Energy Conversion and Management 94 (2015) 159–168

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

Effects of valve lift on the combustion and emissions of a HCCI gasoline engine Can Cinar a,⇑, Ahmet Uyumaz b, Hamit Solmaz a, Tolga Topgul a a b

Department of Automotive Engineering, Faculty of Technology, Gazi University, Ankara, Turkey Department of Automotive Technology, Vocational High School of Technical Sciences, Mehmet Akif Ersoy University, Burdur, Turkey

a r t i c l e

i n f o

Article history: Received 24 November 2014 Accepted 24 January 2015

Keywords: HCCI Valve lift Combustion Engine performance Exhaust emissions

a b s t r a c t As an alternative combustion mode HCCI seems as one of the most effective choice to increase the thermal efficiency and reduce the soot and NOx emissions among the other conventional combustion modes. HCCI combustion has common properties which gasoline and diesel engines have. Although the spark ignition engines have lower thermal efficiency compared to compression ignition engines, they have been commonly used due to their better starting properties in cold conditions, combustibility and controlling the combustion. For both conventional combustion modes, it is needed to use particle filters, catalytic converters and EGR mechanisms. However, these systems are too complex, expensive and not sufficient for future emission restrictions. HCCI combustion has advantages like increasing the thermal efficiency and reducing exhaust emissions. In order to obtain HCCI combustion, using variable valve mechanism is the most effective and practical method in spark ignition engines. In this study, four different valve mechanisms were used in order to extend HCCI operating range in a four stroke, single cylinder gasoline engine. The experiments were performed between 800 and 1900 rpm engine speeds. The test engine was operated at full HCCI combustion mode at different air/fuel ratios ðk ¼ 0:5  2Þ and inlet air temperatures ðT in ¼ 20  120  CÞ. The effects of air/fuel ratio and inlet air temperature were investigated on HCCI combustion, cylinder pressure, heat release rate, engine performance and exhaust emissions. The test results showed that HCCI operating range can be extended using low lift cams on knocking and misfiring operating zones. It was also found that the test engine was run on HCCI combustion mode at leaner air/fuel ratio as the inlet air temperature increased. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Exhaust emissions emitted from motor vehicles damage the human health, environment and atmosphere. Exhaust emissions are restricted each passing day owing to increasing the number of road vehicles and preventing the global warming. For this reason, hybrid and electric vehicles are thought to be used for reducing not only the fuel consumption but also exhaust emissions. However it does not seem practical to use hybrid and electric vehicles in the medium and long terms. Because it is not possible to take a long way with electric energy due to insufficient performance and charging problems of batteries. It is also very expensive and not practical to convert hydrogen into electricity. Therefore, many investigations have been performed in the internal combustion engines [1–7]. There are still some problems which needed to be solved in conventional internal combustion engines in spite of ⇑ Corresponding author. Tel.: +90 312 2028646; fax: +90 312 2028947. E-mail address: [email protected] (C. Cinar). http://dx.doi.org/10.1016/j.enconman.2015.01.072 0196-8904/Ó 2015 Elsevier Ltd. All rights reserved.

technological developments in the engines. For these reasons considerable interest has been focused on developing HCCI (homogeneous charge compression ignition) engines. Because HCCI engine has many advantages such as higher thermal efficiency due to higher compression ratio and lower exhaust emissions [8]. In HCCI engines, compression ratio and air inlet temperature must be increased for autoignition. Air/fuel mixture is also prepared homogeneously with higher excessive air coefficient. Homogeneous and leaner charge mixture is auto ignited simultaneously owing to higher end of compression temperature in the combustion chamber. Hence, combustion occurs without existing richer mixture zones and temperature variations in the combustion chamber. Besides, it causes to start the ignition of the leaner mixture zones [6–9]. It results lower end of gas temperature at the end of combustion. In-cylinder gas temperature is one of the factors affecting the NOx (nitrogen oxides) emissions. Because chemical reactions can not occur between nitrogen and oxygen molecules at lower gas temperatures in the combustion chamber. So, NOx formation can be prevented. NOx formation mechanisms also depend

