Low and medium lift Flow

[digger]

Flow numbers at maximum lift have very little meaning since peak air demand occurs at maximum piston velocity.
If peak piston speed is at 74 deg ATC, then whatever valve lift is present at that crank angle will determine air flow.

On the exhaust side things are a little different. Maximum exhaust gas mass flow occurs from EVO to BDC. Therefore, low valve lit is critical. If EVO is at 75 deg BBC, then the critical valve lift area is from 0 to 75 deg.

David the intake runner tuning and lag mean that mass flow is not highest when piston speed is highest. At low rpm it might be true but at peak hp rpm the peak mass flow is not at peak piston speed

Digger — Should you plot the velocity or mass flow in that graph, which one is more relevant? Best, Tuomo

the goal is to get mass in the cylinder not generate velocity for the sake of velocity. At the velocities in question the compressbility/density matters

Here are the plots of Mass flow inlet and mach number inlet port at the same rpm





and another with less reversion at start

[digger]

further to that here is the mass flow and cumulative mass flow(%) and valvelift(%)

it shows that from 50% lift to peak to 50% the cumulative mass flow went from 6% to 96% so bulk of mass flow happens when the valve lift is medium to high and you simply are not going to make much gains with low lift flow. you are better of focusing on controlling pressure differentials in the engine to help low lift flow by stopping reversion with runner tuning and cam events

you can see with the blue mass flow curve there is reversion at then end which accounts for a 2% loss

[David Redszus]

further to that here is the mass flow and cumulative mass flow(%) and valvelift(%)

it shows that from 50% lift to peak to 50% the cumulative mass flow went from 6% to 96% so bulk of mass flow happens when the valve lift is medium to high and you simply are not going to make much gains with low lift flow. you are better of focusing on controlling pressure differentials in the engine to help low lift flow by stopping reversion with runner tuning and cam events

you can see with the blue mass flow curve there is reversion at then end which accounts for a 2% loss

Digger is quite right in that most flow is related to mid lift rather than highest valve lift or low lift.
There is mass flow (and velocity, discounting density changes), after peak piston speed, due to several factors;
air flow lag, tuned port pressure and cam events (including curtain area).

The following chart indicates the air lag in degrees for a few engine combinations.

RPM..............Stroke (in)
................3"........4"........5"
3000.........3.88......5.17.....6.46 degrees
5000.........6.46......8.62....10.77
7000.........9.04.....12.07....15.00
9000........11.46.....15.05....19.00

It is quite evident that air flow lag is a function of both stroke length and engine speed.
Short stroke, low speed engines produce a shorter air flow lag than long stroke, high speed engines.
So if peak piston air demand occurs at 71 deg ATC, we would expect peak air flow to occur no
later than about 81 deg ATC for most engines.

Peak valve lift angle, relative to peak piston speed angle, can be adjusted in order to optimize
air flow.

Actual air flow lag will also be impacted by sonic velocity which is a function of fuel vapor
density, humidity and air temperature.

Pressure waves in tune ports have little effect on lower speed engines. While they have more
impact as engine speed increases, they contribute relatively little to inlet air flow and exist
only for a limited number of degrees.

If tuned port pressure waves are used to improve induction pressure ratios, then they cannot
provide much help during the overlap process.

So the question remains, as Digger has so elegantly revealed, what causes the very large interval
from peak piston speed angle (plus a few degrees air flow lag) to observed air mass flow peak?

[ptuomov]

“ the goal is to get mass in the cylinder not generate velocity for the sake of velocity. At the velocities in question the compressbility/density matters”

I agree with both.

However, even more than compressibility, the size of the hike matters for mass flow.

“ it shows that from 50% lift to peak to 50% the cumulative mass flow went from 6% to 96% so bulk of mass flow happens when the valve lift is medium to high and you simply are not going to make much gains with low lift flow. you are better of focusing on controlling pressure differentials in the engine to help low lift flow by stopping reversion with runner tuning and cam events”

I’m thinking that the mass flow at low lifts is low because the port doesn’t flow well at low lifts. We don’t know from that mass flow graph much mass flow the port would have at low lifts if the port would flow better at low lifts. Right?

Intuitively, I would use the Mach velocity number as an indication of the lifts at which the port would like to flow more and improved “flow bench CFM” would help. I don’t know if it’s the right intuition, though.

Also, I think that it’s really the highest Mach number point that I’d like to see at the port, which almost always means the Mach number at the valve.

Perhaps this turned from port conversation into a camshaft conversation....

