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Car Forum / Driving, Maintenance, Tuning / Maintenance and Repair / June 2005Burning Lean = more heat?
I understand why burning rich reduces combustion temperatures. I don't understand why burning lean increases combustion temps. Web and usenet google searches haven't turned up much, and there is a lot of contradictory theories. What gives? thanks, Andy Probably too simple, but a lean mixture has a higher ratio of o2 and subseqently the resulting combustion generates more heat up to the point of a misfire or incomplete combustion. My old old race car would turn in it's best times on a given day, at the leanest I could run the car and still maintain acceleration at the 1/4 mile speed traps. Next step lean it would fall down right at the finish line. > I understand why burning rich reduces combustion temperatures. I don't > understand why burning lean increases combustion temps. Web and usenet [quoted text clipped - 5 lines] > thanks, > Andy Andrew Crabtree <andrewcrabtree@earthlink.net> wrote in article <l7one.6324$%u2.5773@news.cpqcorp.net>... > I understand why burning rich reduces combustion temperatures. I don't > understand why burning lean increases combustion temps. Web and usenet > google searches haven't turned up much, and there is a lot of contradictory > theories. > > What gives? There are only so many BTUs in gasoline. Leaner mixtures actually burn cooler since there are fewer BTUs available in a given fuel/air charge, but..... .....a richer mixture cools the cylinder more as it is introduced, so, even though you are burning more fuel - thus more BTUs - the cylinder temperatures run cooler due to the cooling effect of the rich fuel charge. An engine will produce the most power (and the most heat) running at stoichiometry, when the amount of air is exactly the amount needed to burn all of the fuel. This ratio of air to fuel is about 14.7 by weight for gasoline. Before the days of emission controls most engines ran slightly lean at part throttle to ensure complete fuel burning for good economy, and slightly rich at wide open throttle. Since the introduction of three way catalytic converters the engine must run at stoichiometry for the NOx reduction to work and is so critical that an oxygen sensor must be used to control the mixture. Since relatively high compression ratios are used with low octane gas to get good numbers on the government mileage tests the only way for an engine to run without detonation at wide open throttle is to greatly retard the spark and run very rich, as low as 10 to 1 air to fuel ratio. Under this condition only about 2/3 to the fuel is burned, the other third is used to keep combustion temperature down and is not burned. Since all gasoline engines now use knock sensors the easiest way to increase power and efficiency is to use higher octane gas, allowing for wide open throttle with more ignition advance and a mixture that is still rich, but not nearly as extreme as with lower octane gas. > Andrew Crabtree <andrewcrabtree@earthlink.net> wrote in article > <l7one.6324$%u2.5773@news.cpqcorp.net>... [quoted text clipped - 14 lines] > though you are burning more fuel - thus more BTUs - the cylinder > temperatures run cooler due to the cooling effect of the rich fuel charge.  SignatureMike Walsh West Palm Beach, Florida, U.S.A. > An engine will produce the most power (and the most heat) running at > stoichiometry False. An engine will produce emissions most compatible with catalytic cleanup when running at stoichiometry. Greatest power is produced at a mixture richer than stoich (between 12 and 13 to 1) and greatest heat at a mixture leaner than stoich. > Since relatively high compression ratios are used with low octane gas to > get good numbers on the government mileage tests the only way for an > engine to run without detonation at wide open throttle is to greatly > retard the spark and run very rich, as low as 10 to 1 air to fuel ratio. Er...no. No 10:1 AFR on any modern passenger car gasoline-powered engine. And you're forgetting three very important letters (E, G and R). > Since all gasoline engines now use knock sensors the easiest way to > increase power and efficiency is to use higher octane gas, Nope. There is no benefit to the use of fuel of a higher octane than that for which any particular engine management system is calibrated, and most systems are calibrated for regular-grade gasoline (87 AKI at sea level). > An engine will produce the most power (and the most heat) running at stoichiometry, Absolutely not. A gasoline engine will produce the most power at an A/F ratio of about 12.6 (1.17 x stoich). By definition this is also the point at which the most heat, i.e. number of BTU, is produced. However this is not the point at which the maximum flame temperature occurs. Flame temperature peaks at an A/F ratio very slightly richer than stoichiometric - about 1.05 stoich or an A/F ratio of 14:1 when the amount of air is exactly the amount needed to burn all of the fuel. This ratio of air to fuel is about 14.7 by weight for gasoline. Before the days of emission controls most engines ran slightly lean at part throttle to ensure complete fuel burning for good economy, and slightly rich at wide open throttle. Since the introduction of three way catalytic converters the engine must run at stoichiometry for the NOx reduction to work and is so critical that an oxygen