Emissions and Performance Characteristics of an SI Engine with Biogas Fuel at Different CO2 Ratios

Abstract

Keywords: Biogas; CO2; Methane; SI Engine; Emissions; Combustion

Introduction

Nowadays, as the number of vehicles increases, the percentage of diesel vehicles in the vehicle market increases [1]. In 2016, the market of diesel vehicles have 49% share among all vehicles in European countries [2]. In this case, the emission of NOx and PM emissions increases, affecting the environment and human health negatively [3]. Due to this reason, several steps have been taken in recent years to reduce the number of diesel vehicles. As an alternative to diesel vehicles, electric and Fuel Cell vehicles are targeted to be presented to the market [4]. However, because of the high cost and short life spans of PEM fuel cells and high cost of purification of hydrogen and hydrogen storage problems avoids the use of fuel cells in vehicles [5]. On the other hand, because of the need of improvement at current battery technology and the inadequacy of charge stations prevents the widespread use of these systems in the near future [6]. Due to this reason, it is foreseen that fuel cells will be used to supply very small units’ batteries and increase their range which can be used as range extender in small powers and operated as both serial hybrids and parallel hybrids when necessary. In the near future, the use of environmentally friendly, low-cost and alternative gas fuels in SI engines will gain great importance. Because of this reasons, biogas emerges as a clean and environmentally friendly alternative fuel containing methane, CO 2 and other gases that can be produced from natural sources [7]. Due to the methane in its content, biogas is a environmentally friendly fuel, which has low C/H ratio [8]. Because of the methane in its content, the use of biogas in in SI engines provides great benefit in terms of emissions [9]. Related studies in the literature on the use of biogas in SI engines are given below: Karagoz et al. [10] used an ANN model for a spark ignition engine with use of different methane (CH4) ratios to estimate engine performance data. The study shows that with the increase in methane concentration, positive effect is seen in volumetric efficiency but a dramatic decrease occurs in thermal efficiency. Despite that there isn’t any observable change in brake specific fuel consumption. Wang et al. [11] studied the effect of methane content to NOx emissions and indicated thermal efficiency in a biogas fuelled spark ignition engine. It is included that increase in methane content in biogas have positive effect in both indicated thermal efficiency and NOx emissions.

This paper was recommended for publication in revised form by Regional Editor Tolga Taner Istanbul Medeniyet University, Mechanical Engineering Department, İstanbul, Turkey *E-mail address: yasin.karagoz@medeniyet.edu.tr Orcid id: 0000-0002-9193-3011 Manuscript Received 17 June 2018, Accepted 8 November 2018

Journal of Thermal Engineering, Research Article, Vol. 5, No. 6, Special Issue 10, pp. 131-140, December, 2019 Shin et al. [12] studied the effect of using biogas in a small spark ignition engine at different compression ratio and various CO2 dilution (0%-50%) on engine performance. The results show that, engine performance increases by using biogas with decreasing CO2 dilution. Nadaleti et al. [13] made experiments tos see the effect of biogas-syngas dual fuel in a SI engine on engine emission characteristics. The results show that the use of biogas fuel decreases NOx emissions with a increase HC emissions. Porpatham et al. [14] investigated the effect of increased oxygen concentration of intake air on engine performance, emissions and combustion in a biogas fuelled spark ignition engine. At higher oxygen levels, an observable improvement was seen in brake thermal efficiency and brake power. With higher oxygen concentration, NOx emissions increases, but HC and CO2 emissions decreases. There is also an improvement in thermal efficiency as a result of enchancement in combustion. Yadav et al. [15] studied exhaust emissions characteristics of a biogas operated automotive SI engine experimentally. Experiments were performed with %100 biogas fuel via petrol. With the comparison of petrol, the results shows that the use of biogas makes reduction in NOx and CO emissions and causes negligible CO emissions. Considering the work done so far in the literature, it has been found that there is not enough work on the performance and emissions effects of using biogas with different CO2 contents directly on SI engines. In addition, the emission and performance impact of using biogas at different CO2 ratios in SI engines has not been investigated because the naturally purifiable biogas fuel refinement is costly and CO2 in the content of biogas has a high heat transfer capacity and low O2 availability. It is known that biogas usage in SI engines may have some negative impact on thermal efficiency and mean effective pressure. However, decreasing the toxic emissions, such as NOx, to the limits determined by the stringent emission regulations requires high cost after-treatment equipments. Because of being a good alternative to reduce the emissions, the research of biogas usage which has different CO2 ratios on SI engines is performed in this study.

