The effects of diisopropyl ether on combustion, performance, emissions and operating range in a HCCI engine

Fuel 265 (2020) 116919

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Full Length Article

The effects of diisopropyl ether on combustion, performance, emissions and operating range in a HCCI engine

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Ahmet Uyumaza, , Bilal Aydoğanb, Alper Calamc, Fatih Aksoyd, Emre Yılmaze Burdur Mehmet Akif Ersoy University, Faculty of Engineering – Architecture, Department of Mechanical Engineering, Burdur, Turkey Burdur Mehmet Akif Ersoy University, High Vocational School of Technical Sciences, Burdur, Turkey c Gazi University, Ostim High Vocational School, Ankara, Turkey d Afyon Kocatepe University, Faculty of Technology, Department of Automotive Engineering, Afyon, Turkey e Hakkari University, Faculty of Engineering, Department of Mechanical Engineering, Hakkari, Turkey a

b

A R T I C LE I N FO

A B S T R A C T

Keywords: Diisopropyl ether HCCI combustion Performance Emissions Operating range

In the current study, the effects of diisopropyl ether were experimentally investigated on combustion, performance, emissions and operating range in a homogeneous charged compression ignition (HCCI) engine. For this purpose, a single cylinder, four stroke, port injection test engine was run with different lambda values between 1.69 and 3.08 on HCCI mode with pure n-heptane, 20% diisopropyl ether 80% n-heptane (D20N80), and 40% diisopropyl ether 60% n-heptane (D40N60) fuel blends at full load. HCCI engine was operated between 800 and 1600 rpm engine speed at constant inlet air temperature of 60 °C and wide open throttle (WOT). The effects of diisopropyl ether addition were observed on cylinder pressure, heat release rate (HRR), combustion duration (CD), indicated mean effective pressure (imep), brake torque, power output, specific fuel consumption (SFC) and exhaust emissions. Test results showed that the increase of lambda leads to lower in-cylinder pressure and HRR. The addition of diisopropyl ether caused to retard combustion. Indicated thermal efficiency (ITE) increased 4.92% with D40N60 compared that n-heptane at 1200 rpm and λ = 2. Brake torque and power output increased by about 1.03% and 1.18% with D20N80 according to pure n-heptane at 1200 rpm respectively. On the contrary, SFC decreased 24.08% with D40N60 compared to n-heptane at 1200 rpm and λ = 2. HC and CO increased with the addition of diisopropyl ether. The test results also showed that the addition of diisopropyl ether expanded the HCCI combustion towards to knocking and partial combustion zone.

1. Introductıon Homogeneous charged compression ignition engines (HCCI) have received great attention by researchers due to high thermal efficiency and lower CO release. HCCI combustion provides simultaneous reduction on soot and NOx emissions, because they are significant handicap in compression ignition (CI) engines. HCCI engines also give reasonable thermal efficiency in spite that the engine runs with leaner charge mixture. Low temperature combustion (LTC) is seen such as reactive controlled compression ignition (RCCI), premixed charge compression ignition (PCCI) and partial premixed combustion (PPC) [1–6]. That phonomena shows to be enviromentally friendly economic combustion mode compared conventional combustion cycles. Nevertheless, HC formation increases due to lower in-cylinder wall and combustion chamber temperature especially with leaner charge mixtures in HCCI combustion. Exhaust gas after treatment systems such as diesel

particulate filter (DPF), three way catalytic converter have been used in order to reduce exhaust emissions. But, these systems are not cost-effective and enough practical to use fertilely in the internal combustion engines [2–11]. Hence, HCCI is seen to present good potential in view of lower exhaust emisssions and reasonable performance. However, HCCI operating range is restricted by misfiring and knocking at low and high engine load respectively. Knocking is seen due to uncontrollable auto-ignition process, because combustion phasing is directed by chemical kinetics and thermodynamic situation at the end of compression stroke apart from spark ignition (SI) and CI cycles [3–15]. Excess air is needed due to combustion of richer charge mixture in order to reduce knocking tendency and complete oxidation reactions on HCCI mode. On the contrary, auto-ignition chemical reactions are deteriorated because of lower end gas temperature and combustion efficiency with leaner mixture. On the other hand, sudden and simultaneous self-ignition causes to higher pressure rise rate resulting in pressure oscillations in

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Corresponding author. Tel.: +90 248 2132765; fax: +90 248 213 27 04. E-mail addresses: [email protected] (A. Uyumaz), [email protected] (B. Aydoğan), [email protected] (A. Calam), [email protected] (F. Aksoy), [email protected] (E. Yılmaz). https://doi.org/10.1016/j.fuel.2019.116919 Received 11 June 2019; Received in revised form 21 August 2019; Accepted 17 December 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.

