air flow and fuel flow calculators

8/12/2019 Air Flow and Fuel Flow Calculators

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Air Flow and Fuel Flow Calculatorsby Jeff Lucius

This web page is designed to help predict air and fuel flow or the Mitsubishi 3000GT VR4 and DodgeStealth TT turbocharged cars when these cars are operated at wide-open throttle (WOT). Dataloggers

for our cars, at least for 1991-1993 OBDI models, cannot report volume air flow because of a "toofew bits reported" response from the ECU after a query. So another method must be used to determineair flow, and from that to determine fuel requirements for our engine.

For background information on some of the theory and formulas used on this page, please see myPressurization Primer.My Pressurization Primer has a dyno chart that can be used to estimate "naturalcapacity" volumetric efficiency for our engines (explained below). Unit conversion calculators can befound atUnit Convertors.

As you use a calculator, note that some input and result values will be assigned in the empty inputfields in the calculators that follow it. You will have to reset these "lower" calculators before changing

values in the "upper" calculators if you want the values to carry through. Text boxes with a white

background can be changed at any time. Text boxes with a yellow background (maybe still white inNetscape, Opera, and other browsers) are read-only. Please remember that by using your browser to

"View Source" and saving the HTML file to your local disk drive, you can have this page availableoffline.

NOTE: These calculators may not work correctly in Netscape, Opera, or another browser

besides MS Internet Explorer.

For those of you with IE 7 (or beyond), you may get a warning about my web site using ActiveX

controls. It does not. I do use JavaScript for my calculators. If you want the functionality of the

calculators, allow "ActiveX" controls (see instructions by clicking on the IE bar above my web

page, if it is there).

Air Density

Air density can be calculated using the following equation or the JavaScript calculator below.

D = D0x (T0/T) x (P/P0),

where T0= 545.69R (86F) or 303.15K (30C),

P0= 14.7 psi (= 1 atm = 760 mm Hg = 29.92 in. Hg = 1.033 kg/cm2= 1.013 bar = 33.9 ft H2O), andD0= 1.1649 g/L = 32.986 g/CF = 0.072751 lb/CF.

T, P, T0, and P0must be in absolute temperature and pressure. Values for T0, P0, and D0were takenfrom tables in the CRC Handbook of Chemistry and Physics. For F add 459.69 to get Rankine and

for C add 273.15 to get Kelvin. Just use Fahrenheit or Celsius below.

For the Air Density calculator, use the underhood air temperature and the absolute atmospheric

pressure. The underhood air temperature for our cars when driving is usually 5F to 20F aboveambient temperatures. Atmospheric pressure decreases at the rate of about 1 in. Hg or 0.49 psi forevery 1000 feet (304.8 m) increase in elevation. Where I live at (5500' elevation), "standard" airpressure is about 12 psi (24.42 in. Hg), not the 14.7 psi (29.92 in. Hg) found at sea level.

Air Density

Temperature Pressure Density

F C psi in. Hg g/L g/CF lb/CF
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Volume Air Flow

Approximately the same volumeof air is drawn in as the piston moves down regardless of engine

speed (RPM), load (throttle opening), or intake manifold pressure. This volume is the cylinderdisplacement, with the consideration that exhaust gas reversion and other factors may reduce this

volume somewhat. However, the densityof the air that is drawn in varies quite a bit. When the airdensity is less than atmospheric pressure we measure a vacuum in the intake manifold, like duringidle. In a normally-aspirated engine at WOT the air density approaches that of atmospheric pressure.When the density is greater than atmospheric, there is positive pressure (or boost) in the manifold.Rather than measure the air density in the cylinders or manifold, we measure the equivalent volume of

outside air. The Volume Air Flow (VAF) is the amount of outside air that goes through an engineduring a specified time period. The ratio of this volume of outside air to the engine displacement is

what is usually referred to as Volumetric Efficiency (VE). VE rarely exceeds 100% for normally-aspirated engine. VE exceeds 100% for forced-induction engines.

The JavaScript calculator below calculates the VAF (volume of outside air flow) in cubic feet perminute (CFM) and liters per minute (LPM) based on engine displacement (CI or L), speed (RPM),and volumetric efficiency (VE). For this exercise, I define the volumetric efficiency as the product of

the natural flow capacity(NC) of the engine, at a particular RPM and load, and the overall densityratio (DR), due to boost pressure. When the actual volume of air (compressed or at partial vacuum)entering the cylinders equals the engine displacement then the NC is 100%.

The next two formulas calculate VAF in CFM. The following one is for displacement in CI.

VAF = (CI/1728) x (RPM/2) x VE,

where, VE equals the natural capacity (NC) times the overall density ratio (DR). If the displacement isin liters then use the following formula.

