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Title: Theoretical Mass Air Flow Post by: masterj on October 01, 2012, 03:50:19 PM Hi, guys! :)
For the past few hours I was trying to understand how to calculate theoretical Mass Air Flow with given RPM, DISPLACEMENT and VE. So what I did was this: THEORETICAL_CFM = DISPLACEMENT (IN^3) * RPM * VE / 3456 My engine displacement is 1781 CM^3 and somewhere on internet I've converted it to 108.683288 IN^3. VE is 100% (=1?). RPM for testing purposes is selected 800 (idle speed). So THEORETICAL_CFM = 108.683288 * 800 * 1 / 3456 = 25.15817 Now to convert this number to g/s (VCDS) I'm using this formula: G_PER_S = CFM * 0.075 * 453.59237 / 60 G_PER_S = 25.15817 * 0.075 * 453.59237 / 60 = 14,2644???? 4 != 14.2644 For some odd reason I'm not getting 4g/s as VCDS shows me... Did anyone tried to do something similar? Title: Re: Theoretical Mass Air Flow Post by: nyet on October 01, 2012, 04:14:13 PM hint: density of air (MAP)
Title: Re: Theoretical Mass Air Flow Post by: matchew on October 01, 2012, 05:00:17 PM 1 litre of air at 30'c weighs 1.165grams
Ignoring VE and with 1000mb absolute pressure in the intake, your engine at 800rpm consumes 712.4 litres per minute, or 11.87 litres per second. this is 13.83 grams/sec If there is 20inHg of vacuum in the intake then there would be ~4.6 g/s of air. Title: Re: Theoretical Mass Air Flow Post by: masterj on October 02, 2012, 02:08:45 AM 1 litre of air at 30'c weighs 1.165grams Ignoring VE and with 1000mb absolute pressure in the intake, your engine at 800rpm consumes 712.4 litres per minute, or 11.87 litres per second. this is 13.83 grams/sec If there is 20inHg of vacuum in the intake then there would be ~4.6 g/s of air. Could you write formulas? I am going to implement this in excel for already logged data: rpm, intake temp, map Thanks Title: Re: Theoretical Mass Air Flow Post by: masterj on October 09, 2012, 04:06:14 PM bump... really need to figure out how did you get that number...
Title: Re: Theoretical Mass Air Flow Post by: nyet on October 09, 2012, 05:11:40 PM (((1 bar) - (20 inHg)) / (1 bar)) * (14.26 (g / sec)) in g/sec (http://www.google.com/search?q=%09+%28%28%281+bar%29+-+%2820+inHg%29%29+%2F+%281+bar%29%29+*+%2814.26+%28g+%2F+sec%29%29+in+g%2Fsec)
Title: Re: Theoretical Mass Air Flow Post by: masterj on October 09, 2012, 05:57:04 PM (((1 bar) - (20 inHg)) / (1 bar)) * (14.26 (g / sec)) in g/sec (http://www.google.com/search?q=%09+%28%28%281+bar%29+-+%2820+inHg%29%29+%2F+%281+bar%29%29+*+%2814.26+%28g+%2F+sec%29%29+in+g%2Fsec) Thank you, right now formula looks like this: CFM = DISPLACEMENT (in³) * RPM * VE / 3456 MAF = CFM * 0.075 * 453.59237 / 60 MAF_FIX = ((MAP - VACUUM) / MAP) * MAF FINAL_MAF (g/s) = (DISPLACEMENT(in³) * RPM / 3456) * (0.075 * 453.59237 / 60) * ((MAP(bar) - VACUUM(bar)) / MAP(bar)) Does this look OK? The MAP(bar) values are ambient pressure or turbo pressure? How do we incorporate intake air temperature into whole formula? I really need to be this as precise as possible given these inputs: RPM, MAP sensor values, engine displacement, IAT sensor values I'm going to implement something like MAF sensor check via excel and logs, but need to be sure 100% that formula is OK... Title: Re: Theoretical Mass Air Flow Post by: nyet on October 09, 2012, 06:31:56 PM No. That makes no sense.
You supplied some sort of weird map reading (vac) and not abs pressure, so i converted it into absolute pressure for you (1 bar - vac in bar) and then divided it by 1 bar to get a constant (to keep the units right, which google calc will do for you if you are careful). Quit making up formulas randomly :P The goal is to convert standard density into a density relevant for your manifold pressure. If all you have is pre-tb pressure, you're screwed anyway. You'd have to do what ME7 does, which is model manifold pressure based on MAP, IAT, air flow, and throttle plate angle. Title: Re: Theoretical Mass Air Flow Post by: Bische on October 09, 2012, 07:21:19 PM No. That makes no sense. You supplied some sort of weird map reading (vag) and not abs pressure, so i converted it into absolute pressure for you (1 bar - vac in bar) and then divided it by 1 bar to get a constant (to keep the units right, which google calc will do for you if you are careful). Quit making up formulas randomly :P The goal is to convert standard density into a density relevant for your manifold pressure. If all you have is pre-tb pressure, you're screwed anyway. You'd have to do what ME7 does, which is model manifold pressure based on MAP, IAT, air flow, and throttle plate angle. As in msdk_w? Title: Re: Theoretical Mass Air Flow Post by: nyet on October 09, 2012, 09:20:07 PM yes. but if you are trusting msdk_w, it means you are trusting the MAF, which means a sanity check is kinda pointless :)
Title: Re: Theoretical Mass Air Flow Post by: Bische on October 09, 2012, 10:51:38 PM yes. but if you are trusting msdk_w, it means you are trusting the MAF, which means a sanity check is kinda pointless :) Yeah, staring at my logs leads me to believe msdk_w is only ~accurate when lamsoni_w = lamsbg_w? I have not looked this up in FR, just an observation, but if its true it means one could spot places where the mixture is corrected in the wrong end, to attain actual = requested.(which is one major concern of mine, being OCD) Title: Re: Theoretical Mass Air Flow Post by: nyet on October 10, 2012, 09:29:05 AM Lets put it this way: if YOU can detect mixure problems solely from analyzing logs (and no extra sensors, MAP, wideband etc.) then bosch could also, and it probably already exists in ME7 in real time :)
Title: Re: Theoretical Mass Air Flow Post by: Bische on October 10, 2012, 10:36:36 AM Lets put it this way: if YOU can detect mixure problems solely from analyzing logs (and no extra sensors, MAP, wideband etc.) then bosch could also, and it probably already exists in ME7 in real time :) Im not sure I understand what you mean? I do have an wideband ME7.5 ECU, so I have actual lambda to compare all the time. :) |