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  #1  
Old 04-17-2004, 09:44 AM
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NA or SC ... is it that simple ?

I have read ALL the posts in both the "500 HP" thread and the newly closed one with Rob throwing words wildly ....and now I need a BEER ( or 2 ) ...

What hit me was a slight philosofic question ..... everybody talks about NA and SC as that was something clear and absolute ..... is it really?

I mean, what IS really NA? ... is NOS a NA system? .... is a "resonance" system NA ? , a resonance system works very similar to SC in the "resonated" area!

In my Polaris (800 VES) snowmobile, there is a "resonated" intake system, if a just open a millimeter in the intake box ("killing" the resonating pulse amplitude) , I loose 10-15km/h in topspeed, going down from 175km/h to about 160-165km/h !!

Even a 2-stroke exhaust "pipe" works like an SC , making it possible to charge more air/fuel into the cylinders that an "normal" atmospheric pressure can do.

So in theory (AND in practice) it WOULD be possible to reach way over the magical (B)MEP figure (14) that was discussed in those threads as an unbeatable and absolute maximum value.

PS. -why (B)MEP, because I have sometimes learned, and always talked about BreakMeanEffectivePressure BMEP, and not just MEP.

-Soooo, is it so simple to tell what is NA and what is SC then ... ??

PS:2, Sorry if my spelling and use of terms are bad, I hope it is understandable anyway


/Sonny *wondering*
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Old 04-17-2004, 12:05 PM
Chicane Chicane is offline
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NA= naturally aspirated.

Using nitrous is NOT naturally aspirated. Nitrous is basically like adding a ton of oxygen to your car's intake charge (thus why you need bigger fuel injectors with nitrous). Hence, you're shoving more fuel and more oxygen into your engine- this is why it's NOT classified as naturally aspirated.

As for SC... that's ONLY forced induction. I believe what you were looking for was FI, or forced induction. With forced induction, be it turbo, supercharger, or ... KIND OF nitrous, that handy mep equation goes out the window. If I didn't answer everything, please let me know.

- Rob
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Old 04-17-2004, 12:09 PM
Chicane Chicane is offline
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Re: NA or SC ... is it that simple ?

Quote:
Originally posted by Sonar
Even a 2-stroke exhaust "pipe" works like an SC , making it possible to charge more air/fuel into the cylinders that an "normal" atmospheric pressure can do.

So in theory (AND in practice) it WOULD be possible to reach way over the magical (B)MEP figure (14) that was discussed in those threads as an unbeatable and absolute maximum value.
For 2 stroke tuned exhausts, read this:

chicane.myftp.org/2stroketuning.pdf

As for applying mep to 2 strokes, I'm not sure you can do that. They might play with a different set of rules... in fact, I'm pretty sure they do, since they have a power stroke on every rotation. But no, in theory, NOR in practice, can you reach over the mep figure of 14. Think of the insane technology going into modern F1 engines, and even they can't reach it...

- Rob
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Old 04-17-2004, 03:19 PM
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Resonant, or "tuned" systems reach a point of diminishing returns at an exponential rate. The scale reaches a point which is effectively infinite rather abruptly. The most influential variable in this equation is torque. Incredible horsepower can be gained at a huge sacrifice in torque (by raising RPM.) Even by making that sacrifice, I think it would not be possible to move those numbers by a significant margin, especially considering the enormous compromise at other parts of the power band.

An interesting example of this compromise is a ram-jet engine. These engines are generally not capable of significantly greater output [in their 'power band'] than conventional turbine engines, yet the compromise they make is so severe that they won't even 'idle,' so-to-speak; that is, they only work at speeds several times the speed of sound. In contrast, a turbine engine is really nothing more than a supercharged ram-jet, which is capable of stellar performance over a wide, wide power band.

Another consideration is that a snowmobile typically uses a two-stroke engine, which has several advantages. First, the exhaust pulse is able to directly interact with the intake air. In a four-stroke, this interaction is effectively baffled by the valves. Second, because a two-stroke generates an exhaust pulse twice as often, you can tune to a higher frequency at lower RPMs. Third, two-stroke motors use the crankcase to generate intake pressure, further increasing power. Fourth, two-stroke motors are typically small displacement engines, capable of being spun very fast compared to most four-stroke motors which are designed more for torque. (This is not always true, but is generally the case.) Fifth, two-stroke motors are not expected to last for decades and hundreds of thousands of miles. ...because they don't. Even industrial diesel two-stroke engines don't last amazingly long, and their output per displacement ratio is fairly low, particularly when compared to a two-stroke gasoline engine.
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Old 04-17-2004, 04:12 PM
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Sure guys, but .....

That was many DESCRIPTIONS but no DEFINITIONS ....!

Have you never wondered what really is the DEFINITION of NA?

I mean, "Normally Aspirated" .....????? what the *** is that ?

A normal atmospheric pressure is 1 bar .... is that the "intake" pressure that counts for a NA? .... or ...??

In that case a so called resonant intake system, wich can apply, let's say equally to 1.2 bar intake pressure, really is a SC !

