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Statement (I): Air-blast injection in diesel engines could reduce engine efficiency. Statement (II): Air-blast injection in diesel engine is not instantaneous but happens when the piston moves outward with the injection valve remaining open for whatever reason.

Statement (I): Air-blast injection in diesel engines could reduce engine efficiency. Statement (II): Air-blast injection in diesel engine is not instantaneous but happens when the piston moves outward with the injection valve remaining open for whatever reason. Correct Answer Statement (I) is true but Statement (II) is false

In an air-blast injection system, a compressor and a storage tank supply air at a pressure much higher than that existing in the engine cylinder, as means of injecting and atomizing the fuel oil. The air blast engine is too heavy and bulky, the compressor uses 10% of the output of the engine. Therefore the efficiency is also considerably low of the air-blast injection system.

An air blast injection system consists of the following units:

  • An air compressor for providing an adequate supply of air at a pressure several hundred pounds higher than the engine compression pressure
  • High-pressure air bottles of sufficient capacity to provide a steady flow of high-pressure air for injection as well as for engine starting.
  • Atomizers in each engine cylinder for injecting the metered fuel by blasts of high-pressure air into the combustion chamber in an adequately atomized and disbursed state.
  • Cam mechanism for timing and lifting the atomizer valve to control the injection process.
  • A fuel pump to meter and deliver to each atomizer the proper quantity of fuel in accordance with engine requirements of speed and load

 

The valve timing is important for the rate of injection, injection is completed at the maximum lift of the valve when the piston in the cylinder is on the downward stroke, it draws in air from the atmosphere. This air is now trapped in the cylinder and, as the piston is now on the upward stroke, it compresses this air to about one-fourteenth of its original volume. Now, compressed-air is allowed to blow fuel through an atomizer at the end of the compression stroke.

Since the injection is instantaneous the statement II) is incorrect.

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