Science Forums 2

[marcringuette]

I understand your reaction, but the textbooks seem to be saying that lean burning is slower, not faster.

From the book "Lean Combustion" by Derek Dunn-Rankin, p.104 : "A disadvantage of lean operation is that the burning rate is reduced compared to combustion under stoichiometric conditions. ... The reduction in burning rate results in an increase in the overall combustion duration, which in turn leads to increased heat transfer losses to the cylinder walls and a decrease in the overall engine thermal efficiency."

Of course, once the cylinder walls are heated up by this effect, preignition can start happening and the usual process of "flame front from spark outward" is messed up. Before that point, though, lean combustion will start with the spark, as intended, but will be slower and less consistent, with some remaining regions of fuel-air mixture not burning until later in the piston stroke.

Anyhow, because I was only seeing the "no evaporation of excess fuel" explanation for why lean engines run hot, I wanted to introduce this seemingly important alternate explanation. Late burning of fuel leaves heat around in the cylinder for longer and causes a thermodynamically less efficient cycle. Cylinder walls heat up, and badness begins.

Cheers
Marc

Ref: search inside "Lean Combustion" book for term "combustion duration"
at URL amazon.com/gp/reader/012370619X