Cylinder Head Temps not even Rotax 912ULS

[BirdDog]

Hi all,

 

I have a Rotax 912ULS with just over 400 hours on it and I noticed something today that I have not noticed before.  (this was on the ground at around 2300 RPM, after just landing from an hour of flying) The right side CHT would creep up to just below the Yellow Zone (see attached pic) about 117 degrees C.  It would sit there for a 20 seconds, and then it would drop back to down to be equal with the other one. 

 

The EGT are all stable etc, so has me beat.

Thoughts?

 

 

 

 

 

[skippydiesel]

Faulty sensor??? Faulty wiring/connector???

 

We all tend to treat our various engine condition read outs as accurate - at best the automotive type sensors & gauges (if steam) are indicators ie not precise. Faults at both the sensor end and  head (gauge) end, are very common.

[BirdDog]

Yeah - don't know!  I will pay closer attention to it next time, but it did appear like it was say when the thermostat would kick in, and the engine would cool, then it would close, and it would heat up again.  If you know what I mean.  But weird that only on one side.  I need to look at where this sensor is!  I have Left and Right!  So is it one Cyl on the left and one of the right?  

[skippydiesel]

27 minutes ago, BirdDog said:

Yeah - don't know!  I will pay closer attention to it next time, but it did appear like it was say when the thermostat would kick in, and the engine would cool, then it would close, and it would heat up again.  If you know what I mean.  But weird that only on one side.  I need to look at where this sensor is!  I have Left and Right!  So is it one Cyl on the left and one of the right?  

If its a Rotax 912 you will have left & right cylinders - CHT is not coolant temperature (although pretty closely associated) - You have stated that EGT's are even -- suggesting same combustion heat on each side. So I still think sensor or wire.

I always advise going with the simplest lowest cost option first - so check your wiring make sure insulation is good and all connections firm.

If that doesnt fix the problem, try swapping CHT sensors and see if the problems moves with the sensor.

[BirdDog]

Thanks Skippy.

 

So I take it only one cylinder on each side is monitored?

 

[IBob]

The earlier 912s had CHT sensors (on the underside of the heads), the later ones are not CHT but coolant temperature (sensors on the top side of the heads).

The cylinders monitored are 2 & 3, which is front left and rear right.

My 912 has the coolant temp sensor, and L appears to run a bit cooler than right, which makes sense as the L is a front cyl and the R is a rear cyl.

 

[IBob]

So far as I know, the 912 does not have a coolant thermostat.

Have you checked your coolant level? There is a little window in the coolant reservoir on the top of the engine, where all the hoses run to. With the engine cold, you should be able to see a coolant level there.

[facthunter]

That type of response is as if there is a local hotspot or airpocket. Nev

[skippydiesel]

3 hours ago, BirdDog said:

Thanks Skippy.

 

So I take it only one cylinder on each side is monitored?

 

In evry set up I have seen only one cylinder per side is monitored - usually a front/rear diagonal.

[skippydiesel]

59 minutes ago, IBob said:

So far as I know, the 912 does not have a coolant thermostat.

Have you checked your coolant level? There is a little window in the coolant reservoir on the top of the engine, where all the hoses run to. With the engine cold, you should be able to see a coolant level there.

Coolant thermostats are available - just Google to see the range 

[skippydiesel]

1 hour ago, IBob said:

So far as I know, the 912 does not have a coolant thermostat.

Have you checked your coolant level? There is a little window in the coolant reservoir on the top of the engine, where all the hoses run to. With the engine cold, you should be able to see a coolant level there.

I Bob - I think you will find they are are CHT sensors, not coolant. You can purchase a coolant sensor kit - usually located where you expect Max temperature to be  - close to radiator inlet point.

 

Cant say I am familiar with the most recent Rotax 912 installations or with the multitude of variations available, but every one I have seen has no " little window". There is the coolant expansion/overflow tank, normally translucent so you can get an indication of coolant level by looking at this. For a slightly more definitive check you must remove the "radiator cap"  - should be full to the brim (no air gap)

Edited May 17, 2021 by skippydiesel

[facthunter]

If you remove a sensor and coolant runs out you are measuring coolant temp (in principle) Just what the temp difference would be is a bit of a guess.. Nev

[BirdDog]

1 hour ago, IBob said:

So far as I know, the 912 does not have a coolant thermostat.