That same engine would be able to produce 20% more power in a given RPM ... and SURELY break the 14 MEP figure easily! , and that with only a "tuned" air-box !!!

edit: And YES you can apply the BMEP or MEP on a 2-stroker, and YES they have larger figures that a 4-stroke because they do in theory twice the amount of "work" on every revolution of the crank. An example is my 800 VES engine, it produces 137HP @ 7800 RPM but "rotary valve" engines have even higher MEP figures than that.

... go figure ...



/Sonny
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Last edited by Sonar; 04-17-2004 at 04:18 PM.
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Old 04-17-2004, 04:24 PM
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..an update, with SLP's single pipe on a 800 VES, and a "better" reed valve (like W-force), the HP is around 145-148 @ 7800RPM

And THAT my friends is 800cc MEP power



/Sonny
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Old 04-17-2004, 06:10 PM
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Yes Sonny, you do have a point there. I guess we have always classed, any other device, that forces a intake pressure, that is above atmospheric, regardless of the cyclic action of the engine, to be supercharging.

Pressure increase that only appears inside the cyclic action of the engine, such as resonate, inerta inlet systems, or the pressure changes caused by the exhaust system, would have to be classed as NA. As the pressure is only created, by the cyclic action of the engines pistons.

As you know two strokes rely totally on resonate pulses, to develope usefull power, they do fire twice as often, but have only half the time, to move the air in and out.

The only option to increase the usefull torque, is to supercharge the two stroke. As has been done by the Detroit diesel and the opposed piston Commer Knocker.

Harvey.

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Old 04-17-2004, 06:22 PM
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You are correct about MEP on a two-stroke engine; It does apply, and it will yeild much higher figures than on a four-stroke engine. The gross potential of a resonant system on a two-stoke is much larger because of the flow configuration of the motor, and because the combustion cycle rate is twice as high.

N.A. means "Naturally Aspirated." "Normally Aspirated" is also acceptable, but not the correct term. Supercharged means that there is a mechanically driven, mechanical compressor forcing air into the engine. These definitions do not define the net effect on the engine, but rather the method used to attain the effect. The terms are purely semantic. Nitros oxide injection does not count as supercharging because the term 'supercharge' was invented to mean only what I described above. If the guy who invented the term had decided it meant something else, then it would - but he didn't and it doesn't. Over the years, this has been accepted and this is the way the term is used. (If you look up supercharge, the definition does not mention mechanical compressors. I assure you this definition is intended to broadly apply to more than just engines.) Resonant systems and NOS do count as NA, because air is drawn in naturally, by aspiration. (See definition #2.) It comes in at atmospheric pressure, (unless of course it's being used on a supercharged engine.)
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Old 04-17-2004, 07:30 PM
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Nos is supercharging.

No I believe that Nox is supercharging, as the engine is not capable of aspiring that amount of oxygen, from the natural atmosphere. The same applies for other oxygen bearing fuels, nitromethane, ect.

QUOTE.
The gross potential of a resonant system on a two-stoke is much larger because of the flow configuration of the motor, and because the combustion cycle rate is twice as high.
QUOTE.UberRoo.

The two stroke achives a higher potential becauses it uses ports instead of valves. A port opens at a much higher rate than a valve can. This high rate produces a much sharper pressure change, than a valve could ever do. It is this fast pressure change, that produces the intensity of the resonate wave action, that is superior in the two stroke.

Harvey
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Old 04-17-2004, 09:32 PM
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Yes, flow configuration, e.g., the orientation of the valves/ports, exhaust runner and resonator, and intake port that is pressurized due to being oriented in the crankcase. I suspect the biggest gains come from the intake and exhaust port being directly across from each other inside the cylinder, at least on conventional two-stroke design which does not use overhead valves. Talk about a great cross-flow design.

Semantically, aspiration does not specify or exclude what is being aspired. Exotic fuels or oxygen rich atmosphere does not change the way in which the air/fuel mixture is brought into the engine. It is still subject to the laws of physics, which basically mean that if you create a vacuum, a higher pressure zone will try to fill it. More importantly however, is that engineers and whoever the guy was who coined the term "supercharge," have decided that it does not apply to anything other than mechanical forced induction. This means that even turbochargers do not count because they are not mechanically driven. (I suppose that one could argue that exhaust gasses exert a mechanical force, thus a turbo would qualify, but again it's a semantic definition. I can only argue why people have made those distinctions, not what technically (and only technically) falls into the general definition. For some reason, electrically driven compressors seem to qualify as a supercharger, while the clearly more mechanical means used in two-stroke engine (using the crankcase as a primitive compressor) does not. I'm sure this is because the two-stroke's pseudo-compressor is integral to the original design, but another argument could be made that some two-strokes (namely diesels) only use overhead valves and no cylinder ports. Go figure. It's all semantics.
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Old 04-18-2004, 03:04 PM
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Quote:
Originally posted by UberRoo
Semantically, aspiration does not specify or exclude what is being aspired.
This is really interesting, it opens a way to go beyond a MEP figure of 14 pretty easily on , by definition, a NA engine
We only have to feed the engine with a GOOD type of fuel

And maybe "tune" both intakes and exhausts to resonate at the correct RPM (max HP RPM)

PS. I just calculated the MEP of my 800 2-stroke, about 23,5 ...!! , but it should theoretically be twice as high as a 4-stroker, but that's just in theory

/Sonny
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Old 04-18-2004, 03:22 PM
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Re: Nos is supercharging.

