Hymee

iTrader: (1)

After reading some of Richard Pauls material, I decided I would take some of my actual measurements, and plot them.

I have used my own ODBII logging s/w and a CAN interface I aquired. I logged the MAF values (grams per second) against the measured RPM. (All measurements taken in real time from the PCM via OBDII).

I also plotted the Theoretical mass air flow, and calculate a % efficiency. At a couple of spots this is slightly greater than 100%. My reaction to this is the SDIAS in operation.



Notes:[list=1][*]MAF & RPM are actual as reported by OBDII.[*]Measurements taken at ~20 degrees C and 102 kPa BARO, virtually at sea level.[*]Theoretic displacement = 1.3 litres per revolution.[*]Theoretic mass flow calculated at 1.3 grams / litre (mass of air at sea level)[/list=1]

Thought it was interesting, and thought y'all might like to take a peek. I guess the actual displacement is acctually 1.308 litres, so I am out a tinsy bit, but you get the drift.

Hope you like it.

Cheers,
Hymee.

PS - Let me know if I suffed up, so I can make amends!

IKnowNot'ing

Good info, thanks.

Did you correct it for your ambient conditions?

Re VolEff > 100%, Yamagushi says VolEff max = 105 %.

Can you tell us with OBD tool you used?

brillo

I'm not an expert in this field, what can we learn from this? How efficient is the MAF? can anything here be improved?

IKnowNot'ing

It doesn't tell you anything about the MAFS, but about the efficiency of the rotary engine to pump air into the combustion chamber.
Usually, 4-stroke reciprocating engine have a VolEff between 80 and 90.

babylou

Is this an average VE throughout the rev range? I ask because I have seen in excess of 100% VE at a specific engine speed (usually the torque peak).

IKnowNot'ing

Don't know, it's a quotation from Professor Heywood. But I guess you could get 100% at a given RPM on some higly tuned and efficient piston engines. But I wouldn't know, as I used tp work on basic Ford engines ...

shelleys_man_06

BTW, how is VE measured for rotary engines? I can't find any good resources on this.

Hymee

iTrader: (1)

I don't see how it would differ from a piston engine. It is a black box positive displacement air pump. The VE is a measure of the theoretical displacement v's the practical displacement.

Also note, my graph is on Mass Air Flow, not volume. The theoretic mass flow is calculated from the displacement volume based on the mass of air at sea level is 1.3 grams/litre.

Cheers,
Hymee.

Hymee

iTrader: (1)

I didn't make any corrections, but I noted what the temp was, and the BARO according to OBDII.

The CAN interface is manufactured specifically for us.

The software is written by me.

Cheers,
Hymee.

bgreene

Was this run moving or sitting?

From another thread, it was mentioned that intake temp jumped 120F when the car stopped, and matched ambient when moving with clear flow in front of the vehicle. That big of a delta-T could presumably have some impact on intake mass flow (or does the MAF sensor reading somehow compensate for that?)

Hymee

iTrader: (1)

It was on the road, full throttle, under load (I used the brakes to load it up like a dyno does).

As far as I know the MAF reading is the actual grams/second. Temperature does not affect the mass air flow. It affect the density and pressure, but a gram of air is always the same number of molecules of air. That is what is important.

Cheers,
Hymee.

shelleys_man_06

Well, it depends on what formula you are using for mass flow rate.

For example, remember the always abused Ideal Gas Law? If not, here it is -

pV=mR[G]T

'p' is the pressure.
'V' is the volumetric flow rate.
'm' is the mass flow rate.
'R[G]' is the ideal gas constant divided by the molar mass of the element.
'T' is the temperature.

Rearraging slightly, you get

m=pV/(R[G]T)

As you can see, the mass flow rate is a function of pressure and temperature.

The other form of the equation for mass flow rate,

m=pAV

where p and V are the same as the above equation, and A is the area, is only dependent on pressure and volume (flame me if I'm wrong).

Here's the real question: does anyone care what I just wrote?

shelleys_man_06

Actually, my real question

was pretty vague. Sorry about that. To find the volumetric efficiency, you must divide the actual displacement by the theoretical displacement. This is one of my favorite efficiencies because it can exceed 100% :D. Same thing with intercooler efficiency. Thermal efficiency is so depressing . Carnot efficiency, I can sort of live with . Anyways, I have the formula for the theoretical displacement somewhere on my computer, so that's one side of the equation. I suppose the only way to calculate the actual displacement is through experiment, or maybe some sophisticated simulation. Man, I'm a nerd .

Hymee

iTrader: (1)

I think you abused it all right.

I might need some help here, but the formulae is PV = nRT, is it not?

The mass of x molecules of gas is invariant. Keeping all other variables constant, If you increase the pressure, the volume will decrease. If you increase the volume, the pressure will decrease. If you increase the temperature the pressure will increase etc.

500 pounds of a gas will [b]always[b] contain the same number of molecules. 5 grams of air is always 5 grams of air.

That is why I was talking about the Mass Flow Rate, not the volumetric flow rate. That is why the car uses a Mass air flow sensor, not a volume sensor.

Where is RAP when I need him?

Cheers,
Hymee.

Hymee

iTrader: (1)

You will find the "actual displacement" in my very first post.

I used the sophisticated sensors and electronics on the car to tell me the Mass Air flow at each RPM point, and it is plotted on the graph.

The theortical displacement does not really require a computer, as we know each rotor displaces 654cc. That makes the theoretical displacement for the engine 1308 cc / revolution, or 1.3 litres. At 1.3 grams / litre for air (at sea level etc.), the theoretical mass flow can be calculated. That is what I did.

Then I plotted theoretical mass flow against actual mass flow and determined the % efficiency (by diving the theoretical mass flow by the actual mass flow).

So you could calculate the actual displacement from the g/s readings the engine was consuming, based on 1.3 g/litre.

So I gave you all the info you wanted...

Cheers,
Hymee.