An experimental investigation into combustion and ...

Energy Conversion and Management 98 (2015) 199–207

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Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman

An experimental investigation into combustion and performance characteristics of an HCCI gasoline engine fueled with n-heptane, isopropanol and n-butanol fuel blends at different inlet air temperatures Ahmet Uyumaz Mehmet Akif Ersoy University, High Vocational School of Technical Sciences, Department of Automotive Technology, 15100 Burdur, Turkey

a r t i c l e

i n f o

Article history: Received 7 December 2014 Accepted 11 March 2015 Available online 10 April 2015 Keywords: HCCI Isopropanol n-Butanol Alcohol Performance Combustion

a b s t r a c t An experimental study was conducted in a single cylinder, four stroke port injection Ricardo Hydra test engine in order to determine the effects of pure n-heptane, the blends of n-heptane and n-butanol fuels B20, B30, B40 (including 20%, 30%, 40% n-butanol and 80%, 70%, 60% n-heptane by vol. respectively) and the blends of n-heptane and isopropanol fuels P20, P30, P40 (including 20%, 30%, 40% isopropanol and 80%, 70%, 60% n-heptane by vol. respectively) on HCCI combustion. Combustion and performance characteristics of n-heptane, n-butanol and isopropanol were investigated at constant engine speed of 1500 rpm and k = 2 in a HCCI engine. The effects of inlet air temperature were also examined on HCCI combustion. The test results showed that the start of combustion was advanced with the increasing of inlet air temperature for all test fuels. Start of combustion delayed with increasing percentage of n-butanol and isopropanol in the test fuels. Knocking combustion was seen with B20 and n-heptane test fuels. Minimum combustion duration was observed in case of using B40. Almost zero NO emissions were measured with test fuels apart from n-heptane and B20. The test results also showed that CO and HC emissions decreased with the increase of inlet air temperature for all test fuels. Isopropanol showed stronger resistance for knocking compared to n-butanol in HCCI combustion due to its higher octane number. It was determined that n-butanol was more advantageous according to isopropanol as thermal efficiency. As a result it was found that the HCCI operation range can be extended using high octane number alcohols away from knocking combustion and autoignition can be controlled. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Exhaust emissions caused from motor vehicles and decreasing petroleum reserves are main challenges for engine researchers and producers. Exhaust emissions produced by motor vehicles are harmful to people and environment [1–6]. Using alternative fuels and different exhaust gas aftertreatment systems are the most common methods in order to meet exhaust emissions regulations. Furthermore, emissions can be reduced with the increase of thermal efficiency due to increasing the compression ratio in the internal combustion engines. However, the compression ratio of SI engines cannot be increased due to detonation. Although the thermal efficiency of CI engines is higher than SI engines, they emit high NOx and PM. At this point, HCCI combustion has a big potential due to reducing NOx and PM emissions simultaneously with a higher thermal efficiency [7–13]. HCCI combustion is highly dependent on the chemical kinetics, mixture composition and E-mail address: [email protected] http://dx.doi.org/10.1016/j.enconman.2015.03.043 0196-8904/Ó 2015 Elsevier Ltd. All rights reserved.

the temperature before the auto-ignition [9,10]. However, there are still some difficulties on HCCI combustion in order to be used in the internal combustion engines. First, autoignition occurs simultaneously and spontaneously across the combustion chamber. This spontaneous and sudden combustion causes a rapid heat release rate resulting in knocking. In contrast, misfiring problem is seen at lower engine loads. Secondly, CO and HC emissions increase due to leaner mixture and lower combustion temperature in HCCI engines, because CO emissions are strongly affected by combustion temperature Third one is to control the combustion phasing [10–16]. To eliminate these problems possible solutions such as EGR, variable valve timing and variable compression ratio and high octane number fuels are proposed in order to slow down rapid heat release and control the combustion phasing in HCCI combustion. Thus, considerable attention should be directed in HCCI combustion. Fuel composition effect, which determines the operating range of HCCI, should be understood well. At this point, the chemical properties and the molecule structure of the fuel affect the HCCI combustion substantially [17–19]. So, one common

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A. Uyumaz / Energy Conversion and Management 98 (2015) 199–207

