**4.2. Influence of valve timing on combustion duration**

The influence of the valve timing strategy on dilution mass fraction and on the duration of combustion is presented here. As discussed in section 2.3.2, valve timing exerts a strong influence on mixture preparation by altering the amount of exhaust gas internally recirculated from one engine cycle to the following one. Dilution mass fraction measurements as a function of valve overlap are presented in figures 3 and 4 for three representative engine speeds, at each of two fixed engine loads (kept constant by acting on the throttle valve position) and spark timings. The spark ignition advance was kept unvaried at 25 CA degrees BTDC for the low load cases and at 14 CA degrees BTDC for the high load cases. The valve timing setting was changed as described in section 4.1; figure 3 refers to fixed EVC timing and figure 4, which shows similar distributions, refers to fixed IVO timing. Levels of dilution are greatest at lowload, low-speed conditions, because of a stronger exhaust gas back-flow when the intake and exhaust valves are overlapping. As expected, dilution mass fraction is an increasing function of valve overlap and, across regions of positive overlaps, it rises at increasing rate as the overlap value increases. For small values of either positive or negative valve overlap, the dilution fraction is relatively constant. When the valve overlap grows negatively (producing wider valve events separation), relatively small increments in dilution are due to early EVC, which has the effect of trapping more residuals, or to late IVO, which reduces the amount of fresh air trapped inside the cylinder.

degrees ATDC. Figure 9 illustrates the general effect of increasing charge dilution on MFB and burning rate characteristics, for fixed engine speed of 1900 rev/min and fixed intake pressure of 60 kPa. As expected, increasing dilution tends to reduce the strength of combustion, as indicated by the peak burning rate in kg/s, and stretches its duration over larger CA intervals for both the development and the rapid burning stages. Representative results for the variation of FDA and RBA with dilution mass fraction for three engines speed, at each of two engine loads and spark advances, are given in figures 10 and 11. Both combustion intervals are seen to increase linearly at a rate which is essentially independent of engine load and spark timing [45]. As discussed in the previous section, the gradients of these linear correlations, particularly for the RBA, are only slightly biased towards greater engine speeds, as a result of extending combustion further along the expansion stroke, into regions of lower temperature and pressure. When the level of dilution is varied by means of cooled external-EGR, combustion duration increases at a slightly higher rate than the case of dilution changes from increasing valve overlap. This is explained considering that charge temperature and charge density variations, which occur at the same time as dilution changes when the valve timing is modified,

Premixed Combustion in Spark Ignition Engines and the Influence of Operating Variables

http://dx.doi.org/10.5772/55495

21

Figures 12 to 14 illustrate the effects of engine speed, load and spark advance on combustion duration. Experimental data were again recorded under default valve timing setting, and the dilution level was kept unvaried by using appropriate rates of external EGR. In figure 12, the FDA and the RBA increase almost linearly with increasing engine speed, with gradients of variation which appear independent of engine load. The RBA increases more rapidly than the FDA because, as discussed above, greater engine speed would stretch the rapid stage of combustion further into the expansion stroke. Engine speed, as discussed in section 2.2, is directly proportional to turbulence intensity and therefore greater engine speed would lead to an augmented rate of combustion by means of increased unburned gas entrainment into the propagating flame front. However, increasing speed extends the burn process over wider CA intervals and the effect of greater turbulence intensity is only to moderate such extension. Doubling the engine speed between 1500 rev/min and 3000 rev/min stretches FDA by about 1/3 and RBA by 1/2. Figure 13 shows that FDA and RBA decrease linearly when plotted as a function of engine load, in terms of IMEP, at constant level of dilution mass fraction. Both burn angles decrease linearly also with increasing intake manifold pressure. The RBA decreases at an average rate of 2.7 CA degrees per 10 kPa increase in intake manifold pressure. The FDA decreases at a rate which is approximately half of the one calculated for the RBA. Some representative results concerning the variation of the burn angles with the degree of ST advance, at fixed dilution, are illustrated in figure 14. As the ignition timing is advanced towards the MBT setting, combustion initiates earlier in the compression stroke, i.e. at lower temperatures and pressures. Under the influence of these less favourable conditions for flame development, the FDA increases slightly. At the same time RBA, which cover the bulk of combustion duration, tends to decrease as the overall combustion phasing improves. The trends in figure 14 extend to STs more advanced than the MBT values, but the degree of overadvance was limited to 3–4 CA degrees to avoid the inception of knock and this was too small

tend to moderate the influence of dilution on burn rate.

to establish any turning point.

