Temperature vs Boost

[Yellow Rose]

Ever wonder what the inlet air temperature is on a forced induction engine? From thermodynamics, below are various compressor discharge temperatures based on 100 °F ambient temperature.

6 PSI - 130 °F
8 PSI - 138 °F
10 PSI - 146 °F
12 PSI - 153 °F
14 PSI - 160 °F
16 PSI - 167 °F
18 PSI - 173 °F
20 PSI - 179 °F

Somewhere I read that the NSX's ECU objects to throttle body intake temperature greater than 150 °F. (I wonder if this is because of factory fuel map limitations or engine component (valve stem guides come to mind) temperature limitations.) Therefore, from the above theoretical temperatures an aftercooler is required on an NSX with more than 11 PSI of boost. As a data point, I am at 11.7 PSI at 7440 RPM and have measured approximately 30 °F of aftercooling on my setup on an 80 °F day. From another thread, a 12 PSI BBSCer reports maximum intake temperature on an 80 °F to be 99 °F. From the table above, that is a phenomenal 54 °F of aftercooling!

The above temperatures are theoretical. While not exact because the formula is a polynomial, you can approximate resulting temperatures by subtraction. For example if it was only 60 °F outside, the resulting temperature with 20 PSI of boost calculates to 133 °F. Remember, these are textbook numbers for heat of compression only. While some would argue that heat soak would also have an affect, I do not subscribe to that theory and below is why.

The piping on my BBSC is 3". From Paxton's website, the Novi 2000 has a rating of 1700 CFM. In the engineering world, this is meant to be SCFM. To calculate velocity, the flowrate has to be in ACFM. Converting yields 947 ACFM at internal pressure of 11.7 PSI. The cross-sectional area of a 3" is 7.065 IN^2. Crunching the numbers yields internal air velocity at redline of 326 FPS. To put this into perspective, that is 222 MPH! Considering the relative short plumbing of the compressor discharge piping, the air molecules are not in contact with the hot pipe long enough to conduct heat into the charged air. Last weekend I had the aftercooler off of my car. At idle with the BBSC just blowing, I was amazed of how much air was coming out. I had my wife rev the engine a few thousand RPM and it was an effort to hold my hand in place. And the noise! The airflow from the Paxton is much more than my gas-powered leaf blower, which is rated at 120 MPH. Simply put, the compressed air is flowing too fast to be additionally warmed by metal heat soak.

 

[MiamieNeSeX]

Good to have you back!


Armando

 

[WOODY]

Therefore, from the above theoretical temperatures an aftercooler is required on an NSX with more than 11 PSI of boost

do you feel the running 11 lbs without an aftercooler is safe? i was wondering ..... if I tune my car with 11lbs while cranking the heat in the tuning area to 90 degrees farenheit (about as hot as it gets in MI), tuning conservatively and getting no detonation with the final numbers. wouldnt I be safe? Are there instances of cars tuned to a perfect AFR at the dyno that later produce detonation ? It seems to me with consideration to max ambient temps a safe tuning program would always be safe IE: are there other factors that would produce detonation other than increased outdoor temp? (no elevation change to speak of in MI)
woody

 

[seandouble]

Andy,

I am new to the Prime, and even though I read the hell out of the forums, I don¡¯t normally reply to posts. I thought I might start to throw my opinion in, because if no one replied I would not be able to learn from the wise NSXers.

I agree with some of your statements, but I disagree (somewhat) with statement that air temp is not affected by the plumbing. The pipe is affected by the compressed air. The pipe doesn¡¯t increase the compressed air temp, the compressed air increases the pipe temp.

The heat is generated from the compressor. As you stated, the more compression (higher boost), the more temp, and yes it is definitely not linear. I thought I have heard that the air in the blower (turbo in my case) is around 1000¢ªF. So obviously, when you take temp readings (IAT) further downstream, like before it enters the intake manifold (~150¢ªF), it will be much lower than exiting the compressor (1000¢ªF).

As the air exits the compressor, it starts to reduce heat (no such thing as cool, only negative heat, therefore the Kelvin unit). The pipe is acting like very small heat sink. It is dependent on material selection (stainless steel has decent thermal conductivity) and wall thickness (thinner wall = lower thermal capacitance).

I am curious what formula you are using to calculate velocity. I don¡¯t have any of my text books here at work. Regardless of the value of the velocity, as air flows down a pipe, its max velocity is at the center (basically, assuming the pipe has a large strait section). The velocity is basically 0 at the wall, no slip condition (best example I can think of is why small particles of dust don¡¯t blow off the car, not even at 180MPH); therefore, the air has plenty of time to transfer heat to the pipe (conduction), coupled with turbulent air flow (Reynolds number > 2400, now I am straining my brain), which ¡°mixes up¡± the air to produce a more consistent air temp throughout the section. This raises the pipe temp (until steady state). Then it will transfer heat to the air surrounding the pipe (assuming the air is less than the pipe/IAT). There could be a condition where exhaust manifold or engine heat IS causing the heat to transfer into the compressed air.

