Why it is better to be rich ?

[fizeus]

Hi all,
Can anyone, please, explain why rich carburation is better (and safer) then lean carbonation.

I've read many threads but still haven't found a tecnical explanation.

I assume that lean carburation means that mixture has less fuel from the optimal ratio, so you should have less power (and also faster combustion compared to rich) and the engine should run at lower temp.

So even if the engine run with less power lean carburation should be preferred because it can give a longer engine life.

Am I wrong ?

Reading on this forum i see everyone tells rich is safer than lean but none can explain the reason.

Thanks in advance

[Flieger]

In layman's terms, "rich" and "lean" generally refer to deviations from the best power air fuel ratio. In more technical context, rich and lean are relative to stoichiometric. Gasoline makes peak power a little rich of stoichiometric, so this confuses things. When you run "lean" you are actually getting closer to stoichiometric, which is where the combustion temperatures are highest. If you run lean of stoichiometric then the cylinder head temperatures will fall partly because less energy is being released by the fuel but the exhaust gas temperatures will rise because combustion takes longer.

Modern EFI engines run close to or lean of stoichiometric (with periodic rich periods to keep the catalytic converter working) because it gives the best fuel economy and fewest emissions.

Running rich cools the combustion partly because of the extra fuel evaporating. This can allow a higher compression ratio or more aggressive spark timing because it helps raise the knock threshold.

[moneymanager]

A somewhat less sophisticated answer than Max': run lean enough and you may destroy your motor. Run rich, and you will use a lot of gas and maybe foul your plugs but it won't cost $15000 for a rebuild.

[Ian Comerford]

Not sure I agree with the comment about running rich not wrecking the engine. Excess, unburnt fuel washes oil away from the bore and leads to accelerated wear.

Ian

[Tippy]

Lean leaves leftover oxygen, and with enough heat will melt the pistons. The oxygen literally chemically breaks down the aluminum once combined with heat.

[fizeus]

Thank you all.
Your explanations were really interesting end complete.

Thanks again !

[Bob Kontak]

Originally Posted by Flieger

Running rich cools the combustion partly because of the extra fuel evaporating. This can allow a higher compression ratio or more aggressive spark timing because it helps raise the knock threshold.

Nice write up.

Another thing to add FWIW is that premium fuel benzene molecules have a longer molecular chain than 87 octane. Something to do with how they cook the gas. More pressure/more heat in the refining process, I think. The gasoline molecules are more stable when they explode. More even burn allowing more compression and timing advance vs regular without a downside.

[Tippy]

Running rich leaves gasoline behind that does not burn due to lack of previously consumed oxygen thus running cooler.

[ClickClickBoom]

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[Bill Douglas]

Originally Posted by moneymanager

A somewhat less sophisticated answer than Max': run lean enough and you may destroy your motor. Run rich, and you will use a lot of gas and maybe foul your plugs but it won't cost $15000 for a rebuild.

Well put.

That' why I find life is easier being rich.

[SpyderMike]

I think the answer lies in just how rich you run. Just running rich is not necessarily good for the engine. Running rich outside of the high cylinder pressure and high temp zone is okay at a cost of increased gas consumption. Studies I have read indicate that running sufficiently lean of peak reduces CHT and EGTs, reduces fuel consumption, and reduces internal cylinder pressures....all good things. Granted , these studies are based on aircooled aircraft engines where this issue has been debated for years. The data supports that with matched fuel delivery to each cylinder and proper cylinder performance monitoring (CHT and EGT) you get great results running LEAN of peak. I am doing this in my plane.

In my Porsche, I am not sure how this applies. I don't imagine the intake/fuel delivery to be that well matched and I don't have the systems to monitor each cylinder like in a plane.

The good news is that the P-car engine is less expensive and I can pull off the road when it goes south.

Mike

[bugstrider]

Like the air cooled aircraft engine found in the general aviation crowds. Fuel settings can directly and drastically affect engine temps in flight. Lean equals higher cylinder head temps and richer settings can be used to cool those same temps bringing the overall engine temps down a bit.

Just my .02 cents

[SpyderMike]

I am sorry, but in my plane running 25degrees LEAN of peak reduces my cylinder head temperatures drastically (like to between 300 and 320 degrees) and reduces fuel consumption about 30%. As I enrichment the mixture from the LEAN side towards peak, I can watch the CHTs and EGTs rise....

Running rich of peak equates to 350-360 degree CHTs and the higher fuel burn with no increase in horsepower. This is real data on a TCM IO 550N running just under 1,000 hours with no issues so far.

One needs to see the data plots of fuel flow, CHT, EGT, Internal cylinder pressure, and horsepower to see the relationship to draw the conclusion.

Running rich seems to be the way to run as the result of not having closely matched fuel/air flows to each cylinder. It provides the additional cooling to prevent any one cylinder from a bad ending.

Mike

[Flieger]

This is exactly why I explained the way I did. Many older engines are set to run rich of stoichiometric, which is where the most energy is being produced and so the highest CHT's. So in that case, running leaner will increase temps up until you reach stoich, then they go back down (though the EGTs peak a little lean of peak CHT due to the slower burn). This is a simple concept, since you have less and less fuel in the cylinder, so you have less and less energy. Otherwise you'd have infinite temperatures with nothing but air, and that is obviously incorrect.

I've seen this very effect using an instrumented CFR engine. The pilot is right, as you would expect.

BTW, pilots use CHT in lieu of an O2 sensor as an air/fuel ratio meter. It makes sense, since the CHT is what you want to keep tabs on lest the engine blow. It's quite accurate since airplane engines run at a constant rpm and power setting during cruise so the mixture can be tweaked to find the peak and then lean past it. Running lean of peak is running lean of stoichiometric (lambda 1, no excess air) as well as peak cylinder head temperature.