Introduction In order to reduce air pollution and to achieve legal limits regarding the emissions of pollutants the new combustion processes for internal combustion engines (ICE) are constantly being developed. Homogeneous Charge Compression Ignition (HCCI) combustion is suggested as a potential solution for above-mentioned challenges. The HCCI combustion process is a form of Low-Temperature Combustion (LTC) [1] and has the advantages of lower emissions of nitrogen oxides (NOX) (compared to both compression ignition (CI) and SI mode of operation) and at the same time of high efficiency (diesel like high efficiency) and also lower emissions of particulate matter (PM). Major disadvantage of the HCCI engine is the control of the start of combustion due to its sensitivity to the intake air temperature [2]. Although there are a number of published studies of the combustion process in the HCCI engine, there is no experimental comparison of the combustion process in HCCI mode with the SI mode on the same engine and at the same operating points in the engine map which makes results comparable. This research presents a comparison of these two combustion modes in IC engine at similar operating points at optimal operating conditions. Experimental tests that employ an approach significantly different from other research found in the literature were performed, where IMEP was held constant for different types of fuel, compression ratios and modes of operation and optimisation of operating points at different combustion modes was performed resulting in the new knowledge regarding the combustion process phenomena of the engine. In this study, the engine is fuelled with methane, a fuel with high octane number, biogas with volume ratio of methane and carbon dioxide 80:20 and 60:40, respectively, and RON 95 gasoline. Different compression ratios were used for different fuels and combustion modes, i.e. in HCCI mode the CR = 18 was used when the engine was fuelled by methane and biogas, compression ratio of 12, 16 and 18 when the engine was fuelled by gasoline, while in SI mode the CR = 12 was used with gasoline and CR = 18 with methane and biogas. In HCCI mode the intake air temperature was used for control of combustion timing and therefore intake air was heated by the external heater. Due to the physical properties of the methane and biogas, it was not possible to start the combustion at CR = 16 since it required excessive intake air temperature for auto-ignition of the mixture (over 400 °C), thus compression ratio was increased to 18. Fuelled with gasoline, the SI mode is limited to CR = 12 due to its lower knock resistance, while the HCCI mode had to be run at CR = 16 because CR = 12 would also require excessive intake air temperature. Experimental setup In the Laboratory for IC Engines and Motor Vehicles of the Faculty of Mechanical Engineering and Naval Architecture of the University of Zagreb, an experimental setup for testing of IC engines is developed. The core of the setup is a modified single cylinder diesel engine Hatz 1D81Z. The engine is modified so that it can operate in SI and HCCI mode, while the compression ratio is changed by changing the piston and/or by using the head gasket with different thickness. To achieve CR = 12, the top of the original piston was machined by 3 mm and used in combination with head gasket thickness of 1.3 mm. The CR = 16 was achieved with the top of the piston machined by 0.6 mm and the head gasket thickness of 1.3 mm, while the CR = 18 was achieved with the same piston as in CR = 16, but with the head gasket thickness of 0.7 mm. In this study, the engine is fuelled by methane from the pressurised gas bottles that had 99.5% CH4, biogas with different volume ratio of CH4 and CO2 also from the pressurised gas bottles. Port fuel injector HANA H2001 was used for the delivery of the gaseous fuels. The engine was also fuelled by RON 95 gasoline taken from the gas station. Gasoline was delivered to the engine by a port fuel injector BOSCH EV-6-E with fuel supply at a constant pressure of 3 bar. The fuel flow was measured by OHAUS Explorer mass scale for gasoline and with Coriolis mass flow meter Endress+Hauser Proline Promass A 100 for methane. To achieve SI mode of operation, the spark plug was mounted on the engine head at the location of the original diesel fuel injector and accompanied by the corresponding ignition system. The in-cylinder