Have you checked your coolant level? There is a little window in the coolant reservoir on the top of the engine, where all the hoses run to. With the engine cold, you should be able to see a coolant level there.

Hmm... I thought there was - or is it oil?

 

 

[IBob]

16 minutes ago, skippydiesel said:

I Bob - I think you will find they are are CHT sensors, not coolant. You can purchase a coolant sensor kit - usually located where you expect Max temperature to be  - close to radiator inlet point.

 

Cant say I am familiar with the most recent Rotax 912 installations or with the multitude of variations available, but every one I have seen has no " little window". There is the coolant expansion/overflow tank, normally translucent so you can get an indication of coolant level by looking at this. For a slightly more definitive check you must remove the "radiator cap"  - should be full to the brim (no air gap)

Hi Skippy. Rotax moved from monitoring CHT to monitoring coolant a while back (2015?). As noted, the difference is in the heads: you either have sensors on the bottom (CHT) or top (coolant). You don't get to choose (unless you want to swap heads).
All the recent 912 installations I have seen have the little sight glass in the front of the coolant reservoir.
You can't check coolant level by looking at the overflow bottle: in the event of a coolant leak (for instance) you are quite likely to have good level in the bottle, but a lot of air in the coolant system.

[IBob]

12 minutes ago, BirdDog said:

Hmm... I thought there was - or is it oil?

 

 

It's more usual to see a thermostat on the oil system, though I don't know if this is now a factory standard or option. Anyone?

[skippydiesel]

1 hour ago, IBob said:

Hi Skippy. Rotax moved from monitoring CHT to monitoring coolant a while back (2015?). As noted, the difference is in the heads: you either have sensors on the bottom (CHT) or top (coolant). You don't get to choose (unless you want to swap heads).
All the recent 912 installations I have seen have the little sight glass in the front of the coolant reservoir.
You can't check coolant level by looking at the overflow bottle: in the event of a coolant leak (for instance) you are quite likely to have good level in the bottle, but a lot of air in the coolant system.

Fair comment - thanks for the update.

 

As for the overflow tank - that's why I said indication.  It a pretty good indication, because if you have a significant leak you should see evidence of this (coolant residue all over the place) where a small leak/weep will give you a progressive drop in the cold reading/inspection of the reservoir/overflow tank (always fill to a known level so that any drop is obvious).

 

To be absolutely sure , your little window in the top thingy or removing the radiator cap from the same, will certainly give you a clear understanding of where your coolant level is.

 

Just remember - like your oil level, coolant level should be checked when engine is stony cold.

[skippydiesel]

1 hour ago, BirdDog said:

Hmm... I thought there was - or is it oil?

 

 

You can have a "thermostat" one one or both - common in cold climates to aid in warm up and I suppose if cold enough maintaining a constant engine temperature. Anecdotally, the oil thermostat is the most common.

 

The cheapest way of addressing cold weather running is to to block of sections of radiator/oil cooler (very common "fix") - down side risk,  is that you may overdo it and run your engine very hot.

[IBob]

Hi Skippy, I don't disagree, but I think it's potentially dangerous to suggest using the overflow as a coolant indicator.
Someone posted a link to footage of an EFATO here recently........was a Drifter or something similar, I think, Rotax 2-stroke....and some eejit had piped the overflow into the top of the bottle instead of the bottom. I expect that setup showed plenty in the overflow, as the engine progressively took in more air from flight to flight until the cooling failed entirely...

[skippydiesel]

9 minutes ago, IBob said:

Hi Skippy, I don't disagree, but I think it's potentially dangerous to suggest using the overflow as a coolant indicator.
Someone posted a link to footage of an EFATO here recently........was a Drifter or something similar, I think, Rotax 2-stroke....and some eejit had piped the overflow into the top of the bottle instead of the bottom. I expect that setup showed plenty in the overflow, as the engine progressively took in more air from flight to flight until the cooling failed entirely...

Cant account for such errors.