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diesel-Calophyllum Inophyllum Methyl Ester blends. Thermal efficiency decreased 5.3% and specific fuel consumption increased by about 12.5% with the addition of diethyl ether. Polat [22] studied the effects of diethyl ether–ethanol fuel blends on HCCI combustion, engine performance and exhaust emissions. He stated that indicated thermal efficiency increased 23.1% and was obtained as 33.1% at lambda 2 with DEE as compared to E30/D70. Reactivity properties of ethers are generally low as solvents. Diisopropyl ether and dialkyl ether can also be used as oxygenate additive for higher octane number. Hence, diisopropyl ether can be considered as potential alternative fuel with high boiling and octane number in HCCI combustion in order to reduce knocking tendency and extend operating limits [22,37–51]. So, combustion behaviour of diisopropyl ether should be investigated in a detail on HCCI combustion. There is almost no study with diisopropyl ether in HCCI engines relating to combustion and performance analysis. In the current study, the effects of diisopropyl ether addition to n-heptane at different mixture rates were experimentally investigated on combustion, performance and emission characteristics.

the combustion chamber. So, not only oxygenated additives but also high octane number fuels have been focused to enhance auto-ignition reactions and avoid knocking in HCCI combustion. To achieve durable and stable operation of HCCI engines is complex and little hard owing to self-ignition and probable pressure fluctuations compared to SI and CI engines. Engine roughness can be enhanced and smoother operation is provided using high octane number alternative fuels that containing higher oxygen content. The usage of alcohol, ethers and oxygenates additive improves engine performance. HCCI operation is highly affected by fuel auto-ignition kinetics. In this regard, environmentallyfriendly and non-toxic diisopropyl ether is seen one of the best alternative fuel to reduce knocking in HCCI combustion [10–21]. Polat [22] performed an experimental study to determine the effects of diethyl ether-ethanol fuel blends on HCCI combustion. Higher amount of ethanol addition prevented to occur auto-ignition and start of combustion was delayed with the increase of ethanol fraction. But he mentioned that almost zero NOx was released. Dhamodaran et al. [23] compared diisopropyl ether-gasoline fuel blends with pure gasoline in view of performance, combustion and emissions in SI engine. He pointed out that in-cylinder pressure, heat release and brake thermal efficiency increased with fuel blends compared that gasoline. Barari et al. [24] performed HCCI simulation by single-zone and multi-zone engine models with di-isopropyl ketone. They found that there is good agreement between single-zone model of HCCI engine and updated kinetic model. Yang and Dec [25] showed that di-isopropyl ketone is sensitive to self-ignition and temperature. Vuilleumier et al. [26] found that low temperature heat release caused to obtain high intermediate temperature heat release in case gasoline and primary reference fuel blends were used. They conducted the experiments between 0.3 and 0.4 equivalence ratio and mentioned that no LTHR was seen below 1.4 bar intake pressure. Yaşar et al. [27] aimed to predict in-cylinder pressure using single zone, a double-Wiebe function combustion model in HCCI combustion. It was shown that double-Wiebe function approach exhibited best agreement. They showed that a minor fraction (10–20%) burned with reduced ratio using Wiebe function model. Xie et al. [28] investigated the effects of residual gas trapping and preheating in HCCI operation. They have found that fuel economy was improved by 8–12% with prehating and waste heat recovery according to negative valve overlap. They also determined that lower load zone was expanded to 0.8 bar. Daniel et al. [29] researched the effects of 2,5-dimethylfuran (DMF) in a SI engine and compared with gasoline, ethanol, butanol and methanol in view of combustion, performance and emissions with the interval of 3.5 bar to 8.5 bar. DMF presented the highest exhaust gas temperature like gasoline. They stated that DMF was effective in cold start condition. 76% increase was observed with gasoline on CO emissions compared that other oxygenated fuels. Çınar et al. [30] investigated the intake air temperature on combustion and performance in a HCCI engine fueled with the blends of 20% n-heptane and 80% isooctane fuels. In-cylinder pressure and heat release increased with the increase of intake air temperature. They showed that λ = 0.6 presented lower brake torque by about 3.1% compared that λ = 0.7. In addition, the COVimep passed over 10% at 50 °C and 110 °C due to unstable combustion at λ = 0.7. Dhamodaran and Esakkimuthu [31] investigated the properties of diisopropyl ether/gasoline blends such as density, calorific value, resrach octane number, anti-knock index. Masera and Hossain [32] reviewed the thermal barrier coated engines fueled with biodiesel. Zhou et al. [33] presented (Liquid + liquid) equilibria (LLE) data for {water octane + diisopropyl ether (DIPE)} and three quaternary systems of (water + 1-propanol + DIPE + octane, or methylbenzene, or heptane) at T = 298.15 K. Arce et al. [34] determined isentropic compressibility, and molar refraction changes of diisopropyl ether (DIPE) + isopropyl alcohol (IPA) + water at 298.15 K. Awad et al. [35] reviewed the alcohol and ether as alternative fuel. Engine performance was improved with the increase of compression ratio when alcohol fuels. Nanthagopal et al. [36] researched the compression ignition characteristics of diethyl ether in

