A centrifugal supercharger takes in air along the central axis of its rotating impeller, and the fins use centrifugal force to rapidly accelerate this air radially outward to their outer tips. As the heavy air is slung away from the center of the impeller, a partial vacuum is left behind that draws even more air into the inlet. This high velocity air slams into the wall of the volute and piles up against the air that's already packed into the housing. This packing converts the kinetic energy of the high velocity air into static energy in the form of increased pressure, which is the goal for a supercharger. Compared with a Roots-type blower a centrifugal supercharger does a lot with its single moving part, and it doesn't heat the air nearly as much in the process. The downside, though, is that the kinetic energy imparted to the air varies with the square of the tip velocity. This means that boost falls off very rapidly with decreasing engine speed. (Glass-half-full people might say it rises very quickly with increasing speed.) The full-size Merlin used a variable pitch prop and operated over a modest rpm range which helped mitigate this shortcoming.
With the air-packed volute under pressure, a potential leak is present in the space underneath the impeller. This leak is created by the gap between the bottom edge of the impeller and the floor of the housing which is adjacent to the volute. The solution is a thin stationary ring bolted down to the floor of the housing that closely surrounds the impeller. The air flung off the impeller will then continue across the top surface of this ring before hitting the wall of the volute. This ring is referred to as a 'diffuser section ring' on the supercharger drawing. There was no detail provided on its design other than the assembly drawing showing its cross section. I had previously drilled clearance holes in the front-half housing for its eight 1-72 mounting screws. These holes were located and drilled through the centers of thin cast spokes on the front of the housing, and so their exact locations were not on a perfect bolt circle but were affected by the casting geometry. The first thing I did was measure their actual locations and compare them with my original notes.
Since the measured TIR of the mounted impeller came out to under a thousandth, I planned the gap between the impeller and ring to be .005" in order to add plenty of margin for hole uncertainties. I chucked up a 4" diameter aluminum round in the lathe and turned a portion of its end for the o.d. and i.d. of the ring while machining the workpiece extra long for a couple rings - just in case. The chucked workpiece was moved to the mill so I could drill and tap the array of holes. A witness mark was included to keep track of the orientation of the imperfect bolt circle pattern. After returning the chucked workpiece back to the lathe I parted off a ring using a right-side ground parting tool to minimize the burr on its inside i.d.. The first ring fit as hoped and was installed on the front-half housing using lowstrength thread locker on the mounting screws.
Before finally assembling the supercharger, a few miscellaneous items had to be completed including the shaft keys as well as a nice streamlined spacer and bolt for the top of the impeller. For the time being I decided to not use any sealer between the housing halves since my previous flashlight test before adding the impeller showed no light leakage.
I plan to eventually do some systematic bench testing on the supercharger including an attempt to measure any boost it might possibly make, but this will require some special hardware to properly drive it. Rather than create an elaborate dedicated drive for a one-off test, I'd like to re-purpose at least a portion of what I have to machine for use in the electric starter. I'll let my brain work on this in the background while I continue on with the engine.
I couldn't resist, though, spinning it up by blasting compressed air into the intake. I have no idea how fast I got the impeller spinning, but it screamed like a turbine winding up and took nearly thirty seconds to slow to a stop when the air was removed. While holding the assembly in my hand I could feel no resonances or significant vibration throughout the entire rpm range that I ran it, and so for now I'll assume current impeller balance is 'good enough'. I could feel a lot of air leaving the outlet just after removing the air source, and so I know I at least have a decent diffuser.
After gaining some confidence that it probably wasn't going to blow up in my hand, I made several sustained stress runs lasting up to a minute or so. Neither the sound nor its feel in my hand seemed to change after accumulating some ten minutes or so of run time.
I found some 0W-20 full synthetic oil in a local auto parts store and dropped a dozen drops down the oil intake. The sound of the bearings immediately changed - not necessarily better, just different - and there was an obvious increase in drag. With the same compressed air spin up, the impeller came to rest twice as fast as it did without the oil. I don't know if any of this means oil is good or oil is bad in the ceramic bearings, but the extra drag has to mean more friction and therefore more heat. I left the oil inside the bearing cartridge, but right now I don't have plans to add anymore.
So far, I'd have say this supercharger has probably been the most fun portion of this build. - Terry
With the air-packed volute under pressure, a potential leak is present in the space underneath the impeller. This leak is created by the gap between the bottom edge of the impeller and the floor of the housing which is adjacent to the volute. The solution is a thin stationary ring bolted down to the floor of the housing that closely surrounds the impeller. The air flung off the impeller will then continue across the top surface of this ring before hitting the wall of the volute. This ring is referred to as a 'diffuser section ring' on the supercharger drawing. There was no detail provided on its design other than the assembly drawing showing its cross section. I had previously drilled clearance holes in the front-half housing for its eight 1-72 mounting screws. These holes were located and drilled through the centers of thin cast spokes on the front of the housing, and so their exact locations were not on a perfect bolt circle but were affected by the casting geometry. The first thing I did was measure their actual locations and compare them with my original notes.
Since the measured TIR of the mounted impeller came out to under a thousandth, I planned the gap between the impeller and ring to be .005" in order to add plenty of margin for hole uncertainties. I chucked up a 4" diameter aluminum round in the lathe and turned a portion of its end for the o.d. and i.d. of the ring while machining the workpiece extra long for a couple rings - just in case. The chucked workpiece was moved to the mill so I could drill and tap the array of holes. A witness mark was included to keep track of the orientation of the imperfect bolt circle pattern. After returning the chucked workpiece back to the lathe I parted off a ring using a right-side ground parting tool to minimize the burr on its inside i.d.. The first ring fit as hoped and was installed on the front-half housing using lowstrength thread locker on the mounting screws.
Before finally assembling the supercharger, a few miscellaneous items had to be completed including the shaft keys as well as a nice streamlined spacer and bolt for the top of the impeller. For the time being I decided to not use any sealer between the housing halves since my previous flashlight test before adding the impeller showed no light leakage.
I plan to eventually do some systematic bench testing on the supercharger including an attempt to measure any boost it might possibly make, but this will require some special hardware to properly drive it. Rather than create an elaborate dedicated drive for a one-off test, I'd like to re-purpose at least a portion of what I have to machine for use in the electric starter. I'll let my brain work on this in the background while I continue on with the engine.
I couldn't resist, though, spinning it up by blasting compressed air into the intake. I have no idea how fast I got the impeller spinning, but it screamed like a turbine winding up and took nearly thirty seconds to slow to a stop when the air was removed. While holding the assembly in my hand I could feel no resonances or significant vibration throughout the entire rpm range that I ran it, and so for now I'll assume current impeller balance is 'good enough'. I could feel a lot of air leaving the outlet just after removing the air source, and so I know I at least have a decent diffuser.
After gaining some confidence that it probably wasn't going to blow up in my hand, I made several sustained stress runs lasting up to a minute or so. Neither the sound nor its feel in my hand seemed to change after accumulating some ten minutes or so of run time.
I found some 0W-20 full synthetic oil in a local auto parts store and dropped a dozen drops down the oil intake. The sound of the bearings immediately changed - not necessarily better, just different - and there was an obvious increase in drag. With the same compressed air spin up, the impeller came to rest twice as fast as it did without the oil. I don't know if any of this means oil is good or oil is bad in the ceramic bearings, but the extra drag has to mean more friction and therefore more heat. I left the oil inside the bearing cartridge, but right now I don't have plans to add anymore.
So far, I'd have say this supercharger has probably been the most fun portion of this build. - Terry
