174

CHAPTER 8

EFFECT OF INJECTION TIMING ON

SPRAY CHARACTERISTICS AND COMBUSTION

8.1 Introduction

Apart from the swirl ratio and the number of nozzle holes, injection timing

plays an important role on the performance of DI diesel engine. From the literature

survey, it was observed that the injection timing influences the combustion process in

turn affects the emissions. Common rail fuel injection systems have the technical

advantages of controlling the injection pressures and the injection timing. The

injection timing directly influences the formation of emissions.

In the present study, an attempt was made to understand the effect of injection

timing on combustion and spray characteristics. The best configuration that was

analyzed based on the number of nozzle holes was considered here for the further

study. Three fuel injection strategies viz., conventional-injection, early-injection and

retarded-injection are considered for the study. The effect of injection timing on

combustion was analyzed through the pressure, heat release rate, temperature and

emission plots against crank angle. The effect of injection timing on spray

characteristics were studied by considering the spray penetration length, spray cone

angle and sauter mean diameter plots against the crank angles. The analysis was

performed on a DI diesel engine using Ricardo VECTIS.

Figures 8.1, 8.2 and 8.3 present the three injection timings that are considered

for the present study. Conventional-injection profile was shown in the figure 8.1. Here

the fuel injection starts at 12.5 degrees BTDC. The injection ends at 7.5 degrees

ATDC. Early-injection profile was shown in the figure 8.2. Here the fuel injection

starts at 17.5 degrees BTDC. The injection ends at 2.5 degrees ATDC. Retarded-

175

injection profile was shown in the figure 8.3. Here the fuel injection starts at 7.5

degrees BTDC. The injection ends at 17.5 degrees ATDC. Thus the total injection

duration for all the considered cases was maintained as 20 degrees.

Figure 8.1: Conventional-Injection

Figure 8.2: Early-Injection

176

Figure 8.3: Retarded-Injection

8.2 Best Injection Timing for 8 Holes Nozzle.

In this part of the work suitable injection timing among 7.5, 12.5 and 17.5

degrees BTDC for the 8 holes nozzle was determined. Exclusive analysis was made in

the following section in the process of identifying the suitable injection timing at

which the engine can perform better. They are:

8.2.1 Effect of Injection Timing on Pressure.

Figure 8.4 represents the pressure variations against crank angle. This provides

comparison for pressure profile against crank angle between the different the injection

timings viz., SOI 342.5 CA, SOI 347.5 CA and SOI 352.5 CA. Here SOI stands for

the start of injection. It may be observed that the angle at which the raise in pressure is

different for the cases under consideration. There noticed a drop in peak pressure

continuously for the considered cases from SOI 342.5 CA to 352.5 CA. It may also be

observed that the peak pressures are moving away from TDC. Among the three cases

that were reported here, SOI 342.5 CA case is having the maximum peak pressure (14

MPa).

177

Figure 8.4: Comparison of Pressure Variations Amongst Different SOI

Against Crank Angle

In the case of SOI 342.5 CA, the fuel was injected at an early stage of

compression. At this condition the in-cylinder temperature and pressure are normally

low when compared to the conventional-injection case. This increases the ignition

delay. The increased ignition delay improves the air-fuel mixture formation. These are

in good agreement with the discussions of Zhu et al., (2010) [104]. This causes for the

raise in peak pressure.

In the case of SOI 352.5 CA, the fuel was injected at a later stage of

compression (compared to the other cases). At this condition the in-cylinder

temperature and pressure are normally high when compared to the conventional-

injection case. This reduces the ignition delay. The reduced ignition delay doesn’t

permit the appropriate air-fuel mixture formation. These are in good agreement with

the discussions of Zhu et al., (2010,2003) [104, 119]. This causes for a fall in the peak

pressure. The change in delay period between the cases is playing a role on the change

in crank angle at which peak pressures are reported.

