Coughing Rotax 912s

[stillin1]

After about 60 mins today the engine started occasionally missing a few beats at cruise power. Ht = 3000'. I'm asking for ideas why so that I can check em all and avoid this little pants-filler happening next time.
There is no such thing as a stupid question
Ideas guys?

[Genghis the Engineer]

No immediate firm idea, but I'd suggest checking the following...

- Dirt in the carburettor (you'll never find the dirt, but a good clean and blow through should eradicate it).

- Loose plug lead

- Water contamination of the intake filter.

- Lead contamination of the plugs if you've been running on AVGAS rather than unleaded MOGAS (the latter is much more friendly to any of the Rotax products).

Not sure if any of these will help, but I'd have a look at them.

G

[bingoboy]

As G says cleaning out the fuel system is always a good idea.

Other thoughts are possible ice from carb being ingested.

or slightly more worryingly a plug breaking down. Once had this happen (Lycoming) diverted - engineer said nothing wrong - flew back to base and next time out all hell broke loose. It was damned hard to find the flaw/crack in the plug but it was there.

Hope this helps

(ps I am not an engineer so what do I know)

[Fly Stimulator]

My first thought would be carb icing.

I had similar engine behaviour a month ago in a PA28 at night. Applying carb heat made no difference, but fortunately the engine kept running long enough to get down.

A check by an engineer the next day revealed that the carb heat cable had broken, so the control in the cockpit was having no effect.

My normal mount is Rotax 912S-powered and has no carb heat control. Apparently the position of the carbs up under the rear of the cowling means that they are in warm air anyway, and to be fair I've never been aware of icing, but I would feel more secure if I had a nice carb heat knob on the panel!

[Gerry Actrick]

What's it in? Has it just started happening? They do carb ice for fun - that would be my first thought. Some have water heated manifolds - that works great if the water is hot - at this time of year it wont be unless you cover rad.
Gerry

[ChrisVJ]

Join Rotafly and ask there.

Rotafly is a new Yahoo forum for Rotax users. If you are not a member of a yahoo group you will need to sign up (Free and nearly painless.)

In your browser go to "Yahoogroups"

Sign up

Add group.

If you are a member of a group go to:


http//groups.yahoo.com/.group/Rotafly[/URL]

Not sure I have done that right, still learning. ChrisVJ

[Genghis the Engineer]

Rotaxes (with the occasional exception) have a very different approach to carb heat to the classic Lycontinental installation.

The more old fashioned approach of diverting hot exhaust air into the intake clearly works in clearing icing, but it also causes significant power loss. Rotax installations generally use either electrical power (drawn from the lighting circuit) or in the case of liquid cooled engines such as the 912 or 582 more often cooling heat to directly heat the carb body. The electrical systems have an almost insignificant effect on power, and the coolant based systems have a totally insignificant effect on power. Also, the coolant based systems whilst almost totally reliable (if you're losing your coolant over the side, carb icing frankly is probably the least of your problems), would inevitably become less reliable if you introduced any form of on-off control.

So, I'd suggest that the control that FS craves would almost certainly increase the risk of a carb heat or coolant-loss failure.

(Anyhow, why didn't you check carb heat in your engine run-ups i.a.w. the PA28 POH and checklists you silly boy).

G

[Fly Stimulator]

(Anyhow, why didn't you check carb heat in your engine run-ups i.a.w. the PA28 POH and checklists you silly boy).

Answer: I did, and it was fine well into the flight. If you can recommend an aircraft where things only fail when the machine is sitting on the ground, preferably on the morning an engineer is booked to examine it anyway, then I will start saving up for one now!

[Genghis the Engineer]

A fair point, well made.

G

[ChrisVJ]

I have a Rotax 912S and my experience was a little different.

For one thing my plane is a pusher and the carbs are stuck out there in the cold part of the airstream just above the wing. The plane is just 25 hrs old and I have not yet decided whether to install carb air heating.

I went flying in December, it was cold, about 2 or 3 deg C and there was wispy low cloud in the valley, otherwise sunny.

I flew for about 40 mins and decided to to do a couple of circuits before packing in. Circuit and approach were normal. To minimise vibration on the central style mounting I usually keep revs up above 2,500 on approach and roundout and then just cut the throttle when she settles. This time when I did that she just spluttered and quit.