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Nomenclature CA CAI CO DME dP dQ dV dh EGR HC HCCI imep

crank angle controlled auto-ignition carbon monoxide di-methyl ether the variation of in-cylinder pressure heat release rate the variation of cylinder volume the variation of crank angle exhaust gas recirculation hydrocarbon homogeneous charge compression ignition indicated mean effective pressure

on the flame length, flame residence time and oxygen concentration in the reaction zone during the combustion [10]. In contrast, HC (hydrocarbon) and CO (carbon monoxide) emissions increase because of lower end of combustion temperature in HCCI combustion. It can be also said that chemical reactions deteriorate with lower combustion temperatures in the combustion chamber. Flame propagation can be interrupted by cooler cylinder surfaces and unburned hydrocarbon emissions are emitted due to incomplete combustion. Moreover HCCI combustion technology has problems such as controlling the combustion phasing and limited operating range [11,12]. The effects of EGR (exhaust gas recirculation), variable valve timing and valve lifts, air/fuel ratio and inlet air temperatures are investigated in order to increase engine performance and expand the operating range of HCCI engines. As it is known, inlet air temperature and compression ratio are increased for HCCI combustion in spark ignition engines. The compression ratio cannot be increased owing to knocking tendency in spark ignition engines. Spark ignition engines must be also operated at stoichiometric air/fuel ratio in case of using catalytic converters. As a result, it is shown that lower thermal efficiency and higher fuel consumption are obtained in spark ignition engines. Thus, thermal efficiency can be increased and fuel consumption and exhaust emissions can be reduced when spark ignition engine is converted to HCCI engine [5,12]. There are some difficulties to control of the autoignition over the engine speed and load operating range of the spark ignition engines. There are two important factors that limited the HCCI operating range. At high loads, knocking combustion occurs at especially with richer mixtures due to faster heat release rate. Secondly, misfiring problem limits the operating range at low engine loads. At this point, the most significant parameter is inlet air temperature in order to provide HCCI combustion. When an external air heater is used in the engines, it requires extra energy. This is not a practical way to use in spark ignition engines. For this reason, heat energy of exhaust gases are used in order to warm up cold homogeneous charge mixture. In HCCI engines, there is no control mechanism on combustion. It results faster heat release rate and higher pressure rise rate due to auto ignition of all charge mixture. The other advantage of trapped exhaust gases in combustion chamber is to slow down the rapid heat release rate. Because fresh charge mixture is diluted with trapped exhaust gases and knocking combustion is prevented. Consequently, it is seen that the most feasible and effective method is to trap the exhaust gases in order to expand HCCI operating range. Thus, it is needed to use variable valve mechanisms including low lift cams in HCCI engines [11–14]. He et al. [11] investigated the effects of different cams in a single cylinder, four stroke, port type injection system HCCI engine. HCCI combustion was achieved with trapping the exhaust gases using negative valve overlap. It was found that more exhaust gases

k NOx P SACI SCCI T in UEGO Vd VVAS Wc k

the ratio of specific heats oxides of nitrogen in-cylinder pressure spark assisted compression ignition stratified-charge compression ignition intake air temperature universal exhaust gas oxygen swept volume variable valve actuation system net work excess air coefficient

were trapped in the cylinder with earlier exhaust valve closing. They have seen that combustion duration increased and maximum cylinder pressure decreased. They have also seen that lambda, valve timing and engine speed affected the combustion and gas exchange efficiency. Karagiorgis et al. [13] investigated the effects of negative valve overlap on trapped exhaust gases in a HCCI engine. They have concluded that combustion was retarded when exhaust valve closing timing advanced. It was also observed that cyclic variations increased. Zhang et al. [14] have performed an experimental study on the effects of variable valve timing and low lift cams on CAI (controlled auto-ignition) combustion. They have changed the valve lifts from 0.3 to 9.5 mm. They have seen the effects of low lift cams on SI (spark ignition) and CAI combustion, in-cylinder pressure and heat release rate. Zhang et al. [15] have performed another study on the effects of HCCI combustion. It was determined that the end of combustion temperature decreased and stable HCCI combustion prevented due to misfiring problem with the increase of lambda. It was also found that HC emissions increased at full load with the increase of engine speed. Chun-hua et al. [16] tested methanol, ethanol and gasoline in a HCCI engine via warming up the inlet air temperature. It was concluded that maximum cylinder pressure increased as inlet air temperature increased. They showed the reduction in CO, HC emissions with gasoline but slight increase in NOx emissions. It is possible to say that HC emissions increased with the increase of air excessive coefficient for three test fuels. As the air excessive coefficient increased, they measured minimum HC emissions with methanol at 160 °C intake air temperature and 1300 rpm engine speed compared to other test fuels. NOx emissions were obtained very low for three test fuels. They also concluded that zero NOx emission were obtained for three fuels when the excessive air coefficient was fixed larger than 2.5. They have seen that NOx emissions decreased with the increase of lambda. Because, in-cylinder temperature decreases with the increase of lambda, NOx formation deteriorates. Lu et al. [17] investigated the effects of gasoline-diesel fuel mixtures on HCCI combustion and exhaust emissions. It was pointed that maximum cylinder pressure, heat release rate and NOx emissions increased with G40 and G50 (40% and 50% gasoline by vol.) fuels compared to G30 (30% gasoline by vol.). They have also implied that inlet pressure has a significant potential in order to reduce NOx and soot emissions. Xie et al. [18] discussed the effects of positive valve overlap and valve timing on combustion and trapped exhaust gases. The test results showed that engine load could be controlled with EGR and positive valve overlap. Singh et al. [19] have performed an experimental study in order to determine the effects of diesel and diesel–biodiesel fuel mixtures (B20 and B40) on HCCI combustion and exhaust emissions. It was found that more stable HCCI combustion occured with biodiesel compared to diesel fuel owing to lower heat release

