Timeline for How exactly does running fuel-lean increase the temperature of ignition?
Current License: CC BY-SA 3.0
18 events
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| Jan 29, 2016 at 21:02 | comment | added | cdunn | @DrewJordan Yeah, I could not figure out a good way to say what I obviously said so badly that first time, so I just removed it. Sorry for the confusion. | |
| Jan 29, 2016 at 21:01 | history | edited | cdunn | CC BY-SA 3.0 |
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| Jan 29, 2016 at 20:55 | comment | added | NL - SE listen to your users | @cdunn You may want to look at my answer. Modern fuel-injected engines keep the fuel to air ratio balanced because too much fuel = hydrocarbon emissions and CO, too little fuel = hot combustion, and production of NOx, balanced combustion = CO2 (AKA clean exhaust). Of course we still need catalytic converter because even the best computer is only reacting to the sensor reading after the fact and there are small amounts of each of the above as the computer adjusts. | |
| Jan 29, 2016 at 16:53 | comment | added | DrewJordan | "...it's cooler than dry air of the same temperature" (emphasis mine). huh? | |
| Jan 29, 2016 at 12:27 | comment | added | tpg2114 | @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? | |
| Jan 29, 2016 at 12:21 | comment | added | I have no idea what I'm doing | @tpg2114 Hmm, I find the explanation to be a little vague. So the extra oxygen is brought to burning temperature but doesn't burn, total temperature should go down a little. Is the key in the fact that the heated air also helps conduct heat to the block ? | |
| Jan 29, 2016 at 11:19 | comment | added | tpg2114 | And yes, "complete" isn't really "complete" but that isn't the problem here. It's more like in a lean mixture, you get more "bang for your buck" in terms of combustion energy to temperature conversion. It takes less energy to heat the extra air and so even though you have less energy released, it is more effective at raising temperature. Until there just isn't enough energy to go around at all (too lean) and then it doesn't raise temperature as much. | |
| Jan 29, 2016 at 11:17 | comment | added | tpg2114 | @IhavenoideawhatI'mdoing It's hard to fit it all in comments here, but the reason why fuel-rich is cooler is the same reason why fuel-lean is hotter for a certain range. Available energy to heat stuff up and the amount of heat it takes to increase the temperature interplay in a non-linear way. | |
| Jan 29, 2016 at 11:12 | comment | added | I have no idea what I'm doing | @tpg2114 I think you are explaining why a rich mixture runs cooler than stoichiometric, not why lean mixture runs hotter. If the combustion is complete as you said, then there should be the same amount of fuel in both stoichiometric and lean mixtures. So extra oxygen should cool, not heat. Unless "complete" combustion is not exactly complete. | |
| Jan 29, 2016 at 11:11 | comment | added | tpg2114 | energy from the fuel that is there will raise the temperature of the mixture more. But too much air (or really, not enough fuel) and there isn't as much energy released so it doesn't get as hot. That's why in the figure in my comment up above, you see the temperature peaks for equivalence ratios between 1.1 and 1.3 depending on the fuel, and then decreases rapidly past that. And it decreases rapidly on the fuel-rich side because fuel takes more energy to heat up. | |
| Jan 29, 2016 at 11:09 | comment | added | tpg2114 | @cdunn The reason it happens is an interplay between the heat released from the fuel from combustion and the amount of energy it takes to increase the temperature of fuel and air. Fuel takes more energy to get hotter (higher heat capacity) than air, so when there is more fuel, it takes more energy to raise the temperature of the mixture. When there is less fuel, it takes less energy to raise the temperature -- but there is also less fuel to release energy that causes the heating. It is a non-linear effect, so if you add a little more air, the temperature will increase because the | |
| Jan 29, 2016 at 11:04 | comment | added | cdunn | The effect is repeatable and observable. I have no idea why adding oxygen makes the flame hotter. I'll have to go do some more reading in this because now I'm curious as to why the effect happens. | |
| Jan 29, 2016 at 11:00 | comment | added | I have no idea what I'm doing | @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? | |
| Jan 29, 2016 at 10:42 | comment | added | cdunn | The latest answer is incorrect in several ways, but more on that later. The simple answer to your question is that the flame created is hotter because of the presence of more oxygen. A metal in contact with a hotter flame will be hotter. Hotter than it would be for a stoichiometric mixture. | |
| Jan 29, 2016 at 10:37 | comment | added | I have no idea what I'm doing | 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? | |
| Jan 29, 2016 at 10:13 | comment | added | tpg2114 | @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. | |
| Jan 29, 2016 at 7:26 | comment | added | I have no idea what I'm doing | 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? | |
| Jan 28, 2016 at 21:59 | history | answered | cdunn | CC BY-SA 3.0 |
