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The autoignition of Liquefied Petroleum Gas (LPG) in spark-ignition engines
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Proceedings of the Combustion Institute xxx (2014) xxx–xxx
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The autoignition of Liquefied Petroleum Gas (LPG) in spark-ignition engines Kai J. Morganti a, Michael J. Brear a,⇑, Gabriel da Silva b, Yi Yang a, Frederick L. Dryer c b
a Department of Mechanical Engineering, The University of Melbourne, Parkville, Victoria 3010, Australia Department of Chemical and Biomolecular Engineering, The University of Melbourne, Parkville, Victoria 3010, Australia c Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ 08544-5263, USA
Abstract This paper investigates the autoignition of C3/C4 hydrocarbon mixtures in a CFR octane rating engine. The four species examined – propane, propylene (propene), n-butane and iso-butane – are the primary constituents of Liquefied Petroleum Gas (LPG), and are also important intermediates in the oxidation of larger hydrocarbons. In-cylinder pressure data was acquired for both autoigniting and non-autoigniting engine operation at the same test conditions. The latter was used to calibrate a two-zone model of the CFR engine in a prior work, thus enabling the inclusion of the unburned charge chemical kinetics for further examination in this paper. The in-cylinder heat transfer and residual gas composition are both shown to affect autoignition significantly. In particular, physically reasonable concentrations of nitric oxide (NO) are found to be a strong promoter of autoignition in almost all cases, in keeping with several, more fundamental studies. The inclusion of NO in the residual gas is also required to obtain good agreement between the measured and modelled autoignition timing. This in turn suggests that kinetic interaction between hydrocarbon fuels and NO plays a vital role in octane rating, and its inclusion is important when modelling the autoignition of hydrocarbons in spark-ignition engines more generally. Ó 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Kinetic modelling; Engine autoignition; Liquefied Petroleum Gas; LPG; GT-Power
1. Introduction Liquefied Petroleum Gas (LPG) is a mixture of variable content, but is primarily composed of propane, propylene (propene), n-butane and isobutane [1]. LPG has several attractive features as ⇑ Corresponding author. Tel.:+61 3 8344 6722; fax: +61 3 9347 8784. E-mail address: [email protected] (M.J. Brear).
an alternative fuel. First, LPG vehicles can have lower emissions of both regulated pollutants and greenhouse gases than gasoline and diesel vehicles [2,3]. When stored in liquid form, LPG also has an energy density that is comparable to other liquid transport fuels. Finally, LPG is usually of relatively low cost. There are nonetheless only a few studies of LPG’s autoignition and knock in engines. These studies most commonly utilise correlation or reduced chemistry, e.g. [4–6], and so provide
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Table 1 The twelve LPG fuels examined in this paper, along with the corresponding k value for SKI. The engine-out emissions of carbon monoxide (CO), nitric oxide (NO) and the total unburned hydrocarbons (UHCs) were measured with a five gas analyser [9]. Fuel composition (% mol.)
kSKI
CO (% mol.)
NO (ppm)
UHCs (ppm C1)
Propane (Pr) Propylene (Pryl) n-Butane (nB) iso-Butane (iB) 70% Propane, 10% Propylene/n-Butane/iso-Butane 66.6% Propane, 16.7% Propylene/iso-Butane 66.6% Propane, 16.7% n-Butane/iso-Butane 50% Propane/Propylene 50% Propane/n-Butane 41.7% Propane/n-Butane, 16.6% Propylene 66.6% n-Butane, 16.7% Propane/iso-Butane 50% n-Butane/iso-Butane
0.96 1.03 0.90 0.90 0.95 0.96 0.93 0.98 0.91 0.92 0.90 0.90
1.70 0.20 3.69 3.65 2.00 1.70 2.64 1.16 3.33 3.20 3.69 3.62
1530 2410 792 890 1355 1530 1190 1610 920 990 855 875
912 222 720 700 765 792 822 540 750 685 765 722
limited insight into the general autoignition problem. Further, since the components of LPG are intermediates in the oxidation of larger hydrocarbons, an improved understanding of their autoignition contributes to our understanding of autoignition more generally. The aim of this paper is therefore to model the autoignition of arbitrary LPG blends in a sparkignition engine. The autoignition modelling is undertaken using a kinetic model derived from those of Healy et al. [7] and Dagaut and Dayma [8]. This kinetic model is coupled with a two-zone engine model that was validated for non-autoigniting operation in a prior work [9]. The in-cylinder heat transfer and residual gas composition are both shown to affect autoignition significantly. In particular, nitric oxide (NO) is found to be a strong promoter of autoignition, in keeping with several, more fundamental studies. Indeed, the inclusion of NO in the residual gas at physically reasonable concentrations enables good agreement between the measured and modelled autoignition timing in almost all cases. 2. Experimental methods The experimental data presented in this paper was acquired with a standard Cooperative Fuel Research (CFR) engine operated in accordance with the ASTM Research method (RON) test condition [10]. The standard engine was equipped with a gaseous fuel system that prepared mixtures of propane, propylene, n-butane and iso-butane. Each fuel was examined at the air–fuel ratio (k) that registered the maximum Knock Intensity (KI) value on the ASTM Knockmeter unit, as summarised in Table 1. The compression ratio was also set to the barometrically compensated value that corresponded to the measured RON of each fuel [1]. This is referred to as the so-called
Fig. 1. Measured in-cylinder pressure traces for nbutane at the SKI test condition. All data was obtained from a batch of 900 cycles (TDC = top dead centre, CR = compression ratio).
