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Experimental investigation of tetrahydrofuran combustion in homogeneous charge compression ignition (HCCI) engine: Effects of excess air coefficient, engine speed and inlet air temperature
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Experimental investigation of tetrahydrofuran combustion in homogeneous charge compression ignition (HCCI) engine: Effects of excess air coefficient, engine speed and inlet air temperature Q6 Q1
an Bilal Aydog Burdur Mehmet Akif Ersoy University, High Vocational School of Technical Sciences, 15100, Burdur, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 September 2019 Received in revised form 22 October 2019 Accepted 28 October 2019 Available online xxx
In this study, the effects of tetrahydrofuran (THF) which is nontoxic and generated from renewable environmentally friendly lignocelluloses, and n-heptane/THF blends on combustion, performance and emission characteristics were investigated at various lambda, engine speed and inlet air temperatures. Wide ranges of lambda value and engine speed were investigated and the results were presented in comparison to n-heptane as reference fuel. The combustion parameters such as cylinder pressure, heat release rate, in-cylinder gas temperature, CA10, CA50, thermal efficiency, ringing intensity, maximum pressure rise rate and imep, the performance parameters such as brake torque, power output, specific fuel consumption and HC and CO emissions were determined. Operating range of the HCCI engine was also determined. The results showed that, increasing the lambda value decreased both the in-cylinder pressure and the heat release rate for all test fuels. The addition of tetrahydrofuran led to retard combustion phasing. Thermal efficiency increased about 54% for F60N40 compared to n-heptane at 60 C inlet air temperature, 1200 rpm engine speed and l ¼ 2.2. The results also showed that HC and CO emissions increased with the increase of tetrahydrofuran. Furthermore, tetrahydrofuran caused to expand HCCI operating range towards to knocking and misfiring boundaries. © 2019 Published by Elsevier Ltd on behalf of Energy Institute.
Keywords: Tetrahydrofuran HCCI Combustion Performance Emission
1. Introduction Since the energy crisis and environmental pollution are concerned as a priority problem all over the world, it has become very important issue to use alternative fuels and combustion modes in the usage of internal combustion engines due to strict rules in emissions and economic regulations to be applied in the future [1e7]. Great attention has been received by researchers on homogenous charge compression ignition (HCCI) owing to reduced emissions and increased thermal efficiency [8e12]. The main consideration in HCCI mode is the usage of a sufficient lean/diluted, premixed air-fuel mixture that can keep the temperature below 1900 C to minimize the NOx and particulate emissions [13]. Two different heat release rate phases are observed in HCCI combustion. The first heat release rate stage is Low Temperature Heat Release (LTHR) and the second stage, also called the main stage is considered as High Temperature Heat Release (HTHR) [14e17]. These stages are separated by Negative Temperature Coefficient (NTC) and determined by the chemical kinetics of fuel reactions in low and high temperature ranges. Many researchers has already studied about the detailed chemical kinetics of the fuels [18e22]. Knocking, misfiring, uncontrollable ignition and narrow operating range are the main existing problems which limit the usage of HCCI mode [23e27]. Difficulties in HCCI mode led the researchers to investigate various fuels and operating parameters. The work in Ref. [13] studied the combustion and emission characteristics of modified four cylinder and four stroke, port injected HCCI engine fueled with ethanol and methanol at different engine speeds. They analyzed the effects of intake air temperature and air-fuel ratio and indicated that maximum incylinder pressure was observed for the richest fuel mixture (l ¼ 2), and the minimum in-cylinder pressure for the leanest fuel mixture (l ¼ 2.6). It has been observed that the maximum in-cylinder pressure increased as the intake air temperature increased. This increase was explained with the higher initial temperature of the charge and earlier auto-ignition caused an advanced combustion phase. The authors in Ref. [28] searched the effects of octane rating on combustion in dual fuel HCCI-DI engine fueled with n-heptane. They tested n-heptane,
E-mail address: [email protected]. https://doi.org/10.1016/j.joei.2019.10.009 1743-9671/© 2019 Published by Elsevier Ltd on behalf of Energy Institute.
an, Experimental investigation of tetrahydrofuran combustion in homogeneous charge compression ignition Please cite this article as: B. Aydog (HCCI) engine: Effects of excess air coefficient, engine speed and inlet air temperature, Journal of the Energy Institute, https://doi.org/10.1016/ j.joei.2019.10.009
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1 RON25 (25% iso-octaneþ75% n-heptane), RON50 (50% iso-octaneþ50% n-heptane), RON75 (75 iso-octaneþ 25% n-heptane) and iso-octane 2 at 1800 rpm engine speed. The experiments showed that dual fuel sequential combustion was achieved when the fuel with higher octane 3 number was used directly injected fuel. It was also reported that direct injected fuel ignition is strongly depend on the active radicals or 4 thermal-active radicals. Saisirirat et al. [29] evaluated the combustion characteristics of four cylinder, four stroke direct injection HCCI 5 engine. The jet-stirred reactor was fueled with 1-butanol/n-heptane and ethanol/n-heptane blends. The experiments were performed at 0.3 6 equivalence ratio, 16:1 compression ratio, 1500 rpm engine speed and inlet air temperature was fixed at 80 C. The start of the combustion 7 was retarded with the increase of alcohol in the n-heptane/air mixture owing to the higher octane number. The results showed that the 8 addition of alcohol into the mixture increased the ignition delay. The work in Ref. [30] conducted an improvement of HCCI engine com9 bustion fueled with dimethyl ether. The experiments were performed with port fuel injector and direct injector at different air excess ratio 10 (3.1e4.8), different injection timing (20e350 CAD) and different EGR rates (0%e60%). They reported that the start of combustion was 11 delayed with direct injection and the gross indicated mean effective pressure increased by EGR due to the prolonged combustion with EGR. 12 IMEPgross increased 22% and 55% by direct injection with optimal injection and EGR, respectively. Ying et al. [31] investigated the combustion 13 and emissions characteristics of HCCI engine fueled with dimethyl ether at 1400 rpm and different port DME aspiration rate. Combustion 14 duration increased by decreasing of port DME aspiration quantity. CO and HC emissions were lower for HCCI-DI. The authors in Ref. [32] 15 studied combustion and emission characteristics and performance of HCCI engine fueled with methanol. The intake charge temperature has 16 strongly effected the combustion pressure and heat release rate, namely they both decreased with increasing of the intake air temperature. 17 The maximum thermal efficiency was achieved at 7.5 oCA CA50 value. Lawler et al. [33] investigated the effects of controlled glow plug on 18 the combustion characteristics of HCCI engine. They found that combustion was advanced when the glow plug increased. They reported that 19 actively controlled glow plug can also be used for controlling the HRR in HCCI engine. Furthermore, HC and NOx emission decreased and 20 combustion efficiency improved with the usage of glow plug. 21 One strategy to eliminate the problems of the HCCI combustion is the usage of a blend of two fuels that have different auto-ignition 22 characteristics. Tetrahydrofuran is relatively nontoxic and can be obtained directly from the renewable lignocelluloses so that it is widely 23 used ecologically friendly solvent in many applications [34]. The study presented here is one of the first investigations to examine the 24 tetrahydrofuran as a fuel in HCCI combustion mode. The aim of this paper is to show the effects of using tetrahydrofuran (C4H8O) (THF)/n25 heptane fuel blends on combustion, emission and performance. Tetrahydrofuran percentages used in the n-heptane were chosen 20%, 40% 26 and 60% by volume. Experiments were performed between 800 and 1800 rpm engine speed, at different lambda values and different inlet 27 air temperatures. This paper presents the experimental findings on in-cylinder pressure, heat release rate (HRR), start of combustion (SOC), 28 combustion duration, ringing intensity (RI), maximum pressure rising rate (MPRR), indicated mean effective pressure (imep), thermal ef29 ficiency, brake torque, specific fuel consumption and CO and HC emissions. 30 31 2. Experimental setup and procedures 32 33 The experiments were carried out in a HCCI engine which was converted from a single cylinder, four stroke, spark ignition Ricardo Hydra 34 test engine. DC dynamometer that is capable of 30 kW power absorbtion at 6500 rpm was connected to the test engine. The schematical 35 view of the experimental setup was given in Fig. 1 and the specifications of the engine are seen in Table 1. The engine was warmed up before 36 the experiments. Firstly, test engine was run in SI mode and then the ignition was switched off for HCCI mode after reaching the operating 37 range. Coolant and engine oil temperatures were kept constant at 75 C and 55 C, respectively for stable and durable operation. Exhaust gas 38 temperatures were measured with K-type thermocouple. 39 The experiments were performed between 1.69 and 2.99 lambda values, engine speeds from 800 to 1800 rpm and inlet air temperatures 40 of 60e80 C at wide open throttle on HCCI mode. In-cylinder pressure was measured with Kistler 6121 model piezoelectric pressure sensor 41 which was placed at cylinder head. Engine speed was measured and top dead center was determined with encoder which produces 1000 42 pulses per rotation. Cussons P4110 combustion analyzer was used to amplify crude in-cylinder pressure signals. National Instrument data 43 acquisition card converted analog signals to digital signals. Converted digital signals were recorded via the computer. The specific MATLAB 44 programme was used to define the combustion parameters like HRR, CA50, CA10 etc. from the in-cylinder pressure data. Consecutive 50 45 cycles were averaged for each test in order to eliminate cyclic variations. Four types of fuels were used in the experiments. 20% 46 tetrahydrofuran-80% n-heptane (F20N80), 40% tetrahydrofuran-60% n-heptane (F40N60), 60%tetrahydrofuran-40% n-heptane (F60N40) and 47 n-heptane (N100) were experienced. Table 2 presents the properties of the test fuels. As it was mentioned above chemical kinetics of the fuel 48 strongly effect the HCCI combustion. Tetrahydrofuran is flammable and oxygenated fuel and has higher octane number and autoignition 49 temperature than that of n-heptane. 50 HC, CO and lambda measured with Bosch exhaust gas analyzer which was placed at the exhaust line. Both gas analyzer and UEGO sensor 51 was used to measure lambda value and kept constant by controlling the injector pulse width on the dynamometer control panel during the 52 Q3 experiments. The specifications of the exhaust gas analyzer were given at Table 3 (see Table 4). 53 Heat release rate which is total heat release was calculated according to the first law of thermodynamics. Within this scope, charge 54 mixture taken into the cylinder was assumed as ideal gas and gas leakages and heat losses from valve and piston rings were neglected. Eq. 55 (1) was used to calculate HRR. 56 57 dQ k dV 1 dP dQheat P V ¼ þ þ (1) 58 dq k 1 dq k 1 dq dq 59 Eq. (2) was used to compute cyclic variations of imep. 60 61 simep COVimep ¼ 100 (2) 62 X 63 64 Ringing intensity was calculated by Eq. (3) as below. 65 an, Experimental investigation of tetrahydrofuran combustion in homogeneous charge compression ignition Please cite this article as: B. Aydog (HCCI) engine: Effects of excess air coefficient, engine speed and inlet air temperature, Journal of the Energy Institute, https://doi.org/10.1016/ j.joei.2019.10.009
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3
Fig. 1. The schematical view of the experimental setup.
