This is a pretty generalized tech question. I've been working on some theoretical calculations for a motor I've been working on and just wanted some input. Here's the question. At an ambient air temp of about 298 K. About how much will the air heat up in the intake when its drawn into the combustion chamber. Treat it as an otto cycle piston engine at normal operating temp. All aluminum metal if your going to use heat transfer coefficients or anything like that. This is all before compression at BDC. It doesn't have to be right on the money but a pretty good idea. Use a specific motor of your chioce if you need an example system. Thanks in advance for your help.

 

Depends on RPM. You do not have steady state heat transfer since it is a function of time. The faster your motor is drawing air in, the less time the air is exposed to the heat of the intake.

EDIT: There are a ton of factors that will get you close to what you want. Such things as valve overlap (affects volumetric efficiency), wet flow vs dry flow, temp of fuel, volume of intake, CFM of TB. Ect ect...

 

It's a theoretical calculation, don't assume any airflow dynamics. If you need a particular system to work with just pick one and fill in the variables for a rough average estimate. I know someone knows this stuff pretty well because I remember a heated debate (no pun intended) about a radiator's metal composition and heat dissipation years back. Let me make it simpler. All I need to know basically is how aluminum handles heat. The basics I'm looking for is how much heat is conducted to an intake from an engine at normal operating temperature. Here are the 3 things I need: 1) what the normal operating temp of an aluminum block is (rough estimate). 2) the average temp of an aluminum duct stretching around .3 meters away from the heat source. 3) the amount of heat transfered to air over lets say .1 seconds. Anyone have the coefficients or can direct me to a refresher website on heat flow dynamics?

 

If you are truly desperate to have this useless info, I will give you some calculations a little later tonight. You are assuming an engine operating in a static state which is an impossibility. You have to take the temperature of the combustion chamber, calculate how much is tranferred through the head to the engine compartment and how much is transferred the .3m you are asking for. You need the coefficient factors for aluminum and for iron, then you need to calculate the tranfer of heat at even increments along this length. This should get you a pretty accurate useless bit of info as, as stated before, the airflow of a running engine has a tendency to push the heat out the exhaust more than into the intake anyway. In reallity, the air temperature wouldn't be much more than the 76.73 deg F that you originally assumed. (why did you use Kelvins for the units anyway?)

If you don't mind, if I am to waste my time giving you this perfectly useless info, why do you want it?

 

I don't need any iron coefficients just aluminum. Forget this has anything to do with any type of engine. All its for is just quick and dirty ideal gas law calculations (hence the Kelvin). If the air temp is only going to vary by a few Kelvin or so then I can probably not even consider air heating through an intake. That's all I really needed to know.

 

If that is what your after, then you only need to know the combustion temperature in the cylinder, and the time it takes to refresh the mixture.

 

I'd say the intake air heating is pretty insignificant. There isn't enough time for a major amount of heat to transfer given the temperature gradient.

 

Specific Heat of AL: 1005 kJ/kg/C
Thermal Conductivity AL: 200 W/m/C
Density AL: 2700 kg/m^3


Specifig Heat of Air: 1005 J/kg/C
Thermal Conductivity Air: 0.026 W/m/C
Density Air: 1.217 kg/m^3

 

You only need the numbers when you have derived your equation to the point where they will be usefull. You have a system with mutiple variables and you cannot just find a plug and chug equation. It won't work that way. Well it will, but it will be wrong. You would need to derive it to show temp as a function of air speed, volume and time. We all know how much fun multi order Diff EQ's are.

EDIT: It would be much easier to plumb up a thermistor in the intake and take the average values for a certian RPM and time span.

 

Pretty much all I was looking for. It's what I expected but just wanted to confirm it. I'll just bump up the ambient temp by 1 or 2 K, shouldn't make all that much of a difference in the long run. Thanks for the help.

 

I can't say I agree with that statement, although I don't have anything quantified. Ever heard of an "Air Gap" manifold?

I like the idea of the thermistor. Better have one at the air inlet, too.

Having real-world data has a tendancy to do unpleasant things to theories.

 

You can say that agian. Another thermistor at the inlet is a must (forgot got to add that). You can really get awesome and accurate idea of what is going on in any dynamic system with a series of thermistors and thremocouples.

 

There is no real world data in this case as mentioned before. It's not corresponding to any existing engine so taking temperature readings is not an option.

 

The difference in power is immeasearable due to the very complex nature of the system being measured.

 

If you don't feel the need for coefficients of iron, I must assume you are using an aluminum block?

Any calculations you need will be a combination of combustion volume, temperature of combustion and refresh rate at given rpm's.

 

Correct, all aluminum. I think I have all the info I need.

 

Just wanted to mention, if you're using the ideal gas law:

PV = nRT

Then your calculation will be way off compared to the real conditions. The ideal gas law only works well for the lightest gases, hydrogen, helium, etc. For heavier gases (and an air fuel mixture would be a heavy gas), the ideal gas law is just too inaccurate.

However, if this is just an academic exercise, then it doesn't really matter.

(Though if you're doing this for your senior engineering project you'll want a better estimate, cause I don't think your prof. would be impressed.)

 

I've only mentioned air which is inert enough to be estimated this way. I know its tough since I posted on an automotive forum but try not to assme any other conditions of this system not already mentioned since its all just a bunch if theory and doesn't pertain to anything concrete yet. Just think of it as air, alumiunum and a heat source. I'm pretty sure Lonestar and Leigh have given me all the info I need anyway.

 

I did this on my old car.
I measured the air temp at the entrance of the cold air intake tube (down near front bumper), halfway down the tube and then just paf the maf sensor so about .2m from theinlet manifold (also plastic).
Didn't change from halfway down the tube to the maf sensor.

That was with plastic piping though and the temp sensor I had probably couldn't respond quikcly enough (looking in the eectronics catalogue the super fast repsonse ones were big $$$).

You need to be careful how you mount the temp sensor, it can get the treat conducted to it from its mounting hole.