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Chemical Reaction Processes of Fuel Reformation by Diesel Engine Piston Compression of Rich Homogeneous Air-Fuel Mixture.

INTRODUCTION

Diesel engines have become the most commonly used power source for industrial machinery, and along with the strengthening of exhaust emission regulations, much research has been conducted to minimize pollutant emissions. There is strong interest in the development of cleaner combustion modes to utilize thoroughly premixed combustion such as with Premixed Compression Ignition (PCI) and Homogeneous Charge Compression Ignition (HCCI). The HCCI has the potential to achieve extremely low nitrogen oxide (N[O.sub.x]) and particulate matter (PM) emissions with good fuel consumption. In spite of these advantages, the HCCI combustion still presents issues that must be solved before it would be commercially acceptable. One of the biggest challenges is to establish controllability of the ignition timing throughout the entire range of engine operation [1], [2]. Miyamoto et al. [3] determined aspects of the relationship between fuel ignitability and engine operating conditions such as the compression and EGR ratios. Ohtsubo et al. [4] applied the HCCI combustion to natural gas engines and investigated methods of ignition control by optimization of intake air temperatures and compression ratios. A further HCCI issue is the difficulties with ignition timing control even for similar commercially available research octane number fuels. Here, Shibata et al. [5] have investigated the influence of fuel composition on the heat release characteristics and ignitability.

Another approach to control the ignition timing is using dual fuel (DF) engines, which operate on the lean burn principle of gas engines, although the ignition is controlled by the diesel fuel injection [6]. Presently, DF engines have been introduced mainly for marine propulsion and as auxiliary engines [7]. The DF combustion is a promising technology for a wide range of applications, however, in operation it requires two fuel supply sources, and this is a cause of complexity for the engine system layout and a costly drawback.

With the background as suggested above, research on fuel reforming technology which converts liquid fuels (gasoline and gas oil) into light gases (hydrogen and carbon monoxide) in the engine system, has been actively pursued. Ashida et al. [8] reported on a method of rich gasoline mixture reformation with a catalyst, and Sage et al. [9] discussed the possibility of diesel fuel reformation with partial oxidation, steam reformation, and autothermal reaction by using catalyst reformer. The results showed that the reformed fuel combustion increase the indicated efficiency greater than 4% by the shorter combustion duration and reduced heat losses. And by enabling of the exhaust energy recovery, further improvement of system efficiency can be expected.

Alger et al. [10] introduced the dedicated EGR concept. Here the number one cylinder of four inline cylinders in a gasoline engine is used as a "dedicated cylinder" to operate in the rich condition with an equivalence ratio above 1.0. The purpose of the dedicated cylinder is to provide an increased amount of EGR gas to the other cylinders, with hydrogen and carbon monoxide obtained as by-products. These concepts suggest the possibility of converting liquid fuels into reformed fuels with low ignitability utilizing a rich mixture for the actual combustion.

This study proposes diesel fuel reformation by piston compression to be implemented in the engine to generate reformed fuel which can be utilized for lean homogeneous combustion. For the diesel fuel reformation by using catalyst, the reaction temperature above 1000 K is required to achieve high reformation efficiency [9]. Since the In-Cylinder reformation can produce the high temperature reaction field by piston compression, the higher reformation efficiency can be obtained without additional heating devices.

ENGINE SYSTEM CONCEPT

Figure 1 shows an outline of the components of the engine system used here. Specific cylinders were assigned as reformer cylinder for the reformed fuel production and power cylinders for the combustion of the reformed mixture. A rich mixture with an equivalence ratio above 2.0 is pyrolyzed and partially oxidized at high pressure and high temperature conditions created by the piston compression in the reformer cylinder. As the reformed gas consists of hydrogen ([H.sub.2]), carbon monoxide (CO), methane (C[H.sub.4]), with carbon dioxide (C[O.sub.2]), and water ([H.sub.2]O), the octane number of the gas is higher than that of conventional diesel fuels. The reformed gas is diluted with air to form a lean homogeneous mixture and introduced into the power cylinders. The injector equipped on the power cylinders supplies a small amount of diesel fuel at the end of the compression stroke to provide an ignition trigger. In this manner lean homogenous combustion can be achieved over a wide speed and load range by optimization of the timing of the diesel fuel injection.

