Energy Conversion and Management

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1 Energy Conversion and Management 51 (2) Contents lists available at ScienceDirect Energy Conversion and Management journal homepage: An experimental study of the biogas diesel HCCI mode of engine operation S. Swami Nathan, J.M. Mallikarjuna, A. Ramesh * I C Engines Laboratory, IIT Madras, Chennai 6 36, India article info abstract Article history: Received 2 May 28 Received in revised form 4 February 29 Accepted 8 September 29 Available online 2 March 2 Keywords: Biogas Renewable fuel HCCI engine Alternative fuels NO x control In this work biogas was used in a HCCI engine with charge temperature and amount of diesel injected into the intake manifold being used to control combustion. The presence of CO 2 in biogas suppresses the high heat release rates encountered with neat diesel fuelling in HCCI engines. Normally biogas use leads to a drop in thermal efficiency in both SI and CI engines. However, present results indicate that thermal efficiencies close to diesel engine values can be obtained in the HCCI mode. The NO level was less than 2 ppm and the smoke level was less than.1 BSU at all conditions. The best energy ratio was 5%. HC levels were very high and were lowered when the charge temperature was raised. A charge temperature of about C was needed, which can be attained though heating by exhaust gases. On the whole the HCCI mode can be a viable option to utilize biogas in a diesel engine. Ó 29 Elsevier Ltd. All rights reserved. 1. Introduction Gaseous fuels in general are considered to be good for internal combustion engines on account of their ability to readily form a mixture with air. Their wide flammability limits and high self ignition temperatures enable spark ignition engines to operate with high compression ratios along with lean mixtures with consequent benefits of good thermal efficiency and low emissions. Biogas is good for decentralized power generation in rural areas [1] particularly in developing countries. Biogas can be produced from cow dung and other animal waste and also from plant matter such as leaves and water hyacinth. Bacteria, which break down organic material under air less conditions in a process called anaerobic digestion are responsible for its production. Small capacity biogas plants are widely available in several developing countries including India. Biogas contains about 6% of methane and the rest is mainly carbon dioxide as seen in Table 1. The presence of carbon dioxide adversely affects the combustion quality of methane [2] as seen in Table 1. The self ignition temperature is high, the flammability limits are low and the flame speed is also low. Abbreviations: BD-HCCI, biogas diesel homogeneous charge compression ignition engine; BMEP, Break Mean Effective Pressure; BSU, Bosch smoke units; CA, crank angle; CI, compression Ignited; COV, coefficient of variation; EGT, exhaust gas temperature; FID, flame ionization detector; HCCI, Homogeneous Charge Compression Ignition; HRR, heat release rate; NG, natural gas; PM, particulate matter; PP, peak pressure; ppm, parts per million; rpm, revolution per minute; RTD, resistance temperature detector; SI, spark ignited; TDC, top dead center. * Corresponding author. Tel.: ; fax: address: aramesh@iitm.ac.in (A. Ramesh). Biogas can be used in spark ignition (SI) engines directly [3]. However the thermal efficiency is low and HC levels are very high due to the presence of CO 2 [4]. Removal of CO 2 enhances the flammability limits and improves flame speed thereby enhancing thermal efficiency and lowering emissions [5,6]. Though removal of carbon dioxide is often done in large biogas plants, which use municipal waste in cities, this method is not viable for rural applications. In addition, care must be taken to control NO emissions when CO 2 is removed biogas is used in engines. Biogas can be used in compression ignition (CI) engines in the dual fuel mode [7]. In a dual fuel engine, after compression of the charge comprised of biogas and air a small amount of diesel, called the pilot is injected. This injected pilot fuel gets self ignited and then becomes the ignition source for the inducted biogas. The main advantage of dual fuel engines is that they can work with a wide variety of gaseous fuels without engine modifications [8]. However, they have problems of high HC (hydrocarbons), CO (carbonmonoxide) and NO x (oxides of nitrogen) emissions [9]. Biogas dual fuel engines generally have low thermal efficiencies due to the presence of CO 2 []. They also emit high levels of hydrocarbon emissions for the same reason. This particularly becomes significant at low loads [11]. In addition a close control over the pilot fuel quantity with respect to load is required in dual fuel engines [12]. On the whole the use of biogas in SI and CI engines leads to poor thermal efficiency and high emissions. Thus an alternative method of utilizing biogas in diesel engines with high thermal efficiency, low emissions is worth investigating. One of the alternative engine combustion concepts that are being widely studied is Homogeneous Charge Compression Ignition (HCCI) [13,14]. Here a homogeneous mixture of air and fuel is compressed and allowed to self /$ - see front matter Ó 29 Elsevier Ltd. All rights reserved. doi:.16/j.enconman