So in summary, the pipe is not causing the compressed air temp to increase, just the opposite, the pipe is causing the compressed air temp to decrease, assuming the surrounding air is less than the air inside the pipe.

I look forward to learning/contributing/communicating to all of you fellow NSX enthusiasts on the prime, and meeting some of you someday.

 

[Yellow Rose]

do you feel the running 11 lbs without an aftercooler is safe? i was wondering ..... if I tune my car with 11lbs while cranking the heat in the tuning area to 90 degrees Fahrenheit (about as hot as it gets in MI), tuning conservatively and getting no detonation with the final numbers. wouldn’t I be safe?

Two issues here.....keep in mind that my calculations are theoretical, so if somebody is considering 11 PSI with or without an aftercooler at 100 °F ambient temperature, they (in my opinion) are on the ragged edge. Also, if you tune in hot ambient temperature then experience cold ambient temperature, the air-fuel ratio will go lean by approximately 0.5:1 if you have an SS box.

Are there instances of cars tuned to a perfect AFR at the dyno that later produce detonation? It seems to me with consideration to max ambient temps a safe tuning program would always be safe IE: are there other factors that would produce detonation other than increased outdoor temp?

As I understand it, pre-detonation is more a function of too much timing advance vs too lean of an air-fuel ratio. (If there are others out there that have information to the contrary, please chime in.) Too much timing advance (also referred to as not enough timing retard) will mechanically damage piston ring lands due to the explosive nature of incorrect spark ignition relative to piston location. On the other hand, on the other hand, too lean of an air-fuel ratio will result in thermal damage that melts holes in the piston tops.

Two different situations that result in the same condition, which is an engine rebuild.

 

[Yellow Rose]

I thought I might start to throw my opinion in, because if no one replied I would not be able to learn from the wise NSXers.

I'm not wise, just an opinionated wise***.

The pipe is affected by the compressed air. The pipe doesn't increase the compressed air temp, the compressed air increases the pipe temp.

Correct.

I thought I have heard that the air in the blower (turbo in my case) is around 1000¢ªF. So obviously, when you take temp readings (IAT) further downstream, like before it enters the intake manifold (~150¢ªF), it will be much lower than exiting the compressor (1000¢ªF).

Your fonts seem to having been drinking a bit. Do you mean to say that air inside the compressor is 1000 °F instead of perhaps 100 °F ? Respectfully, there is no way a compressor can generate that much heat for two reasons - the rubber boots associated with the metallic plumbing will melt and lube oil (synthetic or organic) will thermally degrade at approximately 250-300 °F. You need to check you font nazi into Betty Ford.

I am curious what formula you are using to calculate velocity.

I converted standard cubic feet to actual cubic feet, then divided by the cross-sectional area of the pipe.

The velocity is basically 0 at the wall, no slip condition (best example I can think of is why small particles of dust don¡¯t blow off the car, not even at 180MPH); therefore, the air has plenty of time to transfer heat to the pipe (conduction), coupled with turbulent air flow (Reynolds number > 2400, now I am straining my brain), which ¡°mixes up¡± the air to produce a more consistent air temp throughout the section. This raises the pipe temp (until steady state). Then it will transfer heat to the air surrounding the pipe (assuming the air is less than the pipe/IAT). There could be a condition where exhaust manifold or engine heat IS causing the heat to transfer into the compressed air.

I assumed a linear velocity profile. Your counterpoints ring true. However, keep in mind the internal air velocity of over 200 MPH and (in my setup) no more than 18" of linear piping, I don't see how significant heat transfer can occur.

 

[seandouble]

Do you mean to say that air inside the compressor is 1000 °F instead of perhaps 100 °F ? Respectfully, there is no way a compressor can generate that much heat for two reasons - the rubber boots associated with the metallic plumbing will melt and lube oil (synthetic or organic) will thermally degrade at approximately 250-300 °F.


Yes, I was trying to type 1000°F.

I reread your original post. I apologize, I though you were reffering to a turbo. I have a turbo, so I have exhaust temp to contend with, and if you mount a rubber boot to the exhaust side of the turbo, yes, it will melt (to say the least). If you place a Fluke on the exhaust side of the turbo at high boost and WOT, it will be close to 1000°F. The reason the EGT is not 1000°F is because the air coming through the air filter into the turbo can't absorb all the energy from the metal housing as it QUICKLY travels through the compressor. Also keep in mind, it starts out at ambient (~85°F) and is almost instantaneously brought up to ~300°F (again using rough numbers). And this is the reason for an intercooler.

However, keep in mind the internal air velocity of over 200 MPH and (in my setup) no more than 18" of linear piping, I don't see how significant heat transfer can occur.

Yes, you are correct. I guess I was tying to point out that the pipe is not heat soaked, it is absorbing heat from the compressed air. The pipe has a lower temp than the compressed air.

There is little heat transfer because of the velocity and more importantly, the contact area (or the lack thereof in the case of the 18" pipe) of the pipe doesn't get much of a chance to absorb the heat from the compressed air. Surface area is the key, this is how the intercooler works. And, the air velocity is very similar through the intercooler, it is the area that is the contributing factor.