TEST Methodology

Schematic diagram of test system is shown in Figure 1. A single cylinder SI engine is used in the experiments. Engine is loaded with an 8 kW AC dynamometer. Fuel system consists of a methane gas tank which is connected to intake and exhaust system of the test engine. Details of the test system is given in the next subsection.

Journal of Thermal Engineering, Research Article, Vol. 5, No. 6, Special Issue 10, pp. 131-140, December, 2019 Test Setup Experiments were performed on a single cylinder, 270 cc, four stroke, naturally aspirated, SI engine. Engine was operated first with only gasoline then with methane at different CO2 contents (10%, 20%, 30%, 40%). All the tests were carried out at constant speed (2800 rpm) and repeated three times. A miniature oval gear type fuel meter was used to measure gasoline consumption. New-flow, TLF series hotwire type mass flow meter was preferred to measure methane and CO2 gas consumption. Mass flow rate of intake air was measured by a mass flow meter. AVL Digas 4000 was used to carry out the emission measurements. Calculation method which is announced by VDMA exhaust emission legislation for diesel and gas engines was used for the calculation of specific emissions by using gasoline, CO 2 + CH4 and intake air mass flow rates [16]. Crank angle position and engine speed was obtained with use of a proximity sensor and an incremental type encoder. In order to measure in-cylinder pressure, Kistler 6118B in-cylinder pressure transducer, which can also be used as spark plug was fixed to the engine head instead of original spark plug of the SI engine. Then obtained in-cylinder pressure data were processed by Kistler Kibox combustion analyzer device. Engine and Dynamometer A 270 cc, single cylinder, four stroke, naturally aspirated, SI engine was used during the tests. Methane and CO2 gas tanks were installed to the intake port of the test engine. Test engine was loaded by a 8 kW AC dynamometer. Detailed information of test system can be seen in Table 1. Tests were performed in Yıldız Technical University, Internal Combustion Engines Laboratory. All of the measurement devices and test equipments in Internal Combustion Engines Laboratory of Yıldız Technical University are calibrated and certificated by the authorized companies in Istanbul. Table 1. Engine specs and dynamometer features Engine Specs Engine manufacturer Honda Number of cylinders 1 Bore × stroke [mm] 77 × 58 3 Cylinder volume [cm ] 270 Aspiration Natural Compression ratio

Data Processing

In order to obtain engine performance and combustion analysis, data which were collected form sensors were used. For the calculation, following equations were used: 2

Journal of Thermal Engineering, Research Article, Vol. 5, No. 6, Special Issue 10, pp. 131-140, December, 2019 where, 𝑚̇𝐵 is mass flow rate of biogas (kg/m3), 𝑣̇ 𝐵 is measured volume flow rate of biogas, 𝜌𝐵 is density of biogas (kg/m3). The equations [17] used to calculate the indicated engine power and indicated engine work are given as: 720

where 𝑊𝑖 is indicated work done per cycle (J), 𝑃𝑚𝑖 is mean indicated pressure (Pa), 𝑉𝐻 is cylinder displacement (m3), P is pressure which was measured from the cylinder (Pa), V is calculated instant cylinder volume (m3), 𝑁𝑖 is indicated engine power (kW). Indıcated engine torque can be calculated as given in equation (8) by the use of indicated power value [17]. 60

