Chapter: 8. Conclusions and Scope for Future work

Transcription

1 Chapter: 8 Conclusions and Scope for Future work 8.1. Conclusions Analytical Investigations The following conclusions can be drawn from the analytical part of present work investigation: Combustion model has been prepared for heat of reaction and stoichiometry. Fuel mole fractions at stoichiometric conditions for engine operation from lean to rich limit has been found analytically. Flammability limits of Gasoline, LPG and CNG fuels have been studied. Validation of the analytical model has been achieved within the domain of the present experimental investigation. For stoichiometric condition, heat of combustion calculated from reaction of Gasoline, LPG and CNG is , and 50.2 MJ/kg respectively. CNG having highest calorific value due to more percentage of methane. Study and analysis of CNG/LPG conversion kit has been done to investigate working of major components for supplying gaseous fuel. It is found that volume contained by second stage low pressure regulator is slightly higher than the volume of cylinder of engine. Mass flow rate and fluid velocity in all stages of LPR have been calculated. Also construction and working of low pressure regulator has been verified analytically. The analytical investigation shows that, the DNS model explains the effects of partial fuel oxidation and reduced oxidation rate. It can be successfully incorporated into a simple one-step Arrhenius model by consideration of variable values of the heat of reaction and activation temperature Experimental Investigations This research work was focused on studying the performance and emission characteristics of the SI engine designed specifically for Gasoline fuel retrofitted for gaseous fuels to compare the performance of engine by changing the spark timing. In this

2 257 study, tests were conducted on 800 cc, three cylinder, four stroke, water cooled, MPFI engine. An experimental set-up is designed and fabricated to generate data for CNG and LPG fuels. The results of this study are based on the data obtained from a research engine that was operated over a range of no load to full load and with spark timing from 20º BTDC to 30º BTDC with Gasoline, LPG and CNG as the fuels. Three fuels have been analysed using Maruti 800 engine under various spark timings. Case- I: Gasoline used as a fuel and various SA: 20º, 25º and 30º BTDC. Case-II: LPG used as a fuel and various SA: 20º, 25º and 30º BTDC. Case-III: CNG used as a fuel and various SA: 20º, 25º and 30º BTDC. Case-I investigates the effect of various spark timings, when Gasoline is used as a fuel and role of MPFI system is studied. Case-II and Case-III studies the effect of various spark timing, when engine is retrofitted with LPG and CNG fuels respectively along with necessary fuel measuring device and other equipments. A comparison of case I, II and III is made to investigate the role of spark timing for all three fuels and to analyse pollutants in each case. The overall arrangement has been kept same in case II and III, except the fuel cylinder and HPR arrangement. The following conclusions are drawn from the experimental investigation: Existing Gasoline engine is successfully converted into dual fuel Gasoline-CNG and Gasoline-LPG engine by installing the CNG and LPG kit. It is concluded that engine operating with CNG fuel at original spark timing of 20º BTDC and at 1500 rpm and 2000 rpm, thermal efficiency obtained is 26.9% and 14.2% respectively. Because at lower speeds, due to slow burning characteristics of CNG, it will get more time to supply its total energy with proper combustion, so thermal efficiency is higher. Combustion is almost complete at lower speed and increase in break power is more than increase in fuel consumption.

3 258 Gasoline fuel operation of the engine gives best performance for 20º BTDC when running at the speed of 2000 rpm. The best performance occurs within a short range of equivalence ratio. i.e. from φ = 3.09 to For Gasoline at higher load, more advanced injection of 30º BTDC is not preferable since more turbulence is created by rapid circulation of mixture in the combustion chamber, results in early fuel ignition, more fuel remains unburned. Due to abnormal combustion knocking occurs which is harmful to the engine. As the load increases, fuel consumption increases. The reason for this is, at low load, fuel-air mixture is lean than its flammability limits and consume less fuel. Because of lower flammability limit of CNG, flame cannot propagate throughout the combustion chamber, so mixture at other part of combustion chamber remains unburned and goes into the exhaust and resulting into lower η bth and higher unburnt HC. At high load, fuel consumption increases because fuel-air mixture crosses its lower flammability limits. Flame accelerates and propagates in the combustion chamber, thus proper combustion of natural gas occurs resulting in increase in break power is more than increase in fuel consumption resulting in higher thermal efficiency at higher load. Moreover, at high load, due to higher temperature, combustion of natural gas is better due to high auto ignition temperature. But this trend is reversed at higher spark timing, at 25º and 30º BTDC compared to original spark timing of 20º BTDC. Since the efficiency is affected by more natural gas substitution. More over, CNG burns slowly and it can not supply its total energy at higher load. Also at high load, natural gas cannot give rich mixture as Gasoline. That is why though spark timing is advanced, efficiency is decreases compared to original spark timing. For CNG fuel, best performance is observed at 20º BTDC compared to Gasoline and LPG within an optimum range of equivalence ratio of φ = 0.73 to This may be due to higher calorific value of CNG compared to Gasoline and LPG. The bsfc is higher at spark timing of 20º, 25º and 30º BTDC for LPG fuelled engine compared to Gasoline and CNG. It is observed that gaseous pollutants are higher for LPG compared to CNG and Gasoline. This is because of incomplete combustion in case of LPG as compared to CNG and Gasoline.

