Carb heat needed?

Question for the VW guys, my new (to me) vmax has a 1600cc great planes conversion running a weber downdraft carburetor mounted above the engine. It has 164.8hrs on this setup and lacks any kind of carb heat. Am i asking for trouble or is the radiant heat from the engine sufficient to prevent icing?

 

Yes, I have even had carb ice with the engine quitting in the VW Bug. Just takes one time. For the piece of mind while flying, I would install carb heat and use it as if I was flying a small Cont engine.

 

Regular Venturi carb needs carb heat. The motorcycle style carbs can get away with not easier.

 

Insurance.

The wrong time to get it is after you need it.

 

I have zero experience/expertise on VW engines. But the cost and complexity and weight of carb heat (whether electric or exhaust heat-based) is very low compared to the benefit IMHO. The radiant heat from the engine would probably "move the risk zone" for carb ice down to a lower ambient temperature, but it will probably not eliminate the concept or cause of carb ice.

 

The Ackerman VW conversion plans originally created for the Evans Volksplane VP-1 include a top-mounted carb just like that as well as an integrated carb heat system. I have those plans somewhere but it will take some digging, if anyone has them handy and could post a photo or scan of the carb heat box page that would be great.

EDIT - The schematic below shows the part that I mean. It's just a rectangular tower with the top cut off at an angle (facing forward) and, IIRC, the bottom is extended to form a drip pan to divert any fuel spillage from over-priming away from the engine via a drain tube. Inside the tower is a simple hinged flapper, open to use outside air or closed to pull from the heat muff.

 

An option is to use the automatic carb heat system devised by Valley Engineering when they were selling their VW conversions. As seen in the photo the system is dirt simple....a small steel tube runs from the exhaust pipe to a fitting mounted just under the carb. They used steel brake line I think, about 1/8" ID and insulated with heat proof wrap.

When the engine is running at full power the manifold vacuum is low and very little hot exhaust is drawn into the carb intake. When the engine throttle is closed the manifold vacuum increases to max and thus draws more hot exhaust to the carb.

Some of my friends have this installed and have not had any icing issues.

 

I like the Valley Eng rig. The 1/2 Hummel my dad bought used a tube wrapped around the carb and then to the exhaust. If I recall the heat moves passively through the tube and radiates to the body of carb keeping it warmer all the time.

 

Just for $hits and giggles I tied in a inlet from bottom of heads (airflow) and ran to vacuum (Carb) into another tee. It seemed to help but craft wasn't at altitude. If your going up make sure and have a heat box you just never know. I sat on the side of the road because a 64 Scout International frosted up one winter.

 

In the VW Bug , Type 1, there is a tube that bolts to the muffler at the head and cross's under and attached to the intake manifold to heat the intake and base where the carb in attached for anti-iceing of the carb. This is marginal in some conditions and sometimes the carb will still ice up. Picture of the intake with the anti-ice tube.
Anti ice with the use of hot air from exhaust pipe heat wrap to heat the intake air is far more efficient.
https://www.cbperformance.com/product-p/3151.htm

 

A friend has a temp probe on his Lycoming carb. At pattern altitude on a 60deg F day his carb reads 32 F before the carb heat. Carb was 25F at 6000 ft where the ground temp was about 40. Don’t remember OAT. Carb heat pushes the temp to about 65F on the 30-40F days.

 

The temperature in the venturi and at the throttle plate when it's closed can be more than 70°F lower than ambient, which explains why carb ice is possible when the OAT is 100°F. Air accelerates through the venturi and past the throttle plate's edge when it's closed or nearly so, and higher speed means lower pressure, and lower pressure means lower temperature. Add the evaporation of the fuel being sprayed into the airstream, and more heat is absorbed from the air, lowering its temperature further.

Certfied airplanes have to meet this requirement:

Sec. 23.1093

Induction system icing protection.

(a) Reciprocating engines. Each reciprocating engine air induction system must have means to prevent and eliminate icing. Unless this is done by other means, it must be shown that, in air free of visible moisture at a temperature of 30° F.--
(1) Each airplane with sea level engines using conventional venturi carburetors has a preheater that can provide a heat rise of 90° F. with the engines at 75 percent of maximum continuous power;
(2) Each airplane with altitude engines using conventional venturi carburetors has a preheater that can provide a heat rise of 120° F. with the engines at 75 percent of maximum continuous power;

A system that can provide a temperature rise of 90°F is a serious source of heat. A lot of homebuilts can't come anywhere close to that, with the result that engine power loss due to carb ice is far too common. Another factor there is the number of pilots who think carb ice is only a wintertime thing.

 

Guaranteed question on a pilot test. Carb heat on a friend ‘s old Piet

 

The KC Dawn Patrol Aviators all use this method for carb heat, they have sent be the concept sketches they use. I have attached them, they claim is it is very effective simple and lite. I have a traditional carb heat source on my revflow carb which claims not to need it. I like the insurance and keeps me in practice of the disapline of applying carb heat before pulling power back.

 

I can't see that that system will prevent icing in the venturi. The venturi is upstream of the throttle plate, and it's not just the throttle plate that ices up.

 

From this I can guess, then, that carb ice is possible in Arizona? (20% humidity at 95f)

 

Lycomings are not immune to ice. We had a bunch of them, various models in various airplanes, and some, like the O-320-A2D in one of the Citabrias, would ice up in a heartbeat even though its sump and carb and everything was identical to the -E2D models in the 172s. And I had carb ice lots of times in the 172s as an instructor. In the summertime. Sometimes right after startup.

Carb ice has been determined by some researchers to be the leading cause of engine failure. AOPA published a list maybe 15 years ago that had carb ice as the leading cause, by a wide margin. Next was fuel starvation, IIRC, then contaminated fuel, then oil starvation, then structural engine failure.

 

That chart refers to temperature and dewpoint. The % RH is shown there for interest's sake, and at 95°F you're just within the limit at 20%. It would be very unusual and that would apply to a very ice-prone setup. But it's possible. That chart is a general thing and one should get to know his system and watch for ice. I've seen carbs icing up and the pilot has no idea that it's doing so. Get him to pull the carb heat and watch the tach and see the RPM fall, then rise, close the carb heat and see the RPM rise to a higher figure than before the carb heat was pulled and the throttle still right where it was. Carb ice.

The temp and dewpoint are given on every METAR, and that's what you use to determine the risk of carb ice for any given flight. When the temp and dewpoint are close, watch out. I typically found pilots unwilling to learn to pay attention to the temp/dewpoint and check the chart and get an idea of the risk. It requires thinking.