Max RPM for Head Flow

[digger]

here is a plot of two exhaust configurations (short vs long collector at 3500 rpm)

It shows the delivery ratio (similar concept to VE) as a function of crank angle. by EVC one of the exhaust configuration has 15-20% more of the theoretical "fill" achieved and this advantage is basically maintained until IVC where one has achieved 10% more than theoretical fill (110% VE) and the other has under filled compared to theoretical by 5% (95%VE) a difference of 15% simply due to one exhaust producing a nice scavenging wave at this rpm and the other producing some reversion.

you can also see that both lose about 5% fill from reversion prior to IVC, kind of unavoidable at such a low rpm. the cam modelled is actually quite small duration, large lift and tight centres so reversion would be alot worse if you wanted to turn more rpm than this cam offers

[hoffman900]

Here is an engine with everything measured as closely as possible (flowbench data, Cam Dr. data, etc). Power curve mimics similar builds very closely.

This is a 2 valve, single cylinder application. Peak power is at 7000rpm, which is what the traces below are from:

Pressure (exhaust and intake taken at the valves):

Units on the x-axis are in Bar. 1 bar = 401.865inH20

Mass Flow (valve throat):


Mach Index (valve throat):

1.0 = SOS.

The 'macro' valve lift measurements:
Intake:
320* @ .010", 275* @ .050, 203* @ .200, . 524" valve lift
Exhaust:
313* @ .010", 267* @ .050, 194* @ .200, .494" valve lift.

LSA: 106

[raynorshine]

The in cylinder pressure should show maybe a slight dip from overlap and it may rise a little, then drop back down as the piston takes over and drop even further. The shape of the pressure curve will be a pretty smooth ellipse at this point up, with pressure increasing at an exponential rate as it travels back up the bore. Ideally the valve will close just at both the intake port and cylinder pressure equalizes.

You're assuming that pressure in the chamber above the piston is for the most part much the same throughout the chamber.
It's not.
Depending on the intake charge stream velocity & how it discharges into the cylinder (does it swirl a lot, does it follow the bore walls, is it coming from the side like an inline valve engine or more central like a hemi etc etc) the pressure gradient from the piston crown to the head chamber can vary substantially.
That charge is moving fast, at 8000rpm the cylinder is filled 66 times per second - air/fuel does not behave anything like the air you think you see when shit is happening that fast, it lags, it 'stretches', it clumps up, it moves in lumps, anything you think you know about air can be thrown out the window coz it just ain't so with the time intervals we're dealing with.

Now, assume we have a head with great pressure recovery, excellent charge velocity, low swirl & a closer to centre intake valve - the incoming charge is moving fast & not loosing velocity quickly to swirl or fast pressure recovery - that charge is going to follow the piston crown down the bore, pressure just above the piston crown will be higher than you'd expect if you made too many assumptions about the behaviour of the charge. The charge closely following the piston down the bore results in lower pressure higher in the cylinder - so you end up with a higher pressure differential across the intake valve than the 'average' pressure in the cylinder would indicate.
On the other end of the spectrum you could look at a head with lacklustre pressure recovery, lower velocity, excessive swirl & an intake valve against the bore wall - the incoming charge is moving slower, swirling around at the top of the cylinder & impeding incoming charge - it is not following the piston down the bore very well, pressure above the piston crown is lower, pressure near the top of the bore is higher, the pressure differential across the intake valve is lower than you'd expect based on the average pressure in the cylinder.

See the difference? - both examples have similar average pressure in the cylinder, but the pressure gradient is markedly different & cylinder fill (& HP) will be wildly different.

This is over simplified, but you get the gist.

Don't bother looking at graphs, flow sheets & data etc - look at dyno sheets & track times. Theory is all great, but we are a long way from being able to take relevant measurements from a running engine (well, at least the vast majority of us are). The snippets we get from the few that can measure this stuff are far removed from the applications we are likely to be playing with & are unlikely to be applicable.
Stick with what works, you don't have to understand everything about an engine to build a good one - you just have to know what works, not why.

