Air/fuel ratio meter on diesel?

Hi Gerry,

Not a chance in hell, AFAIK. A/F meters based on O2 (Lambda) sensors react only over a very narrow range of oxygen concentration, just either side of the stoichiometirc ratio for air and petrol (approx 14.7:1). Your diesel will be running 99% of the time with huge percentages of excess air - and hence, large concentrations of oxygen. I'd expect the A/F meter to be clamped at the 'lean' end of the scale.

The only time it MIGHT read something else is at full throttle/full load, when your A/F ratio might be approaching stoichiometric and the excess oxygen concentration drops into the meter's measurable range. But even then, I'd suspect you'd be well into 'overfuelling', generating black smoke and excessive EGT well before the meter moved off its full 'lean' reading.

A few other considerations: Unless it's a self-heating type of Zirconia probe, your diesel exhaust will probably be too cool for it to measure at all, much of the time (i.e. unless you've got your foot flat to the boards). Then there's the soot, as you mention. And, lastly, what is the reading (if any) going to tell you?

But, all that said, I've haven't actually tried one in a diesel exhaust so I can't say with absolute certainty that it won't work at all.

Strangely enough, I have just about finished a technical article on these fundamental differences between petrol and diesel engines for my website. Hope to have it posted in the next couple of days. So, please keep an eye out for the new "Tech Info" page. Feddback is welcome.