alternate carb heat

[Ray Toews]

Has anyone ever tried using cowling air under the engine as heated air
for carb heat.
Because of tight cowling space I am thinking of building a door in the
lower cowl for fresh ram air and then closing it and drawing air from
under the engine, cooling air coming off the cylinders which seems to
run about 120 degrees as my heated air in case of carb ice.

I have measured the under engine air on two airplanes now and it seems
to run around 120 deg. so this must be fairly typical and would seem
to be more than adequite to get rid of carb ice. I have never had the
opportunity to measure heated air coming off exhaust manifold.

I would build it so fresh ram air is unfiltered and alternate, heated,
air would be filtered. in this way I would always taxi using warm
filtered air but in normal flight use cold ram air.

Makes sense to me, but then so does capitalism.

Ray

[Dave Hyde]

Ray Toews wrote...
> Has anyone ever tried using cowling air under the engine as heated air
> for carb heat.

Serveral RV-builders including me have tried something similar.
Van's sells a mini heat muff that doesn't do mucgh but serve as
a mount for scat tube, so in essence we're drawing undercowl
air into the airbox when carb heat is selected. Sam Buchanan
has a picture on his website:

http://home.hiwaay.net/~sbuc/journal/engine4.html

about 1/2 way down the page. I did the same thing on an
RV-4 and I get a barely perceptible RPM rise when I select
carb heat, which tells me it's not helping much.

Your call.

Dave 'hot air specialist' Hyde

[Kyle Boatright]

"Dave Hyde" > wrote in message
...
> Ray Toews wrote...
>> Has anyone ever tried using cowling air under the engine as heated air
>> for carb heat.
>
> Serveral RV-builders including me have tried something similar.
> Van's sells a mini heat muff that doesn't do mucgh but serve as
> a mount for scat tube, so in essence we're drawing undercowl
> air into the airbox when carb heat is selected. Sam Buchanan
> has a picture on his website:
>
> http://home.hiwaay.net/~sbuc/journal/engine4.html
>
> about 1/2 way down the page. I did the same thing on an
> RV-4 and I get a barely perceptible RPM rise when I select
> carb heat, which tells me it's not helping much.
>
> Your call.
>
> Dave 'hot air specialist' Hyde
>

You get an RPM increase??? Sumpthin' is way, way wrong with that... ;-)

With the exact same setup, I get a barely perceptible RPM decrease...

KB

[Dave Hyde]

Kyle Boatright wrote...

> You get an RPM increase??? Sumpthin' is way, way wrong with that... ;-)

Argh. Something's wrong alright, but not
with the airplane.

Dave 'fatfinger' Hyde

[UltraJohn]

Sorry Dave
In this case I don't think it's the fingers that are 'fat'. ;-)
John


Dave Hyde wrote:

> Kyle Boatright wrote...
>
>> You get an RPM increase??? Sumpthin' is way, way wrong with that... ;-)
>
> Argh. Something's wrong alright, but not
> with the airplane.
>
> Dave 'fatfinger' Hyde
>

[Jerry J. Wass]

I know this is a homebuilt group, but for naturally aspirated engines-CFR
23.1093
says---" each engine shall have a preater to raise the inlet temp. 90 deg
F, from an
ambient of 30deg F w/ no visible moisture present.---AND-at only 75%
power. This is not remotely possible with under cowl air temps. The
pucker factor gets pretty high when your altitude is 600 ft over a forest,
or lake, and the engine starts choking down.
..Jerry

Ray, Toews wrote:

> Has anyone ever tried using cowling air under the engine as heated air
> for carb heat.
> Because of tight cowling space I am thinking of building a door in the
> lower cowl for fresh ram air and then closing it and drawing air from
> under the engine, cooling air coming off the cylinders which seems to
> run about 120 degrees as my heated air in case of carb ice.
>
> I have measured the under engine air on two airplanes now and it seems
> to run around 120 deg. so this must be fairly typical and would seem
> to be more than adequite to get rid of carb ice. I have never had the
> opportunity to measure heated air coming off exhaust manifold.
>
> I would build it so fresh ram air is unfiltered and alternate, heated,
> air would be filtered. in this way I would always taxi using warm
> filtered air but in normal flight use cold ram air.
>
> Makes sense to me, but then so does capitalism.
>
> Ray

[smjmitchell]

I would do nothing less than ensure the system complies with the FAR requirements.

