Follow-on: is a leanest mixture necessarily stoichiometric?

Gasoline engines perform with the greatest horsepower output fuel rich of stochiometric.  This is well studied as it is very important to the various automotive industries.

I would bet that fuel rich would still produce more horsepower, which I would expect to produce more revs.

Phil

I have been reading on this because I know the answer I gave is "correct" but I lack a good "why" (it has been over a decade since studying thermodynamics, engines, and chemistry.)  The best simple answer is that just rich of stochiometric every molecule of oxygen in the air is consumed, and there is enough fuel to consume all the oxygen, regardless of the exhaust products.  This means that there are quite a bit of monoxide products in the exhaust at maximum power.

Phil

OK, thanks for the replies, guys.
With your input, after more thought I've come to the conclusion that it's not something I really need to know a whole lot about after all. 

I thought that the very definition of  the 'stoichiometric' ratio IS that it burns every O2 molecule (along with every molecule of fuel), being rich or out of ratio to that is (by definition) not stoichiometric.

But being richer than the ideal allows for the real world use of an un-ideal engine and helps cool the engine better and prevent detonation under high loads.

So, to answer your question "Does a stoichiometric mixture produce highest rpm? "
In an ideal world and under light loads, yes.
   

But monoxide products are technically able to still be burnt. There is not enough oxygen to fully oxidize all the fuel.  The molecules are not fully oxidized therefore the stoichiometric equation is not reduced to the lowest energy level.

Phil

if you're looking for more power go to 15, 20, or even 25% nitro.  It keeps the motor cool but still provides more power, gives a broader needle valve setting, and generally runs better.  Downside it fuel economy.

Ah, combustion.

My wheelhouse.

It really depends on a lot of variables. The most important is probably the adiabatic flame temperature. You see, the by-products of combustion will vary depending on the flame temperature which determines the ksp of the combustion reaction. That's important because the heat addition will be a product of the energy released by the individual reactions creating the multiple by-products. In the theoretically perfect word we would combust a hydrocarbon with oxygen and get CO2 and H20 as products.

In the real world, we burn air, and thus we get may reactions happening at once determined by the flame temp. Since air is mostly nitrogen and gasoline isn't pure octane your exhaust really contains CO,CO2, NO, NO2, SO, SO2, H2SO4...

So to "dial in" the mixture you really need to be analyzing the exhaust gasses. (Note many light aircraft utilize a EGT gauge)

For racing motors on sports cars we run them slightly rich to make them run cooler and last bit longer. Where we set the mixture is a function of how long the race is. Some cars can adjust it during the race. The driver can lean out a bit and cruise at lower power to save fuel if that's part of the race strategy.

Methanol and juice (aka CH3NO2) together provide more oxygen. Typically we will run a nitro motor rich for a couple of reasons:

We run very close to hydraulic lock when the fuel is all the way up. Since all the fuel doesn't burn and we can take away a little HP on the hit at the cost of a little excess compression since fuel is incompressible until it gets heated up and boils into vapor. We run 100gpm fuel pumps so there's no shortage of nitro. It's coming into the engine like a fire hose.

If you watch fuel cars on TV or go to a race you can watch the crew chief. The car will be running lean during the burnout to get the heat up, after the driver backs up and gets read to stage the car the chief will turn the barrel valve to richen the car up. That's when the sound goes from pop-pop-pop to the menacing whop-whop-whop and the idle drops.

Second, the excess nitro will combust after it leaves the exhaust and give an orange flame. We can see immediately if we lost a cylinder. Also, for night exhibitions it makes the car look like a 320MPH roman candle...

Nitro and methanol burn cool. The long smoky burnouts you'll see fuel cars do is more about bringing the engine to temp than heating the tires.  We run solid blocks with no cooling liquid.

Alcohol engines run so cool that it's very common to see frost forming on the injector butterfly as the car idles, even on a 90 degree day. You have to spray them with de-icer or they may freeze shut!

Anyway, I'm rambling but you can start to see the dance... you lean it out, the flame temp changes and combustion reaction(s) change and the power you get changes, not always for the better. It's a bit artsy-fartsey and then you get into the differences between each individual injector and it's head being slightly different due to the twists and turns of the fuel line. The final answer is always an average because each cylinder is slightly different. So richen it up to be safe.

Perfect Stoichiometry won't necessarily give you the highest RPM, it will give you the most power, but because we burn air and not pure oxygen and get mixed by-products of combustion it's not really applicable. It's a theoretical point rather than practical, i.e., in practice it's the domain of engineers and not scientists, lol!

So Chuck, how does deliberately adding say 20% oil (which may or may not be combustable) effect all of this?

Traditionally a light non combustable oil mist from blowby lowers octane rating drastically, so I am guessing that this is the same with a two stroke?

I can well see that the answer might be 'depends.' Depends on whether or not the oil is a combustable.

The engine has a fixed displacement - I'm assuming you're using a naturally aspirated 2 stroke - so anything you put in the combustion chamber that's not fuel or oxygen takes away power. With good 'ol castor oil, you'll raise the compression ever so slightly with the addition of the oil, since it won't vaporize and it's non-compressible in the liquid state.

Some small portion of the oil will be combusted however, and this will be a lower energy reaction than methanol or nitro. Since this is consuming oxygen for a lower heat producing reaction, the combustion temp will be lower and the gasses don't expand as much - net result is loss in output.

