Intake Manifold Temperature
Intake Manifold Temperature
(OP)
Here's a little bit of a strange question.
What is the typical temperature in the intake manifold for a naturally aspirated spark ignition engine?
Here's why I was wondering. . . .
The intake manifold is, of course, below atmospheric pressure for a NA engine except for full throttle conditions. Since the engine has to perform work on the air to get it below atmospheric pressure, the temperature of the air will drop.
According to my calculations, dropping the pressure of a container of air from one atmosphere of pressure to one-fourth atmosphere will drop the temperature from 80 deg F (assumed ambient) to something like 80 degrees below zero.
This phenomena does, of course, account for icing that is frequently encountered with piston airplane engines.
The intake manifold air will be heated by the surrounding engine, but to how great an extent?
Since I started thinking about this I haven't had a chance to put a temperature probe in an intake manifold.
Has anybody ever done that?
What is the typical temperature in the intake manifold for a naturally aspirated spark ignition engine?
Here's why I was wondering. . . .
The intake manifold is, of course, below atmospheric pressure for a NA engine except for full throttle conditions. Since the engine has to perform work on the air to get it below atmospheric pressure, the temperature of the air will drop.
According to my calculations, dropping the pressure of a container of air from one atmosphere of pressure to one-fourth atmosphere will drop the temperature from 80 deg F (assumed ambient) to something like 80 degrees below zero.
This phenomena does, of course, account for icing that is frequently encountered with piston airplane engines.
The intake manifold air will be heated by the surrounding engine, but to how great an extent?
Since I started thinking about this I haven't had a chance to put a temperature probe in an intake manifold.
Has anybody ever done that?





RE: Intake Manifold Temperature
The manifold casting temp is typically between 150F and water temp (180-195F), but varies greatly due to exhaust cross-over, gasket type, surface area common to head casting, water jacket, etc.
The internal air temp is also reduced by vaporization, so richer mixture cools it, larger emulsion droplets from the venturi boil off, etc.
Too many variables to predict.
RE: Intake Manifold Temperature
i 1st thought it might be around 60 to 80 F
but according to onboard data logger it was 120 to 140 F
on that particular car with weather conditions with gasoline
Larry Meaux (maxracesoftware@yahoo.com)
Meaux Racing Heads - MaxRace Software
ET_Analyst for DragRacers
Support Israel - Genesis 12:3
RE: Intake Manifold Temperature
http://naca.larc.nasa.gov/
If you search this site, I am sure you will find some extensive research data. Most of the work was done during the initial development of piston engines for aircraft, and a lot was done to gain performance for WW11 aircraft
Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
RE: Intake Manifold Temperature
I think you assumed the expansion across the throttle is isentropic, which is far from the case. A turbine, with high isentropic efficiency will take energy (heat) out of a fluid stream and turn it into shaft work, leaving the fluid cooler. A throttle does not extract energy from the air, it simply lowers the pressure, all the heat/energy remains and the temperature stays the same. Of course the air is heated by the manifold surfaces as well.
RE: Intake Manifold Temperature
Then why in the world do so many small airplanes crash because of carburetor icing?
RE: Intake Manifold Temperature
This is with an aluminium inlet manifold seperated from the cylinder heads by a 4mm rubber gasket.
This is of course for a MPI engine which doesn't see any cooling in the inlet manifold due to fuel evaporation.
A.
RE: Intake Manifold Temperature
RE: Intake Manifold Temperature
in just 4 to 5 second acceleration tests,
the manifold around carb area gets around 40 deg F
i'm dyno testing a small block chevy engine saturday
on gasoline ,and will put a probe inside the plenum
and post results :)
Larry Meaux (maxracesoftware@yahoo.com)
Meaux Racing Heads - MaxRace Software
ET_Analyst for DragRacers
Support Israel - Genesis 12:3
RE: Intake Manifold Temperature
The only real risk is in high moisture environments, like cloud. I don't know of any examples of cars suffering carburettor icing...
Mart
RE: Intake Manifold Temperature
At full throttle, the pressure of the air is only slightly dropped, so little cooling should occur. The real cooling and icing should only occur at cruise throttle settings, so dyno full power runs will not indicate the real situation.
The fuel evaporation does complicate the situation, as not all fuel will be evaporated in the manifold.
Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
RE: Intake Manifold Temperature
The reason carb ice forms is that warm air can hold more moisture than cooler air and when there is a temperature drop across the venturi the excess moisture comes out. If the temperature at the throttle is above zero, the water vapor and water goes through the engine, if not it freezes to the throttle plate etc. and the engine quits. At temps below -10F the air does not hold enough moisture to cause icing. The way carb ice is prevented is by the use of heated air (carb heat) which on small A/C is a manual system and controlled by pilot. It is when they don't understand when icing can occur that causes problems. The temp drop is more with a fuel air mixture than with air only. There are other types of icing but fuel injection are not effected by carb ice.
Intake manifolds are often heated after the carb because air intake temps influences the uniformity of air fuel ratios and its evenness between cylinders and degree of atomization. Often an air intake temp control is use on auto engines to speed up heating of the manifold.
http://level2.cap.gov/documents/u_120903081720.pdf
http://www.fishcreekflyingclub.com/safety/carbice/
RE: Intake Manifold Temperature
Carb icing can occur when the ambient temperature is above freezing -- in fact, I think its been reported at temperatures as high as 60 deg F.
The comment about there not being much of a temperature drop at full throttle is correct, since there is little air resistance and therefore pressure drop from the throttle body. I would think the pressure/temperature drop would be greatest at idle.
RE: Intake Manifold Temperature
Hot intake air, ducted from around the exhaust manifold, is of course another cleaner and more reliable method, but not quite as quick to respond, but certainly quick enough.
The air temp in the manifold will change considerably from point to point in the induction system, and with changes in throttle and A/F ratio and fuel discharge point locations.
Whether to insulate, heat, or cool the induction system is an interesting subject on it's own, especially with turbo charged inter-cooled cars. Is the air duct above or below ambient at a particular point, so will insulating hurt or help?
Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
RE: Intake Manifold Temperature
on a Chrysler small block Super Stock in plenum temps range from 120 to 140 F down DragStrip with no HoodScoop
i 1st thought it might be around 60 to 80 F
but according to onboard data logger it was 120 to 140 F
on that particular car with weather conditions with gasoline
==========================================================
the Racer that gave me the feedback on Intake temps, must have meant that the probe was glued or attached "underneath the plenum" and not inside the plenum like i thought he meant ?
---------------------------------------------------------
just finished Dyno testing small block Chevy engine
and here are results with temperature probe "inside" the plenum of the Intake Manifold and with Inlet Air Temp in Dyno Room
RPM CAT Plenum
5500 94 77
5600 94 77
5700 94 76
5800 94 76
5900 94 76
6000 94 76
6100 94 76
6200 94 75
6300 94 75
6400 94 75
6500 94 75
6600 94 75
6700 94 74
6800 94 74
6900 94 74
7000 94 74
7100 94 74
7200 94 74
7300 94 74
7400 94 74
7500 94 73
7600 94 73
CAT = Carb Air Temperature in deg F ( 1 1/2 Feet above carb)
with VP HydroCarbons C-16 race gas
Larry Meaux (maxracesoftware@yahoo.com)
Meaux Racing Heads - MaxRace Software
ET_Analyst for DragRacers
Support Israel - Genesis 12:3
RE: Intake Manifold Temperature
Thanks for the info.
One other question -- what was the intake manifold pressure at these conditions?
RE: Intake Manifold Temperature
Thanks for the info.
One other question -- what was the intake manifold pressure at these conditions?
---------------------------------------------------------
SBBlue , yes i should have measured the vac readings inside the plenum along with the temperature ,
but didn't this time ! i usually always measure plenum vacuum readings, but this time had the temp probe screwed into that hole in the carb spacer
i'm going to modify all the carb spacers in my shop
for both temp and vac for next series of dyno tests.
will post results.
Larry Meaux (maxracesoftware@yahoo.com)
Meaux Racing Heads - MaxRace Software
ET_Analyst for DragRacers
Support Israel - Genesis 12:3
RE: Intake Manifold Temperature
One other slight request;
Could you check the temperature/manifold pressure for perhaps one-quarter and one-half load/throttle?
I guessing from the rpms that it was pretty much "pedal to the medal" time.
RE: Intake Manifold Temperature
turbinator writes to:
SBBlue:
I think you assumed the expansion across the throttle is isentropic, which is far from the case. A turbine, with high isentropic efficiency will take energy (heat) out of a fluid stream and turn it into shaft work, leaving the fluid cooler. A throttle does not extract energy from the air, it simply lowers the pressure, all the heat/energy remains and the temperature stays the same. Of course the air is heated by the manifold surfaces as well.
Isentropic: taking place without a change in entropy
"high isentropic efficeincy" What the hell does that mean? Something takes place without a change in entropy, or something changes entropy but does it with minimal energy input? May be something else? Please explain!