Nomenclature ATDC BTDC CI CO dp dV dh EGR HC HCCI imep k mfuel MTBE NOx

after top dead center before top dead center compression ignition carbon monoxide (%) the variation of cylinder pressure (bar) the variation of cylinder volume (m3) the variation of crank angle (°) exhaust gas recirculation hydrocarbon (ppm) homogeneous charged compression ignition indicated mean effective pressure (bar) the ratio of specific heat values consumed fuel per cycle (kg/cycle) methyl tert-butyl ether nitrogen oxides (ppm)

way to achieve stable autoignition and extend the HCCI operating range is to use the high octane number alcohol fuels based on renewable energy source. The temperature and the production of the radicals can also be increased when high octane number fuels are used as suppression additive fuel in HCCI combustion [17,19–22]. Furthermore, alcohols contain more oxygen resulting improved combustion and less pollution [23,24]. Cooling effect is also observed on charge mixture as the vaporization heat of alcohols is higher than that of gasoline. In this way, higher pressure rise rate may be also prevented when alcohols are used in HCCI engines. In this regard, n-butanol and isopropanol have a big attractiveness on the usage in HCCI engines due to knocking resistance, controlling rapid heat release rate [25–31]. But there is not enough study regarding two fuels on HCCI combustion. A few studies have been applied and discussed on the effects of alcohols in HCCI combustion in recent publications [10,17,26–28]. Lü et al. [22] investigated the effectiveness of inhibition of HCCI combustion using additive fuels (MTBE, isopropanol, ethanol and methanol). They determined that methanol has shown the most suppression effect among the other test fuels (isopropanol, ethanol and methanol). Minimum suppression effect was obtained with MTBE. However, they determined that ethanol was the best additive when the operating range, thermal efficiency and emissions were considered. Saisirirat et al. [17] evaluated the effects of 1-butanol and compared to pure n-heptane and n-heptane/ethanol mixture fuels on HCCI combustion. They performed the modeling of constant volume combustion in order to discuss engine results. Yao et al. [10] studied the effects of the blends of n-butanol and diesel with EGR on combustion, efficiency and exhaust emissions in a direct injection diesel engine. They showed that peak cylinder pressure and heat release rate increased with the increase of amount of butanol at low EGR rates. He et al. [32] conducted an experimental study in order to determine the effects of n-butanol in HCCI engine equipped with variable valve timing and lift mechanisms. The test results showed that the start of autoignition was advanced with engine speed. He et al. [33] presented an another study in order to investigate the effects of gasoline, 30% n-butanol and 70% gasoline by vol., and pure n-butanol in HCCI combustion using negative overlap and variable valve timing. Numerical studies were also conducted in order to observe HCCI combustion. Neshat and Saray [34] developed a new chemical kinetic mechanism for HCCI combustion using multi zone model in order to predict cylinder pressure and emissions. In [35], numerical study was performed in order to examine the role of fuel reactivity gradient in RCCI using Kiva4-Chemkin code. It was shown that fuel reactivity gradient retarded the ignition timing and reduced the heat

OH p PM RCCI SI SOC TDC UEGO V Vd W net dQ dQ heat

gT Q LHV

hydroxyl radical cylinder pressure (bar) particulate matter reactivity controlled compression ignition spark ignition start of combustion top dead center universal exhaust gas oxygen cylinder volume (m3) swept volume (m3) net work (J) heat release rate (J) heat transfer to cylinder walls (J) thermal efficiency the heating value of the fuel (kJ/kg)

release rate. Vuilleumier et al. [36] aimed to examine the intermediate temperature heat release in HCCI engines using ethanol/ n-heptane mixtures. They also modeled the combustion process using single zone HCCI model. They used the simulation results in order to identify the dominant reaction pathways contributing to intermediate temperature heat release. They found good agreement with pre-ignition pressure rise and heat release rate between experimental and modeling results. They also found that H-atom abstraction contributed reaction pathways to intermediate temperature heat release. In this study, the effects of pure n-heptane, and isopropanol/nheptane mixtures and n-butanol/n-heptane mixtures were investigated on HCCI combustion, performance and emissions of a single cylinder, four stroke, port injection Ricardo Hydra gasoline HCCI engine. N-heptane percentages used in the isopropanol and n-butanol mixtures were chosen 60%, 70% and 80% by volume. Experimental study was performed at 1500 rpm engine speed and constant lambda k = 2 at different inlet air temperatures of 313 K, 333 K, 353 K, 373 K and 393 K in order to observe the controlling of HCCI combustion. The variation of cylinder pressures, heat release rates, the starts of combustion and combustion durations was investigated in case of HCCI combustion with isopropanol/n-heptane mixtures, n-butanol/n-heptane mixtures and pure n-heptane. 2. Experimental setup and procedures A single cylinder, four stroke, port injection gasoline HCCI engine was used in the experiments. The technical specifications of the test engine are seen in Table 1. The test engine was coupled

