Skip to main content

Octane Number - Rating of S.I. Engine Fuels

What is Octane Number? How rating of SI Engines (Spark ignition engines) is done using an octane number?

The hydrocarbon fuels used in spark ignition (S.I.) engine have a tendency to cause engine knock when the engine operating conditions become severe. The knocking tendency of a fuel in S. I. engines is generally expressed by its octane number. The percentage, by volume, of iso-octane in a mixture of iso-octane and normal heptane, which exactly matches the knocking intensity of a given fuel, in a standard engine, under given standard operating conditions, is termed as the octane number rating of that fuel. Thus, if a mixture of 50 percent iso-octane and 50 percent normal heptane matches the fuel under test, then this fuel is assigned an octane number rating of 50. If a fuel matches in knocking intensity a mixture of 75 percent iso-octane and 25 percent normal heptane, then this fuel would be assigned an octane number rating of 75. This octane number rating is an expression which indicates the ability of a fuel to resist knock in a spark ignition engine.

Since iso-octane is a very good anti-knock fuel, therefore it is assigned a rating of 100 octane number. On the other hand, normal heptane has a very poor anti-knock qualities, therefore, it is given a rating of zero octane number. These two fuels, i.e., iso-octane and normal heptane are known as primary reference fuels. It may be noted that higher the octane number rating of a fuel, the greater will be its resistance to knock and higher will be the compression ratio. Since the power output and specific fuel consumption are functions of compression ratio, therefore we may say that these are also functions of octane number rating. This fact indicates the extreme importance of the octane number rating in fuels for S. I. engines.

Note: The octane number of petrol, generally available, is 80 to 100.


Popular posts from this blog

Are Engineers Favourite Among Girls?

What do girls think about men who are engineers and what type of engineers are most favourite among girls? A survey was conducted by a team of aboutmech with members chosen from different countries. Each team member carried out a neutral survey in his/ her country to evaluate what females think about most of the engineers from different engineering branches. An overview of different questions asked in survey is given below: Question: Which male engineers do you find more attractive as a husband? Almost 50% women voted for mechanical engineers to be the most attractive husbands among other engineers. So mechanical engineers are most favourite among girls as a husband. They find them more trustworthy, intelligent, and creative. A married women from California said, “If your husband is a mechanical engineer, he will help you in everyday tasks. Because mechanical engineers are good team players and they have creative ideas about everything.” Which male engineers do you find more

Best CV Format for Mechanical Engineers

Fifty percent engineers fail to grab a dream job because of basic faults in their CVs/ Resume. This article tells you about best CV format for mechanical engineers and how you are supposed to design your CV when applying for jobs. You CV is your first impression on future employer. He/ she does not know you. Your CV is the only representation of your personality. He/ she may select or reject you after first glance on your resume.  I have experienced shortlisting and interviewing candidates for different jobs. I was astonished to learn that most of the candidates simply copy paste each others CVs without any changes at all. These types of CVs are rejected immediately. I am not trying to tell you that you should avoid copying, rather i ma telling you to refrain from giving your CV to your friends. An ideal CV consists of following portions: Name/ Address of the applicant Objective Education Record Employment History Professional Skills Awards & Achievements Co-Cu

Engineering Mechanics - Introduction

The Engineering Mechanics is that branch of Engineering-science which deals with the principles of mechanics along with their applications to engineering problems. It is sub-divided into the following two main groups: (a) Statics, and (b) Dynamics The Statics is that branch of Engineering Mechanics which deals with the forces and their effects, while acting upon the bodies at rest. The Dynamics is that branch of Engineering Mechanics which deals with the forces and their effects, while acting upon the bodies in motion. It is further sub-divided into the following two branches: (i) Kinetics, and (ii) Kinematics The Kinetics is that branch of Dynamics, which deals with the bodies in motion due to the application of forces. The Kinematics is that branch of Dynamics which deals with the bodies in motion without taking into account the forces which are responsible for the motion.