How Exhaust Heat Recovery and Recirculation Works

In this increasingly green world, automakers are looking for ways to increase product efficiency. Their focus might surprise you: 

The exhaust pipe.

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.

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.

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.

Electricity from Exhaust

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.

Sources

• Green Car Congress. "Thermoelectrics Gaining More Attention and Development Focus." July 22, 2005. (April 14, 2009)http://www.greencarcongress.com/2005/07/thermoelectrics.html
• Heremans, Joseph. " Material may help autos turn heat into electricity." Eureka Alert. July 24, 2008. (April 15, 2009)http://www.eurekalert.org/pub_releases/2008-07/osu-mmh072108.php
• Thaindian News. "Recycling Exhaust Heat May Power Green Cars." February 26, 2008. (April 14, 2009)http://www.thaindian.com/newsportal/health/recycling-exhaust-heat-may-power-green-cars_10021214.html

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