15 replies to this topic

[GSX-R]

Why lean mixture provide a better thermal efficiency ?

Any reference to serious web documents are welcome.

Regards

GS

[DOHCPower]

I dont have any "serious" web documents to quote from, but my educated guess is that the higher combustion chamber temperatures of the lean mixture would burn all of the fuel more efficiently therefore losing less power due to undburnt fuel in a typical rich mixture.

[GSX-R]

That is the natural thinking to imagine not all the fuel burn.

But actually stochiometric commustion don't seem to exhaust too much HC, so is it only this reason ?

[ray b]

yes a very lean burn is more efficent
on a physic board a guy posted about big 60L stationary motors
runing slow at 60 to 1 ratios to rival diesel motors

but higher RPM and cooling make for melting bits in cars

[J. Edlund]

When going to fuel mixtures richer than lambda 1 there will always be some fuel that doesn't burn. With lambda 1 all fuel should be burned in theory, in reality there are richer and leaner areas in the combustion chamber so some fuel will not be burned. With some air excess, slightly on the lean side, most of the fuel should be burned.

Lean mixtures results in decreased combustion temperatures, it's only when you go from rich to slightly lean you will see temperature increases.

[GSX-R]

Imagine we apply a richness of 1 for 17 (lean), does the combustion burning time is really longer ?

I mean if we don't consider exhausts problems, thermal efficiency like burning rate and lean mixture and ther're antagonists. Am i right ?

I'm talking for gasoline S.I.

[hydra]

The reason lean mixtures increase thermal efficiency (above the stoichiometric point) is because gamma approaches 1.4 (from 1.3) the leaner you go, and gamma features prominently in the air-cycle efficiency equation, which is proportional to r^(gamma-1/gamma) or something like that
Sorry, too lazy to go fetch my Heywood

[GSX-R]

Quote

Originally posted by hydra
The reason lean mixtures increase thermal efficiency (above the stoichiometric point) is because gamma approaches 1.4 (from 1.3) the leaner you go, and gamma features prominently in the air-cycle efficiency equation, which is proportional to r^(gamma-1/gamma) or something like that
Sorry, too lazy to go fetch my Heywood

Do we have some charts indicating gamma according mixture ratio and the kinf of guel (gasoline vs gasoil) ?



What is the part of HC on modern S.I engine running at richness 1 ?

regards

[Stian1979]

Combustion proceses are advanced stuff.
To have a perfect air/fuel mix is imposible.
If you have a theoretical corect mix it's big chance that there will be local lack of air some places in the cylinder and to fat mix other locations.

If you runn a fat mix it will hawe a higher hp potential, but also alot more energy will leave uncombusted.
A fat mix will also giwe high preshure during the stroke, but alot of preshure will be present when the exhaust valve open.
A lean mix will reduce the preshure, but moust of it will be used during the stroke so not that mutch energy will leave the cylinder as preshure.

[hydra]

Quote

Originally posted by GSX-R


Do we have some charts indicating gamma according mixture ratio and the kinf of guel (gasoline vs gasoil) ?



What is the part of HC on modern S.I engine running at richness 1 ?

regards

I can't say that I've ever seen a chart detailing the relationship between gamma and mixture ratio. And to tell you the truth I don't think the differences would be that profound between a 13:1 ratio and a 17:1 ratio, but lean that out to say >25:1 and you'd start seeing a bit of difference... So the main reason BSFCs get better when you lean the mixture (in the operating window seen in a typical homogeneous combustion SI engine) is simply because more of the existing fuel gets burned and thus converted to useful work

[GSX-R]

I tend also to say that this should be marginal for efficiency.

So a lean mixture, within certains limits, could slow the flame and could not fit in fast RPM engines ?

If we except the NOx problem, a compromise should be found beetween the ideal fuel/air ratio and the RPM of the engine ?

I'm goind to try on the only model i've got : lotus engine simulator. For the moment i've found strange if even with combustion efficiency set to 1 (all the fuel burn) and equivalence ratio set to 1 and then to 0.7, the thermal effiency is slightly better for lean..
Somewhat like a couple of %. Didn't take the time to check into the tool why. Will get a more accurate look soon.

Regards

GS

[GSX-R]

Last minute :

i've discovered an interested track... beetween 0.1 and 1 the trapped air temp depends of the fueling equivalence ratio... but not in the sense of cooling when more fuel, in the other sense.

to be continued. exiting.

[Greg Locock]

Yes the flame front speed is slower in lean mixtures (over the normal range of mixtures). That's why a lean mixture needs earlier spark.

[GSX-R]

some results of the simulation experiment :



F/A ratio : 1 -> Trapped Temperature 100.0 C

0.7 -> Trapped Temperature 88.0 C

The "secret" is when the mixture is at 1, the admited air is reheat by residual, when the mixture is leaner, the exhaust are cooler and the mass trapped is higher and then the thermal efficiency regarding mass/surface ratio.. I'm going to investigate to see how many % of efficiency we could imagine to get using a lean mixture and a not too fast RPM. Problem is at too low RPM, wall heat losses will increase.

Regards.

AF ratio has an effect on charging.. great.

[J. Edlund]

That gamma approaches 1.4 with lean mixtures is probably something that mostly will affect diesels since they can operate with very lean mixtures, especially during part load. For SI engines I would expect that the amount of fuel burned/not burned will give the largest difference in efficiency.

Laminar flame speeds usually peaks around lambda 1, perhaps a bit on the rich side. Going richer or leaner will decrease laminar flame velocity. For example toluene peaks at about lambda .86. In a SI engine a slower combustion will result in a decreased efficiency, but this have most likely a very small effect.

[GSX-R]

Quote

Originally posted by J. Edlund
.. I would expect that the amount of fuel burned/not burned will give the largest difference in efficiency.
...

Now, after the simulation and integration of the residual element (especially its temp) into the calculation, i think that on S.I engines, one notable advantage of a slight lean mixture is the reduced temp of the residual and then the admitted mixture than on a 1. Surely, the flame and then the thermodynamic efficiency should be slighty decreased. Possibly the assessment is null or near that value.

It could be interesting to get exprimental values of unburned HC for S.I at 1 to check the first assumption beetween 1 and 0.7 for example.