Standard Knock Intensity (SKI) test condition [10]. 2.1. In-cylinder pressure data In-cylinder pressure data was acquired with a thermally insulated Kistler 6125C piezoelectric pressure transducer. The data was sampled at 0.1 crank angle degree (CAD) intervals using a synchronised rotational encoder. The cycle-tocycle variability observed in all combustion engines was accounted for by determining a ‘representative’ raw pressure trace for each fuel. This trace minimised the sum of squared errors (SSE) for both the crank angle and the in-cylinder pressure at which autoignition was observed (relative to their average values from a batch of 900 cycles). Figure 1 shows the location of the autoignition onset in one such case, with this property defined as occurring when the in-cylinder pressure rise rate exceeds a fixed threshold. Similar results were obtained using other autoignition criteria from the literature, e.g. [11].
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2.1.1. Autoignition suppression using tetraethyllead The crank angle resolved mass fraction burned history can most easily be estimated from incylinder pressure data that does not exhibit autoignition. In this study, the autoignition observed for each fuel under standard octane rating conditions was therefore suppressed by adding a small amount of dilute tetraethyllead (TEL) fuel anti-knock compound to the intake charge. In almost all cases, the amount of dilute TEL required to suppress autoignition did not exceed 2% of the total fuel mass. Further analysis of the in-cylinder pressure data indicated that these small amounts of dilute TEL did not significantly affect the flame propagation [12]. This is consistent with the consensus in the literature, e.g. [13,14].
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Fig. 2. Measured and simulated non-autoigniting pressure histories for a 50/50 mixture of n-butane and isobutane. The simulated pressure history was obtained using the optimum f value.
3. Numerical methods
to propagate spherically from the side-mounted spark plug.
3.1. CFR engine model
3.4. Residual gas sub-models
The two-zone model of the CFR engine was developed using GT-Power [15]. The engine model incorporated the combustion chamber, together with sub-models of the intake and exhaust systems and the gas exchange processes. The full engine model was validated for non-autoigniting operation in a prior work [9].
The term f denotes a convective heat transfer coefficient multiplier, B denotes the cylinder bore, and w denotes the mean gas velocity [16]. The terms p and T denote the mixture pressure and temperature respectively. The heat transfer sub-model was calibrated for each fuel in a prior work [9] using the representative in-cylinder pressure trace with autoignition suppressed. The optimum value of f was obtained when the SSE between the measured and modelled pressure traces was minimised, as shown in Fig. 2. In all cases, the optimum value of f was 1.08–1.20 [9].
The residual gas fraction of the cylinder contains the combustion products that remain after closure of the exhaust valve. In our previous study [9], the residual gas fraction was assumed to contain only the stoichiometric products of combustion with dissociation neglected. Since this resulted in good agreement between the measured and modelled air flow rates through the engine, the modelling of the residual gas fraction should also be reasonable. The residual gas fraction was found to occupy approximately 5.5–7.0% of the cylinder mass at intake valve closure (IVC) [9]. This result is consistent with data reported in other studies that utilise CFR engines, e.g. [17]. These values were then used to compute the residual species concentrations within the cylinder at IVC for each fuel and operating condition. The blending of the residual gas with the fresh air–fuel mixture was performed using the standard GT-Power submodel [15]. This provided the in-cylinder mixture temperature and the concentrations of all species at the start of compression. The same procedure could also be used to determine the in-cylinder concentrations of CO and NO from the measured engine-out emissions data (Table 2). This procedure is discussed in more detail in a prior work [18].
3.3. Prescribing the flame propagation
3.5. Chemical kinetic model
The simulated pressure trace corresponding to the optimum f value was used to compute the crank angle resolved mass fraction burned history for each fuel. This established the amount of mass transported from the unburned to the burned zone at each time step. The flame was assumed
The autoignition modelling was undertaken by coupling the calibrated GT-Power engine model with a comprehensive chemical kinetic mechanism. The latter contained all reactions from the C1–C4 model of Healy et al. ([7]), together with the reactions that related only to
3.2. In-cylinder heat transfer sub-model The in-cylinder convective heat transfer coefficient was modelled using the Woschni correlation without swirl [16], hc ¼ f B0:2 p0:8 T 0:55 w0:8 :
ð1Þ
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Table 2 Estimated residual gas fraction [9] and the resulting in-cylinder concentrations of CO and NO at the start of compression. Fuel composition (% mol.)
Residual Fraction (% mass)
CO (% mol.)
