Is cetane to diesel fuel like octane is to gasoline?

Yes and no. Both are measures of the fuel's combustion quality.

Octane measures the gasoline's ability to burn smoothly and not "explode" in the combustion chamber in a spark engine.

Cetane measures the ability of the diesel fuel to achieve combustion in a compression (diesel) engine. The higher the cetane, the more easily the fuel achieves combustion. This is more important, I think, the higher the normal operating speed in rpm of the diesel engine.

Usually, the minimum cetane rating you will find in the USA is around 40. Some better fuels in the USA are around 45 cetane. In Europe, where there are a lot of smaller, higher revving diesels in automobiles, fuel with cetane ratings of 50 or more are available.

I am sure others more knowledgable than me will refine this explanation.

The reason I asked is I started using Performance Service Diesel Kleen which boosts cetane by either 3 or 6 (depending on whether you add 8 or 16 ounces to 25 gallons of diesel).

All I can say is when you use 16 ounces to a tank, the diesel knock is almost completely gone. Today after using almost 1/2 tank of the added mixture, I almost thought I was driving a gasoline powered truck.

Octane number is the ability of a gasoline fuel to resist spontaneous ignition. It allows higher compression ratio.

Cetane number is the ability of a fuel TO spontaneously ignite.

As such, they are opposites. Whatever kind of molecule (say aromatics) makes high octane, leads to low cetane and vice versa (like for paraffins).

They quantify cetane number by measuring ignition delay. That's the time from when the fuel injection starts to when the pressure rises in the cylinder indicating that it started to burn. It does take a few milliseconds for the burn to start after the fuel starts going in.

The longer the delay, the more fuel is sprayed in before burning, the more sharp the rise in cylinder pressure, the more combustion noise you hear. That's why higher cetane number fuels are quieter. Higher cetane also lets you go to higher RPM. Ignition delay is what limits the RPM of diesels. Too fast and there's not enough time to complete the burn due to the delay...leads to incomplete combustion and smoke, unburnt HC's, etc.

The little pilot injection, sprayed early, warms up the air, and adds radicals, so the delay of the main charge is shorter. That's why pilot injection makes it quieter.

If the delay is shorter, the high pressure is pushing on the piston for more of the stroke, and there's more burn time. That's why is helps mileage and power.

ASTM D975, which sets US diesel fuel specs, lists a minumum cetane number of 40. In Europe it's 50, and in some countries, like Sweden, it's 55.

IMHO, our low cetane spec is the main reason they can't design small diesels in the US that people would buy to replace gasoline engines.

.......great post.....:thankyou2

very nice citi,