Physics and chemistry of why lean-running air/fuel mixes produce more heat?

[fuzzybabybunny]

So the general explanation for why a lean fuel mixture produces more heat and can lead to an engine seizure is that it's the same as if you took a fire in a fireplace and fanned air into it. With the added air the temperature of the fire goes up.

To me this doesn't quite make sense around the subject of engines.

If you have a general combustion formula:

CH4 + 2O2 -> combustion -> 2H2O + CO2 + 100 kJ heat (or whatever)

If you simply add more O2 without increasing fuel (CH4) the amount of energy and heat released should still be the same:

CH4 + 48O2 -> combustion -> 2H2O + CO2 + 46O2 + 100 kJ heat

The only way for it to increase heat is if more fuel was burned.

And this is what I don't understand about lean-running engines producing more heat. When you tune the carb to run lean, you're restricting the amount of fuel that gets into the combustion chamber, right? So less fuel in the combustion chamber should mean less heat is produced, right?

The reason the fire in the fireplace gets hotter when you fan more air is because more fuel is being burned in, say, 1 second than if there wasn't air being fanned onto it. All that extra fuel (wood) is just laying there in the fire itself.

But in the case of engines the combustion chamber only has a set amount of fuel, whatever amount the carb is set to deliver.

 

[brainhulk]

fuel acts as a coolant. google the rest because I forgot the chemistry/physics

 

[fuzzybabybunny]

fuel acts as a coolant. google the rest because I forgot the chemistry/physics

That's a factor on rich fuel mixtures because the excess evaporating fuel has a cooling effect on the engine.

 

[Raizinman]

You are looking at just the fuel and air. The engine is trying to convert the fuel air to mechanical energy. When you restrict the fuel the pistons do not know this and are still trying to make the same amount of energy (which it cannot) and thus starts to overheat due to lack of fuel.

 

[fuzzybabybunny]

You are looking at just the fuel and air. The engine is trying to convert the fuel air to mechanical energy. When you restrict the fuel the pistons do not know this and are still trying to make the same amount of energy (which it cannot) and thus starts to overheat due to lack of fuel.

Huh? What's the actual thing that is causing an increase in heat? Chemical bonds being broken? Friction between molecules? Etc?

If there's not enough fuel there would be a loss in engine power but depending on how much power less there is, the two-stroke cycle should still be able to continue.

 

[tortillasoup]

Lean fuel mixtures are more efficient because they help oxidize the fuel more completely. Just because the injected fuel mixture is technically 14.7:1, doesn't mean it mixes all that well and usually mixes on the rich side slightly. Only until very recently did cars have wideband oxygen sensors whereby previously all they did was alternate the fuel mixture between lean and rich, hoping for an average of stoichiometric or less (rich). Lack of precision readout of fuel mixture mixing is why on non-wideband cars, full throttle completely ignores oxygen sensor read outs.

Other reasons I've read of why lean mixtures increase power and fuel economy at the cost of pollution is because not only is there a lot of heat, but there is excess oxygen, nitrogen, water and other combustion byproducts ready to be split and combined which can make heat in of itself. In a rich or stoichiometric mixture, there are no extra oxygen molecules and the fuel that cannot be burned actually cools the mixture.

By the way, very lean mixtures don't create more power. Mixtures above 16:1 definitely will not overheat an engine but may not run very well or optimally if it isn't designed for it because there are certain scenarios like high load when a lean mixture would be undesirable. Lean mixtures are also unstable which is why on the Hondas, they had a VTEC-E design where it opened one intake valve normally and barely opened the second intake valve when under lighter loads or under 3000RPM I believe. This created turbulence and swirl in the intake charge, helping distribute the lean fuel mixture. Above 3000rpm, the second valve would open at a normal depth. All Hondas with VTEC have this feature today but back in the day of the lean-burn Civics, it was unique to lean-burn VTEC motors.

 

[fuzzybabybunny]

Thanks. Everyone says to run two-strokes slightly rich because lean increases heat and can seize the motor and/or burn a hole in the piston.

Is that just because two-strokes are made very robust and can't handle the heat of a lean mixture?

 

[Meghan54]

2-strokes are vastly different animals vs. 4-stroke engines. The "run rich" camp in the 2-stroke will point out the firing on every stroke of the cylinder, unlike a 4-stroke which has a stroke that has no ignition, which consequently can lead to higher cylinder temps--no time to cool at all. And there's the issue of the gas/oil mix providing all the lubrication the engine gets, which if running lean, may not provide the engine's bearings with enough lubrication at higher engine speeds.

At least that's my understanding of some of the "run richer" camp.

