[arghx7]

Originally Posted by serialk11r

Thanks arghx7, tons of useful stuff as usual...
Offset cranks...doh forgot about those. Is there anything important about them that we should know? I imagine the offset reduces friction at low rpm by having the rod at a smaller angle on the power stroke, but what about at high rpm where the "inertial" forces are much greater?

It reduces friction and piston slap, but I'm not 100% sure if that effect is more or less significant at higher rpm. Check out this document https://docs.google.com/open?id=0B_j...c0hLTURYOWxyZw You can jump to page 38.

Also I was wondering why engines usually run open loop at high rpm, if you don't mind.

In the old days on a production car you had a narrowband sensor before the cat and that's it. That was when there were fewer drive cycles for emissions/fuel economy tests: the FTP75 (can also be referred to as LA4), the HWFET (Highway fuel economy test), and certain Japanese and European cycles. It's conceivable that you could drive through those cycles and never get out of closed loop once the engine warms up. Read this about a certain old 80s fuel injection system, roughly comparable to a simpler Megasquirt by my estimation: https://docs.google.com/open?id=0B_j...LTlBTjBJVWhXUQ

Then the government started adding more test cycles that apply additional load to the engine (SC03 which has the air conditioning on, and US06 which goes at higher speed and acceleration). Now you are going into enrichment more while at the same time emissions and fuel economy standards are getting tighter. So they introduce the closed loop delay to pass the tests and improve fuel economy. You have to be careful with the way it's set up or exhaust temperatures get too hot and the catalyst ages prematurely. As standards got tighter they had started putting in rear O2 sensors (to have a Catalyst monitor) and over time more factory widebands in the front.

Read this for some info on how the narrowband in front and rear system works: https://docs.google.com/open?id=0B_j...Z1ItUjlDWURBQQ

and read this for more info on one type of planar wideband in front + narrowband in the rear catalyst strategy: https://docs.google.com/open?id=0B_j...c3duZTZ5UTBQdw

With closed loop under all rpm and load points, in the end you can boil it down to cost. It takes more development and testing man hours, more sophisticated engine controllers and models, plus better sensors to do it that way. Multiply that by a gazillion cars and the costs add up. That's why the more sophisticated control strategies are mostly on higher end mass production cars and in areas with the tightest emissions standards (US and Europe).

Depending how you put it together, it's more than just having a target AFR table inside the ECM and some kind of basic feedback control. You really need to have rationality checks and monitors to make sure the model and the sensors are working correctly. If you go closed loop at all rpm and something goes wrong with the model (exhaust too lean), the catalyst could burn up from high exhaust temperatures or misfire, or you could severely degrade exhaust components, or even have a significant knock event. If the model is off and the exhaust is too rich, you are back where you started with open loop and you get high CO emissions. Then you can't certify the car, or if you do certify it you get nailed in 4 yours when you start doing in-use testing and the CO fails.