2. Experımental setup and procedures Experiments were conducted at Gazi University, Faculty of Technology, Department of Automotive Engineering, Internal Combustion Engine Laboratuary. A single cylinder, four stroke, spark ignition Ricardo Hydra test engine was converted to HCCI engine. Test engine was coupled with DC dynamometer that is capable of 30 kW power absorbtion at 6500 rpm. DC dynamometer is also used as motor. The schematical view of the experimental setup is seen in Fig. 1. The engine specifications are given in Table 1. Engine operation parameters such as engine speed, load, fuel injection pulse, ignition can be controlled using dynamometer control panel. All experiments were performed after the test engine was warmed up. The test engine was first run in SI mode until the operating temperature was reached and then switch off the ignition for HCCI mode. Engine oil and coolant temperatures were held constant at 55 °C and 75 °C respectively in order to achieve stable and durable operation. Exhaust gas temperatures were also measured using K-type thermocouple from the exhaust line. The test engine was operated at different lambda values from 1.69 to 3.08 at 1200 rpm and wide open throttle on HCCI mode. Inlet air temperature was kept constant at 60 °C with closed loop controller. Fuel consumption was also measured for each test condition using precision balance with the resolution of 0.01gr. Kistler 6121 model piezoelectric pressure sensor was adapted to cylinder head in order to determine incylinder pressure. Encoder that produces 1000 pulses per rotation was mounted to the crankshaft in order to measure engine speed and determine top dead center according to crank angle. Firstly, raw in-cylinder pressure signals were amplified using Cussons P4110 combustion analyzer. Pressure data were then delivered to the National Instrument data acqusition card in order to convert analog signals into the digital signals. Digital in-cylinder pressure data were then recorded to the computer. In-cylinder pressure data were verified to determine combustion characteristics such as heat release, combustion duration, CA10, CA50 etc. using program that prepared with Matlab. In order to eliminate cyclic variations, consecutive 50 cycles were averaged for each test condition. Three type of test fuels were used in the experiments. 20% diisopropyl ether-80% n-heptane (D20N80), 40% diisopropyl ether-60% n-heptane by vol. (D40N60) and pure n-heptane were experimented. The properties of the test fuels are shown in Table 2. Fuel properties highly affects the HCCI combustion phasing. Diisopropyl ether is seen to be good additive to n-heptane, because it is flammable and oxygenate fuel. Moreover, diisopropyl ether can be properly mixed with n-heptane. It is seen that diisopropyl ether has advantage in view of higher octane number in order to control chemical kinetics in HCCI mode. Knocking can be observed especially with fuels having higher reactivity in HCCI combustion. So, high octane number and auto-ignition temperature of diisopropyl ether receive great attention for HCCI 2

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

Table 1 Engine specifications.

