Over the next several weeks, we are going to discuss turbocharging
and supercharging.
We’ll start with the history of turbocharging, followed by
how it works and its main parts. We will then do the same for supercharging.
After completing these, we will discuss the advantages and disadvantages of each
in a comparison.
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| Compressor Section of an Automobile |
Alfred
Buchi of Switzerland who developed a compressor driven by exhaust gas to force
air into the intake of a diesel engine to create more power patented the first
turbocharger in 1905. It still took another twenty years though before an
actual operating turbocharger was built for vehicular use. There were several
attempts by the French to turbocharge some types of airplane engines in World
War I with limited success. Turbocharging of aircraft engines was perfected by
the early 1920s and a short time later the same thing took place on diesel
engines on ships.
The two biggest problems to
developing a turbocharger for automotive use were the ability to scale down the
size and manufacturing a seal that could be small enough but withstand the
pressure and heat inherent to turbocharging. There were some applications to
racecars during the 1940s and 1950s, but many of these were adapted aircraft
turbochargers. The first manufacturer to produce a production built vehicle
with turbocharging was Saab in 1977. Other manufacturers followed,
unfortunately, many of the early turbochargers failed due to heat and seal
problems. Saab started using a turbocharger which was cooled by antifreeze in
1986, and this proved much more reliable. Since that time, and especially in
the last few years, turbocharging has become very popular because of the
ability to derive the same or more power from a much smaller displacement
engine, thereby achieving a much higher gas mileage without sacrificing
performance, and in many cases bettering it.
The theory behind turbocharging is
actually quite simple. In most internal combustion engines, the intake mixture
of gas and air is actually drawn into the engine by the downward movement of
the piston. In a turbocharged engine, that intake charge is forced into the
engine by the turbocharger, resulting in a much larger volume of intake charge,
which when ignited by the spark plug, creates much more power. The pressure to
force that charge into the intake comes from the other half of the turbocharger,
which is spun by the exhaust gases that are escaping from the engine. Also, the
use of pressurizing the charge causes it to burn more fully, which increases
the fuel efficiency of the vehicle.
The control of turbocharging has evolved dramatically in the
last few years and is now quite complex. Many manufacturers now use knock
sensors, all use waste-gates and blow off valves. And many use variable
geometry and intercooling.
Boost is
the term applied to the amount of pressure created by the turbocharger above
normal atmospheric pressure. The level of boost is normally indicated on a
pressure gauge in bar, psi or kPa. Boost pressure must be controlled so that
the design of the engine is not exceeded which would cause it to fail
prematurely. Over-boosting can damage the engine by overheating, over-stressing
of parts or by detonation. Detonation or preignition means that due to the
amount of heat and pressure, the intake charge ignites before the piston is
near the top of its cycle. This exerts undue stress and heat on the internal
parts. This is controlled with a knock sensor, which if it detects detonation,
signals the computer to open to blow-off valve, which will release the boost
pressure. The same thing can be achieved by the waste-gate which is vacuum
controlled.
The main
components of the turbocharger are the turbine, which is a radial flow design
to build pressure on the intake side. The compressor section is where the
exhaust gas passes through to build the pressure. The center housing is where
the seals, lubrication and cooling are contained. The size and design of the
compressor components dictate how much boost it will create and how quickly it
will build to maximum boost. In some turbochargers, it is possible for the
impellor to spin at speeds of up to 250,000 rpm. This is the reason that seal
design, heat dissipation and lubrication are so important.
Next week we will discuss the types of turbochargers and the
other related technologies. Some
information for this article was sourced from www.wikipedia.org.
