flow numbers in ref to power gained

[David Redszus]

I have posted this for David Redszus. Who will add commentary about the post.

Stan

PseudoFlow3graph.pdf

The graph posted earlier shows crank angle (deg), piston air demand (cfm), valve lift (in), curtain area (in^2) and flow velocity (ft/sec). The engine bore = 4.0", stroke = 3.25", cr = 6.0", valve dia = 2.0", rpm = 7000, inlet duration is 280deg (seat to seat).

At TDC, the valve lift is .139" (25% max lift) but there is no piston air demand since there is no piston motion. The only flow that can occur is due to pressure differential of inlet and exhaust ports.

Maximum piston air demand (537cfm) occurs at 76 deg ATC as does maximum piston speed. The valve lift is .555 (90% max lift). Flow velocity is 521 ft/sec.

Maximum valve lift occurs at 113 deg (.614") but piston air demand has fallen to 428cfm and velocity has also fallen to 453 ft/sec. Less air is flowing into the cylinder at maximum valve lift.

Note the dip in the velocity curve and that only the curtain area curve follows the valve lift curve.

As Mike Jones indicated earlier, the proper selection of the camshaft, based on piston air demand, rpm, and valve size can substantially improve the air flow into the engine. Or an improper cam can have the reverse effect.

Thanks for posting the graph Stan.

[nickmckinney]

Really cool stuff, I like that graph.

[GARY C]

I have posted this for David Redszus. Who will add commentary about the post.

Stan

PseudoFlow3graph.pdf

The graph posted earlier shows crank angle (deg), piston air demand (cfm), valve lift (in), curtain area (in^2) and flow velocity (ft/sec). The engine bore = 4.0", stroke = 3.25", cr = 6.0", valve dia = 2.0", rpm = 7000, inlet duration is 280deg (seat to seat).

At TDC, the valve lift is .139" (25% max lift) but there is no piston air demand since there is no piston motion. The only flow that can occur is due to pressure differential of inlet and exhaust ports.

Maximum piston air demand (537cfm) occurs at 76 deg ATC as does maximum piston speed. The valve lift is .555 (90% max lift). Flow velocity is 521 ft/sec.

Maximum valve lift occurs at 113 deg (.614") but piston air demand has fallen to 428cfm and velocity has also fallen to 453 ft/sec. Less air is flowing into the cylinder at maximum valve lift.

Note the dip in the velocity curve and that only the curtain area curve follows the valve lift curve.

As Mike Jones indicated earlier, the proper selection of the camshaft, based on piston air demand, rpm, and valve size can substantially improve the air flow into the engine. Or an improper cam can have the reverse effect.

Thanks for posting the graph Stan.

Is it possible to show how increasing or decreasing valve size and flow by 20 cfm below .300 would effect cylinder filling?

[Orr89rocz]

In a perfect world, would you want the curtain area curve to mimic/follow the piston demand curve to allow the most cylinder fill, with maybe an extended opening after piston reaches bdc to allow intake ramming effects?

[Stan Weiss]

I have posted this for David Redszus. Who will add commentary about the post.

Stan

PseudoFlow3graph.pdf

The graph posted earlier shows crank angle (deg), piston air demand (cfm), valve lift (in), curtain area (in^2) and flow velocity (ft/sec). The engine bore = 4.0", stroke = 3.25", cr = 6.0", valve dia = 2.0", rpm = 7000, inlet duration is 280deg (seat to seat).

At TDC, the valve lift is .139" (25% max lift) but there is no piston air demand since there is no piston motion. The only flow that can occur is due to pressure differential of inlet and exhaust ports.

Maximum piston air demand (537cfm) occurs at 76 deg ATC as does maximum piston speed. The valve lift is .555 (90% max lift). Flow velocity is 521 ft/sec.

Maximum valve lift occurs at 113 deg (.614") but piston air demand has fallen to 428cfm and velocity has also fallen to 453 ft/sec. Less air is flowing into the cylinder at maximum valve lift.

Note the dip in the velocity curve and that only the curtain area curve follows the valve lift curve.

