Quote:
Originally Posted by MetroMPG
2018's emission test will confirm whether the change to the MAF spring tension keeps things within specs. Given the soot in the tailpipe prior to the change (suggesting a rich condition), I'd bet it's still OK.
|
That depends on the emissions test. My guess is that we aren't talking about $5000 per day FTP75 testing on an electric dyno. IM240 testing on an inertia dyno is like a go/no-go gage when it comes to NOx emissions and it isn't even capable of measuring NMHC or NMOG emissions, only total HC. Going off of closed-loop operation to lean of stoichiometry will cause a large increase in NOx emissions under those conditions even if the IM240 test doesn't hit those exact speed/load points during testing - in other words, it will only pick it up if it hits the conditions where you are operating net lean. If you plot exhaust lambda vs. engine speed and torque for U.S. or California emissions compliant vehicles produced since 2004, what you will see is closed loop and near constant exhaust lambda (typically slightly rich of stoichiometry) over the entire speed/load map, even for transient operation, except along a line running from peak power to peak torque, and sometimes at speeds below peak torque, where it is often open-loop and net rich for component protection (exhaust temperatures at higher speeds, preignition at very low speed/high torque). This is largely also for NOx emissions compliance. NOx cannot be reduced over a three-way-catalyst (TWC) when the partial pressure of NO or NO2 exceeds the partial pressure of O2 in the exhaust. NOx reduction over a TWC at stoichiometric conditions exceeds 98%, so NOx literally goes through the roof when lean - it blows right past the TWC unchanged other than oxidation of NO to more toxic NO2. A secondary reason for this type of A/F calibration is catalyst protection. A TWC will rapidly sinter its precious metals when operated even at moderate loads in a net-oxidizing exhaust environment (i.e., 600C exhaust temperature - pretty common). Same temperatures at stoichiometry or rich operation has a much lower sintering rate (in fact, slight-net-rich at 900C is even OK). When precious metals migrate through washcoating material and sinter, they rapidly lose surface area and the TWC begins to turn into an exhaust restriction instead of an exhaust catalyst.
So, again, lets keep it clean. There are other ways to achieve higher engine efficiency besides lean operation. The primary benefit of lean operation is pumping work reduction from dilute combustion (i.e., larger throttle opening for the same BMEP non-dilute). The charge dilution doesn't need to be with air. What is becoming more common with new production spark ignition engines is to use cooled (usually engine coolant heat exchanger) exhaust gas recirculation (EGR) for charge dilution (2nd gen Prius, 2018 HEV and non-HEV Camry, Mazda CX9 turbo, any number of recent Subarus) in order to reduce pumping work and improve cycle efficiency while maintaining exhaust lambda near 0.97 - 1.00. Also, any type of charge dilution also benefit significantly from a higher energy ignition system in order to reduce misfire (misfire is more common lean and/or with EGR), so going to a higher energy ignition system, preferably coil-on-plug with multi-strike capability (I think latest MegaSquirt variants support this) helps both combustion efficiency and hydrocarbon emissions. Increasing spark ignition energy also allows an engine to run even more dilute, which improves part load efficiency even further.