Variables affecting delay period
(i) Cetane number
Both physical and chemical properties of the fuel are important. Ignition quality of the fuel is defined by its cetane number. Straight chain parafinic compounds (normal alkanes) have
highest ignition quality, which improves as the chain length increases. Aromatic compounds, alcohols have poor ignition quality.
-Cetane number can be increased by ignition-accelerating additives like organic peroxides, nitrates, nitrites and various sulphur compounds. Most important (commercially) is alkyl nitrates – about 0.5% by vol in a distillate fuel increase CN by 10.
-Normal diesel fuel has CN of 40 to 55 (high speed 50 – 60, low speed 25 – 45
(ii) Injection timing
-At normal operating conditions min ignition delay (ID) occurs with start of injection at 10 to 15 OCA BTDC
-Cylinder temperature and pressure drops if injection is earlier or later (high at first but decrease as delay proceeds)
(iii) Injection quantity (load)
-Reducing engine load changes AFR, cools down the engine, reduces wall temperatures, reduces residual gas temperatures and increase ID
-Droplet size, injection velocity and rate Ignition quality within practical limits do not have significant effect on ID
-Increase in injection pressure produces only modest decrease in ID Injector nozzle diameter -effects of droplet size but has no significant effect on ID
(iv) Intake air temperature and pressure
-Reducing intake air T and p increase ID
-Strong dependence of ID on charge temperature below 1000 K – above this value effect of intake air conditions is not significant.
(v) Engine speed
Increase in engine speed increases the air motion and turbulence, reduces ID time slightly (in ms), in terms of CA degrees ID increases almost linearly.
-A change in engine speed, changes “temp~time” and “pressure~time” relationships
(vi) Combustion chamber design
-Spray impingement on the walls effect fuel evaporation and ID increase in compression ratio, increase p and T and reduces ID
-Reducing stroke volume, increase surface area to volume ratio, increase engine cooling and increase ID
(vii) Swirl rate
-Change of evaporation rate and air-fuel mixing - under normal operating conditions the effect is small.
-At start-up (low engine speed and temperature) more important, high rate of evaporation and mixing is obtained by swirl
(viii) Oxygen concentration
Residual gases reduce O2 concentration and reducing oxygen concentration increases ID
(i) Cetane number
Both physical and chemical properties of the fuel are important. Ignition quality of the fuel is defined by its cetane number. Straight chain parafinic compounds (normal alkanes) have
highest ignition quality, which improves as the chain length increases. Aromatic compounds, alcohols have poor ignition quality.
-Cetane number can be increased by ignition-accelerating additives like organic peroxides, nitrates, nitrites and various sulphur compounds. Most important (commercially) is alkyl nitrates – about 0.5% by vol in a distillate fuel increase CN by 10.
-Normal diesel fuel has CN of 40 to 55 (high speed 50 – 60, low speed 25 – 45
(ii) Injection timing
-At normal operating conditions min ignition delay (ID) occurs with start of injection at 10 to 15 OCA BTDC
-Cylinder temperature and pressure drops if injection is earlier or later (high at first but decrease as delay proceeds)
(iii) Injection quantity (load)
-Reducing engine load changes AFR, cools down the engine, reduces wall temperatures, reduces residual gas temperatures and increase ID
-Droplet size, injection velocity and rate Ignition quality within practical limits do not have significant effect on ID
-Increase in injection pressure produces only modest decrease in ID Injector nozzle diameter -effects of droplet size but has no significant effect on ID
(iv) Intake air temperature and pressure
-Reducing intake air T and p increase ID
-Strong dependence of ID on charge temperature below 1000 K – above this value effect of intake air conditions is not significant.
(v) Engine speed
Increase in engine speed increases the air motion and turbulence, reduces ID time slightly (in ms), in terms of CA degrees ID increases almost linearly.
-A change in engine speed, changes “temp~time” and “pressure~time” relationships
(vi) Combustion chamber design
-Spray impingement on the walls effect fuel evaporation and ID increase in compression ratio, increase p and T and reduces ID
-Reducing stroke volume, increase surface area to volume ratio, increase engine cooling and increase ID
(vii) Swirl rate
-Change of evaporation rate and air-fuel mixing - under normal operating conditions the effect is small.
-At start-up (low engine speed and temperature) more important, high rate of evaporation and mixing is obtained by swirl
(viii) Oxygen concentration
Residual gases reduce O2 concentration and reducing oxygen concentration increases ID
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