FYI, if you have a 200mph velocity, that is the velocity at the center of the pipe. It is much slower at the wall surface (no slip condition, theoretically 0, depending on laminar or turbulent flow) and this is where you get more heat transfer for this reason.

Thanks for the discussion.

Rick

 

[Marshall1]

Ever wonder what the inlet air temperature is on a forced induction engine? From thermodynamics, below are various compressor discharge temperatures based on 100 °F ambient temperature.

6 PSI - 130 °F
8 PSI - 138 °F
10 PSI - 146 °F
12 PSI - 153 °F
14 PSI - 160 °F
16 PSI - 167 °F
18 PSI - 173 °F
20 PSI - 179 °F

Something is very wrong with those numbers. Assuming 72% compressor efficiency (about right for the Novi 2000 and most decent compressors)

6 PSI - 185 F
8 PSI - 206 F
10 PSI - 228 F
12 PSI - 248 F
14 PSI - 266 F
16 PSI - 284 F
18 PSI - 302 F
20 PSI - 318 F

A 54F drop from inter or aftercooling is nothing. After 300+F outlet temps on my Supra, I see ~40F temps post methanol injection. A good IC should knock you within 20-40F of ambient under full boost - so that aftercooler you mentioned was actually reducing temp by about 150F -pretty normal.

 

[WOODY]

Also, if you tune in hot ambient temperature then experience cold ambient temperature, the air-fuel ratio will go lean by approximately 0.5:1 if you have an SS box.

doesn't the stock ecu adjust for changes in ambient?

 

[Yellow Rose]

Something is very wrong with those numbers.

I used the textbook formula of PV=nRT, which theoretical numbers will be lower than real world. But at 20 PSI, my temperature is 179 °F vs your temperature of 318 °F. How did you calculate your numbers? If you didn't calculate the discharge temperature, how did you measure it?

 

[Yellow Rose]

doesn't the stock ecu adjust for changes in ambient?

Yes, assuming that the engine is naturally aspirated.

With the Split Second controller on a BBSC, the ECU still controls fuel while you are just cruising around town, with the engine pulling a vacuum. However, when the engine is under boost the factory ECU is now being told by the SS how to manage fuel. An air-fuel ratio delta of only 0.5:1 is not that significant, assuming your trended air-fuel ratio is less than 12.5:1. Although there are some CTSCs at 13:1, in my opinion this is a bit too lean for my comfort.

 

[WOODY]

But at 20 PSI, my temperature is 179 °F vs your temperature of 318 °F.

I have spoken with Devin at Payne about this in the past and it was my understanding that at 11 lbs with the novi 2000 my car would be running IAT in excess of 200 F. if my intake temp is only 150 degrees at 11 lbs on a 100 F day, why bother with aftercooler?

 

[Marshall1]

Something is very wrong with those numbers.

I used the textbook formula of PV=nRT, which theoretical numbers will be lower than real world. But at 20 PSI, my temperature is 179 °F vs your temperature of 318 °F. How did you calculate your numbers? If you didn't calculate the discharge temperature, how did you measure it?

Thats a tricky equation to use bc of the volume change. Use T2/T1=((P2/P1)^((gamma-1)/(gamma)))/comp efficiency

http://www.grc.nasa.gov/WWW/K-12/airplane/compexp.html

These will give you real world numbers. Heres a quick calculator if you don't feel like plugging numbers http://not2fast.wryday.com/turbo/glossary/turbo_calc.shtml

You can play with ICs, water/meth/alky injection, ect.

 

[Yellow Rose]

LISTEN UP EVERYBODY

My numbers are conservatively (artificially) low. Using Marshall's numbesr emphasizes even more, that aftercooling of forced induction is needed.

 

[Marshall1]

Water/alky/meth injection can also give you the charge air cooling benefit of the aftercooler as well as absorbing heat in the combustion chamber -all for much cheaper ($300-$400) ...something to look into.

 

[Yellow Rose]

You Are Onto Something

Indeed you can get quite a bit of "bang for your cooling buck" using liquid spray injection. A fellow BBSC'ed NSX'er running 10 PSI of boost has recorded with his AEM, intake air temperatures of approximately 225 °F, so your numbers are more real-world than mine. Good info.

 

[WOODY]

If you are going to go the refillable cooling spray injection route , why not just use NOS from the start and save huge $$$$

 

[Marshall1]

If you are going to go the refillable cooling spray injection route , why not just use NOS from the start and save huge $$$$

Nitrous Oxide is an entirely different animal. It increases oxygen and therefore pressure in the combustion chamber and leads to greater instability, higher EGTs, no margin of error on pump gas, ect. Water absorbs a tremendous amount of heat in the combustion process giving greater stability, lower EGTs, and a much better margin on pump gas. I've seen several cars run 25-26 psi on 93 octane with absolutely no knock and full timing. I've also seen intercoolers act as interHEATERS when methanol (meth will cool the air, water will cool the chamber) is injected upstream.