Article, High comp, power & efficiency

[Anonymous]

Found this on www.coatesengine.com/lookmo...
I found it a little hard to believe. Comments Anyone?

It says. "The first and second laws of thermodynamics dictate that any increase in compression ratio will yeild a disproportionate gain in fuel efficiency over power generation".
"Raising the compression ratio from 8:0:1 to 11:0:1 will add just 5 percent to the engines power but decrease fuel consumption by 20 percent if all the other factors in the engine remain constant.
They add "This is the reason the cars of the mid 70s were so fuel thirsty, the mind set then was to lower the comp ratio to reduce oxides of nitrogen (NOx) emissions".

 

[Asa]

404 error

without reading the article, just what you've said makes a certain amount of sense, i think...
but not at the level of 5% or so
if everything stays the same, but power bumps up a little, then you are using a lower RPM level to get the same power as before, but i don't think a bump that low will show anything

i might be missing something somewhere else, but i'm not sure
i had thought that 70s cars were so down on power because the emissions crap slowed the engine down through parasitic drag and poorly designed systems

 

[rbohm]

8) i dont remember if i posted it here or not, but here goes;

compare two engines one 8:1 engine and one 10:1 engine. we will for the sake of simplifying the math round off the standars pressure from 14.7 to 15, and we will assume 100% volumetric efficiency. the number change some but not enough to worry about. ok when the piston comes up on its compression stroke, cylinder pressure rises by the compression factor, in these cases the 8:1 engine makes 120psi, and the 10:1 engine makes 150psi. when we light the fuel mixture, cylinder pressure rises by a factor of 10 at its peak, thus the 8:1 engine has a peak of 1200psi, and the 10:1 engine hits 1500psi. to get the net pressure on the piston we subtract the gross from the initial pressure and we get for the 8:1 engine 1200-120=1080psi, and fo rthe 10:1 engine we get 1500-150=1350psi. as you can see the great compression engine gets more work done in the same amount time, and on the same amount of fuel, thus you need less throttle pressure to get the same amount of work from a 10:1 engine as you do from an 8:1 engine. hope this helps.

 

[Asa]

so, basically what i said wif a lotta math thrown in?

 

[Fiorelli]

I couldn't find your page but did find this one
http://www.coatesengine.com/engine_of_the_future.html

 

[MustangSix]

5% power change per point of CR is the rule of thumb, within limits. But the fuel economy issue is debatable.

 

[Anonymous]

[No message]

 

[rbohm]

8) well i know both david vizard, and denny wykoff, they were both instructors of mine in college. they both gave me the basic explanation i wrote here. as i said the number were rounded off, and certain assumptions were made to simplify the math, but the end result is still close. and yes the increase in cylinder pressure is not linear, but the peak is still about 10 times the initial cylinder pressure, again rounded off to simplify the math.

 

[ASMART]

1. SOmeone commented about the 70s emmission cars not having power due to the emission garbage stuck on them. It was the loss of compression, less cam timing, EGR, catalytic converters, retarded timing, lower octane, etc.
2. One thing no one mentioned is the better cylinder filling with higher compression. At least that i sthe way I see it. When the piston starts to move down, it had less volume above it to start with. This would lead to more " vacuum" that would in turn put more pull on the air in the intake.
3. I believe the idea is right, I just don't believ ethe 20% economy increase. Even if it could get 20% increase, I would use it up realizing the extra 5% hp.

 

[CobraSix]

Just out of curiosity, looking at the second article posted, what cars where running 12:1 CRs in the 60s? I've yet to find any normal production cars that ran that high.

Slade

 

[Anonymous]

Hemi's ran up to 13.5:1 from the factory.


-=Whittey=-

 

[rbohm]

All that proves is that you didn't understand what they were saying.
For example:
10-1 CR at full stroke X 15 psi = 150
10-1 CR (rod ratio: 2-1) with intake closing @ 60 ABDC = 8.18-1 CR^1.2 X 15 psi = 192 psi gauge pressure. Error: -22%.
7-1 X 15 = 105
7-1 intake closes @ 40 = 6.45-1^1.2 X 15 = 139 psi. Error: -24%
7-1, close at 80 = 4.89-1 = 94 psi. Error: +12%
It's even more inaccurate with your method of disregarding intake closing point:
10-1 X 15 = 150
10^1.2 X 150 = 251 psi. Error: -40%.

A bit more "rounded off" than useful. In fact it doesn't work at any CR or closing point, except by complete coincidence.

8) again, i simplified everything to make the math easy to follow. i realize, and david and denny both said in class that the math was more complicated, but they were also simplifying it so the average person in the class could follow it(this was in 1977, and it was a night class). my explanation is the same as david put in the article, again just simplified, but the effect is the SAME. it show the difference in net cylinder pressure at the peak pressure, and the difference in compression ratios, and the amount of work that was done. that is ALL i was trying to get across in a simplified example. yes i could have used the correct figures, but then most people reading the explanation of how the difference in compression ratios work, would have been hopelessly lost and even more confused. remember the KISS principle, KEEP IT SIMPLE SON.

 

[StrangeRanger]

At the risk of making the math unsimplified:

Thr thermal efficiency of an Otto cycte engine is related to the compression ratio by:

E = 1 - (1/CR)^0.408

For CR = 8, E = 0.572
For CR = 10, E = 0.609
For CR = 12, E = 0.637

Volumetric efficiency also increases with CR, but I cannot find the equation. I assume the proportion is about equivalent

 

[Anonymous]

Of course you're right. Rounded off to the nearest 40% is necessary for the low IQ of the readers here.
I give up.

 

[rbohm]

Of course you're right. Rounded off to the nearest 40% is necessary for the low IQ of the readers here.
I give up.

8) you hav eot get over this high opinion you hold of others . the people on this forum are generally intelligent, but may not grasp certain engineering or physics concepts, or have a vague understanding, but dont quite get it. and not just with automobiles, for example;
addo is a carpenter in austrailia. i can do the same things he does with wood, but it will take me far longer, and wont be as good. does that mean i am retarded because i cant work with wood like addo? no it doesnt. can jack or strangeranger handle hotel night audit and management like i can? perhaps, perhaps not. if not does that make them retarded? again no it doesnt. if everyone here understodd engines like waddell wilson, we wouldnt need the forum we have. i like this forum, and the people here, so please refrain from insulting people you dont know. also recognize the fact that when i try to help people understand certain concepts, i try to make it so they can easily grasp what i am saying, so they have a better understanding. also remember that there are people who want the same info but are afraid to ask because they dont want others to think less of them.

 

[ASMART]

Rbohm is right.

 

[Anonymous]

.

 

[Anonymous]

what ASMART said.

If it weren't for people like RBohm, Mustangsix, Inliner, Addo etc. etc., posting and helping, I would have had to pay someone to convert my drums to discs. Thanks FSP.

 

[Anonymous]

I believe his point was that many here happily accepted and defended a number that was 40% off. We all know nothing is perfect, but 40% is pretty damn far from perfect.

As for people not understanding "certain engineering or physics concepts" that is why we have forums. For people to ask questions. People aren't going to learn much if all you do is simplify things, and they sure as hell aren't going to learn anything by using incorrect information.


-=Whittey=-

 

[rbohm]

8) i will grant that there is a large difference between using accurate numbers, and just using simplfied models, but i only used the simple model to make it easy for people to understand the concept. also the actual numbers may be off, but the difference between the different compression ratios isnt going to be off by that much when using the actual formulas. i merely used a simple formula to show what kind of difference you will have in the amount of work differing compression ratios will do.