recharge the batteries, and perhaps most importantly, run the electric motor in hybrid and plug-in battery electric vehicles. This would be a nearly perfect confluence of several technologies, and would have the side effect of helping to reduce nitrous oxide emissions by further cooling the exhaust before it's mixed with fuel.


Any type of car could benefit from this thermoelectric boost, but once again, it would be most useful when applied to hybrid vehicles. It would extend their range by supplementing the batteries that run the electric motor and reduce the amount of time required to recharge those batteries.

The development of fuel-efficient and low pollution technologies like exhaust recirculation and thermoelectric power will make the cars of the future -- which will have little or no use for fossil fuels -- possible. It's important that we develop these technologies now, before fossil fuels run out and pollution does significant damage to the Earth's atmosphere and climate.


Lots More Information

Related HowStuffWorks Articles

  • Green Car Congress. "Thermoelectrics Gaining More Attention and Development Focus." July 22, 2005. (April 14, 2009)
  • Heremans, Joseph. " Material may help autos turn heat into electricity." Eureka Alert. July 24, 2008. (April 15, 2009)
  • Thaindian News. "Recycling Exhaust Heat May Power Green Cars." February 26, 2008. (April 14, 2009)