. Homogeneous Charge Compression Ignition (HCCI) engine has attracted great attention due to its improved performance and emissions compared to conventional engines . It can reduce both Nitrogen O xides (NOx ) and Particulate Matter ( PM) emissions simultaneously without sacrificing the engine performance . However, controlling its combustion phasing remains a major challenge due to the absence of direct control mechanism . The start of combu stion is entirely initiated by the chemical reactions inside the combustion chamber, resulted from the compression of its homogeneous mixtures. Varying some critical engine parameters can play a significant role to control the combustion phasing of HCCI engine. This paper investigates the characteristics of HCCI combustion fuelled with n -heptane (C7H16) using single-zone model computational software . The model enabled the combustion object to vary from cycle to cycle. Detailed simulations were conducted to evaluate the effects of air fuel ratio (AFR ), compression ratio (CR) and intake air temperature on the in -cylinder pressure and heat release rate . The simulation results show ed that the single -zone model was able to predict the two-stage kinetic combustion of HCCI engine ; the Low Temperature Heat Release (LTHR) and the High Temperature Heat Release (HTHR) regions . It was found that minor change s in AFR, CR and inlet air temperat ure led to major change s in the HCCI combustion phasing. 1. Introduction One of new combustion technologies that attracts numerous atte ntion is Homogeneous Charge Compression Ignition (HCCI) engine. It combine s the best features of both Spark Ignition ( SI) and Compression Ignition ( CI) engines, offering a relatively high efficiency with low Nitrogen Oxides (NOx) and Particulate Matter (PM ) emissions [1]. The homogeneous mixture is achieved without using spark plug or injector to control its combustion phasing [2]. Due to the absen ce of additional equipment to regulate its start of combustion , an HCCI engine does not typically give large gradients of spatial temperature from the flame front [3]. As a result, it can significantly reduce NOx emissions. Mor eover, since the mixture is premixed and very lean, it creates homogen eous charges inside the cylinder thus reducing the PM emissions significantly [4]. In addition to its low emissions characteris tic, an HCCI engine can also be ope rated at high compression ratio, producing more power and achievin g higher efficiency compared to conventional engine s [5]. 2 However, d espite its benefits in terms of high performance and low emissions, the combustion of HCCI engine is difficult to control [6]. Conventional engines such as SI and CI engines use external devices i.e. spark plug in SI engine or injector in CI engin e to control the start of combustion . In HCCI engine, such devices are not available . Moreover, t o be widely used as a commercial engine , the HCCI technology has still a number of challeng es to be solved. These include narrow operating range and the diffic ulty to regulate its heat release rate [2]. It is also important to note that as the mixture s between air and fuel ignite nearly simultaneously at multiple location within the cylinder , knocking and misfir e phenomena may occur [7]. Therefor e, controlling the combustion phasing remains the biggest challenge in HCCI combustion. Since no external source of energy is used to control its ignition timing, the auto-ignition depends entirely on the chemical kinetics and combustion environment withi n the cylinder [8]. Changes in the cylinder environment can affect the combustion phasing of HCCI engine [9]. Varying the air fuel ratio (AFR ), intake air temperature and compression ratio (CR) have the potential to assist the combustion control of HCCI engine [10]. However, if the value of these engine parameters reach to a point where the rich mixtures are created inside the combustion chamber, the early auto-ignition may occur which eventually cau se knocking [11]. On the other hand, if too lean mixtures are formed, misfire and incomplete combustion may take place , increasing the unburned H ydrocarbons (u HC) and Carbon Monoxide ( CO) emissions of HCCI engine [12]. Thus, further investigation is needed to understand the optimised operating range of HCCI for various AFR , CR and intake air temperature s. To investigate the combustion behaviour of an HCCI engine, it is important to have the right fuel and its chemical mechanism. Commercial diesel fuel contains a mixture of alkanes, aromatics, cycloalkanes and ole fins [13]. Consequently, it