By Ryan
When
people searching at the information of different vehicles, they may find the
engine types of those cars are different, some may confused by the distinction
of naturally aspirated, turbocharge and supercharge. To tell the
difference, of course we need to know their operating principle.
In normally aspirated piston
engines, intake gases are "pushed" into the engine by atmospheric
pressure filling the volumetric void caused by the downward stroke of the
piston (which creates a low-pressure area), similar to drawing liquid using a
syringe.
The turbocharger's compressor draws in ambient air and
compresses it before it enters into the intake
manifold at increased pressure. This
results in a greater mass of air entering the cylinders on each intake stroke.
The power needed to spin the centrifugal compressor is derived from the kinetic energy of
the engine's exhaust gases. The main advantage of turbocharger is increasing
fuel efficiency and increasing power at the same time. This is achieved by recovering waste
energy in the exhaust and feeding it back into the engine intake. By using this
otherwise wasted energy to increase the mass of air, it becomes easier to
ensure that all fuel is burned before being vented at the start of the exhaust
stage. The increased temperature from the higher pressure gives a higher efficiency.
In contrast to turbochargers, superchargers are mechanically
driven by the engine. Belts, chains, shafts, and gears are common methods of
powering a supercharger, placing a mechanical load on the engine. Compare to the turbo technology, mechanical
pressurization completely solved the throttle response lag, turbo lag and power
output achieve instant throttle response, linear power with rotational speed,
and can increase the driving performance. In addition, in high torque at low
speed, superchargers can provide instantaneous acceleration, but in another hand,
this is where the principal disadvantage of a supercharger becomes apparent;
the engine must withstand the net power output of the engine plus the power to
drive the supercharger, so it has lower adiabatic efficiency as compared to
turbochargers.
Inevitably, the primary disadvantage of turbocharging is what is
referred to as "lag" or "spool time". This is the time
between the demand for an increase in power (the throttle being opened) and the
turbocharger(s) providing increased intake pressure. Throttle lag occurs
because turbochargers rely on the buildup of exhaust gas pressure to drive the
turbine. In variable output systems such as automobile engines, exhaust gas
pressure at idle, low engine speeds, or low throttle is usually insufficient to
drive the turbine. Only when the engine reaches sufficient speed does the
turbine section start to spool
up, or spin fast enough to
produce intake pressure above atmospheric pressure.
VW's twincharging system
In summary, none of them is perfect solution
in rising the power but reducing the cost at the same time, the latest technology
is twinturbing, which is a combination of an
exhaust-driven turbocharger and an engine-driven supercharger so that the
weaknesses of both can minimized. But we have reason to believe that this is
not the end, the seeking of better solution will never stop.
Sources
Turbocharger
- Wikipedia, the free encyclopedia
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