To obtain higher conversion efficiency of the fuel, the intake air of the reformer cylinder are heated by exhaust gas out of power cylinders with heat exchanger. The part of the exhaust energy can be recovered to the reformed gas, an improvement of the system efficiency can be expected.

DYNAMIC REACTION ANALYSIS

Model Setup

A numerical analysis was carried out to obtain detail of the characteristics of the reformation reaction process and the relationship between engine operation conditions and the composition of the reformed products. The chemistry dynamics simulation software, CHEMKIN Pro, was utilized in this study. The primary input parameter variables for the CHEMKIN simulation are shown in Table 1. The reciprocating cylinder model and the reaction mechanisms of the n-alkane reaction model (2115 species, 8157 reactions) developed by the Lawrence Livermore National Laboratory [11] were used in the simulation. This reaction mechanism contains the detail reaction mechanism of n-alkane up to 16 carbon atoms in the molecule. The dimensions of the cylinder model were set to match the engine used in the experiments. To simplify phenomena, simulations were performed at adiabatic conditions.

The equivalence ratio, and initial temperature are the variable parameters, and the equivalence ratio is adjusted by the oxygen ([O.sub.2]) concentration in the mixture to maintain the introduction of the same fuel quantities. The equivalence ratios of 2.0 and 5.0 correspond to 10% and 4% [O.sub.2] concentrations.

Influence of Equivalence Ratio and Initial Temperature

Figure 2 shows the reformed gas concentrations at various equivalence ratios at a 500 K initial temperature. Hydrogen ([H.sub.2]), carbon monoxide (CO), carbon dioxide (C[O.sub.2]), and water ([H.sub.2]O) are dominant in the gas produced at the equivalence ratio of 2.0. With increases in the equivalence ratio, hydrocarbons of smaller molecular weights such as methane (C[H.sub.4]), ethylene ([C.sub.2][H.sub.4]), and acetylene ([C.sub.2][H.sub.2]) are obtained. At the higher equivalence ratios, the ratios of hydrogen, carbon monoxide and carbon dioxide lowers with increases in the ratio of hydrocarbons. Further, hydrocarbons with 2 or fewer carbon atoms are dominant in the reformed gases at the high equivalence ratios. At the conditions in these simulations, it is considered that large molecule hydrocarbons such as n-heptane are consumed by oxidation or decomposition reactions to form small molecule compounds.

Figure 3 shows contour plots of the equivalence ratio and initial temperature versus the reformed gas composition, hydrogen, water, carbon monoxide, carbon dioxide, and methane. The areas surrounded by the dashed lines show the area where operation with the single cylinder engine was possible.

The hydrogen reaches the highest value at equivalence ratios of 2-3, and the amounts are proportional to those at the initial temperature. The higher initial temperatures, result in higher concentrations of hydrogen at all equivalence ratios above 2.0. The water shows a largely constant value in the range of equivalence ratios of 2 to 5 and an initial temperature range of 300 K to 500 K. Since the hydrogen is immediately consumed by the water production when there is a sufficient oxygen available, the relationship between hydrogen and water at equivalence ratios below 2.0 shows an inverse correlation. The carbon monoxide shows characteristics similar to hydrogen. It reaches a maximum value in the range of equivalence ratios of 2-4 and higher concentrations are estimated at higher initial temperatures. The oxidation of carbon monoxide is suppressed at lower oxygen concentrations, and carbon dioxide is mainly produced at equivalence ratios below 2.0.

Methane shows different characteristics than hydrogen and carbon monoxide. To obtain higher concentrations of products, higher equivalence ratios are required. The area where methane is produced in Figure 3 is affected by both the equivalence ratio and the initial temperature. Methane molecules are consumed by partial oxidation reactions when there is a sufficient amount of oxygen available, and the methane concentration decreases at lower equivalence ratios. At higher equivalence ratios, the partial oxidation reactions are suppressed by the shortage of oxygen, and concentrations of unreacted hydrocarbons increase. If the temperature is low, the thermal decomposition of hydrocarbons with large molecules is not activate and little methane is produced. At extremely high temperatures, the thermal decomposition of methane molecules becomes active and the methane molar fraction is lowered.