where n is the engine speed (rpm) and 𝑀𝑖 is indicated engine torque (Nm). In order to obtain engine performance characteristics, the calculated engine power value, measured pressure and flow rates of gasoline, methane and CO2 can be used. Equation (5) is used for the calculation of indicated thermal efficiency [17]. 𝑁

where, 𝜂𝑇𝑖 is indicated thermal efficiency, 𝑚̇𝑔 and 𝑚̇𝐶𝑂2 are mass flow rates of gasoline and CO2 respectively (kg/s) and 𝐿𝐻𝑉𝑔 and 𝐿𝐻𝑉𝐶𝑂2 are lower heating values of gasoline and CO2 respectively (kJ/kg). For the calculation of emission values, which were also measured by volume and mass flow rate of exhaust gases during tests, measured flow rates of intake air, methane, CO2 and gasoline can be used as seen equation (6) [15]. Thereby, by using these values and indicated engine power, specific emissions can be calculated as given in equation (7) [15].

where 𝑚̇𝑒𝑥 and 𝑚̇𝑖𝑎 is mass flow rates of exhaust gases and intake air, respectively (g/h), 𝐸𝑃𝑖 is specific emission value of relevant gases (g/kWh), 𝐸𝑉𝑖 is relevant gases and total exhaust gases ratio by volume for CO and by ppm for THC and NOx emissions, 𝑀𝑖 and 𝑀𝑒𝑥 are molar mass of relevant gases and exhaust gases respectively (kg/kmol) and 𝑁𝑖 is indicated engine power (kW). The total propagated uncertainty of the Precision and Systematic (Bias) errors measurement determined by Kline and McClintock analysis [18] is:

And the computation of the total uncertainty 𝑊𝑅 is done with:

Journal of Thermal Engineering, Research Article, Vol. 5, No. 6, Special Issue 10, pp. 131-140, December, 2019 2 2 𝑊𝑅 = √(𝑊𝑅 )𝑃 + (𝑊𝑅 )𝐵

where (𝑊𝑅 )𝑃,𝐵 denotes the propagated uncertainty for either precision error (𝑊𝑅 )𝑃 , or systematic (Bias) error (𝑊𝑅 )𝐵 functions. x1, x2,…,xn are the measured variables and w1, w2,…wn are the corresponding uncertainties of the variables. The measurement uncertainty values of test setup were given in Table 2. Table 2. Accuracy values of test devices and obtained total uncertainty values Parameter

Results And Discussion

ITE The efficiency of useful energy conversion from the fuel consumed by the engine is defined by indicated thermal efficiency. In other words, the engine specific fuel consumption is directly affected by ITE [17]. The variation in indicated thermal efficiency due to biogas usage at different CO2 ratios is given in Figure 2.

Figure 2. Effect of different level of CO2 and CH4 addition on indicated thermal efficiency As comparison with the results when the engine is operated with only gasoline, ITE value is reduced at all CO2 ratios with biogas usage as fuel. ITE value decreased 1.8% at 10% CO2 ratio, 2.4% at 20 CO2 ratio, 2.8% at 30% CO2 ratio and decreased 3.9% at 40% CO2 ratio.

Journal of Thermal Engineering, Research Article, Vol. 5, No. 6, Special Issue 10, pp. 131-140, December, 2019 The heat carrying capacity of CO2 in the biogas causes reduction of in-cylinder combustion pressure values [20]. This situation explains the reduction of ITE values with biogas usage which contains CO2. IMEP Through the cycle, in-cylinder pressure of an engine changes continuously. In order to estimate an average value of in-cylinder pressure variation, mean effective pressure parameter is defined [19]. Because of independency of engine speed and size, using mean effective pressure is a good way to compare engines in terms of design and output [19]. Indicated mean effective pressure (IMEP) is obtained by using indicated work to define mean effective pressure value of the engine [17]. In this study, indicated mean effective pressure (IMEP) parameter is used to compare engine performance of gasoline and biogas blends. Figure 3 gives the test results about the variation of IMEP with the usage of only gasoline as fuel and with only biogas at four different CO2 ratios (%10, %20, %30 and %40) at constant engine speed (2800 rpm). In comparison with the results when the engine is operated with only gasoline, IMEP value is reduced at all CO 2 ratios with biogas usage as fuel. IMEP value decreased %8.9 at % 10 CO2 ratio, %9.4 at 20 CO2 ratio, %10.5 at %30 CO2 ratio and decreased %12.2 at %40 CO2 ratio.

