In pursuit of the elusive discharge coefficient

[David Redszus]

An accurate determination of discharge coefficient (Cd) is essential to any engine simulation.
Without such, everything is merely an estimate. Which sometimes can be right on target.

As Blair convincingly demonstrates, Cd is a variable which is dependent on pressure ratio,
area ratio, and flow direction. Since pressure ratio and area ratio change with each crank angle,
we quickly come to the conclusion that a graphic of CD must be three dimensional in nature.

The issue then becomes one of process; how shall we go about measuring CD?

Looking for new ideas and suggestions regarding the determination of CD values.

[Belgian1979]

My guess : 2 high speed pressure sensors, one in the port just above the valve and one in the combustion chamber on the opposite end of the valves (if possible to place it there). Based on the drop of pressure I think you should be able to make some conclusions.

[ptuomov]

I thought that this is done on the flow bench.

The person who did my heads flowed the port at different pressure differentials and different valve lifts. Only in the intended direction, though. At least with those heads, the Cd didn't really vary with the pressure other than at very low lifts.

Were we doing it wrong?

[Belgian1979]

I think that on a live engine, things will vary with a lot of factors. So a flow bench might not actually provide the info you're after.

[ptuomov]

I think that on a live engine, things will vary with a lot of factors. So a flow bench might not actually provide the info you're after.

Please be more specific.

[Belgian1979]

effects of intake tubes, filters,...
Effects of carb, throttles
Effects of intake manifold
Effects of the exhaust on intake
Effects of the total length of the intake tract
etc.

[ptuomov]

effects of intake tubes, filters,...
Effects of carb, throttles
Effects of intake manifold
Effects of the exhaust on intake
Effects of the total length of the intake tract
etc.

Those will impact the pressures on both sides of the valve, but I don’t understand how are they going to make the Cd measured on the flow bench misleading?

[Belgian1979]

the delta pressure across the valve is one of the mayor factors in the discharge coefficient. Thus if these factors influence the pressure delta they will influence the discharge coefficient. So if all of these things are not present, you can not predict what the discharge coefficient will be in the actual engine by only testing the heads.

[ptuomov]

the delta pressure across the valve is one of the mayor factors in the discharge coefficient. Thus if these factors influence the pressure delta they will influence the discharge coefficient. So if all of these things are not present, you can not predict what the discharge coefficient will be in the actual engine by only testing the heads.

It is or it isn’t. If you flow the head at different valve lifts and pressure differentials with the intake runner attached, that will give you a map of CFM as a function of pressure differential and valve lift. Knowing the valve and seat geometry (and port mcsa) will give you the area and thereby coefficient if discharge, Cd, per lift and pressure differential. Right? So there you have it. The pressure waves shouldnt change that, IMO.

[Zmechanic]

As discharge coefficient is a dimensionless number, what do we choose as the best (most appropriate reference)? Straight pipe? Sharp orifice?

Genuinely curious, not just being cantankerous.

[David Redszus]

Most amateur flow benches are limited to about 28" H2O; some only 10" H2O.

To replicate the inlet (NA) pressure ratios found in a running engine, we need to test
at 203.8" H2O; and many points in between. Only a few test benches can do this.

In addition, air is compressible. At 28" H2O, the compressible flow function is about
4%, at 204" H2O it is about 30%.

I would be interested to see a flow schematic indicating how pressure sensors could be used to
capture Cd, since it must also includes water vapor, density, viscosity and type of gas.

The closest solution I have seen is the use of in-cylinder pressure and inlet pressure measurement
using a TFX system or similar. And then use a mass flow sensor for flow measurement.

Perhaps Clint would be willing to post inlet and cylinder pressure curves.

[ptuomov]

The valve only sees those high pressure differentials at very low lifts. So the flow bench doesn’t have to move a ton of air, just hold higher pressure.

[Warp Speed]

When flowing a head on a flow bench, the 28in drop isn't across the valve, but relative to atmosphere correct?

[GARY C]

Most amateur flow benches are limited to about 28" H2O; some only 10" H2O.

To replicate the inlet (NA) pressure ratios found in a running engine, we need to test
at 203.8" H2O; and many points in between. Only a few test benches can do this.

In addition, air is compressible. At 28" H2O, the compressible flow function is about
4%, at 204" H2O it is about 30%.

I would be interested to see a flow schematic indicating how pressure sensors could be used to
capture Cd, since it must also includes water vapor, density, viscosity and type of gas.

The closest solution I have seen is the use of in-cylinder pressure and inlet pressure measurement
using a TFX system or similar. And then use a mass flow sensor for flow measurement.

Perhaps Clint would be willing to post inlet and cylinder pressure curves.

How much of a power gain have you seen possible testing with TFX equipment to improve on the initial flowbench data?

[David Redszus]

Most amateur flow benches are limited to about 28" H2O; some only 10" H2O.

To replicate the inlet (NA) pressure ratios found in a running engine, we need to test
at 203.8" H2O; and many points in between. Only a few test benches can do this.

In addition, air is compressible. At 28" H2O, the compressible flow function is about
4%, at 204" H2O it is about 30%.

I would be interested to see a flow schematic indicating how pressure sensors could be used to
capture Cd, since it must also includes water vapor, density, viscosity and type of gas.

The closest solution I have seen is the use of in-cylinder pressure and inlet pressure measurement
using a TFX system or similar. And then use a mass flow sensor for flow measurement.

Perhaps Clint would be willing to post inlet and cylinder pressure curves.

How much of a power gain have you seen possible testing with TFX equipment to improve on the initial flowbench data?

I have never compared actual TFX data to flow bench data. I consider them to be an apples to bananas comparison.
Further, unless the tuning is optimized, no real comparison can be made; sub-optimal comparisons have limited value.
Even engine simulations can produce sub-optimal results if the inputs are not correct.

Air flow is such an input.