NO (ppm)
Propane (Pr) Propylene (Pryl) n-Butane (nB) iso-Butane (iB) 70% Propane, 10% Propylene/n-Butane/iso-Butane 66.6% Propane, 16.7% Propylene/iso-Butane 66.6% Propane, 16.7% n-Butane/iso-Butane 50% Propane/Propylene 50% Propane/n-Butane 41.7% Propane/n-Butane, 16.6% Propylene 66.6% nB, 16.7% Propane/iso-Butane 50% n-Butane/iso-Butane
5.60 6.60 7.10 6.40 5.80 5.80 5.80 6.10 6.50 6.40 6.90 6.80
0.10 0.01 0.28 0.25 0.12 0.10 0.16 0.07 0.23 0.21 0.27 0.26
89 162 60 60 82 92 72 101 63 67 62 63
nitrogen chemistry from the C1–C2–NOx model of Dagaut and Dayma ([8]). Since the former did not contain nitrogen chemistry, this ensured that no chemical reactions were duplicated. This also provided a systematic means of choosing between competing rate coefficients, with the values from the more widely validated Healy et al. model retained in all instances. The final model contained 254 chemical species that participate in 2432 elementary reactions (refer to the Supplementary material). Validation of the model is discussed in a prior work [12].
timing in a physically intuitive manner. In the case of reducing f, the lower in-cylinder heat transfer between the mixture and the combustion chamber surfaces causes a significant increase in the end-gas temperature, along with a more advanced autoignition timing. This demonstrates that heat transfer is an important part of the autoignition modelling problem.
4. Results and discussion 4.1. Effect autoignition
of
in-cylinder
heat
transfer
on
The effect of heat transfer on the autoignition timing of propane was examined by adjusting the value of the convective heat transfer coefficient multiplier (f) in Eq. (1). The value of f was varied from 0.5 to 1.5, with all other model parameters remaining unchanged. Figure 3 indicates that heat transfer strongly influences the autoignition
Fig. 3. Simulated effect of variations in the value of f on the autoignition of propane.
4.2. Effect of carbon monoxide on autoignition The effect of carbon monoxide (CO) on the autoignition timing of the four neat fuels was next examined by varying the CO concentration at IVC between one and eight times the calculated residual value (Table 2). This upper value is well outside practical limits, but provides an insight into the sensitivity of autoignition to CO. Figure 4 indicates that CO has a weak effect on autoignition. In most cases, the presence of elevated levels of CO does not begin to influence the autoignition timing until its concentration significantly exceeds the calculated residual quantity
Fig. 4. Simulated variation in the autoignition timing for the neat fuels with the addition of CO. The filled symbols represent the calculated residual CO concentration at IVC, as reported in Table 2 (CAD ATDC = crank angle degrees after top dead centre).
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Table 3 Measured engine-out hydrocarbon emissions from a propane-fuelled SI engine (k = 0.96) [23]. Hydrocarbon
Concentration (ppm C1)
Propane Propylene Ethane Ethylene Methane Acetylene
894 138 27 239 97 53
Total
1448 Fig. 6. Measured and simulated pressure histories for n-butane.
Fig. 5. Simulated effect of unburned hydrocarbons on the autoignition timing of propane. Results are presented with and without the inferred NO concentration included in the residual gas.
Given the complexity of these processes, the residual combustion products contain a wide range of hydrocarbon species. These species are not usually identified in standard engine tests [22], as was the case in this study. The effect of these unburned hydrocarbons on the autoignition timing of propane was therefore examined using speciated exhaust emissions data reported for a similar propane-fuelled engine [23]. This data contained the unreacted propane concentration, along with the concentrations of several stable intermediate species (Table 3). The total engine-out UHC concentration measured in this study was approximately 1500 ppm C1, and shows reasonable agreement with the equivalent value measured in our previous study [9]. Assuming a residual gas fraction of 5.60% mass (Table 2), the corresponding residual UHC concentration that should be simulated is 88.1 ppm C1. Figure 5 indicates that the unburned hydrocarbons have a weak promoting effect on the autoignition timing when NO is not present in the residual gas. However, this weak promoting effect is no longer observed when NO is included in the residual gas. This suggests that the promoting effect of NO on the autoignition timing is far stronger than that of the unburned hydrocarbons. This is consistent with more fundamental studies for similar hydrocarbons, e.g. [24]. 4.4. Effect of nitric oxide on autoignition
listed in Table 2. As reported for other hydrocarbon fuels, e.g. [19,20], the addition of CO causes the autoignition of propane, n-butane and isobutane to be marginally suppressed. These results are consistent with the high octane rating of CO [21], which exceeds that of all LPGs [1]. 4.3. Effect autoignition
of
unburned
hydrocarbons
on
Unburned hydrocarbons (UHCs) are the result of several processes, including the desorption of hydrocarbon-rich gases from oil layers and crevice volumes, along with incomplete combustion.
The effect of nitric oxide (NO) on the autoignition of the various LPG fuels was next examined. In almost all cases, NO was found to have a promoting effect on the autoignition timing. As an example of this behaviour, the simulated pressure histories for n-butane are presented in Fig. 6. These results are compared with the representative autoigniting pressure trace acquired at the same engine operating condition. The corresponding measured and modelled autoignition timings are presented for all twelve LPG fuels in Fig. 7. These simulations were performed with the NO concentration varied between zero and twice the
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Fig. 7. Measured and simulated autoignition timing for the twelve LPG fuels. Autoignition was not observed for neat propylene when the residual NO concentration was doubled.
