AH64ID
TDR MEMBER
Cetane number is not a measure or indication of the heating value (BTU) of diesel fuel and has little if any affect on engine power output. The measure of diesel fuel heating value, or sometimes called energy content is API gravity. Typical #2 diesel fuels have about 35 API gravity. Lower than 35 API gravity the fuel would have higher heating value, higher than 35 API would mean lower heating value. Higher heating value fuels usually result in better fuel economy.
Cetane number as defined by ASTM is a measure of the auto-ignition characteristic of the fuel. It works something like this in a Diesel engine: when fuel is injected into the cylinder it must evaporate, mix with air, before burning and expansion of gases can occur. This process takes time (crank angle) and is produced by the heat of compression of air only. This is called heterogeneous combustion. Compare this to a spark ignition gasoline engine where air and fuel are introduced into the cylinder pre-mixed and are ignited by a spark plug. This is called homogeneous combustion. Therefore, a high cetane number would be a low octane number fuel. Cetane and octane are numerical ratings of the auto-ignition characterics of diesel fuel and gasoline respectively. They are basically opposite ends of a spectrum. Diesel engine manufacturers typically require 40-45 cetane fuel for their engines. ASTM D-975 is sometimes referenced. When diesel engine manufacturers discuss cetane number their concern is adequate cetane rating for satisfactory cold start ability.
Respectfully,
Old thread...
While Cetane itself is not a measurement of BTU's it is an indicator. The process in which fuel is refined to increase Cetane will decrease the BTU content. It's a direct correlation.
This is a chart I've seen in several papers and kicking around. As you can see as Cetane increases so does the API, which means a lower BTU content.
Increased Cetane is good in certain applications, but an increase in Cetane isn't always the best thing as a general rule. I like it in winter under certain low temp/low load/low rpm uses, but with normal loads/rpms the benefit to increased Cetane diminishes after starting.