[T3] Mixture vs. Advance?

Jim Adney jadney at vwtype3.org
Fri Apr 5 09:43:31 PDT 2019

On a recent post, I mentioned that richer mixtures burn slower than lean 
ones. As a result of this, one might want a bit of extra advance for a richer 
mixture, to give it time to burn. Turns out that it's not that simple and richer 
sometimes burns faster.  

It took me a LONG time to appreciate the fact that engines need advance as 
the RPMs go up, simply because the burn time is approximately constant 
while the time available for burning goes down as the RPMs go up. The goal 
is to get the pressure peak to arrive shortly after TDC, where it can be used 
most effectively.   

A friend of mine, who knows much more about this than I do, answered my 
question, but then passed the question on to someone who knows even 
more. For anyone who might want to geek out on this, here's the long 
answer, from the friend of a friend. The rest of you may want to delete this 
message right now.

I believe there are some typos in this, but I've left them as-is, so you get to 
figure it out. Enjoy!  ;-)

> To answer the question it really is "how rich" or "how lean". Attached
> is a recent paper that sort of helps to explain it. these are
> fundamental burner measurements of surrogates of gasoline and some
> European gasoline (basically same as the USA gas for all intensive
> purposes). fore reference the PHI used in the plots is 1/lambda
> (inverse of lambda), so PHI>1 is rich and PHI<1 is lean and PHI=1 is
> stoch (i.e. lambda=1).  So the question is kinda complex (as most
> things are). 
> 1. for a given temperature a richer mixture burns faster by just a little bit (up to a point, about PHI=1.1), and this depends on the fuel significantly (e.g. nitromethae burns faster lean according to some studies, but it is goofy as it is a monopropollent).
> 2. running rich increases charge cooling so the charge at intake valve close is usually cooler, so any effect of a rich mixture on flame speed is probably negated (you can calculate me temperature pretty easily with some thermodynamic tables, but you have to assume some factors about when the fuel vaporizes, in the intake, in the chamber, and more importantly when, which is a function of the engine temperature, intake temperature, valve overlap/VE of the engine (trapped residuals), and fuel distillation (ASTM d86 test) which can vary somewhat widely on the fuel composition (e.g. ethanol content, and aromatic content of the fuel).
> 3. a rich mixture actually has a shorter ignition delay(for a given temperature) (more prone to knock), but the charge cooling of the fuel actually reduces the temperature more than the increase in ignition delay. knock behaves as a exponential function (that "e" "number" in the maths thingy) so a small change in temperature has an exponential change on the fuel knock.
> 4. flame speed is a function of temperate and pressure, reduced temperature the slower the flame speed (so again rich cooling the charge down, too much, will reduce the flame speed). this all comes back to what the temperature, pressure, and composition are at the intake valve closing time.
> 5. A richer mixture will have a faster spark-to-flame transition but again only to a point. running at AFR=12.5 or so for gasoline PHI~1.1-1.15, makes the maximum flame speed and increases ignition transition and makes the most power typically, at the expense of fuel economy and emissions.
> 6. The richer mixture also has an effect on the compressed temperature as the richer the mixture the less favorable the gas properties are for compression work, meaning that the ratio of specific heats (called gamma) decreases. This makes the relation (T_tdc/T_bdc)=compression ratio ^(gamma-1), which is a relation the engine follows by polytropic compression. So a richer mixture reduces gamma and thus T_tdc for a given T_bdc  condition. In other words the compression process is less efficient and the temperature is not increased as much so the fuel is less likely to knock (again exponential on temperature thing). Note this also makes a richer mixture less efficient on the thermodynamics of the engine cycle, but the engine can make more power as you are liberating more energy , again just less efficiently, but more energy in general, so you get more power.
> As for the timing with rich and lean, that is really noting with flame speed and more with the kinetic state of the end gas (will it auto ignite or not, in other words will it knock or not)
> So its complex, Im not sure if this helps.

Jim Adney, jadney at vwtype3.org
Madison, Wisconsin, USA

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