Figure 4 shows the simulated cylinder pressure histories at the different equivalence ratios and initial gas temperatures. In general, the oxidation reaction becomes increasingly active at higher oxygen concentrations, and the reaction rate is higher at lower equivalence ratios. The thermal decomposition of fuel molecules is accelerated by higher ambient temperatures, and an earlier start of the reaction was observed at the higher initial temperatures.

Figure 5 shows the reformed gas molar fractions versus the maximum temperature during the reaction process. The hydrogen ([[psi].sub.H2]) increases with the maximum cylinder temperature and becomes constant above 2000 K. The carbon monoxide ([[psi].sub.CO]) reaches a local maximum value near 1400 K and increases steadily from 1700 K to 2000 K. The methane ([[psi].sub.CH4]) and ethylene ([[psi].sub.C2H4]) reach the maximum values in the vicinity of 1400 K. There are inverse relations between hydrogen, carbon monoxide and methane above 1400 K, and from this it may be assumed that the oxidation reactions that consume hydrocarbons

to produce hydrogen and carbon monoxide are increasingly active above 1400 K. Throughout the CHEMKIN simulations, it is suggested that the dominant reactions in the fuel reformation process can be categorized by the maximum cylinder temperature.

Influence of the Compression Ratio

To investigate the maximum cylinder temperature effects, the compression ratio was chosen as the parameter, and figure 6 shows the simulated maximum cylinder temperatures at the compression ratios 16.1, 18.0, and 20.1 for different equivalence ratios and initial temperatures. The higher maximum gas temperature is obtained at higher compression ratios, lower equivalence ratios and higher initial temperatures. Further, the effects of the compression ratio and the initial temperature on the maximum cylinder temperature are more pronounced at lower equivalent ratios.

The production of hydrogen (a), carbon monoxide (b), and methane (c) at different compression ratios are plotted in Figure 7. Overall the gas production is not strongly influenced by the compression ratio, however the amounts of the reformed gases are influenced markedly.

In Figure 7 (a), the area of higher hydrogen concentrations expands toward lower initial temperatures and higher equivalence ratios. Figure 7 (b) shows similar characteristics, suggesting that higher carbon monoxide concentrations may be expected at higher compression ratios.

In Figure 7 (c), the area of high methane concentrations is shifted to the lower temperature side at high compression ratios. This may be explained by Figure 5 where the methane concentration reaches the maximum value at the maximum cylinder temperature of 1400 K. If the compression ratio is increased, a lower initial temperature would be required to maintain the same maximum cylinder temperature. As discussed with Figure 6, the sensitivity of the compression ratio to the maximum cylinder temperature is stronger at lower equivalence ratios. However, no significant changes in the production of gases were observed in the area of lower equivalence ratios in Figure 7 (a), (b), (c).

The simulated results shown in Figure 5, 6, 7 suggest that for large amounts of hydrogen and carbon monoxide, operating conditions with higher maximum cylinder temperatures are necessary; and to obtain more hydrocarbons such as methane and ethylene, maintaining 1400 K as the maximum cylinder temperature through the optimization of the operating condition is required. In addition, the contribution of the reformed gas composition increase in the order of compression ratio, initial temperature, and equivalence ratio.

EXPERIMENTAL SETUP

Engine Setup

Figure 8 is a schematic outline of the experimental apparatus used for the fuel reformation with a single cylinder engine. The engine specifications are listed in Table 2. The bore is 85.0 mm, the stroke is 88.0 mm, displacement 499 [cm.sup.3] and the compression ratio varies from 16.1 to 20.1. First, the engine was operated at 1000 r/min by electric dynamometer, and this was changed to the predetermined experimental operating conditions after warming up.

Control of the Equivalence Ratio

To retain similar gas temperature profiles in the compression stroke with the different equivalence ratios, the equivalence ratio was adjusted by changing the intake oxygen concentration rather than the injected quantity. The fuel injection quantity was maintained at 20 mg/cycle for all experiments and nitrogen ([N.sub.2]) gas was supplied to reduce the concentration of oxygen in the intake air. Nitrogen is a diatomic molecule and the equivalence ratio of the mixture can be controlled without changing of the polytropic index. The dilution nitrogen gas was supplied by the high-pressure cylinder via the pressure regulator and flow control valve (Fig.8). The surge tank, located upstream of the engine intake pipe, entrain fresh air and with the dilution nitrogen gas the low oxygen ratio mixture is formed. The flow rate of the nitrogen was manually controlled by a flow control valve. The intake oxygen concentration was monitored by an [O.sub.2] sensor (Fig. 8, [O.sub.2in]) attached downstream of the surge tank. The flow rates of the fresh air, dilution nitrogen, and fuel, were individually obtained, and the actual equivalence ratio was calculated from the mass flow data for each of these.

