Charger
1. Kinds of charger
The basic of increasing the engine power and torque is 『How more
oxygen can be inhaled into the engine』. For example, if
the density and amount of inlet air is high, then the power and torque shall be
high.
Additionally, by developing the intake system
and combustion chamber, it is possible to intake more air. One of possible
methods is 『to compress the air and to inhale』, that is, to use auxiliary device, the charger.
There are some types in the chargers.
Typically, there are a Turbocharger type in which the charger is driven
by the exhaust turbine, and a Supercharger type in which the
supercharger is driven in mechanically using driving force of some part (such
as crankshaft rotation).
Turbocharger
Supercharger
The Turbocharger is, as the “Turbine driving
charger”, a system compressing air by the compressor rotating a turbine using
the exhaust gas flow. It is possible to get high power output using a small
device. However, when the engine is rotating with low speed, the turbine can
not rotate with high speed, so the compressing power is not enough and the
acceleration will be delayed.
Therefore, even the accelerator is pressed
the engine rotation may not be response immediately, that is Turbo Lag.
The lag means the time delay.
As driven by the interlocking mechanism with
the crankshaft, the Supercharger, the mechanical supercharger, has the well response.
However, in the low engine speed, the engine efficiency will be worse by
increasing leak air through the gap between the rotor and the housing. In the
high engine speed, the driven force loss may be increased. To recover these
defects, the structure of supercharger system may be changed or a the Turbocharger
and the Supercharger may be combined to new system.
The engine without such supercharger is
called as the naturally aspirate engine or NA engine.
2. Turbocharger
As the turbocharger is a combined word of
turbo (turbine) and charger, it comprises of a turbine and a compressor
directly linked, so that the turbine wheel is rotated by the exhaust gas energy
and the inlet air will be compressed by the rotation of the turbine wheel.
The body of turbocharger comprises of turbine
wheel, compressor wheel and axis, and attached near the exhaust manifolds.
The heated air by the compressing is cooled
by the intercooler and supplied into the engine via the throttle valve. The
exhaust gas passes to the turbo charger to rotate the turbine wheel. In order
to prevent the over boosting at high speed, when the boost pressure is over the
predetermined pressure, a waste gate valve (exhaust bypass valve) of the WGT
(waste gate turbocharger) will be
opened.
The turbine wheel is rotated 100,000 rpm ~
160,000 rpm with hot exhaust gas about 900℃ at high speed,
the turbine wheel is made of light material having high heat resistance such as
ceramics.
The smaller and lighter turbocharger is
better for the engine response such as acceleration and deceleration but worse
power at high speed. On the contrary, the bigger turbo charger has a high power
at high speed but slow response. Therefore,
it is important to select the size of turbo wheel according to the displacement
volume of the engine. Generally, the compressor wheel is made of aluminum to
make lighter turbocharger.
For the supporting of the high speed rotor
shaft, lots of the engine oil is supplied to the shaft for the lubrication and
the cooling. If the engine is stopped from the high speed abruptly, the
turbocharger is rotated without the oil by inertia force of turbocharger until
self stopping. So, the rotor shaft may be adhered. For this reason, the turbo
engine should be stopped after idling.
3. Boost Pressure and Compression Ratio
The air pressure which is pressurized by the
turbocharger is the Boost pressure. If the boost pressure is increased, the intake
air amount into the cylinder is also increased and the power output will be
increased. However, the boost pressure can not be increased without limitation.
As increasing the boost pressure, the actual compression ratio is also
increased, so knocking shall be occurred at high compression pressure. The
actual compression ratio represents how the inlet air into the cylinder
compressed actually is. So, the boost pressure is regulated by waste gate
valve.
The knocking is the self ignition
phenomena while the flame is spreading
after the ignition of the spark plug, the un-burn mixture can be easily
self-ignited at high temperature condition by high compression. So, the
knocking is occurred as much as the actual compression ratio high.
By this reason, the compression ratio of the
turbo engine in specification is smaller than that of the NA engine. For
example, if the engine having the represented compression ratio of 10 is
supercharged with 1 atm, then the air amount will be 2 times and the actual
compression ratio will be 20 and then knocking will suddenly be occurred.
Generally, compression ratio of the commercial turbo engine is set lower than
that of the NA engine balancing with the power, torque and fuel efficiency.
The knocking can be prevented by controlling
the ignition timing in the NA engine but the knocking control by the ignition
timing in trubo engine is not easy because
the knocking is affected by the boost pressure.
The maximum power and fuel efficiency is made
just before the knocking is occurred because the combustion speed is most fast
at this situation. By sensing the knocking noise, the ignition timing can be
fully advanced by ECM (electronic control modul) until the knocking is occurred.
The sensor for detecting the shock noise is the knock sensor.
The knock sensor converts the vibration about
7 kHz into an electrical signal. By attaching at the cylinder block, this
signal is treated by computer with the engine rpm, crank angle, and the intake
air amount to control the ignition timing for preventing the knocking
4. Turbo Lag
The turbo charger is an air compressor for 『how much air is supplied』. By using the
negative pressure acquired when the piston goes down and the flow inertia for
intake air, the NA engine can make the charging efficieny of 65∼95%.
In the turbo charger, charging amount is 1.2∼1.5 times
higher than NA engine if the displacement volumes are same. Therefore, it is possible
to make smaller engine than NA engine if the power and the torque are same.