4 259 The engine is observed insensitive to spark timing 25º and 30º BTDC in case of LPG fuelled engine. Here no definite influence of spark advance has been observed. This is due to poor flame establishment and due to higher spark advance, this leads to incomplete combustion. By comparing the mechanical efficiency of various spark timings, it is observed 75%, 69% and 60% for Gasoline, CNG and LPG fuels respectively. It is higher for Gasoline as compared to CNG and LPG for all loads because with CNG and LPG fuels, due to the sudden pressure rise in the cylinder, the heat loss and the friction losses are more compared to Gasoline, resulting in lower mechanical efficiency. The power loss is related to the intake manifold air density. The heat of vaporization of Gasoline helps to decrease the temperature of mixture, producing the dense mixtures. Although propane and methane have higher heat of vaporization value, they are already in gaseous state when inducted into the intake manifold and they do not provide the cooling effect. Development of liquid fuel injection systems for LPG engines should provide better A/F ratio control. Backfire is almost eliminated due to introducing less volume of explosive gases in the inlet system. Cooling effect of endothermic expansion of the liquid increases the resistance to pre-ignition and knock. This leads higher compression ratio which means higher power output. It is concluded that for original spark timing of 20º BTDC, Gasoline fuelled engine gives better performance in terms of thermal efficiency due to MPFI feature of engine as compared to CNG and LPG fuelled engine. The poor performance of CNG and LPG is observed due to its high auto ignition temperature compared to Gasoline. To meet maximum braking torque, engine has to achieve peak combustion pressure which is possible if its ideal burn duration is achieved with advance in ignition timing which improves engine torque. For spark timing of 25º and 30º BTDC, it is observed that CNG fuelled engine performed better than Gasoline and LPG fuelled engine in terms of output power and thermal efficiency. The ignition delay is reduced through advanced injection timing for CNG which can improve bsfc, however best CNG bsfc was still inferior

5 260 to Gasoline bsfc obtained at original spark timing. While for Gasoline and LPG fuels, due to their inherent characteristics, with retarded ignition timing, there is increase in exhaust temperature and also drop in torque at higher load as charge density is a function of gas pressure and gas temperature. Higher engine load lean down air to fuel ratio and combustion temperature increases. Hence they give comparatively poor performance. The equivalence ratio (φ) are within the limit of operating range of lean to rich limit for Gasoline, LPG and CNG for all spark advance. It can be concluded that at 20º BTDC Gasoline, CNG and LPG fuelled engine performed much better compared to the engine operated with 25º and 30º BTDC, when there are no timing advance processor mounted separately on the engine for CNG and LPG. Carbon monoxide being the product of incomplete combustion, therefore, it is totally dependant on the air fuel ratio. CNG forming homogeneous mixture with air easily due to its gaseous phase and having higher diffusivity at higher pressure. Hence, with CNG fuelled engine there is 63% and 66% reduction in CO emission as compared to Gasoline at 25º and 30º BTDC for same operating conditions. The main source of HC is due to the composition of fuel and combustion occurring due to uneven mixture formation. On analyzing the engine exhaust for gaseous pollutants, it is found that CNG fuelled engine exhaust contains HC in the range of 50 to 150 ppm, whereas Gasoline and LPG fuelled engine have about 70 to 650 ppm and 230 to 1175 ppm respectively. The step of adopting CNG will improve the air quality. It is concluded that CO and HC emissions were higher for LPG at 20º, 25º and 30º BTDC compared to Gasoline and CNG. While CNG fuelled engine have lowest value of CO and HC emissions. From the emission data, it can be concluded that CNG fuelled engine produce lesser amount of pollutants than Gasoline and LPG fuelled engine at all spark timings. It indicates that CNG is clean fuel compared to LPG and Gasoline.

