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


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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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  1. 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. 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. 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.

  4. ya but the thing about water vapour is that at a certian point when the concentration becomes to high it starts to rain and then the percentage of water vapour in the atmosphere is greatly reduced so really the only thing that additional water vapor in the sky will do is make alittle more rain

    just another argument for more hydrogen powered vehicles

  5. I think your heart is in the right place, but this article shows a severe lack of physics knowledge on the part of the author. Though many processes create heat on Earth they are absolutely insignificant when compared with the heat that the sun imparts, and the Earth is continually radiating energy into space. The rate of temperature change on Earth is essentially due to the difference between heat entering Earth (Sun, cosmic rays) and heat leaving (EM wave energy), which is a function of cloud cover (as the sun’s output is basically constant) and the greenhouse effect.

    As for the argument against hydrogen fuel cell technology by one commenter, you’re argument is ludicrous as hydrogen gas isn’t taken from natural supplies, but from the electrolysis of water. Water dissolved in air (humidity) maintains an equilibrium based on many factors; excess results in precipitation.

    As a university physics professor I am very concerned with global warming, but don’t put your blood, sweat, and tears into a cause based on original research or intuition. I apologize for some of my vitriol, but over the years I’ve seen, time and again, use precious time and money fo insignificant causes.

  6. A few things:

    Shocked, I want to get to the bottom of your comment

    “As for the argument against hydrogen fuel cell technology by one commenter, you”™re argument is ludicrous as hydrogen gas isn”™t taken from natural supplies, but from the electrolysis of water.”

    1. I’m unclear what part of this conversation you’re referencing to about hydrogen coming from natural supplies.

    2. You’re also saying that hydrogen comes from electrolysis, which is truel, but leaving out a few things…

    Predominately (commercially), hydrogen is formed from steam reforming of natural gas (much cheaper than electrolysis):

    CH4 + H2O → CO + 3 H2

    & then

    CO + H2O → CO2 + H2

    If you consider natural gas (essentially methane) to be a ‘natural supply’, then yes, most hydrogen comes from natural supplies. Unfortunately the cheapest (currently) formation of hydrogen results in CO2 &/or CO….

    My rant:

    Longterm, if hydrogen is to be a viable alternative for fuel, it needs to be produced pollution free (wind electrolysis?)…. or at least at a rate below the earths ability to ‘metabolize’ the pollution (CO2 getting sucked up by plants).



  7. I was responding to one commenter’s concern that water resulting from hydrogen oxidation, whether through combustion or fuel cell technology, would contribute to global warming. I should amend my comment to say that regardless of how much new water is created, the concentration of water in the atmosphere will reach an equilibrium quickly, and excess will precipitate out.

    Your description of H2 production from methane is entirely correct, but I believe the long term goal of hydrogen gas sourcing is indeed from electrolysis, as it is simply more energy efficient to simply burn methane in a turbine and produce electrical energy rather than the convoluted process of extracting H2 from CH4 then transporting it, etc.

    I should note that there are non hydrogen fuel cells in development, which extract the hydrogen in the fuel cell system.

  8. With regard to Jamie Scott’s post:

    While I agree that global warming is driven by changes in the earth’s ability to radiate heat out to space, Jamie’s question about the temperature of automobile exhaust gases actually brings up a very interesting point.

    Mechanical efficiency in reciprocating engines is inversely proportional to the temperature difference between the exhaust and the fuel/air mix.

    Upstream of the catalytic converter, exhaust gas temperature in the “average” car is in excess of 900 degrees fahr. We clearly left a lot of heat energy “on the table”. The question is whether there is anyway to convert more heat energy into mechanical energy without a massive redesign of the engine.

    As a matter of fact, there is: inject water into the combustion chamber along with the fuel/air mix. The water vaporizes and adds to the explosive force of the heated air. More heat energy is successfully converted into mechanical energy and the resulting exhaust gas stream has a lower temperature.

    In this arrangement, the air will provide less expansive force than without water but the water-to-steam conversion will more than make up for it.

      1. Bruce, the water is vaporized so quickly that it will not be able to rust anything. I work at a combined cycle power plant where we do daily water washes. This is where we literally spray (demineralized) water into the natural gas fired turbine to clean off any soot/residue from the combustion process. The is hundreds of gallons of water being sprayed into the firing path and there is no rusting problems because the water flashes to steam so quickly.

        As what Grandpa j. says:
        “As a matter of fact, there is: inject water into the combustion chamber along with the fuel/air mix. The water vaporizes and adds to the explosive force of the heated air. More heat energy is successfully converted into mechanical energy and the resulting exhaust gas stream has a lower temperature.”