 

[uclaLabrat]

As to the chemistry explanation, the heat put out by the combustion is the same, but the temperature is higher because there are fewer molecules to absorb the heat. Same reason an acetylene torch burns hotter than propane, even though propane puts out more heat than acetylene.

 

[jaha2000]

2-strokes are vastly different animals vs. 4-stroke engines. The "run rich" camp in the 2-stroke will point out the firing on every stroke of the cylinder, unlike a 4-stroke which has a stroke that has no ignition, which consequently can lead to higher cylinder temps--no time to cool at all. And there's the issue of the gas/oil mix providing all the lubrication the engine gets, which if running lean, may not provide the engine's bearings with enough lubrication at higher engine speeds.

At least that's my understanding of some of the "run richer" camp.

If your talking two cycle, the heat is generated from the lack of oil getting into the engine in the lean mix.
Keep in mind the VOLUME going into the cylinder is the same. Lean mix = less oil. unless you change your premix ratio.

 

[tortillasoup]

As to the chemistry explanation, the heat put out by the combustion is the same, but the temperature is higher because there are fewer molecules to absorb the heat. Same reason an acetylene torch burns hotter than propane, even though propane puts out more heat than acetylene.

There aren't less molecules, there are simply a different ratio. Unless the atmospheric pressure is changed, all you're getting is a different ratio of molecules.

 

[JulesMaximus]

Stricter emissions control is why the air-cooled engine is going the way of the dodo bird. The last big bore Ducati air-cooled motors were running notoriously lean (and hot). A Termignoni full exhaust and race CPU cured that but of course emissions increase as a result. Now there is only one air-cooled Ducati left and it is the Scrambler.

 

[Mark R]

Huh? What's the actual thing that is causing an increase in heat? Chemical bonds being broken?

Yes. When an engine runs rich, you get incomplete combustion. The limiting factor is air. As a result, not all the bonds in the fuel get broken, and instead of getting CO2 and H2O as in your equations, you get carbon and carbon monoxide. On top of that, the excess fuel absorbs the heat of combustion by evaporation, and the unburned fuel vapor carries the heat out of the exhaust. The general lack of oxygen in a rich condition means that the combustion is slower, and cooler.

When an engine runs lean, the limiting factor is the fuel - but you end up with complete combustion of the fuel, so release the maximum amount of energy. Even at stoichiometric, combustion is not complete, although it is close - the catalytic converter is used to clean up the residual CO, hydrocarbons and NOx. Peak cylinder temperatures are reached at a few % lean of stoichiometric. This is when you get the most complete combustion. Going leaner still reduces temperatures because the heat is diluted in a greater mass of air.

The real problems with lean running is that it reduces detonation margin, because there is less cooling of the charge air by the latent heat of vaporization of the fuel, and because it impairs ignitability, so increases the risk of misfire.

 

[RLGL]

You also have incomplete combustion in a 2 cycle. Some of the initial fuel charge is used to push out the exhaust from the prior burn

 

[fuzzybabybunny]

Yes. When an engine runs rich, you get incomplete combustion. The limiting factor is air. As a result, not all the bonds in the fuel get broken, and instead of getting CO2 and H2O as in your equations, you get carbon and carbon monoxide. On top of that, the excess fuel absorbs the heat of combustion by evaporation, and the unburned fuel vapor carries the heat out of the exhaust. The general lack of oxygen in a rich condition means that the combustion is slower, and cooler.

When an engine runs lean, the limiting factor is the fuel - but you end up with complete combustion of the fuel, so release the maximum amount of energy. Even at stoichiometric, combustion is not complete, although it is close - the catalytic converter is used to clean up the residual CO, hydrocarbons and NOx. Peak cylinder temperatures are reached at a few % lean of stoichiometric. This is when you get the most complete combustion. Going leaner still reduces temperatures because the heat is diluted in a greater mass of air.

The real problems with lean running is that it reduces detonation margin, because there is less cooling of the charge air by the latent heat of vaporization of the fuel, and because it impairs ignitability, so increases the risk of misfire.

I guess the part that I get stuck on is this:

When an engine runs lean, it means that there is less fuel, *not* that there is more oxygen. When I go to adjust a carb for leanness/richness, I adjust the screws that control the flow of fuel. I don't adjust anything that has to do with the flow of air.