Adjustment of Intake Air Temperature

As an index corresponding to the initial temperature in the CHEMKIN simulation, the intake temperature detected by the thermocouple attached upstream of the intake port (in Fig. 8, [T.sub.in]) was used. The intake temperature was maintained at 400 -500 K by the intake heater located between the intake [O.sub.2] sensor and the fuel injector. First, the engine operable range without misfiring and knocking was investigated for a number of equivalence ratios and intake temperatures. Then, the engine tests were conducted at various compression ratios as detailed in Table 3.

Rich Mixture Formation

The purpose of this study is to apply the diesel fuel reformation concept to an engine system (Fig.1). However, the volatility of diesel fuel is low, and this causes an unpredictable lack of uniformity of the in-cylinder mixture. Further, as the commonly available diesel fuel is composed of multi-component hydrocarbons, the analysis of the reaction process becomes complicated. For this reason, n-heptane ([C.sub.7][H.sub.16]), with high volatility and similar ignitability to diesel fuel, was selected as the test fuel. The fuel was pressurized to 300 kPa by an electric pump and supplied to the gasoline injector installed 800 mm upstream of the intake port. Since the fuel is injected downstream of the intake air heater, almost all of the fuel is vaporized and a uniform air-fuel mixture is introduced into the cylinder.

Measurement of Cylinder Pressure

The cylinder pressure measurements were carried out with a pressure sensor installed in the cylinder head. The pressure signals were sampled at a 0.20 crank angle resolution, and 44 cycles of signals were stored with the high-speed A/D converter. The mean cylinder gas temperature and the heat release rate were calculated based on the cycle-averaged cylinder pressure signals.

Analysis of Reformed Gas Composition

Table 4 shows a list of chemical species that are quantitatively analyzed in this study, and the analytical methods applied to each. To maintain gas concentrations within the measurement range of the analytical instrument, the sampled gas was once captured in a sampling bag and diluted 25 times by nitrogen. The molar fraction of carbon monoxide was measured by the IR analyzer directly connected to the exhaust pipe. The dilution ratio of the sampling bag method was corrected by comparing these two types of measurements.

In addition, the degree of filter blackening, corresponding to the amount of smoke in the reformed gas, was measured by a smoke meter. The sampling was performed at least five times per condition, and an average value was used in the determinations.

REACTION CHARACTERISTICS IN THE REFORMER CYLINDER

In-Cylinder Pressure, Temperature, Heat Release

Figure 9 shows the in-cylinder pressure, mean gas temperature and heat release rate in the reforming processes at 500 K of intake temperature. The compression ratio is 18.0 and equivalence ratio is changed from 2.0 to 5.0. At the equivalence ratio of 2.0, there is a sharp heat release at -18 [degrees]CA ATDC, and the maximum gas temperature reached 1630 K. With increases in the equivalence ratio, the total amount of heat release decreased, and the starting timing of the heat release was delayed. The maximum cylinder temperature decreased at higher equivalence ratios. At the equivalence ratio 5.0, little heat release was observed, and the mean cylinder gas temperature does not reach 900 K.

Focusing on the heat release rate, especially at low equivalence ratios, a weak heat release was observed at around -30 [degrees] CA ATDC, and a strong heat release was observed at around -10 [degrees] CA ATDC. Consequently, the initial reaction process of the rich mixture is the same as the reaction process of lean homogeneous compression ignition that starts from low temperature oxidation and high temperature oxidation [12].

From Figures 4 and 9, the starting timings of the reaction are different for the experiments and simulations. At the 2.0 equivalence ratio, there is a rapid pressure rise at -25 [degrees]CA ATDC in the simulations, this was also observed at -12 [degrees]CA ATDC in the experiments, and the gradient of the cylinder pressure was flatter than the simulation. Similarly, the reaction started at the top dead center (TDC) in the actual engine at equivalence ratio 3.3, versus -15 [degrees]CA ATDC in the simulations.