Blow-by is generally from the combustion chamber into the crankcase. if your engine is blowing oil past the pistons into the combustion chamber I'd say you have bigger problems than octane rating...

On big, supercharged motors blow-by is controlled by evacuating the crankcase and cylinder heads. If you look at a fuel car you'll always see a round tank behind the engine. On a dragster there are hoses connecting it to the engine. (On a funny car we utilize the frame rails.) This is the "puke tank" that allows the blow-by to leave the engine. On less powerful motors you'll often see a line from the oil pan to one of the exhaust headers.


Nitro is the key to power.  I digress, but with fuel motors the shear numbers are staggering. A couple years ago someone built a torque transducer that could handle the loads and we found out fuel motors are now in excess of 10,000HP at the crank. If you do some calcs you find it's taking about 1000HP just to turn the blower, so a 500c.i. block running nitro is delivering over 11,000HP! At Charlotte when four leave at once the sound is beyond deafening.

*Nitromethane is the key*

If you need more power, add more nitro. Typical model engines run on a mixture of methanol and nitro, so the stoichometry isn't as clear cut. The methanol will go rich before the nitro does.

Chuck

Bill, my experience with 2 stroke diesels is exactly zero, and my ignorance of them is profound.

That said, for what you describe I'd be looking at the thermal efficiency of the setup.  With all the restrictions you have, I'd be trying to find the temperature the engine is it's most efficient at, and then compromise for power as strategy dictates. The air will only take so much heat before it's at the thermal choking point, so anything you can do to cool the crankcase, cylinder and head and incoming charge will get you a gain.

So if i was so inclined and in full mad scientist/Dr.Evil mode, I would run a jacket around the motor, use the engine heat to power a vapor-compression refrigeration cycle and then enter the air through an intercooler. You'd basically be taking away engine heat and using it to cool the incoming air. It might be easier and lighter than you think - and you'd be a legend in F2C, LOL!

Yes, I do want to live in a hollowed-out volcano.

Chuck

Chuck, probably the wrong verbage from me - by 'blowby' I mean oil leaking by valve guides and rings into the combustion chamber.

Its well know that oil leakage into the combustion chamber significantly lowers the fuels octane rating and causes early onset detonation.

A partial cure for an older poorly sealed engine is too use a higher octane fuel.

So it would appear that oil being present in any IC engines chamber will have a similar effect.

To sum up, the oft quoted ideal fuel air ratio in a methanol powered engine would not seem to apply in a very high oil content model engine.

Would you agree?

Chuck, probably the wrong verbage from me - by 'blowby' I mean oil leaking by valve guides and rings into the combustion chamber.

Its well know that oil leakage into the combustion chamber significantly lowers the fuels octane rating and causes early onset detonation.

A partial cure for an older poorly sealed engine is too use a higher octane fuel.

So it would appear that oil being present in any IC engines chamber will have a similar effect.

To sum up, the oft quoted ideal fuel air ratio in a methanol powered engine would not seem to apply in a very high oil content model engine.

Would you agree?

Well, we're way off topic and I'll take the blame for that, but when your area of research was in supersonic combustion you tend to go deeper than you should. My bad.

Any IC engine with valves, that is leaking enough oil past the seals on the valve guides to affect the combustion output is pretty well knackered anyway, and probably so down on HP that it really doesn't matter. You'll have carbon buildup leading to pre-ignition or even fouling the plugs which screws up the flame front...

And as Tim notes, this has nothing whatsoever to do with CLPA. I run a 4-2-4 setup and even my "2" is a bit on the soggy side.  The high oil concentration often used in CLPA isn't there so much for lubrication as it is to take away heat. That's why changing to really good fuel with the correct amount of castor oil will make your engine run better - IMHO. So the excess oil may actually be keeping the engine cooler and helping more than hurting on the power...in the long run.

Tim, I am under the impression that Team race is run as lean as possible - it is after a range event as much as a speed one and to promote engine life a slight misfire is introduced by backing off the comp slightly.

Chuck, I don't think we are WAY off topic since the original question was so open ended but to me a model two stroke engine is run oil heavy because of sliding contact big ends - put a needle race in there and you could probably get the oil % down to 2 without heat issues.


My point here is that nowhere have  I see a stoichiometric ratio accurately stated for a fuel/air mix for methanol mixed with oil so how do we determine the correct ratio for maximum rpm for that fuel?

The argument is circular.

Stoichiometry isn't the answer.   

If you want more power you put more of the mixture in - at the same ratio.

Look at a carburetor. It controls the amount of air. Fuel injection measures the amount of air coming into the engine and meters the fuel accordingly.

For a glow engine, we open the venturi we get more revs. If we open it enough it will no longer pull fuel and so we go to a bladder tank or crankcase pressure as a very simple fuel injection system.

On a fuel car we compress the air to get more into the cylinder.

But whatever the air intake, we strive to keep the mixture roughly the same.

A little late comment but whatever the mix is that is mixed in carburettor, it's not the same mix that gets combusted in cylinder. It is very important to have a certain degree of mixing of fresh mixture and burned mixture from previous stroke.
So if scavenging is too efficient, or blowdown too much optimized, you cannot get the engine really going well. L

Um, since control line has a fixed throttle a too efficient engine runs away to its peak?

(Keeping the charge 'dirty' is an effective limiting device.)