Representative results for the 0 to 10% MFB duration (FDA) are given in figures 5 and 6; those for the 10 to 90% MFB duration (RBA) are given in figures 7 and 8. The burn angles are plotted for three engine speeds and two levels of IMEP and spark advance. Both FDA and RBA increase consistently with increasing values of positive valve overlap. The increase in RBA is more pronounced than that in FDA, and proportionately greatest at low-load conditions (2.5 bar IMEP). The variations with overlap are similar for fixed intake and fixed exhaust timings, indicating that overlap phasing about TDC is not critical and the influence on combustion is exerted primarily through the overlap extension. The plotted trends are similar at all three engine speeds considered, with a small offset which reflects the inherent increase in burn duration as the speed increases. For small positive overlaps and for negative overlaps the burn angles do not show evident correlation with valve overlap. In these regions, the back-flow into the cylinder is reduced or does not occur at all, indicating that dilution mass fraction is the main cause of combustion duration alterations. Figure 5 to 8 show data which refer to operating conditions at which some variations of combustion duration was actually found. For running conditions exceeding about 6 bar IMEP and 3000 rev/min, combustion duration is almost independent of the valve timing setting.

The analysis of figures 3 to 8 suggests that the influence of dilution accounts for most of the variation in the rate of combustion with variable valve overlap. The effect of valve timing exerted through modifications to bulk motion and turbulence was not apparent in the data. Plots of RBA against dilution (not included here, but available in [42]) depict linear trends with gradients of variation only slightly biased towards greater engine speeds, and also independ‐ ent of the valve overlap phasing. Similar conclusions for part-load running conditions and intake valve-only variations have been drawn by Bozza et al. in [43], whereas Sandquist et al. [44] observed that the linearity between burn angles and charge dilution held only for fixed phasing, indicating that a dependence upon engine design is possible. The FDA also increases linearly with dilution mass fraction, though at a much weaker rate.

#### **4.3. Influence of other operating variables on combustion duration**

The influence of charge dilution upon rate and duration of combustion was explored also by means of separate tests carried out at fixed valve timing setting, to minimize any potential underlying influence on combustion, and using variable amounts of external EGR. Valve timing was set at default configuration, i.e. IVO = +6 CA degrees BTDC, and EVC = +6 CA degrees ATDC. Figure 9 illustrates the general effect of increasing charge dilution on MFB and burning rate characteristics, for fixed engine speed of 1900 rev/min and fixed intake pressure of 60 kPa. As expected, increasing dilution tends to reduce the strength of combustion, as indicated by the peak burning rate in kg/s, and stretches its duration over larger CA intervals for both the development and the rapid burning stages. Representative results for the variation of FDA and RBA with dilution mass fraction for three engines speed, at each of two engine loads and spark advances, are given in figures 10 and 11. Both combustion intervals are seen to increase linearly at a rate which is essentially independent of engine load and spark timing [45]. As discussed in the previous section, the gradients of these linear correlations, particularly for the RBA, are only slightly biased towards greater engine speeds, as a result of extending combustion further along the expansion stroke, into regions of lower temperature and pressure. When the level of dilution is varied by means of cooled external-EGR, combustion duration increases at a slightly higher rate than the case of dilution changes from increasing valve overlap. This is explained considering that charge temperature and charge density variations, which occur at the same time as dilution changes when the valve timing is modified, tend to moderate the influence of dilution on burn rate.

changed as described in section 4.1; figure 3 refers to fixed EVC timing and figure 4, which shows similar distributions, refers to fixed IVO timing. Levels of dilution are greatest at lowload, low-speed conditions, because of a stronger exhaust gas back-flow when the intake and exhaust valves are overlapping. As expected, dilution mass fraction is an increasing function of valve overlap and, across regions of positive overlaps, it rises at increasing rate as the overlap value increases. For small values of either positive or negative valve overlap, the dilution fraction is relatively constant. When the valve overlap grows negatively (producing wider valve events separation), relatively small increments in dilution are due to early EVC, which has the effect of trapping more residuals, or to late IVO, which reduces the amount of fresh air

Representative results for the 0 to 10% MFB duration (FDA) are given in figures 5 and 6; those for the 10 to 90% MFB duration (RBA) are given in figures 7 and 8. The burn angles are plotted for three engine speeds and two levels of IMEP and spark advance. Both FDA and RBA increase consistently with increasing values of positive valve overlap. The increase in RBA is more pronounced than that in FDA, and proportionately greatest at low-load conditions (2.5 bar IMEP). The variations with overlap are similar for fixed intake and fixed exhaust timings, indicating that overlap phasing about TDC is not critical and the influence on combustion is exerted primarily through the overlap extension. The plotted trends are similar at all three engine speeds considered, with a small offset which reflects the inherent increase in burn duration as the speed increases. For small positive overlaps and for negative overlaps the burn angles do not show evident correlation with valve overlap. In these regions, the back-flow into the cylinder is reduced or does not occur at all, indicating that dilution mass fraction is the main cause of combustion duration alterations. Figure 5 to 8 show data which refer to operating conditions at which some variations of combustion duration was actually found. For running conditions exceeding about 6 bar IMEP and 3000 rev/min, combustion duration is almost

