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Does cold gasoline have less energy

  1. May 27, 2012 #1
    Driving at -30 Celsius versus driving at +25 Celsius.

    The gasoline has less kinetic energy at -30 Celsius...

    Yet our cars run fine at -30. You would think that there would be a massive difference because 60 degrees celsius difference (-30 vs +30) is a LOT of kinetic energy.

    Most problems in the winter have to do with thick oil in the engine, and ice in the gas tank from water. Gasoline temp itself doesn't seem to be a problem in the winter. Atomizing the gas is indeed sometimes a problem, but atomization is good enough to ignite even at -30.

    So if you had really cold gas at -50 celisius and you could ignite it, that would contain less kinetic energy initially, but once out the tail pipe... it seems to run the engine generally the same? WTF? It's not like the engine fails to run due to that massive loss of initial kinetic energy of the gas.
     
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  3. May 27, 2012 #2

    Simon Bridge

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    Your engine converts chemical energy (stored in interatomic bonds) by combustion to kinetic energy. It does not use the (thermal) kinetic energy in the fuel.
    So the ambient temp makes no difference because it is still the same chemical reaction releasing the same energy.
     
  4. May 27, 2012 #3
    Simon is correct but as something to think about when talking about heat energy you need to work in K so your temperature difference is 243K to 303 K, about 20%.
     
  5. May 27, 2012 #4
    No matter what time of the year it is, your engine is designed to heat the gasoline up to about the same temperature before it is ignited. It does this using waste heat from the engine. So for the subject currently under discussion, the only difference between summer and winter is that the engine will have to expell less waste heat in the winter.

    Heating the fuel does add energy to the fuel, but it is a very tiny fraction of the total chemical energy that your engine is designed to exploit. I doubt that you could develop a means of measuring the difference without spending a great deal of time, money, and thought designing the instrumentation system.

    Back during WWII in North Africa, they developed a secret system that greatly reduced fuel consumption in the battle vehicles. Back then, they were gas and not diesel, and they used very primative carburators. This system added a heat exchanger to the fuel supply so that the gas would enter the combustion chamber as a vapor and not as a mist. This greatly increased combustion efficiency, and therefore also fuel ecconomy.

    In the 1970's many people were adding similar systems to their cars as home projects. They were equally successful. I was one of the people involved in that technical fad. I had one car, a 1974 Mitsubishi, that flowed engine coolant through a jacket built into the carburator. That car got incredibly great fuel efficiency at the time.

    But then they changed all the cars over to fuel injection. That was even more efficient at atomizing the fuel for better combustion, so we had no need for the heat exchangers any more.

    But the addition of heat to the fuel had no measurable effect on fuel ecconomy. It was all due to improved mixing of the vapors with air for more efficient combustion.
     
  6. May 31, 2012 #5

    russ_watters

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    This is not correct. The thermal energy is absolutely a factor, since the fuel has to be heated before it is burned. It just isn't a big factor because:

    1. ICEs have a lot of waste heat available.
    2. The amount of thermal energy is a lot smaller than the amount of heat energy.

    Also, the conditions of the entering air matter.

    This is a much bigger issue for fuel fired heaters.
     
    Last edited: May 31, 2012
  7. May 31, 2012 #6

    Simon Bridge

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    Well all right - but it is still the base misunderstanding.
     
  8. May 31, 2012 #7

    Dotini

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    A discussion of how gasoline combines with air to make power in an internal combustion engine probably is not really relevant to the reduced General Physics question posed in the OP.

    But if we were in the broader world of multidisciplinary engineering, one could immediately note that under racing conditions, cooler ambient temperatures generally mean more power, since the denser air combined at the correct ratio with gas means more of the charge can be processed through the engine with it's fixed displacement and rpm.

    Another way to consider this is to use an intercooler to reduce the temperature of the incoming charge in a supercharged or turbocharged engine, once again allowing more charge to be processed, generating more power.

    If our engine's power is limited by the volume of fuel and air that it can process, then we may hope to increase the power if we can make that volume denser.

    I apologize if I'm off topic.

    Respectfully,
    Steve
     
  9. May 31, 2012 #8

    russ_watters

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    Just for clarification, the answer to the limited question of combustion energy lies in between some of the previous answers, but with some major complications lurking beneith the surface.

    First, lets compare thermal energy to combustion energy:

    -Gasoline has a combustion energy of 47.2 MJ/kg (wiki)
    -Gasoline has a specific heat of 2.22 kJ/kg-K (wiki)

    So the difference between -30C and +25C is 1.2 MJ or 2.6%. But what does this really mean?

    If you are heating your house with a gasoline (fuel oil is similar) boiler with a constant input of 25C air and an outdoor tank at -30C, then you will get 2.6% less heat out of your heater for the same amount of input gas. Certainly not "massive" but for our purposes, I don't think we should ignore it.

    A car is far more complicated and frankly I'm not completely clear on all of these effects (perhaps one of our car guys can help...):

    1. You don't just put cooler gas into it, you also put cooler air into it. So that implies even less energy input.
    2. But cooler air is more dense, so it brings in more oxygen, allowing you to put in more gas and generate more power.
    3. A car engine is a cycle, where a heating boiler is not. So your lower combustion temperature may actually just shift the entire cycle down a bit on the T-S chart. But that depends a little on how the temperature of the engine is regulated. In a water cooled engine (almost all cars), the heat rejection is regulated so it may not actually lower the combustion temperature. In an air cooled engine, it would -- though the air cooling itself is not regulated which is an additional effect.

    Hope that doesn't muddy the water too much...
     
  10. Jun 1, 2012 #9
    I was incorrect to say it was "massive difference"... it seems like 40 degrees celsius difference is a lot of kinetic energy since the number 40 sounds more than the number 2.6 percent.

    By the way, if we could hook up sterling engines to the exhaust pipe and extract heat from the waste.... we would make our engines more efficient. However hooking up sterling engines along the exhaust pipe is complex and impractical....

    I wish there was some way to trap the heat using a turbo charger that is powered off only heat, and doens't create more heat like turbo chargers do. Turbo's tend to create more power and not increase mileage that much, even though they kind of run off excess heat of the engine (but they end up creating more heat, another mind boggling complex scenario that one can think about).

    Instead of wasting the heat using coolant and ramming air against a rad to release the heat into the surroundings, one could use this heat to produce electricity. That electricity could then power an electric super charger or engine assistant to increase mileage. It could be used to produce more power, but many would prefer more mileage rather than power.
     
  11. Jun 1, 2012 #10

    russ_watters

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    There are car manufacturers looking into using exhaust heat to drive steam turbines.
     
  12. Jun 1, 2012 #11
    Cold gas is also denser, so it should provide more chemical energy even if it is not providing more kinetic temperature energy...

    but not that much denser since liquids don't expand and contract that much.
     
  13. Jun 1, 2012 #12

    sophiecentaur

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    You / they must have been lucky with your particular carburettor design. With many systems there was a real risk of vapour lock even just in hot weather. Then you got no power at all!
     
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