is difficult to understand its chemical mechanism due to its oxidation complexity of all the chemistry compounds. Unlike diesel fuel in the market, surrogate mixture such as n -heptane is free from aromatics and has higher volatility as well as less sulfur content compared to diesel fuel. Therefore, n -heptane is often used as the primary reference fuel (PRF) to substitute commercial diesel fuel in numerous studies. It is suitable to be operated in HCCI as the cetane number of n -heptane is comparable to commercial diesel fuel [14]. As a result, the combustion characteristics of both fuels are almost similar due to its identical cetane number [15]. Given the costly nature of experimentation using engine test bench , simulation work using computer programme is often preferred. A simulation model can provide insight into the effect of different parameters by isolating some of their values [16]. Various simulation models have been propos ed [17– 20]. In general, the models can be grouped into two categories; chemistry model and fluid dynamic model [20]. The chemistry model, also referred to as the zone model, takes into account the chemical reaction in the cylinder using detailed kinetic mechanism. The fluid mechanics and heat transfer phenomena are simplified in this model. On the other hand, in the second model, focus is given on the fluid dynamic and heat transfer phenomena . The fluid dynamic model is usually performed using Computational Fluid Dynamics (CFD) with c hemical reaction being simplified. Considering that HCCI combustion is primarily governed by chemical kinetics and less influenced by the fluid mechanics or heat transfer phenomena, the zone model is preferred in the simulation of HCCI combustion. The zo ne model is normally classified into the single -zone and the multi -zone models [21]. The single -zone model does not require high performance comput ing process as it neglects the temperature and the inhomogeneity of the composition inside the cylinder. The combustion duration is underestimated, while the pressure peak is overestimated [22]. Therefore , it cannot predict accurately a number of extreme conditions such as high Exhaust Gas Recir culation ( EGR ) or very lean mixture , owing to the neglection of the te mperature and the inhomogeneity of the mixtures . On the other hand, the multi -zone model takes into account both the temperature and the inhomogene ity. I t enhances the idea of single -zone model by dividing the cylinder volume into several zones with each zone having its 3 own characteristics of temperature and composition [23]. Multi -zone models may slightly be dif ferent with one another due to the variation between the interaction of one zone with its neighboring zone. Fiveland and Assanis [24] developed a single -zone model to investigate the performance and combustion of HCCI engine fuelled with hydrogen or methane. The effects of AFR, CR, and inlet temperature were investigated. It was found that the development of m ixture preparation and ignition point significantly affect ed the volumetric efficiency and power output. They concluded that HCCI operation fuelled with natural gas need ed high compression ratios as well as lo w intake temperatures to promote the auto-ignition. In terms of the fuel, previous studies have shown that the operating region of HCCI engine fuelled with n -heptane is extremely narrow due to early ignition and knock problem at lower AFR. Hasan et al. [4] used a single zone model to evaluate HCCI combustion and performance using a reduced n -heptane chemical reaction mechanism. The model was successful in capturing the combustion phasing of HCCI engine , particularly the Low Temperature Heat Release (LTHR) region, for different AFR, CR, engine speed, inlet air tempe rature and intake air pressure . It was found that the combustion was advanced and the combustion duration was shortened with the increase of the intake air temperature and the decrease of the engine speed. Previous studies mentioned above have indicated that the problem in HCCI combustion phasing could be effectively controlled by changing a number of engine parameters using computational programme. Fathi et al. [23] investigated both the single -zone and multi -zone approaches in the HCCI combustion modelling . It was found that the single -zone model gave over -prediction on the in -cylinder pressure. Moreover, the maximum pressure rise rate was also significantly overestimated. On the other hand, the multi -zone model provided better predictions on the combustion phasing and the maximum