2 1348 S. Swami Nathan et al. / Energy Conversion and Management 51 (2) Table 1 Biogas properties. Calorific value (MJ/kg) 17 Density at 1 atm and 15 C 1.2 Flame speed (m/s).25 Stoichiometric A/F (kg of air/kg of fuel) 5.7 Flammability limits (vol.% in air) (7.5 14) Research octane number 1 Auto-ignition temperature ( C) 65 ignite. Combustion is controlled by varying the temperature and composition of the charge. This concept was first studied experimentally in a two-stroke engine and low levels of cyclic variations were obtained [15]. HCCI leads to extremely low levels of NO x and particulates. Several fuels have been successfully used [16]. HCCI mode of combustion has been used for utilization of gaseous fuels also (LPG and NG) [17,18]. However it cannot be realized at low loads on account of the lean mixtures that are used and the low temperatures that prevail in the combustion chamber. Further the auto-ignition temperature is high for gaseous fuels. Charge temperature and fuel additives are generally used to control the start of combustion. Intake charge temperature control, variable valve timing to achieve different compression ratios and exhaust gas recirculation have been used successfully [19]. One of the main problems in HCCI engines is that combustion can become too rapid and lead to knock [2]. High combustion rates are seen when the load is high and this limits the range of operation of HCCI engines [21]. This mode of combustion cannot be realized at low loads on account of the lean mixtures used and the low temperatures that prevail in the combustion chamber. Thus the range of loads where in HCCI combustion can be had is limited. Combustion control in HCCI engines can be done using two fuels with different combustion properties in different proportions [22]. Often diluents like recirculated exhaust gases or other gases are used to control combustion rate [23,24]. In an earlier work, the authors found CO 2 to be a good diluent for combustion control in a HCCI engine fuelled with diesel [25]. Thus it was thought that the CO 2 present in biogas itself could control combustion rate in a biogas fuelled HCCI engine. The use of biogas in a HCCI engine does not seem to have been reported in literature. This motivated the authors to under take this investigation on biogas fuelled HCCI mode of operation. Since biogas has a high self ignition temperature, an additive with a low self ignition temperature, in this case diesel was needed for ignition. 2. Present work The objective of the present work is to investigate the potential of the HCCI concept to utilize biogas effectively. Here biogas is the main source of energy and diesel is used to control combustion. A compression ignition engine was operated in the HCCI mode with biogas being inducted along with air like in a dual fuel engine. Diesel which was also injected into the intake air through a high pressure injection system for good atomization, was used to control the start of combustion. The charge temperature was also varied to control the combustion process. The influence of charge temperature and biogas to diesel proportions was studied at different Brake Mean Effective Pressures (BMEPs) under a constant speed of 15 rpm. 3. Experimental setup The base engine used in this work is a single cylinder, watercooled, DI, diesel engine which was modified to operate on the HCCI mode and the schematic of the setup is shown in Fig. 1. The specifications of the engine are seen in Table 2. An electrical heater was placed in the intake air stream to control the charge temperature. There were no changes in the combustion chamber geometry to convert it from CI to the HCCI mode. However, a separate inline injection system run by the cam shaft which could be set at any injection timing desired was developed and used. The required injection timings were set with the help of an external mechanical arrangement. A needle-lift sensor was used to find the actual start of injection. The pintle type diesel fuel injector was placed after the electrical heater. Biogas generated in a fixed dome reactor was collected in a flexible bag and then transported Fig. 1. Schematic of experimental setup. 1. Biogas floating drum; 2. Biogas fuel control and measurement system; 3. Manifold injection system; 4. Intake manifold; 5 and 6. Heater system; 7. Air flow measurement; 8. Pressure transducer; 9. Crank angle encoder;. Data acquisition system; 11. Personal computer; 12. Cooling water inlet; 13. Water flow measurement; 14. Cooling water temperature measurement; 15. Cooling water outlet; 16. Exhaust gas temperature measurement; 17. Exhaust manifold; Exhaust gas analyzers; 22. Dynamometer controller; 23. Eddy current dynamometer; 24. Clutch controller; 25. Electric motor; 26. Electromagnetic clutch; 27. Engine.

