Air/fuel ratio control is theprimary function of any en-gine management system.Note that this statement offact starts with air. Themass airflow (MAF) sensor isa direct measure of the air entering the en-gine, compared to the speed-density sys-tem, which uses manifold absolute pres-sure (MAP) to calculate air delivery.

In this months column, Ill concentrateon understanding the relationships amongairflow, load, fuel delivery and MAF sys-tems, and what effect those interrelation-ships have on driveability. The test vehiclewas a properly running 1.8L 2004 ToyotaCorolla.

The following idle airflow calculationclosely matches the actual idle airflow inFig. 1 below:

750 rpm 2 (for 4-stroke cycle) 60 sec-onds in a minute = 6.25 power cyclesper second x 1.8L x 1.184 grams per liter 15% Volumetric Efficiency = 2.0 gramsof air per second (gm./sec.)

In the pair of graphs in Fig. 1, the idleairflow is 2.07 gm./sec. at idle, no load, inNeutral. Shifting into Drive increased theairflow to 2.4 gm./sec., a load increase ofabout 20%. I turned on the Corollas airconditioning and the airflow rose to 3.7gm./sec. to compensate for the load. Thea/c load increase of 1.3 gm./sec. is a nearly65% increase over the base idle load of 2.0gm./sec.

The general rule for airflow at idle isthat it should match the engine liter dis-placement 20%. You can see that addi-tional loads can change this number con-

MarkWarren

A better understanding of the interrelationships among fuel delivery,

airflow and relative load will help your diagnostic abilities. It begins by

recognizing that fuel delivery calculations are all about the airflow.

smwarren@motor.com

Driveability CornerSc

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Fig. 1

17August 2010

continued on page 18

siderably. Be sure to test idle air-flow with all loads off.

Fig. 2 shown above is a trio ofgraphs that were created during atest drive on the freeway, with awide-open throttle (WOT) in the

center, a deceleration and idle atthe end. While the load/airflow atspeed was uneven due to the vol-ume of traffic, the typical airflow at75 mph was 23 to 25 gm./sec. Theairf low at 5000 rpm WOT was

about 70 gm./sec., or 39 times 1.8L.Typically, wide-open throttle at5000 rpm is 37 to 40 times the en-gine displacement in liters.

The fo l lowing equat ions wi l lhelp you better understand the re-lationship between airflow and fuelconsumption:

70 mph 35 mpg = 2 gals./hr.2840 gm./gal. x 2 gals. = 5680gm./hr.

5680 gm./hr. 3600 sec. = 1.58gm./sec. (gasoline)

23.3 gm./sec. air 14.7 air/fuelratio = 1.58 gm./sec. fuel

The top set of equations providethe amount of fuel consumed, ingm./hr., at 70 mph and 35 mpg:5680. The middle equation is theamount of fuel consumed ingm./sec.: 1.58. The lower equation

Driveability Corner

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Circle #11

18 August 2010

Fig. 2

is a reverse calcu-lation of airflow,multiplying 1.58gm./sec. of fuelby 14.7:1 air/fuelratio. The reversecalculation of air-f low closelymatches the actu-al measured air-flow.

The chart inFig. 3 at r ightshows the mea-sured airflow un-der different loadconditions (firstcolumn). Thesecond column is the measured air-flow in gm./sec.; the third columnis calculated fuel consumption ingm./sec. at 14.7:1 air/fuel ratio (ex-cept for the last row, WOT, calcu-lated at 12.5:1 air/fuel ratio); the

fourth column is fuel consumptionin gals./hr.; the fifth column is thepercent of load measured against5000 rpm WOT; the sixth column ismpg achieved.

Gaining a better understanding

of fuel delivery, airflow and relativeload will help improve your diag-nostic abilities when it comes todriveability problems. Remember,fuel delivery base calculations areall about the airflow.

19August 2010

Circle #12

Fig. 3