pressure rise rate. However, the use of multi -zone model requires significant amount of time and resources. T his study indicated that if the initial conditions were set properly and the heat transfer during the compression stroke w as carefully estimated, the start of main combustion could be predicted accurately using single -zone model. To take the advantage of speed and low cost of computational study , this paper aims to develop a simulation model of HCCI engine fuelled with n -heptane using single -zone model to vary the combustion object from cycle to cycle . The combustion analysis was carried to evaluate the in -cylinder pressure and heat rel ease rate for various air-fuel ratio s, compression ratios and intake air temperature s. 2. Methodology Figure 1 shows detail of the entire model from the inlet part to the engine and exhaust system. The specifications of t he engine cylinder geometry are as follows ; bore = 86 mm, stroke = 86.07 mm, connecting rod length = 175 mm, and TDC clearance height = 1 mm. Figure 1. Schematic diagram of the model 4 The Woschni GT model was used to estimate the heat transfer in the cylinder. This is a modified model from classical Woschni in which the coefficients of heat transfer are obtained when the valves are open. As a result, the Woschni GT model is more accurate since it considers the increase of heat transfer resulted from the inflow velocities at the intake and the backflow velocities at the e xhaust valve. Furthermore, t he modelling wa s performed in a perfectly mixed zone using one -dimensional engine modelling software. The injector delivery rate was set to 7 g/s with 360 degree injection timing angle and 0.3 vaporized fluid fraction. The model allowed the combustion object to vary from cycle to cycle. As a result, this mixed -mode model c ould be used at different cycles during the same transient simulation for HCCI model. To understand the use of n -heptane in HCCI engine for a wide range of operating conditions, varying some engine parameters such as air-fuel ratio (AFR), compression ratio (CR) and intake air temperature were needed. Firstly, the value of AFR was varied between 30 and 50, while intake air temperature, pressure, CR, EGR and en gine speed were kept constant. Afterwards, the value of CR was varied between 10 and 16. Lastly, the intake air temperature was varied between 300 and 360 K, while other engine parameters such as AFR, CR, EGR and engine speed were kept constant. Table 1 pr ovides the simulation matrix of the param eters and variables used in the simulation. Table 1. Simulation matrix No. AFR CR Intake Air Temperature (K) Pressure (bar) EGR (%) Speed (rpm) 1. 30-50 10 325 1 20 1000 2. 45 10-16 325 1 20 1000 3. 45 10 300-360 1 20 1000 The value s presented in Table 1 are sufficient to achieve the HCCI combustion. As mentioned in the introduction, HCCI is limited by its narrow range of operation. At rich mixtures, early auto-ignition may occur resulted from its high in -cylinder pressure and temperature that eventually leads to knocking. At too lean mixtures, the auto-ignition cannot be achieved thus creating incomplete combustion and misfire. Therefore, the selection of variables and constant parameters is of great importance in engine simulation. The values given in the simulation matrix above are within the accepted operating range to achieve the HCCI combustion [4]. 3. Results and d iscussion The use of conventional diesel fuel or n-heptane in HCCI engine tends to produ ce a two -stage heat release region; the Low Temperature Heat Release (LTHR) and the High T emper ature Heat Release (HTHR) . This two -stage heat release characteristic is also observed in other diesel -like fuels such as straight carbon -chain paraffin , Fisher-Tropsch nap htha and Dimethyl Ether ( DME ). 5 3.1. Effect of air -fuel ratio (AFR) The HCCI combustion depends heavily on the homogeneous mixture to produce high efficiency and low emissions level . Varying the air fuel ratio is considered as the most pra ctical approach to assist the homogenization of the mixture. Figure 2 shows the effect of various AFR on HCCI in -cylinder pressure. The value of AFR was varied between 30 and 50 for a constant intake air temperature (325 K), pre ssure (1 bar), CR (10), EGR (20%) and engine speed (1000 rpm). Figure 2 shows that as the value of AFR increases, the in -cylinder pressure decreases and the peak pressure is retarded. As a result, longer combustion delay is observed with higher AFR (lean mixtures). On the other hand , low AFR has relatively shorter combustion delay as its high