sensor must be used to control the mixture. Since relatively high compression ratios are used with low octane gas to get good numbers on the government mileage tests the only way for an engine to run without detonation at wide open throttle is to greatly retard the spark and run very rich, as low as 10 to 1 air to fuel ratio. Nonsense. No engine, modern or otherwise runs at anything like that rich at WOT, nor is ignition advance greatly retarded. For a good compromise between power and economy most production engines are set up to run about 13 to 13.5 A/F ratio at WOT. Under this condition only about 2/3 to the fuel is burned, the other third is used to keep combustion temperature down and is not burned. Since all gasoline engines now use knock sensors the easiest way to increase power and efficiency is to use higher octane gas, allowing for wide open throttle with more ignition advance and a mixture that is still rich, but not nearly as extreme as with lower octane gas. Higher octane fuel is only of benefit if detonation is occuring at the optimum power settings for ignition and fueling on a lower octane fuel. This is not the case for the vast majority of standard production engines. -- Dave Baker - Puma Race Engines (www.pumaracing.co.uk) > > An engine will produce the most power (and the most heat) running at > stoichiometry, [quoted text clipped - 35 lines] > -- > Dave Baker - Puma Race Engines (www.pumaracing.co.uk) Good post Dave. > I understand why burning rich reduces combustion temperatures. I don't > understand why burning lean increases combustion temps. Web and usenet > google searches haven't turned up much, and there is a lot of contradictory > theories. It depends what you mean by lean. Leaner than what? Leaner than stoichiometric, leaner than the optimum A/F ratio for maximum power? Maximum flame temperature in a gasoline/air engine occurs at about 1.05 x stoich or 14:1 A/F ratio. Maximum power, and therefore total heat generated, occurs richer than this at 1.17 stoich (12.6 A/F) The point at which flame temperature peaks depends very much on the fuel and the oxidant. For gasoline burning in pure oxygen rather than air it is at about 1.15 x stoich. For hydrogen burning in oxygen it is very close to stoich and for acetylene in oxygen it is at 1.75 x stoich. There is no simple rule that says a fuel has to produce its peak flame temperature at stoichiometric just because that is theoretically where all the fuel molecules combine exactly with available oxygen ones. -- Dave Baker - Puma Race Engines (www.pumaracing.co.uk) > I understand why burning rich reduces combustion temperatures. I don't > understand why burning lean increases combustion temps. Web and usenet [quoted text clipped - 5 lines] > thanks, > Andy There is an optimum mixture ratio for max heat, close to stoich. Going further lean again reduces temp. Any good book on IC engine design should give this. Try a college library. Running rich reduces temps by vaporizing fuel. Any conversion of a liquid to a gas will absorb heat and excess fuel, in the form of atomized droplets, does this handily though it's an awful polluter. Aircraft engines regularly run full rich on takeoff for this reason, since they're aircooled and tend to be rather powerful engines for their weight. Rich can mean mixtures as rich as 8:1, and mixture in most piston engines is manually controlled. We find best power at around 12:1, highest exhaust gas and cylinder head temps at 15:1, and the thing won't run past 18:1. Dan Dan Well I have a rather unorthodox view of combustion developed as a smog tech. One of my smog instructors said that the difference between a rich mixture and a lean mixture is that the rich mixture burns faster because fuel molecules closer together and a lean mixture burns slower. So the way I look at it is a rich mixture will burn when the piston is near Top dead center producing alot of pressure and heat where you can do the best work for pushing down the piston. Now a lean mixture will not burn fast enough to produce maximum push and produces pressure when piston is further down and volume is too great for maximum power. So heat and pressure of rich mixture is converted more into kinetic energy of piston, which lowers the temperature of the burning mixture as the piston goes down. The heat of the lean mixture is not converted to kinetic energy of piston so it retains more heat as piston goes down and that is why exhaust is hotter and metal is hotter. So with a lean mixture you have more waste heat. Why does a diesel engine have more torque, because it pushes on piston longer as long as injector is open while with a gas engine it just pushes alittle while after top dead center. So a rich mixture produces more instant heat and pressure energy right where it is needed for maximum power while a lean mixture extends the burning time and therefore pushes weaker and at past best time. As you know from physics energy is converted from one form to another. So if heat and pressure of burning gasoline turns into more kinetic energy then it must lower its temperature or heat energy more than a lean mixture. And you get more power from a rich mixture. Of course the unburned gasoline that can't find any oxygen to combine with will absorb some heat energy and also the heat released upon conversion to carbon monoxide is not as much as released with conversion to carbon dioxide (adequate oxygen).. > Well