# How exactly does running fuel-lean increase the temperature of ignition?

I learned in this question that running an engine fuel-lean will increase the temperature of ignition, among other things such as causing problems with the catalytic converter. I have no idea how or why.

How and why exactly does running fuel lean (or oxygen rich) increase the temperature of ignition?

You have a qualitative description of what happens, but let's break it down to a smaller scale. When we talk about "temperature" of something, we are really talking about how fast the molecules are moving around and bouncing off each other. "Temperature" is really "kinetic energy". And it turns out that there are other types of energy besides moving around in space -- molecules can rotate, they can vibrate, and their electrons can get excited and move around relative to the nucleus. Each of these energies can also be a "temperature", so you can have translational temperature (what we normally think of), but you can have rotational temperature, vibrational temperature, and electronic temperatures.

Molecules exchange energy with one another by colliding into each other. When they do this, they also distribute the energy between them. How often they collide determines how quickly the energy becomes uniform, and this defines how quickly they reach what is called equilibrium. When all of the different temperatures are the same, the state is in equilibrium and we don't need to worry about keeping track of all of the different types of temperature. For most of the processes that would occur in an engine, there is more than enough time to reach equilibrium and so we don't need to worry too much about non-equilibrium effects.

Now, in chemical reactions, molecules break apart and form new ones. If the new ones have less energy, the difference in energy is released as heat. If the new ones have more energy, the reaction requires adding energy to make it happen. Obviously engines get hot, so the reactions in them release energy and we harness that energy to move the vehicle.

So, molecules break apart. And they break apart when they start vibrating so hard that the bonds between the atoms cannot hold them together anything. The only way to make the molecule vibrate is to have another molecule collide with it, with enough energy and an efficient enough transfer of energy to start the vibrations. And the energy has to be high enough that the vibration makes the molecules fall apart.

By changing the amount of fuel in the mixture, you are changing the types of collisions that can occur. And it's not exactly straight forward, but some molecules are better at exchanging energy with others. To make the fuel molecule fall apart, they need to collide with other fuel molecules with some energy of with other oxygen molecules with more energy. If you add more than the usual amount of oxygen (run lean), you also need to make that oxygen hotter so the molecules have more energy when they collide and can make the fuel vibrate hard enough to fall apart. Conversely, if you run fuel-rich, you have more fuel molecules that can collide with one another and fall apart, but fewer oxygen molecules for them to combine with and give off heat. This (and some other effects) makes the final flame temperature lower.

Based on some extended conversation on the question, let's put this all back into the context of an engine. For a direct-injection gas engine, the air is sucked into the cylinder, the piston compresses it, and then fuel is sprayed into the cylinder. A spark plug then triggers a spark in the chamber. This deposition of electrons gets the fuel-air mixture molecules all excited -- it actually ionizes the air (strips off electrons from the molecules) and this all adds a bunch of energy to the molecules. This energy is the initial energy required to start the combustion.

For a fuel-lean condition, I said it takes more energy to start the reaction and I phrased it in terms of a higher ignition temperature. The ignition temperature comes from that spark plug (for a cold engine -- hot engines will also contribute heat from the cylinders themselves). For normal operating conditions, spark plugs provide more than enough energy to ignite. As the operating condition gets leaner, the spark plug provides the same amount of energy -- but it is still enough energy to ignite. Eventually, for lean enough conditions, it won't be enough energy. This is a lean misfire.

Diesel engines work differently. For sake of argument, let's stick with a direct injection again. The cylinder fills with air, the piston compresses it, and the fuel is injected. There is no spark to initiate the reaction though. Diesel engines rely solely on creating high enough pressures to ignite the mixture. High pressure means high density and that means more collisions to spread the energy around (molecules don't need to go as far to hit one another). At any rate, the same ideas apply. In lean conditions, it would require a higher pressure to ignite. At ideal conditions, the engine compresses more than is exactly required, so when it runs fuel-lean, it still has enough compression to ignite. If you go so lean that the compression isn't high enough anymore, you will again get a lean misfire. Glow plugs can help all this by heating the cylinders and helping to add heat to the mixture and get the reactions going.

In either engine, once they have been running for awhile, the cylinder walls heat up and it requires less input (from sparks or from compression) to make the reaction occur. But for cold engines, it needs that initial energy deposition to get the reactions moving along. Many ECU's are set to burn fuel-rich when the engine is just starting because it is easier to ignite; as they heat up, the mixture becomes more lean and reduces emissions and fuel consumption. You may be familiar with manual chokes on things like lawn mowers -- the choke is what changes the fuel-air mixture and to get the motor started, you have to set the choke to be fuel-rich.

For those interested, based on the discussion we had in the various comment threads, I went ahead and gave a concrete example of how/why temperature can increase when the flame is fuel-lean. The conversation in chat is bookmarked here.

