MAF based understanding

[Hentai]

There are two types of LOAD specified in SAE J1979

One is CALCULATED LOAD VALUE and the other is ABSOLUTE LOAD VALUE.



Please take a look in this explanation.



Calculated LOAD Value (PID 04)



The OBD regulations previously defined CLV as:

(current airflow / peak airflow @sea level) * (BARO @ sea level / BARO) * 100%

Various manufacturers have implemented this calculation in a variety of ways. The following definition, although a little more restrictive,

will standardise and improve the accuracy the calculation.

LOAD_PCT = [current airflow] / [(peak airflow at WOT@STP as a function of rpm) * (BARO/29.92) * SQRT(298/(AAT+273))]

— Where: STP = Standard Temperature and Pressure = 25 °C, 29.92 in Hg BARO, SQRT = square root,

— WOT = wide open throttle, AAT = Ambient Air Temperature and is in °C

Characteristics of LOAD_PCT are:

— Reaches 1.0 at WOT at any altitude, temperature or rpm for both naturally aspirated and boosted engines.

— Indicates percent of peak available torque.

— Linearly correlated with engine vacuum

— Often used to schedule power enrichment.

— Compression ignition engines (diesels) shall support this PID using fuel flow in place of airflow for the above calculations.

NOTE Both spark ignition and compression ignition engines shall support PID $04. See PID $43 for an additional definition

of engine LOAD.

05 Engine Coolant Temperature A –40 °C +215 °C 1 °C with

–40 °C offset

ECT: xxx °C (xxx °F)

ECT shall display engine coolant temperature derived from an engine coolant temperature sensor or a cylinder head temperature

sensor. Many diesels do not use either sensor and may substitute Engine Oil Temperature instead.



Absolute Load Value (PID 43)



The absolute load value has some different characteristics than the LOAD_PCT defined in PID 04 This definition, although restrictive,

will standardise the calculation. LOAD_ABS is the normalised value of air mass per intake stroke displayed as a percent.

LOAD_ABS = [air mass (g / intake stroke)] / [1.184 (g / intake stroke) * cylinder displacement in litres]

Derivation:

— air mass (g / intake stroke) = [total engine air mass (g/sec)] / [rpm (revs/min)* (1 min / 60 sec) * (1/2 # of

cylinders (strokes / rev)],

— LOAD_ABS = [air mass (g)/intake stroke] / [maximum air mass (g)/intake stroke at WOT@STP at 100% volumetric efficiency]

* 100%.

Where:

— STP = Standard Temperature and Pressure = 25 °C, 29.92 in Hg (101.3 kPa) BARO, WOT = wide open throttle.

The quantity (maximum air mass (g)/intake stroke at WOT@STP at 100% volumetric efficiency) is a constant for a given cylinder swept

volume. The constant is 1.184 (g/litre 3) * cylinder displacement (litre 3/intake stroke) based on air density

at STP.

Characteristics of LOAD_ABS are:

— Ranges from 0 to approximately 0.95 for naturally aspirated engines, 0 – 4 for boosted engines,

— Linearly correlated with engine indicated and brake torque,

— Often used to schedule spark and EGR rates,

— Peak value of LOAD_ABS correlates with volumetric efficiency at WOT.,

— Indicates the pumping efficiency of the engine for diagnostic purposes.

Spark ignition engine are required to support PID $43. Compression ignition (diesel) engines are not required to support this PID.

[Hentai]

here seems two different formulas to come up with air mass as in grams a intake stroke, grams a cylinder

air mass (g / intake stroke) = [total engine air mass (g/sec)] / [rpm (revs/min)* (1 min / 60 sec) * (1/2 # of

cylinders (strokes / rev)]

air mass (g / intake stroke) = [1.8 g\s] / [(798)* (1 min / 60 sec) * (2)]
air mass (g / intake stroke) = [1.8 g\s] / [(798)* (0.016666666666666666666666666666667) * (2)]
air mass (g / intake stroke) = [1.8 g\s] / [25.536]
air mass (g / intake stroke) = 0.067669172932330827067669172935053

grams a cylinder = (total engine air mass (g\sec) * 30 / RPM
grams a cylinder = 1.8 g\s * 30 / 798
grams a cylinder = 54 / 798
grams a cylinder = 0.067669172932330827067669172932331

[SleepyKeys]

It looks like that takes care of the air side of things. Now you need the *fuel side of the formula(lambda, stoich AFR, fuel density), to generate a PW. The cool thing is that you can play with these formulas in tandem, with others, to check for concurrency.

replace "concurrency" with "consistent"

[Fred]

concurrency?

[SleepyKeys]

In other words, "the same".
Equation1 = a;
Equation2 = b;
Equation3 = c;
a, b and c, should be equal.

[Fred]

Ahh, consistent! :-)

[Hentai]

update to the equation, the revs need to be half of what I said, there is a reason to it. But if you divide revs by two. You will get a proper load number now that equals out right to what I would see normally.

air mass (g / intake stroke) = [1.8 g\s] / [(798)* (1 min / 60 sec) * (2)]
air mass (g / intake stroke) = [1.8 g\s] / [(798)* (0.016666666666666666666666666666667) * (2)]
air mass (g / intake stroke) = [1.8 g\s] / [25.536]
air mass (g / intake stroke) = 0.067669172932330827067669172935053

air mass (g / intake stroke) = [1.8 g\s] / [(798 / 2)* (1 min / 60 sec) * (2)]
air mass (g / intake stroke) = [1.8 g\s] / [(399)* (0.016666666666666666666666666666667) * (2)]
air mass (g / intake stroke) = [1.8 g\s] / [0.13533834586466165413533834587016]
air mass (g / intake stroke) = 0.13533834586466165413533834587016

[Hentai]

The actual formula's are in my previous post about Load ABS. I was trying my best to figure out units and deal with conversion data. but everything seems to work now.
To make it seem simple in words you take what current air consumption is and divide it by air consumption at estimated 100% VE of your engine at the same revs using STP.

Now we got a load variable to use.
Now to figure out how to get a PW from all this.
math I used will be in next post for people to go over and ceck over and throw their thoughts in.

[Hentai]

MASS at 100 % VE = ( displacement * STP * revolutions) / 2
MASS at 100 % VE = ( 96ci * .00004671 * 1000) / 2
MASS at 100 % VE = (4.48416) / 2
MASS at 100 % VE = 2.24208 lbs/min

conversion to grams a second = 16.949839682 grams a second at 100% VE

[Hentai]

update on STP
J-1349
77°F \ 25°C and 29.23 inches of mercury