How can something so simple be so complex? The enigma that is the turbo seal ring…
It is the most basic of parts—a steel ring with a rectangular cross-section and an end gap. Sometimes they are referred to as “piston rings”, but there is no piston and it doesn’t go up and down. Or back and forth. They are also often referred to as “oil seals”, but they don’t seal oil. Really. How many turbo engineers have been given assignments to improve an oil seal and spent months fiddling with rings trying to get it to seal oil? End gaps, side gaps, orientation of the end gap, ring tension, width. Even 2 or 3 rings. All you have to do is sit a used turbo vertically for a few hours and you can verify for yourself that the seal ring does not seal oil by the oil puddle in the turbine housing. But it’s not spinning you say! Seals need to be spinning to work correctly! That really makes no difference—the ring still doesn’t seal oil. But that’s not to say that the features for preventing oil leakage are not energized by the turbo operating. It’s just not the seal rings that do it. Although futzing with those parameters won’t make the ring seal, it can make it leak more, which can fool that engineer into thinking they’re making progress when it leaks less. Alas, no. The ring does not seal oil.
What good is an oil seal that doesn’t seal oil? Or put another way, what is the function of the seal ring if it’s not to seal oil? The seal ring’s main function is to reduce blow-by; air from the compressor or exhaust gas from the turbine must be kept from the center housing drain space and the connected crankcase space. High blow-by means crankcase fumes and their organic fraction will be carried to the closed crankcase vent, which is typically sent to the compressor inlet, to be routed back through the engine and burned. Overall, a good idea to burn those. But very bad for all of the surfaces from the compressor inlet to the engine’s intake valves. These wet vapors stick to walls, fouling them and introduce a lot of inefficiency, particularly on the compressor diffuser. That has such a bad impact on fuel economy that very highly efficient air/oil separators must be used. You would be a very rich engineer if you had taken that up when closed crankcase vents became mandatory for diesels. We both missed that boat and now I’m writing blogs and you must be a turbo or engine engineer if you’re reading this. The good thing is we are both likely doing something we have passion for! Well, maybe not much passion for these mundane little rings, but at least engines and turbos. Maybe by the time we get to the end of this blog (if you do), you will have just a little passion for these little buggers as there are a lot of forces in the free-body tug-of-war.
If oil seal rings don’t seal oil, then what seals the oil? Actually, nothing. There is no seal. On either end. But, there are two primary methods to eliminate the leakage of oil. First, don’t let oil reach the seal ring. Second, make sure the pressure gradient is always positive to the drain pressure. If air and exhaust are always flowing into the center housing, oil is pushed back into the housing. High blow-by is an excellent “seal”! So, we see a conundrum here—when we reduce the blow-by, we can often wind up with oil leaks. That puts a premium on the first method to eliminate leaks—don’t let oil get to the seal area.
On the compressor side, this means paying very close attention to the thrust bearing discharge oil. If you can capture 100% of this oil and route it directly to the drain area, you won’t have leaks. “Flingers” are often used on both compressor and turbine seals. In its simplest form a flinger is a spinning disk that has an area of reduced radius on the path from oil to the seal ring. Centrifugal force throws the oil off radially, and oil cannot migrate radially inward by traveling axially along the path to the seal. Or maybe it’s centripetal force that throws the oil off—I could never keep those two straight. But that’s a topic for another blog.
Let’s get back to the seal ring. How does it work? If the seal is pushed up against one side of the groove, then there would be a very good seal. But this never happens and you’ll see why shortly. Theoretically, the only leak path is through the end gap which is typically about 0.1mm.
How often is the seal ring up flush against the face of the groove? The projected area of the seal ring is quite large, so even a side gap of 0.01mm will represent a much larger leak area than the end gap. Let’s start by looking at the forces on the ring. Sorry—this becomes somewhat tedious, but it is the meat and potatoes of understanding seal rings.
1. Ring tension, which holds the ring against the outer bore of the housing.
a. The depth and width of the ring cross-section affects the ring tension.
i. A radially thicker (deeper) ring will affect the stress, especially in installation, so this parameter has limited variability.
ii. A wider ring increases the ring tension without affecting the projected area pressure can act on, which is a big lever. Should we use that lever? We can come back to this point later.
b. Friction in the bore is what resists the ring’s movement axially.
i. Oil (or lack of) in the area will change the coefficient of friction.
ii. High frequency vibration can change the friction from static to dynamic friction. Is this a good thing? We can come back to this later.