So it would be something like this for a "normal" mixture:

50 mol CH4 + 100 mol O2 -> combustion -> 100 mol H2O + 50 mol CO2 + 5000 kJ heat (or whatever)

And for a lean mixture:

40 mol CH4 + 100 mol O2 -> combustion -> 80 mol H2O + 40 mol CO2 + 40 mol O2 + (something less than 5000) kJ heat

So I would think that the "something less than 5000 kj" would mean that it should run cooler since there is simply less moles of fuel being burned. The amount of O2 in the combustion chamber is the same because the carb adjustments only restrict the moles of fuel getting into it. In a lean mixture there is more complete combustion of the fuel, but there is also less fuel. So is it fair to say that despite there being more fuel in a rich mixture, MORE fuel is being burned in a lean mixture anyway? So running an engine slightly lean will produce more power than an engine running slighting rich? And because a rich mixture burns less fuel and has more evaporative effects than the lean mixture, the engine runs cooler?

I get that a stoichiometric mixture in real life won't have complete combustion and that lots of other things will be made from incomplete combustion and all the other elements in air besides O2 when running rich. And there's more excess fuel being evaporated so the heat of evaporation factor is more.

 

[yh125d]

I guess the part that I get stuck on is this:

When an engine runs lean, it means that there is less fuel, *not* that there is more oxygen. When I go to adjust a carb for leanness/richness, I adjust the screws that control the flow of fuel. I don't adjust anything that has to do with the flow of air.

So it would be something like this for a "normal" mixture:

50 mol CH4 + 100 mol O2 -> combustion -> 100 mol H2O + 50 mol CO2 + 5000 kJ heat (or whatever)

And for a lean mixture:

40 mol CH4 + 100 mol O2 -> combustion -> 80 mol H2O + 40 mol CO2 + 40 mol O2 + (something less than 5000) kJ heat

So I would think that the "something less than 5000 kj" would mean that it should run cooler since there is simply less moles of fuel being burned. The amount of O2 in the combustion chamber is the same because the carb adjustments only restrict the moles of fuel getting into it. In a lean mixture there is more complete combustion of the fuel, but there is also less fuel. So is it fair to say that despite there being more fuel in a rich mixture, MORE fuel is being burned in a lean mixture anyway? So running an engine slightly lean will produce more power than an engine running slighting rich? And because a rich mixture burns less fuel and has more evaporative effects than the lean mixture, the engine runs cooler?

I get that a stoichiometric mixture in real life won't have complete combustion and that lots of other things will be made from incomplete combustion and all the other elements in air besides O2 when running rich. And there's more excess fuel being evaporated so the heat of evaporation factor is more.

Look at it more like this, with the assumption that complete combustion occurs when you have equal amounts fuel and air (not true, but just go with it)

Normal conditions running rich are like 130 units fuel + 100 units air = 30 units unburned fuel, B units combustion byproducts, and 100 units energy. Those 30 units unburned fuel carry some 30 units of heat out by evaporative cooling and as absorbed thermal energy that is not enough to cause combustion (due to the lack of sufficient air) = 70 net thermal units energy

Running lean, lets say 90 units fuel + 100 units air = 0 units unburned fuel, B x .9 combustion byproducts, 10 extra units air, 90 units energy. Maybe the 10 units air carry 10 units energy away = 80 net thermal units energy


Maybe I got that right, makes sense to me (specifics of numbers are of course garbage but just illustrate the concepts)

 

[fuzzybabybunny]

Look at it more like this, with the assumption that complete combustion occurs when you have equal amounts fuel and air (not true, but just go with it)

Normal conditions running rich are like 130 units fuel + 100 units air = 30 units unburned fuel, B units combustion byproducts, and 100 units energy. Those 30 units unburned fuel carry some 30 units of heat out by evaporative cooling and as absorbed thermal energy that is not enough to cause combustion (due to the lack of sufficient air) = 70 net thermal units energy

Running lean, lets say 90 units fuel + 100 units air = 0 units unburned fuel, B x .9 combustion byproducts, 10 extra units air, 90 units energy. Maybe the 10 units air carry 10 units energy away = 80 net thermal units energy


Maybe I got that right, makes sense to me (specifics of numbers are of course garbage but just illustrate the concepts)

Ok, yeah, makes sense. Now, if it was even more lean:

80 units fuel + 100 units air = 0 units unburned fuel, B x .8 combustion byproducts, 20 extra units air, 80 units energy. Maybe the 20 units air carry 20 units energy away = 60 net thermal units energy

This extra lean mixture would then make it cooler than the rich mixture above. But then you run into other problems, like loss of power.

 

[Zorba]

Yeah, running more and more lean will produce cooler temperatures.

Jet engine combustors used to start with an extremely rich mixture, then mix in more and more air, so you'd pass from very rich, through stoichiometric, to pretty lean. The hottest temperatures are produced at stoichiometric, which is when the majority of the NOx is made. To reduce NOx production, they had to keep the flame temperature down, so new combustors are actually "Lean of Peak" so they start the mixture out lean so it never passes through the stoichiometric point.