Table 3 The specifications of the exhaust gas analyzer.

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

CO (% vol) CO2 (% vol) HC (ppm) O2 (% vol) λ NO (ppm vol)

Chemical formula Density (kg/m3) Octane number Calorific value (kJ/kg) Boiling point (oC) Auto-ignition temperature (oC) Molar mass (g/mol)

n-Heptane

C6H14O 725 129 39,300 68 443 102.2

C7H16 679.5 – 45,500 98 204 100.16

Accuracy

0.000–10.00 0.00–18.00 0–9999 0.00–22.00 0.500–9.999 0–5000

0.001 0.01 1 0.001 0.001 ≤1

combustion [52–56]. The production process of diisopropyl ether is seen in Fig. 2. Bosch exhaust gas analyzer which of the specifications were given in Table 3 was adapted to the exhaust line for CO, HC and lambda measurement. The first law of thermodynamics was applied for heat release rate analysis. In this regard, charge mixture which delivered into the cylinder was assumed to be ideal gas. In addition, gas leakegas and heat losses from valve, piston rings were neglected. Heat release rate was computed with Eq. (1).

Table 2 Fuel properties. Diisopropyl ether

Operating range

dQ k dV 1 dP dQheat = P + V + dθ k − 1 dθ k − 1 dθ dθ

(1)

dQ , dθ , dQheat , k shows the heat release, crank angle variation, heat transfer to the cylinder wall and the ratio of specific heat respectively. Cyclic variations are seen due to the variation of charge composition, thermodynamic situation which defines the durability and stability of the operation in the test engine. Cyclic variations of imep was calculated with Eq. (2) as below. COVimep =

σimep −

× 100

(2)

X −

In Eq. (2) σimep and X define the standard deviation and average of the indicated mean effective pressure respectively. Ringing intensity was computed with Eq. (3) as below.

Fig. 2. Production process of diisopropyl ether [23,53]. 3

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on the heating value of test fuel and combustion phasing. HCCI combustion was started earlier. Similarly, the increase of in-cylinder temperature started earlier with n-heptane compared to D20N80 and D40N60. The highest in-cylinder gas temperature was computed with D40N60. Auto-ignition chemical reactions were delayed due to higher octane number of diisopropyl ether. When the in-cylinder temperature history during compression stroke was examined, lower in-cylinder gas temperature was observed with n-heptane compared to D20N80 and D40N60. Higher boiling temperature of n-heptane caused to remain lower heat due to vaporization of n-heptane as seen in Fig. 5. Retarded combustion also caused to accumulate higher amount of fuel molecules. Spontaneous and simultaneous ignition towards to combustion chamber resulted in higher in-cylinder gas temperature. Fig. 6 illustrates the variations of CA10 versus lambda with test fuels. In this study, CA10 was assumed to be SOC. There is no mechanism to control self-ignition in HCCI combustion. So, SOC is significant for HCCI combustion, because it affects the rest combustion process. So, fuel construction and charge mixture composition at the EOC vary the auto-ignition temperature. As lambda increased, CA10 was delayed for all test fuels. Lower fuel concentration in the combustion chamber deteriorated the auto-ignition reactions. Besides, incylinder gas temperature at the EOC, decreased with leaner charge mixtures due to lower fuel efficiency. As expected, combustion was advanced with n-heptane compared that fuel blends due to lower octane number and auto-ignition temperature. CA10 was obtained before top dead center (BTDC) with n-heptane. CA10 is quite dependent on auto-ignition temperature. It was found that CA10 was delayed with the increase of diisopropyl ether fraction in the fuel blends. More time is required in order to reach CA10 versus crank angle with diisopropyl ether fuel blends owing to higher auto-ignition temperature. CA10 was obtained as 7.56 °CA, (BTDC), 5.76 °CA, (BTDC) and 0.72 °CA, (ATDC) with n-heptane, D20N80 and D40N60 respectively at λ = 2 and 1200 rpm. Likely, CA50 is crank angle where 50% of charge mixture is completed to ignite. Fig. 7a depicts the variations of CA50 versus lambda. CA50 was delayed like CA10 with the increase of lambda. In-cylinder gas temperature decreases with lower heat energy released at leaner mixtures. Oxidation reactions are deteriorated and combustion temperature decreases with lower fuel energy. So, CA50 was delayed for all test fuels with the increase of lambda. Fig. 6b shows the indicated thermal efficiency (ITE) variations versus lambda. ITE increased until a given lambda value and then started to decrease for all test fuels. Sufficient fuel molecules with enough oxygen concentration are included in the combustion chamber at richer mixture close to stoichiometric ratio resulting in higher ITE due to better oxidation reactions. Nevertheless, too much oxygen concentration reduces the in-cylinder gas temperature and combustion rate at the EOC. Thus, ITE decreased at leaner mixtures. CA50 is important in view of thermal efficiency. If CA50 is obtained nearly after top dead center (ATDC), thermal efficiency can be increased. CA50 was obtained ATDC with D40N60 as seen in Fig. 7a. So, the highest ITE was computed with D40N60. It was depicted from Fig. 7 that there was a good agreement between CA50 and ITE. ITE increased 8.1% and 4.9% with D40N60 compared to D20N80 and n-heptane respectively at λ = 2. The highest ITE was calculated as 23.4% at λ = 2.33 with D40N60. The minimum ITE was computed as 17% with n-heptane at λ = 2.7. Higher density and autoignition temperature cause to obtain prolonged combustion. This evident leads to increase heat energy converted from fuel energy. Heat losses are getting lower at λ = 2 and λ = 2.13. More heat energy is included in the combustion chamber leading higher ITE. Heating value of test fuel decreases with D20N80. So, ITE decreased compared to nheptane at the same lambda values. Besides this, combustion flame is cooled near the cylinder wall with leaner charge mixture because of the lower combustion temperature. This phonomena caused to extinguish flame earlier resulting in lower ITE. D40N60 showed reasonable performance in view of ITE compared to other test fuels.

