Significant problems with Rotax engines?

South Africa with mostly steady dry climate isn’t exactly Northern Europe. These Bings are pressure compensated, but not temperature compensated. The iS, or any other EFI for that matter, is. Manual mixture also

Some interesting comments on Rotax reliability here from a South African Sling operator with 35,000 hours spread over several aircraft, the highest time of them having over 7,000 hrs TT. This was published over a year ago. He’s had gearbox and alternator stator problems with the injected 912is variant, but reports no problems such as those reported elsewhere with the Bing carb’d engines in those airframes, using whatever auto fuel he is burning in that country.

gallois wrote:

Carburetor icing is caused by the temperature drop in the carburetor, as an effect of fuel vaporization, and the temperature drop associated with the pressure drop in the venturi.

Yes, but please. Google some more until you find the relative effect fuel vaporization vs pressure drop. Don’t remember what they are, but in the order of 100 to 1 for fuel vaporization.

@Le Sving
A quick Google
Carburetor icing is caused by the temperature drop in the carburetor, as an effect of fuel vaporization, and the temperature drop associated with the pressure drop in the venturi.

boscomantico wrote:

Just as a tiny datapoint, AFAIK, since my club switched their two BR23s to only SuperPlus or 100LL (I assume at least 90% of which Super Plus, which we get mainly at EDFC), there have have been no issues.

This is in line with all other tiny datapoint Yet another confirmation of facts.

gallois wrote:

When air enters the carb, the venturi effect can lower that temperature by as much as 30°

No. It’s not the venturi effect. It’s the evaporation of fuel that cools down the mix.

Maoraigh wrote:

I don’t understand how partial carb heat can make carb ice possible. I understand the carb ice diagrams, but an increase in air temperature will be accompanied by a reduction in humidity unless water vapour is added.

I agree. What I learned during PPL was it’s a procedural thing. Running without heat, then turning on the heat every now and then for 20 seconds you will learn to listen to the engine. If it behaves the same every time, no carb ice. If ice is formed, the engine will behave different when this ice is cleared, and you will notice. No carb heat gives more power.

Rotax installations has at least 3 different ways (5 actually) of dealing with carb ice that I know of.

1. The usual way (Lycoming method) deflecting heated air into the carb.
2. A shroud heating up the carb using the engine coolant
3. Simply placing the intake in the cowling (always “partial” carb heat)
4. Nothing at all, uncowled engine.
5. EFI (iS engines)

Which one is best? I don’t know. Carb ice and Rotax in the same sentence is a seldom phenomenon. Obviosly EFI is best of course, but carb ice doesn’t really seem to be a problem from the start.

Most ULs actually have wing tanks and an aux pump. Like the B23, one of the aircraft types that seems to suffer from the issue that we are discussing.

In the Super Guépard has fuel tanks behind the seats in the fuselage as @Vic says but there is no problem climbing at 1000ft per min and more on UL95. As we have lots of similar aircraft here, doing the same, I can’t see how the fuel pump is the full answer.
@Maoraigh a mass of warm air can carry more water vapour than a mass of cold air so doesn’t necessarily become less humid with a rise in temperature. That water vapour is turned to water droplets or ice as the temperature drops. Think dew or frost on grass. In the carb this ice or dew forms very rapidly at the mouth of the carb just at the butterfly and that can’t move.
Partial carb heat often does not produce enough heat to melt this frost or evaporate this dew. Again think how strong the sun has to get to melt the frost on the grass on a clear day. In the carb those water droplets when hit by -30° air is more likely to turn to hard clear ice than soft frost and to do it very quickly. You need quite a bit of heat to remove that.:)

Seems most failures with Rotaxes to be related to fuel starvation, a few conditions may lead to this. I wonder if most or all got a feed pump next to the fuel tank. In ultralights I guess most tanks are placed behind the pilot, not many with wing tanks. So high or low wing does not matter much here. We can see lots of ULs doing maximum climb angles after takeoff so in this situation the fuel tank will be quite low and the engine pump will struggle to suck any fuel up into Bings. And sucking is problematic for fuel pumps and will lead to cavitation and “vapour locks” . Simple safety action is getting a Pierburg pump installed next to the fuel tank for feeding up to the engine. No need to adjust for fuel pressure, just get one with 0.4 bar maximum pressure like for any old Beetle – or Vauxhall Viva . . . But then, I don´t know much about Rotax or fuel tank installations and what´s standard in them. Vic

Pierburg pump 0.4 bar

I don’t understand how partial carb heat can make carb ice possible. I understand the carb ice diagrams, but an increase in air temperature will be accompanied by a reduction in humidity unless water vapour is added.