178

8.2.2 Effect of Injection Timing on Heat Release Rate.

Figure 8.5 represents the heat release rate variations against crank

angle. This provides comparison for heat release rate profile against crank angle

between the different the injection timings viz., SOI 342.5 CA, SOI 347.5 CA and

SOI 352.5 CA. It may be observed that the angle at which the raise in heat release rate

is different for the cases under consideration. There noticed a drop in peak heat

release rate continuously for the considered cases from SOI 342.5 CA to 352.5 CA.

Figure 8.5: Comparison of Heat Release Rate Variations Amongst Different

SOI Against Crank Angle

In the case of SOI 342.5 CA, the fuel was injected at an early stage of

compression. This increases the ignition delay. The increased ignition delay improves

the air-fuel mixture formation. These discussions are matching with the discussions of

Gunabalan et al., (2009) [116]. Early injection and longer ignition delay gives a scope

to accumulate more evaporated fuel before the start of combustion. This leads to

improve the combustion rate. This would have caused for the raise in peak heat

release rate.

179

In the case of SOI 352.5 CA, the fuel was injected at a later stage of

compression (compared to the other cases). This reduces the ignition delay. The

reduced ignition delay doesn’t permit the appropriate air-fuel mixture formation.

These discussions are matching with the discussions of Gunabalan et al., (2009)

[116]. This reduces the combustion rate. This would have caused for the fall in peak

heat release rate.

8.2.3 Effect of Injection Timing on Temperature.

Figure 8.6 represents the temperature variations against crank angle. This

provides comparison for temperature profile against crank angle between the different

the injection timings viz., SOI 342.5 CA, SOI 347.5 CA and SOI 352.5 CA. It may be

observed that the angle at which the raise in temperature is different for the cases

under consideration. There noticed a drop in peak temperature continuously for the

considered cases from SOI 342.5 CA to 352.5 CA.

Figure 8.6: Comparison of Temperature Variations Amongst Different

SOI Against Crank Angle

180

In the case of SOI 342.5 CA, the increased ignition delay improves the air-fuel

mixture formation. Early injection and longer ignition delay gives a scope to

accumulate more evaporated fuel before the start of combustion. This leads to

improve the combustion rate. This would have caused for the raise in peak

temperature.

In the case of SOI 352.5 CA, there noticed a reduction in the ignition delay.

The reduced ignition delay doesn’t permit the appropriate air-fuel mixture formation.

This reduces the combustion rate. This would have caused for the fall in peak

temperature.

8.2.4 Effect of Injection Timing on Swirl Speed.

Figure 8.7 represents the swirl speed variations against crank angle. This

provides comparison for swirl speed profiles against crank angle between the different

the injection timings viz., SOI 342.5 CA, SOI 347.5 CA and SOI 352.5 CA.

Figure 8.7: Comparison of Swirl Speed Variations Amongst Different

SOI Against Crank Angle

181

From the figure it may be observed, the profiles of swirl velocity are same up

to 342.5 degrees CA. There is noticed a small change in the profiles beyond 342.5

degrees CA. This is due to the change in the SOI for the cases under consideration.

Due to this there is noticed a difference in crank angles at which combustion has

started. This can be clearly observed in the figure 8.5. This is the reason for the minor

deviations in the profiles of swirl speed.

8.2.5 Effect of Injection Timing on Penetration Length, Sauter Mean Diameter

and Spray Angle.

Figure 8.8 shows the spray tip penetration lengths versus crank angle against

the different the injection timings viz., SOI 342.5 CA, SOI 347.5 CA and SOI 352.5

CA. From the figure it may be observed that the maximum penetration length is

increasing by a small amount with an increase in the angle of SOI. Maximum

penetration lengths for the cases SOI 342.5 CA, SOI 347.5 CA and SOI 352.5 CA

were reported to be 0.052m, 0.053m and 0.056m respectively.

Figure 8.8: Comparison of Penetration Lengths Amongst Different SOI

Against Crank Angle

182

SOI 342.5 CA - is a case of early injection. Fuel injection starts at an early

stage of compression. Piston has to cover a distance in proportion to 17.5 degrees CA

in order to reach TDC. As the fuel being injected is continued for a period of 20

degrees CA from 17.5 BTDC, the span of fuel injection continues up to 2.5 degrees

ATDC. This would have contributed for the reduction in the penetration length.

