why lean is bad

[rppngears]

I have read a great deal of the posts put on this board in hope of learning as much as possible. One thing I have learned is that a lean condition is devastating to an engine, especially if you have boost. Why is this though? It seems to me if you have an insufficient amount of fuel the explosion would just be less intense, so if anything it would be better for your engine. Why is it particularlty bad if you have boost? Also, is it just as bad to run rich?

[RPM427]

During a lean condition, higher boost levels will destroy the internals of your engine. The damage occurs when not enough fuel is being delivered therefore causing the motor to run lean which in turn causes detonation and extremely high combustion chamber temperatures. The detonation alone can cause pistons to crack and apply undo stress to the rods and crank and so on. The high combustion temperatures can actually melt pistons. Usually the top of the pistons as well as the ring lands will melt away causing extreme blow by and excessive crankcase pressures which can blow seals out of the motor and the end result will be lose of oil and oil pressure. Normally detonation and extreme combustion temperatures occur together during a lean condition, so you can imagine the damage that can occur.

[chris91LX]

Yeah, what he said. An example would be if you have a camp fire or a fire in the fireplace, when you blow air into it like with a bellows you can see the flames gow more intense. Or if your familiar with a steel foundry, there is molten iron in a huge furnace and they stick a big air hose into the metal and turn it on, and flames come shooting out of the portals like crazy. Foundry guys call it "opening the gates of hell". That's not something you want to do inside an internal combustion engine. Adding fuel and air at the same time maintains a more stable burn temp. Too much fuel actually will not burn. If you have a bucket of gasoline and throw a match in it, the match will actually be put out in the liquid. However empty the bucket, just leaving the sides of the bucket wet, let it sit for a minute and throw a match and you better run. Its the fumes that burn not the liquid, which is why you are adding air and fuel, instead of just fuel to a combustion process. Damn, I'll shut up now, how's that for a $10 answer to a $5 question.

[302 LX Eric]

The 'ideal' condition you're looking for in a gasoline combustion engine is a mixture of 14.7:1 - I believe this is called Stoich or Stoiometric condition. Basically, it's 14.7 parts air to one part gasoline. This is where the optimum explosion occurs. However, from what I've seen, most Stangs seem to run a little richer than this, say around 12.5:1 or so.

Running too rich can be bad too. Too much fuel and over time you can end up washing the walls of the cylinders with fuel and there go your rings. You can also get massive amounts of carbon like build up on your valve and pistons.

Some of the engineers on this site could give you a more technical answer, I'm just giving you the layman terms.

E

[chris91LX]

Yeah that's the Stoichiometric fuel ratio and I believe the 14.7:1 is what is recommended by Ford, which is the optimum ratio for fuel economy I'm pretty sure. The ratio changes when you are trying to improve performance. N/A combinations are ideally set at about 13:1 and power adders are usually down around 11:1 I think. These are numbers I've gleaned from speaking to other stangers and gearheads at the track and dyno techs and such.

[rppngears]

I understand what being to lean or rich is, and that it causes severe damage to your engine; metling pistons, detonation, etc. What I don't understand is why?

It seems to me if you have a lean condition where you have say 17:1, then it seems to me you would have complete combustion of the fuel that is present and just have some left over oxygen at the end. Now this seems that this explosion would be less intense then say a mixture of 13:1 where there is the same amount of oxygen in the cylinder, we just increased the amount of fuel. Since there is more fuel, that would seem to tell me there would be a greater explosion.

I know of course if you keep decreasing the fuel air ratio, this will not stay true of course because you will eventually have insufficient oxygen, but assuming these ratios, why is the lean condition worse for your engine?

[chris91LX]

If you read the posts you would understand it is the heat generated by a lean air/fuel ratio that kills the motor. More air = hotter temperatures in the cylinder, hence melted pistons.

[SlowGT]

I think the question was 'why does the lean mixture burn hotter?'

Although I don't have the words to explain it, I know that the heat produced is proportional to the amount of each part of the mixture as it is with any chemical reaction. You could probably figure it out if you got into the physical/chemical properties of air & fuel. 'Stoich' is the perfect combination of air and fuel where both are completely consumed in an internal combustion engine. It's the point of efficiency.

Well, I've typed the following paragraph several times and still can't get it straight, so it no longer exists.

In additional thought on running too rich for too long: gasoline in the oil pan doesn't lubricate bearings very well.


I can't believe PKRWUD hasn't been on here already with the nitty gritty on A/F ratios and the combustion process. Chemical reactions just aren't my mug of beer. I'm still waiting for class to begin

[RPM427]

For any combustion or burning you need a fuel and an oxidizer. In this case the fuel is gasoline and the oxidizer is the oxygen in the air. The more oxygen, the more intense the reaction. The reaction is obviously exothermic (gives off heat) so the faster/more intense the reaction, the more heat. Got it?

[SlowGT]

Yeah...that's it. I knew it was something like that.

14 years without a chem lab will do a number on ya

[SlowGT]

Yeah...that's it. I knew it was something like that.