 

[uclaLabrat]

There aren't less molecules, there are simply a different ratio. Unless the atmospheric pressure is changed, all you're getting is a different ratio of molecules.

There are less molecules. One molecule of Acetylene (C2H2) burns to make two molecules of CO2 and one molecule of water (H2O) plus heat. One molecule of ethane (C2H6) burns to make two molecules of CO2 and 3 molecules of water. The difference is in the saturation. There is less energy in C-C double bonds (acetylene) than in C-H bonds, so the ethane has more total energy, but it's distributed over more molecules so the collective temperature is lower.

 

[JCH13]

There's a lot more at play here than just the 'un-burnt fuel absorbs heat' thing.

Maximum flame-front speed is achieved at slightly-rich charge mixes. Simplistically speaking, the faster the charge burns the more power an engine can generate because.

More power is generated because the charge energy is extracted more efficiently by the piston (simplistically speaking). Well, when we vary from this 'slightly-rich, best power output' point there is more opportunity for heat to be transferred to the engine by the charge/combustion gases per unit power output. This can be affected by air fuel mix, timing, fuel atomization, etc.

Cars are fine cruising at leaner AFRs because they're under little load and can handle the required heat rejection (as required heat rejection is ostensibly output power * (1-engine efficiency).

Some related reading/chart sauce: http://www.clubmr2quebec.com/forums/index.php?topic=344.0

More later if anyone is interested...

 

[Yuriman]

You might have some fun with this chart:

Source

The AFR in my Insight goes as lean as 25:1 sometimes, and can remain in excess of 21:1 up to 90% load. My real-world fuel economy is anywhere between 70 and 120mpg, with my last tank coming in at 874.2 miles on a 10 gallon tank.

I don't think brake specific fuel consumption is any lower when running this lean (it's probably slightly lower than just slightly lean, actually), but what it does allow is for the engine to run in a very efficient RPM and load range and still vary the amount of power, which is necessary for maintaining speed in varying conditions.

The reason you can burn valves and such, as Mark R says, is detonation.

 

[JCH13]

The reason you can burn valves and such, as Mark R says, is detonation.

Please explain how detonation can burn valves...

 

[tortillasoup]

Please explain how detonation can burn valves...

If you have an uncontrolled explosion during the exhaust stroke, typically how a valve gets burnt. The valve head is used as a heatsink when the valves are closed so an explosion in the combustion chamber doesn't overheat the valves but if the valve is open during the exhaust stroke and there is an explosion, there is no where for the valve to sink its heat.

 

[JCH13]

If you have an uncontrolled explosion during the exhaust stroke, typically how a valve gets burnt. The valve head is used as a heatsink when the valves are closed so an explosion in the combustion chamber doesn't overheat the valves but if the valve is open during the exhaust stroke and there is an explosion, there is no where for the valve to sink its heat.

But doesn't detonation occur on the compression stroke, not the exhaust stroke? I'd be really interested to hear how combustion would occur on the exhaust stroke (anti-lag systems aside).

 

[JCH13]

There are a few things that I know of that can cause burned valves, and they all relate to the valve not seating to the head properly and thus not conducting heat out of the valve into the head. A leaking valve seal will gunk the valve with carbon and insulate it from the head. A bad valve adjustment or adjuster will prevent the valve from seating properly. Excessive valve wear will also prevent proper seating. Excessively high exhaust gas temperatures can also burn valves.

Nothing to do with detonation or pre-ignition that I know of...

Detonation, or the ignition of air-fuel charge after the spark event, but ahead of the flame front, is a result of the air-fuel charge's inability to withstand the heat and pressure of during the combustion cycle. This will occur only during the power stoke because the pressure inside the cylinder will drop dramatically as soon as the exhaust valves open, preventing any further detonation events.

The way that lean AFRs, detonation, and burning valves are related could be as follows: lean AFRs cool the cylinder less than richer AFRs because there is less fuel to evaporate. Lean fuel mixtures also burn more slowly than richer fuel mixtures, generally speaking. See:

When the air-fuel charge burns more slowly the engine has less time and less piston stroke to extract mechanical energy from the charge during the power stroke. This un-extracted energy remains in the charge as heat, and could cause high exhaust gas temperatures, perhaps high enough to cause a valve to burn. Thus one could clearly correlate detonation to burnt exhaust valves, but there is no direct causal relationship between the two.

Please feel free to disagree with me, I'd be happy to hear a counter argument.