Why do lean cars run hotter?

[BK911]

If a rich car produces more power, and power is essentially heat, why does a lean car tend to run hot?

BK

[pwd72s]

The unburned gas acts as a coolant in rich running cars. Ever watched a Nascar race? Overheating car, the driver lifts off a bit sooner, letting excess gas help cool things...

[TimT]

OUCH


dont make me try to remember my thermodynamics classes



Ideally combution if a gasoline internal combustion engine will exits at the stiochiometric ratioi i think it is 14.7:1

Lean makes the temps to hot

rich cools the combustion temps

[DonNewton]

Has anyone equated running rich with "quenching" the heads? 14.7:1 is correct. . .Lean, you're seeing "theoretical" total combustion (even we all know there's no such thing), while as has been said before, running rich allows the excess fuel/air mixture to "quench" the heads.

[island911]

Originally posted by BK911
If a rich car produces more power, and power is essentially heat, why does a lean car tend to run hot?
BK

I think you get localized "hot". With a lean mix, it burns fast, and hot in the head vicinity. Richening it up will allow the burn to be slower and more in time with the piston speed.
. . .or something like that. It's Friday night and a few later. . . .maybe if I have a few more I'll *see* how the fuel wets down the head to cool it

[stlrj]

Lots of opinions...but is there any hard data to back it up?

Joe

[island911]

hard data !? You want hard data . . .I still havent got my 2¢ from you for the last post. And now you want hard data. Sheesh!
Consider what happens with "pinging". The flame front goes supersonic, all of the energy is let loose early in the piston stroke. This puts all of the heat energy "right there" to melt pistons and hammer at components.
So I'm just saying, there is more to this question than "conservation of energy" and evaporation. Something about why there are fins down the cylinders.
Of course this means you owe me 4¢ now, for all this fine conjecture.

[Sean Hamilton]

Originally posted by island911
Consider what happens with "pinging". The flame front goes supersonic, all of the energy is let loose early in the piston stroke. This puts all of the heat energy "right there" to melt pistons and hammer at components.

Interesting. So, if an engine is tuned with too much ignition - advanced, it will run hotter? Or retarding the timing = cooler?

[island911]

Air+fuel+spark=BANG
gunpowder+spark=BANG . . .that all seems simple enough.
Or simplistic?
Pressure is the missing ingredient in the above equations.
For example; If you make a little trail of gunpowder (or pour a trail of gasoline) on the ground and light it, you get no "BANG" ; just a fizzle or a flame.
If you take a bit of the powder and wrap it in paper (firecracker) that little containment will cause the pressure to rise fast; which in turn causes the chemical reation to go even faster =BANG.

Pressure management:
The piston engine wants to have a fairly fast burn, but not to the point of "bang" (detonation)
Think about the forces you put on a bicycle pedal. You wouldn't want to jump on the pedal only at TDC, and sit back on the seat after 15° of rotation.
So too, the engine wants to capture the burn pressure to extract a nice even push (work, hp).
The engine geometry is infuencing the burn rate by the changing volume (effecting pressure by giving more space).
The advance is infuencing the burn rate by where in the geometry (bicycle pedal) the burn starts building pressure. Too early and the pressure rises fast, puts some of its work into pushing the piston, and a lot of work in to make heat for the head.
So there *is* a lot of balancing going on there beyond just lighting it off.
We're over a nickle on this one.
Buyer beware. .conjecture based on a 10y/o recollection of an IC engine class.

[BlueSkyJaunte]

"caveat lector"?

R&T ran an article recently about "12 super-duper cars". All kinds of tests like 0-150-0, 1/4 mile, etc.

The 600 HP Lexus IS300 entered into the shootout was a DNF because one of the injectors failed and leaned out the cylinder, which resulted in a hole burned through the accompanying piston.

That's evidence/data enough for me, even if it's second-hand!

I always try to remind myself that our engines are air-, oil-, AND fuel-cooled.


PS: Whoop! My bad, it was C&D, not R&T.

[stlrj]

What I would like to see, besides a lot of heresay and conjecture, are how rich vs lean affect CHT and EGT.

What is considered lean, stochiometric? If that's the case then all our cars from '80 on are running too lean.

Joe

[BlueSkyJaunte]

Joe,

You should talk to pbanders on the teener forum. His knowledge of Bosch D-jet is horrifying.

http://members.rennlist.com/pbanders/

[Early_S_Man]

Island ... A few more subtle 'variations' ...

Black powder + special assembly procedures = rocket fuel + spark = controlled burn for a specified time and thrust profile ...

Black powder + incorrect assembly method = 'presumed rocket fuel' + spark = BIG BOOM !!!