I sure as shit don't know 1/4 as much as I'd like to, but I can still build a good engine based on experience & a basic understanding that I don't know shit really & I'm better off looking at results to learn than theory.

i really like the last part...people really get hung up on theories, even when the facts and results don't support

[V Remian]

Alright, I have to ask. Doesn't the pressure chart posted vindicate Vizard's claim of small pressure deltas through most of the intake event?

[4sfed]

Here is an engine with everything measured as closely as possible (flowbench data, Cam Dr. data, etc). Power curve mimics similar builds very closely.

This is a 2 valve, single cylinder application. Peak power is at 7000rpm, which is what the traces below are from:

Pressure (exhaust and intake taken at the valves):

Units on the x-axis are in Bar. 1 bar = 401.865inH20

The 'macro' valve lift measurements:
Intake:
320* @ .010", 275* @ .050, 203* @ .200, . 524" valve lift
Exhaust:
313* @ .010", 267* @ .050, 194* @ .200, .494" valve lift.

LSA: 106

That's a very realistic simulation. What software are you using? Here is measured data from a small 4-cylinder with individual runners. Data taken near peak power. Pressure in psi. One caution . . . the accuracy of the cylinder pressure is suspect at this low pressure. The sensor has a 2000 psi range so an error of only 0.25% results in an error of 5 psi.

Intake:
310* @ .010, 267* @ .050, 202* @ .200, .493 max valve lift
Exhaust:
289* @ .010, 253* @ .050, 192* @ .200, .467 max valve lift

7888_RPM.jpg

[digger]

Alright, I have to ask. Doesn't the pressure chart posted vindicate Vizard's claim of small pressure deltas through most of the intake event?

the pressure differential feeding the cylinder after overlap is finished is fundamentally driven by ambient pressure with respect to cylinder pressure. the pressure differential shown in Hoffmanns graph is the pressure differential across the valve only. remember this is transient accelerated flow not a static flow bench

here is an example showing the pressure along various points within the inlet runner and ports etc

here are the pressure gradients in the inlet system of a porsche NA 6cyl IR engine at peak hp 7000rpm.

Pcyl(6) = cylinder pressure cyl 6
Pin(6) = pressure trace at inlet valve of cyl 6
PinTrace(6) = pressure at position 6 which is between the head and throttles of cyl 6
PinTrace(12) = pressure at position 12 which is between the throttles and bellmouth entry of cyl 6

it shows the accelerated flow and large pressure differential in the inlet system despite the pressure across the valve only not being particularly high. the mach number in the port is 0.43

[MadBill]

The Hoffman graph appears to show at peak mass flow rate (~ 480°) an intake to cylinder delta P of ~ 17" H2O, whereas just after TDC overlap it's over 30"*.However, although the exhaust peak suction pulse is perfectly positioned at TDC, the intake length looks to be significantly short for this RPM. Be instructional to see how the traces would look with the runner matched to the test RPM...

<EDIT>
As I read it, on Digger's graph the red curve would be the equivalent of atmospheric pressure on a flow bench and the blue cylinder pressure curve would represent the equivalent test pressure, so it appears that the delta is indeed much higher ~ TDC, (over 200" H2O!) and much smaller through the high flow portion of the intake stroke, indeed crossing into negative territory at ~ 510° (~ 30° BDC).

The chart also shows a very well-matched intake length.

[4sfed]

The Hoffman graph appears to show at peak mass flow rate (~ 480°) an intake to cylinder delta P of ~ 17" H2O, whereas just after TDC overlap it's over 30"*.However, although the exhaust peak suction pulse is perfectly positioned at TDC, the intake length looks to be significantly short for this RPM. Be instructional to see how the traces would look with the runner matched to the test RPM...

<EDIT>
As i read it, on Digger's graph the red curve would be the equivalent of atmospheric pressure on a flow bench and the black cylinder pressure curve would represent the equivalent test pressure, so it appears that the delta is indeed much higher ~ TDC and much smaller through the high flow portion of the intake stroke. (Also a very well-matched intake length.)

