treyZ28

**disclaimer; numbers used may or may not be realistic but the keep the calculator away!**
I also encourage correction of technicalities, oversights, errors etc.

VE: Volumetric Efficiency
Tq: Torque
P: power
F: fuel
A: Air
TDC/BDC top/bottom dead center
VFR/MFR Volumetric/Mass flow rate

Lets start with some basics:
Density= Volume *Mass
Mass= Volume/Density
Volume = Density/Mass

Air: Using volume to describe air is generally a poor idea. This is because air is elastic and its volume is dependant on MANY factors. Mainly, pressure and temperature. It is compressed and its density will change. Volume is very very dynamic.
Volumetric flow rate: Simply take your cross sectional area and multiply by the average velocity. If you have a 1mile square with water flowing through at 1 mph- you have 1 cubic mile per hour

Mass flow rate: The amount of mass that flows through a system. MFR = Density * volumetric flow rate. This is basically your volumetric flow rate with an added dimension. So the volumetric flow rate of water and air being the same, water will have a higher mass flow rate

Volumetric efficiency: Imagine your cylinder is a jug. This jug placed on your counter can hold 1 pound. Your cylinder, holds 1lb of air. Heads are bolted on, piston is at BTC. Your cylinder is completely filled with air.

Now do you really think that in the few tenths of a second that your intake valve is open, 100% of that cylinder is filled? Nah, typically less than 80% in “normal” cars fill their engine at WOT.
That would mean you get 0.8lbs of air in there.
Easily put: the theoretical ratio of the maximum amount of air you can put in your cylinder(NA) : the amount actually in there (You can get 135+ VE if you area really cool, NA)

More scientifically put: it is the ratio between (A density * Volume of a cylinder) and Actual Mass flow into the cylinder during the intake stroke.

Your VE at any given point corresponds to your toque. VE goes up, TQ goes up. VE * displacement (and a few other factors) is your torque. Differences in efficiency and friction neglected, a 6 liter engine at 50% VE will make as much torque as a 3 liter engine at 100% VE and a 1 liter engine at 300% VE (think big boost).

MEP Now this VE stuff relates to something called mean effective pressure. This is the actual pressure inside your cylinder. The relationship is obvious. Higher VE (more air in there) means higher pressure. So it also follows that MEP, TQ and VE all increase/decrease together. MAX MEP = MAX VE = MAX TQ.
*side note*
Pressure, is what will cause a “spark knock” or your fuel to combust before a spark due to excessive pressure. Ever wonder why your engine knocks at your peak toque?
To find a relationship between something and spark knock, MEP is typically the constant. (Actually BMEP, but I don’t want to get in to indicated , brake and frictional elements)

For example, bigger bores increase the tendency to detonate. One graph in my book snows “Knock limited maximum spark timing vs. Bore at xxxMEP”
On the X axis is bore. Y axis is spark timing. You notice that the bigger the bore gets, the earlier the spark has to occur.
There are also charts showing maximum MEP vs. Bore holding spark timing constant but yeah, whatever.
DCR is a good tool to estimate when you’ll knock because typically, VE numbers don’t vary much from NA to NA street car. (higher compression = more pressure)

Volume flow rate of an engine:
Every two cycles, the for stroke engine will displace exactly 1 of its displacement in terms of air. My LQ9 has a volume flow rate of 6 liters per minute at 2rpm, 6000 liters every minute at 2000rpm- WOT or cruise.
No matter WHAT, the engine is constant volume pump.

Some Clarification if you want to read it:
If you have a 1 liter container under vacuum with a 0.5 liter bottle with air in that 1 Liter container and you open the 0.5 liter container up, what happens? You have 1 liter of air since the air fills that whole one 1 liter. Its not like a liquid that will just occupy the bottom and still be 0.5 liter. Keep in mind the mass is still the same, that is important. The jug is 1 liter displacement at 50% VE. Stretch the Jug, make it a 2 liter jug and you have 2 liters of air and 25% VE. Air just gets less dense.


So if it is moving the same amount of air is going through all the time, why different torque from part throttle to WOT?
Ahh, details. Remember how I said volume flow rate is a poor way to measure things? Its density varies a LOT? Air is elastic?

That 6000 liters of air was air that was very “stretched” to get through a small opening in small throttle body opening and not stretched at WOT. So the density varies. And VE depends on the MASS of the air in the cylinder. It will always be the same volume, since the air will expand and take up the whole space, right?

