Rotax Parts Supply Problems

[pmccarthy]

My coolant and oil temps rarely lift much above the pin, neither has ever reached the green, the gauges are not marked with temps, just coloured zones. Are the gauges wrong, or is it running cool? If I blank off part of the heat exchangers and it was the gauges, I could damage the engine.

[aro]

1 hour ago, skippydiesel said:

Rotax still refers to the head temperature (now liquid/coolant) as CHT or "Coolant temperature limit measured in the cylinder head Engine types"

 

That pretty clearly says "Coolant temperature" not cylinder head temperature. The cylinder head is where it's measured, not the temperature being measured. In all the Rotax documentation they are very careful to distinguish between coolant temperature and cylinder head temperature.

1 hour ago, skippydiesel said:

All of the above are, by convention, verbally & on your instrument panel, referred to as CHT

By who's convention? People who don't know any better? The numbers are different, so it is actually important to know whether your measurement is coolant temperature or cylinder head temperature.

 

If your coolant temperature is marked as CHT, it's wrong.

[skippydiesel]

If your coolant temperature is marked as CHT, it's wrong.

 

So what? - that's the way the instrumentation is designed - not specifically for Rotax. It may be technically wrong but that's life and it doesn't matter a jot.

 

The numbers are different, so it is actually important to know whether your measurement is coolant temperature or cylinder head temperature.

 

Maaate ! - who cares what it's measuring (metal/coolant) or for that matter what the numbers are? - what the pilot needs to know is if the readings (numbers/coloured band) are within the acceptable range, if they have changed since the last reading(s) and what is the correct response, to an abnormal reading or trend. 

[skippydiesel]

Applicable for engine S/N with Suffix -01. Coolant temperature limit measured in cylinder head Engine type Max. 120 °C (248 °F) 912 S/ULS.

 

Coolant temperature: (coolant exit temperature) Max. 120 °C

 

Early days in Rotax Owner Forum responses, to my enquiry, on this matter , however it would seem that there is little, if any need for the pilot of Rotax 912 ULS engine S/N with Suffix -01.  (mine) to monitor Coolant temperature: (coolant exit temperature) Max. 120 °C just duplicating CHT readings.

 

Older engines without Suffice - o1 may benefit from having a coolant exit temperature sensor/gauge.

 

Further Rotax Installation Manual does recommend (I missed it) a location for (coolant exit temperature):

"The measuring of the coolant exit temperature is performed using a separate sensor, which has to be installed in the line between expansion tank and radiator inlet."

[facthunter]

That's the same location as the normal coolant thermostat is  fitted in road vehicles. It's a useful sensor point.  Nev

Edited December 21, 2023 by facthunter
typo

[RFguy]

IMO you want to keep the exit coolant temp at least 10% below  the boiling point of the coolant at the operating pressure. Bubbling, gassing will upset the flow and lead to coolant being ejected into the expansion bottle (bad ish) . 

Vapour spots  'hot spots' have a higher potential heat removal capability than liquid water conduction regions, depending on flow..  It's hard to know how big the hotspots would be because they would vary with flow and viscosity which beleive it or not, varies quite a bit for common coolants.  Remember , Ally Alloy has a fairly high thermal conductivity   so hot spots are unlikely because they'll just conduct the heat to the cooler spot. 
**Plenty up for chance and speculation without accurate physical modelling !** so just keep the coolant 10-15 deg below the boiling pt

I'd keep the temps below boiling pt for an atmospheric pressure of 50/50.  IE below 106C. 

 

Anyway, I dont think you have coolant temp issues, you need to get the oil temp down. 80-90 deg C hottest. 

 

[facthunter]

There's nothing good about having hot spots  The coolant coming out of the heads HAS to be colder than the heads or it wouldn't cool them. Nev

[aro]

23 hours ago, skippydiesel said:

who cares what it's measuring (metal/coolant) or for that matter what the numbers are? - what the pilot needs to know is if the readings (numbers/coloured band) are within the acceptable range

You can simply look at the markings on the gauge, but but some people do believe that its worthwhile to learn the limits as well. This means that you need to know whether the gauge is reading CHT or coolant temperature because the numbers are different.