6 261 The results of present study on the effect of η bth, bsfc, fuel consumption and airfuel ratio are in good correlation with the results of other investigators for Gasoline, LPG and CNG fuels. From cost analysis, it is concluded that as compared to Gasoline, saving in fuel cost by using CNG is about 60%, 66% and 60%, and by using LPG is about 50%, 42% and 58% at 20º, 25º and 30º BTDC respectively. Fixed cost analysis suggests that Gasoline engines are cheaper compared to other fuelled vehicle, while running cost analysis shows that it has highest value of 4.13 Rs./km and lowest running cost of 1.66 Rs./kg for CNG. From the case study taken from GSRTC, it is concluded that the CNG fuelled engine is more efficient to reduce CO, HC, NO x and particulates by 56%, 55%, 56% and 86% respectively compared to similar diesel engine. The performance of CNG bus is almost identical to that of a diesel bus. In the metropolitan city like Ahmedabad, the average of 11 km/hr is observed for both diesel and CNG bus. The acceleration of CNG bus is same or slightly better than the diesel bus. The maximum speed is also same as that of the diesel bus. CNG bus is preferable from the environment point of view. To conclude the performance of entire project, though within internationally reported results, it still requires further modifications. Moreover, it is highly recommended the use of LPG/CNG as a fuel for automobiles and for any low compression ratio engine provided the following parameters to be optimized for LPG/CNG: (a) Compression ratio, (b) Spark timing, (c) Inlet manifold design, and (d) Inlet manifold temperature. The effect of spark advance has been experimentally investigated for CNG and LPG fuels. Studies on design of gaseous fuel kits are limited to classification and function of various components only and no information available on design of kit components. Therefore, to gain further understanding of design variables, performance parameters and design related features, the present work experimentally and analytically investigates the role of CNG and LPG as alternative fuels for a cleaner world.

7 262 From above results, it is concluded that the performance characteristics are comparatively good for CNG and LPG fuels. They are eco-friendly and environmentally preferred fuel as the cost of CNG and LPG is less compared to Gasoline and pollutant emissions are also less Scope for future work As CNG operated engine is running hotter by 100ºC compared to Gasoline operated engine, material of cylinder head, valve and valve seat insert must be changes for CNG compatibility. Cylinder head material should be upgraded with silicon copper high alloy material to retain hardness at elevated temperatures. Valve material should be changed with mono metal with bi-metal valve with satellite coating on valve head in order to avoid premature wear and tear due to dryness of CNG fuel. Cobalt base alloy should be used in valve seat inserts which remain intact even at elevated temperatures. Moreover, there is also a potential for improvement in design of mixture preparation system for CNG and LPG, which are still in the development stage. The development of gas flow controlled by micro-processor, which will regulate the gas flow in intake manifold vaccum, may produce a further reduction in emission levels and improve engine performance. The gas flow rate needs to be adjusted according to the engine operating conditions and load to attain the required air-fuel ratio. It should be noted that the gas flow practically by-passes the carburetor details or the sophisticated electronic fuel injection system, but being gaseous has the inherent advantage of far better miscibility with incoming air. Even an optimized passive flow control device will be altering gas flow close to the required level as the intake vacuum changes, under some operating conditions. For better performance, active control device like electronically controlled gas injection system may be used. Engine parameters like fuel injection rate, exhaust gas recirculation rate, spark advance, etc. are controlled by electronic engine management system to keep emissions at low level and also to maintain an acceptable engine performance. Lean burn is an effective way to improve fuel efficiency and reduce emissions. Lean burn limits are dependent on combustion chamber geometry, ignition

8 263 timings, ignition energy and turbulence. To enhance the power density of natural gas engines, turbo charging technology should be used. To meet stringent emission regulations, lean burn engines need a rather complex deno x system like selective catalytic reduction method. Stoichiometric natural gas engines equipped with three ways catalyst can meet future stringent emission regulations, but it needs precise air-fuel ratio control strategies and highly efficient catalyst to oxidize methane. To get better fuel economy at pure stoichiometric SI operation conditions, the addition of EGR to a stoichiometric mixture is one option. Brake mean effective pressures of natural gas engines are limited by knocking and thermal loading. EGR can improve knock limit by reducing combustion temperature. Modifications required on the engine design itself should be studied for getting optimum performance from a NG fuelled engine. Experiments should be conducted to test the longevity of engine and other parts like cylinder, regulator etc., when running with CNG as the fuel. As NG is a slow-burning fuel, performance and emission characteristics of the engine should be studied with preheated inlet air to determine the optimum inlet air temperature for the NG fuelled engine.