Table 1 The technical specifications of the test engine. Model

Ricardo-Hydra

Cylinder number Cylinder bore (mm) Stroke (mm) Swept volume (cc) Compression ratio Maximum power output (kW) Maximum engine speed (rpm) Valve timing

1 80.26 88.90 540 13:1 15

Valve lift

5400 IVO/EVC 12° before top dead center/56° After bottom dead center Intake/exhaust 5.5/3.5

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Fig. 1. The schematic view of the experimental setup.

with McClure DC dynamometer which was rated 30 kW/6500 rpm engine speed. The schematic view of the experimental setup is given in Fig. 1. The injection pulse, engine speed, engine coolant, oil temperatures and inlet air temperature can be controlled and measured from the dynamometer control panel. The test fuel was injected into the intake port at constant crank angle degree. The injection duration was changed using potentiometer on the dynamometer control panel in order to adjust k = 2. Air heating system was mounted in the entrance of the intake manifold. Inlet air temperature was also measured using K-type thermocouple placed in the intake manifold and was held constant by closedloop controller. In the experiments, inlet air temperature was changed between 313 K and 393 K with step of 20 K. The coolant and engine oil temperatures were fixed at 358 K and 348 K respectively in order to prevent incomparability and measurement failures. So, the tests were conducted at steady state operation conditions. In-cylinder pressure was measured using Kistler model 6121 piezoelectric pressure transducer. Pressure data were amplified using Cussons P4110 combustion analysis device and then converted to digital signals using National Instruments USB 6259 data acquisition card with a timing resolution of 0.36 crank angle degrees. Digital cylinder pressure data were recorded in the computer. The cylinder pressure signals were obtained by averaging the sampled pressure data of 50 consecutive cycles for a specific condition in order to eliminate the cyclic variations. During the experiments fueling ratio and engine speed were kept constant at k = 2 and 1500 rpm respectively. The compression ratio of the test engine can be varied from 5:1 to 13:1. The compression ratio of the engine was fixed at 13:1 for achieving stable HCCI combustion and occurring autoignition easier. Besides, CO, HC and NO emissions were measured using exhaust gas analyzer. The technical specifications of the exhaust gas analyzer are given in Table 2. UEGO sensor was placed in the exhaust line in order to measure air/fuel ratio. UEGO sensor detects the amount of oxygen contained in the exhaust gases. Air/fuel ratio was also measured and controlled from the exhaust gas analyzer. The test

Table 2 The technical specifications of the exhaust gas analyzer.

CO (%) HC (ppm) NO (ppm) CO2 (%) O2 (%) Lambda

Operating range

Accuracy

0–15 0–9999 0–5000 0–20 0–25 0.6–4

0.001 1 ppm 1 ppm 0.1 0.01% 0.001

engine was first operated on SI mode and engine was warmed up, and then HCCI combustion was achieved by switching off the spark ignition. The experiments were performed with pure n-heptane, isopropanol/n-heptane mixtures and n-butanol/n-heptane mixtures at different inlet air temperatures on HCCI combustion. N-heptane was used as the base fuel in the experiments and compared with the other blends of fuels in order to see the effects of alcohols on HCCI combustion. The abbreviations and the chemical properties of the test fuels are given in Tables 3 and 4 respectively. Raw cylinder pressure data were processed using an algorithm prepared in Matlab. Heat transfer occurs from cylinder to the cylinder walls. It was considered and calculated in the algorithm. Cylinder charge mixture was also assumed to be constant. In addition, in-cylinder mass was considered to be ideal gas [7]. Imep was

Table 3 Percentage of fuels and abbreviations. Abbreviation

Percentage of fuels

B20 B30 B40 P20 P30 P40

20% 30% 40% 20% 30% 40%

n-butanol 80% n-heptane n-butanol 70% n-heptane n-butanol 60% n-heptane isopropanol 80% n-heptane isopropanol 70% n-heptane isopropanol 60% n-heptane

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Table 4 The chemical properties of the test fuels [17,31].