As a teenager, Jim Lovell encountered the second method of building model rockets, and fortunately managed to survive to fly around the moon, twice! Hundreds of amateur rocketeers weren't so lucky over the years!

Fortunately, Verne Estes discovered a safe method of assembly 40+ years ago with an unmanned hydraulic press and many millions of safe Black Powder model rocket engines have been built since then!

Kart racers like to run very lean mixtures that produce maximum power ... but occasionally, they 'STICK' pistons, which is an informal way of saying 'seized' piston and cylinder! Obviously, a rather fine line exists between a race-winning engine, and a 'stuck' engine waiting for a rebuild .. along with a DNF!

[RoninLB]

Originally posted by stlrj
What I would like to see, besides a lot of heresay and conjecture, are how rich vs lean affect CHT and EGT.

What is considered lean, stochiometric? If that's the case then all our cars from '80 on are running too lean.

Joe

Well Joe, how about some trivia from here..I have a stock'77..I speak to PMO..I will have E-cams, SSI, etc..so more HP..which should equal more heat.. then he says my PMO, E-cams, SSI engine will run cooler...IMO, a nice running engine and smog requirements in my '77 are at odds...............Ron

[island911]

Originally posted by Early_S_Man
. .Black powder + incorrect assembly method = 'presumed rocket fuel' + spark = BIG BOOM !!!

Heh-heh. Yeah, there is no "off-switch" with solid fuels.

On those Kart racers, running very lean mixtures; That makes sense (even if it's heresay and conjecture )
They have some short-stroke, high reving engines, I presume.
This would put them wanting to have a very fast burn, and thus the leaner mix.
I hadn't really thought about that before, but indeed they are running "a rather fine line." Thanks.

[TimT]

What I would like to see, besides a lot of heresay and conjecture, are how rich vs lean affect CHT and EGT.

I have dyno tuned some turbo and NA engines, when we do this we have a wide band O2 sensor, ( the motec a/f tool) and always have an accurate EGT gauge.

There is a direct correlation between mixture and CHT and EGT

Your engine makes the most power on the lean side of stoi, so when tuning the engine you observe the A/f meter and EGT and try to strike a balance of leaning out the engine without increasing EGT (and hence CHT as well). you run the engine through all sorts of loads and RPM ranges and keep fine tuning the mixture to balance power and minimize harmful EGT's

Oddlly Turbo motors like higher EGTs in the 1650 area
NA engines like 1375 or so

[stlrj]

Tim,

I read that peak EGT occurs at stoi., so running slightly lean of stoi. would be the same as running lean of peak EGT or slightly cooler. At least that corresponds to the findings in general aviation.

According to the charts I have seen, peak CHT seems to occur at around 50* to 100* rich of peak EGT which would also correspond to higher oil temps. that should be noticeable in our engines.

Interestingly, the greatest tendency to knock is at a rich mixture, near 13.5:1 air-fuel ratio according to a study done by Texaco.

http://www.repairfaq.org/filipg/AUTO/F_Gasoline7.html#GASOLINE_009

Joe

[TimT]

I can only tell you what I have seen sitting in a car at a dyno.. spending hours trying to duplicate what the engineering forces at GM and Porsche have already done.

Besides the wide band O2 sensor and EGT gauge, when we tune Haltechs we have a trim control that we can roll to rich or lean

if your EGT goes down you are to rich

Following that you would think " I want higher EGT's" yes till a point

In a NA engine beyond 1400F things happen that reduce power and generallly make an engine unhappy

If I turn the trim to lean power goes up and simultaneously so to EGT temps

and CHT temps

but there is apoint of diminishing returns

Oil temps lag so far behind that they are not even noted

knock is uncontrolled combustion.. something no one wants

[Zeke]

Originally posted by stlrj
Lots of opinions...but is there any hard data to back it up?

Joe

Kart racers like to run very lean mixtures that produce maximum power ... but occasionally, they 'STICK' pistons, which is an informal way of saying 'seized' piston and cylinder! Obviously, a rather fine line exists between a race-winning engine, and a 'stuck' engine waiting for a rebuild .. along with a DNF!

I raced karts for over thirty years and this is absolutely correct. Stuck pistons is about as hard data as you can get. Lean it down and watch the digital guages on the steering wheel climb (EGT and head temp). This may not be a laboratory, but the real world is good enough for me. Oh, and kart racers like to run less oil in their fuel, too. Which doesn't help.

[stlrj]

As far as I know, karts do not run stoichiometric mixtures like modern cars do and are two stroke not four and becuase of this are not designed to run anywhere near stiochiometric mixtures.

Joe