Return to Answer

For those interested, based on the discussion we had in the various comment threads, I went ahead and gave a concrete example of how/why temperature can increase when the flame is fuel-lean. The conversation in chat is bookmarked here.

In either engine, once they have been running for awhile, the cylinder walls heat up and it requires less input (from sparks or from compression) to make the reaction occur. But for cold engines, it needs that initial energy deposition to get the reactions moving along. Many ECU's are set to burn fuel-rich when the engine is just starting because it is easier to ignite; as they heat up, the mixture becomes more lean and reduces emissions and fuel consumption. You may be familiar with manual chokes on things like lawn mowers -- the choke is what changes the fuel-air mixture and to get the motor started, you have to set the choke to be fuel-rich.

Based on some extended conversation on the question, let's put this all back into the context of an engine. For a direct-injection gas engine, the air is sucked into the cylinder, the piston compresses it, and then fuel is sprayed into the cylinder. A spark plug then triggers a spark in the chamber. This deposition of electrons gets the fuel-air mixture molecules all excited -- it actually ionizes the air (strips off electrons from the molecules) and this all adds a bunch of energy to the molecules. This energy is the initial energy required to start the combustion.

For a fuel-lean condition, I said it takes more energy to start the reaction and I phrased it in terms of a higher ignition temperature. The ignition temperature comes from that spark plug (for a cold engine -- hot engines will also contribute heat from the cylinders themselves). For normal operating conditions, spark plugs provide more than enough energy to ignite. As the operating condition gets leaner, the spark plug provides the same amount of energy -- but it is still enough energy to ignite. Eventually, for lean enough conditions, it won't be enough energy. This is a lean misfire.

Diesel engines work differently. For sake of argument, let's stick with a direct injection again. The cylinder fills with air, the piston compresses it, and the fuel is injected. There is no spark to initiate the reaction though. Diesel engines rely solely on creating high enough pressures to ignite the mixture. High pressure means high density and that means more collisions to spread the energy around (molecules don't need to go as far to hit one another). At any rate, the same ideas apply. In lean conditions, it would require a higher pressure to ignite. At ideal conditions, the engine compresses more than is exactly required, so when it runs fuel-lean, it still has enough compression to ignite. If you go so lean that the compression isn't high enough anymore, you will again get a lean misfire. Glow plugs can help all this by heating the cylinders and helping to add heat to the mixture and get the reactions going.