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Old 09-02-2008, 09:57 PM   #7 (permalink)
cfg83's Avatar
Join Date: Dec 2007
Location: Southern California
Posts: 5,864

1999 Saturn SW2 - '99 Saturn SW2 Wagon
Team Saturn
90 day: 40.49 mpg (US)
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Hello -

I noticed this commercial program from Australia. It is designed to work with the ElmScan hardware. It also claims to be able to deduce instant-MPG from OBD II data, which is what I have been looking for :

OBD 2007 - Installation Guide
Fuel Analysis (Page 15 of PDF)

Even though there is no specific pid for fuel consumption in the OBDII specification, fuel consumption can be calculated, if a vehicle supports Pid 0x0D Vehicle Speed and Pid 0x10 Mass Air Flow. All vehicles support vehicle speed and nearly all vehicles support MAF.

MAF, the mass of air moving into the engine is measured in grams per second. A petrol/gasoline engine burns the air/fuel mixture at the stoichiometric air fuel ratio of 14.64:1. The oxygen sensors in a modern vehicle are designed in conjunction with both the short and long term fuel trims to maintain this ratio. Under ideal conditions for every 14.64 grams of air an engine will consume 1 gram of petrol. We can therefore calculate FuelFlowGramsPersSec = MAF/Air Fuel ratio. We can convert the previous figure to FuelFlowLitresPerSecond by dividing FuelFlowGramsPerSec by the density of petrol/gasoline.

We can then readily convert that figure to FuelFlowLitresPerHour -- i.e. litres/hour. Fuel consumption is then simply SpeedKmsPerHour/FuelFlowLitresPerHours -- i.e. litres/kms. All the remaining figures on the OBD 2007 Fuel Analysis tab are returned by simple conversion factors -- for Metric, Imperial (US) and Imperial (UK) units.

Late model vehicles which use Oxygen sensors known as wide range linear Oxygen sensors, can also introduce another Pid 0x44, Commanded Equivalence Ratio. This pid allows fine adjustment of air fuel ratio from the constant 14.64 by modifying the air fuel ratio by a value between 0 and 1.999 which permits air fuel ratios to be measured to greater precision. If a vehicle supports Pid 0x44, OBD 2007 fuel consumption figure is modified by the effect of Commanded Equivalence Ratio.

To assist in understanding what is going on under various driving conditions, the OBD 2007 Fuel Analysis tab also includes gauges for Short and Long Term Fuel Trims. Ideally the short term fuel trims are trying to maintain the long term fuel trims at around 0%. A negative figure is an indication of leanness and a positive figure is an indication of richness.

The final components of OBD 2007’s Fuel Analysis tab are two LEDs to indicate the state of Pid 0x03, Fuel Status. The LEDs can display 5 different colours to indicate the various states of Fuel Status.

Gray -- Unused.

Red -- System is running Open loop -- has not yet satisfied conditions to go Closed loop.

Green -- System is running Closed loop -- using O2 sensors as feedback for fuel control.

Amber -- Open loop due to driving conditions -- either enrichment or enleanment.

Orange/Gray -- Fault detected.

Because the gauge is an instantaneous fuel consumption gauge, it is quite normal for the gauge to swing wildly whenever the vehicle is accelerated quickly (high fuel consumption) or decelerated quickly (low fuel consumption). You may even notice on the digital fuel consumption gauges a reading of -Err-. This doesn’t actually signify an error, but is an indication that the fuel consumption figure is greater than what the gauge can display. E.g. if the fuel consumption is greater or equal to 100mpg, that figure cannot be displayed because there is only room for two significant digits.

When the vehicle is stationary with the engine idling, fuel consumption is zero. However the fuel rates of the right hand digital gauges will show their respective fuel flow rates. The following window below displays a typical fuel consumption window.

Please note like the Dashboard tab the Fuel consumption tab is optimised for viewing in full screen mode.
Picture of "Fuel Analysis" Gauge :
Click image for larger version

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Question: What do you think? Does the equation they are using look valid to you?


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