Since the VFR at any rpm is the same, we see that if we want more torque, we need the air to be denser! What equation does this remind you of? Mass Flow Rate. The mass flow rate of air in an engine is everything. So if air has a density of 1.29 g/liter (take fluids w/ a good prof and try to forget that) that 6.0 liter engine at 2000rpm will need 6000*1.29 grams of air per minute to have 100% VE (and probably around 450ft lbs of torque).
The fluid mechanics of keeping the air dense are outside the scope of this post, sorry

Now what we all want to hear about. HORSEPOWER
Horsepower, or power actually, is your TORQUE at a given RPM. And torque is what? VE. And VE is what? Mass of air in the cylinder.

100 torque at 2000rpm is the same power as:
50 torque at 400rpm.
200 torque at 100 rpm

Clearly, what you want is HIGH VE at HIGH RPM to make power. The durability, streetability and feasibility are out of the score of this post.
To have 100% VE in that 6.0 at 2000rpm we needed about 7.750 Kg of air per minute. And we estimate- ehh… 450ft lbs at 2000rpm. Which is about 170hp )
100% VE at 4000rpm. 450ft lbs @ 4000rpm is about 340hp. VE didn’t change so torque wont (it will decrease because friction increased, but I’m not going into that much detail, so CYA)
Easy as pie right? Wait, what happened to our mass flow rate of air? It doubled! We now need 15.5kg of air per minute! (see how superchargers can be your friend?)


So lets recap here
VE= Torque
The mass of air that was ingested is going to determine VE

Mass flow rate: Everything

Something else I want to point out. The amount of power an engine makes, when friction and other stuff is ignored, can almost be figured out by the mass air flow rate! Even if you don’t know rpm!
Let me prove it: 7.750Kg per minute was 450ft lbs at 2000rpm from above. That made it 170hp.
If 7.750kg at 2000rpm is 100% VE, what is it at 2000rpm? And 4000? 200/50% VE, right? (think about it. If it fills the cylinder 100% every time around, the same amount of air going into a cylinder twice as often is spread thin. The same amount of air has to cover 2 sets of cycles.)
Ok, VE and torque are essentially the same thing. 50%VE = half the torque, 225ft lbs
225* 4000rpm /5252 = (are you surprised?) 170hp.
200VE = 900ft lbs @1000rpm = 170hp. Holy cow!

This is how you can find fuel requirements based on power too. A:F is close to constant amongst engines at WOT. Power can tell your mass air flow and then you can get mass fuel flow (36lbs per hour?)

Mass air flow is everything! So why do we measure the performance of an engine in CFM and PSI? I don’t know, its pretty silly if you ask me. I’m going to refer you to oldsstroker and the “old-as-dirt crew” . They spawned this convention!

Ok, so we got that. Cool. Deduction you should be making: That is why all the power is in the top end! All the power comes from mass air flow and valvetrain regulates air flow!

treyZ28

Take if further: basic fluid dynamics. Flow is created by pressure differential. The bigger the difference in two pressures, the faster/harder a fluid like air will flow.
Piston drop creases a NEGATIVE pressure. So what do you want your intake to do? Be HIGH PRESSURE (yay boost!) to create a LARGE differential and drive more air in. Ever notice vacuum (negative pressure) at cruise, when tq/hp is low? Higher (about 0 or atmospheric in NA cars) at WOT? More air flow, higher VE, higher torque, more power, more smile on face of pedal operator and Exxon Mobil accountant.

Intake Pressure/Boost PSI. All boost is not created equal.
Pressure is resistance to flow. At WOT, you minimize resistance. If there was no resistance, there would be no pressure because the fluid would BE THERE already and equalize things. So, lets just imagine the same tube. Think over inflated balloon and empty balloon connected with a straw, pinched in the middle. Let go of pinch and the air will flow from the high pressure to the low pressure until the pressures are equal, or no difference, or zero pressure differential. No pressure differential, no flow.
If you are flowing water through 2 different tubes. One is big, one is small (cross section). The smaller the cross section, the greater the restriction, right? Duh.
Which one is going to need a larger pressure differential if they are going to flow the same amount? The smaller tube will need a larger pressure differential. Think about this now. If you had the balloons, which one would flow the air faster (higher flow rate). The big one. In order to make the flow rates equal, what would you do? Lower the pressure of the balloon on the small tube.

