First, there's the "flash point of diesel fuel which is the lowest temperature at which a fuel will ignite. For diesel that lies between 110°F, and 160°F.

Other wise, here's a quote from Britannica.com:

"The diesel engine gains its energy by burning fuel injected or sprayed into the compressed, hot air charge within the cylinder The air must be heated to a temperature greater than the temperature at which the injected fuel can ignite. Fuel sprayed into air that has a temperature higher than the "auto-ignition" temperature of the fuel spontaneously reacts with the oxygen in the air and burns. Air temperatures are typically in excess of 526°C (978.8°F); however, at engine start-up, supplemental heating of the cylinders is usually required, since the temperature of the air within the cylinders is determined by both the engine's compression ratio and its current operating temperature."

http://208.154.71.60/bcom/eb/article/6/0,5716,108536+1,00.html

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Scott Longston
Northern California Wine Country...
"Turbos whistle, grapes wine..."