HEAT ENGINES AND FLUID MACHINERY

ASSIGNMENT PRESENTATION-I

Prediction by mathematical modeling of the behavior of an internal

combustion engine to be fed with gas from biomass, in comparison to

the same engine fueled with gasoline or methane

GROUP MEMBERS:

M.MEENAMATHI KARTHIGA(11E131)

R.MEENUSREE(11E132)

M.MOHAMED MEHATAB(11E133)

M.MOHANKUMAR(11E134)

K.MOHAN KUMAR(11E135)

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R.MONICA(11E136)

ABSTRACT:

The performance of a spark ignition internal combustion engine (SI ICE) fuelled with biomass gas (woodgas) is evaluated using an analytical mathematical model. For the evaluation, the model was based on the fuel-air thermodynamic cycle for spark ignition engines . The model can predict of the internal temperatures profiles, heat flow, as well as the work and pressure in relation to crank angle. It was used also to evaluate the influence of the rotation speed, the air ratio and the ignition timing on the engine indicated power. It was found that when feeding the engine with woodgas, a power output between 59 and 65% can be obtained, in comparison it’s powered by gasoline.

INTRODUCTION:

The combustion of synthesis gas is an important issue in advanced power systems based on the gasification of fuel feedstocks and combined cycles Several works has evaluated the possibilities to use syngas* as fuel in gas turbine engines but another possibility is to burn the syngas in stationary reciprocating engines. Whether in spark ignited or compression ignited engines (CI ICE), syngas could serve to power large bore stationary engines, such as those presently operated with natural gas. From the technical, economical and ecological point of view Gasifier/ICE is an attractive technology in comparison with other combustion technologies. The use of biomass gasification gas as fuel for internal combustion engines is a technology that has been used for over a century. Since the efficiency of internal combustion engine depends on various parameters, including the quality of supplied fuel, the geometryand some other engine operating conditions, it is desirable to use mathematical models to evaluate the behavior of an internal combustion engine fueled specifically with woodgas, especially because this gas has a complex composition as a mixture of many components, whose proportions are different, depending on several variables of the gasification process. Internal combustion engine (ICE) modeling is a multidisciplinary task encompassing various areas, including thermodynamics, chemical kinetics, fluids mechanics, heat transference, combustion, and also numerical methods. Despite the complexity, and mainly because engines are traditional machines, there is possible to find in the literature many models of combustion of conventional fuels (e.g., natural gas, propane, gasoline and, diesel) in SI ICEs However there have been only a few published models that explain the simulation of the synthesis gas combustion in an SI ICE This is due to the fact that the synthesis gas is an unusual multicomponent gas fuel (hydrogen, carbon monoxide, and methane), of limited commercial value and variable chemical compositions, because different types of biomass feedstockes and gasifiers could be used to produce the syngas To describe the overall operation of the biomass power generation system a mathematical model of an SI ICE is used, which is based on the fueleair thermodynamic cycle. Such cycle takes into account the composition of syngas as fuel, the heat losses in the cycle due to heat transfer to the walls of the engine cylinders, the dissociation processes which occur during combustion of fuel and

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also the “blow-by” (the leakage of gases between piston sealing rings and the cylinder wall). Additionally, the engine’s model accounts for the influence of residual gases in the cylinder at the beginning of the compression stroke and for variations in thermophysical properties of the fueleair and residual gases mixture and of combustion products.The main contribution of this work is that the model presented is able to provide more detailed information on the behavior of an engine fueled with woodgas. The temperature profile, heat flow,work and pressure are predicted and analyzed; the mathematical model is also able to determine the optimal ignition timing. Among other results, it is found that the power output by woodgas is about 59e65% of that of gasoline. About the mathematical model used An analytical mathematical modelwas used, whose details were presented in a previously published work by the authors [25,26] and based on the original model presented by Ferguson [27]; some changes were made to allow the model to accept biomass gasification gas as fuel. The modification is to introduce mathematical functions to calculate the thermodynamic roperties of the gas (specific heat capacity, enthalpy and entropy) as a function of temperature and the distribution of species (CO, CO2, H2, CH4 and N2). The species present in the biomass gasification gas are usually not fixed and vary according to operating conditions of he gasifier, then the model should be able to predict the thermodynamic properties of woodgas for each temperature within the range of engine operation.

EXPERIMENTAL:

The YANMAR BTD22 engine shown in Fig. was used; originally designed to work with Diesel, but modified to work with natural gas. The engine modifications include the installation of spark plugs in the head of cylinders (one per cylinder) and the implementationof a set of double regulating valves in the engine’s syngas and air induction system. The double valve system provides a finer adjustment of syngas and air mass flow rates. The engine has the cylinder bore and piston stroke equal to 90 mm with the Fig. 1. compression ratio being 12:1. The ignition timing in the cylinders can be regulated. The SI ICE was fueled with

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eucalyptus woodgas, produced in a downdraft gasifier and tested under variable load-constant speed conditions. In the tests the mass flow rates of syngas and air were gradually increased and the engine’s speed was maintained at 1800 rpm, while increasing the engine’s load. The electrical power produced by the engine’s generator varied from 1.45 to about5 kWe. The original engine to operate with LPG produces up to 10 kWe therefore the engine’s power de-rating, when fueled with woodgas, was of about 50%.Validation of mathematical model As mentioned above, Ferguson and Kirkpatrick [27] made the respective validations when working with conventional fuels. This work was limited to compare model simulations with test results obtained by Martínez [28], Martínez et al. [29], Martínez et al. [30],when the engine is fueled with woodgas. In Table 2 the data generated by the model and the corresponding experimental data for five different engine load conditions are showed; also Fig. 2shows a bar graph comparing the electrical power measured by Martínez [28] against the electrical power predicted by the model, in this same way, Fig. 2 shows the indicated power predicted by the model. The refered to the experimental measurements, the modelpredictions have an average error of 11.53%. Some data were not reported in experimental tests, if they are set in the model, could help improve the simulation results. Some of these data are engine wall temperature, thermal conductivity and piston engine wear. In general, the engine’s model provides an acceptable accuracy in predicting the engine’s performance and can be used as a theoretical tool for the analysis of the operation of thepower system."