If want to proceed then I would instrument the carby by placing a thermocouple inside inthe induction system as close to the carby thoat as you can get it. I would also place a second thermocouple outside the aircraft to measure OAT (you could also use an existing thermometer for this if installed). Climb up until the OAT is 30 deg F set 75% power and then turn on the carby heat (i.e. switch to the alternative inlet inside the cowl) and let the system stabilize. If you get the required 90 deg F rise in temp then good .. if not get rid of it an install a conventional system. I have done this in a number of airplanes and it is no big deal to set up and run the tests. Note that in most cases you will get an initial higher temperature and then the intake air will stabilize at a lower temperature. You must stabilize the system at a delta of 90 deg F.

I have reviewed FAR 23.1091 and 1093 quickly ..... I would suggest that some detailed study is in order. These requirements were revised many times since this original amdt until Amdt 51 which set in place the rules that are effective today. The rules at Amdt 51 are very different from those earlier requirements and have now split the requirement between FAR 23.1091 and 1093.

What I think you should do is study the amendment history and understand the way that the FAR's have developed over the years. This may give some insight the basis of the requirements and the reasons for the changes - usually a valuable history lesson. In particular I would study the NPRM's and other preambles may also provide some useful background on the reasons for change.

The earlier amendments (up to Amdt 43 in 1993) had different requirements and specifically state that the induction air from the alternative intake must have a temperature at least equal to the temperature of the cooling downstream of the engine with a carburetter that tends to prevent icing (presumably a fuel injected engine) BUT you must meet the requirement for a 90 deg F increase compared to 30 deg F ambient with a convention venturi carburetor. Don't get confused by this. For reason explained in the NPRM's they ammended the first requirement to require to state that engines with a fuel metering device (fuel injection) tending to prevent icing must demonstrate a 60 deg F temperature rise at 75% power. The 90 deg F requirements remains for convention carby's.

The FAR also specifically state that you must have an alternative air intake (I don't think a door on the front of the cowl qualifies but that may be open to some interpretation. I think they mean physically separate systems with a value in the induction system.

Also FAR 23.1091 provides guidance on where this intake can be located. I would review this.


"Jerry J. Wass" > wrote in message ...
I know this is a homebuilt group, but for naturally aspirated engines-CFR 23.1093
says---" each engine shall have a preater to raise the inlet temp. 90 deg F, from an
ambient of 30deg F w/ no visible moisture present.---AND-at only 75% power. This is not remotely possible with under cowl air temps. The pucker factor gets pretty high when your altitude is 600 ft over a forest, or lake, and the engine starts choking down.
.Jerry
Ray, Toews wrote:

Has anyone ever tried using cowling air under the engine as heated air
for carb heat.
Because of tight cowling space I am thinking of building a door in the
lower cowl for fresh ram air and then closing it and drawing air from
under the engine, cooling air coming off the cylinders which seems to
run about 120 degrees as my heated air in case of carb ice.
I have measured the under engine air on two airplanes now and it seems
to run around 120 deg. so this must be fairly typical and would seem
to be more than adequite to get rid of carb ice. I have never had the
opportunity to measure heated air coming off exhaust manifold.

I would build it so fresh ram air is unfiltered and alternate, heated,
air would be filtered. in this way I would always taxi using warm
filtered air but in normal flight use cold ram air.

Makes sense to me, but then so does capitalism.

