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Consider the following: 1. Increased cetane number 2. Increased compression ratio 3. Increased injection advance 4. Increased air turbulence Which of the factors will reduce the physical delay in diesel engines?

Consider the following: 1. Increased cetane number 2. Increased compression ratio 3. Increased injection advance 4. Increased air turbulence Which of the factors will reduce the physical delay in diesel engines? Correct Answer 1, 2, 3 and 4

Concept:

The combustion in a CI engine is considered to be taking place in four stages. It is divided into

1. Ignition delay period

2. Period of rapid combustion or uncontrolled combustion

3. Period of controlled combustion

4. Period of after burning

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Ignition delay is the time lag between the start of injection to the ignition.

Physical Delay: Since the time of injection to the beginning of chemical reactions is termed as physical delay. In this period, fuel is atomized, vaporized and mixed with air and raised to self ignition temperature.

Chemical delay: In this period, chemical reactions start slowly and then accelerate until complete inflammation takes place.

The physical properties such as Cetane number, viscosity of fuel, nozzle hole size, injected quantity and injection pressure contribute to the delay phenomenon in diesel engines.

  • The cetane number is an important property of diesel fuel and is a measure of the tendency of diesel fuel to knock in a diesel engine. Fuels with a high CN (low self-ignition temperature) have a very short ignition delay time, that is, ignition occurs in a very brief interval of time after injection begins.
  • Increased Compression ratio increases the density, pressure, and temperature of the charge. Increased temperatures and pressure reduces the delay period.
  • Injection timing advance reduces pressure and temperature when the injection begins and thus reduces the delay period.
  • The air turbulence increases the rate of mixing and atomization and thus reduces the delay period.

Note:

The cetane number does not distinguish between the chemical and physical delay times. This makes it difficult to determine the sensitivity of various engines in terms of the limitations created by the injection system design or the combustion chamber design.

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