Intake Manifold Temperature 1

[SBBlue]

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?

 

[ejit]

Pressure will always be below ATM, since there must be a pressure drop at the venturi for carb function, and only really serious FI motors have almost no pumping losses. Typical cruise vacuum: 15" Hg (1/2 ATM)?
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.

 

[MaxRaceSoftware]

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


Larry Meaux (maxracesoftware@yahoo.com)
Meaux Racing Heads - MaxRace Software
ET_Analyst for DragRacers
Support Israel - Genesis 12:3

 

[patprimmer]

I have provided a link to the National Advisory Committee for Aeronautics

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.

 

[Turbinator]

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.

 

[SBBlue]

The temperature stays the same?

Then why in the world do so many small airplanes crash because of carburetor icing?

 

[andyv8]

Having a quick look at some dyno data for a N/A engine, Airbox inlet temp=12°C, Inlet manifold air temp=15°C at full load with an 8°C cell temp.
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.

 

[sreid]

Would there not be a temperature drop due to adiabatic expansion of the air?

 

[MaxRaceSoftware]

many times dyno testing methanol carb engines
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

 

[GraviMan]

Dunno about various predictive methods, but for aircraft temp drop is normally given to be about 15'C. This is only really due to fuel vapourisation, and is why aero piston engines (carburetor) can ice between 0'C and 15'C.

The only real risk is in high moisture environments, like cloud. I don't know of any examples of cars suffering carburettor icing...

Mart

 

[patprimmer]

I had a car ice the carby one frosty morning, The heat riser on the carb was blocked, as I very rarely encountered morning frost conditions.

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.

 

[aviat]

The temperature of the manifold and temperature in carb throat are often different. It is the second one that causes carb ice.

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/

 

[SBBlue]

One thing to add. . . .

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.

 

[patprimmer]

If you heat the manifold with exhaust gas, the manifold quickly becomes hot enough to transfer some heat to the throttle body of the carby, so long as the source of heat is relatively close to the carby, and the throttle body is not insulated from it.

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.

 

[MaxRaceSoftware]

***My previous post *****
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

 

[SBBlue]

Larry. . . .

Thanks for the info.

One other question -- what was the intake manifold pressure at these conditions?

 

[MaxRaceSoftware]

Larry. . . .

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

 

[SBBlue]

Great work, Larry.

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.

 

[Chumley]

Guys, please, lets take a look in reality.

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

 

[SBBlue]

Regarding the continuing debate regarding throttle body and intake manifold temps --

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.