I restarted the engine easily enough, sat on the runway ( the place was otherwise deserted,) and tried a few throttle advances from idle. It seemed sudden advance would make her splutter and even quit but a medium steady advance would not. ( I have not had this before when testing, the engine always seemed to pick up ok now matter how I advanced the throttle.)

After a while the problem seemed to go away and after checking the throttle for quick advance seven or eight times I decided to do a close in circuit. The circuit went fine, absolutely no problem till I closed the throttle on roundout. Same thing, splutter, quit.

Again after about five minutes of ground running I was back almost to normal but decided to ask around before testing any further.

One forum I belong too suggested "Change to Winter jets." though I can see no mention of this in the installation or running handbooks. (I have however jacked up the idle to 1800 pro tem.)

Looking at the pictures on a UK agent for Rotax I see that if icing occurs it can occur right out there where the butterfly is and I am guessing, that if it did, when the butterfly was closed there would be no remaining air passage for the idle air, so the engine would quit when the throttle was closed. Without the cooling effect of Zero deg C airflow the carbs would absorb enough radiant heat from the engine to melt the ice and I'd be fine until we were back in the air when icing would start again.

I flew around both in sunshine and shadow for that 40 mins and there was no apparent effect on power or smoothness and that would have allowed plenty of time for a serious build up of carb ice so I'm wondering if there is something about the shape or operation of the carbs that inherently inhibits larger ice build up.

One concern, with a pusher, is that any parts not perfectly secured etc damage the prop, can even write it off, if they seperate so I tend to be a bit careful about what I put on that area of my plane.

[QNH 1013]

I too would always prefer to see a carb heat control. However, I have mentioned this when flying Rotax 912 powered aircraft and I have been told that (unlike Lyc and Cont engines) they are not prone to carb ice because they have two carbs and so each carb is only evaporating half the AVGAS that the engine burns, and the fuel consumption (= fuel being evaporated and therefore producing the cooling effect) is much less than with a Lyc or Cont engine.
This sounds sort of plausible, but I would really like the definitive answer if anyone has it. Have ROTAX ever published anything about this?

[Gerry Actrick]

I can assure you the 912 will get carb ice unless you do something about it. If its in a cowl it may be sucking in warm air, which will help. Otherwise fit something. As Genghis pointed out, water or leccie works. Many flexwings use constant hot air and suffer the power loss (there is too much power anyway).
Gerry

[Genghis the Engineer]

I'm totally inconvinced by the argument you've been given QNH, it's not fuel that causes carb icing, it's moist air through the venturi. Now given that a Lycontinental is most prone at low power settings (= low airflows), the argument given would make a Rotax more prone.

Now personally most of my Rotax flying has been with single carb 2-strokes, and I've never seen any serious case of carb icing. But when you think about it, the airflow there is far greater than a multi-carb 4-stroke, so that doesn't help.

G

[FlyingForFun]

My Rotax (914 - so a little different, but hopefully will be at least slightly relevant) is prone to rough-running due to clogged fuel filters. Checking the fuel filter is an essential part of the pre-flight, and if it does start running rough in flight the solution is to switch on the boost pump (which uses a different filter to the main pump).

In all the time I've been involved with the aircraft (getting on for a year now) we've come up with many theories about what's causing the filters to become clogged, but haven't yet found the solution. The current thinking is that it's bits of chipped paint coming off the inside of the jerry-can we use for re-fuelling.

Hope that's some help - good luck with fixing it!

FFF
--------------

[QNH 1013]

Hi Genghis,

Like I said, I'm not totally convinced that Rotax 912 engines don't need carb heat but I think I'm right about the carb icing being caused by the fuel evaporation.

The moist air doesn't cause the ice by itself. It just provides the source of water. It has to be cooled by some mechanism for the moisture to freeze. Carb icing can happen when flying through moist air at temperatures well above freezing. Something is therefore cooling the moist air. There will be some cooling caused by the expansion downstream of the venturi but I have always assumed that the main cooling effect is produced by the fuel evaporation in the carb.

The latent heat required in the evaporation of the fuel has to come from somewhere and I assume it comes from the moist air, thereby cooling it to less than zero degrees, hence the ice formation.

I also think that the reason that carb icing is more likely with MOGAS is that AVGAS is optimised to have a lower latent heat of evaporation (therefore less cooling in the carb). Remember that at high power settings (ie max fuel flow) the volume of air is at a maximum too.

At present the above is just my opinion - I haven't got time to do all the calculations. Does anyone have a definitive reference?

