How Hot Is Your Car Exhaust And Which Way Does It Point?

July 14th, 2008 • Related • Filed Under

The following is a guest post from reader Jaimie Scott, who was kind enough to write an article for The Good Human. Thanks Jaimie!

Something occurred to me while I was riding my bike today. I bet not many of the design engineers at the major auto makers ride bikes. I came to this conclusion when I stopped at an intersection alongside a car. As the car took off I was engulfed in a cloud of hot exhaust. So I started looking at cars with this in mind and noticed that the side of the car that the exhaust comes out on and the direction it points is different on nearly every make and model. Of course for cyclists, the worst combination is the passenger side exhaust pipe that comes out the side of the vehicle, followed closely by the passenger side pipe that comes out the back of the vehicle. It will sure be nice when a non-polluting form of transportation replaces the car!

By “cloud” I mean a thermal cloud, not a visual one. It has been very warm in Sacramento this week and the wildfires still burning in Northern, CA are not helping matters. Even though it was almost 90 degrees out at 11am when this happened, I had no problem feeling the hot exhaust hit my face. It made me wonder how hot those car exhaust gases are. I figured they must be at least 50 degrees higher than the ambient air in order to be that easily noticed. Then I thought to myself, “I wonder if the temperature of those hot exhaust gases significantly contributes to global warming?” We all know that greenhouse gases such as CO2 play a role in trapping heat near the earth that results in global temperatures rising. But what about the energy in the form of heat that we create when we drive. Does this play a role and if it does, to what extent? I don’t see how it could not with the exhaust gas temperature being much higher than the air temperature.

So as I always do when I have a question I turned to the web. At first I tried to find out the temperature of those exhaust gases. I couldn’t find much information, but from what I was able to find most cars’ exhaust temperature after it passes through the catalytic converter is in the hundreds of degrees, like somewhere between 300F and 500F. That would explain why I could feel it so powerfully ten feet away on a 90 degree day. This is an incredible amount of heat we are releasing into the atmosphere. I think it’s easy to forget that many thousands of explosions take place every second in your car’s engine. That heat has to go somewhere. Cars are very inefficient at converting that energy into power to drive the wheels. Exhaust gases are only one way in which heat is transferred from our car engines to the atmosphere. Engines produce a great deal of heat that radiates directly from the block and cylinder head, not to mention the heat that the coolant removes which gets transferred to the air by the radiator. Much to my surprise, when I started looking for articles on the contribution of the heat generated by our cars to global warming I was unable to find a single one!

It seems like the calculation should not be too difficult. I mean it’s easy enough to figure out how much heat all of the cars in the world are putting out in their exhaust if we make assumptions for the average exhaust temperature and the volume of exhaust generated per gallon of gas burned. Burning a gallon of gasoline creates about 18 pounds of C02. But is that the only contribution to global warming we should be concerned about with all the driving we do? The difficult part of the calculation for me comes from assessing the impact of that heat on the planet. I’m not knowledgeable enough about thermodynamics to figure out that part of it.

Does anybody reading this have the knowledge or data at their disposal to complete the calculation? Have you read about this topic anywhere else? Do you know a physicist that is well-informed in the ways of auto exhaust and heat transfer that you can forward this too so we can figure it out?

Many thanks to Jaimie Scott for this guest post!

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There Are 3 Responses So Far. »

  1. Comment by ben on 14 July 2008:

    Snow here in NYC never lasts very long because there’s so much heat energy being released from cars, heated homes and lighting etc. I’m sure it has an effect, no matter how small it might be, on the local environment.

    However, I’m sure the sun’s energy is likely a billion times more than what all cars produce.

    What makes the difference is the amount of energy that is unaturally reflected back to earth (greenhouse gases) instead of sent back out to outerspace. Gases which cars help produce.

  2. Comment by Drew on 29 July 2008:

    The surface of the earth receives, on average, about the energy content of 74 trillion gallons of gasoline per day from the sun. Assuming there are 590 million cars on this planet, that would mean each one of us would have to go through about 125,000 gallons of gasoline per day in order to equal the heat output of the sun on the earth.

    It’s all a moot point anyway, because we re-radiate most of that heat out into deep space at night (why it gets cold). Any extra heat would just mean extra radiation at night.

    The real concern with global “warming” is the greenhouse gas increase in our atmosphere, which cause more solar heat to be trapped on earth during the day. The actual heat output from the tailpipe is negligible.

  3. Comment by Chris on 20 November 2008:

    We also need to consider that CO2 is not the most potent or only contributor to the greenhouse gas effect.

    Water vapor, though less potent, is another contributor; and hydrogen cell vehicles put out lots of water vapor as their exhaust.

    Just one more argument against hydrogen powered vehicles.

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