Since the
invention of the first automobile engine, manufacturers have increased its
Compression Ratio in order to,
(A) Reduce
the engine manufacturing cost per unit engine power output.
(B) To
obtain more power from a given engine weight.
The reason
is higher the compression ratio the higher the pressure at the end of the
compression stroke and the higher the pressure exerted on the piston during
the power stroke. The result will be more power from each power stroke.
(Compression ratio is the ratio that the air fuel mixture is compressed. The
more the air fuel is compressed the higher the compression ratio)
In designing automobile
engines with high compression ratios the main constraint faced by the design
engineers was the ignition knock which is the spontaneous
explosion of the air fuel mixture instead of gradual combustion. This
occurs due to high pressure when air fuel intake increases with increasing
engine load. This problem was solved by introducing petrol with high
resistance to ignition knock.
During refining
process several additives are mixed with fuel such as volatility
improvers, knock inhibitors, oxidation inhibitors, anti rust agents,
anti-icers, detergents, dyes etc.
Scientists
wanted to introduce a rating based on ignition quality of fuel. So
Octane Value for petrol and Cetane Value for diesel were introduced.
These values can be considered as a measure of resistance to
combustion. Fuel burning will be more retarded with increasing
octane or cetane values.
Petrol is a
mixture of hydrocarbons from C7 to C8.
(1) Normal
heptane(C7H16) shows low resistance to detonation (Burns quickly)
Its octane value is taken to be 0.
(2) Iso-octane(C8H18)
shows high resistance to detonation (Burns slowly) Its octane value is taken to
be 100.
For example,
a mixture of 10% n-heptane and 90% iso-octane will form 90 octane
petrol.
According to
British classifications octane rating is denoted by star points.
Octane Value Star Points
90 **
94 ***
97 ****
100 *****
By adding
certain compounds called knock inhibitors such as Tetra Ethyl Lead
(TEL) octane value can be increased and such petrol is called leaded
petrol.
Using leaded petrol will turn exhaust
emission contain with lead compounds which cause,
(1) Atmospheric
pollution.
(2) Lead
poisoning a potential health hazard to all living beings.
(3) Turns the
Catalytic Converter (Emission control system) inoperative beyond repair.
In the USA unleaded petrol typically has octane rating of
87 - Regular.
88 to 90 - Mid-grade
90 to 94 - Premium
Super Premium gasoline (100 octane) is usually used for,
1. Piston type Air craft Engines.
2. High performance cars with high compression ratio engines for which high octane petrol is needed to prevent knocking.
Vehicle manufacturer's specifications mention the suitable octane number of petrol for the particular vehicle.