Chemical formula Density (kg/m3) Octane number Lower heating value (MJ/kg) Boiling point (°C) Molar mass (g/mol)

n-Heptane

Isopropanol

n-Butanol

C7H16 679.5 – 44.56 98 100.16

(CH3)2CHOH 809 107 30.447 82 60.10

C4H10O 808 92 35.1 117.7 74.12

calculated using Eq. (1). In Eq. (1). W net and V d refer the net work and cylinder swept volume. Net work was calculated by Eq. (2).

imep ¼

W net ¼

W net Vd

Z

pdV

ð1Þ

ð2Þ

In Eq. (2), p and dV are the cylinder pressure and the variation of cylinder volume. Heat release rate was calculated using cylinder pressure and cylinder volume by Eq. (3).

dQ k dV 1 dP dQ heat ¼ P þ V þ dh k 1 dh k 1 dh dh

ð3Þ

dQ is the heat release dependent on the variation of crank angle dh. dQ heat dh

is the heat transfer to the cylinder walls. k is the ratio of specific heat values. Thermal efficiency can be defined as the ratio between the net work and released energy from fuel. It is also dimensionless performance parameter in the internal combustion engines [7,37]. Thermal efficiency ðgT Þwas calculated by

gT ¼

W net mfuel1 :Q LHV1 þ mfuel2 :Q LHV2

ð4Þ

mfuel represents the consumed fuel per cycle. QLHV is the heating value of the test fuel. 3. Results and discussion The experiments were performed at 1500 rpm engine speed with different inlet air temperatures (313 K, 333 K, 353 K, 373 K and 393 K). Lambda (k = 2) was selected for stable HCCI combustion without knocking. Engine speed was kept constant at 1500 rpm in order to avoid misfiring on HCCI combustion mode. Inlet air temperature plays an important role on HCCI combustion, because HCCI combustion depends on the chemical kinetics. Charge mixture is compressed until autoignition temperature reaches [7,12]. In addition, suitable fuel should be selected avoiding misfire and knocking. In this way, alcohols resist to not only knocking combustion but also reducing the harmful exhaust emissions in HCCI combustion. As an alternative fuel, alcohol seems to be the most promising choices among other fuels in order to control HCCI combustion. Fig. 2 illustrates the effects of isopropanol and n-butanol fuel mixtures on cylinder pressure and heat release rate dependent on the crank angle with different inlet air temperatures. N-heptane was used as base fuel in the experiments and the test results were compared with alcohols/n-heptane mixtures. The pressure rise rate increased too much and knocking combustion occured with B20 and P20 fuels. The octane number of n-butanol is lower than isopropanol. It is clearly seen from Fig. 1 that knocking tendency with B20 is higher than P20 because of higher octane number of isopropanol than n-butanol. It could be also mentioned that SOC was delayed as the amount of alcohols increased in the test fuels for isopropanol and n-butanol. It is possible to say that autoignition was increased with the increase of inlet air temperature for all test fuels. More molecules participated to the chemical