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rate of biodiesel. Chen et al. [20] aimed to expand HCCI combustion operating range in their experimental study by using variable valve timing. It was evaluated to enhance the stable HCCI combustion as inlet valve opened earlier at low loads. He et al. [21] operated the HCCI engine fueled with butanol. They have seen that auto ignition timing was advanced with the increase of engine speed. It was also shown that exhaust gases reduced and auto ignition timing was retarded as lambda increased. Maurya and Agarwal [22] researched the performance, emission and combustion characteristics of HCCI engine fueled with ethanol, methanol and gasoline. It was shown that ethanol and methanol were ignited at lower inlet air temperatures compared to gasoline. Combustion efficiency also increased compared to gasoline. Jang and Bae [23] tested the low lift cams (2.5, 4, and 8.4 mm) in a HCCI engine fueled with DME (di-methyl ether). They showed that higher lift cams had an advantage at high engine loads. They have implied fuel conversion efficiency increased with the increase of exhaust valve lifts. The reason of increasing fuel conversion efficiency is that negative work decreased with the increase of valve lift because of auto-ignition timing. It was seen that combustion efficiency decreased as the valve lift increased. It was also pointed that EGR increased and volumetric efficiency decreased as exhaust valve closing was advanced. Megaritis et al. [24] determined the effects of valve timing and reduced valve lifts on HCCI combustion. They have seen that HCCI combustion was retarded when intake valve timing was advanced or retarded. Peng and Jia [25] researched the turbulence and combustion characteristics of HCCI engine according to valve lifts and different valve timing. The test results showed that turbulence intensity increased before the compression stroke and better mixture obtained in the cylinder. It was also seen that trapped exhaust gas and in-cylinder temperature increased using negative valve overlap. Law et al. investigated [26] the effects of full variable valve mechanism on CAI combustion. They concluded that oxygen addition caused to accelerate the combustion and knocking tendency increased. Lee et al. [27] tried to observe the effects of basics of autoignition. It was investigated the effects of air/fuel ratio, EGR, injection timing and low lift cams on CAI combustion. They have seen that the highest indicated mean effective pressure (5 bar) was obtained at k ¼ 5 and 28% EGR ratio. Mahrous et al. [28] performed a modeling study to see the effects of valve timing on the gas exchange and performance in HCCI engine. They showed that HCCI operating range became much wider using nontypical valve timing. It was also seen that HCCI combustion was achieved at lower engine load with wider range of valve timings. Xie et al. [29] investigated the residual gas trapping and heating the air inlet temperature on HCCI combustion. It was shown that fuel economy was improved according to negative valve overlap method with preheating the inlet air temperature. It was also found that HCCI operating range was extended at the low load boundary. Kozarac et al. [30] analyzed the effects of internal EGR in HCCI engine fueled with biogas. The test results showed that cylinder temperature increased when the greater negative valve overlap was used to increase combustion products. But, dilution effects prevented the thermal effect. In addition, it was found that EGR caused to instability on HCCI combustion. Cinar et al. [31] investigated the effects of intake air temperature on HCCI combustion. They have seen that in-cylinder pressure and heat release rate increased with the increase of intake air temperature. It was also concluded that combustion was advanced with the increase of inlet air temperature. Godino et al. [32] analyzed the multi zone combustion modeling in order to simulate the HCCI combustion using diesel fuel. They applied the model to HCCI combustion with early and late injection strategies. They have seen that the model gave the datas which were close to the experimental findings. Djermouni and Ouadha [33] performed thermodynamic analysis of a turbocharged HCCI engine. They have found that thermal

and exergetic efficiencies increased with the increase of compression ratio. They have also seen that equivalence ratio improved the thermal and exergetic efficiencies. In this study, a single cylinder, four-stroke, gasoline engine was converted to HCCI operation. For this purpose low lift cams were used. The effects of valve lift, intake air temperature and air/fuel ratio on the combustion and emission characteristics of the HCCI engine were investigated. In addition, misfiring and knocking regions of the HCCI operation were determined.