pressure is measured with AVL GH14DK sensor which is synchronised with the low-pressure sensor AVL LP11DA installed into the intake manifold. During measurement, 300 consecutive cycles were sampled and then used in the analysis. To enable the control of the intake air temperature the Osram Sylvania air heater with 18 kW of installed power was used in the intake system after the intake air settling tank. The emissions of carbon monoxide (CO) and carbon dioxide (CO2) are measured by the nondispersive infrared (NDIR) method, the emissions of hydrocarbons (HC) are measured by the flame ionisation detector (FID), while the NOX measurement is performed by a ceramic NOX sensor. The intake air flow is measured by the laminar mass flow meter TSI 2017L [3–5]. Experimental testing This research presents the results of testing of the engine in SI and HCCI mode at operating points which are optimised for combustion phasing (CA50) so that the criteria for IMEP, ringing intensity (RI), knock and coefficient of variation of IMEP (CoV(IMEP)) are satisfied. Limit for CoV(IMEP) is set to 10% [6] for both combustion modes and in all measured operating points, the CoV(IMEP) was significantly below the imposed limit. The method for achieving the optimal operating point in SI mode was to obtain the maximum IMEP while simultaneously satisfying the limits of the CoV(IMEP) and knock [5]. The measure of knock was Maximum Amplitude Pressure Oscillation (MAPO) which was at all operating points under the limit of 0.5 bar since the chosen operating points presented low load in SI mode. MAPO is defined as the absolute peak value of the band-pass filtered pressure trace as described in [7]. Criteria for determining optimal operating point in HCCI mode was maximum IMEP while satisfying the limit on RI [8]. In order to determine the optimal operating point during the engine testing, an online program for monitoring of the engine parameters such as combustion phasing (CA50), RI, maximum pressure rise rate (MPRR), maximum in-cylinder pressure, MAPO, combustion noise level (CNL), etc. is created by using indicating hardware and software package AVL IndiCom [9,10]. The comparison of operation in different combustion modes is performed at three different levels of IMEP labelled as IMEP 1, IMEP 2 and IMEP 3. For each level of IMEP the engine was tested at three different engine speeds: 1200, 1600 and 2000 rpm. Since there are three IMEP levels and three engine speeds, nine operating points are tested for each combination of fuel, combustion mode and compression ratio. In the results, a comparison of in-cylinder pressure, temperature and rate of heat release (ROHR) is shown together with the measured emission levels of the HC, CO, CO2 and NOX. The control mechanism for the CA50 in SI and HCCI mode is different. In SI mode, the CA50 is controlled by the timing of the spark discharge (Spark Timing), while in the case of HCCI mode, the CA50 is determined by the intake air temperature which is controlled by the installed air heater. Combustion phasing is advanced by increasing the intake air temperature and vice versa. The engine load in SI mode of operation is controlled by a throttle valve position that changes the intake manifold pressure, while the air to fuel mixture was stoichiometric or slightly lean. In HCCI mode of operation, the engine load is controlled by changing of the excess air ratio (λ), where richer mixture results in higher engine load, while the intake manifold pressure was kept at the atmospheric pressure. Engine testing in SI mode of operation is done: - at the compression ratio ε = 12 and fuelled by gasoline RON 95 (hereinafter referred to as: SI-B-CR12), - at the compression ratio ε = 18 and fuelled by biogas in the volume ratio of CH4: CO2 = 100: 0 and with stoichiometric mixture, λ = 1 (hereinafter referred to as: SI-BP 100: 0 - CR 18-A), - at the compression ratio ε = 18 and fuelled by biogas in the volume ratio of CH4: CO2 = 100: 0 and with λ = 1.2 (hereinafter referred to as: SI-BP 100: 0-CR18-B), - at the compression ratio ε = 18 and fuelled by biogas in a volume ratio of CH4: CO2 = 80: 20 and a stoichiometric mixture (hereinafter referred to as SI-BP 80: 20-CR18) - at compression ratio ε = 18 and fuelled by biogas in volume ratio of CH4: CO2 = 60: 40 and stoichiometric mixture (hereinafter referred to as: SI-BP 60: 40-CR18). Engine testing in HCCI mode of operation