reactions. Besides, the chemical reactions were improved between fuel and oxygen molecules at higher inlet air temperatures. So, autoignition occured easily. Maximum cylinder pressure was obtained with P20 and P30. Heat release rate increases with the increase of inlet air temperature like the cylinder pressure. It was observed that the most significant effect of inlet air temperature on HCCI combustion was autoignition timing. One of the most important finding was that knocking tendency disappeared when the amount of alcohols increased in the fuel mixtures. So, more stable HCCI combustion occured. Although the SOC was affected by the inlet air temperature, there was a slight differences on cylinder pressure for all test fuels. Fig. 3 shows the variation of the cylinder pressures and heat release rates with pure n-heptane at different inlet air temperatures. It can be concluded from Fig. 3 that the autoignition timing was advanced with the increase of inlet air temperature. Furthermore, it can also be clearly seen from Fig. 3 that rapid heat release rate occurs due to higher pressure rise rate. Thus, HCCI combustion deteriorated due to knocking and cylinder pressure decreased with the increase of inlet air temperature. The knocking resistance of n-heptane is already zero. In addition this, higher inlet air temperatures cause to knocking combustion with n-heptane. The lower heating value of n-heptane is higher than isopropanol and n-butanol. As a result, undesirable knocking combustion occured. As mentioned above ideal HCCI fuel should have knocking resistance. At high inlet air temperatures autoignition can occur easily in HCCI combustion. Fig. 4 shows the results of cylinder pressure and heat release rate variation with test fuels at 373 K and 393 K inlet air temperatures. As can be seen in Fig. 5 the start of combustion was delayed with the increase of the amount of the n-butanol in the test fuels. N-heptane was autoignited earlier than all the other test fuels due to zero octane number and higher heating value. It is clear that maximum cylinder pressure decreased as the amount of n-butanol increased in the test fuels, because the heating value of the n-butanol is lower than n-heptane. Stable autoignition occured. Similar effect was observed with isopropanol at 373 K and 393 K inlet air temperatures. But maximum cylinder pressure and heat release rate increased with P40 comparing to P30. At 393 K maximum cylinder pressure and heat release were obtained with P20. Maximum cylinder pressure decreased with n-heptane due to knocking and earlier autoignition. In-cylinder temperature and pressure should be adequately high at the end of compression stroke in order to start the chemical reactions in HCCI combustion. The in-cylinder temperature at the end of compression stroke increases with the increase of inlet air temperature. However, inlet air temperature is limited because of the higher pressure rise rate and knocking on HCCI combustion. It also causes the cyclic variations which are the indication of durability and stability of the internal combustion engines [2]. It can be also concluded from Fig. 4 that lower cylinder pressure and heat release rate were obtained with B40 and P30. But the test engine operated more stable away from the knocking operation limit. The effects of inlet air temperature with different test fuels on SOC are seen in Fig. 5. SOC is strongly controlled by chemical and physical properties of fuel, temperature and pressure at the end of compression stroke in HCCI combustion. Among them, inlet air temperature is one of the most dominant factor affecting the start of autoignition. In this study, SOC was determined with the heat release rate which rises from zero to positive value. It was found that increased inlet air temperature caused combustion to advance for each test fuel. Experimental values showed that earliest autoignition was obtained with n-heptane. Minimum SOC was obtained with B40 test fuel. It could be also implied that the SOC was delayed as the amount of isopropanol and n-butanol increased in the test fuels, because higher octane number alcohol tends to


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Fig. 2. The variation of cylinder pressure and heat release rate of HCCI combustion at constant lambda k = 2 and 1500 rpm engine speed with different inlet air temperatures and test fuels.

Fig. 3. Cylinder pressures and heat release rates of n-heptane at different inlet air temperatures (n = 1500 rpm, k = 2).

retard the autoignition. Earlier autoignition results in knocking combustion which damages the engine parts. As a result, high octane number alcohol fuel seems to be the most promising alternative fuel in HCCI combustion in order to prevent knocking combustion and to extend operating range. Combustion duration is usually defined as CA10–90 in combustion process. CA10, CA50 and CA90 are determined by the normalization of the cumulative heat release between 0 and 1 depending on the crank angle. The crank angle corresponding to 10% of charge mixture burnt in the combustion chamber is called CA10. Similarly, CA50 defines the crank angle that the half of the charge mixture

completed to combust. In this study, CA50 was determined depending on the crank angle ATDC. The determination of the end of combustion is a little bit difficult because of heat transfer to the cylinder walls and incomplete combustion in the combustion chamber. So, CA10–90 is generally used to determine the combustion duration. CA10–90 is defined as the time interval between the 10% of charge mixture completed to combust and 90% of charge mixture completed to combust depending on the crank angle [7,12,37]. Fig. 6 shows the combustion duration (CA50 and CA10–90) of test fuels at different inlet air temperatures. Fig. 6a shows the variation of CA50 with test fuels at different inlet air temperatures. In Fig. 6a, negative crank angle degrees define the points versus crank angle where the CA50 was obtained before top dead center. CA50 should be obtained slightly ATDC for better thermal efficiency [7,37]. It is possible to say that CA50 closes to TDC as the inlet air temperature increases. It was concluded from Fig. 6a that the half of the charge mixture completed to combust BTDC with n-heptane due to knocking. Besides, CA50 was determined far away from TDC when the amount of n-butanol increased in the test fuel. CA50 was also determined near TDC with P20 due to faster combustion. It can be inferred from Fig. 6b that CA10–90 decreases as inlet air temperature increases. Minimum combustion duration was obtained at 393 K for each test fuel. At high inlet air temperatures, CA10–90 decreases due to improvement of chemical reactions and increasing of the activated molecules during the combustion reaction. It results shorter combustion duration. Furthermore, combustion duration (CA10–90) decreased when the mass fraction of n-butanol increased in the test fuel. It was seen that the increase of octane number of test fuels caused the decrease on combustion duration. The addition of isopropanol also causes a decrease in combustion duration like n-butanol. But there

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Fig. 4. Cylinder pressure and heat release rate variation with test fuels at 373 K and 393 K air inlet temperatures (n = 1500 rpm, k = 2).