How it relates to cars:
I make 25lbs of boost. I make 4lbs of boost. Who makes more power? How can we tell? I4 of V12, Mass air flow.
If 25lbs is going to 4 tiny little Honda ports and 4lbs is going through 8 MASSIVE 427CI LS7 ports- well, the suddenly Mr. 25lbs doesn’t look so cool does he?
Also, don’t forget the negative pressure being created by eight massive pistons dropping a LONG way down vs. 4 little pistons dropping down a little bit.
So why do we talk about psi we run? Everything needs to be rated in mass flow.

Maybe that’s why there is no replacement for displacement huh? Drop that big piston far and create more negative pressure. Draw in more air. Mass air flow = power. The little Honda engine will have to drop all of its pistons 5 times to make the vacuum all the LS7 pistons will create.

Lets apply this pressure thing to exhaust and relate it to intake.
Suck, squish, boom and now the exhaust valve opens. PV=MRT. M and R are constants (mass in the cylinder didn’t change R is a constant; 8.314KJ/g*K? anyone?) High pressure in the combustion chamber, and the valve opens. What makes the exhaust want to leave? PRESSURE DIFFERENTIAL! So high pressure to low pressure is what we want. So why are people trying to get high pressure (not enough backpressure!). I don’t know. I’m not smart enough to figure out how they defy the principals of fluid flow. You want LOW pressure!

Now the intake valve is open exhaust is on its way out. Starting from the intake manifold to the cylinder to the exhaust, what kind of pressure do you want? High xxx(cylinder)xxx Low. Fluid flows from higher to lower. Go from intake to cylinder. And what is in the cylinder to exhaust. The bigger the ratio, the faster it flows.

In fact, the ratio between the intake and exhaust is criticalto VE. When exhaust pressure builds, air exists the piston slow/ more is left behind. If the cylinder is partially full vs. completely empty- which has higher pressure? The partially full. The intake opens, crap! High pressure. Fresh air charge wont flow in as fast (at all!). low VE, low torque, no smile on face, Exxon is still laughing because they have a lot more money than us.

Some more basic concepts:
Pumping loss. Sucking air through the air filter, into the cylinder and out of the cylinder. It takes power to do that. Your engine makes that power. So some of the air you suck in that makes power is used as power to suck in air and push it out. Reduce intake and exhaust restrictions and reduce pumping loss, reduce power “wasted” by the engine. More power, or make the same power using less air. Constant air to fuel ratio and you have what? Less fuel to make the same power.

Compression: Free power. Long story short; same VE at a higher compression ratio will yield more torque. Same VE = Same amount of air? Yup. Air: Fuel in constant…. Whoa! More power, same fuel or ….same power, less fuel. Cool.

Bore: Big bore, unshroud valves. Awesome, big bore away! Not so fast. Big bore means more likely to detonate because the flame has to travel further. So while that flame front is on its way to get to the outer edge of the bore….. boom, the fuel there detonates. So big bore is good, but means less compression/timing or more octane. Ever notice how the LONG stroke SMALL bore 5.4L/4.6L ford engines LOOOOVE boost? Try that kind of MEP on a ford 429, which is basically a 350 bored out to a 429. their strokes vary by 0.02’’

Big bore also means higher hydrocarbon emissions. Other things that boost HC levels are ridges, nooks and crannies in the piston/combustion chamber. So English muffin shaped pistons are a poor idea. Fuel gets caught there, doesn’t burn, spit out. It also creates hot spots, which is why polishing a piston/ rounding sharp cuts and polishing the combustion chamber can help run more compression. Keeping the engine running well so carbon isn’t building up and creating a rough surface is a good idea too.

RPM vs. Fuel consumption:
Higher rpm = higher friction, pay for friction via power = fuel.
So drop that RPM right? Wrong. After a certain point (varies but I see 1400 rpm in my head) fuel consumption to make the same power increases! Why?
During the s….l….o.....w... movement of the piston on the power stroke, the heat escapes in to the cylinder walls, heads, coolant etc. If it goes there, its not moving your piston down. Power wasted. Also, reversion and other crappy things happen when an engine is out of its operating range. Put an F1 engine at 1000rpm and it will kill you. So will its owners.