14 years without a chem lab will really take it out of ya


Thanks

[jimberg]

I love these dicussions.

"Too lean" is a relative term. 14.7:1 is the stoichiometric air/fuel ratio but it is not necessarily the ideal condition. Stoichiometric basically means that for every fuel molecule there will be the necessary number of oxygen molecules so that when a chemical reaction takes place there will be no fuel or oxygen molecules left. Closed loop mode is when the ECM is trying to maintain an air/fuel ratio that will yield the most power for the amount of fuel consumed.

Adding more oxygen to the reaction than is necessary does NOT generate more heat. The amount of releasable heat energy will be maxed out at the stoichiometric ratio. Adding more fuel to the reaction than is necessary will reduce heat, however.

When you add more fuel than is necessary, the remaining fuel will be vaporized which will cool down the cylinder. A water injection system does the same sort of thing. "Too lean" would be a condition where the heat being generated by combustion isn't being given enough time to disapate between each firing of the cylinder which would cause heat to build up. At high RPMs this heat would build up rapidly. Even though 14:1 is rich compared to the stoichiometric ratio, it would be too lean at WOT and could damage your engine as described by the other posters.

[RoadWarrior]

I think it was best decribed above. When you add air its like fanning the flames when your trying to get a fire going. You blow on the fire to add air and heat it up so the fire starts.

[SlowGT]

Hey jimberg

Could you help me understand this:

The same amount of heat is produced at 15.7 as at 14.7 as at 13.7.

Only at 15.7, there's not enough fuel to absorb all the heat, so the heat ends up in the pistons/rings, etc.

And at 13.7 there's an excess of fuel so the heat is more than adequately absorbed therefore giving a 'cooling' effect in the combustion chamber.

and if you're 'cooling' the combustion chamber, subsequent cycles will be losing heat from the reaction to the combustion chamber instead of using all the heat for the combustion process.

Is this enough understanding to be promoted to Grasshopper 1st Class? or have I missed the mark again?

Thanks in advance
Jared.

BTW, nice Vert. Dark blue w/silver is one of my favorite fox paint schemes. I see you've got a Lightning block in there. Do you know if all Lightning blocks have a roller cam from the factory? I've been leaning toward a 347, but I think a stroked 351W would be a lot healthier and beefier. Maybe in the 383-392 range. The only thing is, I've never seen swap headers for an SN95.
Thanks again.

[jimberg]

RoadWarrior, the fanning a flame analogy is really not a good one for describing this particular problem. A camp fire is an open system that is burning slowly. Since it's burning in the atmosphere, you have an unlimited amount of oxygen for a limited amount of fuel.

The chemical reaction that occurs in your combustion chamber is a closed system that has a limited amount of fuel and oxygen. It is also compressed so it explodes.

As far as air/fuel ratio goes, let's say that our engines run with hydrogen as the fuel in a pure oxygen atmosphere. If you have 100 hydrogen atoms and 50 oxygen atoms, the combustion should produce 50 water ( H2O )molecules with nothing remaining. This would be a stoichiometric reaction since no fuel or oxidant remains. If you have 100 hydrogen atoms and 100 oxygen atoms, you'd still only get 50 water molecules and 50 left over oxygen atoms. This would be a lean mixture, but since only 100 hydrogen atoms and 50 oxygen atoms were combined, the same amount of energy (heat) would be released from the chemical reaction. The same would be true if you had 200 hydrogen atoms and 50 oxygen atoms. Only 50 water molecules would be created but you would have 100 left over hydrogen atoms with it being a rich mixture.

There are other factors that generate heat such as the rapid compression of gas and other chemical reactions such as nitrogen combining with oxygen to form NOx, but I hope the above examples will be pretty clear as to what is happening as far as air/fuel ratio.

Jared, I hope the above example helps with your question. The amount of heat generated is based on the amount of fuel that can react completely with oxygen, but yes, I think you have the gist of what I'm saying. There's also the added heat that is generated by rapidly compressing the left over oxygen.

I'm pretty sure that my lightning block came with a roller cam from the factory. The block is definitely roller cam ready. As far as the headers go, that's an interesting question. I would think that the 351W swap headers for a fox would fit the same way. You should call Mac or Holcomb Motorsports to see what the difference would be. I'm really glad that I went with a 351.

I really like the color scheme, too, but after 13 years, it's time for a change. I'm thinking of going with all silver.

[SlowGT]

Thanks!!

[rppngears]

Well, from the whole camp fire analogy, I thought I understood it, but apparently that is not the correct reason. I understand the air/fuel ratio and the H2O example just fine. What I don't understand is if the same amount of heat is generated regardless of ratio, as long as the same amount of fuel is consumed, then why is it so bad for your engine to run lean or rich? Why does this generate the extra heat? After combustion you will be left over with either some oxygen or fuel molecules, but they will be removed from the chamber on the next stroke.

[jimberg]

When you're running on the rich side, the fuel vapor that is left tends to absorb the heat and remove it from the cylinder as SlowGT said. This cooling effect also allows more air to enter the cylinder which will give you more power, as well.