Fig. 5. The variation of SOC with different test fuels at different inlet air temperatures (n = 1500 rpm, k = 2).

was a small increase on CA10–90 when P40 is used in the experiments compared to P30. The longest combustion duration was obtained with n-heptane due to knocking.

Imep is significant performance parameter which indicates the averaged cylinder pressure exerted on piston during a cycle. It was calculated by Eq. (1). The variation of imep with test fuels at different inlet air temperatures on HCCI combustion is seen in Fig. 7. It is possible to say that imep decreases with the increase of inlet air temperature. It also causes the decrease of volumetric efficiency at high inlet air temperatures. The test engine has already operated with leaner mixture at constant excessive air coefficient k = 2 on HCCI combustion. So, the energy driven to the cylinder decreases. Thus, imep decreases especially at high inlet air temperatures as seen in Fig. 7. It can be also concluded from Fig. 7 that imep decreases with n-heptane due to knocking although it has higher heating value compared to isopropanol and n-butanol. There was no remarkable difference on imep when the isopropanol was used as an additive fuel in the experiments. But imep of B30 and B40 was higher than B20. Maximum imep was obtained with B30 as 3.50 bar at 313 K inlet air temperature. Imep increased by about 25.71% when compared to pure n-heptane at 313 K inlet air temperature. The reason of imep increase with B30 and B40 can be explained due to higher calorific value compared to isopropanol. In addition, the density of isopropanol and n-butanol is higher than n-heptane. It may be mentioned that it causes to drive more energy into the cylinder compared to n-heptane. Hence, the pressure at

Fig. 6. The variation of combustion duration (CA50 and CA10–90) on HCCI combustion (n = 1500 rpm, k = 2).


Fig. 7. The variation of indicated mean effective pressure on HCCI combustion (n = 1500 rpm, k = 2).

the end of combustion increases due to more fuel molecules participation into the chemical reactions. Thermal efficiency is defined as the conversion performance of the chemical energy of the fuel into mechanical energy in the internal combustion engines. The effects of test fuels and inlet air temperatures on thermal efficiency are shown in Fig. 8. Thermal efficiency increased with the increase of inlet air temperature with n-heptane, B20, B30, B40, P20 and P40. It can be clearly noticed that autoignition can occur easily at high inlet air temperatures and autoignition conditions improve at each point of the combustion chamber, because combustion occurs close to TDC with the increase of inlet air temperature as seen in Fig. 6a. So, thermal efficiency increases. But, small decrease was seen on thermal efficiency with n-heptane, B40 and P30 at 393 K inlet temperature. It can be said that CA50 was determined far away from TDC with B40 and P30 compared to other test fuel at 393 K inlet air temperature as seen in Fig. 6a, because combustion occured away from TDC. Maximum thermal efficiency was obtained as 49.31% with B20 test fuel at 393 K inlet air temperature. Similarly, CA50 was determined near TDC with B20 as seen in Fig. 6a. Chemical reactions improve when the combustion occurs near TDC due to higher cylinder pressure at smaller volume in the combustion chamber. In contrast, minimum thermal efficiency was obtained as 22.2% with P20 test fuel at 313 K inlet air temperature. Although combustion occured close to the top dead center with P20 at each inlet air temperature, knocking tendency was observed with P20 test fuel. Thus,

Fig. 8. The effects of test fuels and inlet air temperatures on thermal efficiency.

