patman

iTrader: (3)

Hi guys,

Quick question here:
One of the often cited reasons for engine speed limitations is flexing of the eccentric shaft due to the weight of the rotors. There are even lightweight rotors on the market that are alleged to increase maximum safe operating speed.

I have been looking into titanium rotors, and critically evaluating the effect of this on maximum engine speed. This brings up an interesting point: the lateral loading on the eshaft due to centripetal force is significantly less than the loading that results from combustion pressure. Also, cylinder pressure is relatively independent of engine speed, whereas centripetal force increases with speed. More importantly, these forces are in generally opposite directions.

In other words, if you have 10,000lb of peak pressure force on the rotor, at about a 30* angle to the peak eccentric, and 1000 lb of force on the eshaft, at 180 degrees to the peak eccentric, at 6000RPM, the resultant force on the eshaft is ~7660lbs. If at 7000RPM the pressure force is the same, but the centripetal force is now 1500lb, then the resultant lateral force DECREASES to 7160 lbs. (obviously made up numbers, but roughly the proper order of magnitude)

Using this logic, unless you increase engine speed by a HUGE amount, lightening the rotor should actually be WORSE for eccentric shaft deflection.

This is counter-intuitive for me, and I wonder if I am missing something. Note that in this case, the engine in question does NOT have an unsupported center section. Is a lighter rotor only applicable to 2-rotor engines without a center bearing? Or am I missing a more basic fact here somewhere?

Also, does anyone have a picture of a rotor housing that has been damaged by rotor-to housing contact from overspeed/eshaft flex? Mostly I am interested in where around the housing does the rotor touch?

Someone straighten me out on this! (or corroborate my observation, but I hope not)

Thanks,
Pat

C. Ludwig

iTrader: (27)

Very interesting point you make. Honestly, I have no real idea. But off the top of my head it would seem to me you looking at things along a singular, linear axis. When in fact a good deal of the mass of the rotor is moving along a different tangent than the mass of the e-shaft. So the big tip of the rotor that is directly opposite the combustion face that will move in a direction that doesn't directly oppose the force of combustion probably needs to be figured into your equation. Just guessing...

Also, with a piston engine, the greatest rod loading is during the exhaust phase when you don't have the combustion pressure creating an equalizing force. Again guessing, you'd have the same situation with the rotary when the e-shaft lobe swings to the intake side of the engine and combustion pressure is pushing it that way. That's where you'll find the greatest load on the shaft and a heavier rotor will increase the load.

patman

iTrader: (3)

Actually, I found a reference in Yamamoto about this. Should have looked there to start with. For Mazda geometry, the crossover point at which centripetal force=pressure force axial component is roughly 6k RPM. Above this speed, eccentric shaft loading increases exponentially with speed. Lightening the rotor therefore is of large potential benefit for us.

The confusion is because I am not working with Mazda parts, therefore for me the failure modes do not occur at anywhere near the same speeds.

FYI page 28 of Yamamoto if you have it.

j9fd3s

iTrader: (3)

some of the newer papers mention seal speeds as well.

Sgt.Stinkfist

iTrader: (6)

as well as lightening the E-shaft lobes, counter weights and seals. You should check out some of Jim Downings "trimmed" e-shafts and lightened rotors