2. Material and method A single cylinder, four-stroke, port injection, Ricardo Hydra gasoline research engine was used in the experiments. The technical specifications of the test engine are given in Table 1. Before the experiments all the test equipments were calibrated. The test engine was coupled with McClure DC dynamometer which can absorb 30 kW power at 6500 rpm. Strain gauge load cell sensor was used to measure engine load. Engine coolant and oil temperatures were kept constant at 90 °C and 70 °C respectively for stable operation. The fuel was injected to the intake port and air/fuel ratio was altered by adjusting the injection pulse electronically from the dynamometer control panel. The experiments were performed at full load, and the fuel injection quantity was controlled using the fuel injection potentiometer from dynamometer control panel. The fuel was injected into the intake port. The quantity of injected fuel was also changed for desirable air/fuel ratio. The air/fuel ratio was kept constant. UEGO (universal exhaust gas oxygen) sensor was placed on the tail pipe to measure air/fuel ratio. The preheating system was placed upstream of the intake manifold in order to warm up inlet air. Inlet air temperature was also altered using potentiometer on the control panel and kept constant by closed loop controller. The inlet air temperature was measured using K type thermocouple mounted in the suction line. Air consumption was also measured by Merriam Z50MC2-4F model flow meter. The camshaft of the test engine was modified for HCCI operation. Four pairs of cams with low lifts and 124° CA duration were used for intake and exhaust valves. Some amount of exhaust gases were trapped using low lift cams at HCCI combustion mode. The cams used in the experiments are given in Table 2. The valve lifts and duration versus crank angle are seen in Fig. 1. In order to reduce cyclic variations the pressure trace for a specific operating condition was obtained by averaging the sampled pressure data of 50 consecutive cycles. The engine was equipped with an incremental 1000 pulsed shaft encoder. Raw cylinder pressure data were amplified using Cussons P4110 combustion analysis device. Then analog cylinder pressure signals were converted to digital signals using National Instrument USB 6259 data acquisition card with 0.36° CA intervals and transferred to the computer. The schematic view of the experimental setup is shown in Fig. 2. SUN MGA 1500 exhaust gas analyzer was used for measuring the

Table 1 The technical specifications of the test engine.

Model Cylinder number Bore  Stroke (mm) Swept volume (cc) Compression ratio Intake valve opening Intake valve closing Exhaust valve opening Exhaust valve closing Valve lifts (mm)

Spark ignition

HCCI

Ricardo Hydra 1 80.26  88.9 540 5:1–13:1 12° CA BTDC 56° CA ABDC 56° CA BBDC 12° CA ATDC 9.5

Ricardo Hydra 1 80.26  88.9 540 13:1 12° CA BTDC 56° CA ABDC 56° CA BBDC 12° CA ATDC 5.5, 3.5, 2

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Table 2 The cams used in the experiments. Valve mechanism

Intake

Original 1 2 3 4

9.5 mm 5.5 mm 3.5 mm 5.5 mm 5.5 mm

Table 3 The technical specifications of the exhaust gas analyzer.

lift lift lift lift lift

Exhaust

Abbreviations

9.5 mm lift 5.5 mm lift 3.5 mm lift 3.5 mm lift 2 mm lift

IN IN IN IN IN

9.5-EX 5.5-EX 3.5-EX 5.5-EX 5.5-EX

9.5 5.5 3.5 3.5 2

CO (%) HC (ppm) NOx (ppm) CO2 (%) O2 (%) Lambda

Operating range

Sensitivity

0–15 0–9999 0–5000 0–20 0–25 0.6–1.2

0.001 1 1 0.1 0.01 0.001

Table 4 The chemical properties of the test fuels.

