5.4 Laminar burning velocity correlation
5.4.6 Influence of dilution method
As mentioned in Chapter 2, lean operation (diluted by air) and/or EGR dilution are among the key techniques to increase the engine efficiency. The laminar burning velocity of a fuel-oxidizer mixture strongly depends on the quantity of air and/or EGR ratio [187]. In order to better understand the effect of dilution methods, lean mixtures are compared to a stoichiometric mixture diluted with burned gases using the developed correlations. To indicate the dilution of air, EGR and R-EGR mixtures, the fuel-to-charge equivalence ratio φ′is used, as in equation 2.8.
Figure 5.16 shows the uLof methanol and the SG50 blend, when they are diluted by air or EGR mixtures, as well as methanol-air combustion diluted by R-EGR
mixture. This illustration is for a pressure of 20 bar and an unburned temperature of 650 K. For both fuels, MeOH and SG50, the uLfor air dilution exceeds that of the EGR diluted mixture because the uLis related to the mixture O2concentration as well as the reaction front temperature [188]. The heat capacity of the burned gases (including CO2, N2and water vapour) is higher than that of air, which leads to a stronger decrease of the reaction front temperature for the EGR dilution. However, for the dilution with the R-EGR mixture, the laminar burning velocity is faster than that of air-diluted methanol combustion when φ′drops below 0.75 (corresponding to YEGR>0.25 in the R-EGR concept). The necessary Tuas a function of φ′to maintain constant uLat 100 cm/s is presented in Figure 5.17 (again for 20 bar). As can be seen, the required Tuincreases slightly when diluting with R-EGR mixtures for lower φ′. In contrast, the required Tuto maintain constant uL for the air and EGR diluted cases increases dramatically. The ∆Tubetween EGR diluted MeOH and SG50 combustion is almost constant at 44 K.
0
Fuel-to-charge equivalence ratio ϕ' (-)
MeOH - Air dilution
Based on the required moles of methanol to generate one mole of syngas with reaction R3.9, the molar/mass fraction of methanol supplied from the second injector (to the reformer catalyst) has been calculated for different syngas blend ratios. The mass fraction of methanol used for the reforming reaction, YMeOH,catalyst is the ratio of mass flow rate of methanol to the catalyst to the total mass flow rate, as follows:
YMeOH,catalyst=
˙
mMeOH,catalyst
˙
mMeOH,engine+m˙MeOH,catalyst
(5.8)
LAMINAR BURNING VELOCITY 127
Fuel-to-charge equivalence ratio ϕ' (-)
MeOH - Air dilution
Figure 5.17: Required Tuas a function of overall dilution φ′to maintain uL= 100 (cm/s) at 20 bar.
In the R-EGR concept, the mass flow rate of methanol to the catalyst is determined by the water concentration in the EGR gases. The ratio of water to methanol is kept at a constant value, 1.5 as in the previous section. Therefore, YMeOH,catalyst
increases with rising EGR levels (decreasing φ′). YMeOH,catalyst reaches 1 at an EGR ratio of 42.8%, thus constituting the R-EGR limit. The relationship between YMeOH,catalystand φ′is illustrated in the lower graph of Figure 5.18. For the syngas blends, horizontal lines result, as the dilution can be set independently from the fuel composition. For example, to produce the SG50 blend, a blend with 50 vol%
methanol and 50 vol% syngas, around 20% of the total methanol mass is injected into the catalyst, and 80% is supplied directly to the engine. For the R-EGR, there is a fixed relation between YMeOH,catalyst and φ′. Taking the SG50 case as an example, with the same YMeOH,catalyst, the EGR ratio in the R-EGR concept is
∼13% (φ′= 0.87).
The top graph of Figure 5.18 plots the ratios of uL,R−EGR/uL,Air and uL,EGR/uL,Air
as a function of φ′to compare the two engine concepts. The uL,R−EGRand uL,EGR indicate the uLof the mixture diluted by R-EGR and EGR at φ = 1, respectively.