RI ¼
1 2g
b
dP dt
2 max
Pmax
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi g:R:Tmax
(3)
Uncertainty was calculated with Eq. (4).
DR ¼
"
vR Dx vx1 1
2
þ
vR Dx vx2 2
2
þ…þ
vR Dxn vxn
2 #0:5 (4)
3. Results and discussion 3.1. Combustion analyses Start of combustion (SOC) and end of combustion (EOC) are important parameters for HCCI combustion and can be defined by the tendency of heat release. Fig. 2 displays the in-cylinder pressure and heat release rate versus crank angle. SOC can be pointed out with the value where the heat release rate reaches positive value versus crank angle. CA10, CA50 and CA90 that determines the combustion stages can be obtained from normalized cumulative heat release. Just as, the point where the combustion of the 90% charge mixture completed is termed as the end of combustion (EOC). The area between low temperature heat release (LTHR) and high temperature heat release (HTHR) is called as negative temperature coefficient region and can be seen in Fig. 2. Fig. 3 presents in-cylinder pressure and HRR of N100, F20N80, F40N60 and F60N40 versus crank angle for different lambda values at 1200 rpm engine speed and 60 C inlet air temperature. Looking at Fig. 3a, it is apparent that knocking tendency was higher for richer mixtures and it reduced with increasing of lambda value for N100. In the meantime, HRR decreased for high lambda values for all test fuels due to lower fuel energy as a result of lean charge mixture. Retarded combustion was achieved with increasing lambda for all test fuels. When the amount of THF increased in the mixture, it is seen from Fig. 3d that knocking has disappeared. Knocking was observed for F20N80 and F40N60 in rich mixtures at l ¼ 1.71e1.85 but knocking began to disappear as the mixture progressed to become leaner. Pressure rise rate increases much with knocking. At this point molecular construction of the test fuel highly affected physical criterion. Density of tetrahydrofuran is higher than n-heptane. It can be mentioned that higher fuel molecule is sent to the combustion chamber by mass. So it can be thought that particle size of tetrahydrofuran is bigger than n-heptane. This situation can lead to harder vaporization of the fuel. On the an, Experimental investigation of tetrahydrofuran combustion in homogeneous charge compression ignition Please cite this article as: B. Aydog (HCCI) engine: Effects of excess air coefficient, engine speed and inlet air temperature, Journal of the Energy Institute, https://doi.org/10.1016/ j.joei.2019.10.009
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Table 1 Engine specifications. Model
Ricardo-Hydra
Number of Cylinders Cylinder bore (mm) Stroke (mm) Volume (cc) Compression ratio Power output (kW) Maximum engine speed (rpm) Valve timing Valve lift
1 80.26 88.90 540 13:1 15 5400 IVO/EVC 12 BTDC/12 ATDC Intake/exhaust 5.5/3.5
Table 2 Fuel properties [35e37].
Chemical formula Density (kg/m3) Octane number Calorific value (kJ/kg) Boiling point ( C) Auto-ignition temperature ( C) Oxygen content (%) Molar mass (g/mol)
Tetrahydrofuran
n-Heptane
C4H8O 889.2 72.9 34880 66 336.5 11.4 72.11
C7H16 679.5 0 45500 98 204 0 100.16
Table 3 The specifications of the exhaust gas analyzer.
CO (% vol) CO2 (% vol) HC (ppm) O2 (% vol)
l
NO (ppm vol)
Operating range
Accuracy
0.000e10.00 0.00e18.00 0e9999 0.00e22.00 0.500e9.999 0e5000
0.001 0.01 1 0.001 0.001 1
Table 4 Accuracies of the measurements and uncertainties in the calculated results.
Time (s) Temperature ( C) Fuel (g) Heating value (kJ/kg) Engine speed (rpm) Load (N) Torque (Nm) BTE BSFC (g/kW-h)
Accuracy
Uncertainty (%)
±0.5% ±1 C ±0.1 g ±0.1% ±1% ±0.25% e e e
e e e e e e ±0.08 ±1.29 ±1.18
contrary, released heat remains lower owing to lower calorific value of tetrahydrofuran. This phonomena caused to eliminate knocking tendency with fuel blends. Combustion occurred at before dead top center (BTDC) for N100 for all lambda values and closed to TDC with increasing of lambda. F20N80 and F40N60 showed similar combustion tendency but combustion occurred after top dead center (ATDC) for l ¼ 2.24, 2.42 and 2.99 with F40N60. Moreover, combustion was obtained ATDC for F60N40 with all lambda values. THF has higher octane number (72.9) than that of n-heptane (0). The higher the octane number, the higher the resistance to auto-ignition and the more controlled self-ignition. On the other hand, THF showed lower in-cylinder pressures and heat release rates thanks to lower calorific value. F40N60 and F60N40 showed better performance in view of lean combustion. Auto-ignition temperature of tethrahydrofuran is higher than n-heptane. It can be said that in-cylinder temperature increases much at the end of compression stroke due to higher auto-ignition temperature. This phonomena resulted in higher temperature history in the combustion chamber for self-ignition of lean charge mixture. On the other hand, higher oxygen content of tethrahydrofuran improved the oxidation reactions. So, HCCI combustion was achieved with leaner charge mixture. The variation of in-cylinder pressure and heat release rate at constant lambda value (2.2) and two different inlet air temperatures (60e80 C) is shown in Fig. 4. What stands out in Fig. 4 is that retarded combustion was achieved with the increasing of THF in the mixture. Besides, the maximum heat release rate increased for N100, F20N80 and F40N60 when the inlet air temperature increased to 80 C from 60 C. HCCI combustion was achieved with leaner mixtures (l ¼ 2.63, 2.99) and for fuel blends due to lower boiling temperature of THF. an, Experimental investigation of tetrahydrofuran combustion in homogeneous charge compression ignition Please cite this article as: B. Aydog (HCCI) engine: Effects of excess air coefficient, engine speed and inlet air temperature, Journal of the Energy Institute, https://doi.org/10.1016/ j.joei.2019.10.009
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Fig. 2. In-cylinder pressure and the HRR in HCCI combustion mode.
