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Old 07-31-2009, 11:32 AM   #21 (permalink)
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umm....

wouldn't the same vacuum "helping" the down-traveling pistons be a hinderance on the up-traveling pistons?

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Old 07-31-2009, 01:15 PM   #22 (permalink)
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Vacuum and pressure in the block really doesn't affect the pistons' travel at all, actually...

I say this because the engine is designed to run in a balanced configuration, so that while one piston is going down, another matches it's speed on the upstroke. The result is that all the air in the block gets moved around alot, but the pressure doesn't change (other than in minute variations) inside the block as a result of piston travel.

The primary benefit of this is that when you force something into a vacuum, it just goes easier than if you force it into 2 atmospheres of pressure. I.E. The pistons are affected by the density of the air inside the block, not the pressure. Vacuum has much less density... in fact, if you pull true vacuum, there is no density... that's the definition of vacuum in this case. A complete lack of atmospheric density.

So instead of the piston fighting to move air around (not compress it), it's just going up and down in what is essentially a lack of atmosphere, which is much easier, and accounts for a reduction in pumping losses.
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Old 07-31-2009, 01:22 PM   #23 (permalink)
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ah, makes perfect sense there - you're just not churning the fluid (air) back and forth between up piston and down piston if there's no air to churn.
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Old 07-31-2009, 01:27 PM   #24 (permalink)
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Precisely.

Also, the vacuum is trying to pull air down past the compression rings, so they seal better against the cylinder walls, meaning less opportunity for "blow-by" in either direction. Less combustion gasses leaking into the oil, less oil in the combustion chamber, less compresssion loss before combustion, etc.

So basically, it's a combination of small boosts that end up being a decent boost in HP, and, at least in theory, economy.
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Old 07-31-2009, 02:03 PM   #25 (permalink)
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Quote:
Vacuum and pressure in the block really doesn't affect the pistons' travel at all, actually...

What about a single?

Actually there has been quite a bit of work done on trying to reduce pumping losses on a single in the motorcycle world.

Also, there is a fairly marked variance in pressure in the crankcase at higher rpms. Enough so that one or even two quarts of oil are actively drawn into the vortex created by the pressure differential. There are also serious differences in the paths the pumped air needs to take. Compare the pattern of a 180 degree straight four to a 120 degree straight six. Probably the boxer configuration is best to minimize pumping losses. Look at some of the designs with knife and fork connecting rods and disk bearings (fairly low rpm engines).
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Old 07-31-2009, 02:10 PM   #26 (permalink)
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Single - drill a hole in the block and put a bypass filter on it.

The engine in my minivan has been known to pull so much vacuum under load that it "misplaces" the spark plug tube seals at high RPM extended periods. In this case, the PCV system of my engine is a "performance enhancer" in that it's large enough to allow the intake's vacuum to affect internal pressures.

It sucks, literally, because I get oil spray if I start revving the hell out of it though, which is obvious b/c of the smoke in the exhaust.

My old Honda also used a vacuum PCV system, employing a venturi in the intake stream to draw gasses out. Not enough vacuum to pull oil, other than vapor, though.

I can't remember what my racing tractor's engine has, but it will probably end up getting changed, since it's a splash oiling system, and I plan on running it at nearly full throttle most of the time.
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Old 07-31-2009, 02:19 PM   #27 (permalink)
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Quote:
Originally Posted by Christ View Post
Single - drill a hole in the block and put a bypass filter on it.
But what size hole?

Lots of thought has been devoted to that. You have to avoid having the system do mork work than necessary.

The danger with intake PCV (and exhaust extractor systems) is the variability of the vacuum.
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Old 07-31-2009, 02:19 PM   #28 (permalink)
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The crankcase of a single can be regarded as an air spring for the piston. It is pretty efficient at any pressure, but with lower windage losses when run at low pressure. To recover throttle plate losses, replace it with a turbine and bypass arrangement, or a variable pitch turbine.
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Old 07-31-2009, 02:28 PM   #29 (permalink)
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I was joking about the single. I don't work with those too much. Apparently, my joke struck on something that's actually workable, which wasn't my intent.

The one time I worked with a single, it wasn't imperative that it run at high power, and it was a diesel cycle engine. The intake air was drawn through the block, into a small supercharger, then into the cylinder. The exhaust was also routed through the block, to maintain increased combustion temps. It's a project I was working on with a shop teacher (not mine) to demonstrate heat effects on diesel combustion. The engine was only about 30 CC, something he'd made in his days as a casting mill operator, I guess.

What we found out was basically that while it was a good idea to keep the fuel hot and keep the block as hot as possible, the thing DID NOT like hot intake air, at all. The hotter the air got, the less power the engine was capable of producing, and the more fuel it required to produce the same power.

Since RPM remained relatively constant, the shop teacher could test the torque output at that speed by just measuring how much force in opposition it took to make the engine speed vary to the point where it wouldn't pick up speed again. Fairly inaccurate, but it worked for what we were playing with.

We actually found out that to some given extent, it is more efficient to have the coldest air possible going into the engine, and running extremely lean, than to run stoich with hot air. It's been almost 15 years since I was working on this, but if I recall correctly, we made it all the way down to something like 45:1 average AFR before it grenaded, with intake temps as low as 60*F.
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Last edited by Christ; 07-31-2009 at 02:36 PM..
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Old 07-31-2009, 02:51 PM   #30 (permalink)
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The above is true about less windage and better ringseal, but the primary benefits are canceling the forces acting on the pistons themselves.

ON your typical 4 cylinder you'll have one piston continuously exposed to the atmosphere (or lack of), give or take given the intake valve duration, but for simplicity sake suppose the intake valve duration is 180* and omit scavenging, resonances, ect for a moment since they are of a minor influence to the subject.

Imagine each piston in the dead middle of its stroke, and firing order of 1,3,4,2



cyl# ->: ------- 1-----2-----3-----4



90* ----------- INT - COM - EXH - PWR

270*---------- COM - PWR - INT - EXH

450*---------- PWR - EXH - COM - INT

630*---------- EXH - INT - PWR - COM


This is what I sketched up quick. 2 cylinders cancel out the other 2 in the sealed crankcase and the absolute pressure remains the same down there (omitting blowby), BUT the force acting on the top side of the piston is a different story.

Put it to the test: Balance the ambient pressure in the crankcase (if it's vented to an ambient pressure source) by opening the throttle to 100%. Drive at certain speed and at specific landmark, cut the ignition then floor it (this is with a manual trans btw) and at another landmark a few seconds later check your speed. Repeat this again at the same landmarks and same speed, but with the throttle closed. I'll bet your final speed is quite a few mph slower.

So the same thing can be achieved by balancing the absolute pressure in the crankcase to the Intake manifold. The limit is going to be a safety limit at a debatable 10-15 in/hg. Thus, if you cruise at a 15 in/hg of intake manifold pressure, you can have 15 in/hg crankcase pressure.

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