The analysis of figures 3 to 8 suggests that the influence of dilution accounts for most of the variation in the rate of combustion with variable valve overlap. The effect of valve timing exerted through modifications to bulk motion and turbulence was not apparent in the data. Plots of RBA against dilution (not included here, but available in [42]) depict linear trends with gradients of variation only slightly biased towards greater engine speeds, and also independ‐ ent of the valve overlap phasing. Similar conclusions for part-load running conditions and intake valve-only variations have been drawn by Bozza et al. in [43], whereas Sandquist et al. [44] observed that the linearity between burn angles and charge dilution held only for fixed phasing, indicating that a dependence upon engine design is possible. The FDA also increases

The influence of charge dilution upon rate and duration of combustion was explored also by means of separate tests carried out at fixed valve timing setting, to minimize any potential underlying influence on combustion, and using variable amounts of external EGR. Valve timing was set at default configuration, i.e. IVO = +6 CA degrees BTDC, and EVC = +6 CA

linearly with dilution mass fraction, though at a much weaker rate.

**4.3. Influence of other operating variables on combustion duration**

trapped inside the cylinder.

20 Advances in Internal Combustion Engines and Fuel Technologies

independent of the valve timing setting.

Figures 12 to 14 illustrate the effects of engine speed, load and spark advance on combustion duration. Experimental data were again recorded under default valve timing setting, and the dilution level was kept unvaried by using appropriate rates of external EGR. In figure 12, the FDA and the RBA increase almost linearly with increasing engine speed, with gradients of variation which appear independent of engine load. The RBA increases more rapidly than the FDA because, as discussed above, greater engine speed would stretch the rapid stage of combustion further into the expansion stroke. Engine speed, as discussed in section 2.2, is directly proportional to turbulence intensity and therefore greater engine speed would lead to an augmented rate of combustion by means of increased unburned gas entrainment into the propagating flame front. However, increasing speed extends the burn process over wider CA intervals and the effect of greater turbulence intensity is only to moderate such extension. Doubling the engine speed between 1500 rev/min and 3000 rev/min stretches FDA by about 1/3 and RBA by 1/2. Figure 13 shows that FDA and RBA decrease linearly when plotted as a function of engine load, in terms of IMEP, at constant level of dilution mass fraction. Both burn angles decrease linearly also with increasing intake manifold pressure. The RBA decreases at an average rate of 2.7 CA degrees per 10 kPa increase in intake manifold pressure. The FDA decreases at a rate which is approximately half of the one calculated for the RBA. Some representative results concerning the variation of the burn angles with the degree of ST advance, at fixed dilution, are illustrated in figure 14. As the ignition timing is advanced towards the MBT setting, combustion initiates earlier in the compression stroke, i.e. at lower temperatures and pressures. Under the influence of these less favourable conditions for flame development, the FDA increases slightly. At the same time RBA, which cover the bulk of combustion duration, tends to decrease as the overall combustion phasing improves. The trends in figure 14 extend to STs more advanced than the MBT values, but the degree of overadvance was limited to 3–4 CA degrees to avoid the inception of knock and this was too small to establish any turning point.

> **1500rpm 2300rpm 3000rpm**

**1500rpm 2300rpm 3000rpm**

**1500rpm 2300rpm 3000rpm**

**1500rpm 2300rpm 3000rpm**

**FDA -**

**FDA -**

> **FDA -**

**FDA -**

um/high engine load

**CA degrees**

**CA degrees**

um/high engine load.

**CA degrees**

**CA degrees**



**Figure 5.** FDA as a function of valve overlap, at constant EVC timing; top plot: light engine load; bottom plot: medi‐

**Valve overlap - CA degrees**



**Valve overlap - CA degrees**

**Figure 6.** FDA as a function of valve overlap, at constant IVO timing; top plot: light engine load; bottom plot: medi‐

**Valve overlap - CA degrees**

**Valve overlap - CA degrees**

**IMEP = 2.5 bar ST = 25 CA BTDC EVC = 6 CA ATDC**

http://dx.doi.org/10.5772/55495

23

Premixed Combustion in Spark Ignition Engines and the Influence of Operating Variables

**IMEP = 6 bar ST = 14 CA BTDC EVC = 6 CA ATDC**

**IMEP = 2.5 bar ST = 25 CA BTDC IVO = 6 CA BTDC**

**IMEP = 6 bar ST = 14 CA BTDC IVO = 6 CA BTDC**

**Figure 3.** Measured charge dilution mass fraction as function of valve overlap, at constant EVC setting; top plot: light engine load; bottom plot: medium/high engine load.

