Turbocharging
The concept of
turbocharging is very old. In 1907 Dr. A.J. Buchi patented a design for
turbocharging but turbocharging was commercially implemented after the
development of efficient compressors and turbines. The objective of
turbocharging is very similar to supercharging but unlike supercharging,
turbocharger does not draw power from the engine for its working although,
turbocharger utilizes the energy of the exhaust gases.
About 27% to 38% of
the total heat input to the engine goes into the exhaust. Some of this energy
can be utilized to run a gas turbine which in turn supply more air to the
engine by driving a compressor. Such utilization of the exhaust energy boosts
engine power and results in better fuel consumption and thermal efficiency.
The power developed by
the turbocharger is sufficient enough to overcome its resistance and to drive
compressor. The speed of turbocharger ranges from 20,000 to 80,000 rpm. In order
to supply sufficient energy to the turbocharger, the exhaust valve is opened much
before the bottom dead center as compared to naturally aspirated engine. This allows
the exhaust gases to escape a higher pressure and temperature giving
turbocharger enough energy to drive the compressor.
Limitations of turbocharging
1. The use of
turbocharging requires special exhaust manifolds.
2. Fuel injection
has to be modified to inject more fuel per unit time. This requires either
large pumping elements and/or larger nozzles with the same basic fuel injection
equipment. This means overloading of the cams and other components.
3. A naturally
aspirated engine can digest solid particles in the inlet air without undue
stress but turbocharged engines pass only the minutest particles without
damage. It can deal with carbon and other combustion products through after a
few thousand hours of running. Blade erosion is common.
The efficiency of the turbine blades is very sensitive to
gas velocity so that it is very difficult to obtain good efficiency over a wide
range of operation.
Methods of turbocharging
The methods used for
turbocharging a four-stroke diesel engine uses a turbocharger alone while a
two-stroke diesel engine uses another devices in addition to turbocharger for
supply of air to the engine. The main types of turbocharging methods are pulse
operation, constant pressure and pulse converter, two stage turbocharger, Miller
turbocharging, Hyper bar turbocharging.
1. Constant pressure turbocharging
The exhaust from
various cylinders discharge into a common manifold at pressures higher than the
atmospheric pressure. The exhaust gases from all the cylinders expand in the
exhaust valve to an approximately constant pressure in the common manifold and
pass from here to the turbine. Thus, the blow-down energy, in the form of
internal energy, is converted into work in the turbine. The recovery of blow
down energy is higher if the pressure ration of turbine is high. The exhaust
gases are maintained at constant pressure during the whole of the cycle so that
a pure reaction turbine can be used.
2. Pulse turbocharging
Considerable part of
the blow-down energy is converted into exhaust pulses as soon as the exhaust
valve opens. These pulses are led through narrow exhaust pipes by the shortest
possible route to the turbine where this energy is utilized. A large proportion
of energy thus recovered. Towards the end of the exhaust the pressure in the
exhaust pipe drops below the scavenging and large air pressure making
scavenging quite easy.
Separate exhaust pipes
are used so that the exhaust process of various cylinders do not interfere with
one another. A common pipe is used for those cylinders whose exhaust cycles do
not overlap significantly in terms of time. The turbine has separate inlets and
nozzle segments for each exhaust pipe. The rate of the exhaust gas at the
various turbine inlets is different and variable with time.
3. Pulse converter
Pulse converter allows
the advantages of pulse and the constant pressure turbocharging to be utilized
simultaneously, while avoiding most of the drawbacks of both.
Turbocharger turbine
is a constant pressure machine and for maximum efficiency requires steady flow
conditions. With pulse charging turbine operates at relatively lower efficiency
due to partial admission operation. Moreover, the low level of available
exhaust energy especially at part load requires operation with pulse charging
for efficient utilization of this energy and good scavenging. For this reason a
combination of two systems is needed for good efficiency of the turbine. This
is done by connecting the different branches of exhaust manifolds together in a
specially designed venture junction, called pulse converter, before the
turbine. The exhaust manifold system of the pulse-system designed for maximum
pulse energy utilization is retained and the turbine runs at full-admission
conditions to provide good efficiency.
4. Two stage turbocharger
For diesel engine
requiring very degree of supercharging (bmep ranging from 25 to 30 bar) which
cannot be obtained in a single-staged turbocharger can either use two turbines
and two compressors on a single shaft or use two stage turbocharging. Two stage
turbocharging is defined as the use of two turbochargers of different sizes in
series; for example a high pressure stage operating on pulse system and a low pressure
stage on constant pressure operation.
5. Miller turbocharging
In Miller turbocharging
system the basic idea is to increase the expansion ratio relative to
compression ratio by means of early closure of inlet valve as the boost
pressure is increased.
If the inlet valve is
closed early then the cylinder will be partially filled and the charge will
start expanding even before the start of compression. Thus at the start of
compression the air charge temperature will be lower than that for the normal
operation. For a normally aspirated engine such partial filling of the cylinder
would mean a loss of power output. However, in a supercharged engine this loss
is recovered by using a high pressure boost ratio.
6. Hyper-bar turbocharging
Hyper-bar
turbocharging basically consists of a low compression ratio (7:1) diesel
engine, a high pressure ratio turbine (up to 5:1), a by-pass control and auxiliary
combustion chamber located between the diesel exhaust valve and the
turbocharger turbine.
The turbocharger
is started by an electrical starter and can be kept running by bypassing the
air and injecting first into the auxiliary combustion chamber while the engine
at rest. Thus the turbocharger and the by-pass system are operating in simple
gas turbine mode. After some time when the appropriate pressures and
temperatures are reached the diesel is started. During operation the large
compression ratio of the compressor makes this preheating unnecessary and on
the contrary the charge air has to be cooled.
