Analysis of In-Cylinder Process in Diesel Engines P M V Subbarao Professor Mechanical Engineering Department Sudden Creation of Young Flame & Gradual Growth ….

Feel the In-Cylinder Processes Start of injection Start of combustion End of injection

Cut-off Ratio & Nozzle Capacity

Sizing of Nozzle Instantaneous Diesel Flow rate: Mass of Diesel per cycle:

Fuel Exiting into High Pressure& Temperature Air Enviroment : An unexplored Fluid Mechanics

Boiling in Nozzle due Improper Fluid Mechanics???? Nurick’s Number, K p v is the vapor pressure of the fuel.

Optimal Design of Nozzle Hole

Effect of Nozzle Surface Finish : Distribution of Droplets

Distribution of Droplets in A Spray

where do is the discharge orifice diameter and FN is the nozzle flow number defined by Strong Influence of In-Cylinder Conditions on SMD

Where  is the liquid surface tension,  L is the liquid viscosity,  A is the air density,  L is the liquid density,  p L, is the injection pressure differential across the nozzle,  is the half spray angle and t is the film thickness, given by

Simultaneous Occurrence of Multiple Process in CI Engines -10 Start of injection End of injecction

Sequence of Events in CI Engines Cylinder Imagine faster than 0.1 ms !!!!!

0.4 ms after ignition3.2 ms after ignition 5.0 ms after ignition Late in combustion process 1 cm Air flow

Combustion in CI Engine The combustion process proceeds by the following stages: Ignition delay (ab) - fuel is injected directly into the cylinder towards the end of the compression stroke. The liquid fuel atomizes into small drops and penetrates into the combustion chamber. The fuel vaporizes and mixes with the high-temperature high-pressure air. -10TC Start of injection End of injecction

Ignition Delay Ignition delay is defined as the time (or crank angle interval) from start of fuel injection to the onset of combustion. Both physical and chemical processes must take place before a significant fraction of the fuel chemical energy is released. Physical processes are fuel spray atomization, evaporation and mixing of fuel vapour with cylinder air. Good atomization requires high fuel pressure, small injector hole diameter, optimum fuel viscosity, high cylinder pressure (large divergence angle). Rate of vaporization of the fuel droplets depends on droplet diameter, velocity, fuel volatility, pressure and temperature of the air. Chemical processes: Autoignition phenomenon in premixed fuel-air. Complex heterogeneous reactions (reactions on the liquid fuel drop surface) also occur. -10TC Start of injection End of injecction

Estimation of Ignition Delay The sum of times required for sub process preceding ingintion. The most widely reported correlation relating the ignition delay to the ambient gas condition is given by the relation where τ is the ignition delay, p g and T g are the ambient gas mean pressure and temperature before autoignition takes place, A, B and n are experimental constants.

Arhenius-type equation for Ignition Delay An Arhenius type equation for Ignition delay is: p :Premixed air fuel ratio.

Symptoms to be Sensed to Predict Auto Ignition

Effect of Gas Temperature on Ignition Delay

Effect of Equivalence Ratio on Ignition Delay

Auto Ignition of Fuels Diesel or Jet A-1210 °C Gasoline (Petrol)246–280 °C Ethanol363 °C Butane405 °C Paper18–246 °C Magnesium473 °C Hydrogen536 °C

The flammability limits versus the number of carbon atoms in alkanes

Damages due to Wide Flammability Limits

Stages of Combustion in CI Engines -10TC Start of injection End of injecction

-10TC Start of injection End of injecction Homogeneous Combustion in CI Engine Premixed combustion phase (bc) – combustion of the fuel which has mixed with the air to within the flammability limits (air at high- temperature and high-pressure) during the ignition delay period occurs rapidly in a few crank angles.

Premixed combustion It is assumed that the rate of premixed combustion is proportional to the mass of the fuel-air mixture prepared during the ignition delay period and given as where is the Taylor microscale and S l is the laminar flame speed. m mix,j is the mass of the fuel-air mixture in the given element. C p is an arbitrary tuning constant determined to (0.002 to 0.005) to match the test bed data.

The Taylor microscale The Taylor microscale is given as where u' is the rms value of turbulent fluctuation velocity, L is the integral length scale, and is the kinematic viscosity. The constant, A, is set to be close to unity. The integral length scale is given as where the constant, C v, is in the range of 0.06 – 0.12.

Mixing controlled combustion phase (cd) – after premixed gas consumed, the burning rate is controlled by the rate at which mixture becomes available for burning. The burning rate is controlled primarily by the fuel-air mixing process. -10TC Start of injection End of injecction

Diffusion combustion Fuel-air mixing is the dominant mechanism to determine the rate of combustion during the diffusion combustion period.

Post (End of ) Injection Combustion The models during the fuel injection period may not be applicable after the end of fuel injection for the spray detached from the nozzle and moving downstream. The in-cylinder flow effects need to be considered to predict the combustion after the end of fuel injection. This is described as a mixing process with the available air at a rate controlled by turbulence in the fuel jet as, where m ea is the total mass of unused air in the cylinder. The constant, Ce,a, is determined from the continuity of the combustion rate at the end of fuel injection.

Late combustion phase (de) – heat release may proceed at a lower rate well into the expansion stroke (no additional fuel injected during this phase). Combustion of any unburned liquid fuel and soot is responsible for this. -10TC Start of injection End of injecction