I have a rather unorthodox view of combustion developed as a > smog tech. One of my smog instructors said that the difference between [quoted text clipped - 3 lines] > near Top dead center producing alot of pressure and heat where you can > do the best work for pushing down the piston. Any difference in distance (mfp) between molecules is insignificant. Further, the speed of the molecules is very high compared to their mean free path. The flame front speed doesn't vary that much with mixture ratio, at least over the ratios we are talking about. Also, the burning time is insignificant in SI engines in terms of degrees of crankcase revolution angle except at VERY high rpms. > > Well I have a rather unorthodox view of combustion developed as a > > smog tech. One of my smog instructors said that the difference between [quoted text clipped - 12 lines] > insignificant in SI engines in terms of degrees of crankcase revolution > angle except at VERY high rpms. Perhaps you mean the change in burn time (i.e with different mixture ratios) but if not then certainly the burn time occupies a significant portion of an engine revolution or we wouldn't need ignition advance in the first place. I'm also unaware of any engines in which the crankcase revolves :) -- Dave Baker - Puma Race Engines (www.pumaracing.co.uk) >I'm also unaware of any engines in which the >crankcase revolves :) The Gnome rotary engine, used on several WW1 fighter aircraft. A radial two-stroke, with the crank bolted to the firewall and the prop bolted to the case. Fuel/air mixture was fed through the hollow crank into the case. Dan >>I'm also unaware of any engines in which the >crankcase revolves :) > The Gnome rotary engine, used on several WW1 fighter aircraft. A >radial two-stroke, with the crank bolted to the firewall and the prop >bolted to the case. Fuel/air mixture was fed through the hollow crank >into the case. Dan To correct myself: The Gnome was a four-stroke. See http://www.keveney.com/gnome.html Dan >>>I'm also unaware of any engines in which the >crankcase revolves :) > [quoted text clipped - 10 lines] > > Dan The head of the museum that is running the Gnome today keeps referring to it as a 2-stroke induction, 4-stroke cycle. What he means is really crankcase induction, "like" many 2-stroke engines. However, since he is head of the museum, I didn't try to correct him in the interview I taped- I just wish he hadn't said it that way. BTW, the museum will be selling DVDs of the run, providing we don't get rained out this weekend. >>>I'm also unaware of any engines in which the >crankcase revolves :) > [quoted text clipped - 10 lines] > > Dan But oiled like a 2-stroke: http://www.biography.ms/Rotary_engine.html >> To correct myself: The Gnome was a four-stroke. See >> [quoted text clipped - 4 lines] > But oiled like a 2-stroke: > http://www.biography.ms/Rotary_engine.html And it used crankcase induction like many small 2-strokes. >>I'm also unaware of any engines in which the >crankcase revolves :) > [quoted text clipped - 4 lines] > > Dan And lubricated with Castor Oil mixed with the fuel. The exhaust sprayed out of the rotating cylinders and kept the plane (and pilot) immersed in a fog of castor oil vapor. Needless to say, most pilots of those things had no problems with "regularity." :-) I'm not making this stuff up, honest! :-) >Newsgroups: rec.autos.tech >Dan_Thomas_nospam@yahoo.com wrote: >>>I'm also unaware of any engines in which the >crankcase revolves >:) > >> The Gnome rotary engine, used on several WW1 fighter aircraft. >>A radial two-stroke, with the crank bolted to the firewall and >>the prop bolted to the case. Fuel/air mixture was fed through the >>hollow crank into the case. >> Dan >And lubricated with Castor Oil mixed with the fuel. The exhaust >sprayed out of the rotating cylinders and kept the plane (and >pilot) immersed in a fog of castor oil vapor. Needless to say, most >pilots of those things had no problems with "regularity." >:-) >I'm not making this stuff up, honest! :-) Keeping your goggles clean must have been a real bear! Widely believed among motorcycle racers in the late 60's that running castor oil in your 2-stroke would give riders behind you the squirts. Seemed a good tactic for a long race. I sure like the smell, for a while. <G> Tom Willmon near Mountainair, (mid) New Mexico, USA Hello...Incontinence Hotline...Can you hold? Net-Tamer V 1.12.0 - Registered > Perhaps you mean the change in burn time (i.e with different mixture > ratios) > but if not then certainly the burn time occupies a significant portion of > an > engine revolution or we wouldn't need ignition advance in the first place. It has always been my understanding that ignition advance is used to compensate for piston velocity as it approaches TDC. By increasing the advance as RPM's increase, the intent is to time the spark to occur at the optimum "instant", literally. The subsequent combustion and pressure occurs as the piston passes through TDC, creating the power stroke. doug >>Perhaps you mean the change in burn time (i.e with different mixture >>ratios) [quoted text clipped - 7 lines] > optimum "instant", literally. The subsequent combustion and pressure occurs > as the piston passes through TDC, creating the power stroke. That's the job of the centrifugal advance mechanism in a distributor- and is indeed based only on engine speed. The vacuum advance is there because at low power