19-04-2012, 03:27 PM
General Turbocharger system
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Turbocharger -General
Turbocharger utilizes exhaust gas energy, which is normally wasted, to drive a turbine-powered centrifugal air compressor that converts air velocity into air pressure to increase the flow of air to the engine cylinders.
The air drawn into a naturally-aspirated engine is at less than atmospheric pressure but turbocharger engine packs the air into the cylinder more than atmospheric pressure.
Operation & Benefits
Operation
After the engine is started, the exhaust gases flowing from the engine and through the turbine housing cause the turbine wheel and shaft to rotate. The gases are discharged into the atmosphere after passing through the turbine housing. Consequently the compressor wheel which is mounted on the opposite end of the turbine wheel rotates and draws in fresh air, and compresses it with high pressure and delivers it to the engine cylinders.
Benefits
Decrease exhaust emissions, smoke and noise
Increase output power and
Maintain nearly constant power at high altitudes
Better fuel economy
Turbocharger-Control
While there are several types of turbocharger controls, they fall into
two general groups;
- Limit turbocharger speed control type
- Limit compressor outlet pressure control type
The limit turbocharger speed control type keep the turbocharger from destroying itself, while the limit compressor outlet pressure control type keeps the turbocharger from damaging the engine.
Waste gate:
One of the most common methods of limiting turbocharger outlet pressure is with a waste gate, or turbine bypass valve. Because the typical modern turbochargers can produce more pressure than any engine can use.
VGT- General
Boost pressure governing is used to control an exhaust turbocharger with Waste-gate as well as a turbocharger with Variable Geometry Turbine(VGT).
For governing an exhaust turbocharger with Waste gate, the actuator is a bypass valve through which the exhaust flow is directed to or past the turbine as mentioned in turbocharger control.
But for governing a VGT turbocharger, the actuator which is activated by ECM PWM (Pulse With Modulation) control is the variable geometry turbine.
Operating principle
Today’s diesel customers demand engines with excellent transient response, high torque backup and maximum fuel economy at all operating points. Large, high flow turbines allow for high positive energy and good fuel economy at high engine speeds, but typically have sluggish transient response and cannot provide as much low-end torque as a turbocharger with a low flow turbine. Technically it is called as “Turbo Lag”.