**Figure 4.** Measured charge dilution mass fraction as function of valve overlap, at constant IVO setting; top plot: light engine load; bottom plot: medium/high engine load.

0.04 0.09 0.14 0.19 0.24

0.04

**1500rpm 2300rpm 3000rpm**

**1500rpm 2300rpm 3000rpm**

0.09

0.14

0.19

0.24

**1500rpm 2300rpm 3000rpm**

22 Advances in Internal Combustion Engines and Fuel Technologies

**1500rpm 2300rpm 3000rpm**

> 0.04 0.09 0.14 0.19 0.24

> > 0.04

0.09

0.14

0.19

0.24



**Valve overlap - CA degrees**

**Figure 3.** Measured charge dilution mass fraction as function of valve overlap, at constant EVC setting; top plot: light


**Valve overlap - CA degrees**


**Valve overlap - CA degrees**

**Figure 4.** Measured charge dilution mass fraction as function of valve overlap, at constant IVO setting; top plot: light

**IMEP = 2.5 bar ST = 25 CA BTDC EVC = 6 CA ATDC**

**IMEP = 6 bar ST = 14 CA BTDC EVC = 6 CA ATDC**

**IMEP = 2.5 bar ST = 25 CA BTDC IVO = 6 CA BTDC**

**IMEP = 6 bar ST = 14 CA BTDC IVO = 6 CA BTDC**

**Dilution Mass Fraction**

**Dilution Mass Fraction**

engine load; bottom plot: medium/high engine load.

**Dilution Mass Fraction**

**Dilution Mass Fraction**

engine load; bottom plot: medium/high engine load.

**Figure 5.** FDA as a function of valve overlap, at constant EVC timing; top plot: light engine load; bottom plot: medi‐ um/high engine load.

**Figure 6.** FDA as a function of valve overlap, at constant IVO timing; top plot: light engine load; bottom plot: medi‐ um/high engine load

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

0 0.02 0.04 0.06 0.08 0.1

**FDA -**

**CA degrees**

**Pin = 44 kPa ST = 26 CA BTDC**

**ST = 26 CA BTDC**

**RBA -**

**CA degrees**

**Burning rate - kg/s** **combustion**

**Spark**

**TDC of combustion**

**Spark**

**Mass Fraction Burned**

338 348 358 368 378 388 398 408 418 428 438 448 458

**Dil. = 8% Dil. = 12% Dil. = 15% Dil. = 18%**

http://dx.doi.org/10.5772/55495

25

**N = 1900 rev/min Pin = 60 kPa ST = 22 CA BTDC**

> **Dil. = 8% Dil. = 12% Dil. = 15% Dil. = 18%**

**N = 1900 rev/min Pin = 60 kPa**

**N = 1500 rev/min N = 1900 rev/min N = 2700 rev/min**

**N = 1500 rev/min N = 1900 rev/min**

 **- CA degrees**

338 348 358 368 378 388 398 408 418 428 438 448 458

**ST = 22 CA BTDC Dilution**

Premixed Combustion in Spark Ignition Engines and the Influence of Operating Variables

 **- CA degrees**

**Figure 9.** Charge burn characteristics at increasing dilution (by external EGR), for fixed operating conditions (N = 1900

0.06 0.09 0.12 0.15 0.18 0.21

**Figure 10.** Influence of charge dilution (by external EGR) on burn angle, at low engine load and fixed valve timing.

0.06 0.09 0.12 0.15 0.18 0.21

**Dilution Mass Fraction**

**N = 2700 rev/min Pin = 44 kPa**

**Dilution Mass Fraction**

rev/min; Pin = 60 kPa, intake manifold pressure; ST = 22 CA° BTDC) and fixed valve timing setting

**TDC of Dilution**

**Figure 7.** RBA as a function of valve overlap, at constant EVC timing; top plot: light engine load; bottom plot: medi‐ um/high engine load.

**Figure 8.** RBA as a function of valve overlap, at constant IVO timing; top plot: light engine load; bottom plot: medi‐ um/high engine load.

> **1500rpm 2300rpm 3000rpm**

**1500rpm 2300rpm 3000rpm**

**1500rpm 2300rpm 3000rpm**

**1500rpm 2300rpm 3000rpm**

**RBA -**

**RBA -**

> **RBA -**

**RBA -**

um/high engine load.