RON Chemical formula Molar mass (g/mol) Density (kg/cm3 at 20 °C) Boiling point (°C)

Fig. 1. The valve lifts and duration versus crank angle.

exhaust emissions. The technical specifications of the exhaust gas analyzer are given in Table 3. The experiments were performed at 13:1 compression ratio. Intake air temperature was changed from 20 °C to 120 °C and air/ fuel equivalence ratio was changed from k = 0.5 to 2. Stable HCCI combustion was achieved between 800 and 1900 rpm engine speeds. The knocking and misfiring regions were determined and the effects of air/fuel ratio, intake air temperature and cam lifts on HCCI operating range, in-cylinder pressure, heat release rate, engine performance and exhaust emissions were investigated. For this purpose, the test engine was first operated at spark ignition mode until the engine reached to the operating temperature. Full HCCI operation was achieved by switching off the spark-

Isooctane

n-Heptane

100 C8H18 114.23 0.69 99

0 C7H16 100.2 0.68 97–98

ing from the dynamometer control panel. During the experiments, RON 80 fuel (20% n-heptane and 80% isooctane by vol.) was used. The chemical properties of the test fuels are given in Table 4. The test fuels were obtained from Merck. Indicated mean effective pressure (imep) is one of valuable parameters indicating engine performance. Imep is a constant average pressure which acted on the piston over a cycle. It directly affects the engine performance. Imep is calculated by dividing the net work with cylinder volume [34]. Imep was calculated by Eq. (1) as below. W c and V d refers the net work and swept volume respectively.

imep ¼

Wc Vd

ð1Þ

Net work was calculated by Eq. (2).

Wc ¼

I

pdV

ð2Þ

In-cylinder pressure was used in order to calculate the heat release rate. Heat release rate was calculated by Eq. (3) by applying the first law of thermodynamics. Single zone combustion model was used in order to calculate the net heat release rate [34]. It was assumed that charge mixture was ideal gas in the cylinder and there were no leakages from piston, rings and valves.

Fig. 2. The schematic view of the experimental setup. 1. Test engine 2. DC dynamometer 3. Port type fuel injection system 4. ECU 5. Electronic balance 6. Inlet air heater 7. Laminer air flow meter 8. Cylinder pressure transducer 9. Combustion analysis device 10. Encoder 11. Computer 12. Data acqusition card 13. Exhaust gas analyzer 14. UEGO sensor 15. Dynamometer control panel 16. Battery 17. Digital lambda indication.

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

ð3Þ

3. Results and discussion 3.1. HCCI operating range

163

away from knocking and misfiring boundaries on HCCI combustion mode with. As a result maximum HCCI operating range was obtained with IN 5.5-EX 3.5 valve mechanism. Knocking can be stated with pressure rise rate in the internal combustion engines. In addition, knocking is influenced by residual gases, temperature– pressure history, the temperature of end gas region, compression ratio, throttle position, fuel properties, combustion modes and engine sizes. It is assumed that the knocking limit is 10 bar/° CA in the internal combustion engines [34]. So, knocking combustion occurred at k ¼ 0:8 with IN 5.5-EX 3.5 valve mechanism. Minimum HCCI operating range was obtained with IN 5.5-EX 2 valve arrangement. Because, the fresh homogeneous charge are mixed with exhaust gases. Trapped exhaust gases prevent the chemical reactions between fuel and oxygen molecules. Very low exhaust valve lift (2 mm) prevents to discharge the exhaust gases from the cylinder well. It also led to decrease the fresh charge as percentage in the combustion chamber. Thus, HCCI combustion deteriorated and operating range was limited. The other important factor affecting the HCCI combustion is the inlet air temperature. The effects of reduced valve lifts on HCCI operating range versus inlet air temperature are depicted in Fig. 4. Largest HCCI operating range was obtained with IN 5.5-EX 3.5 valve mechanism. It was concluded that the increase of inlet air temperature allowed to occur HCCI at leaner mixtures. At lower inlet air temperatures, auto ignition could not be able to start and misfiring problem was observed. HCCI combustion was achieved even low inlet air temperature (20 °C) using IN 5.5-EX 3.5 valve mechanism. It was also pointed that HCCI combustion occurred only at higher inlet air temperatures because of misfiring when IN 5.5-EX 5.5 and IN 3.5-EX 3.5 valve mechanisms were used. The narrowest HCCI operating range was obtained with IN 5.5-EX 2 valve mechanism. It is clear that residual exhaust gases were replaced instead of fresh charge and oxygen concentration decreased in the combustion chamber. As a result, auto ignition reaction mechanism and HCCI combustion prevented.