(I'm still looking for any evidence of a turbine at any throttle body. SBB blue never said a throttle body takes heat takes heat out of the intake, but in the exhaust a turbine for sure takes heat out, at the expense of adding back pressure. No free lunch)
For 75 years engineers design systems that add heat to a naturally aspriated spark ignited engines intake air. (Are we just stupid or what?)
Look at turbo charged engines. Many eliminate the intake air heat function. Why? (Here's the big clue: We're just stupid.) No, the additional intake turbulence heats the air too! Because of that, it could be a problem both without and under boost so we add an . . . intercooler to reduce the intake air temperatures. Form follows function.
Chumley
RE: Intake Manifold Temperature
I wasn't so much thinking of a throttle body removing heat from air as the fact that when you pump air out of an enclosed volume, you cause a drop in the temperature of the remaining air.
If you don't believe me ---- Take a small air tank and connect it to the intake system of your air compressor so the air compressor is pumping the air out of your axillary tank. Don't forget to attach a vacuum gage to the tank.
You will note two things; 1) It doesn't take all that long for the air compressor to produce a fairly good vacuum, and 2) the vacuum tank will get noticably cold.
So in answer to the query about the throttle body taking away heat -- I was thinking more along the lines of the process of pumping air out of the intake manifold causing a drop in the temperature of the air.
RE: Intake Manifold Temperature
I am interested in comments from those who can connect thermodynamics to practical application. Temperature across a throttle is usually considered constant. Therefore the cooling observed must be due to fuel evaporation. Correct or no?
RE: Intake Manifold Temperature
http://bednorzmuller87.phys.cmu.edu/demonstrations/thermodynamics/manyparticlesystems/demo524.html
If a gass is compressed isothermally, there will be a temperature increase. If a gas is expanded isothermally there will be a temperature decrease (this is not always true-see the Joule-Thompson coefficient from a previous post but it is true for air at standard temperature and pressure).
Having said that, the gas temperature decrease is probably much greater due to the vaporization of the gasoline and there is most certainly a temperature rise in the intake manifold due to the temperature of the intake manifold.
RE: Intake Manifold Temperature
"Having said that, the gas temperature decrease is probably much greater due to the vaporization of the gasoline and there is most certainly a temperature rise in the intake manifold due to the temperature of the intake manifold."
=============================
Actually, we can do some calculations relatively easy to determine the temperature drop due to vaporization of fuel.
First, an assumption; all of the fuel will be assumed to vaporize completely in the intake manifold. That, of course, is not correct, but it will give us the "worse case" scenario.
Second: Heat of vaporization for gas. Actually, I'm not sure -- but I do know the heat of vaporization for Butane (~140-170 BTU/lb, depending upon temperature), Isobutane (same), propane (about the same), and turpetine (again, about the same). I would suspect Octane (the gasoline equivalent) would be about the same.
Third, we will assume stoichometric combustion, so the air/fuel ratio will be about 14.5:1.
So what do we get? One pound of hydrocarbon will take, say 150 BTUs of heat for vaporization. This 150 BTUs will be supplied by 14.5 lbs of air, which means that each lb of air will lose about 10.3 BTUs of heat. Air at a temperature of 80 deg F will drop to about 36 deg F.
Of course, you must remember that not all of the gasoline will be vaporized in the intake manifold. I'm not completely sure, but I suspect that the bulk of it would be vaporized in the cylinder.
RE: Intake Manifold Temperature
Let's get some context here:
My post was in reply to SBBlue's opening post:
"According to my calculations, dropping the pressure of a container of air from one atmosphere of pressure to one-fourth atmosphere will drop the temperature from 80 deg F (assumed ambient) to something like 80 degrees below zero."
I believe the formula he used in this calculation was that for isentropic expansion (Efficiency=100%)
T2=T1*(P2/P1)^0.286
(SBBlue, please correct me if I'm
wrong). That would explain his calculated temperature drop of 160 deg. F or so.
As I said earlier, the formula does not work for a throttling proccess, which simply lowers the pressure without taking energy (heat) out of the air stream. Here's the extended version of the formula:
T2=T1*[1-efficiency*(1-[p2/p1]^0.286)].
If the isentropic efficiency is 100%, you get the simpler formula. 100% isentropic efficiency means the maximum amount of energy is being extracted from the fluid stream. For a throttling process, no energy is being extracted. Therefore, the isentropic efficiency is 0%. Plug that into the formula, and you get T2=T1. So, !!as far as the expansion process alone!!, their is no temperature difference between the inlet and outlet of a throttle. Now, as explained by other members their which does cause the temperature to lower.
