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Burning Lean = more heat?



 
 
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  #11  
Old June 3rd 05, 06:55 AM
edmechanic
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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)..

Ads
  #12  
Old June 3rd 05, 02:35 PM
Don Stauffer
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edmechanic wrote:
> 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.


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.
  #13  
Old June 3rd 05, 02:47 PM
Dave Baker
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Don Stauffer > wrote in message
...
> edmechanic wrote:
> > 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.

>
> 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.


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)


  #14  
Old June 3rd 05, 03:01 PM
Dave Baker
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edmechanic > wrote in message
oups.com...
> 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
> 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.


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)


  #15  
Old June 3rd 05, 10:12 PM
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>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

  #16  
Old June 3rd 05, 11:18 PM
doug
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> 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


  #17  
Old June 4th 05, 12:51 AM
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>>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

  #18  
Old June 4th 05, 01:53 PM
Don Stauffer
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Dave Baker wrote:
>
> 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)
>
>


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.
  #19  
Old June 4th 05, 01:57 PM
Don Stauffer
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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
>
>
> To correct myself: The Gnome was a four-stroke. See
>
>
http://www.keveney.com/gnome.html
>
> 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.

  #20  
Old June 4th 05, 02:56 PM
Dave Baker
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Don Stauffer > wrote in message
...
> Dave Baker wrote:
> >
> > 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)
> >
> >

>
> 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.


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)


 




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