value inferred from the engine-out emissions data presented in Table 2. For the fuels composed primarily of paraffins, the simulated autoignition timing typically occurs 1–3 CAD (0.28–0.83 ms) more prematurely when the measured NO concentration is included in the residual gas. This is consistent with the promoting effect of NO on the oxidation of these hydrocarbons observed in more fundamental studies, e.g. [25–28]. For the paraffinic fuels, the simulated autoignition timing also shows reasonable agreement with the experimental autoignition timing. This suggests that the model is largely capturing the autoignition behaviour of these simple paraffins. The simulated autoignition behaviour for neat propylene differs considerably to that of the other fuels examined. As an example of this behaviour, Fig. 8 indicates that the inclusion of NO in the residual gas causes the autoignition timing to be inhibited. This effect becomes more prominent with increasing NO, and eventually leads to autoignition being fully suppressed once the NO concentration exceeds approximately 200 ppm. Given that the limited data in the literature indicates propylene can be effective promoter of NO to NO2 conversion [26,28], one might therefore expect its simulated autoignition behaviour in Fig. 8 to be more similar to that of the neat paraffins. The poor agreement observed between the simulated and measured autoignition timing for propylene in Fig. 7 can potentially be attributed to several factors. First, the kinetic model employed here does not contain fuel-specific hydrocarbon– NOx reaction pathways for hydrocarbons larger than ethane. The kinetic interactions between larger hydrocarbons and NOx are therefore only included via the pathways specific to methane and ethane. Given the good agreement observed
Fig. 8. Simulated variation in the autoignition timing for the neat fuels with the addition of NO. Negative values represent a more premature autoignition timing, and vice versa.
Fig. 9. Simulated fraction of the total fuel consumed by pre-flame kinetic activity for n-butane.
Fig. 10. Simulated end-gas temperature increase due to pre-flame kinetic activity for n-butane.
for the majority of fuels, this could indicate that the propylene–NOx specific interactions are more important than the equivalent interactions for propane, n-butane and iso-butane, with their absence leading to a net inhibiting effect on the autoignition timing. Propylene also has the highest residual NO concentration at IVC of the twelve fuels examined (Table 2). These results could therefore alternatively indicate an underlying sensitivity to elevated NO concentrations.
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Consistent with these uncertainties, the development of improved kinetic models that specifically address propylene oxidation remains an active area of research, e.g. [29–31]. 4.5. Pre-flame kinetic activity within the end-gas The simulated fraction of the total fuel consumed by pre-flame kinetic activity is presented for n-butane in Fig. 9. These results indicate that approximately twice as much fuel is consumed in the lead up to autoignition when NO is present in the residual gas (4% vs. 8% mass). This behaviour coincides with the more premature autoignition timing observed previously in Fig. 6, and ultimately leads to a significant increase in the fraction of the total fuel consumed by the final autoignition event itself (18% vs. 36% mass). These observations are consistent with those reported in motored engine studies for n-butane, e.g. [32]. The two-stage autoignition behaviour observed in more fundamental studies is also apparent, e.g. [33]. The effect of this pre-flame kinetic activity on the simulated end-gas temperature is considered in Fig. 10. This profile represents the difference between the calculated end-gas temperature with chemical kinetics implemented in the end-gas, and a second test case which assumed chemical equilibrium [9]. Even prior to the start of combustion, the pre-flame heat release is shown to raise the end-gas temperature by approximately 20 K. This is followed by a more rapid temperature increase once combustion has commenced, particularly in the test case where the measured NO concentration was included in the residual gas. This behaviour was also observed for the other paraffinic fuels examined in this paper. Overall, these results suggest that compression of the unburned charge by the piston is an important modelling consideration if accurate predictions of the end-gas temperature during combustion are to be achieved.
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inclusion of the unburned charge chemical kinetics for further examination in this paper. The in-cylinder heat transfer and residual gas composition were both shown to affect autoignition. In particular, nitric oxide (NO) was found to be a strong promoter of both fuel oxidation and autoignition for propane, n-butane and isobutane. For these fuels, the inclusion of NO in the residual gas at physically reasonable concentrations also enabled good agreement between the measured and modelled autoignition timing. The same agreement was not observed for neat propylene, suggesting that further research is required for this fuel. In all cases, physically reasonable concentrations of both carbon monoxide (CO) and unburned hydrocarbons (UHCs) had an insignificant effect on autoignition in comparison with NO. More broadly, these results suggest that kinetic interaction between hydrocarbon fuels and NO plays a vital role in octane rating, and its inclusion is important when modelling the autoignition of hydrocarbons in spark-ignition engines. Nonetheless, the fuel chemistry is not the only challenge. This work showed that several other phenomena, particularly the flame propagation, in-cylinder heat transfer and the residual gas composition also need to be modelled accurately if truly predictive models of autoignition are to be achieved. Acknowledgments This research was supported by the Advanced Centre for Automotive Research and Testing (ACART) and the Australian Research Council. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http:// dx.doi.org/10.1016/j.proci.2014.06.070. References
