Lean Burn in Cars: What Happens & Why Hotter?

[Curl]

What happens when there is lean burn in a combustion engine? Does the reaction complete quicker? Is that why it is hotter?

 

[Pkruse]

I have pondered that at length over the years. The only hypothesis that has survived is that the engine is working less efficiently and therefore has to work harder.

 

[joelwatts]

A lean burn is bad for engines. Fuel acts as a coolant inside the combustion chamber. Too little fuel means temperatures are much higher. Higher temps means pre-detonation aka knock. Knock is bad because it ignites the fuel/air mixture by high cylinder temps. It starts burning the fuel way to soon. Then the spark plug fires and now you have multiple flame fronts in the cylinder. When the flame fronts meet, you get knock. More flame fronts equal more heat coupled with an already hot cylinder from the lack of cooling fuel, you will quickly reach the melting point of most piston materials. Not to mention that when the intake valve opens, you start allowing those hot temps into the intake tract. That usually doesn't help matters. Lean mixtures make more power to a point an help fuel economy but just like everything else, moderation is the key. Hope that helps.

 

[Curl]

I'm not convinced that the extra fuel really has any significant effect in cooing the combustion chamber. An extra 0.0002 grams of fuel is nothing, keeping in mind that the latent heat of vaporization of gasoline is even lower than water, and the heat capacity of the chamber is significantly higher.

 

[AlephZero]

I'm not convinced that the extra fuel really has any significant effect in cooing the combustion chamber. An extra 0.0002 grams of fuel is nothing, keeping in mind that the latent heat of vaporization of gasoline is even lower than water, and the heat capacity of the chamber is significantly higher.

The latent heat effects are probably negligible compared with the change in the conbustion chemistry.

This was very easy to see back in the days before electronic engine management, when you tuned mechanical carburettors using "color-tune" spark plugs which had a glass insert around the electrode, so you could see the flame color inside the cylinder. There was a very obvious change from yellow (too rich) to dark blue (correct) to light blue and possibly a pink tinge (too weak).

Apparently they still sell them for bikes ... http://www.carbtune.co.uk/colortune.html

 

[256bits]

I'm not convinced that the extra fuel really has any significant effect in cooing the combustion chamber. An extra 0.0002 grams of fuel is nothing, keeping in mind that the latent heat of vaporization of gasoline is even lower than water, and the heat capacity of the chamber is significantly higher.

You would be referring to complete engine cooling, and in tha case it is negligable. What joelwatts is referring to is the cooling of the air-fuel charge that goes into the engine. A lean is a certain amount of degrees warmer than a rich mixture when entering the chamber.

 

[256bits]

What happens when there is lean burn in a combustion engine? Does the reaction complete quicker? Is that why it is hotter?

This is a curious subject and not well understood.
The hottest flame temperature would be at a stoichiometric mixture of fuel and oxygen. Being lean or rich means that excess fuel or excess air has to be heated and this will lower the flame temperature on both sides of stoichiometric.
So one would naturally conclude that with a lean or rich mixture an engine should run cooler.
But other things are going on. One is the advancement of the flame front. A lean mixture will take a longer time to burn so during the cycle of piston movement the mean temperatutre has increased. The exhaust gas temperature has also increased as the mixture has been burning farther down in the piston stroke.
Since the flame front advances more slowly, the mixture has more time to increase in temperature to the point where hot spots within the cylinder can cause pre-ignition.
Joelwatts pretty much hit the answer completely, even with the heat entry into the intake ports whein the fuel-air charge will now be elevated to a higher temperature.

 

[joelwatts]

Thanks for the 2nd opinion. I don't know much but this is one of the first things you learn about an internal combustion engine. You can't fix something unless you know how it broke. Lean burns are a common failure especially in the aftermarket world of modifying when people start tossing forced induction and nitrous on. Most people don't know that an engine needs to be properly tuned for these things and they end up grenading it.