One of the factors that explain these differences in the reaction profile is the difference in the gas temperatures. In the simulations, the heat loss was not considered, and the estimated gas temperature is higher than that of the engine tests. The higher gas temperature may lead to a faster start of reaction. Further, the degree of the pressure and temperature rise after the start of the reaction also becomes higher than the in actual engine.

Maximum Cylinder Gas Temperature

Figure 10 shows the maximum cylinder temperature obtained at various compression ratios, initial temperatures, and equivalence ratios in the experiments. As discussed in Figure 6, the higher maximum gas temperature is observed at lower equivalence ratios, and the contribution of the equivalence ratio to the maximum cylinder temperature is stronger at lower equivalence ratios. The maximum cylinder temperature becomes constant above equivalence ratio 5.0. This can be explained by the extinction of the heat release at equivalence ratio 5.0 in Figure 9. The influence of the compression ratio on the maximum cylinder temperature is relatively smaller than the equivalence ratio.

Reformed Gas Properties

Figure 11, 12, 13, 14, 15, 16 show plots of reformed gas concentrations versus the equivalence ratio, with the initial temperatures and compression ratios detailed in Table 3.

In Figure 11, the hydrogen molar fraction plotted against the equivalence ratio shows higher concentrations at lower equivalence ratios. Where the equivalence ratio is higher than 5.0, the produced amount of the hydrogen was close to zero. A similar trend can be observed at the simulation result in Figure 3.

Figure 12 shows the concentrations of reformed carbon monoxide versus the equivalence ratio. There are similar characteristics to those of hydrogen that shows a higher molar fraction at lower equivalence ratio. Since about 2.0% of carbon monoxide is produced at equivalence ratio 5.0, the production of carbon monoxide under high equivalence ratios appears as higher than that of the hydrogen.

Figure 13 shows the reformed methane molar fractions plotted against the equivalence ratio. The methane molar fraction increases with the increase in the equivalence ratio from 2.0 to 3.0, and it shows an inverse correlation with hydrogen. Further, it reaches the maximum value at equivalence ratios of 3.0-4.0, and decreases at the higher equivalence ratios above 4.0. In Figure 14, the ethylene molar fraction increased with the increase in the equivalence ratio from 2.0 to 4.0, however the concentration does not increase above equivalence ratios of 4.0. In Figure 15, the total hydrocarbons (THC) show characteristics similar to ethylene at lower equivalence ratios, however the THC concentration continues to increase above equivalence ratio 4.0. From Figures 13, 14, 15, it is considered that the hydrocarbons of small molecules such as methane are dominant at equivalence ratios below 4.0, and that hydrocarbons of two or more carbon atoms are dominant at higher equivalence ratios.

Figure 16 shows the smoke emissions plotted against the equivalence ratio. No smoke was observed at equivalence ratios above 3.0. In Figure 10, the maximum cylinder temperature at equivalence ratio 3.0 is 1200 K. Considered with the predictions of the soot generation characteristics in the [phi] -T map proposed by Kamimoto et al [13], extremely low smoke emissions can be achieved even in the high equivalence ratios by regulating (lowering) the ambient temperature.

DISCUSSION OF REFORMED REACTANT CHARACTERISTICS

Gas Production Characteristics with Respect to the Maximum Cylinder Temperature

To validate the CHEMKIN simulations, the simulation results were compared with the engine test results. Figure 17 shows a plot of the hydrogen production versus the maximum gas temperature. As mentioned in the previous chapter, the maximum gas temperatures in the actual engine were lower than the simulations for similar operating conditions, and the amount of hydrogen gas available in the cylinder in the simulations and experiments differ.

The simulated molar fraction of hydrogen produced (solid figures in Fig.17) versus the maximum cylinder temperature shows a linear relation with the experimental results (open figures in Fig.17). That there is no strong correlation between the experiments and simulations for the qualitative trends of hydrogen production. However, similar to the simulations, it is confirmed that the hydrogen molar fraction shows a close relationship to the maximum cylinder temperature at the experiments.