5. Conclusions This paper investigated the autoignition of C3/C4 hydrocarbon mixtures in a CFR octane rating engine. The four species examined – propane, propylene (propene), n-butane and iso-butane – are the primary constituents of Liquefied Petroleum Gas (LPG), and are also important intermediates in the oxidation of larger hydrocarbons. In-cylinder pressure data was acquired for both autoigniting and non-autoigniting engine operation at the same test conditions. The latter was obtained by adding a small amount of dilute TEL to the intake charge, and was used to calibrate a two-zone model of the CFR engine in a prior work. This enabled the
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Proceedings of the Combustion Institute xxx (2014) xxx–xxx
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The autoignition of Liquefied Petroleum Gas (LPG) in spark-ignition engines Kai J. Morganti a, Michael J. Brear a,⇑, Gabriel da Silva b, Yi Yang a, Frederick L. Dryer c b
a Department of Mechanical Engineering, The University of Melbourne, Parkville, Victoria 3010, Australia Department of Chemical and Biomolecular Engineering, The University of Melbourne, Parkville, Victoria 3010, Australia c Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ 08544-5263, USA
Abstract This paper investigates the autoignition of C3/C4 hydrocarbon mixtures in a CFR octane rating engine. The four species examined – propane, propylene (propene), n-butane and iso-butane – are the primary constituents of Liquefied Petroleum Gas (LPG), and are also important intermediates in the oxidation of larger hydrocarbons. In-cylinder pressure data was acquired for both autoigniting and non-autoigniting engine operation at the same test conditions. The latter was used to calibrate a two-zone model of the CFR engine in a prior work, thus enabling the inclusion of the unburned charge chemical kinetics for further examination in this paper. The in-cylinder heat transfer and residual gas composition are both shown to affect autoignition significantly. In particular, physically reasonable concentrations of nitric oxide (NO) are found to be a strong promoter of autoignition in almost all cases, in keeping with several, more fundamental studies. The inclusion of NO in the residual gas is also required to obtain good agreement between the measured and modelled autoignition timing. This in turn suggests that kinetic interaction between hydrocarbon fuels and NO plays a vital role in octane rating, and its inclusion is important when modelling the autoignition of hydrocarbons in spark-ignition engines more generally. Ó 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Kinetic modelling; Engine autoignition; Liquefied Petroleum Gas; LPG; GT-Power
1. Introduction Liquefied Petroleum Gas (LPG) is a mixture of variable content, but is primarily composed of propane, propylene (propene), n-butane and isobutane [1]. LPG has several attractive features as ⇑ Corresponding author. Tel.:+61 3 8344 6722; fax: +61 3 9347 8784. E-mail address: [email protected] (M.J. Brear).
an alternative fuel. First, LPG vehicles can have lower emissions of both regulated pollutants and greenhouse gases than gasoline and diesel vehicles [2,3]. When stored in liquid form, LPG also has an energy density that is comparable to other liquid transport fuels. Finally, LPG is usually of relatively low cost. There are nonetheless only a few studies of LPG’s autoignition and knock in engines. These studies most commonly utilise correlation or reduced chemistry, e.g. [4–6], and so provide
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Table 1 The twelve LPG fuels examined in this paper, along with the corresponding k value for SKI. The engine-out emissions of carbon monoxide (CO), nitric oxide (NO) and the total unburned hydrocarbons (UHCs) were measured with a five gas analyser [9]. Fuel composition (% mol.)
kSKI
CO (% mol.)
NO (ppm)
UHCs (ppm C1)
Propane (Pr) Propylene (Pryl) n-Butane (nB) iso-Butane (iB) 70% Propane, 10% Propylene/n-Butane/iso-Butane 66.6% Propane, 16.7% Propylene/iso-Butane 66.6% Propane, 16.7% n-Butane/iso-Butane 50% Propane/Propylene 50% Propane/n-Butane 41.7% Propane/n-Butane, 16.6% Propylene 66.6% n-Butane, 16.7% Propane/iso-Butane 50% n-Butane/iso-Butane
0.96 1.03 0.90 0.90 0.95 0.96 0.93 0.98 0.91 0.92 0.90 0.90
1.70 0.20 3.69 3.65 2.00 1.70 2.64 1.16 3.33 3.20 3.69 3.62
1530 2410 792 890 1355 1530 1190 1610 920 990 855 875
912 222 720 700 765 792 822 540 750 685 765 722
limited insight into the general autoignition problem. Further, since the components of LPG are intermediates in the oxidation of larger hydrocarbons, an improved understanding of their autoignition contributes to our understanding of autoignition more generally. The aim of this paper is therefore to model the autoignition of arbitrary LPG blends in a sparkignition engine. The autoignition modelling is undertaken using a kinetic model derived from those of Healy et al. [7] and Dagaut and Dayma [8]. This kinetic model is coupled with a two-zone engine model that was validated for non-autoigniting operation in a prior work [9]. The in-cylinder heat transfer and residual gas composition are both shown to affect autoignition significantly. In particular, nitric oxide (NO) is found to be a strong promoter of autoignition, in keeping with several, more fundamental studies. Indeed, the inclusion of NO in the residual gas at physically reasonable concentrations enables good agreement between the measured and modelled autoignition timing in almost all cases. 2. Experimental methods The experimental data presented in this paper was acquired with a standard Cooperative Fuel Research (CFR) engine operated in accordance with the ASTM Research method (RON) test condition [10]. The standard engine was equipped with a gaseous fuel system that prepared mixtures of propane, propylene, n-butane and iso-butane. Each fuel was examined at the air–fuel ratio (k) that registered the maximum Knock Intensity (KI) value on the ASTM Knockmeter unit, as summarised in Table 1. The compression ratio was also set to the barometrically compensated value that corresponded to the measured RON of each fuel [1]. This is referred to as the so-called
Fig. 1. Measured in-cylinder pressure traces for nbutane at the SKI test condition. All data was obtained from a batch of 900 cycles (TDC = top dead centre, CR = compression ratio).
