ENERGY AND EXERGY STUDIES ON RECOVERY FROM WET-ETHANOL FUELLED HCCI ENGINE FOR PERFORMANCE ENHANCEMENT AND AIR CONDITIONING

Abstract

Blending of ethanol especially with gasoline has become quite common in developing countries like India but the barriers for its application as a primary fuel to internal combustion engines have not been overcome yet. Detailed study on ethanol production from corn shows a slightly energetic advantage and estimated the net energy gain of 20%. In order to obtain further net-energy gain and hence to lower the energetic cost of changing ethanol to utilizable form, development of new utilization techniques needed. Utilization of ethanol-water mixture (wet-ethanol) in combustion engines by eliminating the energy spent in removal of water may significantly increase the net energy gain, from 21% to 55%. HCCI engines can exploit wet-ethanol more effectively than the traditional engines and hence can be considered as a suitable mode of combustion for the direct utilization of wet-ethanol as a primary fuel. Renewable fueled HCCI engines have been analyzed using a traditional method of analysis based on first law of thermodynamics which cannot show how or where the irreversibilities in the system or process occur and hence it does not provide further insight into the overall thermodynamics of the phenomena. On the other hand, second law of thermodynamics allows the identification of various processes where most exergy destruction of working medium occurs. Reduction of those irreversibilities and exergy losses can lead to an effective exploitation of a renewable fuel in HCCI engines. Second law investigations carried out on HCCI engines shows that a majority of thermodynamic losses occurs during the HCCI combustion process and thermal energy exhaust to the ambient. It is calculated that a larger portion of the fuel energy is expelled as waste enthalpy in the hot exhaust gases, resulting in a substantial quantity of exergy being emitted from the system. It has been determined that recovering part of the thermal energy contained in exhaust gases is worthwhile in the development of more effective and environmentally friendly combustion engines. iv In this regard, present thesis provides a two-folded methodology to increase the work output and fuel conversion efficiency of HCCI engine fueled by wet-ethanol. First, a detailed exergy analysis of HCCI engine was developed and performed operating on an advanced combustion strategy with wet-ethanol. Second, a theoretically driven, computational exergy analysis methodology of exhaust flows to characterize the exhaust exergy was introduced and implemented on a HCCI engine. In this regard, an exergy analysis framework is developed to quantify fuel exergy transformations in HCCI engine combined with the waste heat driven cooling systems. Then, the study leading to variation of exergy components and the effect of operating conditions on exergy distribution and irreversibilities were conducted. A detailed investigation was made first, to recover the energy and exergy accompanied by engine exhaust gases to drive the novel thermodynamic cycles producing the cogeneration of power and cooling. Additional generation of cooling and power through engine exhaust heat is expected to provide the economic benefits and improves the engine overall efficiency without supplying the additional fuel. Obligatory standards concerning the weight of the empty vehicle disregard the use of absorption refrigeration cycles (ARC) as they are bulky, expensive and complex in design. To surmount this limitation, ERC (ejector refrigeration cycle) is found to be a promising option as it avoids the use of mechanical compressor and CFC’s. Moreover, ejectors are more compact and easier to maintain than compression and absorption cooling systems. In this context, a novel configuration consists of an organic Rankine cycle combined with the ejector is applied as the most potential means to recover the wet-ethanol fueled HCCI engine exhaust heat to produce cooling and power in an energy efficient, less expensive and eco-friendly manner. The developed combined system of cooling-power cogeneration was simulated by Engineering Equation Solver (EES) software. Combined system responses to altering the operative conditions on the energy and exergy performances are ascertained to obtain guidance for system design. The results are computed for R134a, R290, and R600a working fluids. Increase in turbocharger pressure ratio from 2.5 to 3.5 v raises the thermal efficiency of cooling-power cogeneration from 47.87% to 50.09% when R134a is used as working fluid. Cooling capacity and exergy of refrigeration are decreased by greater than 2.0% in case of R134a operated system when the vapor generator pressure is elevated from 1800 kPa to 2200 kPa. Increase in evaporator pressure of ERC from 327.4 kPa to 348.7 kPa is greatly beneficial to thermodynamic performance of cogeneration and its cooling capacity is improved by 11.34% when R134a is utilized as the working. When PEvap rises from 175.7 kPa to 186.9 kPa and R600a is employed as the working fluid, the cooling capacity is increased by 12.58%. In order to determine the comparative waste heat