Fig. 3. In-cylinder pressure and the HRR of a) N100, b) F20N80, c) F40N60 and d) F60N40 in HCCI combustion mode versus crank angle for different lambda values.
an, Experimental investigation of tetrahydrofuran combustion in homogeneous charge compression ignition Please cite this article as: B. Aydog (HCCI) engine: Effects of excess air coefficient, engine speed and inlet air temperature, Journal of the Energy Institute, https://doi.org/10.1016/ j.joei.2019.10.009
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Fig. 4. In-cylinder pressure and the HRR of test fuels in HCCI combustion mode versus crank angle for constant l ¼ 2.2 at a) 60 C and b) 80 C inlet air temperature.
Higher heat is able to trap owing to lower boiling temperature with THF/n-heptane blends in the process of combustion reactions. Higher heat is absorbed to reach auto-ignition temperature due to higher boiling temperature of n-heptane. This event causes to decrease self ignition temperature when pure n-heptane was used. So, combustion could not be performed with leaner mixtures. Fig. 5 shows the variation of in-cylinder gas temperature in HCCI combustion mode versus crank angle at constant lambda and inlet air temperature. Heating value of the fuel and the combustion stages are the main factors effect the in-cylinder gas temperature. Combustion started earlier for N100 and F20N80 than the other test fuels. In other words, combustion retarded with increasing amount of THF in the mixture. This means, the increase of in-cylinder gas temperature started earlier for N100 and F20N80. N-heptane (98 C) has higher boiling temperature than tetrahydrofuran (66 C) which cause higher vaporization cooling. THF addition decreased the maximum in-cylinder temperature. The maximum in cylinder temperature was measured as about 1752 C and 1683 C for N100 and F60N40, respectively. Tetrahydrofuran has higher oxygen content. This characteristic showed better self ignition condition in the combustion process. In addition, knocking resistance of n-heptane is zero compared to tetrahydrofuran. Combustion reactions occur simultaneously and spontaneously. Combustion flame proceeds fastly during combustion. Combustion rate and speed also increase with n-heptane. Hence higher in-cylinder temperature is obtained with n-heptane. CA10 is estimated as the start of combustion (SOC) by many researchers in literature [38,39]. It was shown in Fig. 2. SOC was determined according to crank angle degree where the heat release rate value takes positive value. Fig. 6 demonstrates the variation of CA10 versus lambda for test fuels. As it is well known, the self-ignition can not be controlled directly in HCCI combustion. From the graph below we can see that, CA10 was retarded for all test fuels when the lambda increased. Auto ignition reactions are collapsed due to high lambda values resulting lower fuel concentration in combustion chamber. CA10 was delayed with the increase of THF in the charge mixture. However, CA10 was obtained BTDC with N100 and F20N80. Addition of 60% THF into n-heptane significantly delayed CA10 and it was observed ATDC for all lambda values. This interesting finding might be explained by the fact that n-heptane has higher vaporization cooling than THF due to higher boiling temperature and it is also known that CA10 is markedly effected by auto-ignition temperature. N-heptane (204 C) has lower auto-ignition temperature than that of THF (336.5 C). That means, THF needs more time to reach CA10 versus crank angle due to higher auto-ignition temperature. Chemical oxidation reactions improved self ignition condition at higher inlet air temperature because of higher
Fig. 5. In-cylinder gas temperature of test fuels in HCCI combustion mode versus crank angle for constant l ¼ 2.2.
an, Experimental investigation of tetrahydrofuran combustion in homogeneous charge compression ignition Please cite this article as: B. Aydog (HCCI) engine: Effects of excess air coefficient, engine speed and inlet air temperature, Journal of the Energy Institute, https://doi.org/10.1016/ j.joei.2019.10.009
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Fig. 6. The variation of CA10 versus lambda at a) 60 C and b) 80 C inlet air temperature.
combustion chamber temperature at the end of compression stroke. Higher inlet air temperature helps to reach auto-ignition temperature earlier. So, combustion was advanced at 80 C inlet air temperature. 3.2. Engine performance CA50 is one of the most challenging and major parameter because combustion is considerably influenced by charge and fuel properties. The fact remains that the balance between engine load and CA50 can control the emissions. It was reported that shifting CA50 caused higher CO and HC emissions but NOx emission increased when CA50 was advanced towards to TDC. Besides, thermal efficiency was also increased when CA50 was nearly observed ATDC [40,41]. Fig. 7 shows the variations of CA50 values and thermal efficiency of test fuels versus lambda. In Fig. 7a there is a clear trend of increasing CA50 and is closed to TDC with the increase of lambda value. A possible explanation for this might be that local lean mixture zones are formed in the absence of fuel concentration. Self ignition reactions are getting difficult. It is obvious that increasing the amount of THF in the mixture significantly increased CA50. Especially F60N40 showed the highest CA50 values for all lambda values and all CA50 values observed ATDC. It seems possible that this result is due to higher octane number of THF. CA50 values of N100, F20N80, F40N60 and F60N40 were 3.96 (BTDC), 2.16 (BTDC), 2.16 (ATDC) and 8.28 (ATDC) oCA, respectively, at 1200 rpm engine speed, 60 C inlet air temperature and l ¼ 2.2. Higher thermal efficiency is obtained when CA50 is occurred ATDC [36]. Fig. 7b represents the variation of thermal efficiency of test fuels at 1200 rpm engine speed, 60 C inlet air temperature. It was found that thermal efficiency increased by increasing lambda value. The maximum thermal efficiency was achieved as 34% for F60N40 at 1200 rpm engine speed, 60 C inlet air temperature and l ¼ 2.17. There is good agreement between CA50 and thermal efficiency. Maximum thermal efficiency is obtained when CA50 appears in 8e10 oCA ATDC in internal combustion engines. Thermal efficiency increased when CA50 was determined ATDC. Higher oxygen content of THF resulted in higher thermal efficiency owing to higher released heat and improved chemical reactions.
Fig. 7. The variations of a) CA50 and b) thermal efficiency.
an, Experimental investigation of tetrahydrofuran combustion in homogeneous charge compression ignition Please cite this article as: B. Aydog (HCCI) engine: Effects of excess air coefficient, engine speed and inlet air temperature, Journal of the Energy Institute, https://doi.org/10.1016/ j.joei.2019.10.009
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Fig. 8 depicts the variation of ringing intensity and maximum pressure rise rate versus lambda at 1200 rpm engine speed, 60 C inlet air temperature. RI and MPRR are the major parameters which characterize the intensity of knocking in HCCI and also used to determine the upper zone of HCCI operation [42e45]. From Fig. 8, it is apparent that both RI and MPRR decreased with increasing lambda for all test fuels. As it was expected that more amount of premixed charge is burned in rich mixtures resulting high pressure rise rate due to high combustion rate [46]. From the data in Fig. 8, it can be seen that F60N40 resulted in the lowest value of both RI and MPRR owing to more controllable self ignition reactions. Moreover, sudden and simultaneous heat release was eliminated due to higher octane number of THF. Hence, lower MPRR and RI were observed. Fig. 9 represents the variation of imep versus cycle number at 1200 rpm engine speed, 60 C inlet air temperature and l ¼ 2.2. Imep was calculated by dividing the net work to swept volume. The leaner conditions result in lower heat release at the end of combustion [25]. Besides, the velocity of flame decreases and oxidation reactions get worse with leaner premixed charge mixture. In Fig. 9 there is a clear trend of increasing on Imep with the increase of THF in the mixture. The addition of THF into n-heptane leads to obtain maximum incylinder pressure nearly ATDC because of higher octane number of THF. HCCI combustion occurred in a smaller in-cylinder volume with THF/n-heptane fuel blends. These phenomena also caused to obtain higher imep. Furthermore, THF includes more oxygen in its chemical structure. This property provides easily to meet oxygen and fuel molecules in the combustion chamber. The effect of engine speed on engine performance characteristics such as brake torque, power output and specific fuel consumption for test fuels was given in Fig. 10. Brake torque and power output shows similar tendency that firstly increased to a certain engine speed and then started to decrease. Likely, brake torque and power output decreased after maximum torque and power output speed due to gas leakages and heat losses. It can be clearly seen from Fig. 10 that increasing amount of THF into the mixture increased both brake torque and power output. This result may be explained by the fact that some addition of THF provided stable HCCI combustion because of higher octane number. The position of maximum in-cylinder pressure changed towards to ATDC with THF fuel blends. Although, gas leakages and absence of oxygen increased at higher engine speeds, THF supplied sufficient oxygen in order to complete self ignition reactions. So, higher torque
Fig. 8. The variation of a) ringing intensity and b) MPRR.