Fig. 3. In-cylinder pressure history and heat release rate.

1 RI = 2γ

(β ( ) ) dP dt max

Pmax

2

γ . R. Tmax

(3)

dP , dt

γ, Pmax and Tmax describe the polytropic index, pressure rise rate, maximum in-cylinder pressure and maximum in-cylinder temperature respectively. 3. Results and discussion Fig. 3 defines the in-cylinder pressure history and heat reelase rate versus crank angle. The start and end of combustion, combustion stages can be determined using heat release trace. The start of combustion (SOC) can be determined where the heat release rate takes positive value versus crank angle. Combustion stages can be determined normalized cumulative heat release such as CA10, CA50 and CA90. End of combustion (EOC) was accepted versus crank angle where the 90% charge mixture completed to combust (CA90). Negative temperature coefficient region between low temperature heat release (LTHR) and high temperature heat release (HTHR) is seen in Fig. 3. Fig. 4 shows the in-cylinder pressure and heat reelase rate versus crank angle with different lambda values. It was clearly seen that knocking was observed with n-heptane especially with richer mixtures. Heat decreased during combustion of n-heptane due to higher boiling point. Then whole charge mixture tend to participate oxidation reactions. Hence, pressure rise rate increased. However, knocking tendency reduced with the increase of lambda. HRR decreases as fuel energy decreases with higher lambda for all test fuels. Combustion was retarded with the increase of lambda for all test condition. Besides, two stage HRR is shown that are called low and high temperature heat release as seen in Fig. 2. Knocking tendency was also found at richer mixtures (λ = 2, λ = 2.13, λ = 2.15) with D20N80 and D40N60. But, it can be pointed out that pressure rise rate decreased with higher lambda resulting in reducing knocking combustion. Maximum in-cylinder pressure decreased with D40N60 compared to other test fuels. Test results showed that test engine could not be operated under lambda values of 2 with fuel blends at 1200 rpm. So, diisopropyl ether is seen environmentally friendly and economical additive for HCCI combustion. On the other hand, diisopropyl ether showed higher resistance to auto-ignition due to higher octane number and auto-ignition temperature. Oxygen content of the diisopropyl ether presented stable combustion, because local richer combustion zone disappeared due to higher oxygen content. In addition, fuel molecules could be well oxidized in the presence of sufficient oxygen in the combustion chamber. This phonomena helped to control self-ignition. Lower in-cylinder pressure is obtained and lower heat is released with fuel blends due to lower calorific value of diisopropyl ether compared to n-heptane at a given lambda value. Fig. 5 demonstrates the in-cylinder gas temperature variation at λ = 2 and 1200 rpm. In-cylinder gas temperature is highly dependent 4

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Fig. 4. The effects of diisopropyl ether on cylinder pressure and heat release rate with different lambda values.