RE: Intake Manifold Temperature
Now, as explained by other members there is evaportaion which does cause the temperature to lower.
RE: Intake Manifold Temperature
For a steady flowing gas, if there is a temperature change, energy has either been added to it or removed. For relatively low velocity change between points 1 and 2:
Change in Energy=Cp x (T2-T1).
where Cp is Specific heat at constant pressure, assumed constant for small temperature differences.
Energy is either heat being transfered or shaft work, neither which occurs to a significant amount across a throttle. So, if there is no change in the energy, their is no change in the temperature.
Again, this is related only to the throttling process and not to evaporative cooling effects of the fuel or moisture in the air.
RE: Intake Manifold Temperature
Rick
RE: Intake Manifold Temperature
http://www.public.asu.edu/~dmatyus/teaching/chm341/lecture_notes/chapter3/chap3.pdf
One can also search the Web for Isenthalpic and Throttle and find similar results.
RE: Intake Manifold Temperature
If you have a sealed container -- no throttle at all -- and pump half of the gas out, you will see a significant pressure drop in the gas that remains.
The pumping process is what is causing the temperature drop -- not the throttling process. And we can regard the pumping process as isoentropic for calculation purposes.
RE: Intake Manifold Temperature
I think the reason it didn't work is that the air needs to be cooled before it gets to the carburetor (or today, FI). Cold air definately makes more HP, you correct dyno HP for air temperature and pressure. Cooling the air would also precipitate out water vapor which also hurts HP.
A side note. The F1 regs have three separate rules against cooling the gasoline.
RE: Intake Manifold Temperature
When the piston starts its downstroke, and lowers the pressure, that process of lowering the pressure may lower the temperature, but once the valve opening throttles the air flow (which it may not do). But that is totally not a steady state process. A throttle is very close to steady state (except during transients, i.e. angle changing).
So, when the 70 degree air sees a throttle and a lower pressure, it doesn't go in and get colder, it just looses energy across the throttle, like a hole in a balloon. On one side of the throttle is 1 bar 70 degree air, on the other, there is .5 bar 70 degree air. That pressure differential keeps the air flowing, the pistons are filled with that pressure and temperature of air.
If this were a single cylinder engine, with the manifold volume at 1bar 70 F momentarily, and then the piston is dropping the pressure, the temperature may drop momentarily, but the inflow of 70 F air is still fighting it.
But, since the situation is a manifold with several exits, creating a constant flow (whether it's pistons or a fan sucking, same difference to the manifold), and this manifold is staying the same size, and there is a leak (throttle) into it, and equal flow out of it, it's just an intermediate area, the temperature stays the same. It's fluid flow, not thermodynamic expansion.
If it were a container of air, with no inlet, and either a piston pulling down, or some pressure drop through some hole, then its contents would cool, but engines can't cool ambient air, since it's not in a container of constant volume. If it were the piston pulled down, and the container volume were still the same (and insulated), and there was the original pressure on the opposite side of the piston, then the work done on that piston could have been recouped, as it is sucked back in. In that case, it would have cooled from the expansion, and warmed back to original during the contraction (assuming 100% efficiency).
blah blah blah, yada yada yada.
I'm just trying to paint the right pictures in your heads here.
Vaporization (of fuel) on the other hand, that'll suck the heat outta the air and manifold quickly, especially at reduced pressures (such as in a manifold).
A dry manifold, such as a MPFI manifold, will not drop the air temperature below ambient.
I'm tired so I'll stop now, but the REASON for the temperature change (if any) is not the thermodynamics of a throttle, manifold, and dry air.
RE: Intake Manifold Temperature
It may come as a surprise to some of the previous posters to know that in some cases temperature of dry gas actually will rise across a throttle, depending on the sign of the Joule-Thompson coefficient!
RE: Intake Manifold Temperature
It is true for an ideal gas, but nitrogen and oxygen aren't ideal gases.
If what TJ Schmidt said was true for nitrogen, then liquid nitrogen and oxygen couldn't be produced. It is produced by pressuring gas, cooling the air down, and then letting it cool itself as it expands. This cycle is repeated several times until the temperature gets several hundred degrees below zero and and the remaining air liquifies.
Another example where it isn't true can be seen with explosive depressurization that can be seen with high flying aircraft. Anybody who's ever taken an altitude chamber "ride" is very familiar with this. What happens? There is immediate condensation of water vapor in the air, creating a cloud. The temperature drops so much that the vapor causes frost on the inside of the windows.
