MikeS

383,

Conservation of mass and conservation of energy are different balances. Each one is a conservation of itself.

Conservation of mass:
The change in mass of a system in a given length of time is equal to the difference of all mass entering the system and leaving the system in that given time. This considers all entries and all exits. In the case of an internal combustion engine cylinder, the combined mass of the intake air and the fuel must be equal to the mass of the exhaust.

Conservation of energy:
Internal energy, kinetic energy, and potential energy are included in conservation of energy. The difference of the sum of these three energies at all entries from the sum of the three energies at all exits must be equal to the difference between the energy put into the system and the work done by the system (assuming no energy storage). This is where there is a loss in the form of heat and sound. They should both be taken as energy leaving the system through an exit.

So, I'm still going to stand by what I said. Mass is conserved and energy is conserved. The mass entering the cylinder is the same, not greater than the mass leaving it.

As for the gasses inside the cylinder expanding to the same volume that they occupied before they were compressed...
I don't believe that that is correct. You failed to take into account the chemical reaction that is occuring. Given a volume of combustible gasses inside a cylinder/piston system, the volume of the system will increase when combustion occurs, provided the piston is not held in place.

So, if anybody cares to read what I've written (383), please tell me where I was wrong. What have I written that is incorrect? I know this is off the initial topic of the thread, but maybe once we've got our thermodynamics worked out we can move on to fluid dynamics.

"Junior"

Apeiron

No, he is correct. When the exhaust valve opens, the exhaust in the chamber will exit the chamber until the pressures inside and outside the chamber are equal. This will leave some exhaust remaining in the chamber, which the piston drives out. Since unless we're using forced induction we only have atmospheric pressure to force the intake charge in, but we can use the piston to drive the exhaust out, it makes sense to make the intake valve larger. This isn't the only way the engine works against itself. It takes energy to compress the intake charge in the next cylinder so it can fire too.

I'm afraid you're the one who is misinformed. The energy from combustion comes from the release of the chemical energy in the molecular bonds, not from the conversion of mass into energy.

Quote:

Also, the expanding gases are what forces the piston down....however, another form of conservation applies here. After the intake charge enters the chamber, it is compressed, and then expands to roughly the same size again once the piston reaches BDC. It provides power by expanding again to its original size. But that does not mean pressure is equal allowing for the smaller mass.

If that was true, the engine would never run. Even if there was no loss of energy to friction, it would take the same amount of energy to compress the intake charge as would be released when it expanded again. There would be no energy to propel the car.

fast89RS

I think we can all agree that high revving motors have more overlap. Right? Ok, why is this? Well, like we've been talking about for most of this thread, scavenging helps pull more fuel into the cylinder. You need this at high rpms, because it gets harder and harder to fill the cylinder with the necessary amount of fuel. So, more overlap, more scavenging. And we all know if you put a big cam in a street motor, it will have bad low end torque. Why is this? Because excessive scavenging will start pulling fuel into the cylinder and then out of the cylinder via the exhaust valve putting less fuel in the cylinder, which will make it have less torque. Backpressure has a role in all of this because it creates less scavenging. So, we can see that having backpressure to some degree will help a street motor retain it's low end torque. That's why it is uncommon to find full-length headers in a mildly built motor (or it should be uncommon).

Rage13

that seems to make alot of sence to me, well put fast89RS

CamaroDriver

Since when did this become a discussion about cam shafts. I was just trying to point out to King-whatever that there's a diff between cylinder scavenging and exhaust scavenging.
Yea..... it's called cylinder pressure. It doesn't "create" anything. It's a restriction. That's all it is. Nothing more, nothing less. Full length headers help BUILD low RPM torque, which is what you'd want in a "mildly built motor".

I did not quote David Vizard earlier (as someone else pointed out. I think it was King-whatever). The reason I mentioned his name is because he wrote it in his book. If you want PROOF then go read it. If you want to keep living in your "Comp Cams says this" world, then go ahead. Let me know when they make 100K mile aluminum roller rockers... OK?

fast89RS

Whoa, buddy, settle down. Your responses to my comments were very poor, along with your attitude. Your first response about cylinder pressure didn't really tell me anything. I don't really know what you were trying to say, but it didn't seem to refute what I said very well. And I apologize, I didn't mean it created anything. And if you weren't looking for a fight I don't think you would have said anything about it. Lastly, I should clarify my full length header statement. I've always been under the assumption that when you get full length headers, you normally get ones with giant primaries. However, you were right, the longer the primaries the better the low-end torque. Having larger (diameter) primaries will reduce low-end torque. This was the point I was trying to make.