Ray

[Matt Whiting]

Kyle Boatright wrote:

> "Dave Hyde" > wrote in message
> ...
>
>>Ray Toews wrote...
>>
>>>Has anyone ever tried using cowling air under the engine as heated air
>>>for carb heat.
>>
>>Serveral RV-builders including me have tried something similar.
>>Van's sells a mini heat muff that doesn't do mucgh but serve as
>>a mount for scat tube, so in essence we're drawing undercowl
>>air into the airbox when carb heat is selected. Sam Buchanan
>>has a picture on his website:
>>
>>http://home.hiwaay.net/~sbuc/journal/engine4.html
>>
>>about 1/2 way down the page. I did the same thing on an
>>RV-4 and I get a barely perceptible RPM rise when I select
>>carb heat, which tells me it's not helping much.
>>
>>Your call.
>>
>>Dave 'hot air specialist' Hyde

>
>
> You get an RPM increase??? Sumpthin' is way, way wrong with that... ;-)
>
> With the exact same setup, I get a barely perceptible RPM decrease...

I thought the same at first, but he may be referring to turning on the
carb heat after he's already accumulated enough carb ice to noticeably
decrease his RPM. In that case, applying the carb heat should increase
the RPM as the ice is removed.

Matt

[Cy Galley]

When you measured the under cowl temperature, I'll bet it was on the ground.
In this situation there is little air moving and poor cooling leaving higher
temps than one would find when flying.

When carb ice occurs, the already low temp is rapidly reduced due to lack of
power. If you do not apply carb heat quickly enough, you become a glider.

The trick is to get carb heat applied before the engine quits so you have
heat to melt the ice. You need a lot of heat and quickly,

<Ray Toews> wrote in message ...
> Has anyone ever tried using cowling air under the engine as heated air
> for carb heat.
> Because of tight cowling space I am thinking of building a door in the
> lower cowl for fresh ram air and then closing it and drawing air from
> under the engine, cooling air coming off the cylinders which seems to
> run about 120 degrees as my heated air in case of carb ice.
>
> I have measured the under engine air on two airplanes now and it seems
> to run around 120 deg. so this must be fairly typical and would seem
> to be more than adequite to get rid of carb ice. I have never had the
> opportunity to measure heated air coming off exhaust manifold.
>
> I would build it so fresh ram air is unfiltered and alternate, heated,
> air would be filtered. in this way I would always taxi using warm
> filtered air but in normal flight use cold ram air.
>
> Makes sense to me, but then so does capitalism.
>
> Ray

[Steelgtr62]

Some British engines used an oil passage through the carb venturi, a far more
elegant solution. However using fuel injection or mounting the carburetor on
top of the engine instead of underneath would largely obviate the problem.

[Matt Whiting]

Steelgtr62 wrote:

> Some British engines used an oil passage through the carb venturi, a far more
> elegant solution. However using fuel injection or mounting the carburetor on
> top of the engine instead of underneath would largely obviate the problem.

Fuel injection will help a lot, but mounting the carb on top makes
almost no difference.


Matt

[Dave Hyde]

Matt Whiting wrote...

> ...but he may be referring to turning on the carb heat
> after he's already accumulated enough carb ice to noticeably
> decrease his RPM.

Yeah, that's it. <g> No, I'm afraid UltraJohn was right.
I haven't seen anything remotely resembling carb ice,
and from what I've heard it's not very common in RVs.
I get a very slight RPM *drop* with carb heat, presumably
to a slight temperature *rise* from switching to undercowl
air.

Dave 'cold throat' Hyde

"My wallet's fat and so is my head" - Greenday, 'Walking Contradiction

[Dan Thomas]

Matt Whiting > wrote in message >...
> Steelgtr62 wrote:
>
> > Some British engines used an oil passage through the carb venturi, a far more
> > elegant solution. However using fuel injection or mounting the carburetor on
> > top of the engine instead of underneath would largely obviate the problem.
>
> Fuel injection will help a lot, but mounting the carb on top makes
> almost no difference.
>
>
> Matt

The Gipsy Major took its carb heat from up next to the
crankcase. As with using undercowl air, the time between startup and
useable heated air is too long and carb ice can form while warming up.
We see carb ice shortly after startup rather regularly here, and
we live in fairly dry country. Need to pull carb heat soon after
startup some mornings. It's a good thing the exhaust piping heats up
quickly.