RESULTS AND DISCUSSION:

The engine model was used in the prediction of the behaviour of the internal combustion engine running on woodgas. For the simulations it was used the YANMAR BTD22 engine geometry, which was the same engine used in the experiments. The results are shown below.

Evolution of gas temperature and heat flux in the motor, in relation to crank angle Fig. 3 present the evolution of temperature and Fig. 4 the heat flux (from inside the engine to the outside) in relation to crank angle, it’s possible to appreciate that both figures are very similar, due to the fact that the heat flux is proportional to the temperature inside

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the engine. It is possible to verify that the temperature of the unburned gases for all gasoline, methane and woodgas is practically the same; while for combustion gases are different, being shorter for woodgas supply compared to methane and gasolinepowered. Another important points that in the range between 20_ before engine’s top dead center (TDC) and 50_ after TDC, there were reported temperature data for both burned gases and unburned gases, this is due to the fact that this is the length of burning time and during this time there exist both types of gas mixtures inside the engine. Fig. 5 shows the heat flow from inside the motor to the surrounding, related to the crank angle. It could be noted that the heat transferred during the compression stage (from _180_ to _20_) is practically the same in the cases when the engine is fueled with woodgas, gasoline and methane, but it should be noted that the values of heat transfer in the compression stage (_180_ to_20_) are slightly negative, because the fresh fuel mixture being compressed has a temperature lower than the one in the engine walls and therefore, in this interval, the heat flows from outside to inside of the motor. From_20_ (20_ before TDC) on the heat loss in the ICM fuelled with gasoline or methane is greater than in the case of biomass.

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Pressure evolution inside and engine work in relation to crank angle

Fig. 6 shows that the pressure inside the engine in all cases (when fueled with gasoline, methane or biomass gas) are practically equal, until the crank reaches 20_ before TDC; thereafter the pressure inside the engine fueled with biomass gas is lower than with gasoline. This is the expected behavior as the calorific value of biomass gas is lower than of gasoline. As in Fig. 6, Fig. 7 shows that working with methane or gasoline the engine has virtually the same behavior as when fueled with biomass gas, up to 20_ before TDC, from this angle the pressure in the ICE fueled with gasoline is greater than in the case when fuelled with biomass.

Indicated power in relation to the speed of rotation of the combustion engine Fig. 8 shows that in all cases studied (gasoline, methane and biomass gas), the ratio of indicated power and the engine rotation speed is linear and proportional. When varied the engine speed within the simulated range, the output power of the engine running with gas from biomass is about 65% of the one operating with gasoline under the same conditions, these results are in agreement with the observations of Tinaut et al. [24], which states that when operating with gas from biomass it could be obtained 2/3 of the power output than with gasoline. On the other hand, when operating with pure methane, the ratio is about 90% of the one with gasoline. Note that, theoretically, all the lines representing the relationship to power on gasoline, the line representing relationship for methane and the line representing the relationship for biomass gas supply should be straight lines through the

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origin of the coordinate system and therefore the relationship between these should be a constant (a horizontal line). In Fig. 8 it can be verified at high speed, but Fig. 8 also shows that at low speed it moves away from this performance.

Relation of indicated power with the amount of air fed to the engine Fig. 9 shows the variation of the power output in accordance with the amount of air fed to the motor. This figure illustrates the expected behavior. That is that the power increases up to the stoichiometric air ratio and starting from this point begins to decrease with the increase in the amount of air. It is important to note that near the stoichiometric air ratio, the power output of the engine running on woodgas is around 60% of the power outputwhen operating with gasoline. On the other hand, the power output of the engine running with methane is around 90% of the power output when operating with gasoline.

Indicated power in relation the spark timing Fig. 10 shows that for angles greater than 25_ before TDC start of combustion does not get a significant increase in the power. This is true when woodgas is used as fuel, because, when feeding methane or gasoline with ignition timing close to 30_, it could be obtained a slightly higher power. When feeding the engine with woodgas only 65% of the indicated power is obtained in comparison when feeding with gasoline. When using methane as fuel 90% of the power is obtained in comparison with.

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Indicated power in relation with the engine compression ratio As shown in Fig. 11 the test of the engine fueled with gasoline was simulated only up to 12:1 compression ratiowas simulated due to the fact that for higher values problems of uncontrolled selfignition are found. For biomass gas the engine was simulated to a compression ratio of 18:1 because usually this fuel can be fed to a gasoline engine (compression ratio of about 12:1) or a modified diesel engine (with a compression ratio ranging from about 12:1 to 18:1). It is noted that the power increases with the compressionratio and therefore, when a higher power output is required from an engine fueled with biomass gas it is recommended to use an engine with high compression ratio, for example, a diesel engine modified for spark ignition.

CONCLUSIONS: When operating an internal combustion engine with biomass gas the power output is 59 and 65% of the one obtained when operating with gasoline. There is an optimum ignition timing that depends on the type of engine and of the fuel fed, up to 24_ before TDC; this angle can change depending on the engine and the fuel; the mathematical model presented could be used as a theoretical tool to determine the optimal ignition timing. For more powerswhen biomass gas is supplied, it’s recommended to use higher engine compression ratio. For example, as the case of a diesel engine modified to spark ignition.

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