 

 

9 hours ago, skippydiesel said:

Early days in Rotax Owner Forum responses, to my enquiry, on this matter , however it would seem that there is little, if any need for the pilot of Rotax 912 ULS engine S/N with Suffix -01.  (mine) to monitor Coolant temperature: (coolant exit temperature) Max. 120 °C just duplicating CHT readings.

You have confused yourself here by referring to the coolant temperature as CHT.

 

Rotax want you to have a coolant temperature gauge. If the coolant temperature is measured in the head, you do not need a second measurement - that is your coolant temperature.

 

If the head sensor is measuring CHT (i.e. metal temperature) Rotax want a sensor in the coolant exit to measure coolant temperature.

 

It's all in the manuals, but you do need to track fairly carefully which engine variation each section in the manual is referring to.

[skippydiesel]

You have confused yourself here by referring to the coolant temperature as CHT.

 

No confusion - I know exactly what I am writing/talking about as do 99% of Rotax drivers who have even the most basic knowledge of their engine function.

 

You are absolutely correct in your pedantry however, your position would seem to be about semantics, rather than how people actually communicate, on this matter and how your instrument panel usually reports/shows the readings.

[aro]

 

25 minutes ago, skippydiesel said:

No confusion - I know exactly what I am writing/talking about as do 99% of Rotax drivers who have even the most basic knowledge of their engine function.

If you know what you are talking about you wouldn't be asking about measuring coolant exit temperature on a suffix-01 engine.

 

You asked:

On 19/12/2023 at 9:57 PM, skippydiesel said:

I may try to raise the coolant temperature a bit, so that there is less of a gap between it and the oil temp.

"Coolant temperature: (coolant exit temperature) Max. 120 °C  Cylinder head temperature: Max. 150 °C "

The problem is, the only coolant measurement I have is CHT (in Rotax this is cylinder head coolant temp). I don't have coolant exit temperature - I presume this is as it comes out of the Radiator. There is no minimum temp or preferred range mentioned (stand to be corrected). The CHTof 85C seems a tad low.

You listed CHT max as 150C. That's the CHT max for 80hp not 100, but its even more wrong if you want to call coolant temperature CHT - it should be 120C. Then you talk about measuring radiator exit temperature with a max of 120C which would be way too high. "The problem is..." There is no problem - just confusion about which numbers to use - largely, it seems because you are calling the coolant temperature measurement CHT and then applying the actual CHT limits instead of coolant temperature limits.

 

Then you start calling people who give you the correct numbers pedants...

[skippydiesel]

Aro me old mate - your like a "dog with a bone", great tenacity.

 

All the best for Christmas/New Year and more good aircraft debates😈

[Blueadventures]

On my ULS engine oil temp is always higher than the CHT temp. usually about 8 degrees C.  I also have a two position toggle for CHT feed from #2 and #3 heads and these read differing temps (one gauge and I can select temp source).

Edited December 24, 2023 by Blueadventures

[RFguy]

Rotax installs vary enormously.  Much of the problem is too much pressure inside the cowls  (leading to loss of differential pressure across the radiators) , due to excessively sized nostrils. The cylinder bores DO need airflow though. (see rotax install manual) , but it really needs to be ducted from a low drag inlet like a naca rather than huge ram air  foreced airflow which abotu 5% makes it across the cylinder bores and 95% just pressurizes the cowling without use. 

Oil and cooling radiators ideally need to be ducted out of the cowling separately to the  nostril / cowling  instake... and forward facing ram air flow facing radiators is a must unless you want to get very fancy with sealing.  forget 'low drag' naca ducts, they can't produce any pressure to drive through the coress. They are good for general cooling ducts though (like cylinder bores, cabin airflow etc) where backpressure is minimal.  

 

[danny_galaga]

45 minutes ago, Blueadventures said:

On my ULS engine oil temp is always higher than the CHT temp. usually about 8 degrees C.  I also have a two position toggle for CHT feed from #2 and #3 heads and these read differing temps (one gauge and I can select temp source).

That's a neat idea! I have one spare switch on my instrument panel, maybe that's what it'll be for 😎

Of course it's designated differently right now 😁

[Blueadventures]

38 minutes ago, danny_galaga said:

That's a neat idea! I have one spare switch on my instrument panel, maybe that's what it'll be for 😎

Of course it's designated differently right now 😁

Switch at top right above gauges.