        This is absolutely 100% correct. When we do the water washes we have to lower our plant power output by approximately 2 megawatts because of the water injection. A big factor in power output is “mass flow” of the fuel. The flow rate of the fuel stays the same (same volume of fuel) but when we inject the water the mass of the water increases the “mass flow” of the fuel. This makes up for the 2 megawatts of power we had to take off simply because we increased the mass of the fuel. It is the same thing on a humid day too. When the humidity is high we are actually able to put more power out than on a low humidity day. The same is true with a cold day, cold air has more density same the air/fuel mix has more mass and we can put out more power.

        All of the stands true with your vehicle. Injecting water will theoretically increase your HP/efficiency but with the caveat of cooling your firing temperature. This would reduce HP but also lower emissions. It’s all give and take but I think water injection would be a better option all the way around. You would get the same (or better) HP, increase your efficiency, and lower emissions… theoretically, no system is perfect, or cheap. That is another big problem with this type of system, but that is another day/another conversation.

  9. Carrying all that distilled water would be like an extra tank of gas. The weight would cause the engine to work harder = hotter. Plus fuel economy would sink.

  10. The real question regarding the temperature of CO2 at the moment it’s introduced to the atmosphere is why haven’t the Climate Change Advocates included it in their argument. Let’s not forget that all CO2 (from all sources) is introduced at elevated temperatures – including that from human lungs – 37 degrees celsius. So, it’s a more significant factor than the above think it is.

    More important is you have all forgotten about a little thing called gravity. Carbon dioxide is heavier than air (O2) and therefore it will not stay in the air to warm up. When it falls onto plants they absorb it retain the carbon and release the oxygen.

    Carbon dioxide can’t accumulate for those two reasons gravity and plants. Also if the current amount of vegetation were unable to keep up with the current production of CO2 the build up would occur at (and below) ground level and the effect would be obvious and observable in the behavior of insects, gophers and other life forms that dwell at or below ground level. In fact you wouldn’t be able to lay on your lawn without it affecting your breathing.

    Let’s forget about CO2 and start looking for the real cause.

  11. I know this is an off topic comment but regarding your statement …

    “I think it”™s easy to forget that many thousands of explosions take place every second in your car”™s engine”

    … a standard 4-stroke car engine would have to be running at 30 thousand RPM for a single thousand explosions to take place in 1 second.

    In reality, pushing an engine to 6000 RPM is …

    6000 (revs per minute) / 2 (power strokes a rev) / 60 (seconds per minute) = 50 power strokes per second.

    But I agree, it would be nice if we could replace cars with something cleaner.

  12. This is a ridiculous argument. While it may seem viable, the amount of heat we produce as humans has absolutely no effect on global warming, especially because it isn’t happening. There are no statistics or facts that contribute successfully to the claim of global warming(it’s technically not a theory because there isn’t enough evidence to even get to that point). By the way, the claim “that many thousands of explosions take place every second in your car”™s engine” is off by many thousand. At 3000 rpm(which is probably around 80 mph, depending on your car) your pulling 50 rps. That is how many times your crankshaft is spinning. For every two spins of the crankshaft you have one explosion per cylinder. So, for a six cylinder engine you have ((3000 rpm / 60 seconds / 2 spins) * 6) cylinders = 150 explosions per second. And at a 1000 rpm idle you have 50 explosions per second. Just a few less than “many thousand”.

  13. “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!”

    That’s because they don’t even correlate and your also an idiot.

  14. As already stated the heat generated is negligible compared to the energy from the sun, likewise, from the earth itself, which is motlen core at the centre and able to supply enough heat for everyones heating needs on the planet. It’s as if you’re thinking this heat will always be trapped in the earths’ atmosphere and adding to the global temperature of the earth. Heat is radiant and rises, conducts through molecules whether solid, liquid or gas ( albeit at different rates ), just as heat from the sun is able to reach the earth. You have to remember the earth is part of the solar system and the solar system is part of the universe. . Isn’t it a bit like asking “what affect does the temperature of Londons’ traffic have on Plutos’ global temperature” ?

  15. Heat is radiant and rises???
    For god’s sake!! If you can’t grasp basic physics shut up! Radiant heat passes through space without molecules, as opposed to conduction or convection, as for the global warming issue, we can’t even manage to predict the weather accurately over long periods since the variables are too great. Look up reductionism in your sociolgy books.

  16. its an important point from a thermodynamics point of view: all that heat = waste that could have been turned into mechanical work in the engine… if only the engineers were good enough to utilize it – lol.

    Even on a cold and wet day, with the wind (at speed) blowing on the exhaust, it still manages to evaporate water to steam! Thats some waste of energy!!

  17. yes.The heat, which is coming from exhaust is high. when i arrange vapor absorption system at exit of gases, the waste heat can be recovered for air conditioning in the vehicle.

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