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Old 05-19-2008, 06:36 PM   #81
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Quote:
Originally Posted by R.I.D.E. View Post
Better aero combined with much higher top gear ratios (import autos are now up to 8 speeds) mean you can capitalize on better aero by using lower engine speeds at higher loads.
Egads, 8 speeds! Do they increase the total range or do they just keep making them closer together?

In bicycle gearing, as they increase speeds, they just make closer and closer ratios. I can't understand these people who say they NEED closer ratios because their legs can't crank at different speeds, but have nothing to say about going up or down hills. My legs can handle 5rpm difference between gears much easier than climbing in a tall gear, and they can produce lots of speed going downhill with tall enough gearing.

I know my VW would have a better 0-60 time if the ratios were a little wider -- 2nd redlines at 55mph, then you have to take time to shift to 3rd for the last few mph.

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Originally Posted by 1993CivicVX View Post
on fe.gov a 2004 4 speed automatic Celica gets 33hwy 25 city and 5 speed manual gets 30hwy and 23 city... how does that work? The site makes no indication the engines are different.
What are the ratios? Is the automatic geared taller? I think in general autos are geared taller, because they are equipped with torque converters which make that easier.

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Originally Posted by monroe74 View Post
Same here. I spend a huge amount of time under 1500. Sounds like you do the same. But every now and then it's fun to rev it up. And believe it or not, Honda actually recommends you do that occasionally. To blow out carbon.
Oh, don't worry about me. I do. All this slow driving occasionally builds up some impatience in me and I blast out a difficult pass at 6000 rpm. That, and I have one really tough left turn on my commute and often have to really open it up to avoid getting creamed in this tiny (by my standards) car.

Quote:
edit: oops, I was a bit off with those numbers. That should say 70mph=2357 rpm. And above I should say that 75mph= 2525 rpm.
I am jealous. 70mph=3000rpm, 60mph=2500rpm, 50mph=2000rpm, 40mph=1600rpm, 30mph=1300rpm. I'm pretty sure this 2.5l I-5 has enough torque to do 50 at 1300 and 70 at 2000.
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Old 06-26-2009, 05:25 AM   #82
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Quote:
Originally Posted by theholycow View Post
Ok, after posting that it led me to this question that I've never quite understood: Why does a more free-flowing exhaust reduce torque? What I've heard, though not experienced for myself or learned the theory behind, is that backpressure is required to make low-end torque, but I have no idea why.

some
back pressure is good for:

keeping the combustion chamber hot
keeps the af charge in the cylinders during overlap
and keeps the exhaust valve hot after shut down so they don't burn.
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Old 06-29-2009, 06:16 AM   #83
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Originally Posted by guest001 View Post
[B]
keeping the combustion chamber hot
keeps the af charge in the cylinders during overlap
and keeps the exhaust valve hot after shut down so they don't burn.

Not to sound like a *ssh*le, but every one of those was urban legend. But don't take my word for it.

ADiF
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Old 06-30-2009, 12:02 AM   #84
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Originally Posted by Allargando DiFortissimo View Post
Not to sound like a *ssh*le, but every one of those was urban legend. But don't take my word for it.

ADiF


quoted from a pretty reputable recently published book. automotive service by tim gilles. It's make text book. all the info in the book is great no reason for me to disbelieve the dam near all of it.
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Old 07-01-2009, 12:41 PM   #85
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Quote:
Originally Posted by guest001 View Post
quoted from a pretty reputable recently published book. automotive service by tim gilles.
Really? I found his website here:

Tim Gilles

...but the links don't show any of the book's contents. Is there any chance you might've misunderstood what you read? He should be a very knowledgeable chap, but all two of the three are oft-repeated (but incorrect) dogma, and the other is just... well, wrong.

Here, repeated, are the supposed "benefits of exhaust backpressure" as quoted by your message:

1) keeping the combustion chamber hot
2) keeps the af charge in the cylinders during overlap
2) keeps the exhaust valve hot after shut down so they don't burn


I'm not addressing Tim Gilles's words since I haven't read them. I'm commenting only on what was written here.

1) keeping the combustion chamber hot
Combustion temperature routinely gets to 1800-degrees Fahrenheit. Since most aluminum melts around 1100-degrees (and gets VERY unstable before that), keeping the combustion chamber hot is not something anyone would want to do. Even if backpressure COULD keep the chamber hot (and it can, to some extent, make it too hot, depending upon the obstruction), why would anyone want to?

The only reason the piston crown doesn't melt in mere seconds of WOT is the fact that combustion temperature, as hot as it is, lasts only a fraction-of-a-second. Peak pressure is (or should) be reached at about 14-15-degrees of crank rotation ATDC (After Top Dead Center), and the chamber begins cooling from this point onward. First due to the rapid expansion of the combustion gases, then the incoming rush of cool intake air (or A/F).

