There is an old motorhead rule that goes something like this

 "Never run lean or yer gunna burn a hole in yer pistons"

To avoid confusion, when referrig to this post, the definition of LEAN will be considered A/F greater than stoich and RICH will be considered less than stoich.

In the application I'm working on a lean burn with max lambda of 1.1  would be very advantageous however before breaking this rule I want to ensure that I understand the problem entirely.

Emissions aside, With instrumentation of a N/A engine I have found the following.

While running rich:

Power increases as A/F in decreased until roughly 13.5:1 (.9 Lambda)
Exhaust temperatures decrease/ball vavle
From A/F of 13.5 to 14.7 the amount of ignition timing that could be added without detonation did not change.
Increasing intake air temperature requires retardation to prevent detonation.

While running lean:
Power decreased
Exhaust temperatures ????  <-- (no instrumentation at time)
Ignition timing required retardation to prevent detonation.
Increasing intake air temperature requires retardation to prevent detonation

From this testing I believed that the motorhead rule evolved due to the following two reasons.

1) Decreased resistance to knock may result in detonation causing engine damage.

2) Motorhead carburated systems typically run rich .8-.9 lambda to take advantage of the power gains running leaner causes EGT's to increase which may lead some to believe that if you go too lean you will "melt the pistons"

I have read the following which seems to be a reasonable explanation of change in knock resistance.

A lean air/fuel mixture burns with most efficiency, so much that the insulating boundary layer also gets consumed and the flame front touches the metal walls.  At those locations, there is a dramatic rise in temperature, high enough to cause subsequent charges of air and fuel to spontaneously ignite.

Question 1:  is this explanation of the decreased knock resitance correct

Question 2:  If detonation and EGT's are controlled will damage still occur.

Question 3:  What is the chemical/thermodynamic reason for the increase of power when running with enrichment?
On the surface one would expect that the complete combustion of a stoichiometric fuel/air ratio would produce the maximum power.

A simple thermodynamic reason why more power is made at rich mixtures is that hydrocarbons release most of their energy on combustion to CO; so for any given displacement/compression ratio/engine speed, etc. complete combustion doesn't lead to highest power output.

Consider the combustion of methane as an example:

CH4 + 2O2 --> CO2 + 2H20

If you took introductory chemistry, recall that standard enthalpies of formation (Hf) allow calculation of the enthalpy (or heat output) of reaction:

Hrxn = Hf,products – Hf,reactants

Tables of Hf can be found in most general Chemistry textbooks.  So for methane combustion we have:

Hrxn = [-395 + 2(-242)] – [-75 + 2(0)] = -804 KJ/mol

Now for incomplete combustion the reaction is:

CH4 + 3/2O2 --> CO + 2H2O
Hrxn = [-110 + 2(-242)] – [-75 + 3/2(0)] = -519 KJ/mol

So essentially 2/3 of the energy available from hydrocarbon combustion is generated in the first step (CH4 --> CO), leaving only 1/3 in the second step (CO --> CO2).  Conceptually at least, for a given displacement/compression ratio/engine speed the most power would be produced IF we could stop the reaction after the first half of combustion, dump the CO and pull in a fresh charge of CH4.  This is hardly practical, but does occur to a certain extent when the A/F ratio is slightly stoichiometrically rich.  There are plots of emissions and power vs. A/F ratio in the literature showing that often maximum engine output and maximum CO emission (untreated) coincide.

Clearly in a fired engine both kinetic and thermodynamic effects are operative.  But the size of the heat of combustion difference suggests to me that the chemical effect may be the dominant factor.

To question 3:
When running rich, the evaporative effect of excess fuel tends to absorb combustion chamber heat, which will thus lead to lower combustion temperatures and help reduce detonation/pre-ignition.  Running rich is commonly seen on small air cooled gasoline implements (mowers, generators, motor cycles, etc.) for this very reason.

When using a liquid fuel, running rich allows for a more ideal air-fuel saturation in the cylinder.  This is less than Lambda=1, where the ideal thermochemcial process is complete.  When running greater than Lambda=1, there is excess oxygen which can lead to higher combustion temperatures.  One clear example is operating with Methanol.  It has a much lower energy density than gasoline, almost half, but as it is mixed with air in the cylinder, it evaporates quickly and leads to a much cooler and denser air-fuel mixture.

As a side note, much of this theory is tossed out the window when running gaseous fuels since there is no evaporative effect of the liquid fuel.

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