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I found some peculiar relationship between mixture and detonation. It's more noticeable on low-octane fuel. So far I've tested only Mk.I

When you give (way) too much MP for the rpm the engine is running, rich mixture seems to detonate easier on Core 1.4. I made some tests flying but for ease repeatability (making airspeed and density variables into constants) I also tested it on tarmac while holding brakes and tail down. Both for full coarse and full fine pitch setting it could obtain higher MP without detonation with lean mixture than rich.

One of the key benefits of rich mixture is to cool down the engine (preventing overheating). The other, is to cool down the air-fuel mixture (preventing detonation). Or am I somehow mistaken? Anyway, lean mixture seems to be less likely to detonate with Core 1.4.

So, a bug?

Accu-Sim models detonation using cylinder pressure, carb air temp, and fuel octane.

The only way Mixture directly affects detonation in accusim is how it affects the power of the combustion. So, using FULL RICH should have a positive effect on reducing detonation. However, if you heavily lean the engine, you can pass past the peak temp and well into engine cooling. It's very hard to know exactly where this is happening especially with auto mixture.

So essentially, if you are experiencing detonation, you need to reduce one or all of these three things. Increase RPM (or pull back the throttle), lower your carb air temp, or use a higher octane fuel.

Scott.

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A2A Simulations Inc.

So, in your modeling, rich mixture doesn't cool the intake air due to evaporation of fuel? Because it should.

If detonation is indeed modeled only by MP, carb air temp and octane rating, it's clearly insufficient modeling. Carb temp is not the only variable which affect air temperature before start of compression: adding water or fuel in liquid form into the intake manifold will, once evaporated into gas, subtract a great deal of heat from the surrounding air.

I know that ADI has not yet been implemented as part of Core Accusim but it would be better for future products' sake to add some amount of "ADI effect" for fuel enrichment, to add extra resistance to detonation for rich mixture. As the modeling is currently done, rich detonates easier than lean. I don't know why - I'm just observing the simulation. Try it out and you'll see what I'm talking about.



This sort of engine behavior doesn't seem accurate. It's either some bug (like "rich" and "lean" conditions have been swapped in the code). Or the intake air cooling effect of adding liquid fuel into the system isn't modeled.

Detonation occurs because there's too much fuel with too little octane rating left after the charge is ignited. The purpose of adding extra fuel during high power operations is to prevent this by "overcharging" the fuel/air mixture and ensuring that the mixture is too rich to detonate and thus only burns. However, you can exceed the beneficial limit of this and get detonation anyway. The way to resolve it is to lower the amount of fuel in the charge to limit or eliminate the excess fuel in the after-charge, thus preventing the detonation by going lean.

However, by leaning, the cylinder temperature will increase because you are now burning more efficiently and completely with no fuel to evaporate after the main ignition.

Here's an article that I found on Detonation and some of the misunderstandings about it -

http://www.avweb.com/news/pelican/182132-1.html

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That link (and other articles regarding engine management under /news/pelican/) is interesting.

This is also interesting (from another topic):


But... what is the most interesting of all, is when you combine the graph from Pelican about mixture vs detonation tendency
http://www.avweb.com/newspics/pp43_deto ... ale_lg.gif
to the data on the Merlin XX (assuming early Merlins run as similarish air-fuel ratios)
http://www.enginehistory.org/members/im ... ble-04.jpg

...you come up with the conclusion that running Merlin in "rich" setting gives you air-fuel mixture that's the most likely to detonate of all possible mixtures the engine could potentially receive (i.e just slightly richer than stoichiometric). Leaning out during high performance flight would indeed reduce likelihood of detonation, against common conception. This might be because running lean slows down the burning and reduces peak pressure after spark.

So, indeed, CAPFlyer might be onto something here. It would mean that both I and Scott were wrong... but modeling on A2A Spit with Core 1.4 would on the other hand be accurate because it clearly behaves exactly the opposite way as I and Scott would have assumed it to behave.

This is the benefit of modelling procedurally instead of statistically. As Accusim models the individual acts instead of the results of those acts, it allows the system to actually produce results that are found in the real thin without having to "force" it do do so. This is why Accusim is such a nice step forward in simulation technology and a proof of where we've come in simulations since we're able to create programming (at home no less) that can mimic the very complex operation of something like a combustion engine and get the correct results without realizing you've done so.

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There are times when Accu-Sim has been modeled on aircraft and we have found out that it performs closer than expected. Specific traits becoming more noticeable once everything is in place and having found to match pilot reports. In some cases these characteristics were not known until modeled, then upon further research the engine produced results which were surprisingly accurate.

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