3 S. Swami Nathan et al. / Energy Conversion and Management 51 (2) Table 2 Engine specifications. Bore stroke 8 1 mm Connecting rod length 231 mm Compression ratio 16:1 Rated output 3.7 kw at 15 rpm Displacement volume.553 m 3 Injector NOP 22 bar Table 3 Instrument accuracy. Sl. no. Parameter Instrument type Accuracy 1 Biogas flow Positive displacement.1 m 3 2 Temperature Thermocouple ±1 C 3 HC emission FID 5 ppm 4 NO emission Chemiluminescence 5 ppm 5 CO emission NDIR.1% 6 Pressure Piezoelectric.25 bar 7 Torque Load cell ±.5% Table 4 Instrument uncertainty. Sl. no. Measured parameter Uncertainty (%) 1 Biogas flow rate ±.5 2 Intake charge temperature ±.5 3 NO emission ±4.1 4 HC (FID) ±2.1 5 CO (NDIR) ±1.5 6 Cylinder peak pressure ±1.3 7 Brake thermal efficiency ±.7 to the laboratory. It was then sent into a floating drum meant to maintain a constant pressure. The flow rate of the biogas was controlled by a valve and was measured with a gas flow meter before being admitted into the intake manifold. Intake air and exhaust gas temperatures were measured by means of the thermocouples and the coolant water outlet temperature was measured by using RTD. An optical-encoder and a flushmounted, water cooled, piezoelectric pressure transducer were used to measure in-cylinder pressure history. During experiments, in-cylinder pressure data was acquired for consecutive cycles. Heat release rate was calculated from the cylinder gas pressure history based on a method proposed in literature [26]. A Flame Ionization Detector for HC, Non-Dispersive Infrared (NDIR) analyzer for CO, chemiluminescent analyzer for NO x and Bosch smoke meter for smoke emissions were used. Tables 3 and 4 provide information about instrumentation accuracy and uncertainties. (at high biogas flow rates i.e. low diesel flow rate) and knock (at low biogas flow rates i.e. high diesel flow rate). Subsequently, the BMEP was raised from 2.5 bar in steps to 4 bar at different constant charge temperatures of 8, and 135 C. The engine could not run at BMEPs below 2.5 bar in the HCCI mode as the intake charge temperature could not be raised beyond 135 C due to system limitations. Beyond a BMEP of 4 bar, combustion rate became very high and very careful control over the charge temperature and biogas flow rate was needed. Results have been reported at different biogas diesel energy ratios. Here energy ratio is defined as the ratio of energy supplied by the biogas alone to the sum of the energy supplied from biogas and diesel expressed in percentage. Engine performance, emission and combustion characteristics were evaluated in all the tests. 5. Results and discussion The engine could operate in the biogas diesel HCCI (BD-HCCI) mode satisfactorily in the BMEP range of bar with charge temperatures in the range of 135 C. Measurements were not done at BMEPs below 2.5 bar as the charge temperature needed was very high. In the case of BMEPs above 4 bar control was difficult as the charge temperature had to be lowered and biogas quantity had to be carefully elevated Brake thermal efficiency The brake thermal efficiency in the biogas diesel HCCI (BD- HCCI) mode of operation at a BMEP of 2.5 bar is seen in Fig. 2. Biogas was supplied till the efficiency dropped down significantly. It may be noted that in the diesel-hcci mode even a charge temperature of C cannot be used as it will lead to severe knocking due to high heat release rates. With biogas, higher charge temperatures could be tolerated due to the presence of CO 2 and the high self ignition temperature of methane present in it. For example, at the charge temperature of C the thermal efficiency increased as the biogas quantity was elevated due to the fact that the combustion was progressively delayed towards TDC for good thermodynamic efficiency. At the charge temperature of C, a maximum of 4% substitution by biogas was possible. Here the energy ratio is defined as the amount of energy from biogas to the total energy (diesel + biogas) supplied to the engine. High levels of biogas lowered the heat release