Fig. 5. In-cylinder gas temperature at λ = 2.

Fig. 6. The variations of CA10 versus lambda with test fuels.

Combustion duration (CD) was accepted crank angle range between 10% and 90% of charge mixture was completed to combust. Fig. 8 presents the CD versus lambda with test fuels. It was seen that CD increased with the increase of lambda for all test fuels. It was concluded that there was no big difference on CD with n-heptane versus lambda. But, the addition of diisopropyl ether prolonged the CD since diisopropyl ether oxygenates in the combustion chamber. It improves the chemical auto-ignition reactions. The completion of combustion was delayed due to sufficient oxygen concentration with the addition of

diisopropyl ether. At richer mixtures, shorter combustion was observed due to lower oxygen concentration in spite of the fact that fuel concentration was highly enough. It can be also mentioned that higher density of diisopropyl ether deteriorates the homogeneity of the charge mixture resulting in weaker combustion reactions especially with richer mixtures. It was seen that HCCI combustion needs more air concentration for complete combustion when diisopropyl ether was used as additive to n-heptane. CD was determined as 34.2, 33.12 and 31.68 °CA with n-heptane, D20N80 and D40N60 at λ = 2. Fig. 9 refers to cyclic variations of imep versus lambda with test 5

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Fig. 7. The variations of CA50 and ITE versus lambda.

cycle that prevents to perform stable operation. It was found that COVimep decreased with the increase of lambda for all test fuels due to lower combustion rate and speed. The highest COVimep was determined as 11.03% with n-heptane. Other COVimep values are under 10% that is acceptable for durable and stable combustion. Fuel blends presented higher cyclic variations according to n-heptane owing to higher resistance to combustion. COVimep was determined as 5.87%, 6.54% and 7.85% with n-heptane, D20N80 and D40N60 at λ = 2. Fig. 10 shows the variations of MPRR and RI versus lambda at 1200 rpm. MPRR is the indication of knocking that is significant problem in HCCI combustion. RI is dependent on the combustion temperature, engine speed and MPRR. MPRR and RI decreased with the increase of lambda. But, lower MPRR and RI were obtained with fuel blends owing to higher octane and auto-ignition temperature. Spontaneous and simultaneous auto-ignition reactions can be slow down with diisopropyl ether addition. As expected, n-heptane presented the highest MPRR and RI especially with richer mixtures. Because, rapid and sudden heat reelase was seen during combustion of n-heptane. Knocking tendency was also realized due to lower resistance to knocking with n-heptane. So, diisopropyl ether reduced the knocking tendency resulting in wider operating region at knocking boundaries. 14.61 bar/°CA, 14.54 bar/°CA, 12.72 bar/°CA MPRR values and 24.61 MW/m2, 19.43 MW/m2, 19.74 MW/m2 RI values were obtained with n-heptane, D20N80 and D40N60 respectively at λ = 2. Imep that is significant performance indication defines the averaged in-cylinder pressure exerted on the piston during a cycle. Fig. 11 represents the imep values with consecutive 50 cycle with test fuels. Although the calorific value of n-heptane is higher than diisopropyl ether, lower imep was obtained with n-heptane compared that fuel blends. It should be also mentioned that higher density of diisopropyl ether caused to inject more fuel molecules by mass per unit volume. It caused to release more heat energy resulting in higher in-cylinder pressure applied to the piston. It can be also said that higher imep was obtained with the addition of diisopropyl ether. Not only prolonged combustion but also reasonable calorific value of diisopropyl ether lead to increase imep. Maximum imep was determined as 2.47, 2.24 and 2.08 bar with D40N60, D20N80 and pure n-heptane respectively at 1200 rpm. Minimum imep was computed as 1.59 bar with n-heptane. Stable HCCI combustion was achieved between 800 and 1600 rpm with test fuels and different lambda values at WOT. Full load speed characteristics were also determined with test fuels. Fig. 12-a shows the variation of brake torque and power output versus engine speed. Heat losses and gas leakages are too much at low and higher engine speed leading lower brake torque and power output. These leakages reduces for each stroke until maximum brake torque speed that the highest charge mixture was delivered to the cylinder. Likely, charge mixture

Fig. 8. Combustion duration.