Standard Knock Intensity (SKI) test condition [10]. 2.1. In-cylinder pressure data In-cylinder pressure data was acquired with a thermally insulated Kistler 6125C piezoelectric pressure transducer. The data was sampled at 0.1 crank angle degree (CAD) intervals using a synchronised rotational encoder. The cycle-tocycle variability observed in all combustion engines was accounted for by determining a ‘representative’ raw pressure trace for each fuel. This trace minimised the sum of squared errors (SSE) for both the crank angle and the in-cylinder pressure at which autoignition was observed (relative to their average values from a batch of 900 cycles). Figure 1 shows the location of the autoignition onset in one such case, with this property defined as occurring when the in-cylinder pressure rise rate exceeds a fixed threshold. Similar results were obtained using other autoignition criteria from the literature, e.g. [11].
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2.1.1. Autoignition suppression using tetraethyllead The crank angle resolved mass fraction burned history can most easily be estimated from incylinder pressure data that does not exhibit autoignition. In this study, the autoignition observed for each fuel under standard octane rating conditions was therefore suppressed by adding a small amount of dilute tetraethyllead (TEL) fuel anti-knock compound to the intake charge. In almost all cases, the amount of dilute TEL required to suppress autoignition did not exceed 2% of the total fuel mass. Further analysis of the in-cylinder pressure data indicated that these small amounts of dilute TEL did not significantly affect the flame propagation [12]. This is consistent with the consensus in the literature, e.g. [13,14].
3
Fig. 2. Measured and simulated non-autoigniting pressure histories for a 50/50 mixture of n-butane and isobutane. The simulated pressure history was obtained using the optimum f value.
3. Numerical methods
to propagate spherically from the side-mounted spark plug.
3.1. CFR engine model
3.4. Residual gas sub-models
The two-zone model of the CFR engine was developed using GT-Power [15]. The engine model incorporated the combustion chamber, together with sub-models of the intake and exhaust systems and the gas exchange processes. The full engine model was validated for non-autoigniting operation in a prior work [9].
The term f denotes a convective heat transfer coefficient multiplier, B denotes the cylinder bore, and w denotes the mean gas velocity [16]. The terms p and T denote the mixture pressure and temperature respectively. The heat transfer sub-model was calibrated for each fuel in a prior work [9] using the representative in-cylinder pressure trace with autoignition suppressed. The optimum value of f was obtained when the SSE between the measured and modelled pressure traces was minimised, as shown in Fig. 2. In all cases, the optimum value of f was 1.08–1.20 [9].
The residual gas fraction of the cylinder contains the combustion products that remain after closure of the exhaust valve. In our previous study [9], the residual gas fraction was assumed to contain only the stoichiometric products of combustion with dissociation neglected. Since this resulted in good agreement between the measured and modelled air flow rates through the engine, the modelling of the residual gas fraction should also be reasonable. The residual gas fraction was found to occupy approximately 5.5–7.0% of the cylinder mass at intake valve closure (IVC) [9]. This result is consistent with data reported in other studies that utilise CFR engines, e.g. [17]. These values were then used to compute the residual species concentrations within the cylinder at IVC for each fuel and operating condition. The blending of the residual gas with the fresh air–fuel mixture was performed using the standard GT-Power submodel [15]. This provided the in-cylinder mixture temperature and the concentrations of all species at the start of compression. The same procedure could also be used to determine the in-cylinder concentrations of CO and NO from the measured engine-out emissions data (Table 2). This procedure is discussed in more detail in a prior work [18].
3.3. Prescribing the flame propagation
3.5. Chemical kinetic model
The simulated pressure trace corresponding to the optimum f value was used to compute the crank angle resolved mass fraction burned history for each fuel. This established the amount of mass transported from the unburned to the burned zone at each time step. The flame was assumed
The autoignition modelling was undertaken by coupling the calibrated GT-Power engine model with a comprehensive chemical kinetic mechanism. The latter contained all reactions from the C1–C4 model of Healy et al. ([7]), together with the reactions that related only to
3.2. In-cylinder heat transfer sub-model The in-cylinder convective heat transfer coefficient was modelled using the Woschni correlation without swirl [16], hc ¼ f B0:2 p0:8 T 0:55 w0:8 :
ð1Þ
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Table 2 Estimated residual gas fraction [9] and the resulting in-cylinder concentrations of CO and NO at the start of compression. Fuel composition (% mol.)
Residual Fraction (% mass)
CO (% mol.)
