c.i Delay Period

7/30/2019 c.i Delay Period

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FACTORS AFFECTING THE DELAY

PERIOD


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The important ones are: compression ratio engine speed output atomization of fuel and duration of injection injection timing quality of the fuel

intake temperature intake pressure


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The increase in the compression temperature as well asthe decrease in the minimum autoignition temperaturedecreases the delay period. The peak pressure during thecombustion process is only marginally affected by thecompression ratio (because delay period is shorter withhigher compression ratio and hence the pressure rise islower).


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The increase in the compression temperature as well asthe decrease in the minimum autoignition temperaturedecreases the delay period. The peak pressure during thecombustion process is only marginally affected by thecompression ratio (because delay period is shorter withhigher compression ratio and hence the pressure rise islower).


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The delay period could be given either in terms of absolute time (in milliseconds) or in terms of crank angle degrees. Fig.12.15 shows thedecrease in delay period in terms of millisecondswith increase in engine speed in a variable speedoperation with a given fuel.


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With increase in engine speed, the loss of heatduring compression decreases, resulting in therise of both the temperature and pressure of the compressed air thus reducing the delayperiod in milliseconds.


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However, in degrees of crank travel the delay periodincreases as the engine operates at a higher rpm. The fuelpump is geared to the engine, and hence the amount of fuel injected during the delay period depends on crankdegrees and not on absolute time. Hence at high speeds,there will be more fuel present in the cylinder to take partin the second stage of uncontrolled combustion resulting inhigh rate of pressure rise.


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Atomization and Duration of Injection

Higher fuel-injection pressures increase the degree of atomization.The fineness of atomization reduces ignitiondelay, due to higher surface volume ratio. Smaller dropletsize will have low depth of penetration due to lessmomentum of the droplet and less velocity relative to airfrom where it has to find oxygen after vapourisation.Because of this air utilization factor will be reduced due tofuel spray path being shorter.


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Also with smaller droplets, the aggregate area of inflammation will increase after ignition, resulting in higherpressure rise during the second stage of combustion. Thus,lower injection pressure, giving larger droplet size may givelower pressure rise during the second stage of combustionand probably smoother running.Hence, an optimum groupmean diameter of the droplet size should be attempted asa compromise.


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Also the fuel delivery law i.e., change in thequantity of fuel supplied with the crank angletravel will affect the rates of pressure riseduring second stage of combustion thoughignition delay remains unaffected by thesame.


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Injection Timing

The effect of injection advance on the pressure variation is shownin Fig.12.16 for three injection advance timings of go, 18, and 27before T DC.

The injected quantity of fuel per cycle is constant. As the pressureand temperature at the beginning of injection are lower for higher

ignition advance, the delay period increases with increase ininjection advance. The optimum angle of injection advance depends on many factors

but generally it is about 20 0 bTDC


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Quality of Fuel

Self-ignition temperature is the most importantproperty of the fuel which affects the delay period. Alower self-ignition temperature results in a lower delayperiod.

Also, fuels with higher cetane number give lower delayperiod and smoother engine operation. Otherproperties of the fuel which affect the delay period arevolatility, latent heat, viscosity and surface tension


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Intake Temperature

Increase in intake temperature increases thecompressed air temperature resulting in reduceddelay period.

However, preheating of the charge for thispurpose would be undesirable because it wouldreduce the density of air reducing the volumetricefficiency and power output


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Intake Pressure

Increase in intake pressure or,superchargingreduces the autoignition temperature and hencereduces the delay period. The peak pressure willbe higher since the compression pressure willincrease with intake pressure. Table 12.2 givesthe summary of the factors which influence thedelay period in an engine


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c.i Delay Period