Why does running lean burn pistons? (Warning: Chemistry involved)
 

It's a fact that running lean can lead to burnt pistons while running rich is generally considered "safer." Now what is mystifying me is, whether you are running rich or lean you have less combustion (than the perfect mixture) and so less heat released by the reactants. What is it about having more air that leads to higher temperatures?

Ponder that :willy: :banghead: :skep:

-Chris

I see you have way too much time on your hands.

My shadetree mechanic understanding is that you are really burning air and gasoline; gas, the fuel is there to 'fuel' the burning process. Intro of the fuel also helps to 'cool' things down in the combustion chamber; air temperatures are known to drop considerably from the air box to the intake runners with intro of fuel; NOx cools things off even more. So, one could say it gets too hot in the combustion chamber with insufficient fuel.

I'm actually in an Automotive Powerplants class in college right now. (No this isn't a homework question, just something that came up in class that the instructor told us to seek out an answer for, for our own knowledge.) While he was talking about this conundrum another student suggested fuel cooling things... as I remember he didn't say "no" exactly... I think that guy might have been saying the fuel itself splashes the piston and cools it. Maybe it is the expansion of the fuel from liquid to vapor that cools the charge? (Just like NO2 does)

?

-Chris

Cool link on the internal combustion engine and history:

http://en.wikipedia.org/wiki/Internal_combustion_engine

Chris,
Fuel being vapourised in the chamber sucks up a lot of heat (latent heat of vapourisation), so maybe that's what he was talking about? Less fuel = less heat removed. I was under the impression that lean mixtures promote detonation which is a big killer of pistons (had to fix a lot of engines with piston crowns blown out due to running lean). Detonation, apparently, blasts small particles of metal from the crown until it's so weakened that a hole opens up. Most of the particles (which are very small) are lost out of the exhaust, but some get by the rings & several engines I've worked on have shown bad scoring of piston skirts/bores even though the oil was changed regularly & they were running decent filters. Investigation (usually asking the owner about it down the pub over a beer :) ) invariably came up with the fact that the engine had been running lean for a while & had been pinging (it was a major problem here with certain engines when leaded fuel was withdrawn). Maybe it's the detonation that's the major cause?
I'm not sure of the details, I just know that uncontrolled lean mixtures are dangerous. Controlled ones are good as they produce more power (hence the performance of computer controlled lean burn engines being so much better than traditional ones that were always set up slightly rich for "safety").
If you do find the reason then can you post it? I've spent so much time fixing blown pistons it'd make sense to understand exactly what caused it :D
:cheers:
edit:
Shot in the dark here, but I remember that lean mixtures burn slower. Could it be that combustion is still occuring during the valve overlap period, so some incoming fuel is immediately burnt & pushed out of the exhaust as it burns? This may sound a bit dumb at first, but it's a common occurance on old bikes where the mixture is set extremely lean (or it's running out of gas) - more than one bike has toasted it's riders nether regions after a backfire (spit-back) through the carb has set the carb on fire :(

I would like to know why combustion temperatures rise with a leaner A/F mixture... anyways, with higher combustion chamber temps you are more likely to get premature ignition (engine knocking) which can damage your pistons.

Actually, the temperature curve is a bell curve: Combustion temperature rises as you lean out towards 14.7. Combustion temp drops on both sides of this ratio, so you can actually run extreme lean and have the excess air run you cooler. However, power output drops dramatically when using this technique, whereas power will actually increase as you richen up below 14.7 down to about 12.5, when power, again, starts dropping. The over-lean air-cool technique is actually used in light carbureted aircraft, where you can pull the mixture control knob during cruise and lean the mixture up to max EGT (exhaust gas temp) and then keep leaning just a tad until EGT drops. You can get really good fuel economy that way without burning pistons. But it does not work well for good power or throttle response, so the over-lean air cool method does not work well in an automitive application where you are constantly accelerating and decelerating.

A lean A/F mixture ignites much more readily, thus any residual heat or 'hot spots' in a cylinder under load will pre-ignite ('knock'/detonation) before the timed spark. Cylinder pressure and temp both rise dramatically when this happens as the piston continues to rise in the cylinder. Can also smoke an exhaust valve with ease.