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thermal efficiency decreased. At 393 K inlet air temperature, thermal efficiency increased by about 28.8% with B20 compared to nheptane. Minimum thermal efficiency was calculated with P20 at each inlet air temperature compared to other test fuels. It is also clear that the addition of n-butanol improves the thermal efficiency compared to isopropanol due to higher heating value of n-butanol except for P30. The thermal efficiency of B30 and B40 test fuels was lower than to B20, because CA50 was also determined far away from TDC with B30 and B40 compared to B20. NOx and PM emissions can be simultaneously reduced in HCCI combustion, because HCCI engines operate with leaner homogenous charge mixtures. As NOx emissions were produced at high combustion temperatures, NO formation mechanisms could not occur due to lower end of combustion temperature. This is one of the most important advantage of HCCI combustion. In the present study, NO emissions were measured almost zero with all test fuel at each inlet air temperatures. However, NO emissions were only measured as 1 and 2 ppm with n-heptane and B20 at high inlet air temperatures due to knocking. Knocking results higher pressure rise rate and faster combustion. It can also be mentioned that higher inlet air temperatures increase the tendency of knocking. In HCCI combustion, CO and HC emissions are unfortunately generated because of lower end of combustion temperature and incomplete combustion [7,37]. The variation of CO emissions is presented in Fig. 9. As seen in Fig. 9, minimum CO emissions were produced with n-heptane because of higher combustion temperature and faster combustion due to knocking. It can be also concluded from Fig. 9 that CO emissions decrease with the increase of inlet air temperature, because CO could be oxidized due to higher inlet temperatures. So, CO2 formation is improved and the amount of CO emissions decreased. Maximum CO emissions were measured at 313 K inlet air temperature for all test fuels. It is also possible to say that CO emissions increase with the increase of the amount of n-butanol in the test fuel. CO emissions also increase with the increase of the amount of isopropanol except for P40. There was a reduction on CO emissions with P40 test fuel compared to P30 test fuel. B20 and P20 can be easily ignited due to lower octane number such as n-heptane. The autoignition occurs more difficult with other test fuels. It can be pointed that lower combustion temperature was obtained B30, B40, P30, P40 according to n-heptane, B20 and P20. Maximum CO emissions were measured as 0.144% with B40, 0.138% with B30 at 313 K inlet air temperature. The other important emission generated from the HCCI engines is HC emissions. The flame goes out on cold cylinder surface in the combustion chamber due to insufficient temperature during the combustion [7,37]. This is the most common way of allowing to

Fig. 9. The variation of CO emissions.

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controlling the combustion, preventing knocking combustion and reduction of CO and HC emissions in HCCI combustion.

References

Fig. 10. The variation of HC emissions.

produce HC emissions. Besides, flame cannot progress through the piston rings and any crevices in the combustion chamber. Hence, autoignition deteriorates and incomplete combustion occurs. In Fig. 10, the variation of HC emissions is seen. Fig. 10 shows that HC emissions decrease with the increase of inlet air temperature. The reason of this reduction is that chemical reactions improve and rapid combustion occurs at high inlet air temperatures. The productions of radicals accelerate with the increase of inlet air temperature and combustion reactions. Moreover, warmer inlet air temperature decreases the cooling effects of homogeneous leaner charge mixture. It can be mentioned that minimum HC emissions were measured with n-heptane and B20 compared to other test fuels. When isopropanol was used as an additive fuel, higher HC emissions were obtained especially at lower inlet air temperatures. Apart from n-heptane minimum HC emissions were measured with B20. It is possible to say that autoignition properties are deteriorated and HC emissions are generated when each test fuel is obtained by blending with alcohols. Maximum HC emissions were measured as 440 ppm and 438.88 ppm with P30 test fuel at 313 K and 333 K inlet air temperatures respectively.

4. Conclusions Test results showed that combustion was delayed with the increase of the amount of isopropanol and n-butanol in the test fuels. It was also seen that HCCI combustion was advanced with the increase of inlet air temperature. It can be also said that isopropanol is more suitable as fuel for controlling the HCCI combustion phasing compared to n-butanol due to higher octane number. When thermal efficiency is examined, n-butanol has advantage according to isopropanol. Thermal efficiency increased by about 28.8% with B20 compared to n-heptane at 393 K inlet air temperature. Knocking combustion occured and imep decreased at all inlet air temperatures with n-heptane. Almost zero NO emissions were measured with test fuels. However, NO emissions were only measured as 1 and 2 ppm with B20 and n-heptane at high inlet air temperatures due to knocking. It can be said that CO emissions increased with the increase of n-butanol in the test fuels. Higher HC emissions were obtained especially at lower inlet air temperatures when isopropanol was used as an additive fuel in the experiments. It is hoped that this experimental investigation contributes to the determination of proper fuel mixture and inlet air temperature for the problems such as extending the HCCI operation range,

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