• Love your angle of approach on this. +1 for unique an thoughtful. TY! – DucatiKiller Jan 29 '16 at 1:04
• It is an interesting angle of approach, though I'm not sure it answers the question completely. So you need to make the extra oxygen hotter, where does the extra heat come from? – I have no idea what I'm doing Jan 29 '16 at 7:24
• As I understand it the underlying question is basically "why does an oxygen-rich flame burn hotter". Your answer explains why more heat is necessary to ignite a lean mixture, but I am still not sure where the extra heat comes from. – I have no idea what I'm doing Jan 29 '16 at 10:29
• @IhavenoideawhatI'mdoing "Ignition temperature" is the heat required to start a flame and that's what the question is asking, and that is different from why does an oxygen-rich flame burn hotter (because it doesn't always, it burns hotter when you add some oxygen, but then not as hot if you add too much). That's a wholly different question -- ignition temperature (OP's question) is different from flame temperature (your question). – tpg2114 Jan 29 '16 at 10:45
• You are probably correct. I think some people, including myself, misinterpreted the question. The other two answers address increased cylinder temperature in the presence of a lean mixture, not ignition temperature. – I have no idea what I'm doing Jan 29 '16 at 10:56

Funny you should ask this Max :)

First lets make sure of our definition. Running an engine lean means changing the air / fuel ratio to have more air than is ideal (14.7:1 air to fuel).

In my reading there are two effects.

First, the fuel is an atomized liquid which has a cooling effect on the combustion chamber. So less fuel, less cooling effect.

Second, flames burn faster and hotter in the presence of more oxygen. More air relative to fuel than usual, means more oxygen than usual. So the flame burns hotter and faster than it should. Both are going to raise the temperature of the combustion chamber.

I hope that helps!

• Would it be correct to state that more fuel is burnt with the presence of more oxygen (or the existing fuel is burnt more completely, to be more accurate)? I don't believe adding more oxygen alone would make the flame hotter, would it? – I have no idea what I'm doing Jan 29 '16 at 7:26
• @IhavenoideawhatI'mdoing When the equivalence ratio is exactly 1, all of the combustion is "complete" and that is what is called stoichiometric. So for any equivalence ratio of 1 or greater (lean), you burn up everything completely and there is no excess fuel. But lean flames do burn hotter just by adding more air, see this figure for example. It's a non-linear process -- there is less heat release because there is less fuel, but air heats up with less energy than fuel does. – tpg2114 Jan 29 '16 at 10:13
• Not sure I understand, and this also seems to contradict the newest answer. Could you elaborate on the mechanism by which the cylinder walls heat up more in a lean mixture than a perfectly stoichiometric? Does the extra oxygen conduct heat to the walls? – I have no idea what I'm doing Jan 29 '16 at 10:37
• @cdunn Well then why is the flame hotter? If the reaction between fuel and oxygen emit heat, then adding oxygen alone shouldn't increase the heat emitted (if the combustion is complete in both cases). So what's the underlying mechanism then? – I have no idea what I'm doing Jan 29 '16 at 11:00
• @IhavenoideawhatI'mdoing I think the back and forth in all the comment threads is getting a little crazy -- would you like to discuss this further in chat where we have some more room to type things out? – tpg2114 Jan 29 '16 at 12:27

If you have ever seen an oxy-acetylene torch being used, you will have noticed that before the oxygen is turned on, the torch has a bright yellow flame. This is the fuel burning in a less than ideal amount of oxygen. The flame is relatively cool and it produces a lot of soot.

When the oxygen is turned on, tthe flame turns blue and becomes hot enough to melt steel.

You may have also seen when too much oxygen is turned on, the flame goes out with a pop.

Fuel lean is the same as oxygen rich.

In an engine, the fuel wants to burn efficiently, but not too hot that it starts to melt the pistons, or maybe even violently explode, which will also cause damage.

From Wikipedia - A stoichiometric mixture unfortunately burns very hot and can damage engine components if the engine is placed under high load at this fuel–air mixture. Due to the high temperatures at this mixture, detonation of the fuel–air mix shortly after maximum cylinder pressure is possible under high load (referred to as knocking or pinging). Detonation can cause serious engine damage as the uncontrolled burning of the fuel air mix can create very high pressures in the cylinder. As a consequence, stoichiometric mixtures are only used under light load conditions. For acceleration and high load conditions, a richer mixture (lower air–fuel ratio) is used to produce cooler combustion products and thereby prevent detonation and overheating of the cylinder head.

https://en.m.wikipedia.org/wiki/Air–fuel_ratio

Sorry I can't get the link to work - copy and paste into browser.

• yes I realized it now. Deleting the comment. – Evren Yurtesen Jan 29 '16 at 11:08
• @NathanL. I hope you don't mind me correcting you, but a violent explosion is exactly what you don't want in an internal combustion engine. That is why engines have knock sensors on them to stop damage from explosions. What is required is a controlled flame front that passes through the fuel to give a steady expansion of the gases. I am pleased that you think my answer is generally correct however. – HandyHowie Jan 29 '16 at 21:31
• @NathanL The fact that you refer to it as an explosion shows that you do not understand what is going on. Please read the extract from Wikipedia that I have added to my answer. The fuel is always ignited before the piston reaches TDC. – HandyHowie Jan 29 '16 at 22:14
• @NathanL I have added a reference to the Wikipedia page. – HandyHowie Jan 29 '16 at 22:26
• – NL - Apologize to Monica Jan 29 '16 at 22:31

The temperature engine rises because the ignition of the fuel is slower. It takes longer for the fuel to burn because there is less of it.

