| Author | Message | ||
     Reepicheep |
Stole this from another bike public forum... please keep all attributions in place (included below) if you use any of this. I thought it was a great writeup, the guy has been a huge help to me in the past, even if he does not get the whole "Buell thing". I can't recommend him highly enough if you have an import that needs work... From: Derek Capito <dcapito@moto-lab.com> Reply-To: radian@yahoogroups.com To: radian@yahoogroups.com Date: Apr 5, 2005 10:21 PM Subject: RE: [radian] Re: Greetings Reply | Reply to all | Forward | Print | Add sender to Contacts list | Report phishing | Show original Dean, The idea is that under perfect conditions (i.e. 100% efficiency), there is some amount of oxygen required to burn a known quantity of fuel with a minimum of leftovers. Unfortunately, 100% efficiency only occurs under the controlled conditions of a laboratory experiment. There must then be some "average" combustion engine efficiency that is used to calculate "A/F ratio" from oxygen content. So, we've arrived at the first problem: There is no direct correspondence between oxygen (or any other gas) content and "A/F ratio". Second, the ideal "A/F ratio" varies from engine to engine and from one rpm/throttle position/load combination to another. Imagine we equip a vehicle with a fuel flow meter and a mass air flow sensor. We then add or subtract fuel so that we have a stoiciometric mixture (as evidenced by the fuel flow and air mass measurements) at a particular chosen throttle position/rpm. We then screw a wideband "lambda sensor" into the exhaust. Do you think it will show a lambda value of 1.0? We then taylor the amount of fuel to yield some desired BSFC or HP. Do you think the A/F ratio as measured by our fuel flow meter/mass air flow sensor combo and our "lambda sensor" will match, or even be offset by the same amount? Say we try this experiment at a different throttle position/rpm. Do you think the results will match, or the offset from the previous comparison will be predictable, and therefore transferable to other throttle positions/rpm? Say we map this all out so that we know what these values are at all throttle positions/rpm. Will it carry over to another engine of the same model? And what if there have been modifications? Third, I'd like to consider what the goal of tuning is in the first place. Is it to get a particular "A/F ratio"? It's more likely that we would like the engine to make horsepower and be efficient (i.e. get good mileage). Looking at oxygen content has limited use when trying to achieve these ends. However, don't get me wrong, oxygen measurements are actually good for something. Once you have tuned for max power by adding and subtracting fuel, the oxygen numbers can help you to detect an ignition timing or stagger issue. If you go straight to a particular oxygen content, you completely ignore these factors. Even once you've corrected these, the oxygen content can still vary substantially. So it can help to point you in the right direction, but isn't the final arbiter of anything. CO on the other hand can, once a proper value has been established through bracketing, be used throughout as a target to set the mixture strength close to optimum for power and mileage. CO is also not the final arbiter of anything, but it tells you much, much more about whether an engines map is in the ballpark than O2. You can hit an oxygen target right off the bat while the engine has the completely wrong amount of fuel and/or wrong timing, whereas with CO, fuel will be close, guaranteed. As an example, there are spots on two Aprilia Futura dyno charts I have, where the O2 content is 0.2%. One has a CO of 12.6% (this is drowning rich!) and the other has a CO of 3.2% (perfect at this particular throttle position/rpm combination). I'm sure you can draw your own conclusions from this example. Certainly the best A/F ratio is where the engine makes best power. How do we know where best power is? By tuning until we’ve found it! Modern Fuel injection systems use O2 sensors to facilitate the use of catalytic converters. They need the O2 content to be in a particular range to function. Most closed loop ECUs only change the pulsewidth from the basemap based on O2 sensor input when the O2 content begins to go out of the range of where the cat will work efficiently as well as survive. The rest of the time, the ECU is just working off of the basemap with pulsewidth corrections based on TP, rpm, temps, pressures, etc. The map is never permanently modified by O2 sensor input, nor is the ECU ever running in a full closed loop mode, where pulsewidth is fully controlled by O2 sensor voltage. Regards, Derek motolab www.moto-lab.com <http://www.moto-lab.com/> | ||
| Blake |
Bill, Isn't all that just a long winded way of saying that we should tune for max power not to match a theoretically ideal A/F as deduced from O2 sensor, and that optimum A/F varies with TP, engine speed and ignition timing? Or did I miss something? Oh yeah... "Modern Fuel injection systems use O2 sensors to facilitate the use of catalytic converters." Better inform Buell of that fact so they can hurry up and add a catalytic converter to their DDFI models.  |