Kingtal0n

why are you still picking on me? Im done! ive admitted defeat! ok you win! im not arguing over the internet like a retard! I dont live in comp cams world, I hate comp cams! they messed up 2 of my grinds!

why are you trying to beat everyone up that says anything?
we are just adding our thoughts and insights! if we are off a little who cares? dont bash us because we want to learn. thats it im through! Im king-whatever and im king of my computer! no more internet for me!

CamaroDriver

Why do you guys take it that I'm upset?

I'm simply trying to educate.

If you want to learn more about camshafts, valve timing, duration, and cylinder pressure, then start a new thread or do a search. That's all.

My intentions are not to critisize, nor bash. Never have I called anyone dumb, or stupid, or anything like that. I just try and joke about stuff to enlighten the situation. I guess I need to use these more ----> :lala:

Sorry. And the only thing I meant by King-whatever is I don't understand your logon name. That's all. Mine's pretty evident. I'm a guy that drives a Camaro. Do you own an Eagle Talon? Is that what it means?

Just try and take this FWIW. I'm not trying to pick on anyone. It's just some literature is written so that it appears simple, but by doing so they leave out VOLATILE information. It can be misleading.

Again...... Sorry :hail:

383backinblack

ok first of all mass and energy are always conserved.......seriously dude its a law of the universe. When something appears to lose mass, that lost mass is converted into energy. if what you said was true, there would be no cost of the energy released in the combustion process. do you honestly think you can complete a combustion with no loss in mass? if you do you have alot to learn about physics my friend. want an example of the law of conservation of mass and energy......when you fuse 2 hydrogen atoms and produce a helium atom....accounting for all the changes that take place in the structure of the new atom it still has slightly less mass than expected...this is because some of the mass is converted into energy. its a simple, well known aspect of physics dude. i think you need to read up.

and as far as only having atmospheric pressure to force intake charges into the cylinder, come on dude.....its called vaccum...which is produced by the piston moving down in the bore towards bottom dead center. which is the opposite of how it expels the exhaust gasses. so that is NOT why exhaust ports are smaller.

also, the gases inside the cylinder absolutey expand to the same size as the original volume of the intake charge. It cant get any bigger cause its inside a closed chamber.

and i didnt say that the energy produced by combustion is from converting mass to energy then cars would be nuclear dude......thats how some of the mass is lost......it is converted to energy.

Apeiron

The fusion of two hydrogen atoms into a helium that you describe is a nuclear process, you don't even have the same elements or number of atoms at the end. Combustion is a chemical process which involves only the outer electrons on the atoms concerned. At the end of a chemical reaction, the same numbers of atoms of the same elements are present as before, and the total mass of the system hasn't changed. I don't believe I can complete a combustion process with no loss in mass, I know it. There's a wonderful device called a calorimeter you should play with sometime, you'd learn a lot from it.

I don't think you understand how vacuum works. Vacuum doesn't "suck" anything, it's the surrounding pressure that pushes into the vacuum. Atmospheric pressure is normally only about 14.7 psi, so even if the piston could create a perfect vacuum inside the chamber, there'd only be 14.7 psi of pressure pushing the intake charge into the chamber. Before the exhaust valve opens, there can be several hundred psi of pressure inside the chamber because we've greatly increased the number of gas molecules inside the chamber during the combustion process (by the way, no we haven't changed the mass of the system anywhere).

As soon as the exhaust valve opens it's no longer a closed system and the gas expands to a much larger volume. Besides, didn't you say earlier that "the volume of exhaust gasses is actually quite a bit less than that of the intake charge"?

Is that phrased right, because it looks like you just contradicted yourself. You didn't say the energy produced by combustion is from convertine mass to energy, but in your previous post you said "a good amount of the mass of the intake charge is converted into heat, and sound energy...as well as contributing to the power output of the engine"? Which is it?

None of the mass is lost and converted to energy, that would only happen in a nuclear process. Like I said before, mass is conserved in chemical reactions. The mass of the intake charge is the same as the mass of the exhaust charge. Period.

As for reading up, I did more than enough reading when I took my degree in physics, so I don't really feel like doing much more, but you might want to consider it for yourself.

