Okay, I know that adiabatics is a thermodynamic principle whereby changes in volume, pressure etc are performed without ensuing heat transfer to surrounding bits and pieces. I assume the advantage, in F1 engine terms, would be regulated heat transfer within engine components themselves. But how the heck do they implement it practically and physically? Any info appreciated.
Adiabatics 101
Started by
Rich
, Sep 15 1999 05:39
3 replies to this topic
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#2
Christiaan
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Posted 15 September 1999 - 16:08
Well Rich, its actually quite simple. The time is which a the mix in a cylinder gets compressed and ingnited and blah blah blah is so short that you can assume that no heat is transfered outside the cylinder. This works even for road cars, one of my design project entails verifying the results published by CAR magazine for BMW Z3. There is a neat maximum engine power calculation done using a thermodynamic model. We made an adiabatic assumption. The theoretical power was within 10% of what CAR had published.
For F1 cars this works out even better because they rev very high.
For F1 cars this works out even better because they rev very high.
#3
narhuit
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Posted 15 September 1999 - 23:47
Wow! someone who claims that there is a simple answer to a thermodynamical question... Unbelievable!!!
I do not have such an answer, but I think the question was more: if the engine is adiabatic, how is it that it does not overheat (and then melt ;-) )?
I do not have such an answer, but I think the question was more: if the engine is adiabatic, how is it that it does not overheat (and then melt ;-) )?
#4
Christiaan
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Posted 16 September 1999 - 15:09
Okay this is as I understand it. It would be possible to run a cold adiabatic engine for maybe 10 minutes or so if the engine could start from 0 and instantly rev to 10,000rpm. This is because the the starting process is not adiabatic and not very efficient. This is why you sometimes get dirty exhaust when you start the car, or when you are revving low.
Now, the flame temperature in an ordinary car is of the order of 2000°+, but it is obviously evident that the engine temperatures are nowhere near that. This is because the whole process is dynamic and the 2000°C+ temperature is instantaneos. If you stuck a thermometer in your cylinders during reving you would see a much lower average temperature, I'm guessing around 900°C.
For the calculations we did for the Z3, the proces was like this: When the spark plug fires the fuel/air mix instantaneosly heats up to about 2000°C. At that point, no volume change is assumed. It then starts to burn and it expands. Expansion from the heat,and also expansion because the products (CO2 ond H20 mostly) have a higher specifc volume than the reactants. The second case causes the the gases to cool sort of adiabatically. This makes the temperature drop remakably to the exhaust gas temperature. Water also has a higher specific heat capacity than the reactants.
There is a very small amount of heat leakage outside the clnder engine per stoke. But you see, 5000rpm (in a road car) is a lot of stokes. Because the inside of the cylinder has higher entropy than the outside the heat will always flow to in that direction. And that is why engines heat up.
There is also the consideration of the heat conductivity of the actual air/fuel mixture. It turns out that lean fuel mixes have a very high conductivity and when you run an engine too lean, you can melt a hole through pistons.
Now, the flame temperature in an ordinary car is of the order of 2000°+, but it is obviously evident that the engine temperatures are nowhere near that. This is because the whole process is dynamic and the 2000°C+ temperature is instantaneos. If you stuck a thermometer in your cylinders during reving you would see a much lower average temperature, I'm guessing around 900°C.
For the calculations we did for the Z3, the proces was like this: When the spark plug fires the fuel/air mix instantaneosly heats up to about 2000°C. At that point, no volume change is assumed. It then starts to burn and it expands. Expansion from the heat,and also expansion because the products (CO2 ond H20 mostly) have a higher specifc volume than the reactants. The second case causes the the gases to cool sort of adiabatically. This makes the temperature drop remakably to the exhaust gas temperature. Water also has a higher specific heat capacity than the reactants.
There is a very small amount of heat leakage outside the clnder engine per stoke. But you see, 5000rpm (in a road car) is a lot of stokes. Because the inside of the cylinder has higher entropy than the outside the heat will always flow to in that direction. And that is why engines heat up.
There is also the consideration of the heat conductivity of the actual air/fuel mixture. It turns out that lean fuel mixes have a very high conductivity and when you run an engine too lean, you can melt a hole through pistons.