****something Id like to note***
Just because something, like an intake manifold, allows an engine to rev higher doesn’t mean its more restrictive. The internal combustion engine is magnificently complex in all its simplicity. The intake manifold is tuned for a different rpm.
You could hook up a flow bench, put 28psi of pressure differential on two different intake manifolds are get the SAME flow. Put it on an engine and one is making power at low RPM and one at high RPM. Don’t confuse “restriction” and “tuning”
There are tons of harmonics in an engine and intake manifold and wave pulse tuning are well beyond my area of expertise. They usually involve supersonic flow as well, which drives me insane.
With that, a lot of what I said is extremely simplified. Please don’t think that you can apply the above principals linearly, directly or anything like that. I didn’t even get into brake, indicated and frictional numbers (BMEP, Brake torque, indicated power) which are extremely important in more advanced calculations.

Hopefully, this will help some of the questions about VE, D/SCR vs. knock etc

I highly recommend Charles F Tayler’s (Taylor?) books for those of you who have taken thermo the internal combustion engine in theory and practice both volumes
And MIT professor John Heywood’s “basics” or “principals” of ICE publication.

I don’t doubt that I made a ton of mistakes, I read through it once. I’m tired. I wanted to talk about BSFC but kind of beat around the bush, sorry. Max Brake Torque Spark Timing, etc- someone else can pick up where I left off.

treyZ28

Holy crap, I really didn't want to study tonight did I?

Vertigo

iTrader: (1)

Very nice write up!
I wanted to ask a really dumb question here...When i was reading about boost (I hope i get this right) The larger the bore and the longer the piston travels downward creates more negative pressure = good as long as you have a lot of postive pressure ready to come in..yes. So ideally, for a high boost application you would want a mid size bore (reduce knock) with a long stroke creating negative pressure that would have a low compression (a thick deck head?) correct me if i am wrong but i don't know exactly what a thick deck means...basically the piston would travel down a lot but at TDC it would have low compression. Is this possible with a stroker motor? For some reason i have always related a stroker motor with high CR. Again nice write up!

treyZ28

The negative pressure is created by the drop of the piston, quickly. That should be fairly intuitive at this point right?
Its a small hole (valve) feeding a HUGE (by comparison) piston dropping down pretty far.
If you ever had to administer yourself shots (or are a heroin addict) you can try picturing a hypodermic needle. pull back on plunger real fast and it will give you some resistance- because you created negative pressure inside the needle. Air cant get fill in the extra volume you create as fast as you create it... or you make space faster than space can be filled.


I really am not 100% sure beyond that- because I know compression ratio and such come in to play. I'll wait for an expert to chime in.

Rick@Synergy

iTrader: (5)

I take it thats why a long rod motor makes the cylinder head breathe better then? Just like you said, less velocity inthe TDC area. So more time for the air to fill.

Good comparisson to the drug addiction. It helped me understand better.

Vertigo

iTrader: (1)

Who told you?

Right this makes sense. So would larger valves in general allow more VE into the cylinder on a stroker motor? I would imagine with a large difference in pressure at the time the intake valve opens, if the intake valve is small this would cause the incoming air to become more elastic and spread itself lowering VE right and just the opposite with a larger valve. But I also know that port velocity is very important too, so at what point do you think a valve would be too large?

Also, I figure this would be a good time to ask although its a newb question. I have heard that LS1's like backpressure. And i have heard and experience more responsiveness/tourqe at lower RPMs with stock headers and no cutout vs with headers and a cutout. Now wouldn't an LS1 get more VE/ more torqe having exaust gases flow out more effenciently? This seems counter intuitive...

Rick@Synergy

iTrader: (5)

This is why most of my exhaust work is 2.5" after the colector. I just dont see much need for 3" until about 450hp. I just want to keep the velocity higher. Good response too.

Rick

Vertigo

iTrader: (1)

Right, but how does having 2.5" help? Wouldn't having 2.5" cause the exaust gases to escape slower >> creating less negative pressure = a lower VE for the incoming air? VS. 3" = more exaust gases escape due to less pressure or resistance allowing more bad air out = more good air in?

Yet, I know that having 2.5" does create more tourqe down low. Why is this. See where i am confused.

I have a feeling that the answer has something to do with the fact that the engine is not operating at its most effiecent RPM. This would be why having free'er flowing exuast would net you higher overall tourqe at higher RPM.
OR
Having 2.5" like the aerodynamics of a wing causes the exaust gases to flow faster through a smaller outlet creating more velicity which would then in turn create more of a pressure difference for the incoming air UNTILL higher RPMS where the 2.5" outlet becomes restrictive...That's my guess....