It is known that the HCCI engines operate well at part loads. At high engine loads, rapid heat release rate led to increase pressure oscillations especially with richer mixtures, because the autoignition occurs simultaneously across the combustion chamber. So, all charge mixture contributes to combustion. This combustion treatment result in knocking. Therefore, homogeneous charge mixture should be diluted with trapped exhaust gases in order to prevent rapid heat release rate. In contrast, in-cylinder temperature decreases at the end of combustion at low engine loads. In addition, the dilution effect of trapped exhaust gases causes the misfiring of the charge mixture. These two significant problems limit the HCCI operating range at large engine speed and loads [34,35]. Fig. 3 shows the HCCI operating range with low lift cams versus engine speed. Misfiring and knocking regions were also seen from Fig. 3. As shown in Fig. 3, HCCI combustion was not achieved at low engine speed with leaner mixtures. Flow rate and the kinetic energy of the charge mixture decrease at lower engine speeds as homogeneous charge mixture can not be created. Homogeneous charge mixture can not be formed as flow rate and the kinetic energy of the charge mixture decrease at lower engine speeds. At higher engine speeds, in complete combustion was observed when the test engine was operated with leaner mixture. It was also found that the increase of engine speed allowed achieving HCCI combustion at leaner mixtures. It was also depicted from Fig. 3 that HCCI combustion did not occur with IN 5.5-EX 3.5 valve mechanism at higher engine speed. At this point, it is possible to say that trapped exhaust gases diluted the fresh charge and prevented the HCCI combustion. However, trapped exhaust gases led to provide HCCI combustion at higher imep. It was seen that fresh charge was heated by the exhaust gases which trapped in the combustion chamber. It causes the increase of temperature at the end of compression stroke. It was also observed that HCCI combustion was achieved near the upper boundary of knocking when the test engine was operated especially with richer mixtures. Maximum imep was obtained by about 11.007 bar with IN 5.5-EX 3.5 valve mechanism at 1300 rpm engine speed. So, the test engine operates

The effects of lambda on thermal efficiency are seen in Fig. 5 at constant intake air temperature Tin = 100 °C. It was found that thermal efficiency increased with the increase of lambda. However thermal efficiency started to decrease at leaner charge mixture with IN 3.5-EX 3.5 valve mechanism at 1000 rpm engine speed. It can be also said that volumetric efficiency and homogeneity of

Fig. 3. The effects of reduced valve lifts on HCCI operating range versus engine speed.

Fig. 4. The effects of reduced valve lifts on HCCI operating range versus inlet air temperature.

3.2. Engine performance

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cams. It was also found that higher valve lift of IN 5.5-EX 3.5 allowed to take more charge mixture to the cylinder compared to IN 3.5-EX 3.5. The technical challenges facing both gasoline and diesel HCCI combustion are their restricted operational range because of the lack of direct control mechanism on the start of combustion. So, engine load and combustion can be controlled with variable valve mechanism due to trapped hot exhaust gases in the combustion chamber. It is seen that exhaust gases absorb the heat during combustion. So, it is necessary to extend the high load limit of HCCI combustion [4,34]. 3.3. The effects of air/fuel ratio and inlet air temperature on in-cylinder pressure and heat release rate on HCCI combustion

Fig. 5. The effects of lambda on thermal efficiency with low lift cams.

the charge mixture decrease at lower engine speed. Moreover, leaner mixture causes to decline the thermal efficiency. It was depicted from Fig. 5 that higher thermal efficiency was obtained with IN 3.5-EX 3.5 valve mechanism compared to IN 5.5-EX 3.5 valve mechanism. The heat energy of the charge increases with IN 3.5-EX 3.5 valve mechanism. Thermal efficiency increases with the increase of lambda due to heating effect of hot residual gases. Imep is another significant performance indication. The influence of low lift cams on imep are shown in Fig. 6 versus 50 consecutive cycles. It was determined that more stable HCCI combustion occurred without knocking and when the test engine was operated with reduced valve lifts. Beside the test engine operated away from knocking boundary without existing rapid heat release rate. Rapid heat release rate has been slowed down with especially IN 5.5-EX 3.5 valve mechanism and maximum cylinder pressure decreased when the engine was operated at 1000 rpm, k ¼ 0:6 and constant inlet air temperature (Tin = 100 °C). When the engine speed increased the amount of residual gases in the combustion chamber increased. At 1400 rpm engine speed minimum imep values were obtained with IN 3.5-EX 3.5 valve mechanism. Because taking less charge mixture to the cylinder and dilution effect of residual exhaust gases resulted lower cylinder pressure due to low lift