[Kyle Communications]

I fitted a Thermobob water thermostat on The Girfriend...It worked very well but you couldnt see anything other than the warm up time was halved

 

[facthunter]

Yes, your pipe into the overflow bottle has to be going right to the bottom.  I'm not convinced that a normally operating motor should vent into the bottle at all. Your pressure cap should seal to allow the temp of the coolant to be above 100 C.  At altitude the pressure cap won't achieve the temp as the pressure is differential and only maintains  it's pressure above outside pressure. Water boils at a lower temp as the pressure drops. Nev

[IBob]

Nev, the coefficient of expansion of the coolant  will be much higher than the coefficient of expansion of the metal of the engine. So when engine and coolant heat up, the coolant  increases in volume more than the galleries in the engine, and since it is not compressible, has to vent somewhere.

 

Same with a hot water boiler, they have an expansion vessel, usually sized 2 x 4% of the system volume, 4% being the expansion of water from approx 4deg to boiling. Without that you'd lift the safeties on the boiler each time it fired.

 

If the Rotax had no venting, the damage would probably be to the hoses. What happens instead is that the level in the overflow bottle rises each time the engine is run up to temperature, and falls each time it cools. Or that's what mine has done, since new. I know this because initially I had a self-inflicted leak that took a lot of fixing (the engine, not me) and have watched my coolant level carefully ever since.

[pluessy]

My Tecnam has a very small overflow bottle (200ml) and I get a 10-15mm level difference between summer and winter (engine at ambient, temp around 20deg different). Very important to know your cold (ambient) level.

Older 912s don't have the "window" so if in doubt, you need to remove the radiator cap to ensure the coolant level is at the neck.

The fluctuating gauge reading could indicate an air lock/bubble, faulty wiring or sensors usually result in fast movements of the gauge. Being an EMS, that will depend on the dampening factors.

[skippydiesel]

2 hours ago, IBob said:

Nev, the coefficient of expansion of the coolant  will be much higher than the coefficient of expansion of the metal of the engine. So when engine and coolant heat up, the coolant  increases in volume more than the galleries in the engine, and since it is not compressible, has to vent somewhere.

 

Same with a hot water boiler, they have an expansion vessel, usually sized 2 x 4% of the system volume, 4% being the expansion of water from approx 4deg to boiling. Without that you'd lift the safeties on the boiler each time it fired.

 

If the Rotax had no venting, the damage would probably be to the hoses. What happens instead is that the level in the overflow bottle rises each time the engine is run up to temperature, and falls each time it cools. Or that's what mine has done, since new. I know this because initially I had a self-inflicted leak that took a lot of fixing (the engine, not me) and have watched my coolant level carefully ever since.

Back in the "good old days" of automotive cooling, best  practise was to fill the radiator to just above the core,  leaving the bulk of the  "top tank" volume for coolant expansion.

This worked, sort of okay (excess fluid being jettisoned by the radiator cap "lifting" due to pressure).

The down sides to this concept was, a heavy and bulky system, that when  cooled, went into considerable negative pressure (vacuum) potentially collapsing radiator hoses (particularly the lower one). The fix was to have a brass spring inside the bottom hose or a metal reinforced hose - not great.

As far as I know all "modern" automotive (& Rotax 91's) have the expansion tank system where the excess hot coolant is allowed to escape the system, be contained (in the expansion tank) and return as the system cools - a two way pressure cap facilitates this flow.

The main benefits  are a more efficient cooling system (it should always be completely full of coolant) facilitating a smaller lighter cooling system  and very low negative pressures - no collapsed hoses

Edited May 17, 2021 by skippydiesel

[Thruster88]

9 hours ago, facthunter said:

Yes, your pipe into the overflow bottle has to be going right to the bottom.  I'm not convinced that a normally operating motor should vent into the bottle at all. Your pressure cap should seal to allow the temp of the coolant to be above 100 C.  At altitude the pressure cap won't achieve the temp as the pressure is differential and only maintains  it's pressure above outside pressure. Water boils at a lower temp as the pressure drops. Nev

Service ceiling is 23,000 feet for the rotax 915. How high do we need to go.