**CA degrees**

**CA degrees**

um/high engine load.

**CA degrees**

**CA degrees**

24 Advances in Internal Combustion Engines and Fuel Technologies



**Figure 7.** RBA as a function of valve overlap, at constant EVC timing; top plot: light engine load; bottom plot: medi‐


**Valve overlap - CA degrees**


**Figure 8.** RBA as a function of valve overlap, at constant IVO timing; top plot: light engine load; bottom plot: medi‐

**Valve overlap - CA degrees**

**Valve overlap - CA degrees**

**Valve overlap - CA degrees**

**IMEP = 2.5 bar ST = 25 CA BTDC EVC = 6 CA ATDC**

**IMEP = 6 bar ST = 14 CA BTDC EVC = 6 CA ATDC**

**IMEP = 2.5 bar ST = 25 CA BTDC IVO = 6 CA BTDC**

**IMEP = 6 bar ST = 14 CA BTDC IVO = 6 CA BTDC**

**Figure 9.** Charge burn characteristics at increasing dilution (by external EGR), for fixed operating conditions (N = 1900 rev/min; Pin = 60 kPa, intake manifold pressure; ST = 22 CA° BTDC) and fixed valve timing setting

**Figure 10.** Influence of charge dilution (by external EGR) on burn angle, at low engine load and fixed valve timing.

**RBA -**

**CA degrees**

**FDA -**

**CA degrees**

**RBA -**

**CA degrees**

**Dilution = 11.3% ST = 18 CA BTDC**

**Dilution = 11.3% ST = 18 CA BTDC**

**Dilution = 12%**

**IMEP = 4 bar Dilution = 12%**

**FDA -**

**CA degrees**

1 2 3 4 5 6 7 8

Premixed Combustion in Spark Ignition Engines and the Influence of Operating Variables

1 2 3 4 5 6 7 8

6 10 14 18 22 26 30 34

6 10 14 18 22 26 30 34

**Figure 14.** Influence of spark timing advance on burn angles, at constant level of dilution and fixed valve timing.

**Spark Ignition Timing (ST) - CA degree BTDC**

**2700rpm IMEP = 4 bar**

**MBT ST**

**MBT ST**

**Spark Ignition Timing (ST) - CA degree BTDC**

**IMEP (net) - bar**

**Figure 13.** Influence of engine load on burn angles, at constant level of dilution, and fixed valve timing.

**1500rpm 2700rpm**

http://dx.doi.org/10.5772/55495

27

**2 CA ST retard**

**1500rpm 2700rpm**

**2 CA ST retard**

**1500rpm**

**1500rpm 2700rpm**

**IMEP (net) - bar**

**Figure 11.** Influence of charge dilution (by external EGR) on burn angles, at medium/high engine load and fixed valve timing

**Figure 12.** Influence of engine speed on burn angles, at constant level of dilution and fixed valve timing

Premixed Combustion in Spark Ignition Engines and the Influence of Operating Variables http://dx.doi.org/10.5772/55495 27

26 Advances in Internal Combustion Engines and Fuel Technologies

**RBA -**

**CA degrees**

**FDA -**

**CA degrees**

**RBA -**

timing

**CA degrees**

**N = 1500 rev/min N = 1900 rev/min N = 2700 rev/min**

**N = 1500 rev/min N = 1900 rev/min N = 2700 rev/min**

**IMEP = 2.5 bar IMEP = 6.0 bar**

**IMEP = 2.5 bar IMEP = 6.0 bar**

**FDA -**

**CA degrees**

0.06 0.09 0.12 0.15 0.18 0.21

0.06 0.09 0.12 0.15 0.18 0.21

**Figure 11.** Influence of charge dilution (by external EGR) on burn angles, at medium/high engine load and fixed valve

**Dilution Mass Fraction**

1200 1500 1800 2100 2400 2700 3000 3300

1200 1500 1800 2100 2400 2700 3000 3300

**Figure 12.** Influence of engine speed on burn angles, at constant level of dilution and fixed valve timing

**Engine speed - rev/min**

**Engine speed - rev/min**

**Pin = 70 kPa ST = 16 CA BTDC**

**ST = 23 CA BTDC Dilution ~ 11%**

**ST = 15 CA BTDC Dilution ~ 7.8%**

**Dilution Mass Fraction**

**Figure 13.** Influence of engine load on burn angles, at constant level of dilution, and fixed valve timing.

**Figure 14.** Influence of spark timing advance on burn angles, at constant level of dilution and fixed valve timing.
