Primarily for general aviation discussion, but other aviation topics are also welcome.
  • 1
  • 3
  • 4
  • 5
  • 6
  • 7
  • 8
User avatar
By bvq
#1787029
Having owned a couple of 912 UL engines over the past 18 years I have come to understand the following:

1. The oil level doesn't always show on the dip stick until it is gurgled.

2. Whilst the 912 doesn't usually burn oil between oil changes, this all changed on my first engine. It would burn about 120ml per hour, so gurgling was the only way of accurately assessing how much oil to add between flights until I finally got fed up and replaced the engine.

3. A 912 will fire if two conditions are satisfied:

a. the high voltage capacitor in the ignition has sufficient charge to generate a spark. This cannot occur immediately; it would take several prop movements at a ‘quicker’ speed causing the flywheel magnets to pass the charging coils a sufficient number of times. There are multiple magnets in the flywheel, and it rotates about 1.5 times faster than the prop.
b. one of the poles of the flywheel passes a trigger coil with sufficient instantaneous speed to generate trigger pulse with sufficient amplitude to trigger the ignition.

4. There is a failure mode of the 912 ignition modules where the kill circuit stops working. This is usually detected when the engine cannot be stopped by switching off both ignition modules. It is however possible that the failure could be temperature related meaning that the failure could be present on a cold engine but not on a hot engine. I have not seen this myself; I have only witnessed this as a hard fault. i.e. it is faulty all the time.

5. Whilst it is unlikely that hand propping a 912 will result in a start it is definitely possible. Take a look at this:
By cmoreflyer
#1787038
The rotax engines are a “dry sump system”
Which means the majority of the oil is stored in a remote sump ie the oil tank.
Which is why the engine has to be gurgled or burped so you can get an accurate reading of the oil level.

Any one who thinks that by rotating the prop a dozen times will remove every last drop of oil from the engine internal surfaces into the oil tank And will cause damage to moving parts clearly has never had an engine apart..

I’ve seen engines being drained overnight tipped every which way to get all the oil out before a rebuild and as soon as you remove the cylinder head out comes a pool of oil..
User avatar
By Miscellaneous
#1787046
cmoreflyer wrote:Any one who thinks that by rotating the prop a dozen times will remove every last drop of oil from the engine internal surfaces into the oil tank And will cause damage to moving parts clearly has never had an engine apart...

Why do you make such a claim? No one has suggested every drop of oil will be removed from the engine's internal surfaces, nor has anyone said certain damage will occur.

Let me explain in a little more detail to help your understanding.

Most pistons, including the Rotax 912 I believe, have three piston rings. The top two are compression rings, their purpose is to 'seal' the cylinder to maximise the pressure in the combustion chamber at the top of the compression stroke, thereafter, on the power stroke, their purpose is to prevent the the burning expanding gases from leaking between the piston and the cylinder wall. The third piston ring is an oil control ring, it's purpose is to control the amount of oil on the cylinder wall, generally it does that by taking away excess oil, after all we don't want a load of oil left on the cylinder walls when ignition takes place. Now, if a cylinder is repeatedly 'scraped' by the oil ring without the oil being replenished by the lubrication system there is potential for the cylinder to become 'dry'. Granted the oil ring is designed to leave sufficient oil for lubrication, however in situations such as described where the engine is being turned and turned and turned without the cylinder walls being re lubricated there is the possibility of too much oil being scraped off the walls. If that does happen and the cylinder is left dryer than it should be then there is potential for the piston rings to scrape the cylinders on start up.

Now how likely is it to be a problem, well that can depend on many factors. But heh, because it it shouldn't be…. Just consider what @ArthurG posts
Just my experience of 300+ hours of 912 operation, still got both arms and all fingers

Long may he keep his arms and fingers, no reason he shouldn't, but then let's consider the reason this thread was started. :wink:

Past performance and performance of others is no guarantee... :wink:

And for the record, I have had an engine or two stripped and rebuilt in my time, although admittedly not recently. Tell me, having misunderstood the technicalities of what was being discussed what qualifies you to assess whether I, or others, have or have not stripped engines?
By cmoreflyer
#1787077
patowalker absolutely if it's students, if it is I had missed that. :thumright:

If it is not students then there is no need to do it. In addition to any risk that does exist it avoids scraping the bores clean immediately prior to start. That's my main reason for not doing it. :D

This a Quote from your earlier post Misc

You have quoted that it will scrape your bores clean immediately prior to start..

If the engine has a good compression ratio it should gurgle within 12 turns of the prop so the chances of the oil scraper ring cleaning the oil from the cylinder wall is minimum.