Fig. 9. The variation of imep versus cycle number.
an, Experimental investigation of tetrahydrofuran combustion in homogeneous charge compression ignition Please cite this article as: B. Aydog (HCCI) engine: Effects of excess air coefficient, engine speed and inlet air temperature, Journal of the Energy Institute, https://doi.org/10.1016/ j.joei.2019.10.009
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and power output were obtained. The maximum value of the brake torque was obtained as 7.2 Nm at 1000 rpm engine speed for F60N40 and it is higher about 63% than that of N100 test fuel. Power output of N100 was 0.58 kW at 1400 rpm engine speed. The corresponding value was found as 0.96 kW for F60N40 at 1400 rpm engine speed and it is higher about 65% than N100. When Fig. 10c was examined, it is possible to say that SFC decreased until 1400 rpm engine speed and then started to increase. Lower SFC was computed with the increase of THF. At higher engine speeds there is no enough time to suck oxygen into the combustion chamber for complete combustion. But, the oxygen content of THF caused to oxidize more fuel molecules resulting in lower SFC compared to n-heptane. The operation limits of HCCI combustion is restricted by misfiring and knocking. Especially the fuels with higher octane number are used to prevent knocking. The operating range of HCCI combustion versus engine speed with lambda values for test fuels was given in Fig. 11. Nheptane showed the narrowest operating range. In other words, HCCI combustion did not occur in a large misfiring and knocking zones for N100. Besides, it is obviously seen that operating range was expanded with increasing amount of tetrahydrofuran in the mixture. It was realized that largest operating range was obtained for F60N40. HCCI combustion was achieved at l ¼ 3.28 which is very lean charge mixture for F60N40 at 800 rpm engine speed and 60 C inlet air temperature. Similarly, test engine was run with F60N40 at 2.81 lambda value at knocking zone. When Fig. 11 was examined, the addition of THF leads to operate HCCI engine with higher imep in knocking boundaries. It was found that HCCI operating range was enlarged at misfiring zone with F40N60. The test engine could run until 1600 rpm for N100 and F60N40, and 1800 rpm for F20N80 and F40N60 on HCCI mode. Homogeneity of charge mixture improves at higher engine speeds. In spite of the fact that auto ignition conditions are getting worse in view of higher octane number of THF, fuel blends succeed to complete self ignition reactions due to higher content of oxygen. Hence, F40N60 showed reasonable performance in terms of stable HCCI operation at higher engine speeds. It can be said that F60N40 presented larger misfiring zone. Higher oxygen concentration of tetrahydrofuran improved chemical oxidation reactions. Combustion temperature increases with oxygen molecules. It also leads to prolonged combustion. Thus, selfignition can be also performed even with leaner mixtures. It was clearly seen from Fig. 11 that higher octane number of tetrahydrofuran resulted in higher resistance to knocking.
Fig. 10. The variation of a) brake torque, b) power output and c) specific fuel consumption versus engine speed.
an, Experimental investigation of tetrahydrofuran combustion in homogeneous charge compression ignition Please cite this article as: B. Aydog (HCCI) engine: Effects of excess air coefficient, engine speed and inlet air temperature, Journal of the Energy Institute, https://doi.org/10.1016/ j.joei.2019.10.009
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Fig. 11. The variation of imep versus engine speed.
3.3. Emissions In HCCI combustion mode, NOx emissions are negligible owing to low combustion temperature. Nevertheless, insufficient oxidation of the fuel occurs at low combustion temperature and HC emissions increase. The variation of HC emissions was given at Fig. 12a. As shown in Fig. 12a, HC emissions increased with the increase of lambda. The observed increase in HC may be explained by the fact that the amount of the fuel taken into the cylinder decreased and leaner conditions occurred and led to be seen low temperatures of burnout in-cylinder gasses with increasing value of the lambda. Low combustion temperatures resulted in incomplete combustion and HC emissions increased. On the other hand, adding THF into mixture increased HC emissions. This result may be attributed to the retarded combustion. Prolonged combustion of THF leads to reduce cylinder wall temperature with the moution of piston to the bottom TDC. The charge mixture is cooled and oncoming flame slows down on combustion chamber surface. Thus, HC is formed. The variation of CO emissions can be seen in Fig. 12b. As previously stated that HCCI combustion leads to low combustion temperatures which cause incomplete combustion. CO generation increases with increasing the rate of incomplete combustion. So it can be said that lambda is the major parameter which controls the CO generation. CO emissions increased with the increase of the lambda due to leaner mixture conditions which led to more incomplete combustion owing to low combustion temperature. Oxidation reactions slows down with low combustion temperatures and oxidation does not occur as intended. It can be seen from Fig. 12b that CO emissions slightly increased with the addition of THF into the mixture except F60N40. The increase of CO emissions for F60N40 was found higher than that of the other fuels especially at higher lambda values. The CO emissions of N100, F20N80, F40N60 and F60N40 were found as 0.07%, 0.083%, 0.086%, 0.114%, respectively, at 1200 rpm engine speed, 60 C inlet air temperature and l ¼ 2.2. These findings suggest that in general the usage of tetrahyrofuran/n-heptane blends help to improve HCCI combustion. The most important problems called knocking and misfiring which restrict the HCCI operating range can be eliminated with the addition of tetrahydrofuran into n-heptane due to higher octane number of the fuel. The second major finding was that the engine performance was
Fig. 12. The variation of a) HC and b) CO emissions versus lambda.
an, Experimental investigation of tetrahydrofuran combustion in homogeneous charge compression ignition Please cite this article as: B. Aydog (HCCI) engine: Effects of excess air coefficient, engine speed and inlet air temperature, Journal of the Energy Institute, https://doi.org/10.1016/ j.joei.2019.10.009
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enhanced with the THF/n-heptane blends. The principal limitation of this analysis was the increasing of HC and CO emissions as in all HCCI combustion mode owing to low combustion temperatures. 4. Conclusions The aim of the present research was to examine the effects of tetrahydrofuran and n-heptane blends on combustion, performance and emissions characteristics of HCCI engine were examined with varying lambda, inlet air temperature and engine speed. The results showed that in-cylinder pressure and heat release rate decreased with increasing the value of lambda for all test fuels. It was found that HCCI mode was generally obtained between l ¼ 1.66 to l ¼ 3.0 with test fuels. HCCI combustion was advanced with increasing inlet air temperature. Addition of tetrahydrofuran into the mixture retarded the combustion owing to higher octane number. Start of combustion was observed as 6.48 oCA (BTDC), 4.68 oCA (BTDC), 0.36 oCA (BTDC) and 4.32 oCA (ATDC) for N100, F20N80, F40N60 and F60N40, respectively, at 1200 rpm and l ¼ 2.2. Thermal efficiency was also increased with increasing amount of THF in the mixture. Thermal efficiency was obtained as 25% and 34% for N100 and F60N40, respectively. Ringing intensity significantly decreased with the usage of F60N40 compared to N100. The ringing intensity was obtained as 12.24 MW/m2, 18.52 MW/m2, 13.71 MW/m2 and 6.97 MW/m2 for N100, F20N80, F40N60 and F60N40, respectively, at 1200 rpm and l ¼ 2.2 Brake torque and power output increased with increasing THF addition into the fuel blend. The brake torque and power output increased about 64% and 63%, respectively, for F60N40 compared to N100. The HC emissions increased about 27% for F60N40 compared to N100 at l ¼ 2.2. Test results also showed that HCCI operating range can be expanded with the addition of THF in the Q4 n-heptane. Nomenclature
Wnet Vd P dV dQ dq dQheat dq
A2 hc n Tg Tw k mfuel QLVH
simep X
g dP dt Pmax
Tmax
DR xn
Net work (joule) Cylinder swept volume (m3) Cylinder pressure (bar) Variation of cylinder volume Heat release (J) Crank angle ( CA) Heat transfer to cylinder walls (J/ CA) Heat transfer surface area (m2) Heat transfer coefficient (W/m2K) Engine speed (rpm) In-cylinder average gas temperature (K) Combustion chamber wall temperature (K) Ratio of specific heat values Consumed fuel per cycle (kg) Lower heating value (kcal) Average of the indicated mean effective pressure Standard deviation Polytropic index Pressure rise rate Maximum in-cylinder pressure (bar) Maximum in-cylinder temperature (K) Uncertainty of the measurement Independent variables of the function and uncertainty
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an, Experimental investigation of tetrahydrofuran combustion in homogeneous charge compression ignition Please cite this article as: B. Aydog (HCCI) engine: Effects of excess air coefficient, engine speed and inlet air temperature, Journal of the Energy Institute, https://doi.org/10.1016/ j.joei.2019.10.009
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Experimental investigation of tetrahydrofuran combustion in homogeneous charge compression ignition (HCCI) engine: Effects of excess air coefficient, engine speed and inlet air temperature Q6 Q1
an Bilal Aydog Burdur Mehmet Akif Ersoy University, High Vocational School of Technical Sciences, 15100, Burdur, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 September 2019 Received in revised form 22 October 2019 Accepted 28 October 2019 Available online xxx
In this study, the effects of tetrahydrofuran (THF) which is nontoxic and generated from renewable environmentally friendly lignocelluloses, and n-heptane/THF blends on combustion, performance and emission characteristics were investigated at various lambda, engine speed and inlet air temperatures. Wide ranges of lambda value and engine speed were investigated and the results were presented in comparison to n-heptane as reference fuel. The combustion parameters such as cylinder pressure, heat release rate, in-cylinder gas temperature, CA10, CA50, thermal efficiency, ringing intensity, maximum pressure rise rate and imep, the performance parameters such as brake torque, power output, specific fuel consumption and HC and CO emissions were determined. Operating range of the HCCI engine was also determined. The results showed that, increasing the lambda value decreased both the in-cylinder pressure and the heat release rate for all test fuels. The addition of tetrahydrofuran led to retard combustion phasing. Thermal efficiency increased about 54% for F60N40 compared to n-heptane at 60 C inlet air temperature, 1200 rpm engine speed and l ¼ 2.2. The results also showed that HC and CO emissions increased with the increase of tetrahydrofuran. Furthermore, tetrahydrofuran caused to expand HCCI operating range towards to knocking and misfiring boundaries. © 2019 Published by Elsevier Ltd on behalf of Energy Institute.