The fuel itself has the same amount of BTUs available by burning it whether you use extra oxygen or not. PERIOD. When you blow on the coals in your fire, they get hotter but burn faster. They release the same amount of heat, but in a much shorter period of time.

Imagine your cylinder as a cabin in the winter. If you took a log and burned it in one minute, the items near the stove where that log was burning would heat up significantly and maybe melt, but most of the heat would go out through the chimney. If you only had one log per hour, the room would be very cold most of the time. Take that same log and burn it slowly for an hour before replacing it with another and less heat goes out through the exhaust and stays in the room.

The reason the engine gets hotter is that the slower burning fuel transfers more heat to the surrounding parts of the engine.

• So, we're talking about a lean condition where there is more air relative to a 14.7:1 desired AFR. Are you saying that adding more air without adding more oxygen will make the engine run cooler? – cdunn Jan 29 '16 at 21:06
• Are you saying fuel burns slower in an oxygen rich environment? – HandyHowie Jan 29 '16 at 21:41
• @handyhowie I'm saying the fuel burns slower in a fuel-lean environment. Usually lean doesn't mean more oxygen; it means less fuel. – NL - Apologize to Monica Jan 29 '16 at 22:02
• @cdunn That depends on if there is enough oxygen to burn all the fuel present, doesn't it? And no, if there is sufficient oxygen to burn all the fuel, more air won't cool the engine. It will cause the combustion temperature to be lower and slower giving more time to transfer heat to the engine during the combustion process. – NL - Apologize to Monica Jan 29 '16 at 22:06
• Nathan - The two are relative to each other. In an engine, if you don't put in a controlled amount of fuel to keep the mixture slightly fuel rich, then the fuel will burn very hot and cause damage to the engine. It can also explode rather burn in a controlled manner. – HandyHowie Jan 29 '16 at 22:08

You all forget something, the reason slight leanburn can be hotter than stochiomnetric ratios is very simple. It has to do with the injection of the fuel. For stochiometric ratio to work as intended every single oxygen atom would have to pair up perfectly with a fuel molecule before ignition. That just isn't possible, so you have unburned fuel molecules in your combustion.

By adding slightly more air to the mix you can ensure that all of your fuel combusts to a higher degree, which in term will raise the temperature of your combustion, add too much and the heat capacity of the excess air will lower the temperature.

I think the answers are incorrect. Because the question assumption is incorrect. First we have to decide hotter compared to what? and also we need to know this is a fact, is it really hotter or is it a myth? in addition the amount of the fuel/oxygen ratio is important, is this condition always true for all lean ratios? Perhaps the correct question is why the "slightly" lean mixture is hotter than "slightly" rich mixture perhaps?

The thermal energy output from fuel is simply related to how much of it you burn. You burn less, less heat is generated. You burn more, more heat is generated. As simple as that. Here what creates the heat is the energy stored in the fuel (for our example other factors such as pressures, frictions etc. are not important).

If you are comparing a rich mixture with lean mixture, of course the lean mixture will have higher energy output because you would convert all the fuel into energy. (more burned fuel = more heat) But it still depends on your mixture ratios, because if you have almost no fuel in your mixture, obviously then it wont generate so much energy.

If you are comparing an ideal mixture with lean mixture, then I think it should be even cooler (less thermal energy generated from combustion) since you would be getting less fuel and more oxygen into the chamber.

• This answer asserts that the heat produced is only related to how much fuel is being burned. That is not true. Burn the same amount of fuel in the presence of more oxygen and you will get more heat. – cdunn Jan 29 '16 at 10:46
• That is only true if your original burning process did not have enough oxygen to burn the fuel completely. The heat generated is purely related to the amount of burned fuel + type of fuel. You can see easily from the heat of combustion tables of different types of fuels. The value does NOT depend on oxygen amount. en.wikipedia.org/wiki/… – Evren Yurtesen Jan 29 '16 at 11:06
• Ahhh, interesting. Sounds like I have more reading to do, starting with the link you provided. – cdunn Jan 29 '16 at 11:25
• @cdunn The confusion may be that "heat" is not "temperature" in the combustion world. Heat is energy, and Evren's statement about energy released is correct. But how that energy gets turned into temperature is based on heat capacities of the mixture. And the heat capacity of the mixture depends on it's temperature and it's composition (it's not a trivial combination of effects). Tying back to my answer, the heat capacity is related to how much freedom a molecule has to move, rotate, vibrate and electronically excite. – tpg2114 Jan 29 '16 at 11:27
• One of our competing research labs has a pretty good site where you can see the equations and examples and you can also compute flame temperatures for different fuels or watch lectures on the topic. – tpg2114 Jan 29 '16 at 11:31