Dan

[Ray Toews]

Thanks for the insightful help, I planned to install some sensors to
check the rise. I think I will glue one rite to the outside of the
carb at the intake for as realistic a measurment as possible.
I use those little automotive outside air temp sensors I buy from
Canadian Tire, I noticed Wally wallmart has them as well but the ones
I bought from wally were celsius only, irritating but not
unsurmountable. They have about six feet of wire and a sticky back
wich seems to hold well.
God bless all the little borocrats who consented to give us the
homebuilt movement.:)

ray

On 19 Oct 2004 16:55:42 -0700, (Dan
Thomas) wrote:

>Matt Whiting > wrote in message >...
>> Steelgtr62 wrote:
>>
>> > Some British engines used an oil passage through the carb venturi, a far more
>> > elegant solution. However using fuel injection or mounting the carburetor on
>> > top of the engine instead of underneath would largely obviate the problem.
>>
>> Fuel injection will help a lot, but mounting the carb on top makes
>> almost no difference.
>>
>>
>> Matt
>
> The Gipsy Major took its carb heat from up next to the
>crankcase. As with using undercowl air, the time between startup and
>useable heated air is too long and carb ice can form while warming up.
> We see carb ice shortly after startup rather regularly here, and
>we live in fairly dry country. Need to pull carb heat soon after
>startup some mornings. It's a good thing the exhaust piping heats up
>quickly.
>
> Dan

[Bob Hoover]

Ray Toews wrote in message >...
> Thanks for the insightful help, I planned to install some sensors to
> check the rise. I think I will glue one rite to the outside of the
> carb at the intake for as realistic a measurment as possible.
> I use those little automotive outside air temp sensors I buy from
> Canadian Tire,

-------------------------------------------------------------------

Dear Ray (and the Group),

Take your outside air temp sensor and hit it with a hammer. Okay,
okay... squeeze it with a pair of pliers. IF (Big IF) IF... you're
lucky, inside you will find a thermistor about the size of a grain of
sand; the the tiny bit of **** that seems to be welded to the wires.

To measure temperature at the carb, drill a hole in it large enough to
accept the thermistor. (#80 works for the little thermistors.) SOP
is one hole just above the throttle plate (if you have a throttle
plate). (If you want to measure inlet air, put a thermistor on the
carb-side of your carb heat box.) Now put a piece of tape over the
hole on the INSIDE of the carb and go find yourself some metal-filled
epoxy. The Good Stuff comes from Kodak, contains silver and transfers
heat with virtually no loss at all. Mix up a dot of it, dip the
thermistor into the epoxy and plug it into the hole. Now let it cure.
(Don't forget to remove the tape.)

You may install a suitable connector, if you wish. Should I mention
that it's a good idea to only drill where the carb is solid metal?
(Nah!)

A carburetor makes a nice refrigerator. Outside air temp has
surprisingly little to do with it. The pressure drop across the
venturi (traditional carb assumed) and gasoline's endothermic
change-of-state is enough to put frost on a can of beer quick like a
bunny. With a suitable temperature probe in place you can sit there
warm & cozy in the cockpit and see it all happen long before the
engine starts running ruff and the Bad Silence ensues. (You may want
to pull out that red knob over there...)

-R.S.Hoover

[Blueskies]

May want to test the thermistor after you free it from the evil probe and before you capture it in the refrigerator....


