The ignition characteristics of diesel are expressed in terms ofa.Octane number b.Cetane numberc.Viscosity d. Flash and fire pointClear my choice

The ignition characteristics of diesel are expressed in terms of the Cetane number.

Here's why:

a. Octane number: This is used to rate gasoline, not diesel. It measures a fuel's ability to resist 'knock' or 'pinging' during combustion, which are caused by the air-fuel mixture detonating in more than once place at a time.

b. Cetane number: This is used specifically for diesel fuel. It measures the combustion speed of diesel fuel and compression needed for ignition. The higher the cetane number, the better the fuel will burn within the engine of a diesel vehicle.

c. Viscosity: This is a measure of a fluid's resistance to flow. It does not specifically express the ignition characteristics of diesel.

d. Flash and fire point: These are safety specifications, not performance specifications. The flash point is the lowest temperature at which the fuel can vaporize to form an ignitable mixture in air. The fire point is the temperature at which the vapor of the fuel will continue to burn for at least 5 seconds after ignition by an open flame. While these are important characteristics of diesel, they do not express its ignition characteristics in the way the cetane number does.