Fig. 9. COVimep.

fuels. Cyclic variations should be lower and should not exceed 10% as mentioned in the literature for stable operation in the internal combustion engines [1,2]. Cyclic variations has important role in HCCI mode, because combustion reactions occur spontaneously. Besides, HCCI is deeply affected by charge composition and fuel reactivity at the end of compression stroke. So, combustion characteristics vary cycle by 6

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Fig. 10. The variations of MPRR and RI versus lambda.

density and more controllable combustion avoiding knocking. Similar trend was seen on power output. 0.913, 0.678 and 0.519 kW power output was computed wth D40N60, D20N80 and n-heptane at 1600 rpm and λ = 2. Fig. 12b defines the SFC variation versus lambda with test fuels at full load. SFC refers the consumpted fuel per unit time for power production. At low engine speeds, effective power ouput is zero despite the fact that fuel is consumed. So, SFC takes infinite value. In additon, fuel could not be well oxidized due to lack of oxygen concentration and homogeneity. Gas leakages and heat losses are too much owing to lower piston and valve speeds. Homogenetity improves and gas leakages reduces with the increase of engine speed resulting in lower SFC. SFC decreased with the additon of diisopropyl ether in the fuel blends. Oxidation reactions were improved with higher oxygen additive. Auto-ignition reactions could be easily performed with diisopropyl ether. Minimum SFC was computed as 306.18 g/kWh, 424.69 g/kWh, 571.68 g/kWh with D40N60, D20N80 and n-heptane at 1400 rpm λ = 2 respectively. HC and CO variations versus lambda are seen in Fig. 13. HC is formed by low combustion temperature. Fuel could not be oxidized owing to lower in-cylinder gas temperature and is discharged from the cylinder in exhaust stroke. HC increased with the increase of lambda. In-cylinder temperature at the end gas region reduces with the combustion of leaner mixture. This prevents the chemical oxidation reactions. Especially, flame goes out on cooler cylinder wall and fuel could not be oxidized on cylinder surface with leaner mixture. Hence, HC is formed. Moreover, diisopropyl ether caused to release more HC due to

Fig. 11. Imep values with consecutive 50 cycle.

decreases at higher speed due to higher piston and valve speeds. As volumetric efficiency decreased, power output decreased. The addition of diisopropyl ether resulted in higher brake torque and power. Brake torque and power output increased by 23.4%, 24.7% with D40N60 compared to D20N80 and n-heptane respectively at 1000 rpm and λ = 2. The reasons of the brake torque increase are seen to be higher

Fig. 12. The variations of brake torque, power output and SFC versus engine speed at λ = 2. 7

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Fig. 13. The effects of diisopropyl ether on HC and CO emissions versus lambda.

molecules can be oxidized through narrow regions and piston cavity in the combustion chamber due to better homogeneity. HC increased by 1.53% and 47.1% with D40N60 compared to D20N80 and n-heptane respectively at 1600 rpm. CO remained same with n-heptane versus engine spped. There is no big variation with n-heptane at a specific lambda versus engine speed. However, CO increased with fuel blends and it can be said that CO increased with the increase of engine speed and diispropyl ether addition. Lower calorific value of diisopropyl ether reduced the released heat energy during combustion. Chemical oxidation reactions can not be completed with lower temperature. CO increased by 16.3% and 29.09% with D40N60 according to D20N80 and n-heptane respectively at maximum brake torque speed of 1000 rpm and λ = 2. Fig. 15 depicts the operating range of HCCI with test fuels. The test engine was stably run on HCCI mode between 800 and 1600 rpm at full load. HCCI combustion was achieved λ = 1.69–λ = 3.08 lambda range at 60 °C. Knocking combustion was seen with n-heptane especially with richer mixtures. The highest imep was obtained with n-heptane in the knocking zone. D20N80 enhanced to achieve HCCI combustion at higher engine speeds and partial combustion zone. Diisopropyl ether fuel blend (D20N80) reduced imep at lower engine speeds. In addition, HCCI combustion was achieved with leanest mixture for all engin speeds. So, operting range was anlarged in misfiring zone with D20N80. When Fig. 15b is examined, larger operating range was obtained with nheptane. The test engine was run with n-heptane and richer mixture. It shows that HCCI combustion was achieved in larger misfiring zone. However, D40N60 presents wider field of stable operating zone

