Oil Temperature Thermostat

by Paul Winder in 912 / 914 Technical Questions 13 years ago

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Re: Oil Temperature Thermostat

by Paul Winder » 13 years ago

Paul Winder

2 6 0

Thanks, Roger, The Rotax bulletin and video actually inspired my question as I have previously purged the system using these instructions. The video advises extra precaution when purging systems using a thermostat but gives no details. Do you agree with James and Bill that the thermostat bypassses enough oil when cold to purge the cooler side assuming a full cooler with inlet and outlet at the top?

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Re: Oil Temperature Thermostat

by Roger Lee » 13 years ago

Roger Lee

125 5.12k 4.53k

Hi Paul,

I do agree that the thermostat does bypass oil through a small hole all the time. Is it fully enough for a good purge, that's a good question. It should be so long as you don't quit rotating the prop during a purge at the first sign of oil. I have seen some purge with it cold then take a heat gun to it and make it open and then continue the purge.

I have many that have had an oil thermostat and didn't think they did much good. Our oil system is so small that the thermostat would work much better if we had a larger volume system. For me it's harder to work around just to get a minute worth of quicker warm up. I have some clients that didn't like them at all and just removed them.
The oil thermostat max is 180F for the Perma Cool brand which is the most popular. I rarely see below that anyway so I don't personally have one. The 2" wide aluminum tape for the winter puts me up at 190F-210F.

If you like them that's good and if you don't that's okay too.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

Thank you said by: Paul Winder

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Re: Oil Temperature Thermostat

by Mike Brown » 13 years ago

Mike Brown

0 39 11

Thanks, Roger, The Rotax bulletin and video actually inspired my question as I have previously purged the system using these instructions. The video advises extra precaution when purging systems using a thermostat but gives no details. Do you agree with James and Bill that the thermostat bypassses enough oil when cold to purge the cooler side assuming a full cooler with inlet and outlet at the top?

Paul: I have designed an oil thermostat for the Rotax 912 and would be happy to offer my perspective. First of all, the oil thermostat is a bypass thermostat and functions quite differently to the thermostat in the cooling circuit of your car, which is an isolation thermostat. The key point to understand is that when the oil is cold, the center bypass of the oil thermostat is open and all the oil flowing in the oil circuit can bypass the oil cooler. I have attached a graphic to my comment which shows how an oil thermostat works.

Your concern is well-founded as, with an oil thermostat installed, it's theoretically possible for the oil cooler to contain some air even after the oil circuit is purged according to Rotax SI-912-018. The reason for this is that the oil may flow primarily through the oil thermostat center bypass during the purging process.

I suggest the following additional step when purging the oil circuit with an oil thermostat installed: disconnect the oil hose that is at the highest point in the oil circuit (usually the outlet hose from the oil tank) and use a small funnel inserted in the end of the hose to fill the oil cooler, the oil thermostat and all the hoses between the oil pump and the oil tank with oil. Depending on how your oil circuit is configured, you may need to use more than one fill point, but the idea is to fill everything between the tank and the oil pump with oil before you begin the purging process.

Rotax912OilThermostatFunctionality.jpg (You do not have access to download this file.)

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Re: Oil Temperature Thermostat

by Paul Winder » 13 years ago

Paul Winder

2 6 0

Thanks, Mike, the designer's perspective is greatly appreciated. I have seen thermostats advertised at 180F, 190F. and 205F. Do you think the higher range thermostats would tend to raise the temp of the oil when it exceeds the opening temp of the thermostat? If I see 215F. on a hot day with high power settings without a thermostat would I see any highter readings under identical circumstanmces with the 205F. thermostat? My main concern is lower oil pressure readings at anything under 180F. with high power settings apparantly due to oil pump starvation. Maybe the 190F. is a good place to start. Incidently, I am using the semi-synthetic 20/40 Shell oil. Thanks again.

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Re: Oil Temperature Thermostat

by Mike Brown » 13 years ago

Mike Brown

0 39 11

Do you think the higher range thermostats would tend to raise the temp of the oil when it exceeds the opening temp of the thermostat?

Strictly speaking, the oil thermostat does not "open". It's always in one of two states: (1) cold oil, thermostat bypass open and unobstructed flow to oil cooler; (2) hot oil, thermostat bypass closed and unobstructed flow to oil cooler.

To answer your question - the Perma-Cool oil thermostat allows some flow through the bypass even when the bypass is "closed", so the Perma-Cool will allow the oil to get hotter than it would with no oil thermostat installed with high power settings and high air temperatures. However, this situation is unusual for temperate climates and is not really a cause for much concern.

If I see 215F. on a hot day with high power settings without a thermostat would I see any highter readings under identical circumstanmces with the 205F. thermostat?

No, you wouldn't. The nominal operating temperature of the oil thermostat will have no effect on your oil temperature when your oil temperature is above the nominal operating temperature of the thermostat, providing there is no bypass flow. When your oil temperature is above the nominal operating temperature of the oil thermostat, the oil thermostat bypass will be closed and all the oil flow will be through your oil cooler. In this situation your oil temperature is controlled solely by the cooling capacity of your oil cooler. So, your oil will still be at 215F regardless of whether you have a 180, 190 or 205F oil thermostat installed.

My main concern is lower oil pressure readings at anything under 180F. with high power settings apparantly due to oil pump starvation. Maybe the 190F. is a good place to start. Incidently, I am using the semi-synthetic 20/40 Shell oil. Thanks again.

The 190F unit is a good place to start and conforms to Rotax's recommendation in SL-912-011.

I'd like to offer a suggestion. I once heard Phil Lockwood say the number one problem with Rotax 912 installations was the routing of the oil hose at the oil pump. A hose that's installed with too tight a bend radius at the oil pump inlet will partially collapse and cause low oil pressure. Rotax offers a kit to address this potential problem - part number 956 585. You can read more about this in the attached document. I would take a good hard look at your oil circuit. Look for places where the minimum bend radius of your hoses is exceeded. That's more likely to be the cause of your apparent oil pump starvation than is oil viscosity under 180F. I would also add that the oil pressure sender is notoriously finicky and a low oil pressure indication at high power settings may well be an instrumentation problem. I have seen exactly the issue you describe where the problem turned out to be the oil pressure sender.

SL-912-011OilThermostat.pdf (You do not have access to download this file.)

Lockwood-Ideal912Installation-2.pdf (You do not have access to download this file.)

Thank you said by: Paul Winder

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