Keywords: Tetrahydrofuran HCCI Combustion Performance Emission
1. Introduction Since the energy crisis and environmental pollution are concerned as a priority problem all over the world, it has become very important issue to use alternative fuels and combustion modes in the usage of internal combustion engines due to strict rules in emissions and economic regulations to be applied in the future [1e7]. Great attention has been received by researchers on homogenous charge compression ignition (HCCI) owing to reduced emissions and increased thermal efficiency [8e12]. The main consideration in HCCI mode is the usage of a sufficient lean/diluted, premixed air-fuel mixture that can keep the temperature below 1900 C to minimize the NOx and particulate emissions [13]. Two different heat release rate phases are observed in HCCI combustion. The first heat release rate stage is Low Temperature Heat Release (LTHR) and the second stage, also called the main stage is considered as High Temperature Heat Release (HTHR) [14e17]. These stages are separated by Negative Temperature Coefficient (NTC) and determined by the chemical kinetics of fuel reactions in low and high temperature ranges. Many researchers has already studied about the detailed chemical kinetics of the fuels [18e22]. Knocking, misfiring, uncontrollable ignition and narrow operating range are the main existing problems which limit the usage of HCCI mode [23e27]. Difficulties in HCCI mode led the researchers to investigate various fuels and operating parameters. The work in Ref. [13] studied the combustion and emission characteristics of modified four cylinder and four stroke, port injected HCCI engine fueled with ethanol and methanol at different engine speeds. They analyzed the effects of intake air temperature and air-fuel ratio and indicated that maximum incylinder pressure was observed for the richest fuel mixture (l ¼ 2), and the minimum in-cylinder pressure for the leanest fuel mixture (l ¼ 2.6). It has been observed that the maximum in-cylinder pressure increased as the intake air temperature increased. This increase was explained with the higher initial temperature of the charge and earlier auto-ignition caused an advanced combustion phase. The authors in Ref. [28] searched the effects of octane rating on combustion in dual fuel HCCI-DI engine fueled with n-heptane. They tested n-heptane,
E-mail address: [email protected]. https://doi.org/10.1016/j.joei.2019.10.009 1743-9671/© 2019 Published by Elsevier Ltd on behalf of Energy Institute.
an, Experimental investigation of tetrahydrofuran combustion in homogeneous charge compression ignition Please cite this article as: B. Aydog (HCCI) engine: Effects of excess air coefficient, engine speed and inlet air temperature, Journal of the Energy Institute, https://doi.org/10.1016/ j.joei.2019.10.009
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1 RON25 (25% iso-octaneþ75% n-heptane), RON50 (50% iso-octaneþ50% n-heptane), RON75 (75 iso-octaneþ 25% n-heptane) and iso-octane 2 at 1800 rpm engine speed. The experiments showed that dual fuel sequential combustion was achieved when the fuel with higher octane 3 number was used directly injected fuel. It was also reported that direct injected fuel ignition is strongly depend on the active radicals or 4 thermal-active radicals. Saisirirat et al. [29] evaluated the combustion characteristics of four cylinder, four stroke direct injection HCCI 5 engine. The jet-stirred reactor was fueled with 1-butanol/n-heptane and ethanol/n-heptane blends. The experiments were performed at 0.3 6 equivalence ratio, 16:1 compression ratio, 1500 rpm engine speed and inlet air temperature was fixed at 80 C. The start of the combustion 7 was retarded with the increase of alcohol in the n-heptane/air mixture owing to the higher octane number. The results showed that the 8 addition of alcohol into the mixture increased the ignition delay. The work in Ref. [30] conducted an improvement of HCCI engine com9 bustion fueled with dimethyl ether. The experiments were performed with port fuel injector and direct injector at different air excess ratio 10 (3.1e4.8), different injection timing (20e350 CAD) and different EGR rates (0%e60%). They reported that the start of combustion was 11 delayed with direct injection and the gross indicated mean effective pressure increased by EGR due to the prolonged combustion with EGR. 12 IMEPgross increased 22% and 55% by direct injection with optimal injection and EGR, respectively. Ying et al. [31] investigated the combustion 13 and emissions characteristics of HCCI engine fueled with dimethyl ether at 1400 rpm and different port DME aspiration rate. Combustion 14 duration increased by decreasing of port DME aspiration quantity. CO and HC emissions were lower for HCCI-DI. The authors in Ref. [32] 15 studied combustion and emission characteristics and performance of HCCI engine fueled with methanol. The intake charge temperature has 16 strongly effected the combustion pressure and heat release rate, namely they both decreased with increasing of the intake air temperature. 17 The maximum thermal efficiency was achieved at 7.5 oCA CA50 value. Lawler et al. [33] investigated the effects of controlled glow plug on 18 the combustion characteristics of HCCI engine. They found that combustion was advanced when the glow plug increased. They reported that 19 actively controlled glow plug can also be used for controlling the HRR in HCCI engine. Furthermore, HC and NOx emission decreased and 20 combustion efficiency improved with the usage of glow plug. 21 One strategy to eliminate the problems of the HCCI combustion is the usage of a blend of two fuels that have different auto-ignition 22 characteristics. Tetrahydrofuran is relatively nontoxic and can be obtained directly from the renewable lignocelluloses so that it is widely 23 used ecologically friendly solvent in many applications [34]. The study presented here is one of the first investigations to examine the 24 tetrahydrofuran as a fuel in HCCI combustion mode. The aim of this paper is to show the effects of using tetrahydrofuran (C4H8O) (THF)/n25 heptane fuel blends on combustion, emission and performance. Tetrahydrofuran percentages used in the n-heptane were chosen 20%, 40% 26 and 60% by volume. Experiments were performed between 800 and 1800 rpm engine speed, at different lambda values and different inlet 27 air temperatures. This paper presents the experimental findings on in-cylinder pressure, heat release rate (HRR), start of combustion (SOC), 28 combustion duration, ringing intensity (RI), maximum pressure rising rate (MPRR), indicated mean effective pressure (imep), thermal ef29 ficiency, brake torque, specific fuel consumption and CO and HC emissions. 30 31 2. Experimental setup and procedures 32 33 The experiments were carried out in a HCCI engine which was converted from a single cylinder, four stroke, spark ignition Ricardo Hydra 34 test engine. DC dynamometer that is capable of 30 kW power absorbtion at 6500 rpm was connected to the test engine. The schematical 35 view of the experimental setup was given in Fig. 1 and the specifications of the engine are seen in Table 1. The engine was warmed up before 36 the experiments. Firstly, test engine was run in SI mode and then the ignition was switched off for HCCI mode after reaching the operating 37 range. Coolant and engine oil temperatures were kept constant at 75 C and 55 C, respectively for stable and durable operation. Exhaust gas 38 temperatures were measured with K-type thermocouple. 39 The experiments were performed between 1.69 and 2.99 lambda values, engine speeds from 800 to 1800 rpm and inlet air temperatures 40 of 60e80 C at wide open throttle on HCCI mode. In-cylinder pressure was measured with Kistler 6121 model piezoelectric pressure sensor 41 which was placed at cylinder head. Engine speed was measured and top dead center was determined with encoder which produces 1000 42 pulses per rotation. Cussons P4110 combustion analyzer was used to amplify crude in-cylinder pressure signals. National Instrument data 43 acquisition card converted analog signals to digital signals. Converted digital signals were recorded via the computer. The specific MATLAB 44 programme was used to define the combustion parameters like HRR, CA50, CA10 etc. from the in-cylinder pressure data. Consecutive 50 45 cycles were averaged for each test in order to eliminate cyclic variations. Four types of fuels were used in the experiments. 20% 46 tetrahydrofuran-80% n-heptane (F20N80), 40% tetrahydrofuran-60% n-heptane (F40N60), 60%tetrahydrofuran-40% n-heptane (F60N40) and 47 n-heptane (N100) were experienced. Table 2 presents the properties of the test fuels. As it was mentioned above chemical kinetics of the fuel 48 strongly effect the HCCI combustion. Tetrahydrofuran is flammable and oxygenated fuel and has higher octane number and autoignition 49 temperature than that of n-heptane. 50 HC, CO and lambda measured with Bosch exhaust gas analyzer which was placed at the exhaust line. Both gas analyzer and UEGO sensor 51 was used to measure lambda value and kept constant by controlling the injector pulse width on the dynamometer control panel during the 52 Q3 experiments. The specifications of the exhaust gas analyzer were given at Table 3 (see Table 4). 53 Heat release rate which is total heat release was calculated according to the first law of thermodynamics. Within this scope, charge 54 mixture taken into the cylinder was assumed as ideal gas and gas leakages and heat losses from valve and piston rings were neglected. Eq. 55 (1) was used to calculate HRR. 56 57 dQ k dV 1 dP dQheat P V ¼ þ þ (1) 58 dq k 1 dq k 1 dq dq 59 Eq. (2) was used to compute cyclic variations of imep. 60 61 simep COVimep ¼ 100 (2) 62 X 63 64 Ringing intensity was calculated by Eq. (3) as below. 65 an, Experimental investigation of tetrahydrofuran combustion in homogeneous charge compression ignition Please cite this article as: B. Aydog (HCCI) engine: Effects of excess air coefficient, engine speed and inlet air temperature, Journal of the Energy Institute, https://doi.org/10.1016/ j.joei.2019.10.009
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3
Fig. 1. The schematical view of the experimental setup.