[Gerry Actrick]

Mr. Fun,
Jerry cans are deadly. I filter mine with a plastic coffee filter in the funnel - without exception. What I do find in the fuel filter is belly button fluff - where does that come from?
Gerry

[100LL]

Sounds to me like you got a spot of carb ice. 912's will ice up i've had it plenty of times especially first thing in the morning warming up on the grass. My advise is to fit a proper carb heat system, Rotax do a nice Air box kit that bolts directly onto the carbs or fit an airbox off a Katana.

Found This (http://www.flycorvair.com/carbice.html) link about carb ice

[kabz]

Not sure you really need carb ice to make it cough, I've had a 172 misfire like ******* on a missed approach at 400 AGL, just heading back into the clouds ...

I was all for maintaining 400 and putting it on the runway, but my instructor had me continue ...

It probably wasn't cold enough for ice, but a low power approach through visible moisture probably collected enough water to make it cough a bit once I opened the throttle.

And cough it did :confused:

[stillin1]

Thank you one and all,
Replies have confirmed my thoughts and given me a course of action I can have faith in.

Be safe and have fun.

[El Thermidor]

QNH 1013:


The moist air doesn't cause the ice by itself. It just provides the source of water. It has to be cooled by some mechanism for the moisture to freeze. Carb icing can happen when flying through moist air at temperatures well above freezing. Something is therefore cooling the moist air. There will be some cooling caused by the expansion downstream of the venturi but I have always assumed that the main cooling effect is produced by the fuel evaporation in the carb.

I was always under the impression that carb ice was caused by the vacuum effect of low throttle openings. Given that, at idle, an inlet manifold can experience 0.1 Bar, this effect must be significant. I don't follow your argument re. fuel evaporation - largely because this would alter according to air-fuel ratio, not air flow. Given that, for the fuel evaporation theory to be correct the icing should be more pronounced at higher throttle positions, as an engine on full chat will often be set up to run rich.

However, I'm sure some spoddy type with too much time on their hands can do the maths and show us all up.

stillin1:

G the E looks to be on the money (quelle surprise!;) ) so I'd listen to him.

[Genghis the Engineer]

Oh no, somebody's taking me seriously again.

I have a bit of a problem with the vapourisation of fuel argument. So far as I can identify from the few books I've got on the innards of a carburettor not all of the fuel is vapourised, I think that a large proportion is atomised - that is split into very small droplets. There is no cooling effect from this in the way so although the textbooks (I'm not going to work this out, I gave up that sort of difficult thermodynamics years ago) suggest a temperature drop due to vapourisation in excess of 30°C, I think a much smaller figure is likely within the carb body itself - I suspect that a lot of the vapourisation cooling happens later, probably much in the actual cylinder where it's cooling effect is largely irrelevant (although it's clearly essential for proper combustion).

Supporting this, we know that carb icing in most engines is most likely at low throttle settings. Whilst this is blamed upon the position of the butterfly valve - it is notable that at that condition the fuel flow will be at a mininum, so vapourisation cooling (such as it exists) will also be inevitably at a minimum.

So, although without doubt vapourisation cooling in the carb must be a player of some form, I don't personally believe it's as big a player as expansion cooling. If I'm wrong somebody tell me.

G

[ChrisVJ]

Ghengis makes a good point. If fuel evaporated in air at the rate needed to feed an engine from that iddy biddy jet area in the carb, then a jar of gas left on the ground would evaporate from that large surface area in about five minutes. No good claiming it happens because of elevated temp either!

I too, without really thinking about it, assumed that gas was evaporated and not atomised but of course it is atomised.
BTW First Daimler car used a carburetor made from a scent bottle atomiser, why did I not remember that?)

Just thinking.

1. When pressure is lowered air can support less moisture which then condenses out on throttle passage walls.
2. Pressure is lowered, air expands, gets cooler. Continuous process in flowing air in throttle passage. Absorbs heat from passage walls.
3. Eventually throttle passage walls get cold enough for ice to form.

[QNH 1013]

Good point Genghis, I'd forgotten that the bulk of the fuel is atomised (not vapourised) within the carb. This ratio may well change with different fuel flows and throttle settings. So it looks like the dominant effect must be the expansion downstream of the venturi. There will still be an effect within the carb from that part of the fuel that is evaporated there to explain the different behaviour of MOGAS and AVGAS.