Champion Service Letter: Lean-of-peak ops

[Dan Thomas]

In an ideal world, you would be advancing the spark as the mixture went lean for maximum return on enleanment.

No, the spark would retard somewhat to get the peak pressure after TDC and avoid the detonation risk. In an automobile the spark is advanced the farthest at high RPM and low manifold pressure; as MP rises (throttle opens) the spark retards. Aircraft in cruise are at relatively high MP and generating 75% or so power; the auto cruises as a lower throttle setting and maybe 25-30% power.

Here's Deakin's chart showing the movement of pressure peak with mixture leaning. Note the locaation of the spark; advancing it further would make the peak come sooner. Not productive at all. Aircraft mags are set at some sort of "average" that is ideal at only one RPM and MP setting.

This chart shows max power happening before peak EGT. Read this right-to-left. Best power is typically at around 12:1 air:fuel by weight; stoichiometric is considered to be 15:1, which, as Tom says, will be near peak EGT. Gasoline is combustible between 8:1 and 18:1, so the roughness isn't far past peak EGT.

Dan

 

[weilke]

Dan

Oh right, that's the diagram where Lycoming tells us to operate LOP right before they tell us in capitalized block letters that we shouldn't operate LOP :wink2: .

 

[Capt. Geoffrey Thorpe]

No, the spark would retard somewhat to get the peak pressure after TDC and avoid the detonation risk. In an automobile the spark is advanced the farthest at high RPM and low manifold pressure; as MP rises (throttle opens) the spark retards. Aircraft in cruise are at relatively high MP and generating 75% or so power; the auto cruises as a lower throttle setting and maybe 25-30% power.

Here's Deakin's chart showing the movement of pressure peak with mixture leaning. Note the locaation of the spark; advancing it further would make the peak come sooner. Not productive at all. Aircraft mags are set at some sort of "average" that is ideal at only one RPM and MP setting.

This chart shows max power happening before peak EGT. Read this right-to-left. Best power is typically at around 12:1 air:fuel by weight; stoichiometric is considered to be 15:1, which, as Tom says, will be near peak EGT. Gasoline is combustible between 8:1 and 18:1, so the roughness isn't far past peak EGT.

Dan

As your chart shows, the burn rate slows down as you go lean. If we were to assume that you had optimal spark (MBT) at the rich condition, then as you lean you advance the spark to compensate for the slower burn rates.
(In general, peak pressure around 15 degrees after TDC is close to MBT)

The comment about "leverage" is nonsense. Work, by definition, is the lintegral of pressure times delta volume. For the same fuel, late peak pressures result in less work. Which is less efficient.

You have to integrate over the whole cycle.

You get best power at about 12:1 as shown in the chart - this happens due to the preferential burning of hydrogen - you get more power from that and just throw the carbon out the back. But best power is not best brake speific fuel consumption (power per unit fuel) as shown in the second chart. If you have the air available lean will use much less fuel than rich for the same actual power output.

I do know what is done in an automobile - I write the algorithms to do it.