This is why I run a slightly 'wet' engine on the street, even with modern cams that bleed off cylinder pressure on the compression stroke. I can run stock static compressions and still have a minimum of pre-ignition that is easily managed.

The down side is I have to change oil more frequently due to increased combustion residues, but thats fine with me.

Hans

Hi Lars,
Yes, I've heard about that method on aircraft, but some of us nutters are running old (ancient) motorcycles using the same method :D You lean it out until it starts running rough (spluttering) & then richen it up. Quite often, just before it starts running really badly there's a point at which it smooths out. Get it in this area and it seems to go for ever on a thimble of fuel, but there's very little power for acceleration & not enough for higher cruising speeds ("higher" being a relative term for something made while steam was still in vogue!!! :D ). To accelerate, or cruise at a higher speeds, it needs to be richened up. In fact, when it's richened it's probably running at about the same ratio as a normal carb would give, it's just that being able to lean it off seems to make it run nicer at low speed.
So, are you saying that the combustion temp bell curve peaks at the ideal stoic ratio? My experience of engines set up lean is that they actually do get hotter (air cooled engines - maybe water cooled would mask the condition?). Why is this? Detonation? Combustion during the valve overlap period?
:cheers:

Yes and no.
First, lean mixtures are generally harder to ignite, as there are fewer fuel molecules residing in/near the spark plug gap. This is why wider plug gaps are generally called out for in lean burn emission controlled engines. The wider gap increases the chances of sufficient initial ignition occuring.
Second, preignition and detonation are two different things. Preignition is generally caused by combustion chamber hot spots, and are very dangerous and expensive as they generally show up prior to TDC, and the piston has to work against the building pressure. Detonation, on the other hand, is caused by high localized gas temperatures in the cylinder and occurs well past TDC.

:iagree:
I agree with 69 437 i would add in that the lean air/fuel mixtures at light throttle cruise mixtures in most cases will not burn a piston (because cylinder pressure is low), but at wide open throttle or high load driving conditions a lean air/fuel mixture can and often will burn a piston. a 14.7 to 1 a/f mixture is chemically ideal for light load/cruise mixture but a power mixture of 12.5 to 1 for mostengine designs to as lean as 13.5 to 1 for a race designed engine with modern combustion chamber design such as NHRA Pro-Stock or a NASCAR Nextel cup engine is as lean as should be run to avoid the damage a lean air/fuel mixture can cause.

henry @ oles carb

could it be that running rich burns less lb/hr of fuel (reaction limited by too little air), therefore less heat released, while running lean burns more total lb/hr of fuel due to more "complete" combustion with the excess air?

This is a picture of a lean burnt piston. It just flash burns right through like a cutting torch.

http://temp.corvetteforum.net/c3/gkull/542bm1.jpg
http://temp.corvetteforum.net/c3/gkull/bm20.jpg

Looks like the piston went plastic. Must have been making a lot of power when it went lean.

The computer monitering graph. showed a .008 second lean out spike where the injector might have hickuped. The cylinder temp also spiked over 1600 for an instant.

this is out of an alki motor. I have a collection of pistons and parts like this that I have personally made or worked on the engine that made them.

Isn't rich by definition more fuel than stoichometric... Ouch It's been a while and I dont think I won the spelling bee. :lol:

Anyway It is a bell curve and peak power usual occurs just rich of stoich. How you deliver the fuel is an application specific thing....

Anybody here ever tried to start a Twin Comanche when the engines were not warm or cold ?

With an airplane you learn to just keep crankin cause it sucks the FIRE and Fuel into the Engine... :cool:

And aluminum goes liquid at around 1220 degrees F. It isn't hard to imagine a hot spot developing well above what the piston can conduct away.

When you say "cylinder temp" do you mean "exhaust gas temp"? That makes more sense...if I remember right alcohol burns at about 1900. The exhaust would be much lower.

Geez, I hope I'm making sense here - I'm half asleep. :ack:

Easy comparison, A cutting torch. Air and fuel mixed to ideal then hit the extra oxygen and melt things instantly.:cheers:

Yes it was 1600 on the one primary tube. Normal runs never see over about 1380. All 8 tubes usually run within about 30 degrees. Unless you blow away a spark plug tip and it goes to less than 600