is done: - at compression ratio ε = 12 and fuelled by gasoline RON 95 gasoline (hereinafter referred to as: HCCI-B-CR12), - at compression ratio ε = 16 and fuelled by gasoline RON 95 (hereinafter referred to as: HCCI-B-CR16), - at the compression ratio ε = 18 and fuelled by gasoline RON 95 (hereinafter referred to as: HCCI-B-CR18), - at compression ratio ε = 18 and fuelled by biogas in a volume ratio of CH4: CO2 = 100: 0 (hereinafter referred to as: HCCI-BP 100: 0-CR18), - at compression ratio ε = 18 and fuelled by biogas in volume ratio CH4: CO2 = 80: 20 (hereinafter referred to as: HCCI-BP 80: 20-CR18), - at compression ratio ε = 18 and fuelled by biogas in volume ratio CH4: CO2 = 60: 40 (hereinafter referred to as: HCCI-BP 60: 40-CR18). Results and discussion For the purpose of comparisons, each combination of CR, fuel and combustion mode was measured at equal loads and at optimised operating points. Based on the results of this research the following facts can be highlighted: - The engine in HCCI mode compared to the SI mode of operation for the same load and the same engine speed has higher in-cylinder pressure in the entire cycle due to significant differences in the fuel mixture and the difference in engine load control. The highest peak pressure in the engine cylinder of 60.78 bar was measured at the highest engine load (IMEP3) and engine speed of 1200 rpm at HCCI-B-CR16 mode. The highest peak in-cylinder pressure was in the HCCI mode fuelled with biogas in volume ratios of CH4 : CO2 = 80 : 20 and CH4 : CO2 = 60 : 40 (approximately 46 bar), which is 20% less than in HCCI-BP-80:20+B-CR18 mode of operation. - Pressure rise rate increases with the increase of the engine load in the HCCI mode fuelled with RON95 gasoline at the compression ratio 12 : 1, and in the HCCI mode fuelled by biogas (CH4 : CO2 = 100 : 0) pressure rise rate decreases, but is still higher than in all SI modes of operation with all fuels at all engine speeds. The highest pressure rise rate of 7.83 bar/degCA is measured at HCCI-B-CR16 mode of operation at the engine load IMEP3 at 1200 rpm. The average value of the pressure rise rate in HCCI operating mode fuelled by the biogas in the volumetric ratios of CH4 : CO2 = 80 : 20 and CH4 : CO2 = 60:40 is 2.45 bar/degCA or 2.83 bar/degCA which is 28% less than in the HCCI mode of operation fuelled by gasoline (the mean value for all three compression ratios is 3.7 bar/degCA). - Indicated efficiency of the HCCI mode fuelled by gasoline at CR = 12 is higher than in the HCCI mode fuelled by biogas in the volumetric ratios of CH4 : CO2 = 100 : 0 and at the same mode of operation fuelled with the same fuel at the CR = 18. In SI mode of operation to achieve the same load at the same engine speed it is necessary to lower the intake pressure. The indicated efficiency of the HCCI mode fuelled with gasoline at CR = 16 is higher than in HCCI mode fuelled by biogas (CH4 : CO2 = 100 : 0), and it is higher than in all corresponding SI modes (SI-B-CR12, SI-BP 100:0-CR18-A and SI-BP 100:0-CR18-A) for all measured operating points. The average value of the indicated efficiency in the HCCI mode fuelled by gasoline at CR = 16 is 25% higher than in the SI-BP 100:0-CR18-A mode at λ = 1. The highest indicated efficiency is determined at the highest engine load (IMEP3) at 1200 rpm at HCCI-B-CR16 mode (36.99%). The highest mean indicated efficiency (same load at three different engine speed) of 36.77% is determined at the HCCI-B-CR16 mode at the highest engine load (IMEP3). The average indicated efficiency (30.56%) is determined at HCCI mode fuelled by biogas in the volumetric ratios of CH4 : CO2 = 80 : 20 and CH4 : CO2 = 60 : 40, which is higher by 8% than in SI mode (27.82% ) fuelled with the same fuel at the same compression ratio. - For optimum performance, engine in the HCCI mode of operation (HCCI-B-CR12 and HCCI-B-CR16) requires 29% less fuel for the same engine load at the same engine speed in comparison to the SI mode (SI-B-CR12). Comparison of the HCCI mode of operation fuelled by biogas with the 100% CH4, the average specific fuel consumption is equal to 233.9 g/kWh, for 80% CH4 the average specific fuel consumption is equal to 399.8 g/kWh, which is 70% higher and for the 60% CH4 which is equal to 670.31 g/kWh and it is higher by 186% compared to biogas with 100% CH4. The lowest specific fuel consumption (215.5 g/kWh) was determined for the HCCI-B-CR16 