In the present case, 87.5% of fuel was injected while the piston was moving

from BDC to TDC and 12.5% of fuel was injected while the piston was moving from

TDC to BDC. Maximum quantity of the fuel was injected during compression stroke.

This would have caused for the reduction in the penetration length.

SOI 347.5 CA - is a case of conventional injection. Fuel injection starts at a

later stage of compression when compared to that of SOI 342.5 CA case. In this case,

piston has to cover a distance in proportion to 12.5 degrees CA in order to reach TDC.

As the fuel being injected is continued for a period of 20 degrees CA from 12.5

BTDC, the span of fuel injection continues up to 7.5 degrees ATDC. This would have

favored for the increase in penetration length when compared to that of SOI 342.5

CA.

In the present case, 62.5% of fuel was injected while the piston was moving

from BDC to TDC and 37.5% of fuel was injected while the piston was moving from

TDC to BDC. The quantity of the fuel that was injected during compression stroke is

a less when compared to that of SOI 342.5 CA. And the quantity of the fuel that was

injected during expansion stroke is a more when compared to that of SOI 342.5 CA.

This would have caused for the increase in penetration length.

SOI 352.5 CA - is a case of retarded injection. Fuel injection starts at a later

stage of compression when compared to that of SOI 347.5 CA case. In this case,

piston has to cover a distance in proportion to 7.5 degrees CA in order to reach TDC.

183

As the fuel being injected is continued for a period of 20 degrees CA from 7.5 BTDC,

the span of fuel injection continues up to 12.5 degrees ATDC. This would have

favored for the further increase in penetration length when compared to that of SOI

347.5 CA.

In the present case, 37.5% of fuel was injected while the piston was moving

from BDC to TDC and 62.5% of fuel was injected while the piston was moving from

TDC to BDC. The quantity of the fuel that was injected during compression stroke is

a less when compared to that of SOI 347.5 CA. And the quantity of the fuel that was

injected during expansion stroke is a more when compared to that of SOI 347.5 CA.

This would have caused for the increase in penetration length.

Figure 8.9: Comparison of Sauter Mean Diameters Amongst Different SOI

Against Crank Angle

Figure 8.9 shows the sauter mean diameter (SMD) versus crank angle against

different injection timings viz., SOI 342.5 CA, SOI 347.5 CA and SOI 352.5 CA.

This provides a comparison amongst the droplet SMDs for various injection timings.

184

From the figure, it is observed that the SMD values are continuously

decreasing for all the cases under consideration. The SMDs are observed to lie

between 19.78 and 19.97 µm.

It may also be observed that the highest value of SMD for the SOI 352.5 CA

case is 19.88 µm whereas it is about 19.97 µm for the other two cases. And the lowest

value of SMD for the SOI 352.5 CA case is 19.78 µm whereas it is about 19.81 µm

for the other two cases. This deviation is due to the fact that-in the case of retarded

injection (SOI 352.5 CA), majority portion (about 62.5%) of the fuel was injected in

to an environment having higher pressure and temperatures when compared to the

other cases. This would have contributed for this deviation.

Figure 8.10: Comparison of Spray Angles Amongst Different SOI

Against Crank Angle

Figure 8.10 shows the spray angle (or spray cone angle) versus crank angle

against different injection timings viz., SOI 342.5 CA, SOI 347.5 CA and SOI 352.5

CA. This provides a comparison amongst the spray angles for various injection

timings. This behaviour is totally supporting the discussions of Sovani et al., (2001)

[149].

185

8.2.6 Effect of Injection Timing on Emissions – NOx, Soot and CO.

Figure 8.11 shows the NOx mass fraction versus crank angle against different

injection timings viz., SOI 342.5 CA, SOI 347.5 CA and SOI 352.5 CA. This

provides a comparison amongst the spray angles for various injection timings.