rate and thus the thermal efficiency. When the charge temperature was elevated a similar trend was seen. Higher biogas rates could be tolerated with better thermal efficiencies. At the charge temperature of 135 C, it was possible to use a biogas energy ratio of 4 57%. Still higher thermal efficiencies and biogas rates could have been achieved if the charge temperature was elevated. This was however not tried in this work 4. Experimental procedure In order to achieve HCCI, the engine was initially motored with the intake charge temperature being increased steadily using a heater. Then the engine was started when diesel was injected into the inlet manifold. An intake charge temperature of 85 C was required for starting the engine. Subsequently, the engine was loaded to a Brake Mean Effective Pressure (BMEP) of 2.5 bar, i.e. 5% of the maximum for this engine. Then the biogas was slowly allowed along with the intake charge and the amount of diesel that was injected was reduced simultaneously and the engine speed was maintained at 15 rpm throughout. The coolant outlet temperature was maintained at 5 C always. Three intake charge (air + biogas + diesel) temperatures namely 8, and 135 C were tried at different diesel biogas proportions at the BMEP of 2.5 bar. The limits of operation in the HCCI mode were misfire Brake Thermal Efficiency [%] Intake charge Temp C Fig. 2. Variations of brake thermal efficiency of biogas diesel HCCI operation at 2.5 bar BMEP.

4 135 S. Swami Nathan et al. / Energy Conversion and Management 51 (2) as the air had to be heated to as high as 2 C to achieve a charge temperature of 135 C. At these conditions the exhaust gas temperature is above 25 C. Hence, a heat exchanger could be used to preheat the charge by the exhaust. Subsequently the engine was run at different BMEPs with three charge temperatures namely 8, and 135 C. It may be noted that here the biogas energy ratio was fixed at the condition where knock was just avoided. The highest thermal efficiency obtained with this constraint (lowest biogas energy ratio without knock) at each BMEP in the BD-HCCI mode is seen in Fig. 3 for each charge temperature. The optimum charge temperatures are 135 C at 2.5 and 3 bar, C at 3.5 bar and 8 C at a BMEP of 4 bar. The thermal efficiencies with the BD-HCCI mode are lower than the case CI mode with diesel by a maximum of 3% absolute (i.e. from % (CI mode) to 27% in the BD-HCCI mode at a BMEP of 4 bar). It may be noted that even in the case of biogas diesel operation in the dual fuel mode the thermal efficiencies are lower than the neat diesel mode of operation []. In the case of dual fuel operation with biogas and diesel higher drops in thermal efficiency have been reported [11]. The shaded area in Fig. 4 indicates the biogas energy ratio that gives the best thermal efficiency at different BMEPs. The corresponding charge temperatures are also easily read off the graph. In all cases a ratio of about 5% delivers best result. The curves in Fig. 2 also show how the energy ratio varies when the engine is operated at the condition where knock is just avoided Emissions Brake Thermal Efficiency [%] Charge Temp C CI Mode Best Operation of BD-HCCI 5 Fig. 3. Variations of brake thermal efficiency of biogas diesel HCCI operation for the HC Emission [ppm] Intake charge Temp C Fig. 5. Variations of HC emissions of biogas diesel HCCI operation at 2.5 bar BMEP. The variation of HC at a BMEP of 2.5 bar under two different charge temperatures is seen in Fig. 5. As the amount of energy from biogas increases the HC level goes up probably because of the unburned portion of the charge which is thought to be composed of biogas, which becomes richer. Beyond a particular substitution there is sudden rise in the HC level due to an increase in the partial burn and misfiring cycles. Fig. 6 also shows that the HC levels go up with rise in BMEP or rise in charge temperature. As the charge temperature rises, at any BMEP more biogas has to be used to suppress knock. As the BMEP increases, we find from Fig. 4 that