383backinblack

hey ***, try reading my posts with an intelligent thought...and dont read syntax. I phrased that wrong too i think though...

your not going to lose mass, i got hung up on that explanation....i was trying to use it as an example in explaning the change in volume.....there is absolutely a change in volume in a combustion process. Take a piece of paper and burn it inside a closed chamber, then compress it as much as possible. your going to find that it can be compressed much more than it could be before the combustion process. you could do the same thing with fuel...but it would be harder and more dangerous.

and as for the vacuum comment.....your a retard dude...your just looking for something to whine about...i know how vacuum works. but i shoudnt have to write a 15 page dissertation on every concept brought up in a discussion. of course the pressure outside the vacuum forces the charge into the chamber...but you should know that and i shouldnt have to explain it.

id like to hear why you think the exhaust ports are always much smaller than intake ports. because it isnt that there is more pressure to force the gases out. if it was then it would be a self contradicting concept because the bigger you made the port, the less restriction there would be. if the port is smaller the engine has to supply more work to expel the exhaust gases. i dont care how big an engine is....the intake valve is always smaller than an exhaust valve

AJ_92RS

Um..... don't you mean the other way around?

383backinblack

yup i meant the exhaust valve is always smaller than the intake valve...whoops

86SCBNJ

A fan in the exhaust? How about we then take what the fan is blowing out and duct it back to the engine. I think they call that a TURBO

Apeiron

I applaud and appreciate your efforts to maintain the levels of intelligence and maturity in this discussion.

I'm not sure what you're trying to get at. How do you define "as much as possible". Do you mean compress it into as small a volume as possible with a given pressure? Do we do it while the system is still hot, or after it has cooled to the surrounding temperature? How could we compress paper while it's a solid anyway? We can discuss this further if you like, but the exhaust charge must occupy a larger volume than the intake charge at equal pressures in order to develop the energy to drive the piston down and still have enough energy to derive meaningful work from the engine.

My apologies. When you objected to my description of atmospheric pressure acting on the intake charge to fill the cylinder and instead claimed that it was vacuum rather than atmospheric pressure that filled the cylinder I understandably mistook this as a sign that you didn't understand the relationship between pressure and vacuum.

It's not because there is more pressure to force the gasses out, it's because we have the capacity to develop more pressure to force the gasses out. Of course the engine will have to work to expel the gases through a smaller exhaust valve, but the point is that it can provide the work, and we have a rapidly expanding exhaust charge that wants to expand beyond the cylinder anyway. On the intake stroke of a naturally aspirated engine we can only ever have at most 1 atmosphere of pressure pushing the intake charge into the cylinder (or 1 atmosphere of vacuum pulling it in if you prefer), and actually less than that since the pressure differential decreases as the cylinder fills. Without some form of forced induction, there is nothing we can do that will allow us to develop a greater pressure differential to push more air more quickly through the restriction created by the valve. On the exhaust stroke however, we have a piston with enough energy to develop several hundred PSI of pressure if necessary to force the exhaust out. Since there's only so much area inside the head to place valves, it makes sense to sacrifice exhaust flow for intake flow and make the intake valve larger so that we don't impede the intake flow any more than necessary. Most dual pattern cams attempt to make up for this sacrifice in exhaust valve size by increasing exhaust life and duration.

rx7speed

yes this might have already been explained and since I am too lazy to read it I'm not going to know for sure yet


but here it is

oh yeah made this a few days ago that is why I'm still putting in on here just didn't have the chanec to post it then


never knew you could create a heluim atom by using to hydrogen atoms

and as for the conservation of energy
the engery used to create the pressure in the cyl in heat. that heat gets absorbed by the cyl walls, pistons, head, your coolant system. it is conserved but transfered to another location
isn't engegy something that is not tangible but has an effect on things around it none thelss? it is something that can be mesured but yet you can't touch, put on a scale, feel, or anything like that so if you say mass is converted to energy and vise versa what shape does that mass take now? mass is a something that is physical, so where does that physical property of mass go if it is converted to energy? also as far as taking two He atoms and converting them to one hydrogen atom.... though it prolly could not be done by normal means if you did figure out how to combine them together yes it might weight less then what two HE atoms combined would be.... but that is because two HE atoms combined have one more positron then one hydrogen atoms has so you would have to get rid of it hence less weight.


and as far as the spent exhuast gas not getting any larger then the cyl... true only if the valve stayed close all the time
but once that valve opens it can become larger then the cyl since it can also take up exhuast port and maniold space


isn't nuke engergy when the nutron or positron gets seperated from the rest of the atom creating that energy
so the mass doesn't get converted to engery but the energy stored up by holding that nutron inside the atom getting released

I admit my physics and chemistiry stuff is a little fuzzy but I can almost sweeear that mass does not change to energy but how the mass reacts will release energy that was stored up


and we need to quit finding things to argue on here 383


hey it is just easier to take each post one at a time, but glad you noticed


could it be that due to the pulse nature of the exhuast that though it might create more work for the piston to push the gas out it might also create such a low pressure zone behind it that it helps scavange the exhuast out?
maybe it could be a way to get more air into the motor
the piston is going to push almost all the exhuast out as it is... but you are not always going to get near being 100% VE so by allowing a larger intake valve in might allow more air in which might overcome the pumping loss by getting rid of the exhuast gas

these are just ideas though but think about them for a moment

also by pressuring something that doesn't change mass right?