Following an ignition event, every degree of combustion chamber temp that the chamber holds onto puts you just that much closer to detonation. Not a desirable situation.

You want the properly-timed combustion event to be as hot as practical during the power stroke (more heat from the combustion = more energy), but immediately afterward you want it to cool as much as possible 'til the next event. Otherwise, the chamber's latent heat works against cylinder filling.

Conversely, every degree of combustion heat you can get rid of (after the power stroke) allows for more ignition advance and/or a higher compression ratio (on a given fuel octane rating); both meaning rising BSFC numbers, that is: More power because you're making more due to better cylinder filling, and also more efficiency because you're wasting less of what power was made.

With a cool(-as-can-be) chamber, there is that much more room for the incoming A/F mixture to fill. Any latent heat means there's less room available room for fresh incoming A/F. That means less power.

2) keeps the af charge in the cylinders during overlap
This is the job of a correctly-timed reverse pressure wave of a tuned exhaust, and this pressure wave is sound, not "back-pressure."

Following WWII, Chrysler did a lot of work in acoustics as they apply to the sonic wave of an engine's intake event. Their work was groundbreaking and you can read about it here:

http://www.allpar.com/history/memories/bob-scott.html

Below is a link to a picture of the intake being described. It's a wild-looking piece of alien hardware but, believe me, it was a regular Chrysler production item. A friend of mine has one the few Chrysler 300s made with this setup:

MOPAR long-runner RAM intake

Anyway, we're talking exhausts here, but Chrysler did most of the heavy lifting so we could understand the acoustics of intake/exhaust events.

Get in a long hallway (with the door closed on the other end) and clap your hands really loud. What you hear coming back is not "air," but an actual sound wave. Of course, sound moves air molecules around, but the air at the end of the hall stays there. During this reverse sound wave event, the molecules around you vibrate as the sound wave returns, but no air from the end of the hall comes back to you. Instead, the sound wave travels through the air, and vibrates the air around you. This knowledge can be used to make the reverse sonic wave seal off the exhaust port during the valve overlap period.

And this soundwave principle is what a tuned exhaust uses to hold the mixture in during valve overlap period... not backpressure (as far as an impediment to out-flow) is involved. You'll note that NASCAR doesn't run any "backpressure," nor do dragsters, nor do any other competition engines. Whenever race engines have silencers (mufflers) of any sort fitted it's a noise issue, and the rules require it. BTW: Some silencers are very efficient, and cause no (or almost no) additional impediment to flow. Lower-rpm engine power is derived from the intake-exhaust parameters, and then careful matching of a properly sized (length, diameter, collector, layout, etc), free-flowing exhaust. Not through partially obstructing the exhaust tract.

3) keeps the exhaust valve hot after shut down so they don't burn
I haven't heard this one in 30+ years. I thought it had finally died.

The exhaust valve gets every bit of the combustion event's 1800-degree fury unleashed upon it. Milliseconds later, it's surrounded with cold A/F mixture (air/fuel or just air, depending on it it's a DFI engine) that is over 1700-degrees cooler than the combustion temp just was. If cold air could hurt the exhaust valve, this violently-sudden temperature drop is when it would do it.

Also, if "cold air" could somehow migrate up the exhaust (which it can't, because colder air cannot ascend into hotter air) it would be hot air long before it finished its journey. Perhaps, with open ports (no exhaust stacks at all) a red-hot exhaust valve, suddenly exposed to cold air, might warp, but wouldn't burn, For any burning to take place, there would need to be some kind of fuel present... which there isn't.

Again, don't shoot me; I'm only the messenger. Look outside the book in your possession and, by all means, do not take my word for it. Act as if I'm full of bull manure and go out into the world and ask around. I didn't come up with all this by myself, and I'm certainly not privvy to any inside information. This is the stuff that's taught wherever there are hungry minds who must find out.

When I was in the 4th grade, my daddy introduced me to two of the above (and disproved them), and the third one is... well, from outer space or something.

ADiF
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Old 07-01-2009, 01:51 PM   #86
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Hmmm interesting debate, although a little far off from the original pumping losses thread.

Exhaust back pressure will reduce cylinder peak combustion temperature.

If you don't think that is true just plug your exhaust system until it shuts the engine off. Measure the exhaust temperature at a header on a non catalyst car and you would be surprised at how low it actually is at idle speed.