Rick@Synergy

iTrader: (5)

I am with you on this as well. So what I actually do, again depending on HP of car, is after the header, I reduce it smoothly down to 2.5. Then the x-pipe and the rest 2.5.

Now on higher HP stuff, its 3" off the colector into the x-pipe, then 2.5 out. I am a big believer that with anything, you need to keep reducing along the way to keep the air speed high.

Rick

treyZ28

You guys are confusing "tuning" with "backpressure"

The exhaust scavaging, waves etc- stuff I only understand but dont know how to control, just so happens to be optimal there. If you could have that "tune" with less backpressure, it would ALWAYS be ideal.

The analaysis above is very "static" and steady state*

This also does not consider the momentum of air, another big deal. Long story short, its not the backpressure.
*edit. steady state, not steady phase. Its well assumed nothing is solidifying

Vertigo

iTrader: (1)

Pfffft..What do you know.

treyZ28

not much, thats for sure. That is all ultra basic stuff you can teach yourself for $30 an hour with a book. Actually, I think alll of the above is 1 chapter of Dr. Heywoods book.

Pretty interesting, but imo the coolest part of the ICE is tuning and associated compromises.

NOx, HC, piston design, combustion chamber design, cam compromises, compression vs sparktiming, etc. All good stuff. Especially with this E-85 stuff coming out. It would be great if grades of gasoline became dependant on ethanol content. This way us E-85 guys can run 105 pump gas! Id have high compression and advanced spark timing overnight. It would be really cool because higher octane would, in theory, be cheaper!

I would like to learn more and take advance ICE classes, but in my senior year I have a job offer to go in to aerospace (turbine jet stuff, military related) so I need to take up that stuff.

For the better I suppose. OEM Automotive industry sucks, regarless of who you work for. The automotive industry has negative cash flow, not exactly something Id like to bet my future financial security on.

Motorsports, on the other hand is cool, but pay is generally terrible when the hours at work and away from home are considered. I spent some time in that and those were the best months of my life as well as my greatest learning expierence. However, stability and motorsports dont exactly overlap and neither do high wages, family life or retirement.

/rant

avitet

the reason a smaller exhaust works to a point is because it keep velocity up because you still have the same amount of gases leaving with less space for them to go so pressure and speed increase now this dosnt mean that you want back pressure but you do wont velocity the trick is finding the right balance well i see this has already been covered better than i can explain but i didnt see anybody talk about tri y's but i suppopse thats another post

treyZ28

Avitet, please use punctuation! I'm no grammar wiz or spelling champion, but a lack of punctuation makes your post incredibly difficult to read!

Bombguy99z28

My analogy as to why sometimes smaller exhaust diameter is sometimes better:

Look at a garden hose. If you run a 1" hose into a 3" pvc pipe, the water will slow down dramatically. Hook a fire hydrant up to a 3" pvc pipe and it will shoot out quickly. Stock/near stock motors = garden hose, don't need as much. big HP moters = fire hydrant

back pressure is like putting a 1" garden hose on a fire hydrant. Its just pointless.

Slow exhaust velocity is bad because you want that exhaust gas moving fastly out of the cylinder, otherwise it won't all get out and exhaust gas left in the cylinder is hot and doesn't contain as much oxygen. Want nice cool air in the cylinder, cause cool air is guuuud.

Bombguy99z28

FYI, the only problem I see with E85 is having to use like ~60lb injectors on a 400hp motor.. I think target AFR for an NA motor using E85 is something like 8 or 8.5:1 with a stoichiometric of 9.6xxx or so?

treyZ28

Does the E-85 option change the injectors or fuel pump?

hrm, i have an Lq9 in a 4.8 silverado. I'm pretty sure all the Lq9's were FFV, but mine was also returnless rail, which brings in to consideration fuel pressure for a fuel system with return rails

Adrenaline_Z

I mean this in the nicest way (not directed to anyone):

Please use the term High Pressure in place of backpressure.


Many people (Including myself) cannot stomach that 'term'.

Bombguy99z28

I believe flex fuel vehicles have some sort of sensor in the fuel line or tank that can detect what fuel it is and adjust fuel pressue accordingly, which is how they compensate for the different stoichiometric values.

I was mostly referring to my plans to take the camaro and run E85 in it, and the fact that injector flow has to go way way up.