Red would be chamber pressure, black - intake, green - exhaust

I believe you could also say the intake length is too long . . . being properly tuned at peak torque.

Going back to David's graph . . . his line marked "Cylinder Pressure" looks like it could be the differential between intake port and chamber pressure.

[MadBill]

Yes, I'd hoped to correct that before getting caught out.. (but per his decoder above the chart, all but the blue are intake traces.)

Re runner length, maybe it's set up for the banking at Daytona, or perhaps the long white dyno I believe you frequent...

[digger]

<EDIT>
As i read it, on Digger's graph the red curve would be the equivalent of atmospheric pressure on a flow bench and the black cylinder pressure curve would represent the equivalent test pressure, so it appears that the delta is indeed much higher ~ TDC and much smaller through the high flow portion of the intake stroke. (Also a very well-matched intake length.)

atmosphere on flow bench would be a horizontal line at 1 on that graph, the red line is still in the flow path and peak velocity there is 0.42. it is not the pressure within an airbox or plenum as this engine model hasnt got one

blue is cylinder pressure or test pressure equivelent on bench, all others are within the actual runner

[MadBill]

..blue is cylinder pressure or test pressure equivelent on bench, all others are within the actual runner

How far downstream from the airbox in the runner is the red sensor? Being upstream of the throttle blade, I ASSumed it was pretty much equivalent to airbox pressure, I thought that to represent this data on a flow bench at various crank angles the red trace would be reasonably equivalent to atmospheric pressure and so it minus the blue absolute cylinder pressure would simulate the test depression on a bench.

[digger]

..blue is cylinder pressure or test pressure equivelent on bench, all others are within the actual runner

Respectfully, I think to represent this data on a flow bench at various crank angles the red trace (being the sensor closest to open air) would be equivalent to atmospheric pressure and it minus the blue absolute pressure would be the test depression.

respectfully i dont agree

..blue is cylinder pressure or test pressure equivelent on bench, all others are within the actual runner

How far downstream from the airbox in the runner is the red sensor? Being upstream of the throttle blade, I ASSumed it was pretty much equivalent to airbox pressure, I thought that to represent this data on a flow bench at various crank angles the red trace would be reasonably equivalent to atmospheric pressure and so it minus the blue absolute cylinder pressure would simulate the test depression on a bench.

that portion of the runner is 178 mm long i dont know if its mid length or something else ill have to email Neels as you dont appear to be able specify location for that one

ill do another model and add an airbox to it and repost

[digger]

here is engine with a single airbox 10L common to all runners and big entry 6" diameter which acts as basically an open runner due to relatively large volume. A smaller volume box would show more oscillation peaks

Pab is the airbox trace

[MadBill]

My bad Digger. Looking at the fairly modest fluctuations of the red trace 12, I was visualizing the pulsations in a small plenum, such as under a 4 bbl. carb, and so considered its instantaneous value to be the test bench equivalent of atmospheric pressure. Showing no fluctuations, obviously the airbox is very large so any test bed equivalent depression would be from a p.r. of 1.0 to that of the cylinder pressure at whatever crank angle is chosen. In this case this means the pressure drop is even larger around TDC, dropping to zero at ~ 500°, or 40° BBDC, thus supporting DV's assertion.

Speaking of same, I initially thought your data was from a running engine, but it dawns on me that it may be from a high end simulation?

[digger]

Speaking of same, I initially thought your data was from a running engine, but it dawns on me that it may be from a high end simulation?

this is in ENGMOD4T.

In this case this means the pressure drop is even larger around TDC, dropping to zero at ~ 500°, or 40° BBDC, thus supporting DV's assertion.

the pressure drop across inlet valve alone can indeed be higher at TDC due to the small aperture of the valve curtain at this point. near max piston speed there is too much area for the pressure across the valve alone to be extremely high.

IMO this doesnt mean the piston isnt doing the bulk of the work or that you should be testing at low depression which was David's argument was. p

he is right that exhaust is critical, if you get it wrong you can throw ~20-25% VE out the door