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Internal combustion engine thermal energy loss

  1. Dec 19, 2008 #1

    I know that something around 65% of the potential energy stored in fuel is lossed in an engine through heat. Im trying to find exactly how its lossed, I know some is through piston friction, and some is lossed through the exhost, but I cant find how much is lossed for each.

    Im not an expert so let me knowwhat I get wrong here.
    When compressed fuel is ignited the heat makes the fuel/gas expanned. Does the (thermal or knetic?)energy from the expanding gas all get forced into driving the piston down, or is most of the energy used up pushing on the areas opposite of, and around the pistion, turning into heated metal?

    Is it like a rifle?. when a shot is fired half the energy goes into pushing the bullet and the other half goes into the recoil pushing the user? Minus some thermal energy into the barrel, brass caseing.

    Can someone breakdown exactly how much energy is lossed due to what?

  2. jcsd
  3. Dec 19, 2008 #2


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    Breaking this down exactly for a general case is impossible. Very broadly speaking:

    35% to power (useful work)
    35% up the exhaust pipe
    30% to the jacket water (including lubricating oil)

    Exact figures vary depending on lots of things (fuel type, engine load, engine design, application, surroundings). The engine manufacturer will give you a much more accurate heat balance if required (and break it down by heat to lube oil, friction, any charge cooling etc).
  4. Dec 19, 2008 #3
    I don't think this is quite what he asked for. He stated;

    I think he is referring to the chemical bonding energy in the fuel itself being lost as heat instead of work.

    In a nutshell, an engine is not extracting all the chemical engine from the fuel for two main reasons. The first being that the piston is not entirely expanding enough to allow the combusted gases to reach their initial/inlet temperature and pressure. The second being that the process is not adiabatic or there is heat transfer from the combusted fuel and air to the engine itself. If the process was adiabatic and the exhaust temperature and pressure was the same as the inlet reactants, then you essentially have the Carnot cycle. This is of course neglecting changes in specific volume of the products and reactants.
  5. Dec 19, 2008 #4


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    That is what brewnog answered.
    Those are some of the reasons why it is inefficient - brewnog quantified where the heat lost to that inefficiency goes.
  6. Dec 19, 2008 #5
    I was asking about both. Where wasted energy goes (brewnog), and how it gets wasted (Topher925). The questions wern't worded the best. Most of what i was asking was pretty much aswered.

    From what I understand,
    Around 30% of the energy is lossed through heat transfer from hot gas to in the cylider wall, and goes to heating up oil, coolent, and the engine block itself. Another 35% Is lossed due to "leftover" pressure to weak to push the cylinder head any faster than it was already traveling from the initial burst. The leftover pressure is pushed out the exhost. Some also goes to friction of moveing parts, but from what i've read, not much.

    What I'm still unclear about;

    When the fuel is first ignited, 100% of the energy released is thermal? Some thermal energy creates ?, or turns into -> kinetic energy by forcing the gas to expand and build pressure, pushing on its surroundings.
    So, is some of the kinetic energy converted back to thermal when it pushes on stationary areas in the chamber other than the cylinder. Or is all the pressure/ kinetic energy (or at least most of it) forced into pushing down the cylinder. moveing the car.


    Is the thermal energy not converted into kinetic untill it moves the cylinder. Meaning all the energy in the chamber is thermal, and is lossed through cylinder walls through heat transfer. I would think that pressure is stored kinetic energy in a way.

    It a little hard for me to explain, im not trying to make it complicated.
    Last edited: Dec 19, 2008
  7. Dec 20, 2008 #6


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    It isn't just leftover pressure: the exhaust is hot.
    Kinetic energy is thermal energy.
  8. Dec 20, 2008 #7
    I think hes referring to an organized kinetic energy from the gas having to accelerate and move with the piston. This is a rather negligible amount of energy and the majority of it will not become any type of thermal energy. The gas not only travels down with the piston but also out of the cylinder and out the exhaust pipe so my intuition tells me that most of the kinetic energy will be conserved.

    I don't really think this is a good estimate. The majority of thermal energy exits the exhaust pipe and comparatively not a whole lot exits the heat exchanger. And I can't think of any ICE otto cycle that is 35% efficient. I don't have any hard numbers but my educated guess would be;

    28% Mechanical work
    52% Heat and work expelled from the exhaust
    20% Heat transferred to the cylinder wall
  9. Dec 20, 2008 #8


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    As I said, this varies and depends on a lot of things. I quoted ballpark figures for a good spark ignition engine operating at peak output. Your estimate probably better describes a lower duty cycle, or an engine operating at part load where throttling losses are greater. To quibble over 5-10% is meaningless without quantifying a whole host of other details (engine type, fuel, compression ratio, any supercharging, altitude/ambient conditions, ignition timing, load, jacket water temperature, throttle position....).
  10. Dec 20, 2008 #9
    That makes sense, The majority of the energy is lossed through conduction, or i guess for exchost it would be convection into the air.
    I was thinking a small amount of usable kinetic energy is lossed moving the engine block. If you rev you engine you will notice it moves a little, it also constintly vibrates, along with vibrating the car. This is a kind of recoil.
    Although I dont know how much energy this could be, it's most likley not much. So most of the energy that can be used for moveing is forced into pushing the piston.
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