 

[IBob]

2 hours ago, Blueadventures said:

On my ULS engine oil temp is always higher than the CHT temp. usually about 8 degrees C.  I also have a two position toggle for CHT feed from #2 and #3 heads and these read differing temps (one gauge and I can select temp source).

I have the later model ULS with the CH coolant temp readouts for cylinders 2 and 3.

2 always reads cooler than 3.

But given that 2 is front LH, with the sensor at the front of the head, while 3 is rear RH, with the sensor at the rear of the head, that has never surprised me, and since they are both well below max temp, it has never bothered me.

[facthunter]

Running heads too cool will reduce  efficiency and engine life. Run a car without a thermostat and find out. Now I know the Rotax doesn't HAVE a thermostat but the point is still made.  Nev

[skippydiesel]

50 minutes ago, facthunter said:

Running heads too cool will reduce  efficiency and engine life. Run a car without a thermostat and find out. Now I know the Rotax doesn't HAVE a thermostat but the point is still made.  Nev

My understanding:

 

In general thermostats are not for maintaining a cooling system temperature, more to speed getting to the desired temp, then staying open  ie the cooling system itself should (ideally) run within the required range.

 

Rotax operated in colder climates (than Australia) often have aftermarket thermostats fitted to oil & sometimes coolant systems, specifically to speed warm up, however the system should have some way of restricting air flow, so that once warm it will stay warm ie not reliant on the thermostat(s).

 

"Back in the day", ground vehicles, in colder climates, could be fitted with driver operated radiator blind/shutters, to reduce air flow (I used a sheet of cardboard). The idea being that the driver closed the blind/shutter before engine start. At some stage, after engine start, when the engine temperature gauge showed normal, the blind/shutter may be opened progressively (or not in very cold weather), to maintaining steady temperature condition.

 

It should be remembered ground vehicles commonly have oversized cooling system (unlike liquid cooled aircraft) so that the engine will not cook when run at low speeds/stationary (eg commuting), for extended periods.

 

Aircraft engines generally operate within much less variability, hence Rotax not providing an oil/coolant thermostat. They have a relativly small radiator, often larger than you might expect Oil Cooler, relying on the fan up front (when on the ground) and forward motion when in the air to cool the heat exchangers.

[facthunter]

Nowhere did I suggest a thermostat be part of the solution.. It was just an example of when engines run too cool.. On  road vehicles thermostats DO regulate engine temps  and the needle just sits in the one spot all day if the system is working properly. It does not operate over a range.  In a Plane you try to minimise cooling drag for aerodynamic  efficiency. Always been a great fan  of cowl flaps. Nev

[RFguy]

but a bypass thermostat I think is part of any solution.  and perhaps cowl flaps etc of sorts (IE airflow control) .

[facthunter]

Four separate heads makes things more difficult.  Nev

[RFguy]

I see with the rotax installs front cylinders can get blast cooled while rear get not much airflow .  It's not quite all bad, since hotter cylinders transfer more heat into the water cooled heads. But there is plenty of thermal resistance- the aluminium alloys used  for these applications have relatively low thermal conductivity compared to pure ally (which peolpe look at for reference but it's not applicable) .   A duct really will make a large difference overall - IE ducted inlet over the cylinder bores- avoids flooding and pressurizing the cowling (no good for radiators that are also being vented into the cowling) .  I'd also like to see some blast air cooling around the carb bowls since that sit above the exhuast headers (or some heat shields ) . Skippy - are you taking care of any overflow from the carbies in a good fashion ? floats since, or seats come out, carb floods, fire ensues.
 

[facthunter]

I doubt air on the outside of the water jacket contributes much to the overall cooling As these engines deliver more power extra  heat will be  required to be removed from places like the cylinders. Baffles anyone?  That provides an opportunity to have blast tubes etc working to target areas that need it. The Jacketed parts work on temperature differential with the coolant.  Nev

Edited December 25, 2023 by facthunter
expand

[RFguy]

yeah but the cylinders are not in water jackets

 

[facthunter]

That's why you need to direct the air better around them when the power is upped. Nev