Fig. 7 shows the effects of air/fuel ratio and inlet air temperature on cylinder pressure and heat release rate with IN 3.5-EX 3.5 valve mechanism at 1000 rpm engine speed. As shown in Fig. 7a cylinder pressure and heat release rate increase with the increase of inlet air temperature. Higher inlet air temperature increases the temperature at the end of compression stroke. It helps to reach auto ignition temperature earlier. The number of molecules which is activated for oxidation reactions increase when inlet air temperature increases. Thus, reaction rate increases and combustion duration decreases. These cause to increase cylinder pressure and heat release rate. It was clearly seen that combustion was advanced and higher cylinder pressure was obtained with the increase of inlet air temperature. Higher inlet air temperature improves the combustion reactions in the combustion chamber. Higher inlet air temperature also improves the combustion rate owing to warmer combustion chamber. Hence auto ignition oxidation reactions can perform easily with the increase of inlet air temperature. Fig. 7b illustrates the effects of air/fuel ratio on cylinder pressure and heat release rate at constant inlet air temperature (Tin = 100 °C). It was clearly observed that cylinder pressure and heat release rate increased until stoichiometric air/fuel ratio (k ¼ 1). After the stoichiometric air/fuel ratio (at leaner air/fuel ratio) cylinder pressure starts to decrease. Maximum cylinder pressure was obtained with k ¼ 1 at 1000 rpm engine speed. It was also determined that combustion was advanced until the stoichiometric air/fuel ratio and then started to retard with leaner mixtures. Combustion rate was improved at stoichiometric air/fuel ratio.

Fig. 6. The variation of imep versus consecutive 50 cycles with low lift cams.

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165

Fig. 7. The effects of air/fuel ratio and inlet air temperature on cylinder pressure and heat release rate.

The energy given to the cylinder increases at richer mixtures. So, cylinder pressure and heat release rate increase. But, cylinder pressure decreased owing to lack of oxygen percentage in the combustion chamber at richer mixtures. It causes the incomplete combustion on HCCI mode. It can be seen from Fig. 7b that minimum cylinder pressure decreased with 100 °C inlet air temperature at lambda 0.5. It is clear to say that richer mixtures deteriorate the HCCI combustion. It can be also clearly observed from Fig. 8a that richer mixtures lead to higher cylinder pressure and heat release rate with IN 5.5-EX 3.5 valve mechanism. The highest cylinder pressure was obtained with k ¼ 0:8 which is richer mixture. Similar results were obtained with IN 5.5-EX 3.5 like IN 3.5-EX 3.5 valve mechanism. Fig. 8b shows the variation of cylinder pressure and heat release rate with different reduced valve lifts. The maximum cylinder pressure was obtained with IN 5.5-EX 5.5 valve mechanism due to higher charge mixture. In contrast, minimum cylinder pressure was obtained with IN 5.5-EX 2 valve mechanism. It could be stated that combustion was advanced with IN 5.5-EX 3.5 and IN 3.5-EX 3.5 valve mechanisms compared to IN 5.5-EX 5.5. In the case of by the use of low lift cams the amount of trapped exhaust gases increase in the combustion chamber. Hence, the heating effect of hot exhaust gases increases the temperature of fresh charge mixture and auto ignition conditions are improved at the end of compression in the combustion chamber. So, HCCI combustion occurred earlier.

3.4. Exhaust emissions As shown in Fig. 9a HC emissions decrease with the increase of inlet air temperature. Higher inlet air temperature reduces the cooling effect of charge mixture and increases the combustion efficiency. Moreover higher inlet air temperature accelerates the production of radicals which helps to occur oxidation reactions. The temperature at the end of combustion increases with the increase of inlet air temperature. Higher combustion temperature improves the oxidation reactions [16,34]. It causes to reduce unburned hydrocarbon molecules. Lower combustion temperatures decrease the oxidation of hydrocarbons and cause to incomplete combustion. In Fig. 9b, it is shown that HC emissions increase with the increase of lambda. The end of combustion temperature decreased as the engine was operated with leaner mixtures [15]. When the engine was operated with leaner mixtures, combustion efficiency decreased. It causes to prevent oxidation reactions. Thus HC emissions increase. As shown in Fig. 10a CO emissions decrease as inlet air temperature increases. CO is an incomplete combustion product because of unsufficient oxygen and temperature in the combustion chamber. CO emissions are sensitive to gas temperature at the end of combustion and chemical reaction kinetics [34]. Oxidation reactions are accelerated with the increase of inlet air temperature and the gas temperature at the end of combustion increases. So, CO formation reduced. Fig. 10b depicts the variation of CO

Fig. 8. The effects of air fuel ratio and low lift cams on cylinder pressure and heat release rate.

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Fig. 9. The variation of HC emissions.