mode of operation. - Emission of the NOX in SI mode of operation (SI-B-CR12, SI-BP 100:0-CR18-A and SI-BP 100:0-CR18-A) is considerably higher (25 times on average) than in HCCI mode (HCCI-B-CR16 and HCCI-BP 100:0-CR18) due to the higher peak in-cylinder temperature that exceeds the nitrogen oxide formation limit of 1800 K. Furthermore, in HCCI mode (HCCI-B-CR16 and HCCI-BP 100:0-CR18), emissions of the NOX are close to the permissible limits for EURO VI heavy duty engines while in SI mode of operation emissions are above the permissible limit. The lowest emission of the NOX for all engine loads was measured at the HCCI-B-CR16 mode. The values are approximately equal to zero, better to say that the values are within the limits of the measuring range of the device. The emissions of NOX are at HCCI mode fuelled by biogas in the volumetric ratios of CH4 : CO2 = 80 : 20 and CH4 : CO2 = 60 : 40 9 times lower than in the SI mode under the same operating conditions. - The emissions of the HC in the HCCI modes (HCCI-B-CR16 and HCCI-BP 100:0-CR18) are in some cases three times higher than in the SI modes (SI-B-CR12, SI-BP 100:0-CR18-A and SI-BP 100:0-CR18-A), due to lean mixture (λ up to 3.2). Emissions of the HC in HCCI mode fuelled by biogas (CH4 : CO2 = 100 : 0) are increased by the increase of the engine speed, and in some cases by the increase of the engine load, while in SI mode, HC emissions are decreasing at the same operating points. Due to the lean mixture, emissions of the HC are the highest in HCCI-BP 60:40+B-CR18 mode at medium engine load (IMEP2) at engine speeds of 1600 rpm and the value is equal to 21.52 g/kWh, which is far above the permissible emission limit of the EURO VI heavy duty engines (0.13 g/kWh) which means that it is necessary to use exhaust after-treatment systems. Emissions of the HC are approximately 70% lower in SI mode of operation with the same fuel (biogas in CH4 : CO2 = 80 : 20 and CH4 : CO2 = 60 : 40) and under the same operating conditions as in the HCCI mode. - The emission of the CO decreases by the increase of the engine load and engine speed in the SI mode of operation (SI-B-CR12, SI-BP 100:0-CR18-A and SI-BP 100:0-CR18-A), while in HCCI mode (HCCI-B-CR16 and HCCI-BP 100:0-CR18) the emission of the CO increases with the increase of the engine load and a decrease of the engine speed. Emission of the CO in the HCCI mode (HCCI-B-CR16) is higher than in the SI mode (SI-B-CR12) when the gasoline is used as a fuel, while in the HCCI mode of operation fuelled by the biogas (CH4 : CO2 = 100 : 0) emission of the CO is lower than in the SI mode operated with the same fuel and with stoichiometric mixture. The lowest value of the CO emission of 1.98 g/kWh was measured at the SI mode fueled with methane at λ = 1 at the medium load (IMEP2) at 1600 rpm. The emission of the CO in HCCI mode fuelled by biogas (CH4 : CO2 = 80 : 20 and CH4 : CO2 = 60 : 40) is 30% lower than in SI mode of operation under the same conditions and it is 4 times lower than in the HCCI mode fuelled by gasoline. Conclusion Comparison of the two types of the combustion processes in ICE, SI and HCCI combustion, is presented in this research. Engine testing in the SI mode of operation was carried out with four different fuels (RON95 gasoline and biogas with three different ratios of CH4 and CO2), where the compression ratio for gasoline fueled engine was equal to 12 : 1 while when using biogas the compression ratio was equal to 18 : 1 in all CH4/CO2 compositions. In the HCCI mode, engine testing was also carried out with four different fuels and at different compression ratios. The compression ratios equals to 12 : 1, 16 : 1 and 18 : 1 were used when engine was fuelled with RON 95 gasoline. When the engine was fuelled with biogas, because of the required high intake air temperature to achieve the combustion (temperature above 400 °C) it was not possible to test the engine at compression ratio of 16 : 1 and for that reason the compression ratio was increased to 18 : 1. With this compression ratio the engine was tested in HCCI mode of operation fuelled by biogas in all CH4/CO2 compositions. From this comparison a general conclusion is that there is a clear benefit of HCCI mode of combustion in mid-load region of the standard engine map, especially with lower octane number fuel, under the assumption that the control of combustion phasing can be efficiently achieved.