Crank Case Pressure/Vacuum Theory

[retro]

Yes, with a pressurized air supply the motor would run in space. But not if the motor had to pressurize that air.

 

[wcorey]

False.

An increase in piston speed (swept feet per second/minute) will correlate 1:1 with the volume of air entering the crankcase until the RPMs reach a point where the port can no longer flow enough air to keep up with the demand. Beyond that RPM point the peak crankcase pressures after compression will be in decline, due to incomplete filling.

Remember, all pressure levels inside the crankcase are characterized and dependent upon the atmospheric pressure level available outside. You'd need to add a blower (increase pressure) if you want a small port to flow more air above that limiting RPM point.

I could still be wrong but I believe the quote I was responding to referred to the pumping diaphragm in the carb...

 

[retro]

Ya, I caught that after I had already posted. Sorry about that. But the same principle applies to the pump diaphragm as it does ports. At no point will internal pressure on the diaphragm rise as RPMs limits are exceeded. It will fall, because of incomplete filling of the crankcase, causing the pulse signal to decay.

 

[wcorey]

Ya, I caught that after I had already posted. Sorry about that. But the same principle applies to the pump diaphragm as it does ports. At no point will internal pressure on the diaphragm rise as RPMs limits are exceeded. It will fall, because of incomplete filling of the crankcase, causing the pulse signal to decay.

I agree with what you're saying and it's a good point, as well as the post about crank case filling.

However, it doesn't relate to the point I was evidently failing to make.

I meant the result of higher rpm/more pulses per minute would be higher volume of fuel pumped and/or higher fuel pressure, not an increase of the actual force applied by the impulse to the diaphragm.

Although, while we're on the subject..., it would seem that WOT would potentially tend to cause the least amount of vacuum in the cc and therefore result in a stronger pulse...

As long as 'RPM limits are not exceeded', whatever that entails...

 

[retro]

Another little brain squirmy is the fact that our atmospheric air, as we know it, can not exist as we might expect it to in space. Because there is no natural pressure component (or gravity) to organize & stratify the gases precisely, that our natural air consists of. They get air close enough in space for astronauts to survive in, but its not the same stuff as our natural air.

 

[retro]

I agree with what you're saying and it's a good point, as well as the post about crank case filling.

However, it doesn't relate to the point I was evidently failing to make.

I meant the result of higher rpm/more pulses per minute would be higher volume of fuel pumped and/or higher fuel pressure, not an increase of the actual force applied by the impulse to the diaphragm.

Aye yeah, sorry... you are 100% correct!

 

[retro]

Although, while we're on the subject..., it would seem that WOT would potentially tend to cause the least amount of vacuum in the cc and therefore result in a stronger pulse...

As long as 'RPM limits are not exceeded', whatever that entails...

WOT allows the CC to completely fill, up to its designed RPM limit. So the signal in the crankcase cannot be characterized as being a dominantly vacuum one... or a dominantly pressure one... compared against atmospheric at WOT.

All while the CC is completely filling and emptying within its most efficient WOT RPM range, both the low pressure signals and the high pressure signals will be at their peak amplitudes. Fuel pump operation/capacity should be peaking right there too.

While the throttle is closed (or less than WOT) the crankcase can not get completely filled. So both the low pressure signals and the high pressure signals will be weak. The fuel pump operation/capacity will be weak there too.

And I'm sure thats exactly what you just said.

EDIT: I should clarify that at closed or partially open throttle both low and high pressure signals will be very weak, because the entire low/high amplitude range will shift down. Atmospheric pressure maintains its constant against the diaphragm, limiting its stroke while inhibiting all flutter movement.

 

[XP_Slinger]

Youch! It’s too early for my brain to catch up on all that...lol!

Port timing was mentioned a couple times. I’d like to get into this once we figure out this carb stuff. Things like insufficient blowdown causing backstuffing in the transfers, changing intake port duration etc etc.

 

[drf256]

This thread hurts my little brain.

 

[XP_Slinger]

This thread hurts my little brain.

Mine too, I feel as though I’ve created a monster...lol! But, still interesting

 

[Stump Shot]

Hmmm, wonder why then they make auto tune carburators if the atmospheric pressure is a constant???

 

[Stump Shot]

No need to hurt your brains here boys, there's nothing new going on that hasn't been for a long, long time. All we need to know about crankcase pressure is it has to exist in a two stroke for it to work, meaning sealed up tight and not leaking.

 

[Termitebuffet]

.

Sawhawgz, ruining the internet since 2012...

 

[CJ Brown]

You'd find it impossible to operate a chainsaw motor outside of earths atmosphere no matter how much air and fuel you provided, due to the lack of an existing, sufficient pressure.

So, what would happen if you brought along a positive displacement blower and installed it on a chainsaw motor (in an effort to replace your missing atmospheric force) in deep space?

Nothing. The device would require more input power to function than it would output.

A 2 stroke Detroit diesel does not use atmospheric pressure to fill the cylinders - it uses a blower. The energy required to power the blower comes the energy stored in the diesel fuel, and released upon ignition/combustion.

 

[brandonstclair20]

Case in point...if you set the metering lever too high it supplies more fuel than the engine needs at idle causing spit back and it won’t idle. But, if you can get past idle to WOT throttle it will typically clear up and tune properly at max rpm. Something to chew on

Interesting, maybe that’s what’s wrong with an 028 I have that’s doing that exact thing. People said it was a worn out intake skirt on the piston but that does not appear to be the case.



Sent from my iPhone using Tapatalk

 

[Al Smith]

The two cycle Detroit/GMC /Electro Motive and Fairbanks-Morse diesels are often confused with being super charged .They are not .What they are is a scavenging air type engine normally using a roots type positive displacement blower .I think the more modern ones might use exhaust driven turbine blowers . The last big one I was around was a 20 cylinder EMD 5,000 HP .I think it had 5 turbos . It was large .
The idea is as old as the hills .Some of the early oil field pumping engines were two cycle ,used a scavenging cylinder with a piston compressor but they are not like a chainsaw engine using impulse from the crankcase .
The larger pipeline compressors built on the order of a Corliss engine made in the 40's by Cooper -Bessemer in Mt Vernon Ohio are still in service and also used a compressor scavenging cylinder .

 

[CJ Brown]

The 2 stroke Diesels that I familiar with require a blower to pressurize the crankcase to enable the cylinders to fill (positive pressure differential). In a chainsaw the piston(s) performs this function on the up/compression stroke. That same method cannot be used on a multi cylinder 2 stroke diesel using a shared crankcase. I'm not sure how a turbocharger could perform this function at cranking speeds, but I won't say it isn't possible. My original post was mostly to address the assertion that an ICE requires atmospheric pressure to create the pressure differential required to fill a cylinder, and using a blower to perform this function would require more power than the energy produced by the engine. That is not true at all.

 

[RI Chevy]

2 stroke diesel???

 

[Marshy]

This thread hurts my little brain.

Atmospheric barometer says there's a high pressure moving in today. Maybe that's why?

 

[Stump Shot]

2 stroke diesel???

Detroit's, you ever hear one running you will know it by the sound.