fuel composition increases the speed of its charge chemical reactions. As a result, the lowest AFR in the graph has the most advanced and highest peak pressure. However, the use of low AFR or ri ch mixtures may lead to excessive pressure and damage the engine due to its early auto-ignition and high peak pressure. To prevent these problems, higher amount of AFR or highly diluted mixture is required to lower the speed of chemical reactions so that e xtreme pressure can b e avoided. Results from F igure 2 indicates that varying the value of AFR can be used as an effective method to control the in -cylinder pressure of HCCI combustion. Figure 2 . Effects of AFR on in -cylinder pressure As mentioned previously, the use of n -heptane in HCCI engine tends to produce a two -stage heat release region namely the Low Temperature Heat Release (LTHR) and high tempera ture heat release (HTHR) . Figure 3 shows that the simulation result is able to prov ide both LTHR and HTHR phenomena which are natural characteristics of HCCI combustion fuelled with n-heptane or diesel. The result shows that as the AFR increases , the phasing of LTHR is slightly advanced, while that of HTHR is significantly 0102030405060 -40 -33 -26 -18 -11 -4 3 11 17 25 32 39Pressure [bar] Crank Angle [deg] AFR = 30 AFR = 35 AFR = 40 AFR = 45 AFR = 50 6 retarded . The retarded HTHR leads to the delay in the auto-ignition . This is because the molecule of hydrocarbon mixes poorly with oxygen at leaner mixtures (higher AFR). As a result, as the heat release rate of the HTHR decreases , the cumulative heat release rate also reduces . This finding shows that varying the value of AFR has the potential to control the heat r elease rate of HCCI combustion thus preventing the tendency of both misfire and knocking. Figure 3 . Effects of AFR on heat release rate 3.2. Effect of compression ratio (CR) Another important engine parameter that is normally used to control HCCI combustion phasing is compression ratio (CR). Figure 4 shows the effect of various compression ratios on HC CI in -cylinder pressure. The value of CR was varied between 10 and 16 for a constant intake air temperature (3 25 K), pressure (1 bar), AFR (45 ), EGR (20 %) and engine speed (1000 rpm). Theoretically, increased CR will raise the peak in -cylinder pressure an d advance the combustion process, resulted from the increase of compression temperatures and pressures. Figure 4 shows that the in-cylinder pressure increase s significantly with the increase of CR. However, the peak pressure of all compression ratios has r elatively the same crank angle position (around -5 before top dead centre) except for CR of 10 . The auto-ignition of high CR is also well advanced in the crank angle position compared to that of lower CR. The reason why the CR of 10 gives the lowest peak p ressure and the most retarded combustion phasing is caused by the poor oxidation of n-heptane at low compression ratio. With low CR, the temperature and pressure are not sufficient enough to ignite the fuel. 0.000.100.200.300.400.500.600.70 -30 -23 -15 -8 -1 6 13Heat Release Rate [1/deg] Crank Angle [deg]AFR=30 AFR=35 AFR=40 AFR=45 AFR=50 7 Figure 4 . Effects of CR on in -cylinder pressure Figure 5 shows the effect of various CR on HCCI heat release rate. Heat release indicates the start of the chemical reactions and the release of massive energy that will increase the in -cylinde r temperature and press ure. It is shown in the figure that the higher the CR is, the more advance the heat release is . One of the main concern of using diesel fuel or n -heptane in HCCI engine is its low volatility characteristic which can cause problem in its fuel evaporation pr ocess. In some cases, wall wetting is unavoidable thus causing the incomplete combustion and increasing both unburned hydrocarbons and CO emissions. Results from Figure 5 indicate that by varying the CR, the heat release can be advanced or retarded. Theref ore, changing the value of CR can be a useful strategy to control the heat release rate of HCCI engine . In addition to that, varying CR can also improve the evaporation process of low volatile fuel such as diesel and n -heptane. 