Rotax 912 high CHT

[BlueSideUp]

Hi all, first post for me

 

We have a Pulsar with a 912 and have issue's with CHT being to high. The engine only has 135 hours on it and has been checked for any issue's by LAME.

 

The oil Temprature is fine at about 90 degress however the CHT is between 112-118 at cruise depending on RPM and load and how it feels on the day !!.

 

The engine has been checked for everything from carby balance to compression etc. and nothing is out of place. New gauge installed and probe checked as well.

 

Changes have been made to radiator posistion to improve airflow and venting to increase low pressure area behind radiator.

 

Coolant has been changed to Evans as well.

 

About now we have just run out of ideas as to what can possible be making temps that high. Particularly when we are told that most people have problems with the engine running to cold.

 

As a last resort we are having the entire aicraft rewired with new aircraft wiring and joiners, regulator etc in the next couple of weeks.

 

If anyone has any suggestions I would love to hear from them. Any suggestions no matter how out there are welcome

 

Edit: It is the 80horsepower 912 BlueSideUp

 

 

[Ed Herring]

Hi BlueSideUp!

 

Welcome to the Forum...! I suspect that if every thing has checked out with the engine it may be simply airflow. You have allready played with the radiator ect to improve airflow and i suspect more work is needed in this area. Is the cowl standard for a Pulsar? The Cowl and Baffles are often critical to CHT.

 

Regards

 

Ed

 

 

[Guest Guest]

There are many others that know much more about this than I do, however you need to realise that the release of air is probably more important than the intake itself. I saw a similar situation on an aircraft recently and the intake was half blocked off and this actually reduced the CHT temperatures because it "balanced" the airflow.

 

There is or was a 912 Pulsar at Lethbridge Vic - I could put you in touch with the owner if you wish.

 

 

[BlueSideUp]

A number of people have looked at the air flow and made suggestions for change. They include builders and LAME's. We have significantly changed the radiator position and increased the vacum pressure behind it a lot. Looking at other aircraft with same engine it would appear that we in fact have better radiator (or at least more prominant) posistion and low pressure area behind. We even built an enclosure for the radiator with dedicated venting but even that made no difference.

 

The radiator is a standard one for the 912 and we have had it checked and flow tested and it appears OK.

 

In fact for all the changes that we have made little change in temp has been achieved.

 

Am about to hook up a spare gauge with its own power supply and a new lead to sensor to see if it is anything to do with wiring or power supply.

 

It is a puzzle thats for sure :yuk:

 

The aircraft is the one from Lethbridge :)

 

 

[Guest Guest]

That's funny... I almost wrote... "there's one at Lethbridge, if in fact that is not your one."

 

I'd be changing over probes and guage. That could be the cause. There are alternatives to the standard CHT too, by placing heat sensitive tape onto the heads that measure the maximum temperature.

 

You could just ignore it and hope it goes away, or perhaps move to Tasmania where it's colder.

 

I know many Skyfoxes and Gazelle's had high temperatures and they introduced a baffle on the top of the engine to promote cooling. I don't think I have any pictures of these though - perhaps Carl Nilsson has one.

 

 

[Captain]

Does a 912 have a thermostat that might need checking?

 

 

[Guest ozzie]

Does the cooling system have a couple of bleed valves to bleed any air out of the engines cooling galleries.

 

If you only have one cht probe can it be moved from cyl to cyl. does the problem follow.

 

To test a thermostat; remove it from the engine and place a piece of cotton thread between the body and the part that opens, place in a can of cold water, place a thermometer in the water and suspend thermostat by cotton thread and heat water, observe the temp were thermostat falls off cotton and check against manufactures specs for opening temp. full open can be checked by looking at it as the water temp increases

 

.

 

Has a gasket been placed upside down and blocked a cooling port? (I did this on a Ford V8 once.)

 

One way to check if the flow around the engine is impaired is to remove the hoses and pour in chilled water and measure the temp around the heads to see if it drops and all read about the same.

 

Timing not to advanced? spark plugs correct heat range?

 

One other problem i had with a hotrod years ago that was running to hot was that the flow rate was too fast as the water moved thru the radiator. If it is flowing too fast it will not have time to offload the heat to the airflow. this is the primary function of a thermostate. to slow the water flow down so as to transfer the heat. It can also affect the reading from the probe

 

You can also try checking the Ohms value of the probe from the gauge end. some one may have shortened the wires or used the wrong type. remove wires from gauge first. then test the gauge. compare with someone elses a/c that uses same type

 

I think that you may just have a faulty probe or gauge. you should also get a corresponding increase in other temps as well.

 

you only mention the cht temp do you have a water temp fitted as well? also do you have a EGT temp gauge. They should all show slightly higher than normal readings.

 

something that may also be worth checking is that in most cars the gauges are actually run on 8 volts not 12. this allows for voltage drop when you have everything on. fans, lights ect. (so the you wont run out of fuel at night) you may have a 12volt gauge that really runs on 8 volts, long shot, but worth checking. what is the aircrafts running voltage. do you have too many toys plugged in? whereare the gauges power supply sourced from, do you have an item that draws lots of power between the gauge and the power supply point?