As Mike Jones indicated earlier, the proper selection of the camshaft, based on piston air demand, rpm, and valve size can substantially improve the air flow into the engine. Or an improper cam can have the reverse effect.

Thanks for posting the graph Stan.

David,
You are welcome.

Where is your velocity at (cutain area)? It looks to be zero @ TDC and not @ BDC but the graph looks to stop @ 175 ATDC.

Stan

[David Redszus]

Where is your velocity at (cutain area)? It looks to be zero @ TDC and not @ BDC but the graph looks to stop @ 175 ATDC.

Stan

Velocity at TDC is zero since there is no piston motion. At BDC, even though the piston has slowed and reversed direction, there is continued air velocity due to flow delay and air momentum. The data was computed for the entire cycle but the graph only displays to 175 ATC.

[user-9274568]

Demand is 537cfm on that?

[Stan Weiss]

Demand is 537cfm on that?

Chad,
Most programs like mine calculate and show piston flow demand @ 28 inches of water. I believe David is showing the raw flow demand. Which in this case is just about 2 times what I calculate.

Stan

ab-pflow-david.gif

[ijames]

David, where is that flow velocity measured? At the valve, back in the port, or? Thanks.

[John Wallace]

I would think piston air demand (cfm) is around 655 cfm at 7000 RPM.
(depends on VE usually)

The CFM demand for the heads would depend on the flow of the head?
(and VE, intake flow, carb, etc)

[David Redszus]

David, where is that flow velocity measured? At the valve, back in the port, or? Thanks.

The flow velocity is measured at the valve/seat interface (valve curtain). When the valve curtain area becomes larger than the port area, it no longer controls air flow. At higher valve ifts and higher piston speeds, the flow is primarily controlled by the port area.

The CFM demand for the heads would depend on the flow of the head?
(and VE, intake flow, carb, etc)

The piston air demand is a function of piston area and piston speed. It assumes the head has been removed and that there is no flow restriction, i.e. Cd = 1.0.
Obviously, in a real engine we would encounter various flow restrictions such as carbs and inlet runners, so that the actual piston air demand would be somewhat lower. But it would still occur at the crank angle that matches maximum piston speed.

It should become obvious that matching (as closely as physically possible) the valve lift curve to piston air demand curve would improve air flow. For maximum filling velocity, the port should be kept small until the velocity approaches a choke point (or reduced Cd) due to high piston speeds. Then the port should be increased in area. All bends and port shapes can be represented by use of empirically derived Cd values.

Note that we have not adjusted CFM flow due to changes in differential pressures across the valve. At low valve lift values, the air flow is very subject to reversion. If flow reversion exists, then poor low lift air flow is a benefit and not a detriment to performance. At inlet valve open during overlap (low lift), the flow is controlled by pressure differential between inlet and exhaust ports. At inlet valve close (again, low lift), the flow is controlled by pressure differential between inlet port and cylinder.

Bear in mind that none of these flows are steady state nor constant pressure. And that both the air temperature and air density will vary with crank angle.

The real objective should be to maintain as high a flow velocity (short of choking or reduced Cd) as possible since there is only a limited amount of time for cylinder filling to occur. At 6000rpm, each complete cylcle lasts 10ms and the filling cycle lasts only 5ms. That's 5 one thousandths of a second.

[user-9274568]

David, where is that flow velocity measured? At the valve, back in the port, or? Thanks.

The flow velocity is measured at the valve/seat interface (valve curtain). When the valve curtain area becomes larger than the port area, it no longer controls air flow. At higher valve ifts and higher piston speeds, the flow is primarily controlled by the port area.

The CFM demand for the heads would depend on the flow of the head?
(and VE, intake flow, carb, etc)

The piston air demand is a function of piston area and piston speed. It assumes the head has been removed and that there is no flow restriction, i.e. Cd = 1.0.
Obviously, in a real engine we would encounter various flow restrictions such as carbs and inlet runners, so that the actual piston air demand would be somewhat lower. But it would still occur at the crank angle that matches maximum piston speed.

It should become obvious that matching (as closely as physically possible) the valve lift curve to piston air demand curve would improve air flow. For maximum filling velocity, the port should be kept small until the velocity approaches a choke point (or reduced Cd) due to high piston speeds. Then the port should be increased in area. All bends and port shapes can be represented by use of empirically derived Cd values.