Figure 18 compares the molar fractions of the produced carbon monoxide plotted against the maximum cylinder temperature for the experiments and simulations. The gas production characteristics are proportional to the maximum cylinder temperature from 800 K to 1400 K. The molar fraction of carbon monoxide in the experiments reaches a local maximum near 1400 K and shows a slight decrease above 1400 K. Those characteristics are also observed in the simulations, however, the absolute amount of the products do not agree at temperatures below 1600 K. In the temperature range above 1600 K, the actual molar fractions of carbon monoxide show a good correlation with the simulations. In conclusion, there is a slight mismatch in the production characteristics of simulations and experiments, the produced amount of carbon monoxide is determined with respect to the maximum gas temperature.

Figure 19 is a plot of the carbon dioxide production versus the maximum gas temperature in the experiments and simulations. The experimental and simulation results show monotonic increases with the maximum cylinder temperature until 1700 K. The molar fraction of the experiments are twice the simulations. From Figure 18 and 19, in the simulations, the carbon monoxide increases and carbon dioxide decreases above the maximum cylinder temperature of 1700 K. It is assumed that the thermal dissociation reactions of carbon dioxide to produce carbon monoxide is gradually activated at higher the maximum gas temperatures. Since the maximum cylinder temperatures did not reach 1700 K in any operating conditions with the actual engine, it is considered that there was no thermal dissociation reaction of carbon dioxide.

Figure 20 shows the simulation and experiment results of methane production plotted against the maximum cylinder temperature. There is a significant difference between the experiments and simulations. In the simulation, the expected maximum concentration was 2.0% at the maximum cylinder temperature of 1400 K. However, in the experiment, the maximum value was 1.0% at 1200 K. The similar characteristics are observed for ethylene ([C.sub.2][H.sub.4]) and propylene ([C.sub.3][H.sub.6]).

Figure 21 shows plots of the molar fraction of different hydrocarbons at the compression ratio of 18.0 and intake temperature of 500 K. The "other HC" in the figure stands for the methane-based concentration of hydrocarbons other than the species shown in Table 4. Acetylene ([C.sub.2][H.sub.2]) would be one candidate for other HCs, and undegraded hydrocarbons fuel (n-heptane) and hydrocarbons containing 3 or more carbon atoms are also included.

The ratio of methane was approximately 6% of THC , and the respective percentages of ethylene ([C.sub.2][H.sub.4]) and propylene ([C.sub.3][H.sub.6]) were 25% and 14% at maximum cylinder temperatures below 1000 K. In this condition, the THC molar fraction reached 10%, and the percentage of the other HCs in the total hydrocarbon exceeds 50%.

With the increase in the maximum cylinder temperature, the THC molar fraction decreases and the percentage of hydrocarbons with small molecules increased temperatures of 1000 to 1200 K. This is explained by the activation of the thermal dissociation reactions. It is observed that the both hydrocarbons with 1-2 carbon atoms and THC are reduced at the maximum cylinder temperature above 1200 K. From

Figure 5, the oxidation reactions that consume hydrocarbons to produce hydrogen and carbon monoxide are increasingly active at higher maximum cylinder temperatures. In the actual engine, the consumption and production of hydrocarbons of small molecules are approximately equal between the maximum temperatures of 1200 and 1400 K.

Different from the simulation results shown in Figure 2, the experimental results show higher concentrations of other HCs such as the hydrocarbons of large molecules at all operating conditions. More than 50% of THC consists of the other HCs even at higher maximum cylinder temperatures. It is considered that the thermal decomposition of the fuel does not proceed in the actual engine, and the reformed gas contains a large amount of large molecule hydrocarbons. And this can be the reason why methane shows different characteristics in the experiments and simulations.

SUMMARY/CONCLUSIONS

Fuel reformation by diesel piston compression of rich homogeneous mixtures was investigated in this report. First, the chemistry dynamics simulation package CHEMKIN Pro, was used to understand details of the reformed gas compositions and chemical processes at various operating conditions of the operation with the reformed fuel. Based on the simulations, a single cylinder diesel engine was operated to determine the validity of the simulation results. The conclusions may be summarized as follows.

1. Reformed fuel composed mainly of hydrogen, carbon monoxide, and hydrocarbons with small molecules were observed above equivalence ratio 2.0.

2. The maximum cylinder temperature of the reformer cylinder can be used as an index for the reformed gas properties. The equivalence ratio, intake temperature, and compression ratio are the effective input factors for the control of reformed gas compositions.

