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.