Fig. 10. The variation of CO emissions.

emissions versus lambda with different low lift cams at constant inlet air temperature (Tin = 100 °C). It could be concluded from Fig. 10b that minimum CO emissions were measured at stoichiometric air/fuel ratio. As air/fuel ratio increases the oxygen concentration of homogeneous charge mixture increases and the gas temperature at the end of combustion decreases. This prevents to oxidise the carbon monoxide. It also causes the decrease of engine power output. At richer mixtures, oxygen concentration decreases in the combustion chamber. As a result, oxidation reactions are prevented. Higher engine speed increases the homogeneity of the charge mixture and causes to increase the end of combustion temperature. It is clear to notice that incomplete combustion is prevented due to higher amount of oxygen. NOx emissions are very sensitive temperature history at the end of combustion. NOx emissions are generated at higher gas temperatures (after 1800 K) at the end of combustion [34–36]. NOx formation can be reduced with trapped exhaust gases. Because the heat capacity of the charge mixture increases with the trapped hot exhaust gases. Residual exhaust gases also dilutes the fresh charge. So, rapid heat release is prevented. Fig. 11a shows the variation of NOx emissions versus inlet air temperature with IN 5.5-EX 3.5

valve mechanism. It was found from Fig. 11a that NOx emissions increased with the increase of inlet air temperature. The increase of inlet air temperature caused the higher gas temperature at the end of combustion. Higher gas temperature at the end of combustion led to accelerate the reactions between oxygen and nitrogen molecules and NOx formation increased. It was also concluded that more NOx emissions were generated at 1200 rpm engine speed compared to 1000 rpm as more charge mixture was taken to the cylinder. Thus, gas temperature at the end of combustion increased. In Fig. 11b, the effects of air/fuel ratio was shown on NOx emissions with different valve mechanisms (IN 5.5-EX 3.5 and IN 3.5-EX 3.5). As shown in Fig. 11b maximum NOx emissions were measured around the stoichiometric air/fuel ratio because maximum gas temperature was obtained. It was obtained that less NOx emissions were measured at leaner and richer mixtures. Gas temperature at the end of combustion decreased with leaner mixtures. So, NOx formation decreased. In spite of that oxygen concentration decreased in the combustion chamber when the engine was operated with richer mixtures. It caused to slow down the oxidation reactions and gas temperature decreased. Hence, NOx emissions reduced.

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Fig. 11. The variation of NOx emissions.

4. Conclusions The aim of this study is to determine the effects of reduced valve lifts, air/fuel ratio and inlet air temperature for the problem of limited HCCI operating range. In this way variable valve mechanisms were used in order to determine the knocking and misfiring regions. The most important handicap is that there is no direct control mechanism on HCCI combustion. One of the most feasible and practical method is to trap the hot exhaust gases via variable valve mechanisms in order to control HCCI combustion phasing. For this purpose HCCI combustion was achieved using variable valve mechanisms including low lift cams in a single cylinder, four stroke gasoline engine. The conclusions of the present study can be drawn as below.  Test results showed that the test engine was operated at 800– 1900 rpm engine speed and 20–120 °C inlet air temperatures. It was seen that HCCI combustion operating range was limited knocking and misfiring.  It was found that HCCI operating range can be extended using low lift cams. Because it was shown that residual hot exhaust gases diluted the fresh charge mixture and has slowed down the rapid heat release rate because of higher heat capacity of cylinder charge mixture. Because it showed that residual hot exhaust gases diluted the fresh charge mixture and slowed down the rapid heat release rate because of higher heat capacity of cylinder charge mixture. Thus, it allowed to realize the HCCI combustion near the knocking boundary.  Test results also showed that HCCI combustion has been achieved at leaner mixtures with higher inlet air temperature at misfiring boundary. It is also possible to say that the HCCI combustion has been achieved at lower engine speeds. It caused to realize HCCI combustion at larger engine speed range.  When the test engine was operated with low lift cams, thermal efficiency increased with the increase of inlet air temperature. It was clearly observed that larger HCCI operating range was obtained with IN5.5-EX 3.5 valve mechanism.  HCCI combustion was achieved with IN 5.5-EX 5.5 valve mechanism at high engine speeds. Cylinder pressure and heat release rate increased and combustion was advanced with the increase of inlet air temperature. However in-cylinder pressure increased until stoichiometric air/fuel ratio and then started to decrease at leaner mixtures.

 HC and CO emissions decreased with the increase of inlet air temperature but NOx. It is also possible to mention that HC emissions increase with the increase of lambda. But CO emissions first decreased until stoichiometric air/fuel ratio and then start to increase slightly at leaner mixtures with low lift cams. Maximum NOx emissions were measured around the stoichiometric air/fuel ratio. It was also seen that NOx emissions decreased at leaner and richer mixtures.

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