Back pressure causes the exhaust to back up in the cylinder and reduce the flow of incoming air and fuel. You have proven that by the extreeme of plugging the exhaust completely. Partial restrictions are inevitable unless the exhaust is completely open at the cylinder head.

Headers with equal length pipes create pressure waves that when timed correctly and with minimal back pressure actually create low pressure (relative to atmospheric) when the next exhaust pulse enters the junction of the individual header pipes. This helps to scavenge the exhaust gases from the cylinder.

Exhaust Gas Recirculation reduces the peak combusition chamber temperature from 3500 degrees to about 3200 degrees farenheit, to dramatically reduce NOX emissions at some sacrifice in combustion chamber peak pressure.

As far as sound waves, they are moving air. Remember no sound in outer space without the air to transfer the pressure waves necessary for sound to travel. Even in water sound waves are pressure differentials, even in solid material like a seismograph reading in an earthquake.

Now the air does not travel a significant distance, but the pressure waves emanating from a sound source are indeed movement of the air, the same way the waves from the impact of a stone in a body of water emanate from the impact source. There is some movement of the water, but nothing like in a tidal wave where there is actual displacement of the water volume itself.

Pulse and glide capitalizes on the "effective compression" of individual combustion events. When you run any engine at low speeds and high loads you need very little throttle imput to eliminate manifold vacuum.

In general terms the lowest engine speed and lowest vacuum will produce the best poower for any given amount of fuel. The limiting factor to that general statement is when you go to low in engine speed you will not get the best potential power per unit of fuel ratio.

Generally speaking (again) the best engine speed for best BSFC is in the 1600 to 2400 RPM range. That will change due to other factors like cam timing and bore to stroke ratio. Higher performance engines designed to make max power at very high RPM will not produce best BSFC at low speeds. Neither will engines with relative large intake and exhaust valves, which reduce turbulence and atomization at low speeds.

A "better breathing" engine will generally require higher RPM for peak efficiency while an engine like the old Buick 3.8 liter 2 valve V6 will produce better BSFC numbers at very low speeds, as low as 1200 RPM. Longer stroke to bore ratio engines with smaller valve survace areas are good low speed engines. The new Honda Insight ISDI a 1.3 liter engine is a 2 valve design that sacrifices peak power for good low end torque and power, by utilizing turbulence and two plugs per cylinder to vary the plug ignition sequence intervals at various speeds and loads.

Yes the Chrysler engine development efforts in the late forties through the sixties were way ahead of the other American manufacturers. The cross ram big blocks in the early 60s letter series 300s were great engines, but that was not lomited to performance engies, as is demonstrated by the slant sixes with their ram air manifolds. In fact that 225 aluminum block 4 barrel slant six with a 4.5 inch stroke produced power on the same level as the lesser performance small block chevrolet engines of the same era.

Of course the 426 Hemi blew tham all away with 805 horsepower in stock trim from 426 cubes in 1966. 200 more real hp than anything from Ford or Chevy until Ford tried to get the SOHC 429 approved by Nascar. The Hemi was so good Nascar made them reduce the displacement to 366 cubic inches to keep them from walking away from the competition.

regards
gary
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Old 07-02-2009, 09:42 PM   #87
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Well, I do have the wrong car for this, but 60mph is like ~3000 RPM!
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Old 07-03-2009, 05:30 AM   #88
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Slurp 812,

An 02 SI may not be the best engine for max mileage, but it is a great engine for pulse and glide, becasue you can pulse very quickly and your pulse to glide ratio will be better than a less powerful engine that takes longer to reach the peak pulse speed.

pulse to glide ratio is the pulse time divided into the glide time, you want at least twice the glide as the pulse.

You may be able to equal or beat the percentage over EPA of many others as long as you use the correct strategy that specifically utilizes the advantages of a high performance engine while avoiding the light load operation that will kill your mileage.

In fact there may be very little difference between the mileage you can obtain with an SI versus a regular vehicle, it just will require slightly more effort.

Not trying to make you feel at a disadvantage, with dedicated hypermiling effort you could probably get close to 50 MPG if you wanted to work very hard at the effort and sadly drive that nice little performance car totally outside of its design parameters.

A simple vacuum guage that helps you learn to use the least throttle imput for the lowest vacuum will give you a good idea of where to postion you throttle on the pulse portion.

regards
gary
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Old 07-03-2009, 12:05 PM   #89
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Originally Posted by R.I.D.E. View Post
Slurp 812,

A simple vacuum guage that helps you learn to use the least throttle imput for the lowest vacuum will give you a good idea of where to postion you throttle on the pulse portion.

regards
gary

Thanks! I just want to get better than average. I dont really want to work too hard at it. My thought is if we ALL did a little to help curb our consumption the world would be a better place...
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