Thermal management on roots type supercharger 3

[yoshimitsuspeed]

Many of my customers run 4AGZE engines that run an old Ogura two lobe roots supercharger.

Many people are running these well outside their original design parameters. At a certain point the lobes hit the housing and it wrecks the supercharger. Whenever I can I try to talk people into going a different direction and use something designed for higher pressure ratios but sometimes people just want to get as much as they can out of the stock unit.
Most discussion is usually focused on cooling the compressor housing but I started wondering if cooling the outside may actually do more harm than good. The temp of the housing will have very little effect on the temp of the lobes. On the other hand if the lobes heat up and expand a lot and the housing is cool and expands less it will be more likely for the lobes to hit each other or the housing. This has me wondering if it may actually be better to coat the housing with a ceramic thermal barrier which would help keep the housing temp more even and closer to the temp of the lobes. If everything heats and expands a similar amount then the gap between the lobes themselves and the lobes and housing should stay closer to nominal shouldn't it?
I know it would be more ideal to work out a way to cool the lobes but that's not going to be a viable option.

 

[1gibson]

Talking about failure from rotor to case contact, how sensitive are these to mounting face flatness, bolt torque, and tightening sequence? On an eaton M90 you can lock the rotors up (or at least cause them to drag) with a cold blower being spun by hand if any of those things is a problem.

You might find it difficult to attribute that type of failure to only high temperatures. Is there room to fit water/air intercoolers on these? If so, more variables.

 

[yoshimitsuspeed]

Tmoose you bring up some good points.
I some more pressing things to take care of but will post up in more detail when I get some time.
You bring up a great point about inspecting wear marks and it so happens that the SC I am working on right now does have more wear than most I have opened up. It also has significantly deeper scoring on the intake side supporting the theory that it has a lot to do with temps.
I have had probably 5 blowers apart. All still functional but all have had some scoring on the lobes and housing.

tbuelna

if you think about surface area the lobes will be exposed to ambient roughtly half the time or half the surface area at any given time.
The housing on the other hand has the entire exterior surface exposed to ambient temps and roughly half the interior surface. On top of that the sides of the lobes and inside the lobes will have some average between inlet and outlet temps. The SC housing will have both sides outside and half the side insides exposed to ambient. This means significantly more than 3/4s of the housing is exposed to ambient temps. This means that the housing will dissapate much more heat than the lobes will. The housing also has significantly more thermal mass so it will take longer to get up to it's peak temp.
These things all support the real world experiences of rotors contacting the housing.

I'll post up some pics and any new findings when I get some time.

 

[tbuelna]

yoshimitsuspeed,

In your OP, you brought up the issue of the two-lobed Rootes type supercharger shown in the picture experiencing contact between the rotor tips and inner housing surface when operated to produce higher manifold pressure levels than it was originally designed for. In my opinion, the rotor tip/housing contact is likely caused by greater distortions in both the rotors and housing due to a combination of increased heat transfer rates and higher pressure loading.

With regards to simple TE changes due to higher charge air temps, what matters is the increase in temperature difference between the rotor and housing. If both components are constructed of a similar metal they should have similar linear TE rates, and if simple TE is causing the rotor to grow more than the housing ID then it would be logical to assume the rotor temp is rising more than the housing temp.

However, with distortion/shape changes in the rotor and housing due to temperature there are other important factors to consider, such as uneven temperatures within the part body and how the part is structurally constrained. The situation with the rotor is fairly straightforward. The rotating rotor faces are constantly exposed to alternating high/low air temps so the rotor body should have a uniform temperature. The rotor body also has only one structural constraint at its axis of rotation, so the part body is free to expand with temp increases and does not experience strain distortions. On the other hand, the housing has an uneven temperature distribution between the inlet and discharge faces. The housing is also more structurally constrained (mounting points, seal interfaces, etc), which can add to distortions in the housing from thermal expansion strain.

Lastly, there is the issue of greater structural deflections caused by higher pressure forces on the housing and rotors faces. The situation with a two-lobed Rootes rotor at high pressure delta can be tricky. Consider just how much net pressure force there is acting on the rotor. Even a medium displacement rotor at 1.5bar boost levels can experience close to half a ton of bending force. The increased pressure loads can also cause serious problems with the bearings supporting the rotors.

