For the last several weeks, we have examined turbochargers
and superchargers regarding the history, methods of operation, design, innovation
and performance characteristics of each. Today we will compare the two systems
as far as advantages and disadvantages. Both
supercharging and turbocharging have significant disadvantages in their
inherent designs, but the gain in horsepower outweighs those disadvantages.
When you can achieve the same horsepower from a four cylinder as a V-6, or the
same power from a V-6 as a V-8, it is possible to overlook some other factors.
The first
major disadvantage of supercharging is the amount of power that it takes from
the engine to operate it. On some large engines such as the single stage
supercharged Rolls Royce Merlin; the supercharger requires about 150 horsepower
(HP) at maximum operating pressure.
However, the benefit outweighs the power demand because the supercharger
generates an extra 400HP, so there is a net gain of 250HP. The other problem
that is created is that the engine must be built to withstand the stress of the
extra 400HP, not the resulting gained 250HP. While centrifugal superchargers
such as those the Rolls Royce Merlin use operate at about a 65-80 percent
efficiency, a Roots blower by comparison often only results in an efficiency of
about 40-50 percent at maximum boost.
The second
major disadvantage is the amount of heat generated. In technical terms, it is referred
to as lower adiabatic efficiency. Adiabatic efficiency is the measure of a
compressor’s ability to compress air without adding excess heat to the charge
air. A compressor will always generate heat as a byproduct of the compression
process however; more efficient compressors generate less heat. Turbochargers
generate the least amount of heat, next best are the centrifugal superchargers,
and while the Roots type superchargers have the poorest efficiency, thereby
generating the highest temperature spike. For a given volume of air, the turbocharged
air is cooler, more dense and containing a greater concentration of oxygen.
This means that it will generate more potential power, all other things being
equal, than a supercharged engine. In practical application, that power difference
can be as much as 15-30 percent depending on the designs and types of units
being tested.
This is the
main reason why most manufacturers using supercharging or turbocharging install
an intercooler (an air to air radiator) in the system. An intercooler can often
restore between 20-30 percent of the power lost through turbocharging and
supercharging. But, even with the cooling, the turbocharger will retain its
higher efficiency rating, which usually means a gain in fuel economy and more
power.
The
greatest drawback to turbocharging is throttle response. Termed “lag,” there is
a time that lapses between when the throttle is depressed and when the turbo
“spools up” to the point where it is generating boost. This is because at idle
or low engine speed, there is little exhaust pressure to spin the turbocharger.
There is a brief delay until there is enough pressure in the exhaust to spin
the turbo fast enough to generate the desired amount of boost. This lag is
reduced by the use of smaller, lighter weight turbochargers and by using
variable pitch or twin turbochargers, but it cannot be eliminated. This is the
one distinct advantage of supercharging. With supercharging, the response to
throttle demand is almost instantaneous due to the fact that it is directly
driven off the engine.
In places
where instant power is demanded, such as drag racing or tractor pulling,
supercharging is the method of choice. Increasingly, for vehicle use,
turbocharging has become the choice for most applications. Even Mercedes Benz,
whose “Kompressor” models are well known and have been around since the early
days of supercharging, has started using turbocharging on some of its gasoline-powered
vehicles as well as its diesel models over the last few years. No matter which method you choose, you can
derive more power from a smaller engine, which equates to less fuel used. In today’s world, that is an important factor
to consider.
