# Thread: Calculating Diesel Fuel Consumption / MPG (OBD2) - Help needed

1. ## Calculating Diesel Fuel Consumption / MPG (OBD2) - Help needed

Hi there!

I'm a new user, but have been lurking as guest for quite a while. Anyways, I'm seriously hoping for some kind of help to achieve some accurate diesel MPG readings for my Car-PC application.

I've been trying to calculate MPG using Bruce Lightner's Formula found here:
http://www.lightner.net/lightner/bruce/Lightner-183.pdf
http://www.mp3car.com/engine-managem...from-obd2.html

I'm developing a WPF/C# application with some basic information and a really nice design. For that application I would like to display Fuel Consumption as well. Screenshots can be posted if you're interested, however I'm not that far into the development yet since the MPG issue has been stalling it.

First of all, here is a readout from my OBD2 application while driving ~50 Km/h. The data has been verified thru multiple OBD applications. The car I'm driving is a 2011 - Peugeot 206+ 1.4 HDi (Diesel).

Explanation:
MAF = MAF Air Flow (Grams/Sec)
VSS = Veichle Speed (Km/h)
MAP = Intake manifold absolute pressure (kPa (absolute))
IAT = Intake Air Temperature (°C)
CER = Command Equivalence Ratio
EL = Calculated Engine Load Value (%)

-- 02-04-2013 21:13:34 --
MAF: 11,83
VSS: 49
MAP: 103
IAT: 29
CER: 48
EL: 38,0392156862745
----
-- 02-04-2013 21:13:37 --
MAF: 11,63
VSS: 51
MAP: 104
IAT: 31
CER: 51
EL: 29,4117647058824
----
-- 02-04-2013 21:13:40 --
MAF: 11,99
VSS: 50
MAP: 102
IAT: 29
CER: 50
EL: 25,4901960784314
----

Using Bruce Lightner's Formula with the last readout looks like this (C#.NET code):

// Diesel Density
double dieselDensity = 7.079;
// Veichle Speed (Km/h)
double VSS = 50;
// MAF Air Flow Rate (grams/sec)
double MAF = 11.99;

double result1 = (14.7 * dieselDensity * 454 * (VSS * 0.621371))
// result1: 1467797,30076021

double result2 = ((3600 * MAF) / 100);
// result2: 431,64

double MPG = result1 / result2;
// MPG: 3400,512697526202

That gives me 3400,51 Miles Per Gallon! Thats quite an economic car I got there if it was accurate, but obviously they are not. I must be forgetting or overseeing something, I just don't know what? So any help would be appreciated and please dont hesitate to ask if you need any further information.

Other formulas and calculation methods are welcome, right now I'm just eager to get something to work since I'm kinda stuck with these results.

Thanks in advance for any help

// tren

2. Using a petrol formula on a diesel engine will always produce wrong results.
A diesel works quite different than a petrol engine.
Have you seen this topic: http://www.mp3car.com/engine-managem...el-engine.html

3. Originally Posted by p2psmurf
Using a petrol formula on a diesel engine will always produce wrong results.
A diesel works quite different than a petrol engine.
Have you seen this topic: http://www.mp3car.com/engine-managem...el-engine.html
I was told by serveral OBD software developers that Bruce Lightner's formula would also work on Diesel cars, all you had to do was to change the Fuel Density from gasoline/petrol to diesel. Also, using the MAF calculation option in both OBDwiz and Dashcommand provides some accurate data, so it must be possible somehow?

4. Find out a little bit more on HOW a diesel management system works and HOW the amount of diesel injected is calculated.
Then you will see that the so-called "OBD developers" have no idea whatsoever on how diesel engines and engine management systems work.

5. So what your saying is that it is not possible to properly calculate diesel fuel consumption?

Or can it actually be done? If so, where do I start to look for some information?

6. I'm still trying to achieve instant MPG for my diesel car, but still I can't seem to get anything to work properly!

Is this impossible or where would I look to get any help? This is becomming an obsession to get working!!

7. It can be done, my past three Ford Super Duty's had overhead console computers that showed avg MPG/ or L/100 km, and km/miles to empty. The miles to empty was VERY accurate if you ran for a lot of miles without shutting it off as it came down to empty. Several times I ran it to about 10km to empty and stuffed about 150L into a tank rated at 144L so I know it was empty.