for all you physics buffs

[Piersol]

How hot does the air get inside the cylinder of a cummins engine at the top of the compression stroke if the outside air temp is 30 degrees?

 

[cruiser_2]

Well, first there is an intake air heater on these vehicles, and more importantly Charles Law and Boyles laws of physics apply. Simplified version is if you compress a gas the temperature is directly porportional to the amount of compression. The reverse is true about expansion which is where we get our ability to Air condition our trucks, homes and businesses

 

[JApol]

Is this at idle with no boost or WOT with ?? boost. how hot is the engine?

 

[Batphreak]

That heat depends on a lot of factors, engine temp, boost level, ammount of fuel in the mixture. I know this much, hot enough to start combustion of the air/fuel mix (if it wasn't the engine wouldn't run).

 

[RKerner]

If you simplify things and make a few assumptions, you can use this formulation and get a ballpark number:



T=To*[Po/P]^(k-1)/k where is k~1. 4 for air.



Remember to use Kelvin and not Fahrenheit for calculations.



What I don't have for you is the pressure ratio at TDC between ambient and in-cylinder conditions. You may want to first apply it as the air goes through the turbo, then again as it goes through the intercooler then finally in the cylinder.



You'll probably find that it will be close to or slightly greater than the ignition point of diesel fuel - hence ignition. Hope this helps.

 

[Jong]

It's simple and it isn't... . PV=nRT where P is pressure, V is volume, n is number of moles of gas present, R is the ideal gas constant, and T is system temperature. The variables come in with amount of air admitted to the cylinder and its initial pressure at BDC (bottom dead center), inital temperature of the system, cylinder ring seal, cylinder cranking pressure versus static pressure, and a few other things. All said and done, your air temp should be around 1600-1650 degrees at TDC, more than enough to ignite diesel fuel. Thank Newton for the laws of thermodynamics... . :-laf

 

[kiwi bird]

I'd guess 540 degrees - give or take a bit if we assume 18:1 compression

 

[Grey Wolf]

As I remember the compressed air is around 750 degrees...

 

[RustyJC]

If you want to treat the process as adiabatic compression where no heat is transmitted to or from the air from the cylinder surfaces during compression, then you can use the simplified formula:



T2 / T1 = ( V1 / V2) ^ 0. 41



Where



V1 = Initial volume in cubic inches



V2 = Final volume in cubic inches



T1 = Initial temperature in degR



T2 = Final temperature in degR



If we assume a 17:1 compression ratio, 60 cubic inch displacement per cylinder, 100 degF initial temperature (559. 7 degR), then the calculation looks like:



T2 / 559. 7 = ( 63. 75 / 3. 75) ^ 0. 41



T2 / 559. 7 = 17 ^ 0. 41



T2 / 559. 7 = 3. 195097



T2 = 3. 195097 x 559. 7



T2 = 1788. 296 degR



T2 = 1328. 596 degF



Rusty

 

[Piersol]

Thanks for the info guys, The reason I asked is because it seems with all that heat there would be a way to make a steam engine. Inject water into all that heat it seems steam would be created to push the piston down. I know this isn't the case because a little water in our fuel and they don't run for shi* but it still seems logical.

 

[GFritsch]

If you want to treat the process as adiabatic compression where no heat is transmitted to or from the air from the cylinder surfaces during compression, then you can use the simplified formula:



T2 / T1 = ( V1 / V2) ^ 0. 41



Where



V1 = Initial volume in cubic inches



V2 = Final volume in cubic inches



T1 = Initial temperature in degR



T2 = Final temperature in degR



If we assume a 17:1 compression ratio, 60 cubic inch displacement per cylinder, 100 degF initial temperature (559. 7 degR), then the calculation looks like:



T2 / 559. 7 = ( 63. 75 / 3. 75) ^ 0. 41



T2 / 559. 7 = 17 ^ 0. 41



T2 / 559. 7 = 3. 195097



T2 = 3. 195097 x 559. 7



T2 = 1788. 296 degR



T2 = 1328. 596 degF



Rusty

Thanks for making it "simple". :-laf Math is pretty cool when one knows how to use it, which you obviously do. I wonder what the average temperature going into the cylinder is after running throught the turbo and intercooler.

 

[AMink]

I touched the exhaust manifold after a run the other day. Temperature is between Son of a biscuit and Gol Dang! Imagine it's even hotter inside! :-laf

 

[RustyJC]

I wonder what the average temperature going into the cylinder is after running throught the turbo and intercooler.

I "assumed" 100 degF on a 30 degF day (see post #1), but one can plug in whatever temperature they might wish, as well as an effective compression ratio (taking intake valve closing into account) in lieu of a mechanical compression ratio.



Rusty

 

[nps]

I "assumed" 100 degF on a 30 degF day (see post #1), but one can plug in whatever temperature they might wish, as well as an effective compression ratio (taking intake valve closing into account) in lieu of a mechanical compression ratio.



Rusty

Is that assuming a recent valve grind and overhead? :-laf

 

[HoleshotHolset]

I touched the exhaust manifold after a run the other day. Temperature is between Son of a biscuit and Gol Dang! Imagine it's even hotter inside! :-laf

That's funny stuff right there! :-laf



Matt