higher octane and auto-ignition temperature in spite of oxygenated fuel. The highest HC was measured as 436 ppm at λ = 2.76. Higher density of D40N60 made difficult to obtain homogeneous charge during combustion. Minimum HC was measured with n-heptane for all lambda values. HC was measured as 325, 335 and 350 ppm with n-heptane, D20N80 and D40N60 respectively at λ = 2. CO variation is also seen in Fig. 13b. CO is the product of incomplete combustion. CO is generated due to unsufficient oxygen and temperature in the combustion chamber. CO increased with the increase of lambda. Excess air coefficient reduced the temperature during combustion. It prevents the oxidization of CO in spite of oxygen concentration. The highest CO was measured as 3.77% at λ = 2.76 with D40N60 at 1200 rpm. CO increased with the addition of diisopropyl ether in the fuel blends. CO increased by about 17.9%, 33.8% with D40N60 compared to D20N80 and n-heptane respectively at λ = 2 and 1200 rpm. Chemical oxidation reactions could not be completed with lower auto-ignition kinetics. Hence, CO is formed. Besides, higher density of diisopropyl ether caused to form inhomogeneity mixture which deteriorates oxidation reactions and slows down the combustion rate. Lower heating value od diisopropyl ether also leads to release lower heat energy resulting in CO formation. Fig. 14 concludes the variations of HC and CO versus engine speed at λ = 2. It is possible to say that HC reduced with the increase of engine speed. It was found that HC increased with the addition of diisopropyl ether at a given engine speed and lambda. Kinetic energy increases and homogeneity of the charge mixture is improved at high engine speeds. Chemical oxidation can be completed even fuel

Fig. 14. The variations of HC and CO versus engine speed at λ = 2. 8

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Fig. 15. The effects of diisopropyl ether on HCCI operating range.

References

avoiding knocking due to higher octane number. The highest imep was obtained with n-heptane and D40N60.

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4. Conclusions Misfiring and knocking are the most important handicap that should be overcome in HCCI combustion, because operating range of HCCI engine is limited. This experimental study aims to investigate the effects of diispropyl ether on combustion, performance and emission characteristics. In addition, diisopropyl ether is considered to be good additive to n-heptane for larger operating range on HCCI combustion. The test engine was run on HCCI mode between 800 and 1600 rpm at full load. In-cylinder pressure and heat release decreased with the increase of lambda for all test fuels. Test results showed that SOC was retarded and CD was prolonged with fuel blends. 23.4% and 24.7% increase was found with D40N60 compared to D20N80 and n-heptane on brake torque and power output respectively at 1000 rpm and λ = 2. ITE increased 8.1% and 4.9% with D40N60 compared to D20N80 and nheptane respectively at λ = 2. Maximum ITE was computed as 23.4% at λ = 2.33 with D40N60. It was seen that D20N80 has advantage in view of misfiring, because diisopropyl ether addition increased the oxygen concentration. It results in better auto-ignition reactions. Otherwise, D40N60 shows good performance in view of knocking. Higher amount of diisopropyl ether addition caused to resist knocking. So, operating range on knocking zone was extended with D40N60. Diisopropyl ether has significant effects on HCCI combustiobn and experimental results showed that diisopropyl ether has good potential to extend HCCI operating range avoiding knocking and misfiring. CRediT authorship contribution statement Ahmet Uyumaz: Formal analysis, Methodology, Project administration, Software, Supervision, Writing - review & editing. Bilal Aydoğan: Formal analysis, Resources, Validation, Writing - original draft. Alper Calam: Investigation, Resources, Validation, Visualization, Writing - original draft. Fatih Aksoy: Conceptualization, Data curation, Project administration, Supervision, Writing - review & editing. Emre Yılmaz: Conceptualization, Data curation, Investigation, Methodology, Software, Visualization. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 9

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