A. To obtain higher concentrations of hydrogen and carbon monoxide, higher maximum cylinder temperatures are required.

B. To obtain small molecule hydrocarbons such as methane and ethylene, the maximum cylinder temperature must be maintained in the 1200 to 1400 K range.

C. To avoid the contamination in the reformed gas, a lower maximum cylinder temperature is necessary.

3. The THC concentration is higher with higher equivalence ratios. Most of the hydrocarbons are not pyrolyzed and remain as large molecules until the end of the reaction process at low temperatures. The thermal dissociation of hydrocarbons into small molecules increases from above 1000 K, however the percentage of large molecule hydrocarbons in THC retains high even above 1600 K.

4. By utilizing of the in-cylinder fuel reformation technology, the wide range operation of the power cylinder can be relied on. To achieve the high load operation without early ignition, the low ignitability gas should be produced by the reformer cylinder at high temperature operation. To avoid misfiring at low load, lower operating temperature of the reformer cylinder is reqired.

REFERENCES

[1.] Thring, R., "Homogeneous-Charge Compression-Ignition (HCCI) Engines," SAE Technical Paper 892068, 1989, https://doi.org/10.4271/892068.

[2.] Ryan, T. and Matheaus, A., "Fuel Requirements for HCCI Engine Operation," SAE Technical Paper 2003-01-1813, 2003, https://doi.org/10.4271/2003-01-1813.

[3.] Sakai, A., Takeyama, H., Ogawa, H., and Miyamoto, N., "Fuel Ignitability and Compression Ratio Dependence of a Premixed Charge Compression Ignition Engine" JSME Technical Paper (B), 2005, 71-703, pp.993-999

[4.] Ohstubo, H., Yamane, K., Kawasaki, K., Yamauchi, K., Nakazono, T., "PCCI Combustion for Multi Cylinder Natural Gas Engine (First Report)" JSAE Technical Paper , 2008, Vol.39, no.1, pp.51-57

[5.] Shibata, G. and Urushihara, T., "Improvement of HCCI Engine Performance by Fuel Composition" JSAE Technical Paper, 2008, Vol.39, no.5, pp.71-76

[6.] Zhang, Y., Ghandhi, J., and Rothamer, D., "Effects of Fuel Chemistry and Spray Properties on Particulate Size Distributions from Dual-Fuel Combustion Strategies," SAE Int. J. Engines 10(4):1847-1858, 2017, https://doi.org/10.4271/2017-01-1005.

[7.] Astrand, U., Aatola. H., and Myllykoski, J., "Wartsila 31 - World's most efficient fourstroke engine" CIMAC Congress , 2016, No.225

[8.] Ashida, K., Hoshino, M., Maeda, H., Araki, T. et al., "Study of Reformate Hydrogen-Added Combustion in a Gasoline Engine," SAE Technical Paper 2015-01-1952, 2015, https://doi.org/10.4271/2015-01-1952.

[9.] Sage, K., Flavio, C., "High efficiency dual-fuel combustion through thermochemical recovery and diesel reforming", Applied Energy, 195(2017), 503-522

[10.] Alger, T. and Mangold, B., "Dedicated EGR: A New Concept in High Efficiency Engines," SAE Int. J. Engines 2(1):620-631, 2009, https://doi.org/10.4271/2009-01-0694.

[11.] LLNL Mechanism, https://www-pls.llnl.gov/7url = science_and_technology-chemistry-combustion-c8c16_n_alkanes.

[12.] Kuwahara, K., Tada, T., Furutani, M., Sakai, Y. et al., "Chemical Kinetics Study on Two-Stage Main Heat Release in Ignition Process of Highly Diluted Mixtures," SAE Int. J. Engines 6(1):520-532, 2013, https://doi.org/10.4271/2013-01-1657.

[13.] Kamimoto, T. and Bae, M., "High Combustion Temperature for the Reduction of Particulate in Diesel Engines," SAE Technical Paper 880423, 1988, https://doi.org/10.4271/880423.