Apeiron

Mass and energy can be changed into one another under certain circumstances. Some Austrian guy wrote something about it once, something about <B>E=mc<SUP>2</SUP></B>.

Drastius

Ok, let me set some stuff straight, info came from my university professors.

First of all the exhaust valve is smaller than the intake valve because it's easy to get exhaust out of the cylinder. The exhaust pressure at BDC after the power stroke is at around 5 atmospheres. So once you open the exhaust valve, exhaust flows past the valve quite quickly. In fact the flow goes supersonic (choked flow) this creates shock waves and the shock waves are what makes your car loud.

Second of all there is no conversion of mass to energy or vice versa in a combusion engine.

And finally there is conservation of mass, the mass which enters the combustion chamber exits as well.

The same goes for energy, but the energy balance is much more complex.

383backinblack

yes, your making some valid points about exhaust port size.....still no one has made a valid argument about why they are always smaller, and not the same size.....because using the reasoning you used, although the engine can provide the work to drive out the exhaust.....it wouldnt have to if the valve/port were larger....but this doesnt create power gains. if you made the exhaust valve the same size as the intake valve you would reduce to amount of work required to expel the exhaust gases from the chamber, however it would probably not show a measured increase in power, and perhaps a loss.

Apeiron

How much simpler of an explanation do you need? Using a smaller exhaust valve allows a larger intake valve to be used to improve flow through the intake.

383backinblack

thats not an explanation dude......because you could make the chamber big enough to accomodate any size valve you want. with a given cylinder volume, and all other things equal making the exhaust and intake valve the same size should maximize performance if your explanation were correct, and this obviously not the case.

Apeiron

Well no, we can't make the chamber any size we want or else we lose compression. Making the exhaust and intake valves the same size would equalize the intake and exhaust flow if the same pressure differential existed across each valve during the intake and exhaust strokes, which doesn't happen.

CamaroDriver

I've already explained why they're smaller.

Granted this is a simplified description, for yet a simple topic. Why does it need to be complicated?

I have no degree in physics so I can't get into the technical jargen with you, but it would seem pretty obvious to anyone who knows why a hot air balloon rises.

The increased molecular activity of the hot air makes it easier to move. A hot air balloon doesn't rise because the air inside is lighter than the outside air. It's just more active than the air outside the balloon. So why wouldn't the same principle apply to exhaust?

You can use all the technical names for what I've described if you like, but that's just going to put in "scientific" words. I prefer the leman terms myself.

Mark A Shields

Instead of fighting, the answer to this post is BACKPRESSURE IS BAD, YOU DON'T WANT ANY BACKPRESSURE.

I can't be anywhere near as technical as you guys, but for those that know what they're talking about, they'll have the same answer as me.

Apeiron

A hot air balloon rises because of buoyancy, the balloon is less dense than the surrounding air. That doesn't really apply to exhaust.

deadbird

Sorry guys.. I have to..

CamaroDriver

Yes it does apply to exhaust because exhaust air is HOT!!!!! And what happens to hot air?

I wasn't comparing a balloon rising to exhaust rising, I was simply trying to point out of WHY a balloon rises... from HOT AIR

I got this from a kid's website for you so maybe it'll help you understand what I'm talking about.

http://www.howstuffworks.com/hot-air-balloon6.htm

So as you can see... heating up the air causes the particals (sorry I said molecules earlier ) to move faster, therefore lowering density, while still maintaining the same pressure.

THAT'S WHY THE EXHAUST VALVES CAN BE SMALLER!!!!!!

fast89RS

Uhhh, no.

92Transam

Uhh yes

Mark A Shields

Oh my freakin gawwwwddddd


Even the guys on PHRTV know it, they did a segment on it.