NO (ppm)
Propane (Pr) Propylene (Pryl) n-Butane (nB) iso-Butane (iB) 70% Propane, 10% Propylene/n-Butane/iso-Butane 66.6% Propane, 16.7% Propylene/iso-Butane 66.6% Propane, 16.7% n-Butane/iso-Butane 50% Propane/Propylene 50% Propane/n-Butane 41.7% Propane/n-Butane, 16.6% Propylene 66.6% nB, 16.7% Propane/iso-Butane 50% n-Butane/iso-Butane
5.60 6.60 7.10 6.40 5.80 5.80 5.80 6.10 6.50 6.40 6.90 6.80
0.10 0.01 0.28 0.25 0.12 0.10 0.16 0.07 0.23 0.21 0.27 0.26
89 162 60 60 82 92 72 101 63 67 62 63
nitrogen chemistry from the C1–C2–NOx model of Dagaut and Dayma ([8]). Since the former did not contain nitrogen chemistry, this ensured that no chemical reactions were duplicated. This also provided a systematic means of choosing between competing rate coefficients, with the values from the more widely validated Healy et al. model retained in all instances. The final model contained 254 chemical species that participate in 2432 elementary reactions (refer to the Supplementary material). Validation of the model is discussed in a prior work [12].
timing in a physically intuitive manner. In the case of reducing f, the lower in-cylinder heat transfer between the mixture and the combustion chamber surfaces causes a significant increase in the end-gas temperature, along with a more advanced autoignition timing. This demonstrates that heat transfer is an important part of the autoignition modelling problem.
4. Results and discussion 4.1. Effect autoignition
of
in-cylinder
heat
transfer
on
The effect of heat transfer on the autoignition timing of propane was examined by adjusting the value of the convective heat transfer coefficient multiplier (f) in Eq. (1). The value of f was varied from 0.5 to 1.5, with all other model parameters remaining unchanged. Figure 3 indicates that heat transfer strongly influences the autoignition
Fig. 3. Simulated effect of variations in the value of f on the autoignition of propane.
4.2. Effect of carbon monoxide on autoignition The effect of carbon monoxide (CO) on the autoignition timing of the four neat fuels was next examined by varying the CO concentration at IVC between one and eight times the calculated residual value (Table 2). This upper value is well outside practical limits, but provides an insight into the sensitivity of autoignition to CO. Figure 4 indicates that CO has a weak effect on autoignition. In most cases, the presence of elevated levels of CO does not begin to influence the autoignition timing until its concentration significantly exceeds the calculated residual quantity
Fig. 4. Simulated variation in the autoignition timing for the neat fuels with the addition of CO. The filled symbols represent the calculated residual CO concentration at IVC, as reported in Table 2 (CAD ATDC = crank angle degrees after top dead centre).
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Table 3 Measured engine-out hydrocarbon emissions from a propane-fuelled SI engine (k = 0.96) [23]. Hydrocarbon
Concentration (ppm C1)
Propane Propylene Ethane Ethylene Methane Acetylene
894 138 27 239 97 53
Total
1448 Fig. 6. Measured and simulated pressure histories for n-butane.
Fig. 5. Simulated effect of unburned hydrocarbons on the autoignition timing of propane. Results are presented with and without the inferred NO concentration included in the residual gas.
Given the complexity of these processes, the residual combustion products contain a wide range of hydrocarbon species. These species are not usually identified in standard engine tests [22], as was the case in this study. The effect of these unburned hydrocarbons on the autoignition timing of propane was therefore examined using speciated exhaust emissions data reported for a similar propane-fuelled engine [23]. This data contained the unreacted propane concentration, along with the concentrations of several stable intermediate species (Table 3). The total engine-out UHC concentration measured in this study was approximately 1500 ppm C1, and shows reasonable agreement with the equivalent value measured in our previous study [9]. Assuming a residual gas fraction of 5.60% mass (Table 2), the corresponding residual UHC concentration that should be simulated is 88.1 ppm C1. Figure 5 indicates that the unburned hydrocarbons have a weak promoting effect on the autoignition timing when NO is not present in the residual gas. However, this weak promoting effect is no longer observed when NO is included in the residual gas. This suggests that the promoting effect of NO on the autoignition timing is far stronger than that of the unburned hydrocarbons. This is consistent with more fundamental studies for similar hydrocarbons, e.g. [24]. 4.4. Effect of nitric oxide on autoignition
listed in Table 2. As reported for other hydrocarbon fuels, e.g. [19,20], the addition of CO causes the autoignition of propane, n-butane and isobutane to be marginally suppressed. These results are consistent with the high octane rating of CO [21], which exceeds that of all LPGs [1]. 4.3. Effect autoignition
of
unburned
hydrocarbons
on
Unburned hydrocarbons (UHCs) are the result of several processes, including the desorption of hydrocarbon-rich gases from oil layers and crevice volumes, along with incomplete combustion.
The effect of nitric oxide (NO) on the autoignition of the various LPG fuels was next examined. In almost all cases, NO was found to have a promoting effect on the autoignition timing. As an example of this behaviour, the simulated pressure histories for n-butane are presented in Fig. 6. These results are compared with the representative autoigniting pressure trace acquired at the same engine operating condition. The corresponding measured and modelled autoignition timings are presented for all twelve LPG fuels in Fig. 7. These simulations were performed with the NO concentration varied between zero and twice the
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Fig. 7. Measured and simulated autoignition timing for the twelve LPG fuels. Autoignition was not observed for neat propylene when the residual NO concentration was doubled.