[Walter R. Malik]

Just curious ... when the valve lifts only .410" or less, is .300" considered to be high-lift or mid-lift...?

[frnkeore]

Hi Randy,
In my OP, the lift I cited was .650 and most head mfg, cite lifts to at least .600 so, that's the range I'm interested in but, you have a point if someone is using a common, mild 218 @ .050 type cam of ~.450 lift.

From what I have read, in researching this question is, high, low lift flow can be beneficial on the Ex side but, can cost some HP on the Intake side, during overlap.

[travis]

Along the lines of what Walter is asking...what about a “lift rule” engine? If you’re limited to a .400-.450” max lift, what do you do?

[plovett]

Along the lines of what Walter is asking...what about a “lift rule” engine? If you’re limited to a .400-.450” max lift, what do you do?

Dwell cam?

[68corvette]

The following chart indicates the air lag in degrees for a few engine combinations.

Between what points have you calculated those lags?
From piston to valve, to atmosphere or back from atmosphere to valve?

[David Redszus]

Along the lines of what Walter is asking...what about a “lift rule” engine? If you’re limited to a .400-.450” max lift, what do you do?

Dwell cam?

A flat topped cam, technically double dwell cam is often used in bikes.

Between what points have you calculated those lags?
From piston to valve, to atmosphere or back from atmosphere to valve?

The lag was calculated from piston to valve, using sonic velocity (1169 ft/sec) and a limit of Mach 1.
The minimum lag is at TDC and increases to a maximum at BDC. The actual speed of sound will vary with
air/fuel ratio and humidity.

[GARY C]

further to that here is the mass flow and cumulative mass flow(%) and valvelift(%)

it shows that from 50% lift to peak to 50% the cumulative mass flow went from 6% to 96% so bulk of mass flow happens when the valve lift is medium to high and you simply are not going to make much gains with low lift flow. you are better of focusing on controlling pressure differentials in the engine to help low lift flow by stopping reversion with runner tuning and cam events

you can see with the blue mass flow curve there is reversion at then end which accounts for a 2% loss

Does this program allow you to account for valve size vs port size, for example a 2.08 valve on a 230cc port vs 2.08 on 180cc port?

[digger]

further to that here is the mass flow and cumulative mass flow(%) and valvelift(%)

it shows that from 50% lift to peak to 50% the cumulative mass flow went from 6% to 96% so bulk of mass flow happens when the valve lift is medium to high and you simply are not going to make much gains with low lift flow. you are better of focusing on controlling pressure differentials in the engine to help low lift flow by stopping reversion with runner tuning and cam events

you can see with the blue mass flow curve there is reversion at then end which accounts for a 2% loss

Does this program allow you to account for valve size vs port size, for example a 2.08 valve on a 230cc port vs 2.08 on 180cc port?

Yes you input flow numbers and sizes of port throat, port entry and valve/seat sizes

[GARY C]

further to that here is the mass flow and cumulative mass flow(%) and valvelift(%)

it shows that from 50% lift to peak to 50% the cumulative mass flow went from 6% to 96% so bulk of mass flow happens when the valve lift is medium to high and you simply are not going to make much gains with low lift flow. you are better of focusing on controlling pressure differentials in the engine to help low lift flow by stopping reversion with runner tuning and cam events

you can see with the blue mass flow curve there is reversion at then end which accounts for a 2% loss

Does this program allow you to account for valve size vs port size, for example a 2.08 valve on a 230cc port vs 2.08 on 180cc port?

Yes you input flow numbers and sizes of port throat, port entry and valve/seat sizes

Cool, It seems a bigger valve would be more prone to reversion on opening and a smaller port would be less prone to velocity lag but that is just an assumption on my part.

[digger]

Does this program allow you to account for valve size vs port size, for example a 2.08 valve on a 230cc port vs 2.08 on 180cc port?

Yes you input flow numbers and sizes of port throat, port entry and valve/seat sizes

Cool, It seems a bigger valve would be more prone to reversion on opening and a smaller port would be less prone to velocity lag but that is just an assumption on my part.

here is a comparison by taking the same engine and shrinking the entire inlet 1/8" including the valve and manifold (head flow is same so the Cd goes up)

port Mach No



mass flow

[ptuomov]

Isn’t the right experiment for simulations to increase and reduce the low lift flow map of the head and then see what happens to mass flow and power? The increase could be done either by increasing the area keeping the coefficient constant or increasing the coefficient keeping the area constant.