It may be observed that the NOx levels are higher for SOI 342.5 CA when

compared with the cases SOI 347.5 CA and SOI 352.5 CA. In this case fuel injection

was started early by 5 degrees CA. Because of this maximum quantity of fuel was

made available within the critical period i.e., between the start of combustion and

shortly after the occurrence of peak cylinder pressure. This would have contributed

for the rise in NOx levels.

Figure 8.11: Comparison of Mass fractions of NOx Amongst Different SOI

Against Crank Angle

Considering the SOI 347.5 CA case, quantity of fuel that is available within

the critical period is less than that of SOI 342.5 CA case. Due to this the peak

temperature (figure 8.6) and peak pressure (figure 8.4) are observed to be less than

that of SOI 342.5 CA. This is the reason for the reduction in the levels of NOx.

186

Considering the SOI 352.5 CA case, quantity of fuel that is available within

the critical period is less than that of SOI 347.5 CA case. Due to this the peak

temperature (figure 8.6) and peak pressure (figure 8.4) are observed to be less than

that of SOI 347.5 CA. This is the reason for the further reduction in the levels of NOx.

Figure 8.12: Comparison of Mass Fractions of Soot Amongst Different SOI

Against Crank Angle

Figure 8.12 shows the soot mass fraction versus crank angle against different

injection timings viz., SOI 342.5 CA, SOI 347.5 CA and SOI 352.5 CA. This

provides a comparison amongst the soot profiles for various injection timings.

The soot formation takes place in the diesel combustion environment at

temperatures between about 1000 and 2800 K, at pressures higher than 5 MPa up to

10 MPa and with sufficient levels of air to burn the fuel completely.

From the figure 8.12 it may be observed that the soot profiles are more or less

following the same trend. There observed a small deviation at the beginning of the

soot formation in the case of SOI 342.5 CA. This is due to the fact that, the fuel was

injected in to the combustion chamber prior to the critical period.

187

Figure 8.13 Comparison of Mole Fractions of CO Amongst Different SOI

Against Crank Angle

Figure 8.13 shows the carbon monoxide mole fraction with respect to crank

angle against different injection timings viz., SOI 342.5 CA, SOI 347.5 CA and SOI

352.5 CA. This provides a comparison amongst the CO profiles for various injection

timings.

In general, the diesel engines operate at lean side of stoichiometric air-fuel

ratios, the CO emissions are normally found to be very low. In spite of that their

behaviour was taken in account to understand the deviation that might reflect on

combustion behaviour.

From figure 8.13 it is observed that, the point of soot formation is different for

the cases under consideration. This is due to the fact the SOI is differing from case to

case. It may also be observed that the CO profiles are more or less following the same

trend.

There is noticed some deviation in the CO formation rates between the cases

under consideration. This is due to the difference in the combustion rates between the

cases. From the figure 8.5 it can be observed that the maximum peak heat release rate

188

is for SOI 342.5 CA case when compared to the other two cases. From the same plot

it may also be observed that the peak heat release rate is decreasing from SOI 342.5

CA to SOI 352.5 CA. From this it can be understood that most of the fuel would have

burned in the case of SOI 342.5 CA. This is the reason behind the lowest CO

formation for SOI 342.5 CA case. The CO formation is observed to increase as the

peak heat release rate is decreasing. This is due to the fact that the quantity of fuel that

is participating in combustion would be decreasing from SOI 342.5 CA to SOI 352.5

CA.

8.3 Conclusions

In the process of determining the best injection time, study was carried out for

three injection times viz., SOI 342.5 CA, SOI 347.5 CA and SOI 352.5 CA and

various results were compared. NOx emissions are found to be low for SOI 352.5 CA

case when compared to the other two. Soot levels are found to be more or less same

for all the three cases. Whereas, CO levels are found to be low for SOI 342.5 CA.

Considering the peak pressure, maximum peak pressure is observed at SOI 342.5 CA.

A conclusion was derived by compromising on the NOx levels that the SOI 342.5 CA

case can be judged as the best one out of the three cases.