 

[yoshimitsuspeed]

tbuelna
That is why I have been wondering about insulating the outside of the housing. This should keep more heat in and keep it more evenly distributed throughout the housing.
These blowers have a gap on the outlet side where the lobe gets further away from the compressor housing just before the lobe passes the port. The only reason I can think of this is to allow the pressure a little more time to equalize before it opens completely to the port. This also means there is enough room for a coating here without concern of interference so I have definitely decided I will coat this area with thermal barrier. This will reduce the amount of heat going into the hottest area of the housing.
I think I will do this as well as thermal barrier on the outside of the housing. These two things combined should help even out the temp throughout the housing and hopefully keep the housing temps more even as well as increasing the gap between the lobes and the housing.
I will also apply a dry film lubricant to the inside of the housing and the lobes to give it a little more gall and gouge resistance.

Beyond that I definitely agree that pre compressor injection will be the best solution for longevity.
My buddy was running his twincharged with the turbo feeding the SC and the SC running a pretty high pressure ratio with no intercooler between the turbo and SC but he was running Methanol pre SC and his housing and lobes looked great when I inspected them.

 

[yoshimitsuspeed]

I'm curious where you get your numbers for the forces applied to the SC.
These rotors are probably about 25 square in across the broad face. At say 15 PSIG even when the rotor is straight up and down taking the majority of the force that would still only equate to 375 lbs of force being applied to it. As the rotors spin they will alternate between that and much less load when they are in different areas of their rotation.

 

[tbuelna]

yoshimitsuspeed-

I have no idea what the projected area (LxD) of your Rootes supercharger rotor is, nor do I know what the pressure delta across the rotor is. I was simply providing an example of the influence of bending force created by the pressure delta across the rotor.

One other issue I would point out with regards to TE of the rotors causing contact with the housing ID surface due to higher operating temps, you should also see contact between the tips of each rotor and the root surface of the mating rotor, since the radial clearance between these surfaces should be similar to that between the rotor tips and inner housing surface.

 

[bentwings2]

I have had Roots blown cars for many years. My current streetrod is a 41 Willys with a SBC 355 cu in with a 6-71 GMC blower. this car has over 25k miles on it now. It has been driven n weather from -5 F to 103 F (record high for the year). Needless t say that heat and AC were on full as required.

In building this one I looked at various coatings for the blower as well as the typical highly polished variants. I saw 3 polished blowers fail due to insufficient end clearance at the rear of the rotors and one fail due to case to rotor clearance. This prompted me to try to measure at least outside case temps. Very few of us have manifold temp gages so I'm lacking on this. Even the typical heat gun is not very accurate however at least it indicates considerable difference in temp from polished to natural finish. I elected to build my 6-71 with a sandblast finish and just live with the scruffy looks. You can't see it anyway unless I open the hood. Only the scoop shows. I've seen almost 100 degrees difference in case temp between mine and similar blower cars at some of the events I've attended. 2 750 carbs flush plenty of fuel thru it but I still get 14-15 mpg cruising.

Last summer I removed the blower to replace intake gaskets and inspected the inside of the blower. There are no marks anywhere that were not there on the original build. The blower is quite loose and is quite easily spun by hand. It will give 8 psi boost at very low rpm. About 3000 rpm. The water/alcohol injection seems to work very well controlling detonation. You can only step on this one about 5-6 seconds before vastly exceeding the highest speed limits so it is just an image car. I can't afford a ticket so I'm pretty careful with it. I drive it daily in all weather 3 seasons here in Minn.

You may just forgo coating the outside of the case and just sandblast it.

As a side note I tested my headers before and after Jet HOT coating. 850+ uncoated and 350 coated. It made a huge difference in under hood temps. A real plus is the headers do not rust at all.

 

[IceStationZebra]

BW2 - My buddy calls IR guns "random number generators" for a reason! You will not get a good number from a polished/plated surface unless it can be recalibrated for the proper emissivity. One trick around this is to put a piece of tape on the part - then you will be reading the tape's temp which should be almost the same. Rubber and painted surfaces are fine with the default calibration unless you have metal flake. ISZ

 

[Tmoose]

Hi IceStationZebra,

"My buddy calls IR guns "random number generators" ". That's pretty good. May I use it in future? I'll try to remember to give credit ech time.

Dan T

 

[pontiacjack]

Water/methanol injection ahead of the blower is the best of all worlds- even for an extreme-output port-fuel-injected engine. And there are now a few systems on the market which make installation seamless- with sophisticated control and metering based on load/temperature/RPM.