Quote:
Originally posted by oab_au
No I believe that Nox is supercharging, as the engine is not capable of aspiring that amount of oxygen, from the natural atmosphere. The same applies for other oxygen bearing fuels, nitromethane, ect.
Woot! I said it first, and Harvey backed me up! If harvey says it, it's probably right.

- Rob
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Old 04-18-2004, 08:19 PM
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Yes UberRoo,

The deeper you go, the more involved it gets. Thats whats kept me involved in the learning process, of Auto Engineering for so long.

I think to define "supercharge" as a particular object, is a bit narrow. I think it is an action, more than a object. All the units that you mention, would have to be pumps driven by the engine. regardless if driven by mechanics, exhaust gas, electrically or a seperate piston. They are all aimed at getting a higher concertration of air/fuel into the combustion chamber, to increase the amount of gas and heat, to give a gain in pressure on the piston.

There are other ways of achiving this, In gas and fuels, that don't have to be bolted on. These must also be classed as super charging the engine. But we have the popular term, "Supercharger", and everybodys mind sees a 6-71 GM blower on a bent 8.

On the two stroke, the opposed ports that you speak about, are a bit out dated, I know they still have favour in US outboards, but thats the US. Out in the real world, they gave way to the Schneurle system of twin transfer ports, one on each side of the barrel. This got away from the defector lump on the piston that hindered power.

The crankcase compression is only in theory, It does not pressurise the cylinder, as the exhaust port is still open, after the transfer ports have closed. The big gain, was when the piston ported inlet, gave way to rotary and reed inlet systems. This allowed the exhaust, transfer and inlet to all be open at the same time. This allowed the exhaust action to, 1. Remove the spent gas, 2. Pull fresh gas up from the crankcase, 3. Pull new gas in the inlet to continue the filling. This not only filled the cylinder but also filled the exhaust header pipe with fresh gas. The last action of the exhaust was 4. To then push this gas back into the cylinder as the exhaust port closed.

This is where the stroker gets is huge volumetric effecience from. Is this Supercharing the cylinder? It certinly forces more charge in than the swept volume of the cylinder.

Good topic.

Harvey.

Oh, ps, GM strokers use four exhaust valves in the head, ring inlet ports at the bottom of the cylinder.
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Old 04-18-2004, 09:32 PM
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I wonder how many variants of the two-stroke design there are. I've only seen three; Detroits with two-valve cylinder heads and cylinder ports, primitive weedeater-style with two cylinder ports (which can amass some serious displacement as I've seen as much as 450cc,) and the rotary valve system. Shoot, then we have the two-recycle engines; diesels that are two cycle, but only breath every fourth cycle. (Wierd stuff you see on big ships.)

I think what I'm saying is that the term "supercharger" means something roughly based off that big roots-type, or other stereotypical compressor. The two terms "super" and "charge," or "super-charge" would apply much more broadly to anything that would yield a charge of fuel/air that is greater in some way.

I would agree that on a two-stroke, the crankcase compression [probably?] does not compress the charge like a conventional compressor. I've never analyzed the timing of the ports, but if it does nothing else, it does compress the charge before it enters so that it would enter with greater velocity and density. Perhaps it would achieve much lower than atmospheric pressure if it didn't have that extra kick? I suppose if you timed the ports differently...but I'm sure they don't because there is some other loss in efficiency caused by changing the timing.

I'd love to see one of those 'GM Strokers' you mention modified to run gasoline or something livelier. I've always been curious to see a two-cycle rat motor aimed at big numbers, like an NHRA application or similar.

One thing I can completely agree upon: Very good topic.
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Old 04-19-2004, 07:53 AM
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Quote:
... The last action of the exhaust was 4. To then push this gas back into the cylinder as the exhaust port closed.
And here is where mr Kaaden (German I think) comes in

He invented the "tuned pipe" system for 2-strokers, wich means that you have a forth-and-back-going pulse in the chamber (from the piston/header section and back to the baffle section) wich really DRAWS the mixture through the transfer ports and up to the cylinder head and finally out of the exhaust port, and THEN when the piston almost closes the exhaust port, the pulse bounces to the baffle and "back" compress the mixture being drawn out of the port, back into the cylinder again...and thus making it contain more mixture that its volume could contain with athmospheric pressure alone

PS. Has anyone heard about the German "Ladepumpe" system ?
That was an amazing thing back in the 40-50's (I'm not sure), the thing was that they added an "extra" cylinder and piston, smaller then the "real" one.
This extra cylinder had no ports at all! , it just amplified the crankcase under/over pressure, to suck in more mixture that it normally could, and then push the mixture up into the combustion chamber! , acting like a form of supercharger , if I remember correctly, it was DKW motorcycles that used that system.




/Sonny
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