 

Air flow around cylinders. as mentioned above cowls and baffells must be tight, no leaks. have you noticed how they control the airflow on cessnas with the cowl flaps. they open and close at the exit point not the entry. are you using the same exit point for both the cooling air and the radiator air? try seperating them. one may be affecting the other.

 

Air flow around a aircraft engine is a whole uni course in itself.

 

do you know a commercial air con installer? he will have all the gear to measure airflow inside ducting ect. try and use them. no use guessing what is going on inside that cowl.

 

is the above "out there" enough for you? ozzie

 

 

[BlueSideUp]

Yep Ozzie some of them were "out there"

 

Few things:

 

Rotax 912 does not have a thermostat.

 

Changed probe's between oil and CHT (same probe) with no difference in temps.

 

Gauge and probe are both VDO (put innew gauge as well)

 

Agree that it could be wiring and power supply so plan on running new wire to probe and connecting to gauge along with stand alone power to gauge from gel cell battery.

 

EGT seems OK.

 

So the only temp that seems to be causing a problem is CHT.

 

And yeah Nosmo, Tasmania might be the go ...........lol

 

It is frustrating I can tell you as I am not one to just ignore things and hope they go away.

 

 

[BlueSideUp]

Well after chasing the problem of high CHT for several months the fix was simple.

 

Drained the system of Evans and gave it a good flush. Nothing of any note came out and flow was fine.

 

Replaced Evans with Glycol and demineralised water 50/50 mix and the CHT dropped from 118 degrees to 90 degrees . Test flight was done an hour after flying with the Evans in 25 degree outside air temp.

 

Must admit that I had a lot of trouble believing the gauge to start with. After a further three hours flying it has remained the same so problem solvered .

 

So after a long and frustrating search with many changes including to the cowl and radiator position it would appear that the main problem was with the Evans coolant.

 

 

[Wilfred]

I dont know if this information is of any use to you but with evans cooland the CHT guage will read approx 30 degreesC higher than with Gyycol/water mix. this is i am told due to the better heat conductivity of the evans coolant.

 

The AD from CASA regarding this is at the link below

 

http://www.casa.gov.au/airworth/airwd/adfiles/piston/rotax/rotax-021.pdf#search=%22evans%20coolant%22

 

 

[hiperlight]

Pure water has a higher coefficient of heat conductivity than glycol(either ethylene or propylene) or Evans Coolant.

 

HOWEVER, water boils at 100 celcius and Evans NPG+ boils at 190 celcius.

 

For those interested, read the contents of the Technical Info tab onwww.evanscooling.com

 

Also, there is a recent amendment to the Rotax 912/914 service bulletin dealing with coolant dated August 2006.

 

http://www.rotax-owner.com

 

Bruce hiperlight

 

 

[BlueSideUp]

Yes I understand that Evans has a much higher boiling point. I still don't like running an engine constantly at close to or in the hot range for the engine.

 

Regardless how good Evans is regarding it's boiling point it is not as good at heat transfer as a Glycol/water.

 

Does not matter what is used in the engine hot is hot

 

 

[Ross]

Does the temp probe in the Rotax record the head temp or the temp of the coolant in contact with the head?

 

For the radiator heat transfer rate is proportional to amongst other things the difference in temperature between the coolant in contact withthe radiatorandthe temperature of the air in contact with the radiator.

 

A point was made earlier that Rotax cannot use leaded fuels because the combustion chamber does not get hot enough to oxidze the lead residue and thus get rid of it.

 

Pressurizing the radiator by having higher pressure caps effectively raises the boiling point of whatever is being used for the coolant. This means it is still a liquid and not a gas (steam) which has a lower capacity to absorb heat.It takes 540 Calories of heatper gram of water to turn itinto steam if it is already at the boiling point temperature whatever that may be for the current pressure condition. It takes one calorie of heat to raise the temperature of one gram of water one degree C.

 

So pressurising the radiator allows the radiator to run hotter allowing a better heat transfer from the radiator to the surrounding air. According to my old Physics book you would need a radiator pressurised to 33.6 psi absolute to withstand a water temperature of 125 degrees C without boiling.

 

That is about what a normal car 14 psiradiator cap does (assuming pure water)because it is already pressurised by the atmosphere to about 14.7 psi plus the pressure cap for an extra 14 psi. At 150 degrees C the pressure required is slightly more than double that example. Other coolants would have different saturated vapour pressures (boiling points).

 

Pure water boils at approximately sea level at 100 degrees C and at lower and lower temps at higher altitudes (lower pressures). If you went high enough you could boil a non pressurised radiator (without a cap) dry without starting the motor and getting the water hot.

 

The CSIRO at Hyatt in Melbourne did some research on using ceramics in piston engines some years ago. Ceramics are hard, have a low coefficient of heat transfer and a very high melting point.