"Bob Hoover" > wrote in message ...
> Ray Toews wrote in message >...
>> Thanks for the insightful help, I planned to install some sensors to
>> check the rise. I think I will glue one rite to the outside of the
>> carb at the intake for as realistic a measurment as possible.
>> I use those little automotive outside air temp sensors I buy from
>> Canadian Tire,
>
> -------------------------------------------------------------------
>
> Dear Ray (and the Group),
>
> Take your outside air temp sensor and hit it with a hammer. Okay,
> okay... squeeze it with a pair of pliers. IF (Big IF) IF... you're
> lucky, inside you will find a thermistor about the size of a grain of
> sand; the the tiny bit of **** that seems to be welded to the wires.
>
> To measure temperature at the carb, drill a hole in it large enough to
> accept the thermistor. (#80 works for the little thermistors.) SOP
> is one hole just above the throttle plate (if you have a throttle
> plate). (If you want to measure inlet air, put a thermistor on the
> carb-side of your carb heat box.) Now put a piece of tape over the
> hole on the INSIDE of the carb and go find yourself some metal-filled
> epoxy. The Good Stuff comes from Kodak, contains silver and transfers
> heat with virtually no loss at all. Mix up a dot of it, dip the
> thermistor into the epoxy and plug it into the hole. Now let it cure.
> (Don't forget to remove the tape.)
>
> You may install a suitable connector, if you wish. Should I mention
> that it's a good idea to only drill where the carb is solid metal?
> (Nah!)
>
> A carburetor makes a nice refrigerator. Outside air temp has
> surprisingly little to do with it. The pressure drop across the
> venturi (traditional carb assumed) and gasoline's endothermic
> change-of-state is enough to put frost on a can of beer quick like a
> bunny. With a suitable temperature probe in place you can sit there
> warm & cozy in the cockpit and see it all happen long before the
> engine starts running ruff and the Bad Silence ensues. (You may want
> to pull out that red knob over there...)
>
> -R.S.Hoover

[smjmitchell]

Just use a standard K or J type welded junction type thermocouple (i.e. two
wires welded together in a small bead). I would be cautious of any other
type of sensor ... they may have too much mass and will introduce lag or
damping into the readings because of the time required to heat the mass of
the sensor housing.

Also don't bond the thermocouple to the carby housing. You want to measure
the air temperature, not the temperature of the metal inlet flange to the
carby.

Just stick the thermocouple wire into the inlet area - depending on the
engine you can loosen a cable clamp in the alternative air system and just
slide it in and retighten the clamp. I would slide in approx 1" of
thermocouple wire so that the thermocouple is somewhere near the middle of
the inlet hose.



<Ray Toews> wrote in message ...
> Thanks for the insightful help, I planned to install some sensors to
> check the rise. I think I will glue one rite to the outside of the
> carb at the intake for as realistic a measurment as possible.
> I use those little automotive outside air temp sensors I buy from
> Canadian Tire, I noticed Wally wallmart has them as well but the ones
> I bought from wally were celsius only, irritating but not
> unsurmountable. They have about six feet of wire and a sticky back
> wich seems to hold well.
> God bless all the little borocrats who consented to give us the
> homebuilt movement.:)
>
> ray
>
> On 19 Oct 2004 16:55:42 -0700, (Dan
> Thomas) wrote:
>
> >Matt Whiting > wrote in message
>...
> >> Steelgtr62 wrote:
> >>
> >> > Some British engines used an oil passage through the carb venturi, a
far more
> >> > elegant solution. However using fuel injection or mounting the
carburetor on
> >> > top of the engine instead of underneath would largely obviate the
problem.
> >>
> >> Fuel injection will help a lot, but mounting the carb on top makes
> >> almost no difference.
> >>
> >>
> >> Matt
> >
> > The Gipsy Major took its carb heat from up next to the
> >crankcase. As with using undercowl air, the time between startup and
> >useable heated air is too long and carb ice can form while warming up.
> > We see carb ice shortly after startup rather regularly here, and
> >we live in fairly dry country. Need to pull carb heat soon after
> >startup some mornings. It's a good thing the exhaust piping heats up
> >quickly.
> >
> > Dan
>