RI ¼
1 2g
b
dP dt
2 max
Pmax
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi g:R:Tmax
(3)
Uncertainty was calculated with Eq. (4).
DR ¼
"
vR Dx vx1 1
2
þ
vR Dx vx2 2
2
þ…þ
vR Dxn vxn
2 #0:5 (4)
3. Results and discussion 3.1. Combustion analyses Start of combustion (SOC) and end of combustion (EOC) are important parameters for HCCI combustion and can be defined by the tendency of heat release. Fig. 2 displays the in-cylinder pressure and heat release rate versus crank angle. SOC can be pointed out with the value where the heat release rate reaches positive value versus crank angle. CA10, CA50 and CA90 that determines the combustion stages can be obtained from normalized cumulative heat release. Just as, the point where the combustion of the 90% charge mixture completed is termed as the end of combustion (EOC). The area between low temperature heat release (LTHR) and high temperature heat release (HTHR) is called as negative temperature coefficient region and can be seen in Fig. 2. Fig. 3 presents in-cylinder pressure and HRR of N100, F20N80, F40N60 and F60N40 versus crank angle for different lambda values at 1200 rpm engine speed and 60 C inlet air temperature. Looking at Fig. 3a, it is apparent that knocking tendency was higher for richer mixtures and it reduced with increasing of lambda value for N100. In the meantime, HRR decreased for high lambda values for all test fuels due to lower fuel energy as a result of lean charge mixture. Retarded combustion was achieved with increasing lambda for all test fuels. When the amount of THF increased in the mixture, it is seen from Fig. 3d that knocking has disappeared. Knocking was observed for F20N80 and F40N60 in rich mixtures at l ¼ 1.71e1.85 but knocking began to disappear as the mixture progressed to become leaner. Pressure rise rate increases much with knocking. At this point molecular construction of the test fuel highly affected physical criterion. Density of tetrahydrofuran is higher than n-heptane. It can be mentioned that higher fuel molecule is sent to the combustion chamber by mass. So it can be thought that particle size of tetrahydrofuran is bigger than n-heptane. This situation can lead to harder vaporization of the fuel. On the an, Experimental investigation of tetrahydrofuran combustion in homogeneous charge compression ignition Please cite this article as: B. Aydog (HCCI) engine: Effects of excess air coefficient, engine speed and inlet air temperature, Journal of the Energy Institute, https://doi.org/10.1016/ j.joei.2019.10.009
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Table 1 Engine specifications. Model
Ricardo-Hydra
Number of Cylinders Cylinder bore (mm) Stroke (mm) Volume (cc) Compression ratio Power output (kW) Maximum engine speed (rpm) Valve timing Valve lift
1 80.26 88.90 540 13:1 15 5400 IVO/EVC 12 BTDC/12 ATDC Intake/exhaust 5.5/3.5
Table 2 Fuel properties [35e37].
Chemical formula Density (kg/m3) Octane number Calorific value (kJ/kg) Boiling point ( C) Auto-ignition temperature ( C) Oxygen content (%) Molar mass (g/mol)
Tetrahydrofuran
n-Heptane
C4H8O 889.2 72.9 34880 66 336.5 11.4 72.11
C7H16 679.5 0 45500 98 204 0 100.16
Table 3 The specifications of the exhaust gas analyzer.
CO (% vol) CO2 (% vol) HC (ppm) O2 (% vol)
l
NO (ppm vol)
Operating range
Accuracy
0.000e10.00 0.00e18.00 0e9999 0.00e22.00 0.500e9.999 0e5000
0.001 0.01 1 0.001 0.001 1
Table 4 Accuracies of the measurements and uncertainties in the calculated results.
Time (s) Temperature ( C) Fuel (g) Heating value (kJ/kg) Engine speed (rpm) Load (N) Torque (Nm) BTE BSFC (g/kW-h)
Accuracy
Uncertainty (%)
±0.5% ±1 C ±0.1 g ±0.1% ±1% ±0.25% e e e
e e e e e e ±0.08 ±1.29 ±1.18
contrary, released heat remains lower owing to lower calorific value of tetrahydrofuran. This phonomena caused to eliminate knocking tendency with fuel blends. Combustion occurred at before dead top center (BTDC) for N100 for all lambda values and closed to TDC with increasing of lambda. F20N80 and F40N60 showed similar combustion tendency but combustion occurred after top dead center (ATDC) for l ¼ 2.24, 2.42 and 2.99 with F40N60. Moreover, combustion was obtained ATDC for F60N40 with all lambda values. THF has higher octane number (72.9) than that of n-heptane (0). The higher the octane number, the higher the resistance to auto-ignition and the more controlled self-ignition. On the other hand, THF showed lower in-cylinder pressures and heat release rates thanks to lower calorific value. F40N60 and F60N40 showed better performance in view of lean combustion. Auto-ignition temperature of tethrahydrofuran is higher than n-heptane. It can be said that in-cylinder temperature increases much at the end of compression stroke due to higher auto-ignition temperature. This phonomena resulted in higher temperature history in the combustion chamber for self-ignition of lean charge mixture. On the other hand, higher oxygen content of tethrahydrofuran improved the oxidation reactions. So, HCCI combustion was achieved with leaner charge mixture. The variation of in-cylinder pressure and heat release rate at constant lambda value (2.2) and two different inlet air temperatures (60e80 C) is shown in Fig. 4. What stands out in Fig. 4 is that retarded combustion was achieved with the increasing of THF in the mixture. Besides, the maximum heat release rate increased for N100, F20N80 and F40N60 when the inlet air temperature increased to 80 C from 60 C. HCCI combustion was achieved with leaner mixtures (l ¼ 2.63, 2.99) and for fuel blends due to lower boiling temperature of THF. an, Experimental investigation of tetrahydrofuran combustion in homogeneous charge compression ignition Please cite this article as: B. Aydog (HCCI) engine: Effects of excess air coefficient, engine speed and inlet air temperature, Journal of the Energy Institute, https://doi.org/10.1016/ j.joei.2019.10.009
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Fig. 2. In-cylinder pressure and the HRR in HCCI combustion mode.