However, it has a defect, so called the turbo
lag. The turbo lag is the time delay from starting the acceleration to the
actual rpm increasing of the engine. Especially, it is occurred when the car is
started, accelerated suddenly, or speed up from the low speed.
The process is like that; at first, the
throttle valve is opened, then the air amount is increased, then the combusted
gas is increased and the exhaust gas temperature is increased. After that, the
rpm of the turbine is increased by the increasing exhaust gas, so that the
supplied air amount by the compressor is increased. By this process, the intake
air amount is increased more. According to this process, the engine
acceleration is delayed from the starting of acceleration to the actual rpm
increasing of the engine.
To minimize the turbo lag, there are many
researches and developments. For example, as the simplest method, there is a
method for increasing the exhaust speed pushed into the turbine wheel. By
reducing the diameter of nozzle of exhaust pipe, as the exhaust speed can be
increased with the same displacement volume, the turbo lag can be minimized. However,
in this case, the maximum power is limited.
To attach small two turbo instead of large
one, it is possible to reduce the turbo lag. For example, in the 6-cylinder
engine, one turbo is attached at each 3-cylinder. Doing so, it is possible to
prevent the exhaust interference as well as to increase the power. This method is called as the twin turbo type. On the other hand, the
two way twin turbo type also accepts
the two turbo, but just one turbo will be driven at the low speed to maintain
better response and two turbo will be driven at the high speed for enhancing
torque.
Hybrid turbo
For another example, there is a hybrid turbo type in which the
supercharger is used for the low speed, and the turbocharger is used for high
speed.
In the electronic control engine, VGT
(Variable Geometry Turbocharger) is used to reduce the turbolag and to increase
the torque of the engine.
The VGT uses a
set of adjustable vanes, or nozzles, to direct flow into the turbocharger
turbine. When the vanes are closed, flow is directed tangentially into the
turbine wheel. This imparts maximum energy into the turbine, causing the
turbocharger to spin faster. Conversely, when the vanes are opened, they direct
flow into the turbine in a more radial direction.
VGT (Variable Geometry Turbocharger)
This reduces the
angular momentum of the flow going into the wheel, producing less turbine work
and ultimately slowing down the turbocharger.
5. Supercharging System and Heat
The turbine shaft of turbocharger is supplied
lots of engine oil to cool and to lubricate. Therefore, engine oil in the turbo
engine is deteriorited easier than NA engine.
As combusting more mixture, the charging
system can not avoid increasing temperature of combustion chamber.
As the turbocharger performs the boosting
using the exhaust gas energy, the boosting efficiency will be better at the
higher temperature of exhaust. For example, by adopting the heat resistance material
to the parts such as the exhaust valve with natrium and the stainless exhaust
manifold, and making the supercharger of high heat resistance material, the
performance shall be enhanced.
The inlet air should have as lower
temperature as possible. Because the air density is lowered as the temperature
is higher, the actual compression ratio will be decreased as the temperature is
increased with the same volume and the oxygen amount will also be decreased. If
the inlet air is hot, the compress mixture at the compressing stroke has higher
temperature easy to make a knocking.
When the air is compressed the temperature is
increased. This is the same in the charging process. So, the boosting effect is
reduced as the amount of increased temperature. Therefore, the heated air
should be cooled using a radiator before it reaches at the throttle valve. This
cooling device is the intercooler.
There are two types in the interclooler, air
cooling and water cooling system.
Air cooling type
Water cooling type
The
air type intercooler cools the inlet air temperature using the wind get
from the running of the vehicle. The intercooler is attached at the front or
side of the radiator. The structure of intercooler is similar with the
radiator, but the flow is not the water but the compressed air by the charger.
The
water type intercooler cools the compressed hot air using an additional
cooling water separated from the engine cooling water. The water has higher
heat capacity than air, so the water type is more effective than air type, but
it has high cost for parts and maintenance.
6. Supercharger
The supercharger drives the blower and
compressor to boost using the engine power. It can make higher torque and not
make any response delay such as the turbo lag. However, the driving force for
supercharger is from the rotation of the crankshaft, so the engine power shall
be consumed. The maximum power output is lower than turbocharger.
There are some kinds in the supercharger,
typically, the Roots Blower and the Lysholm Compressor.
The
Roots Blower has been used in vehicle engine. The boost control is performed by
computer. It is operated when the high output is needed such as for
accelerating and driving in high speed.
The structure of the roots blower is, to send
the air from one side to other side by rotating two elliptical shaped aluminum
rotors coated with special resin, in the oval housing. If the boost pressure is
over charged, the valve is opened to return some amount of the charged air.
Lysholm compressor
The
Lysholm compressor had been used in industrial field not in vehicle
engine. The structure is that two rotors including 3 and 5 screw blades
respectively are combined in the elliptical shaped housing. It is driven by a V
belt linked with the engine. The rotor is made of aluminum alloy coated with
resin of teflon. When it is rotating, airs supplied from one side is
accumulated and transmitted to other side so the air is compressed. Consequently,
the intake air is compressed up to 2 times.
In the supercharger, the roots blower is not
a compressor but a fan as know from the name. In the NA engine, the air is
charged by the negative pressure generated when the piston goes down, but the
supercharger is additionally equipped the blower for enhancing the charging
efficiency by positively sending the air. The Lysholm compressor is a
compressor so that it can send the compressed air like the turbocharger.
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