01020304050607080 -40 -33 -26 -19 -11 -4 3 11 18 25 32 39Pressure [bar] Crank Angle [deg]CR=10 CR=11.5 CR=13 CR=15.5 CR=16 8 Figure 5 . Effects of CR on heat release rate 3.3. Effect of intake air temperature To control the HCCI combustion, heating the intake air temperature is often used as an effective way to control the combustion phasing. Figure 6 shows the effect of intake air temperature on HCCI in -cylinder pressure. The value of the T in was varied between 300 and 36 0 K for a constant a mbient pressure (1 bar), AFR (45 ), CR (10), EGR (20%) and engine speed (1000 rpm). Figure 6 shows that as the intake air temperature increases, the auto-ignition timing advances significant ly. Due to the advanced of auto-ignition timing, the pressure rise rates will increase thus resulting in rapid heat release rate. In reality, a preheating device is often used to heat the intake air. However, it is important to note that as the hot intake air enters the combustion chamber, it will mix with the fuel and is compressed during the compression stroke. This will eventually increase the charge temperature to the auto-ignition temperature of n -heptane. As a result, too e arly auto-ignition may occur and cause knocking. Therefore, the range of intake air temperature value has to be considered care fully. Results from Figure 6 indicate that varying the intake air temperature is a practical method to achieve the HCCI combustio n using n -heptane fuel. 0.000.100.200.300.400.500.600.70 -31 -24 -16 -9 -2 6Heat Release Rate [1/deg] Crank Angle [deg]CR=10 CR=11.5 CR=13 CR=15.5 CR=16 9 Figure 6. Effects of intake air temperature on in-cylinder pressure Figure 7 shows the effect of various intake air temperature on HCCI heat release. The figure reveals that as the intake air tempera ture increase s, both the LT HR and HTHR are more advanced . Consequently, knocking occurs as the peak of heat release rate is observed before TDC thereby deteriorating the HCCI combustion. As previously explained, the use of n -heptane in HCCI engine is known to produce LTHR that will increase the in -cylinder temperature [25]. In the low -temperature phase, the most important reactions of key chemical components occur i.e. C7H15O2=C 7H14OOH. This reaction will retard the process of low -temperature oxidation and cause the negative temperature coefficient (N TC) phase. Furthermore, Figure 7 also shows that a minor change in the intake air temperature will result in a major change in the heat release rate of HCCI e ngine. Therefore, changing the intake air temperature can play a significant role in controlling the HCCI combustion phasing. 05101520253035404550 -41 -33 -26 -19 -12 -5 2 10 17 24 31 39Pressure [bar] Crank Angle [deg]Tin=300 Tin=315 Tin=330 Tin=345 Tin=360 10 Figure 7. Effects of intake air temperature on heat release rate 4. Conclusion s HCCI fuelled with n -heptane is known to hav e narrow operating ranges. Its start of combustion is also difficult to regulate due to the absence of direct control mechanism. Varying a number of engine parameters can be an effective way to control its combustion phasing. A single -zone model was establ ished to evaluate the HCCI combustion fuelled with n -heptane for different AFR, CR and intake air temperature s. The main conclusions can be summarised as follows: • The fast and low cost of computational single -zone model was able to capture the two -stage kinetic combustion of HCCI engine namely the Low Temperature Heat Release (LTHR) and the High Temperature Heat Release (HTHR) regions. • Decreasing the value of AFR was found to increase the pressure and te mperature of combustion due to its rich mixtures compo sition . As the pr essure and temperature increase d during the compression stroke, the oxid ation of n -heptane also improved thus advancing the auto-ignition timing. As a result , the pressure -rise rates increased thus causing rapid heat release rate that was prone to knock problem. • The increase of CR and intake air temperature resulted in the in crease of pressure and temperature combustion. During the compression stroke, the reactions of key chemical components depend ed on th e in -cylinder pressure and temperature. Therefore, higher compression ratio and inlet air temperature would advance the start combustion of HCCI engine. • Minor change s in the value of AFR, CR and intake air temperature led to significant cha nges in the comb ustion phasing . This finding indicates that both knocking and misfire can be effectively avoided if critical engine parameters are set properly . Moreover, the start of HCCI combustion can also be controlled by varying the AFR, CR or inlet air temperature. 0.000.100.200.300.400.500.600.70 -31 -23 -16 -9 -2 5Heat Release Rate [1/deg] Crank Angle [deg]Tin=300 Tin=315 Tin=330 Tin=345 Tin=360 11 Acknowledgment The authors acknowledge the financial support from Ministry of Higher Education Malaysia ( MOHE ) under the Fundamental Research Grant Scheme (FRGS) - (R.J130000.7824.4F884 ).