CONTACT INFORMATION

Go Asai

YANMAR Co., Ltd. R&D Center

2481, Umegahara, Maihara City, Shiga Pref., 521-8511, Japan go asai@yanmar.com

Gen Shibata

Hokkaido University

Kita 5 Nishi 8, Kita Ward, Sapporo, Hokkaido Pref. 060-0808, Japan

g-shibata@eng.hokudai.ac.jp

DEFINITIONS/ABBREVIATIONS

PCI - Premixed Compression Ignition

HCCI - Homogeneous Charge Compression Ignition

EGR - Exhaust Gas Recirculation

DF - Dual Fuel

PM - Particulate Matter

THC - Total Hydro Carbon

CA - Crank Angle

TDC - Top Dead Center

ATDC - After Top Dead Center IR - Infrared Gas

FTIR - Fourier Transform Infrared Spectroscopy

CR - Compression Ratio

A/D - Analog / Digital

APPENDIX

[phi] - Equivalence Ratio [-]

[[psi].sub.i] - Molar percentage of species i [mol%]

[O.sub.2in] - [O.sub.2] concentration of intake gas [vol%]

[T.sub.in] - Intake temperature [K]

[T.sub.ini] - Initial temperature in CHEMKIN simulation [K]

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of SAE International.

Positions and opinions advanced in this paper are those of the author(s) and not necessarily those of SAE International. The author is solely responsible for the content of the paper.

Go Asai YANMAR Co., Ltd.

Yusuke Watanabe, Shuntaro Ishiguro, Gen Shibata, Hideyuki Ogawa, and Yoshimitsu Kobashi Hokkaido University
Table 1. Parameter variables for CHEMKIN analysis

Fuel                     n-Heptane
Oxidizer                 Air
Starting crank angle     -160 [degrees]CAATDC
Calculated crank angle    285 [degrees]CAATDC
Connecting rod ratio        3.51
Displacement              499[cm.sup.3]
Engine speed             1000 r/min
Initial pressure            1.0 atm
Initial temperature       300-800 K
Compression ratio          18.0
Equivalence ratio           1.0-8.0
Cooling Loss             Adiabatic

Table 2. Specifications of the tested engine

Test engine type      Nissan SC-77
Number of cylinders     1
Bore x Stroke          85.0mm x 88.0mm
Number of valves        1 intake valve
                        1 exhaust valve
Displacement          499 [cm.sup.3]
Compression ratio      16.1, 18.0, 20.1
Fuel                  n-Heptane (C7H16)

Table 3. Experimental conditions

Compression   Intake        Equivalence ratio
ratio         temperature   [-]
[-]           [K]           (Intake [O.sub.2] concentration
                            [vol%])

16.1          400             2.0-3.3
                            (10-6)
              450             2.0-3.3
                            (10-6)
              500             2.0-5.0
                            (10-4)
18.0          400             2.0-3.3
                            (10-6)
              450             2.0-3.3
                            (10-6)
              500             2.0-5.0
                            (10-4)
20.1          400             2.5-5.0
                             (8-4)
              450             2.5-5.0
                             (8-4)
              500             2.5-5.0
                             (8-4)

Table 4. List of quantitatively analyzed chemical species

Species        Chemical             Measurement   Sampling
Name           symbol               method

Hydrogen       [H.sub.2]            MS            Diluted
Carbon         CO                   IR            Diluted
monoxide
                                    FTIR          Diluted
                                    IR            Direct
Carbon         C[O.sub.2]           IR            Diluted
dioxide
Total hydro-   THC                  FID           Diluted
carbon
Nitrogen       NOx                  NDIR          Diluted
oxide
Methane        C[H.sub.4]           FTIR          Diluted
                                    FTIR          Direct
Methanol       C[H.sub.3]OH         FTIR          Diluted
Formaldehyde   C[H.sub.2]O          FTIR          Diluted
Ethane         [C.sub.2][H.sub.6]   FTIR          Diluted
Ethylene       [C.sub.2][H.sub.4]   FTIR          Diluted
Propylene      [C.sub.3][H.sub.6]   FTIR          Diluted
iso-Butene     [C.sub.4][H.sub.8]   FTIR          Diluted
Butadiene      [C.sub.6][H.sub.6]   FTIR          Diluted
Benzene        [C.sub.6][H.sub.6]   FTIR          Diluted
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Title Annotation:INTERNATIONAL
Author:Asai, Go
Publication:SAE International Journal of Engines
Article Type:Report
Date:Dec 1, 2017
Words:5638
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