2. High pressure (compressor or turbine) pushes the ring inboard.
a. The projected area of the ring converts the pressure to a force.
b. The area that this pressure works on is in close proximity to a rotating surface, the groove. Typically this gap is .05-.08mm, which may reduce the pressure due to Coanda effect pumping.
c. This pressure can also add to the ring tension by acting on the inner surface of the ring. This requires the OD of the ring to have no gap to the bore.
3. The rotating groove stops the ring from being pushed too far inboard. However, this turns into a wear couple, with the net force on the ring towards the bearings loading the ring against the groove.
a. Coefficient of friction is impacted by oil (or lack of).
b. Contact stress is impacted by the cross-section of the ring that is in contact with the groove. This area can be surprisingly small.
c. If the coefficient of friction is high enough during high speed operation, the ring is heated. If the temperature is high enough, the ring loses its temper, loses its tension and proceeds to complete failure quickly. Sometimes this failure looks like there was no ring installed. It’s hiding in the engine oil pan.
4. In the case of a step-bore design, if the step-bore comes into contact with the inboard surface of the ring, it stops the wear and prevents the self-heating ring scenario.
a. If the gap between the ring and the step-bore is too large, then the ring will lose its temper and fail before it reaches the step-bore.
b. If the gap between the ring and the step-bore is too small, the ring can become the thrust bearing when the aero load is pushing the rotor group towards the compressor. This will fail the ring very quickly.
c. The stack-up is critical for the location of the step-bore to the seal ring groove. This is where working with manufacturing engineering on the machining operation sequence, chucking and locating features, pay off.
5. To make things worse, the thrust bearing clearance will add to all of the stack-up dimensions. Usually the clearance is around .05-0.7mm depending on the size of the turbo. That is about the same clearance as the ring to the ring groove. But the forces pushing the rotor group and the forces pushing on a ring can be in opposite directions. The thrust in a turbo is constantly changing with operating points and transients, so the sealing surface of the groove is an axially moving target, especially since it is combined with thermal transients that change the lengths of housings versus shafts.
6. The flatness of the ring is of paramount importance. If the ring is not flat, then it will not be pushed up against one side of the groove—it will act as a compressed spring where the opposite sides of the ring groove constrain the “spring”. Rings can be bent easily during installation and handling. Some rings are wound in a helix on a mandrill. Then they are “straightened” to get the helix angle to zero. Seems a bit dicey to me, but there are a lot of rings made that way. If the ring is not extremely flat, it is just not going to have low blow-by.
7. It is amazing how many problems can be traced back to shaft motion. Here’s another one. If the rotor group is orbiting with sub-synchronous vibration, the ring is constantly being pushed away from the sealing surface—hundreds or thousands of times per second. It is difficult to reduce blow-by if your ring is constantly being pushed around. Some turbo designs have quite high shaft motion at ultra-low speeds. As in turbo speed at engine idle. What harm could that cause?
That’s quite the list for our little seal ring. It makes you wonder how they work as well as they do. If you throw a lot of these forces out of consideration, it would seem you want a seal ring that stays put in the bore. That would be a very wide ring. And being more rigid, it would have a lower variance on flatness. It would have a low probability of being pushed into the side of the groove hard enough to self-heat, lose its temper and wind up in the engine oil pan.
But with shaft motion moving the seal groove in a weird Lissajous dance, pressure against the ring changing and pulsing, thermal transients changing all of the dimensions in an un-harmonious way; maybe you want a very compliant ring? One that gets pushed right back against the groove by the aero pressure. But can it keep up with shaft motion? Maybe not.
And don’t forget—we’re not talking about sealing oil, but reducing blow-by. Because we know that turbo oil seal rings--- don’t. seal. oil.
If you read this far, you might like our training. Unlike this dry treatise, training has lots of charts and graphs. And pictures with lots of arrows going different directions. Maybe a few equations (but only ones you actually would use). You know—all the stuff that engineers love!
Steve