the biogas energy ratio increases if we need operation at the knock border. Thus it is evident that the HC level goes up whenever the biogas energy ratio increases. These levels are far higher than the base diesel values. Biogas diesel dual fuel engines also show significantly increased HC levels as compared to normal diesel operation []. Fig. 7 indicates that at a BMEP of 2.5 bar the CO level drops to a minimum and then rises as the biogas energy level is increased. Near the operating condition of highest thermal efficiency the CO level is a minimum. When misfire occurs the CO level shoots up as in the case of HC. However, the HC level always goes up with an increase in the biogas energy ratio. We find from Fig. 8 that HC Emission [ppm] " Charge Temp C CI Mode Fig. 6. Variations of HC emissions for different charge temperatures for the Fig. 4. Variations of energy ratio for different charge temperatures for the operation window. CO Emission [% Vol] Intake charge Temp C Fig. 7. Variations of CO emissions of biogas diesel HCCI operation at 2.5 bar BMEP.

5 S. Swami Nathan et al. / Energy Conversion and Management 51 (2) CO Emission [% Vol] Charge Temp C. Fig. 8. Variations of CO emissions for different charge temperatures for the Heat Release Rate [J/ CA] C-Chrage Temp 4% Biogas 45% Biogas 47% Biogas 51% Biogas 54% Biogas 57% Biogas Fig. 11. Heat release rate for 135 C charge temperature at 2.5 bar BMEP. the CO level falls with increase in BMEP when the biogas ratio is maintained to avoid knock. Here the biogas energy ratio increases (i.e. the diesel share decreases) with rise in BMEP. In general it is seen that CO falls when the diesel share reduces. The NO level was found to reduce as the biogas energy ratio was raised because the combustion rate came down with a decrease in the amount of diesel injected. Even when the engine was knocking the NO levels in the BD-HCCI mode was below 6 ppm is seen in Fig. 9 due to the effect of CO 2. Fig. indicates that at different BMEPs the NO level when the engine was at the knock border is less than 2 ppm as compared to values between 25 and 47 ppm with neat diesel operation Combustion characteristics Fig. 11 shows the heat release rate in the BD-HCCI mode at biogas energy ratios in the range of 4 57% at a charge temperature of 135 C and a BMEP of 2.5 bar. We find that the addition of biogas delays the combustion and also reduces the heat release rate. This is because of the presence of CO 2 and also because methane present in biogas has a high self ignition temperature. The highest thermal efficiency occurred at a biogas energy rate of 51% due to optimal phasing of combustion. In this case neat diesel operation in the HCCI mode was not possible due to knock. The corresponding calculated cylinder peak temperatures were reduced from 16 K to 13 K when the energy ratio changed from 4% to 57% is shown in Fig. 12. There was a reduction in NO levels also. Figure 13 indicates that biogas can significantly reduce the maximum rate of pressure rise which can be very high with HCCI operation. At the best efficiency condition the maximum rate of pressure rise was about 4 bar at both the charge temperatures at the BMEP of 2.5 bar. The cycle by cycle variations in peak pressure show a sharp increase beyond the best thermal efficiency point due to misfiring and partial combustion as seen in Fig. 14. Subsequent graphs indicate the effect of BMEP on combustion parameters when the biogas energy ratio was maintained at a con- NOx Emissions [ppm] Intake charge Temp C Fig. 9. Variations of NO x emissions of biogas-diesel HCCI operation at 2.5 bar BMEP. Tempearture [ K] % Biogas 45% Biogas 47% Biogas 51% Biogas 54% Biogas 57% Biogas C-Chrage Temp Fig. 12. In-cylinder temperature for 135 C charge temperature at 2.5 bar BMEP. NOx Emissions [ppm] Charge Temp C CI Mode Fig.. Variations of NO x emissions for different charge temperatures for the Maximum Rate of Pressure Rise [bar/ CA] Intake charge Temp C Fig. 13. Variations of MRPR of biogas diesel HCCI operation at 2.5 bar BMEP.