recovery potential of heat driven cooling systems an HCCI engine fueled by wet-ethanol was proposed to be bottomed with the ERC and ARC, separately, to recover the exhaust heat for refrigerating the thermal load of vehicle air conditioning. A comparative thermodynamic analysis of proposed cooling-power cogeneration was conducted by considering the quality and quantity of energy transfers during the energy conversion processes of the cycle. Energetic and exergetic performances of cogeneration cycle was assessed by altering the following parameters; turbocharger pressure ratio, turbocharger compression efficiency, and ambient temperature. In addition, the distribution of fuel energy supplied in terms of energy produced and energy lost as well as the breaking down of exergy of fuel supplied into the exergy produced, exergy destruction in the major components of cogeneration cycle, and the loss of exergy due to thermal exhaust to ambient is also computed, graphed, and discussed. The contribution of exergy destruction is scrutinized and debated in regard of cycle performance improvement. Results are derived for the employment of R134a as the refrigerant of ERC and LiBr-H2O mixture as the working fluid pair for the ARC. Further, the COP of both ERC and ARC were computed with the variation in the entrainment ratio and generator temperature, respectively. Results show that elevated pressure of turbocharger results in the enhancement of HCCI engine power and increase of the refrigeration of thermal load, simultaneously. However, ambient vi temperature rising shows the decline of HCCI engine efficiencies and energy efficiency of cogeneration while the cogeneration cycle exergy efficiency is found increasing. Furthermore, the results are reported for the refrigeration performed by LiBr-H2O operated ARC, and R134a and R290 operated ERC, respectively. Mapping of exergy destruction for the cogeneration cycle studied discovered HCCI engine, boiler of ERC, generator of ARC, and catalytic convertor as the components of significant exergy destruction. Entrainment ratio and type of refrigerant employed in ERC and the generator temperature of ARC shows a marginal impact on the COPs of these cycles. It was noticed that an increase in pressure ratio across the turbocharger from 2.5 to 3.5 raises the HCCI engine first law efficiency from 44.09% to 46.32%, and for cogeneration it is increased from 47% to 49.21% when R134a is used as ERC refrigerant and from 46.06% to 48.29% when R290 is the ERC refrigerant, and in case of ARC bottoming it is increased from 55.08% to 57.25%. Furthermore, a relatively new thermodynamic model was also developed in EES to compare the performance of HCCI engine operating on natural gas and wet-ethanol, and a new type of ejector technology was employed to recover the exhaust heat of a natural gas fueled HCCI engine to simultaneously refrigerate the thermal load of air conditioning and low temperature refrigeration. A shaft power driven two-phase ejector which consists of a hermetic reciprocating compressor, an air cooled condenser, a separator, and two evaporators for combined production of refrigeration and air conditioning and integrated to HCCI engine operated on natural gas engine was employed as it was found novel for current investigation. The development of such a cooling-power cogeneration has been found to possess three fold benefits; cooling produced by low temperature evaporator will refrigerate a thermal load for food and vaccine preservations and the cooling produced by high temperature evaporator will provide cabin cooling or air conditioning of vehicle, and decrease in exhaust temperature due to waste heat recovery will reduce the thermal pollution. Energetic and exergetic investigations were carried out to study the role of equivalence ratio, vii engine speed, condenser temperature, refrigeration evaporator temperature, air conditioning evaporator temperature, and ejector nozzle efficiency on the thermodynamic performance parameters of the combined cycle. The analysis of two-phase ejector cooling cycle using three working fluids including R717, R290, and R600a is conducted. Results reveal that the thermal efficiency of HCCI engine is increased from 47.44% to 49.94%, and for the R600a operated combined cycle it is increased from 60.05% to 63.26% when the equivalence ratio is promoted from 0.3 to 0.6. Distribution of fuel exergy results show that out of 100% exergy input, in case of R717 operated combined cycle, 139.79 kW (38.72%) is the total exergy output and 164.21 kW (45.49%) and 57 kW (15.79%), are the values for exergy destruction and exergy losses. It is further shown that change in refrigerant minorly influence the percentages of exergy distribution. Finally, an important existing knowledge gap was covered by introducing a methodology for performing a detailed exergy analysis of exhaust flows from the perspective of exhaust waste energy recovery in HCCI engine for cooling production. The physical exergy of a flowing stream and its thermal and mechanical components along with the chemical exergy of fuel and the subsequent mixture of gases are computed by combining the thermodynamic formulations developed in this study.