Note that we have not adjusted CFM flow due to changes in differential pressures across the valve. At low valve lift values, the air flow is very subject to reversion. If flow reversion exists, then poor low lift air flow is a benefit and not a detriment to performance. At inlet valve open during overlap (low lift), the flow is controlled by pressure differential between inlet and exhaust ports. At inlet valve close (again, low lift), the flow is controlled by pressure differential between inlet port and cylinder.

Bear in mind that none of these flows are steady state nor constant pressure. And that both the air temperature and air density will vary with crank angle.

The real objective should be to maintain as high a flow velocity (short of choking or reduced Cd) as possible since there is only a limited amount of time for cylinder filling to occur. At 6000rpm, each complete cylcle lasts 10ms and the filling cycle lasts only 5ms. That's 5 one thousandths of a second.

David. clear this up for people. Would that be MIN port area or AVG port area?

[BrazilianZ28Camaro]

I have posted this for David Redszus. Who will add commentary about the post.

Stan

PseudoFlow3graph.pdf

Cool graph Stan!

I'd love to see the exust excavenge @ overlap added into equation.

[Brian P]

It should become obvious that matching (as closely as physically possible) the valve lift curve to piston air demand curve would improve air flow.

... Bear in mind that none of these flows are steady state nor constant pressure. And that both the air temperature and air density will vary with crank angle.

The real objective should be to maintain as high a flow velocity (short of choking or reduced Cd) as possible since there is only a limited amount of time for cylinder filling to occur. At 6000rpm, each complete cylcle lasts 10ms and the filling cycle lasts only 5ms. That's 5 one thousandths of a second.

There is a lag between the piston air demand and the actual flow into the cylinder due to inertial and wave effects.

During the early part of the intake stroke, the piston air demand is partially showing up as flow and partially pulling a vacuum on the port, causing the air column to accelerate as the suction-wave (I'll call it that) travels up the intake runner. Then when it reflects off the open end and comes back down as a positive pressure wave, that increases flow through the runner and then the valve, and if things are sized correctly, that bump in flow happens towards the end of the intake stroke. You don't want that positive wave to reflect off a mostly-closed valve ... you want it to fill the cylinder. Even though piston demand is lower at that point, the air flow is higher because it is essentially filling the partial vacuum in the cylinder. That's when you want the least resistance across the valve ... that's why peak valve lift/flow should be after peak piston demand and for that matter, why the valve has to stay open for a bit after BDC.

Matching curtain area to piston demand only ought to work if you have a near-zero-length intake runner or a low-revving engine with too-big ports and you aren't making use of the wave and inertia-ramming effects. Otherwise, you want the least resistance to happen when the cylinder is being charged up towards the end of the intake stroke.

[David Redszus]

There is a lag between the piston air demand and the actual flow into the cylinder due to inertial and wave effects.

Indeed there is a lag between piston air demand and actual flow into the cylinder. For this specific engine it is:
at 76deg =4.5 deg, at 113deg = 7.6deg and at BDC = 10.1deg. The lag delay is in real time units so the higher the engine rpm, the greater the lag in degrees. But even accounting for the lag, there is still a large difference between peak piston air demand and max valve curtain area. This can also be affected by camshaft and valve size.

Even though piston demand is lower at that point, the air flow is higher because it is essentially filling the partial vacuum in the cylinder. That's when you want the least resistance across the valve ... that's why peak valve lift/flow should be after peak piston demand and for that matter, why the valve has to stay open for a bit after BDC.

After BDC, the piston air demand is reversed and begins to build cylinder pressure working against the momentum of the incoming air flow. When the cylinder pressure exceeds inlet pressure the flow becomes reversionary. The purpose of a late inlet close is to allow the air column to catch up to the piston air demand.

The arrival of a positive pressure pulse just before inlet valve close is not to increase inlet air flow but rather to prevent flow reversion.

David. clear this up for people. Would that be MIN port area or AVG port area?

The MIN port area would be the critical dimension. But in reality, since air is compressible, the area would be used to create a Cd value that would be applied across the AVG port area.