Figure 3. Effect of using CH4 at different CO2 levels on indicated mean effective pressure As seen in Figure 3, with use of biogas and increasing CO2 content, IMEP values decreases. The main reason of this result is the decrease in maximum in-cylinder pressure because of operating the engine at near stoichiometric conditons. Also, because of using gaseous fuel, low density of methane, methane’s rendering high cylinder volume and dilution of combustion chamber by CO2 (EGR effect) are the other reasons of the decrease in IMEP values. CO Carbon monoxide is a colorless, odorless, poisonous gas, which is mostly generated when engine is operated with rich fuel ratio [19]. When there is not enough O2 to convert all carbon to CO2, some fuel does not get burned and some carbon ends up as CO [19]. Figure 4 gives the test results about the variation of CO emission with the usage of only gasoline as fuel and with only biogas at four different CO2 ratios (10%, 20%, 30% and 40%) at constant engine speed (2800 rpm).

Journal of Thermal Engineering, Research Article, Vol. 5, No. 6, Special Issue 10, pp. 131-140, December, 2019

Figure 4. Effect of different level of CO2 and CH4 addition on specific CO emissions As the comparison with only gasoline usage as fuel, CO emissions are low with only biogas usage at all CO2 ratios. CO emission reduces about 22,5% at 10% CO2 ratio, reduces about 20,8% at 20% CO2 ratio, reduces about 20% at 30% CO2 ratio and reduces about 19,5% at 40% CO2 ratio from CO emission released by engine operated with only gasoline. Methane composes the main structure of biogas. Methane has low C/H ratio and low carbon concentration in its composition. Because of its low C structure, methane increases the oxygenation of the fuel [21]. So, this situation explains the reduction in CO emission with use of biogas as fuel. Methane has low C content, so it provides low CO emissions. As CO2 content of mixture increases, CO emissions slightly increases. However, methane is a reactive gaseous fuel, and with the use of methane, a nearly homogenous mixture occurs in combustion chamber. So, the changes in CO emissions at all CO contents are nearly negligible. THC As one of the reasons of lung and blood diseases, hydrocarbons are another harmful emission [22]. So, for human health, it is essential decrease in THC emission. THC is the definition of the hydrocarbon emissions which are emitted from tailpipe. It contains both partially burned and unburned hyrocarbons. So, CH4 emissions are counted in total hyrocarbons and more accurate results are obtained. The measured THC emission values with only gasoline fuel and with biogas at four different ratios (10%, 20%, 30% and 40%) at constant engine speed (2800 rpm) are given in Figure 5. As compared with only gasoline usage as fuel, hydrocarbon emissions were increased considerably with the use of biogas at all CO2 ratios. THC emissions increased at about 2.4% at 10% CO2 ratio, increased about 6.6% at 20% CO2 ratio, increased about 8.7% at 30% CO2 ratio and increased about 11.2% at 40% CO2 ratio in comparison with gasoline.