Fig. 3. In-cylinder pressure and the HRR of a) N100, b) F20N80, c) F40N60 and d) F60N40 in HCCI combustion mode versus crank angle for different lambda values.
an, Experimental investigation of tetrahydrofuran combustion in homogeneous charge compression ignition Please cite this article as: B. Aydog (HCCI) engine: Effects of excess air coefficient, engine speed and inlet air temperature, Journal of the Energy Institute, https://doi.org/10.1016/ j.joei.2019.10.009
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Fig. 4. In-cylinder pressure and the HRR of test fuels in HCCI combustion mode versus crank angle for constant l ¼ 2.2 at a) 60 C and b) 80 C inlet air temperature.
Higher heat is able to trap owing to lower boiling temperature with THF/n-heptane blends in the process of combustion reactions. Higher heat is absorbed to reach auto-ignition temperature due to higher boiling temperature of n-heptane. This event causes to decrease self ignition temperature when pure n-heptane was used. So, combustion could not be performed with leaner mixtures. Fig. 5 shows the variation of in-cylinder gas temperature in HCCI combustion mode versus crank angle at constant lambda and inlet air temperature. Heating value of the fuel and the combustion stages are the main factors effect the in-cylinder gas temperature. Combustion started earlier for N100 and F20N80 than the other test fuels. In other words, combustion retarded with increasing amount of THF in the mixture. This means, the increase of in-cylinder gas temperature started earlier for N100 and F20N80. N-heptane (98 C) has higher boiling temperature than tetrahydrofuran (66 C) which cause higher vaporization cooling. THF addition decreased the maximum in-cylinder temperature. The maximum in cylinder temperature was measured as about 1752 C and 1683 C for N100 and F60N40, respectively. Tetrahydrofuran has higher oxygen content. This characteristic showed better self ignition condition in the combustion process. In addition, knocking resistance of n-heptane is zero compared to tetrahydrofuran. Combustion reactions occur simultaneously and spontaneously. Combustion flame proceeds fastly during combustion. Combustion rate and speed also increase with n-heptane. Hence higher in-cylinder temperature is obtained with n-heptane. CA10 is estimated as the start of combustion (SOC) by many researchers in literature [38,39]. It was shown in Fig. 2. SOC was determined according to crank angle degree where the heat release rate value takes positive value. Fig. 6 demonstrates the variation of CA10 versus lambda for test fuels. As it is well known, the self-ignition can not be controlled directly in HCCI combustion. From the graph below we can see that, CA10 was retarded for all test fuels when the lambda increased. Auto ignition reactions are collapsed due to high lambda values resulting lower fuel concentration in combustion chamber. CA10 was delayed with the increase of THF in the charge mixture. However, CA10 was obtained BTDC with N100 and F20N80. Addition of 60% THF into n-heptane significantly delayed CA10 and it was observed ATDC for all lambda values. This interesting finding might be explained by the fact that n-heptane has higher vaporization cooling than THF due to higher boiling temperature and it is also known that CA10 is markedly effected by auto-ignition temperature. N-heptane (204 C) has lower auto-ignition temperature than that of THF (336.5 C). That means, THF needs more time to reach CA10 versus crank angle due to higher auto-ignition temperature. Chemical oxidation reactions improved self ignition condition at higher inlet air temperature because of higher
Fig. 5. In-cylinder gas temperature of test fuels in HCCI combustion mode versus crank angle for constant l ¼ 2.2.
an, Experimental investigation of tetrahydrofuran combustion in homogeneous charge compression ignition Please cite this article as: B. Aydog (HCCI) engine: Effects of excess air coefficient, engine speed and inlet air temperature, Journal of the Energy Institute, https://doi.org/10.1016/ j.joei.2019.10.009
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Fig. 6. The variation of CA10 versus lambda at a) 60 C and b) 80 C inlet air temperature.
combustion chamber temperature at the end of compression stroke. Higher inlet air temperature helps to reach auto-ignition temperature earlier. So, combustion was advanced at 80 C inlet air temperature. 3.2. Engine performance CA50 is one of the most challenging and major parameter because combustion is considerably influenced by charge and fuel properties. The fact remains that the balance between engine load and CA50 can control the emissions. It was reported that shifting CA50 caused higher CO and HC emissions but NOx emission increased when CA50 was advanced towards to TDC. Besides, thermal efficiency was also increased when CA50 was nearly observed ATDC [40,41]. Fig. 7 shows the variations of CA50 values and thermal efficiency of test fuels versus lambda. In Fig. 7a there is a clear trend of increasing CA50 and is closed to TDC with the increase of lambda value. A possible explanation for this might be that local lean mixture zones are formed in the absence of fuel concentration. Self ignition reactions are getting difficult. It is obvious that increasing the amount of THF in the mixture significantly increased CA50. Especially F60N40 showed the highest CA50 values for all lambda values and all CA50 values observed ATDC. It seems possible that this result is due to higher octane number of THF. CA50 values of N100, F20N80, F40N60 and F60N40 were 3.96 (BTDC), 2.16 (BTDC), 2.16 (ATDC) and 8.28 (ATDC) oCA, respectively, at 1200 rpm engine speed, 60 C inlet air temperature and l ¼ 2.2. Higher thermal efficiency is obtained when CA50 is occurred ATDC [36]. Fig. 7b represents the variation of thermal efficiency of test fuels at 1200 rpm engine speed, 60 C inlet air temperature. It was found that thermal efficiency increased by increasing lambda value. The maximum thermal efficiency was achieved as 34% for F60N40 at 1200 rpm engine speed, 60 C inlet air temperature and l ¼ 2.17. There is good agreement between CA50 and thermal efficiency. Maximum thermal efficiency is obtained when CA50 appears in 8e10 oCA ATDC in internal combustion engines. Thermal efficiency increased when CA50 was determined ATDC. Higher oxygen content of THF resulted in higher thermal efficiency owing to higher released heat and improved chemical reactions.
Fig. 7. The variations of a) CA50 and b) thermal efficiency.
an, Experimental investigation of tetrahydrofuran combustion in homogeneous charge compression ignition Please cite this article as: B. Aydog (HCCI) engine: Effects of excess air coefficient, engine speed and inlet air temperature, Journal of the Energy Institute, https://doi.org/10.1016/ j.joei.2019.10.009
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Fig. 8 depicts the variation of ringing intensity and maximum pressure rise rate versus lambda at 1200 rpm engine speed, 60 C inlet air temperature. RI and MPRR are the major parameters which characterize the intensity of knocking in HCCI and also used to determine the upper zone of HCCI operation [42e45]. From Fig. 8, it is apparent that both RI and MPRR decreased with increasing lambda for all test fuels. As it was expected that more amount of premixed charge is burned in rich mixtures resulting high pressure rise rate due to high combustion rate [46]. From the data in Fig. 8, it can be seen that F60N40 resulted in the lowest value of both RI and MPRR owing to more controllable self ignition reactions. Moreover, sudden and simultaneous heat release was eliminated due to higher octane number of THF. Hence, lower MPRR and RI were observed. Fig. 9 represents the variation of imep versus cycle number at 1200 rpm engine speed, 60 C inlet air temperature and l ¼ 2.2. Imep was calculated by dividing the net work to swept volume. The leaner conditions result in lower heat release at the end of combustion [25]. Besides, the velocity of flame decreases and oxidation reactions get worse with leaner premixed charge mixture. In Fig. 9 there is a clear trend of increasing on Imep with the increase of THF in the mixture. The addition of THF into n-heptane leads to obtain maximum incylinder pressure nearly ATDC because of higher octane number of THF. HCCI combustion occurred in a smaller in-cylinder volume with THF/n-heptane fuel blends. These phenomena also caused to obtain higher imep. Furthermore, THF includes more oxygen in its chemical structure. This property provides easily to meet oxygen and fuel molecules in the combustion chamber. The effect of engine speed on engine performance characteristics such as brake torque, power output and specific fuel consumption for test fuels was given in Fig. 10. Brake torque and power output shows similar tendency that firstly increased to a certain engine speed and then started to decrease. Likely, brake torque and power output decreased after maximum torque and power output speed due to gas leakages and heat losses. It can be clearly seen from Fig. 10 that increasing amount of THF into the mixture increased both brake torque and power output. This result may be explained by the fact that some addition of THF provided stable HCCI combustion because of higher octane number. The position of maximum in-cylinder pressure changed towards to ATDC with THF fuel blends. Although, gas leakages and absence of oxygen increased at higher engine speeds, THF supplied sufficient oxygen in order to complete self ignition reactions. So, higher torque
Fig. 8. The variation of a) ringing intensity and b) MPRR.
