Originally Posted by RoadWarrior
I think the problem with lean burn the way honda did it was not that it produced more NOx than a regular motor, because they used a kind of stratified charge approach, but that it didn't have enough hydrocarbons spewing out for the cat to reduce NOx further. Cat needs something to do with the O2 it yanks away from the N, so it needs unburned HC to make H2O and CO2 with. (Yeah I know in a perfect world it should just release O2, but it's "stuck" to the cat and needs something energetic to yank it off again)
The 1996 VW TDI diesel (as high as 100:1 air/fuel at idle) suffered from this same lack of post engine fuel to work in the cat. It had an added fifth injector spitting raw fuel into the cat based on a pair of temperature sensors, one before, one after the cat just to provide the cat with needed fuel. If the front sensor was hot enough to ignite, but the rear one was cold indicating low cat conversion, the fuel was spritzed in.
Or so it was supposed to work.
It worked well enough to get the new engines certified for 50 state sale, but in practice was a smoky, fuel consuming mess. The remedy was a new computer that kept this supplemental injector turned off, new injectors that opened at 3,000 psi instead of 2,750, and a delay of the onset of injection to drop peak combustion temperatures, all for lower NOx.
The device was discontinued for the 1997 model year TDI, but now has returned in a much improved form for the 50 state approved 2009 "Clean Diesel" Jetta TDI.
Time will tell if it keeps working better than the 1996 predecessor (from the Latin for already dead!)