The uL,Air indicates the uL when diluted with air (lean operation). For the EGR and air dilution cases, six fuel mixtures are plotted: MeOH to SG50. Because in the developed correlation, the influence of XSGon the dilution term is neglected, the uL,EGR/uL,Airratio versus φ′is independent of the syngas ratio. Therefore, only one curve of uL,EGR/uL,Air is plotted in the top graph of Figure 5.18. Because the denominator, uL,Air of methanol/syngas blends, improves (increases) with increasing syngas ratio (as shown in Figure 5.15) and there is only one value
of uL,R−EGR at each φ′ (numerator), the uL,R−EGR/uL,Air ratio decreases with the higher content of syngas in the blends. Six curves of uL,R−EGR/uL,Air vs. φ′ are illustrated for different syngas ratios, with φ′being confined by the R-EGR limit.
The ratio uL,R−EGR/uL,Airat φ′= 1 respectively equals 1, 0.975, 0.947, 0.917, 0.884 and 0.847 for XSGincreasing from 0, 0.1 to 0.5.
At the same φ′ (YEGR) and the same YMeOH,catalyst, the combustion diluted by R-EGR mixtures is faster than that of normal EGR dilution. For instance, the uL,EGR/uL,Airequals ∼0.7 and the uL,R−EGR/uL,Airequals ∼0.81 for SG50 at a φ′of 0.87. This means LBV in the R-EGR concept is faster than in the EFR concept with the same YMeOH,catalyst and EGR ratio. A similar trend is found for other syngas molar fractions. This can be explained by the increase of hydrogen concentration and the reduction of water vapour in the reactants, which is shown in Figure 5.9.
0.4
Fuel-to-charge equivalence ratio ϕ' (-) MeOH
Fuel-to-charge equivalence ratio ϕ' (-) R-EGR SG50
Figure 5.18: Ratio of uLat 20 bar, 650 K for different dilution methods, EGR relative to air, and mass fraction of methanol delivered to reformer catalyst as a function of φ′.
If NOx emissions are not considered, dilution with air is preferable because it produces a faster laminar burning velocity if a small amount of burned gases (less than 25%) is recirculated to the intake. As can be seen in Figure 5.18, the uL,R−EGR/uL,Air ratio is greater than 1 when φ′is smaller than 0.744, 0.722, 0.7,
LAMINAR BURNING VELOCITY 129
0.682, 0.659 and 0.643 (corresponding to EGR ratios greater than 25.6%, 27.8%, 30%, 31.8%, 34.1% and 35.7%, respectively) for a syngas ratio XSGof 0, 0.1, 0.2, 0.3, 0.4 and 0.5, respectively.
Figure 5.19 compares the uL of an undiluted methanol-air mixture at φ = 1 to four diluted mixtures at constant dilution factor, φ′ = 0.87, for an isentropic compression starting from atmospheric conditions (γ = 1.35). Compared to other diluted mixtures, the air dilution case (lean operation at φ = 0.87) offers the fastest burning velocity. Comparing the other dilution methods (at φ = 1), it can be seen that the EGR diluted SG50 and R-EGR diluted case have an increased uLcompared to normal EGR dilution due to the combustion of hydrogen-rich mixtures. In both cases (SG50 and R-EGR), the same XMeOH,catalyst and YEGR are used, 0.2 and 0.13 respectively. Although it is not possible to recover the same uL as without EGR, the R-EGR concept provides faster burning velocities than the EFR concept.
Additionally, the required volume of the vehicle’s fuel tank in the R-EGR concept is much smaller than that of the EFR one (as there is no need for a water tank) [189]. For these reasons, the R-EGR concept is recommended for supporting high dilution levels when an SI engine is operated with a stoichiometric mixture.
0 40 80 120
10 20 30 40 50
uL(cm/s)
Pressure (bar)
MeOH, Φ = 1 MeOH, Φ = 0.87 MeOH, Φ = 1 SG50, Φ = 1 R-EGR, Φ = 1 Isentropic compression, γ = 1.35
Reference condition:
p = 1 bar, Tu= 300 K
YEGR= 0.13 ϕ'= 0.87
Figure 5.19: uLas a function of pressure under isentropic compression.