Fig. 9. The variation of imep versus cycle number.
an, Experimental investigation of tetrahydrofuran combustion in homogeneous charge compression ignition Please cite this article as: B. Aydog (HCCI) engine: Effects of excess air coefficient, engine speed and inlet air temperature, Journal of the Energy Institute, https://doi.org/10.1016/ j.joei.2019.10.009
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and power output were obtained. The maximum value of the brake torque was obtained as 7.2 Nm at 1000 rpm engine speed for F60N40 and it is higher about 63% than that of N100 test fuel. Power output of N100 was 0.58 kW at 1400 rpm engine speed. The corresponding value was found as 0.96 kW for F60N40 at 1400 rpm engine speed and it is higher about 65% than N100. When Fig. 10c was examined, it is possible to say that SFC decreased until 1400 rpm engine speed and then started to increase. Lower SFC was computed with the increase of THF. At higher engine speeds there is no enough time to suck oxygen into the combustion chamber for complete combustion. But, the oxygen content of THF caused to oxidize more fuel molecules resulting in lower SFC compared to n-heptane. The operation limits of HCCI combustion is restricted by misfiring and knocking. Especially the fuels with higher octane number are used to prevent knocking. The operating range of HCCI combustion versus engine speed with lambda values for test fuels was given in Fig. 11. Nheptane showed the narrowest operating range. In other words, HCCI combustion did not occur in a large misfiring and knocking zones for N100. Besides, it is obviously seen that operating range was expanded with increasing amount of tetrahydrofuran in the mixture. It was realized that largest operating range was obtained for F60N40. HCCI combustion was achieved at l ¼ 3.28 which is very lean charge mixture for F60N40 at 800 rpm engine speed and 60 C inlet air temperature. Similarly, test engine was run with F60N40 at 2.81 lambda value at knocking zone. When Fig. 11 was examined, the addition of THF leads to operate HCCI engine with higher imep in knocking boundaries. It was found that HCCI operating range was enlarged at misfiring zone with F40N60. The test engine could run until 1600 rpm for N100 and F60N40, and 1800 rpm for F20N80 and F40N60 on HCCI mode. Homogeneity of charge mixture improves at higher engine speeds. In spite of the fact that auto ignition conditions are getting worse in view of higher octane number of THF, fuel blends succeed to complete self ignition reactions due to higher content of oxygen. Hence, F40N60 showed reasonable performance in terms of stable HCCI operation at higher engine speeds. It can be said that F60N40 presented larger misfiring zone. Higher oxygen concentration of tetrahydrofuran improved chemical oxidation reactions. Combustion temperature increases with oxygen molecules. It also leads to prolonged combustion. Thus, selfignition can be also performed even with leaner mixtures. It was clearly seen from Fig. 11 that higher octane number of tetrahydrofuran resulted in higher resistance to knocking.
Fig. 10. The variation of a) brake torque, b) power output and c) specific fuel consumption versus engine speed.
an, Experimental investigation of tetrahydrofuran combustion in homogeneous charge compression ignition Please cite this article as: B. Aydog (HCCI) engine: Effects of excess air coefficient, engine speed and inlet air temperature, Journal of the Energy Institute, https://doi.org/10.1016/ j.joei.2019.10.009
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Fig. 11. The variation of imep versus engine speed.
3.3. Emissions In HCCI combustion mode, NOx emissions are negligible owing to low combustion temperature. Nevertheless, insufficient oxidation of the fuel occurs at low combustion temperature and HC emissions increase. The variation of HC emissions was given at Fig. 12a. As shown in Fig. 12a, HC emissions increased with the increase of lambda. The observed increase in HC may be explained by the fact that the amount of the fuel taken into the cylinder decreased and leaner conditions occurred and led to be seen low temperatures of burnout in-cylinder gasses with increasing value of the lambda. Low combustion temperatures resulted in incomplete combustion and HC emissions increased. On the other hand, adding THF into mixture increased HC emissions. This result may be attributed to the retarded combustion. Prolonged combustion of THF leads to reduce cylinder wall temperature with the moution of piston to the bottom TDC. The charge mixture is cooled and oncoming flame slows down on combustion chamber surface. Thus, HC is formed. The variation of CO emissions can be seen in Fig. 12b. As previously stated that HCCI combustion leads to low combustion temperatures which cause incomplete combustion. CO generation increases with increasing the rate of incomplete combustion. So it can be said that lambda is the major parameter which controls the CO generation. CO emissions increased with the increase of the lambda due to leaner mixture conditions which led to more incomplete combustion owing to low combustion temperature. Oxidation reactions slows down with low combustion temperatures and oxidation does not occur as intended. It can be seen from Fig. 12b that CO emissions slightly increased with the addition of THF into the mixture except F60N40. The increase of CO emissions for F60N40 was found higher than that of the other fuels especially at higher lambda values. The CO emissions of N100, F20N80, F40N60 and F60N40 were found as 0.07%, 0.083%, 0.086%, 0.114%, respectively, at 1200 rpm engine speed, 60 C inlet air temperature and l ¼ 2.2. These findings suggest that in general the usage of tetrahyrofuran/n-heptane blends help to improve HCCI combustion. The most important problems called knocking and misfiring which restrict the HCCI operating range can be eliminated with the addition of tetrahydrofuran into n-heptane due to higher octane number of the fuel. The second major finding was that the engine performance was
Fig. 12. The variation of a) HC and b) CO emissions versus lambda.
an, Experimental investigation of tetrahydrofuran combustion in homogeneous charge compression ignition Please cite this article as: B. Aydog (HCCI) engine: Effects of excess air coefficient, engine speed and inlet air temperature, Journal of the Energy Institute, https://doi.org/10.1016/ j.joei.2019.10.009
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enhanced with the THF/n-heptane blends. The principal limitation of this analysis was the increasing of HC and CO emissions as in all HCCI combustion mode owing to low combustion temperatures. 4. Conclusions The aim of the present research was to examine the effects of tetrahydrofuran and n-heptane blends on combustion, performance and emissions characteristics of HCCI engine were examined with varying lambda, inlet air temperature and engine speed. The results showed that in-cylinder pressure and heat release rate decreased with increasing the value of lambda for all test fuels. It was found that HCCI mode was generally obtained between l ¼ 1.66 to l ¼ 3.0 with test fuels. HCCI combustion was advanced with increasing inlet air temperature. Addition of tetrahydrofuran into the mixture retarded the combustion owing to higher octane number. Start of combustion was observed as 6.48 oCA (BTDC), 4.68 oCA (BTDC), 0.36 oCA (BTDC) and 4.32 oCA (ATDC) for N100, F20N80, F40N60 and F60N40, respectively, at 1200 rpm and l ¼ 2.2. Thermal efficiency was also increased with increasing amount of THF in the mixture. Thermal efficiency was obtained as 25% and 34% for N100 and F60N40, respectively. Ringing intensity significantly decreased with the usage of F60N40 compared to N100. The ringing intensity was obtained as 12.24 MW/m2, 18.52 MW/m2, 13.71 MW/m2 and 6.97 MW/m2 for N100, F20N80, F40N60 and F60N40, respectively, at 1200 rpm and l ¼ 2.2 Brake torque and power output increased with increasing THF addition into the fuel blend. The brake torque and power output increased about 64% and 63%, respectively, for F60N40 compared to N100. The HC emissions increased about 27% for F60N40 compared to N100 at l ¼ 2.2. Test results also showed that HCCI operating range can be expanded with the addition of THF in the Q4 n-heptane. Nomenclature
Wnet Vd P dV dQ dq dQheat dq
A2 hc n Tg Tw k mfuel QLVH
simep X
g dP dt Pmax
Tmax
DR xn
Net work (joule) Cylinder swept volume (m3) Cylinder pressure (bar) Variation of cylinder volume Heat release (J) Crank angle ( CA) Heat transfer to cylinder walls (J/ CA) Heat transfer surface area (m2) Heat transfer coefficient (W/m2K) Engine speed (rpm) In-cylinder average gas temperature (K) Combustion chamber wall temperature (K) Ratio of specific heat values Consumed fuel per cycle (kg) Lower heating value (kcal) Average of the indicated mean effective pressure Standard deviation Polytropic index Pressure rise rate Maximum in-cylinder pressure (bar) Maximum in-cylinder temperature (K) Uncertainty of the measurement Independent variables of the function and uncertainty
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an, Experimental investigation of tetrahydrofuran combustion in homogeneous charge compression ignition Please cite this article as: B. Aydog (HCCI) engine: Effects of excess air coefficient, engine speed and inlet air temperature, Journal of the Energy Institute, https://doi.org/10.1016/ j.joei.2019.10.009
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an, Experimental investigation of tetrahydrofuran combustion in homogeneous charge compression ignition Please cite this article as: B. Aydog (HCCI) engine: Effects of excess air coefficient, engine speed and inlet air temperature, Journal of the Energy Institute, https://doi.org/10.1016/ j.joei.2019.10.009