Journal of Thermal Engineering, Research Article, Vol. 5, No. 6, Special Issue 10, pp. 131-140, December, 2019

Figure 5. Effect of different level of CO2 and CH4 addition on specific THC emissions Biogas contains high concentration of methane. Because of the high quenching distances and zones of methane and its volatile HC structure, THC emissions increase with biogas usage as fuel [23]. For optimal usage of biogas, crevice zones, electrode gaps, valve seats, piston ring gaps and sealing gaps in the engine should be minimized. The unsuitable design of the SI engine for biogas fuel is another reason of the increase in THC emissions. NOx Oxides of nitrogen, which is emitted by an engine, can reach to 2000 ppm value. Nitrogen in the air is the main source of NOx. Released NOx is one of the major reasons of photochemical smog [19]. Nowadays, emission regulations become more stringent, especially for NOx emissions. Because of stringent emission regulations, SI engines are being named as positive ignition engine as they are compared with diesel engines in terms of released amount of NOx emissions. Methane has additive effect on NOx emissions. In this study, because of high methane percentage of fuel, NOx emissions are higher than that of pure gasoline at 10% and 20% contents.

Figure 6. Effect of different level of CO2 and CH4 addition on specific NOx emissions NOx emissions are shown in Figure 6 with biogas at different CO2 ratios (10%, 20%, 30% and 40%) and with only gasoline at constant engine speed (2800 rpm).

Journal of Thermal Engineering, Research Article, Vol. 5, No. 6, Special Issue 10, pp. 131-140, December, 2019 When compared to only gasoline usage, NOx emissions raise about %4.8 with biogas at %10 CO2 ratio, and raises about 2.4% at 20% CO2 ratio. Then, NOx emissions decreases at about 2.4% with biogas at 30% CO2 ratio and decreases at about 6% at 40% CO2 ratio. As seen in the results, with the usage of biogas, NOx emissions increase because of the increased local peak in-cylinder pressure, but when CO2 ratio in the biogas reaches to 30% and upper values, there can be seen a decrease in NOx emissions, The main reason of this decrease is the heating capacity of CO2 and its decreasing O2 availability in combustion process [24].

Conclusions

In this study, in a single cylinder, 270 cc, naturally aspirated, four stroke SI engine, at stoichiometric condition and constant engine speed (2800 rpm), effect of biogas fuel at different CO 2 ratios (10%, 20%, 30% and 40%) on engine performance and emissions was investigated with comparison of only gasoline usage. The main conclusions were summarized below: • With the increasing CO2 ratio, there has been a decrease in indicated mean effective pressure. The main reason of this situation is stoichiometric working conditions and the low heating value (by volume) of biogas. ITE value decrease with use of biogas and increasing ratio of CO2. • With use of biogas and increasing CO2 content, IMEP values decreases. The main reason of this result is the decrease in maximum in-cylinder pressure because of operating the engine at near stoichiometric condition. Also, because of using gaseous fuel, low density of methane, methane’s rendering high cylinder volume and dilution of combustion chamber by CO 2 (EGR effect) are the other reason’s of the decrease in IMEP values. • CO emissions decrease with biogas usage. The low C/H ratio of methane, which is main component of biogas, is the main cause of the decrease in CO. THC emissions increases as the CO content in biogas increases. The unsuitable design of the engine for methane fuel, the crevice zones in combustion chamber and the structure of methane which is already an unburned HC are the other reasons of increase in THC emission. • Because of the increase in local peak in-cylinder pressure values with biogas usage, NOx emissions increase also. However, when CO2 ratio in biogas reach 30% and upper values, because of CO2’s heating capacity and decreasing O2 availability, NOx emission values are under the values which are taken with only gasoline usage. Although it has some negative impact on engine performance, biogas usage in SI engines can simulate EGR effect, so it can be possible to control the NO x emissions.

Nomenclature

CA Crank angle CH4 Methane CO Carbon monoxide CO2 Carbon dioxide EGR Exhaust gas recirculation HC Hydrocarbons IMEP Indicated Mean Effective Pressure ITE Indicated Thermal Efficiency NOx Oxides of nitrogen PM Particulate matter SI Spark ignition THC Total unburned hydrocarbons

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