Why the cc-rating of a car is a useless quantity
The Sunday Nation
Nairobi,
01 July 2012
Kepha Bukindo asks three seemingly straightforward questions about cars:
first, “what is ‘cc’ as used in 2000cc,4000cc, 5000cc etc and which is
the formula of finding it?”; second, “what is the meaning of WHP and
BHP? and finally, “what is
the formula of finding horsepower?”
The answer to the first question is that “cc” stands for Cubic
Centimetres or Centimetres Cubed. This is the volume of one cylinder of
the engine; that is, the combustion chamber where the fuel is burned to
produce motion energy.
The formula for calculating this volume is the cross-sectional area of
the cylinder multiplied by the distance over which the piston runs
during its up-and-down movements. This explains why the figure is
normally referred to as “displacement”: it is the volume displaced by
the piston.
The only way to determine it independently is to dismantle the engine
and take the necessary measurements. If you are not a highly skilled
mechanic, this is not something I would recommend! Therefore, you have
to rely on the figures given by manufacturers.
Going to the second question; the letters HP stand for “Horse Power”. As
explained in an earlier article, this is the rate at which the engine
produces energy. One HP is equivalent to the working rate of a
workhorse; not a racehorse!
BHP stands for “Brake Horse Power”. It the power generated by the engine
when it is standing alone; not connected to anything (no gears, not
differentials, no drive shafts etc).
WHP on the other hand is the “Wheel Horse Power”; that is, the power
available at the wheels. Thus WHP takes into account the energy losses
suffered along the transmission system of the car.
Now Kepha asks for the formula for calculating these HP figures. I
assume he means in terms of the engines displacement (cc). The answer to
that is NO; there is no formula. Given the cc-rating of a car, it is
impossible to calculate its power out.
The reason is that there are too many other variables that affect the
power; for example, the number cylinders (is it one or two or three or
four or six or what?), how the engine sucks in air (naturally or via
turbo), how it takes in fuel (carburettor or electronic injection), the
cylinder’s aspect ratio (is it wide and short or narrow and long?) and
so many others.
The power can only be measured experimentally in an engineering
laboratory. Other than that, we have to rely on the manufacturer’s
figures. But even these are for the prototype when new!
Nevertheless, general rule of thumb is that the bigger the displacement
the more powerful the engine…and the more fuel it consumes. This was the
wisdom behind former Finance Minister Uhuru Kenyatta’s limit of 1,800cc
for vehicles used by government officers.
However, this assumption is not often wrong, very wrong: small engines
can produce much more power than larger ones and also consume more. A
good example is the Mazda RX-8 sports car. It has a 1,300cc engine that
produces more power than a 4,000cc conventional engine…and also consumes
more fuel!
In a nutshell: the cc-rating of a car engine is a completely meaningless
(and useless) quantity to a motorist!
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