A question on FA ratios

[Major_Tom]

Hey all, I was hoping someone could clarify something for me. When talking about lean/rich conditions, it often comes up that lean will overheat exhaust valves, and the ideal mixture is somewhat richer than stoich. As I understand it stoich will be the hottest, burning all the oxygen and all the fuel with as little dilutants as possible, leading to the most efficient burn and the biggest expansion force. Its hard for me to see how too much air will increase the temp, and it seems to me that the eco weenies and trackday weenies should unite at stoich for the most power and cleanest burn.

I do get that the vaporisation of unburned fuel will absorb energy and cool the cylinder, but it seems to me that this is also pulling power away from pushing the pistons, therefor sapping at the hrsprs.

 

I'm sure there's something I'm missing, please let me know why I'm an idiot!

Edited June 27, 2019 by Major_Tom

[Grant31781]

 

This   Mixture video explains it pretty well.

[robbo10]

My 1940s bike's (as avatar) engine seized when I leaned out to max (carb). Engine got too hot. But experts will be along shortly....

[mossrider]

On 6/27/2019 at 9:06 PM, Major_Tom said:

I'm sure there's something I'm missing, please let me know why I'm an idiot!

Well I don't know if enough space exists to cover all said reasons you are, but, with respect to air-fuel ratios here are a couple of good resources;

Air-Fuel Ratio Meter Tuning- Car Craft Magazine

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It used to be that tuning a hot street engine or race motor was one of those black arts that only those carburetor and ignition wizards seem to understand...

Air–fuel ratio - Wikipedia

Engine Air/Fuel Ratios

Suffuce it to say that the 'best burn' may not the same as 'cleanest burn' or 'most complete burn' or 'most  powerful burn' which are all subjective depending on your stated objectives, needs or design parameters. There are a miriad of other variables that come in to play such as speed of flame propagation, combustion ratio/speed/chamber shape, crank rotation vs burn speed, heat buildup in engine internals, cooling capacity of engine construction, engine load, designed rpm range vs fuel type etc etc. Then let's consider things like Nitro Methane makes tremendous power but offers abysmal fuel econmy,  alcohol based fuel burns cooler than gasoline but contains less energy per pound so more fuel must be burned for a given amount of recoverable energy. Diesel fuels have a huge amount of energy per pound but the high compression ratios required to utilize it demand heavy construction, long piston stroke and offer a narrow operating rpm range limiting their applications,  and on and on... 

 

I'm an idiot too, just get used to it, 

Edited July 1, 2019 by mossrider

[norcal616]

The 14:7 gas AFR ratio for example is just a reference to use... Some gases have a 13:1 AFR ratio and if you don't pay attention to that you will think its running rich but it's not...

Believe it or not both of my bikes are running 13:1(rolling off throttle-deccel injection)-15:1(on throttle)ratios across the maps, according to my AFR guage... 14:0 is the avg... I will run the AFR gauge a few times a month just to see the engine is still okay... 

[longrider1951]

4 hours ago, Grant31781 said:

 

This   Mixture video explains it pretty well.

I learned a lot from that video, but you really had to pay attention.  After watching it twice I would say that running slightly rich is all around better than too lean.

[Major_Tom]

30 minutes ago, longrider1951 said:

I learned a lot from that video, but you really had to pay attention.  After watching it twice I would say that running slightly rich is all around better than too lean.

Also that some people consider stoich to be lean

[wrxer]

Max power comes from a richer mixture.  Its a pipe dream to expect perfect combustion with a 14.7 ratio will happen with every molecule of fuel meeting every molecule of oxygen.  For starters "air" as your engine breathes it, is not standard & varies in composition so you don't know for sure how much oxygen you really have at a given time.  2nd, inefficiencies in combustion will leave some fuel without a dancing partner if you only provide the exact theoretical amount needed for your "air".  Air is the limiting factor in the mechanics of an engine.  The ability to draw in air is finite, your engine is just a self powered pump.  Flow into & out of the pump is determined mainly by displacement & flow dynamics.  Everything done to an engine is to increase air--airbox mods, exhaust, porting, forced induction, etc.  You can always dump in more fuel.  You want to make sure to use all of your finite resource---air.  Not using all the air drawn into the engine leaves power on the table.  You provide a slightly richer mixture so there is a surplus of fuel to make sure to use up all the air.  If fuel = women  & air = men would you have better luck at a party with 50 dudes & 40 ladies (lean) or 40 guys & 50 women (rich)?