[turbo]

an rv buddy went to a seminar at osh and heard of using a bit of carb heat during leaning. the presenter said to lean till rough then richen till smooth. add a bit of carb heat , then reduce throttle a bit. any thoughts on this. what would carb help with?

 

[Ironflight]

This has been discussed a number of times in passing Ed. the theory is that the carb heat valve might create a bit of turbulence which can give better mixing, and a smoother air/fuel charge. The same thing is true about going slightly less than full throttle - with the throttle plate slightly tilted, you can get some turbulence off the trailing edge of the plate and give a better distribution.

These are things that seem to work for some people (I find that the throttle trick helps my engine), but you have to decide for yourself whether they are of benefit. I have never seen either documented in anything resembling a scientific study.

Paul

 

[Finley Atherton]

My usual practice for cruise above about 7500 ft is to go WOT and add sufficient heat to get a carb temp of about 15 degrees C.
I did some speed/FF tests recently and got the following results:

DA 8,500 ft
WOT
RPM 2,250
MAP 21.8
OAT 2 degrees C
Carb Temp 16 degrees C
IAS 134
TAS 153 kts (from 4 leg GPS spreadsheet)
FF 24.7 L. (6.5 US gal) - leaned manually to rough then in to just smooth.

Immediately after this I did another run without carb heat and adjusted the throttle to give the same FF using the same manual leaning method. As you can see the TAS drops by 4 kts for the same FF:

DA 8,500 ft
RPM 2,250
MAP 21.1
OAT 2 degrees C
Carb Temp 3 degrees C
IAS 131 kts
TAS 149 kts
FF 24.7 L. (6.5 US gal) - leaned manually to rough then in to just smooth.

I am not sure if the improved efficiency with carb heat was due to better fuel atomization, the throttle plate being wide open or a combination of the two but the 4 kt benefit for the same FF is significant. In the future I will do some more testing to see if there is a throttle/carb temp combination that gives an even better result.

Fin.
9A, 0-320, Hartzell, 9:1 CR, LSE Plasma III and mag.

 

[petehowell]

Carb Heat

Hi Ed,

Fin and I have compared notes before and we are pretty close(he is faster top end - dang it!!). My sweet spot is partial carb heat, WOT, 8500 ft, LOP. I get 150KTAS on 6.1 GPH. (O-320, CS, Pmags)

As Paul said, I can't tell you if the warmer air provides better atomization, or if the carb heat valve adds some turbulence, but it seems to work. I know it is doing something, b/c if I am cruising smooth, dumb and happy at these settings and dump the carb heat open, it runs rough, add the Carb Heat back in, and it smooths out.(how's that for rigorous scientific data!!)

I corresponded with the one GAMI guys and he told me he thought it was heat that was doing it, but I don't know.

Timing advance also plays a part. I have varied my timing advance and it definitely affects how well the engine will run LOP. Big advance helps the LOP ops, but you pay for it with higher CHT on climbout. Small advance = cool CHT's but not so good LOP ops.

Lots of things play in the mix.

 

[Andy_RR]

From my day job experience calibrating gasoline auto engine ECUs, I think heat is the primary factor here. Most gasoline engines require quite a bit of heat into their fuel to provide optimum combustion.

On sequential port fuel injected engines, the lay person thinks that the injector injects through an open valve, but this is so far from the truth it's almost funny. Optimum injection timing usually starts just after the inlet valve closes in order to maximise the time the fuel sits on the back of the inlet valve and hence amount of heat that is absorbed into the fuel. In fact, sequential injection is actually used to avoid injection through the open valve.

Injecting through an open inlet valve will produce noticably bad combustion stability (partial misfiring) due to the lack of mixture preparation, despite rapid gas velocities and turbulence through the ports and over the valve head. So, from my experience, heat, rather than turbulence is the main factor here.

Of course, AVGAS with its lower vapour pressure compared to MOGAS, will suffer more significantly from this problem too.

 

[alcladrv]

At cruising altitude, (7500' to 9500'), my practice is to use partial carb heat, pulling the knob out about 1/2" to 1". I do wonder how much real heat is input with Van's standard carb heat arrangement on my O-360. I set the throttle for my fixed pitch prop to 2500 - 2600 rpm and then start leaning for 8 gph or less.

I don't know how much the carb heat helps, but I can say that I can get my EGTs within a 15 to 20 degrees F max spread, which I think is pretty tight for a carbureted, fixed-pitch setup. True airspeed is usually about 155 Kts.

The cylinders range range from 10 to 40 degrees F lean of peak. I don't have a carb temp probe.

If I run it flat out and don't care about the fuel flow it will give 168 Kt. TAS, which is darn close to Van's published numbers.

Mike