Maybe check out www.phrtv.com for their videos on exhaust , I think it may be there.

rx7speed

I didn't think the mass was getting converted into energy but rather the mass has the ability to store and give off energy

rx7speed

I think I gave some ideas on that one


you might reduce the amount of work to push the exhuast gas out but then again you make it more work to get the intake in

exhuast gas has help on the way out while the intake doesn't really have that much help and so by making the intake valve larger to reduce the restriction it will make up for the extra power needed to push the exhuast gas out

rx7speed

ok well I just saw that you guys already got into it

oh well

383backinblack

apeiron, you need to stop being such a pseudo intellectual and actually explain what your talking about instead of circularly reasoning to make yourself feel important with respect to a theory that doesnt hold water under scrutiny. but when it is scrutinzed you refer back to your theory as being right, or restating something you already said instead of supporting your position.

brodyscamaro

fast89RS

You can't just say that no backpressure is good for every engine setup. That's all I'm arguing. If you have a car that is carbeurated and it doesn't have computer controls, which would mean it couldn't tell if the engine was leaning out, then it can cause problems. With no backpressure, cars like these will run lean (one reason why people think no backpressure causes valve burning) thus reducing power. We all know that each engine is unique. Why can't you agree that "no backpressure is good" isn't universally applicable?

Mark A Shields

that makes no sense.

fast89RS

Will you please elaborate. Refuting in this manner doesn't help anything. I realize you're a hot shot moderator and all, and I'm some lowly peon with 20 posts, but that in no way means that I make no sense. Actually, being a moderator should mean that you are above replying how you just did. Please, tell me why I'm wrong, if you make sense I not against agreeing with you. I have read that exhaust modifications will allow more air to be pulled into the engine thus leaning out the mixture. Maybe they were wrong. Maybe they weren't. But you haven't addressed this, now have you?

rx7speed

can't you change something.... I think there is a way.... I don't know maybe I'm wrong but I could have sworn on it... that you can make a carb run richer based upon the current setup... but again I'm prolly wrong

only motor I can see backpressure being good is one that has a lot of overlap and that would only be at low rpms......

those are motors with lots of overlap.... like a 2 stroke motor

fast89RS

Which is something I said before.

Apeiron

Tell me what you don't understand about what I'm saying and I will try to explain it better.

Mark A Shields

First off, I'm not the most knowledgable guy, and being a mod, doesn't mean I'm an expert.

But, if you search on this board for Five7kid's replies on this subject he explains it quite well, I may search for you. I used to think it was good too, then I read peoples replied on here and then I even saw it on PHRTV to back it up.

Backpressure causes an engine to struggle, which is not good.

If it messes with the fuel mixture, fix it then by adjusting the carb, and you will create more hp.

If you need to run richer or leaner on a carb, you just play with the jets, correct?

Mark A Shields

here's what five7kid said...

rx7speed

you didn't really take the whole quote

a 4 stroke motor will prolly not do all that great if you add b a ckpressure to it

the period of overlap as well as how the ports are setup doesn't quite lead to the issues you would get with a 2 stroke motor....


also on a 4 stroke motor if you are building a motor for high rpms where you are using lots of overlap you are prolly not worried about low end torque

brodyscamaro

tune your setup in the conditions it will be ran in...

MikeS

I've read everything since I last posted on this, and it appears that, other than the continued "backpressure good vs. bad", there is still a discussion about the relationship between intake valve size to exhaust valve size. I assume that in the hundred and some odd years of development of the internal combustion engine, a somewhat optimal ratio was determined. I am guessing that through the years, different ratios have been tried (equal sizes included) before arriving at the current ratio of areas. Maybe the first internal combustion engines had equal size valves, somebody realized they worked better with the intake valves larger, and the trend continued until no more improvement was noticed?

By the way, deadbird,
Don't think that horse can get off that easy, do you?

dans82bird

BACKPRESSURE = RESTRICTION = LESS HP

IN CONCLUSION: BACKPRESSURE = GAY


Have a nice day

fast89RS

Again, this is what I said before, engine specific. For everyone else, I'm not completely sold on this whole deal, but you do make great points. I'm going to research further.

rx7speed

the whole port timing on a 2 stroke is so different from one another that you can't even really compare them to a 4 stroke

yeah you might get some pull through on a big cam motor but prolly not enough to worry about

most of the power loss is prolly going to come from such a late intake closing point as well as a really early exhuast valve opening point

with the intake closing so early you are going to bleed a lot of that intake charge back through the intake and exhuast

yeah that will as I said suck some air and gas through the exhuast but even if you add backpressure or something like you would on a two stroke at which point would just push it back up the intake then.... still you would be bleeding off that pressure a/f mixture

then with the early exhuast opening you are going to bleed of pressure from not letting that combustion fully press against the piston and instead shove some of it out the head and backpressure isn't really going to help keep that much force inside the combustion chamber, a little draw through of a/f is one thing but the exhuast... no way


so backpressure still would be a bad idea