And if it doesn’t gurgle quickly the compression of the engine is low which usually means the rings are shot.
And don’t worry there will always be tiny amount of oil in the groove of the piston to lubricate the cylinder for the nano second it takes for the oil to reach the cylinder wall upon pushing the starter button.
User avatar
By Miscellaneous
#1787081
cmoreflyer wrote:You have quoted that it will scrape your bores clean immediately prior to start..

Where did I say that? :wink:

I'm afraid you are showing your lack of understanding of the technicalities again. :whistle:
cmoreflyer wrote:And if it doesn’t gurgle quickly the compression of the engine is low which usually means the rings are shot.

The better the compression the better the seal the compression rings are achieving, the less pressure passes the rings to the crankcase to purge the oil. Worn rings allowing the pressure to escape the combustion chamber means lower compression AND greater crankcase pressure to purge.

The fact remains there is no need to purge the engine providing the oil verified in the tank without gurgling is sufficient. Simples. :thumright:
Flyin'Dutch' liked this
User avatar
By RichJordan
#1787090
Apologies if it's already been mentioned in the thread and I've missed it.

Is there another reason to gurgle the 912, that if left for long enough, enough oil could leak into the cylinders to hydrolock, causing damage if it's turned on the starter?

Fairly sure I've been told this by more than one person, so I've believed it, but having read the thread I'm not sure....
User avatar
By carlmeek
#1787094
I’m pretty sure that’s nonsense unless the engine was mounted wonky with a cylinder pointing down!
RichJordan liked this
User avatar
By Flyin'Dutch'
#1787095
Hydrolock. as I understand it, is a feature of radial engines and those with upside-down mounted pistons. Never heard it mention for boxer engines or normal-side-up engines.
By cmoreflyer
#1787103
In addition to any risk that does exist it avoids scraping the bores clean immediately prior to start. That's my main reason for not doing it. :D

This is from your post on top of page 3!
Misc


To get an accurate oil reading it is necessary to gargle the engine..
User avatar
By carlmeek
#1787107
I don’t think an accurate oil reading is generally necessary. We just need to know there is SUFFICIENT. So if it’s just above min, all is good.
Miscellaneous liked this
By patowalker
#1787110
cmoreflyer wrote:If the engine has a good compression ratio it should gurgle within 12 turns of the prop so the chances of the oil scraper ring cleaning the oil from the cylinder wall is minimum.


The time it takes to gurgle is dependent on the difference in height between the sump and the oil tank. Engines with the same compression in different types of aircraft will take a different number of turns to gurgle.
User avatar
By Miscellaneous
#1787112
cmoreflyer wrote:In addition to any risk that does exist it avoids scraping the bores clean immediately prior to start. That's my main reason for not doing it. :D

This is from your post on top of page 3!

Indeed it is, I'll explain that for you too.

Consider; saying not gurgling avoids the risk of the engine starting and causing injury does not mean the engine will start and cause injury if one gurgles. :wink: Same applies re scraping the bores clean really. :D

cmoreflyer wrote:To get an accurate oil reading it is necessary to gargle the engine..

Indeed that's true, as I have said. Now tell me why you need to know exactly how much oil there is before going flying? Is the purpose of checking the oil level not to establish there is sufficient?

If the engine burns a few hundred ml in 25 hrs (as has been suggested) and you verify the oil in the tank is above half way between min and max prior to your 90 min flight, what benefit is there in gurgling?

Happy and safe gurgling. :thumright:
By cmoreflyer
#1787115
There is no internal sump on a rotax engine the dry sump system is the oil tank.

So when you dip the tank for oil all you have checked is the level of oil in the tank and not the whole system.
When you gargle the engine by rotating the prop you are returning the vast majority of the oil to the tank (sump) for a more accurate reading.
User avatar
By lobstaboy
#1787122
Miscellaneous wrote:
cmoreflyer wrote:To get an accurate oil reading it is necessary to gargle the engine..

Indeed that's true, as I have said. Now tell me why you need to know exactly how much oil there is before going flying? Is the purpose of checking the oil level not to establish there is sufficient?

Happy and safe gurgling. :thumright:


I'm sorry Misc, as far as I'm concerned that just won't wash. I DO want to know exactly how much oil, not simply if there is "enough". Why? Because a change in the oil level that is unusual is an early sign of a problem with the engine.
Rotax 912 and 914 engines are superbly reliable and should use very little oil, as we have said. But to get blasé about it and treat it the way you would a car engine may be expensive and/or dangerous.
Nick, ivor.phillips liked this
  • 1
  • 3
  • 4
  • 5
  • 6
  • 7
  • 8