 

The idea was to protect the engine structure by stopping or reducing the heat getting to the metal. This has two advantages. It allows more efficient combustion in the combustion chamber because it is hotter and secondly it raises the engineefficiency dramatically by virtually eliminating the loss of energy that occurs by needing to havea large cooling system to remove a large percentage of energy from the engine basically to stop the engine seizing up or melting.

 

Regards Ross

 

 

[BlueSideUp]

As a follow up to the saga of using Evans another interesting fact has emerged in the last few days.

 

Stemme the manufacturers of the Stemme Motor Glider (the current model uses a Rotax 914) have issued an AD that if you are using Evans it must be replaced with a 50/50 coolant water mix.

 

According to the AD it has been found that Evans will burn. Not a nice thought to have an engine fire that gets fed by the engine "coolant" if the pipes rupture .

 

http://ad.easa.europa.eu/page-2/ BlueSideUp

 

 

[Guest ozzie]

Ronnie Smith, South Mississippi Light Aircraft

 

Ronnie talks answers another listener issue about what the oil pressure and temperature readings should be on a Rotax 912. He also has a coolant update for the engine. Ronnie can also answer your four stroke and two stroke ultralight aircraft engine questions. Do you have questions? When the show is live, IM them in on Yahoo Instant Messenger to UFRadio. [email protected] is the address you can use to E-mail those questions in. South Mississippi Light Aircraft (10/10/06)

 

 

 

[WestCoast]

Thank you for that information 'Blueside'........as a user of Evans NPG+ it is of concern and I amcontacting Rotax and Evans to see what their intentions arein relation to the AD.

 

Regards

 

Dave

 

 

[Guest Crezzi]

Rotax have recently amended SB-912-043. The latest revision no longer mandates using NPG. The "old"50/50 water/glycol mix is allowed again provided you change theradiator cap (& possibly install a coolant temperature sensor andcockpit mounted coolant temperature gauge)

 

http://www.auf.asn.au/airworthiness/sb912043914029R1_UL.pdf.

 

 

[WestCoast]

I have only recently changed over to Evans NPG+ and find that it is a huge improvement on the previous 50:50 mix.

 

I would like to continue using NPG+ and I consider that EASA may have over-reacted as there is not much that isn't flammable in an ultra-light aircraft.

 

Rotax are looking into it as are Evans and we will have to wait and see what the outcome is.

 

Regards

 

Dave

 

 

[BlueSideUp]

It will be interesting to see what Rotax decide Dave.

 

My main worry would be that if there is any sort of engine fire then it would get a whole lot worse if the hoses or plastic elbows give way (or more to the point when they do in that situation) as there is no way to stop the "coolant" from continouing to feed the fire.

 

You can turn the fuel off but a couple of litres of Evans would sure keep things warm .

 

CASA may well have an opinion given they issued a directive a while ago for Rotax engines to use Evans and later amended it to allow both Evans and 50/50 coolant.

 

http://www.casa.gov.au/airworth/airwd/adfiles/piston/rotax/rotax-021.pdf

 

 

[Guest coyote2]

CHT

 

Don't worry to much about CHT As Rotax engines have water cooled heads the water temp is what you should be concerned about. 160-180 degrees C is ok any more than that you have a problem. High water temps are probably caused by poor airflow through the radiator or air in the coolant. Been there done that.

 

c2

 

 

[Guest Jacc]

Yep Ozzie some of them were "out there" Few things:

 

Rotax 912 does not have a thermostat.

 

Changed probe's between oil and CHT (same probe) with no difference in temps.

 

Gauge and probe are both VDO (put innew gauge as well)

 

Agree that it could be wiring and power supply so plan on running new wire to probe and connecting to gauge along with stand alone power to gauge from gel cell battery.

 

EGT seems OK.

 

So the only temp that seems to be causing a problem is CHT.

 

And yeah Nosmo, Tasmania might be the go ...........lol

 

It is frustrating I can tell you as I am not one to just ignore things and hope they go away.

Bluesideup

Had the same problem with my trike engine. Not a 912, but a 582. Was reading very high on the water temp. I cannot remember the temp reading but it was well over normal. I could not figure it out as the EGT readings were fine and the motor just did not feel that hot.

 

Changed the sensor and the gauge. No difference. Finally I used a cup with hot water in it and inserted 2 thermometers (1 digital & 1 analogue) took the sensor out of the head and put that in the cup. I found that the gauge was reading 30 degrees high. Finally traced the problem to a bad earth. Fixed that and all was right with the world.

 

Something to look at. At least by comparing temps with a known temp you can maybe prove the sensor and gauge.

 

Cheers

 

John

 

 

[gregrobertson]

CHT of 112-118 deg C is not a problem for a 912 running on Evans NPG coolant. with a 1.2 atmos cap. They run a little warmer on NPG than on the old 50/50 coolant. Check with Bert Flood but I think you will find that Max CHT is 130 deg C. Ideal oil temp should be 100 so that any moisture in the oil boils off. My 912ULS runs at up to 125deg C CHT no problems.

 

Regards Greg