This effect can also be seen if you look at air just before it enters a gas turbine at full power. In some instances a water vapor cloud is produced. What is the mechanism for that other than the fact that the air temperature suddenly decreases as the air pressure in front of the turbine decreases from air being sucked into the turbine.
RE: Intake Manifold Temperature
Z is a function of:
Reduced Pressure, P/P-critical, where for P-critical is 547 psia for air
and Reduced Temperature, T/T-critical, where T-critical is 238.5 R for air.
Looking at the chart, the compressibility factor Z gets farther from 1 as P-reduced increases and as T-reduced decreases.
So, using an extreme example:
Say the air before the throttle has been compressed to 65 psia, giving a Reduced Pressure of 0.12, and the Temperature is -100 F (360 R) giving a Reduced Temperature of 1.5.
According to the chart in front of me, the Compressibility factor would be Z=0.99, which is about the largest deviation from the ideal gas law (Z=1) as would be encountered at the throttle of a common air breathing engine.
RE: Intake Manifold Temperature
Isn't some of the kinetic energy being taken away to push the gas into a bigger volume?
Wouldn't the expansion be similar to warm air rising? Cooling due to expansion is often cited as the reason for clouds forming.
RE: Intake Manifold Temperature
"The investigations of Joule and Lord Kelvin showed that a gas drops in temperature when throttled. This is not universally true. For some gases, notably hydrogen, the temperature rises for throttling processes over ordinary ranges of temperature and pressure. ..... The ratio of observed drop in temperature to the drop in pressure, i.e. dT/dp, is the Joule-Thomson coefficient."
RE: Intake Manifold Temperature
I don't have time or energy to research this, but if anyone else does, please post it here.
RE: Intake Manifold Temperature
EXAIR, who make all kinds of air ejectors, does not seem to claim any cooling benefit, aside from the air flow itself.
http://www.newton.dep.anl.gov/askasci/eng99/eng99082.htm
http://www.mindspring.com/~divegeek/lift.htm
RE: Intake Manifold Temperature
Chevron 92 premium pump gas
Part Throttle, very light Dyno load
Engine----CAT----Vacuum----Manifold
RPM-------degF----in Hg---- deg. F.
1435-------105----16.1--------111
1754-------106----16.3--------111
2064-------105----17.6--------110
2305-------104----17.3--------108
2632-------104----15.8--------107
3141-------104----13.7--------104
3712-------105----12.3-------- 99
Full Load at 600 Rpm/Sec Acceleration Rate SF-901 Dyno
Engine----CAT----Vacuum----Manifold
RPM-------degF----in Hg---- deg. F.
3200------102------0.0------95
3300------102------0.0------95
3400------102------0.0------95
3500------103------0.0------95
3600------103------0.0------94
3700------103------0.0------94
3800------103------0.0------94
3900------103------0.0------94
4000------103------0.0------94
4100------103------0.0------94
4200------103------0.0------94
4300------103------0.0------94
4400------103------0.5------94
4500------103------0.5------94
4600------103------0.5------93
4700------103------0.0------93
4800------103------0.5------93
4900------103------0.5------93
5000------103------0.5------93
5100------103------0.6------93
5200------103------0.6------93
5300------103------0.7------92
5400------102------0.7------92
5500------102------0.7------91
5600------102------0.8------91
5700------102------0.8------91
5800------102------0.8------91
5900------102------0.7------91
6000------103------0.9------90
6100------103------0.8------90
6200------104------0.8------90
6300------104------0.9------90
6400------104------0.9------91
6500------104------0.9------90
Larry Meaux (maxracesoftware@yahoo.com)
Meaux Racing Heads - MaxRace Software
ET_Analyst for DragRacers
Support Israel - Genesis 12:3
RE: Intake Manifold Temperature
Is that with a carburetted engine?
The fuel vaporization will pull heat energy from the air if it is, explaining the temperature drop for the most part. I still think the argument is whether the throttle itself is causing the temp drop which I'm pretty sure it isn't.
RE: Intake Manifold Temperature
It looks to me that there is minimal temperature drop across the throttle. I suspect the slight increase in temperature seen at idle is secondary to heat from the manifold being transferred to the air in the manifold.
The temperature drop at high loads might be explained by the Joules-Thompson effect. I seem to recall some calculations I did several weeks ago suggested that there would be about a 10 deg F drop from that effect.
RE: Intake Manifold Temperature
Larry Meaux (maxracesoftware@yahoo.com)
Meaux Racing Heads - MaxRace Software
ET_Analyst for DragRacers
Support Israel - Genesis 12:3