6 1352 S. Swami Nathan et al. / Energy Conversion and Management 51 (2) COV of Peak Pressure Intake charge Temp C Fig. 14. Variations of COV of peak pressure of biogas diesel HCCI operation at 2.5 bar BMEP. dition where the engine was just about to knock. In Fig. 15, we see that the Maximum Rate of Pressure Rise (MRPR) falls with BMEP increase and also rise in charge temperature. As mentioned earlier as the BMEP increases or the charge temperature rises in the following graphs, the biogas energy ratio goes up. Thus we see from Fig. 13 that the MRPR falls with increase in the biogas energy ratio. The combustion duration also goes up as seen in Fig. 16. Combustion duration was calculated using the cumulative heat release curve between % and 9% of total heat release. The COV of PP also follows a similar trend indicating that the biogas energy ratio has a significant effect (Fig. 17). The variation of heat release rates at a charge temperature of 8 C (Fig. 18) indicates that though the peak heat release rate increases with BMEP, combustion gets delayed. This is because at higher BMEPs larger biogas energy ratios had to be used while working at the knock limit for good thermal efficiency. Even though the maximum heat release rate rises with BMEP the MRPR reduces as seen earlier in Fig. 15. This is because of the delayed Maximum Rate of Pressure Rise [bar/ CA] Charge Temp C Fig. 15. Variations of MRPR for different charge temperatures for the operation window. Combustion Duration [ CA] Charge Temp C Fig. 16. Variations of combustion duration for different charge temperatures for the COV of Peak Pressure combustion phasing. Fig. 18 also indicates a small early heat release portion (cool flame) at the BMEP of 2.5 bar where the biogas energy ratio is low (diesel energy ratio is highest). At higher BMEPs this phenomenon is not clearly seen because the fraction of diesel energy is low. The absence of cool flames in the case of lower hydrocarbons like methane is reported in literature [27]. Figure 19 indicates that as the charge temperature is raised at fixed BMEP (3 bar) while the engine operates on the knock border, combustion starts earlier and the heat release rates also become higher. This occurs even though the biogas energy ratio is raised as the charge temperature is elevated. The maximum rate of pressure rise thus significantly increases with charge temperature. Thus it is evident that a close control over the biogas energy ratio and charge temperature is needed to achieve good combustion. 6. Conclusions Charge Temp C Fig. 17. Variations of COV of peak pressure for different charge temperatures for the Heat Release Rate [J/ CA] C-Chrage Temp bar BMEP 3. bar BMEP 4. bar BMEP Fig. 18. Heat release rate for 8 C charge temperature at 2.5 bar BMEP. Heat Release Rate [J/ CA] bar BMEP 135 C Charge Temp C Charge Temp 8 C Charge Temp Fig. 19. Heat release rates for different charge temperatures at 3 bar BMEP. This work has shown that biogas can be effectively used in the HCCI mode with manifold injected diesel and charge temperature

7 S. Swami Nathan et al. / Energy Conversion and Management 51 (2) being employed for controlling combustion. The presence of CO 2 in biogas suppresses the high heat release that normally occurs in HCCI engines fuelled with diesel. The high self ignition temperature of methane helps delay the combustion process to favorable crank angles. Efficiencies close to diesel operation along with extremely low levels of NO and smoke were attained in a BMEP range of bar. This can be extended to still higher BMEPs with proper control over the charge temperature and biogas energy ratio. This mode of operation seems to be better than the biogas diesel dual fuel mode in the range of BMEPs tried as regards thermal efficiency and emissions of NO and smoke. The thermal efficiency at a BMEP of 4 bar is 27.2% in the biogas diesel HCCI mode as against % with diesel operation. The NO level is less than 2 ppm and the smoke level is less than.1 BSU at all conditions in the biogas diesel HCCI mode. 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