value inferred from the engine-out emissions data presented in Table 2. For the fuels composed primarily of paraffins, the simulated autoignition timing typically occurs 1–3 CAD (0.28–0.83 ms) more prematurely when the measured NO concentration is included in the residual gas. This is consistent with the promoting effect of NO on the oxidation of these hydrocarbons observed in more fundamental studies, e.g. [25–28]. For the paraffinic fuels, the simulated autoignition timing also shows reasonable agreement with the experimental autoignition timing. This suggests that the model is largely capturing the autoignition behaviour of these simple paraffins. The simulated autoignition behaviour for neat propylene differs considerably to that of the other fuels examined. As an example of this behaviour, Fig. 8 indicates that the inclusion of NO in the residual gas causes the autoignition timing to be inhibited. This effect becomes more prominent with increasing NO, and eventually leads to autoignition being fully suppressed once the NO concentration exceeds approximately 200 ppm. Given that the limited data in the literature indicates propylene can be effective promoter of NO to NO2 conversion [26,28], one might therefore expect its simulated autoignition behaviour in Fig. 8 to be more similar to that of the neat paraffins. The poor agreement observed between the simulated and measured autoignition timing for propylene in Fig. 7 can potentially be attributed to several factors. First, the kinetic model employed here does not contain fuel-specific hydrocarbon– NOx reaction pathways for hydrocarbons larger than ethane. The kinetic interactions between larger hydrocarbons and NOx are therefore only included via the pathways specific to methane and ethane. Given the good agreement observed
Fig. 8. Simulated variation in the autoignition timing for the neat fuels with the addition of NO. Negative values represent a more premature autoignition timing, and vice versa.
Fig. 9. Simulated fraction of the total fuel consumed by pre-flame kinetic activity for n-butane.
Fig. 10. Simulated end-gas temperature increase due to pre-flame kinetic activity for n-butane.
for the majority of fuels, this could indicate that the propylene–NOx specific interactions are more important than the equivalent interactions for propane, n-butane and iso-butane, with their absence leading to a net inhibiting effect on the autoignition timing. Propylene also has the highest residual NO concentration at IVC of the twelve fuels examined (Table 2). These results could therefore alternatively indicate an underlying sensitivity to elevated NO concentrations.
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Consistent with these uncertainties, the development of improved kinetic models that specifically address propylene oxidation remains an active area of research, e.g. [29–31]. 4.5. Pre-flame kinetic activity within the end-gas The simulated fraction of the total fuel consumed by pre-flame kinetic activity is presented for n-butane in Fig. 9. These results indicate that approximately twice as much fuel is consumed in the lead up to autoignition when NO is present in the residual gas (4% vs. 8% mass). This behaviour coincides with the more premature autoignition timing observed previously in Fig. 6, and ultimately leads to a significant increase in the fraction of the total fuel consumed by the final autoignition event itself (18% vs. 36% mass). These observations are consistent with those reported in motored engine studies for n-butane, e.g. [32]. The two-stage autoignition behaviour observed in more fundamental studies is also apparent, e.g. [33]. The effect of this pre-flame kinetic activity on the simulated end-gas temperature is considered in Fig. 10. This profile represents the difference between the calculated end-gas temperature with chemical kinetics implemented in the end-gas, and a second test case which assumed chemical equilibrium [9]. Even prior to the start of combustion, the pre-flame heat release is shown to raise the end-gas temperature by approximately 20 K. This is followed by a more rapid temperature increase once combustion has commenced, particularly in the test case where the measured NO concentration was included in the residual gas. This behaviour was also observed for the other paraffinic fuels examined in this paper. Overall, these results suggest that compression of the unburned charge by the piston is an important modelling consideration if accurate predictions of the end-gas temperature during combustion are to be achieved.
7
inclusion of the unburned charge chemical kinetics for further examination in this paper. The in-cylinder heat transfer and residual gas composition were both shown to affect autoignition. In particular, nitric oxide (NO) was found to be a strong promoter of both fuel oxidation and autoignition for propane, n-butane and isobutane. For these fuels, the inclusion of NO in the residual gas at physically reasonable concentrations also enabled good agreement between the measured and modelled autoignition timing. The same agreement was not observed for neat propylene, suggesting that further research is required for this fuel. In all cases, physically reasonable concentrations of both carbon monoxide (CO) and unburned hydrocarbons (UHCs) had an insignificant effect on autoignition in comparison with NO. More broadly, these results suggest that kinetic interaction between hydrocarbon fuels and NO plays a vital role in octane rating, and its inclusion is important when modelling the autoignition of hydrocarbons in spark-ignition engines. Nonetheless, the fuel chemistry is not the only challenge. This work showed that several other phenomena, particularly the flame propagation, in-cylinder heat transfer and the residual gas composition also need to be modelled accurately if truly predictive models of autoignition are to be achieved. Acknowledgments This research was supported by the Advanced Centre for Automotive Research and Testing (ACART) and the Australian Research Council. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http:// dx.doi.org/10.1016/j.proci.2014.06.070. References
5. Conclusions This paper investigated the autoignition of C3/C4 hydrocarbon mixtures in a CFR octane rating engine. The four species examined – propane, propylene (propene), n-butane and iso-butane – are the primary constituents of Liquefied Petroleum Gas (LPG), and are also important intermediates in the oxidation of larger hydrocarbons. In-cylinder pressure data was acquired for both autoigniting and non-autoigniting engine operation at the same test conditions. The latter was obtained by adding a small amount of dilute TEL to the intake charge, and was used to calibrate a two-zone model of the CFR engine in a prior work. This enabled the
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