What are the advantages of pressure charging?
There are two distinct advantages namely,
1. Depending on the pressure of the charge air, more fuel can be burned in the engine giving more power from same engine.
2. Better scavenging (removal of exhaust gases from previous cycle) of the cylinder is achieved during each cycle resulting in increased efficiency of engine.
3. Waste heat in the exhaust gases is used to operate the turbocharger resulting in more power produced for same fuel compared to normally aspirated engines.
4. Engine operating temperatures are reduced due to cooling effect of excess charge air resulting in reduced thermal stresses in the engine parts, which may otherwise get overheated. This reduces the maintenance costs and results in longer life for engine parts.
While turbocharged four stroke diesel engines face any problems, turbocharged two-stroke diesel engines particularly the large ones, are difficult to start from stop position and, are also difficult to run at low speeds. This is due to the fact that it takes little time for the turbocharger to pick up speed from stationary position as exhaust gases do not have sufficient energy in the beginning to cause quick rotation of the turbine wheel.
Why the charge air is cooled prior to delivery into engine?
On compression in the blower, the air gets heated which is cooled in charge air cooler prior to delivery to the engine. Cooling has two advantages namely;
1. Hot air has lower density. Hence, less weight of air is contained in the cylinder combustion of fuel resulting in less power produced per cycle. By cooling the engine, its density is increased resulting in more fuel burned per cycle producing increased power.
2. Hot air will increase the operating temperatures of the engine resulting in extra thermal stresses, increased maintenance and reduced life for engine parts.