settings (throttle mostly closed) the absolute cylinder pressure is greatly reduced and the burn gets MUCH slower. To get maximum efficiency at low throttle opening/high manifold vacuum, it takes a lot more ignition advance to get the charge completely burned before the exhaust valve opens. But that's not a function of mixture, just of absolute cylinder pressure. > Perhaps you mean the change in burn time (i.e with different mixture ratios) > but if not then certainly the burn time occupies a significant portion of an [quoted text clipped - 3 lines] > -- > Dave Baker - Puma Race Engines (www.pumaracing.co.uk) Actually, I am doing a video on an engine today in which the crankcase revolves and the shaft stays still :-) It is a pre-WW1 aircraft engine. Maybe the back of my mind was playing tricks- I've been obscessing on that run for weeks. But yeah, on this newsgroup I really meant the crankshaft. The engine runs are scheduled today and tomorrow- rain forcast both days. It is a 1909 Gnome engine, and it will be a heartbreak of it is rained out. Done lots of prefilming and interviews so far. > > Perhaps you mean the change in burn time (i.e with different mixture ratios) > > but if not then certainly the burn time occupies a significant portion of an [quoted text clipped - 13 lines] > days. It is a 1909 Gnome engine, and it will be a heartbreak of it is > rained out. Done lots of prefilming and interviews so far. A friend of mine is into aircraft and aircraft engines (he had flying lessons for a while but ran out of money) and from past chats I do actually know that some of the radial ones had the crank stationary and the rest of the engine rotating round it but adding all that to my line above would have rather devalued the intended humour so I left it out. I suppose I could have said I'm unaware of any car engines in which the crankcase revolves. -- Dave Baker - Puma Race Engines (www.pumaracing.co.uk) > A friend of mine is into aircraft and aircraft engines (he had flying > lessons for a while but ran out of money) and from past chats I do actually [quoted text clipped - 4 lines] > -- > Dave Baker - Puma Race Engines (www.pumaracing.co.uk) Actually, the rotary engine WAS first designed for automobiles. The first that I am aware of was designed and built by DeDion. I don't believe it was ever used in a production car. Another was the Balzer engine in US. This engine was modified by Manly for Langley's aerodrome, for which he made it stationary. However, Balzer designed it originally for cars, though he never could perfect the engine. For cars it had two advantages. First, since it was air cooled, the spinning of the cylinders provided cooling air motion when the vehicle was stopped or at low speeds. Secondly, no flywheel was needed, since the rotating crankcase and cylinders had enough mass and moment of inertia to act as a good flywheel. I mention this only for information- Dave is right, no production automobile used such an engine, as far as I know. > Well I have a rather unorthodox view of combustion Yes you do and not a correct one I'm afraid to say. developed as a > smog tech. One of my smog instructors said that the difference between > a rich mixture and a lean mixture is that the rich mixture burns faster [quoted text clipped - 11 lines] > goes down and that is why exhaust is hotter and metal is hotter. > So with a lean mixture you have more waste heat. The change in burn time over the range of mixtures used in a modern SI engine (14.7:1 at cruise to about 13:1 at full power) is not significant. In any case the ignition advance can be mapped to take account of this if needed. The mixture that gives maximum power does so because the most fuel molecules are burned in a given time. It's that simple. What does alter burn time very considerably is the amount of cylinder filling which is why engines need more ignition advance at part throttle. > Why does a diesel engine have more torque, because it pushes on > piston longer as long as injector is open while with a gas engine it > just pushes alittle while after top dead center. Diesel engines don't have 'more torque' as the most cursory look at the torque per litre figures from normally aspirated diesel and petrol engines will tell you. Even if they did it would have nothing to do with how long something pushed on the piston. > So a rich mixture produces more instant heat and pressure energy > right where it is needed for maximum power while a lean mixture extends > the burning time and therefore pushes weaker and at past best time. Poppycock. -- Dave Baker - Puma Race Engines (www.pumaracing.co.uk) >> So a rich mixture produces more instant heat and pressure energy >> right where it is needed for maximum power while a lean mixture extends >> the burning time and therefore pushes weaker and at past best time. >Poppycock. A lean mixture does indeed burn more slowly. It's one of the reasons why detonation occurs more easily in lean burn, since there is more time for the complex aromatic molecules to break down into simpler autognitable compounds, and why the exhaust gas temperature rises as mixture is leaned from best power through stoichiometric. Leaning past that point results in a falling EGT. But running really rich doesn't speed up burn or give a lot more power; it just drowns everything as the excess fuel absorbs heat. And fouls the plugs. Dan |
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