VAF = (L/28.317) x (RPM/2) x VE.

Negative input values are not allowed. In addition, NC must range from 0 to 100 (percent), and theDR cannot be greater than 5. For a stock, turbocharged, Mitsubishi 6G72 2.972-L engine, NC rangesfrom 75% to 95% at WOT. Overall density ratios are usually less than 2.5 (that is boost pressure isless than 24 psi). The overall density ratio is less than the ratio of absolute boost pressure toatmospheric pressure (the pressure ratio) because of frictional and heating losses. For normally-aspirated engines use a DR equal to 1. Engine redline is around 7300 RPM. However, peak NCusually occurs between 5000 and 6000 RPM in the 6G72 engine.

Volume Air Flow

Displacement RPM Volumetric Efficiency Volume Air Flow

cubicinches liters

Naturalcapacity,%

Densityratio CFM LPM

Mass Air Flow


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Mass air flow (MAF) is calculated using the volume air flow as determined and explained above, anda value for density calculated from the temperature and pressure near the air filter. Once the mass airflow is known, the engine electronic control unit (ECU) can calculate the amount of fuel to add toachieve a pre-determined air/fuel mixing ratio. The same data entry restrictions used above apply tothe MAF calculator with the additional restriction that air density is in grams per cubic foot (g/CF)and must be greater than 0. For air density, use the value from the air density calculator foruncompressed, outside air. For normally-aspirated engines use a DR equal to 1.

Mass Air Flow

Displacement RPM Volumetric Efficiency Density Mass Air Flow

cubic

inches litersNatural

capacity,%Density

ratio g/CF g/min lb/hr

Fuel Flow

Of course, once we know MAF we can calculate fuel flow using aspecific air/fuel mixing ratio (A/F). MAF must not be negative,

and A/F can range from 6 to 22. Using an average density ofgasoline of 6 pounds per gallon, then lb/hr (mass) fuel flow times10.5 is equivalent to (volume) fuel flow in cc/min.

Optional input: You can predict engine power (HP) output byassuming a certain brake specific fuel consumption (BSFC). BSFC

must be a value between 0 and 1. High-performance race engines

can have BSFC values (they vary a little across the RPM range) below 0.4. The average street enginehas BSFC values near 0.5. Turbocharged vehicles' BSFCs are often near 0.6. I think values from 0.55to 0.60 are representative for our stock to mildly modifed Mitsubishi 6G72 turbocharged engines. Ourhighly modified engines may have a BSFC as low as 0.45.

Fuel Flow

Mass Air Flow A/F Fuel Flow BSFC Power

g/min lb/hr lb/hr cc/min HP

12.0

0.50

Fuel Injector Sizing

Air/Fuel Ratio Limits

6.0:1

9.0:111.5:1

12.5:1

13.2:1

14.7:1

15.5:1

16.2:1

18-22:1

Rich run limit

Low power, black smokeRich best torque at WOT

Safe best power at WOT

Lean best torque at WOT

Chemically ideal

Lean light load, part throttle

Best economy, part throttle

Lean run limit


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The JavaScript calculator below will calculate total fuel flow based on the injector size (static fuelflow rating), fuel line pressure, number of cylinders, and the injector duty cycle (IDC). The industryrecommends a continuous, maximum IDC of no more than 80% (that is, injectors opened 80% of thetime avaliable to them). 90% is probably OK for brief intervals. Injectors may actually flow less fuelabove 95% IDC than predicted as they may just cycle between partly opened and partly closed. Youshould leave the injector rated pressure (@ psi) at 43 unless you know that the injector was tested atanother pressure.

When the base fuel line pressure is different than the rated injector pressure, then the injectors willflow differently than rated according the following formula.

FN= F0x SQRT(PN/P0),

where FNequals the new flow rate when the differential fuel line pressure is PNand the injector has arated flow rate of F0at a rated pressure of P0. The differential fuel line pressure (psi) is the differencebetween the actual line pressure and the boost pressure in the manifold. Our cars are designed tomaintain a 43 psi differential pressure.

Optional input: If you assign a value for BSFC then engine power (HP) will be estimated. If an A/F

value from 6 to 22 is entered then mass air flow (lb/hr) is calculated. In addition, if you also assignvalues for the underhood air temperature and the ambient air pressure, then volume air flow (CFM) iscalculated.

Fuel Injector Sizing

Injector Rated Flow Fuel line Cylinders IDC Total Fuel Flow BSFC Power

cc/min lb/hr @ psi psi % cc/min lb/hr HP

43

43

6

90

0.55

Temperature Pressure A/F Total Air Flow

F C psi in. Hg CFM lb/hr12.5

air flow and fuel flow calculators

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