| The application of green alternative fuels to replace the traditional fossil fuels is an important developing trend for internal combustion engines.The alcohols could be regarded as one of the most promising alternative fuels for internal combustion engines,since it could be produced by kinds of ways.The alcohol contains oxygen atom in its molecule.This could be helpful for improving the combustion and emissions characteristics of the traditional internal combustion engines.At present,the application of ethanol as the fuel has been widely accepted worldwide,particularly in countries where the manufacturing cost of ethanol is relatively low and the production of cash crop of corn is relatively high,such as America and Brazil,etc.In these countries,the ethanol-blended fuels have become one of the major energy sources for vehicle application.In northeast of China,the ethanol-blended gasoline was also used to be applied.Since China is rich in coal resources,as one of the major product in coal industry,the methanol-blended gasoline was widely adopted in Shanxi province.The carbon fraction in butanol is relatively high.Compared with methanol and ethanol,the combustion and physicochemical characteristics of butanol is much closer to those of gasoline.Thereby,the butanol is also regarded as one of the most promising alternative fuels for internal combustion engines.However,currently,the alcohol fuel is usually blended with traditional fossil fuels for vehicle application.However,the alcohols also have several drawbacks that make it not suitable to be applied in all operating conditions of internal combustion engines,such as high latent heat and low lower heating values.Therefore,it is necessary to improve the fuel property by blending with green alternative fuel to further improve the performance of alcohol engines.The hydrogen has low ignition energy,wide flammability,high burning and diffusive velocities.These characteristics of hydrogen make it theoretically possible to improve the alcohol engine performance by adding proper amounts of hydrogen to the engine intake gas.The current work investigated the effects of hydrogen addition on the combustion and emissions characteristics of methanol,ethanol and butanol engines.Firstly,a quasi-dimensional model which could be adopted to simulate the performance of methanol engine under various operating conditions was constructed and validated through experimental data.The simulating results showed that the addition of hydrogen was effective on stimulating the turbulent flame propagating speed of methanol-air mixtures.At a hydrogen volume fraction in the intake of 3% and an equivalence ratio of 0.8,the turbulent flame propagating speed and indicated thermal efficiency were respectively increased by 18.7% and 18.0%,compared with the original methanol engine.After the modeling,a series of experimental investigations were conducted on the test bench to explore the combustion and emissions characteristics of the methanol,ethanol and butanol engines under different hydrogen volume fractions in the intake,excess air ratios and spark timings at the cold start,idling and part load conditions.The results from cold starting test of methanol engines showed that adding small amount of hydrogen could effectively improve the starting stability of methanol engines.Meanwhile,the hydrogen addition was also feasible to ensure the stably starting the methanol engine under relatively low ambient temperatures.The results indicated that the peak engine speed and cylinder pressure were both heightened through the hydrogen addition.When the hydrogen flow rate to the engine increased from 0 to 3.15 L/min,HC,CO and particulate emissions were respectively reduced by 68.7%,75.2% and 72.4% during the cold start process.Meanwhile,the addition of hydrogen could improve the methanol engine indicated mean effective pressure and fuel economy at the idling condition.Moreover,the test results demonstrated that,under part load conditions,the addition of hydrogen could effectively shorten the flame development and propagation periods,reduce the cyclic variation,extend lean burn limit and decrease HC and CO emissions.The adoption of lean combustion was effective on controlling the engine NOx emissions.Results from the hydrogen-blended ethanol engine testing showed that the addition of hydrogen was also able to improve the cold start stability of ethanol engine and reduce HC,CO and particulate emissions during the engine starting process.Both the engine fuel economy at idle and the lean burn limit of the ethanol engine under part load conditions could be improved through hydrogen addition.Furthermore,a controlling strategy which could realize the excess air ratio-controlled engine brake mean effective pressure under the wide open through conditions was proposed.In this strategy,the hydrogen addition,lean combustion and cylinder cutoff was coordinately controlled to ensure the engine stability.The test results showed that the application of this strategy could effectively reduce the engine pumping loss under middle and small loads conditions.For an engine speed of 1400 r/min,by using this strategy,the engine brake thermal efficiency could be averagely improved by over 10.0% under various tested load conditions.The butanol has a relatively high C/H ratio.Thus,the butanol engine could expel larger amounts of HC emissions than the methanol and ethanol engines.The test results confirmed that the addition of hydrogen and the application of lean combustion could effectively reduce HC and CO emissions from the butanol engine under idle and part load conditions.Under the part load condition,for an excess air ratio of 1.4,CO and HC emissions could be respectively reduced by 27.6% and 81.8% through adding 3% hydrogen in the intake.Moreover,since the addition of hydrogen could stimulate the charge burning velocity,the dropped post combustion tendency and the decreased cooling loss were realized for the hydrogen-blended butanol engine.This makes the hydrogen addition is beneficial for improving thermal efficiency of the butanol engine.This paper verified the effects of hydrogen addition on improving the combustion process of methanol,ethanol and butanol engines through bench test and simulation.It was found from the test that the addition of hydrogen was more effective on improving the charge flame development process than the flame propagation process.At the part load conditions,under a hydrogen volume fraction in the intake of 3% and the stoichiometric condition,for the methanol,ethanol and butanol engines,flame development period was averagely reduced by 4.3°after the hydrogen addition.Comparatively,flame propagation period was only reduced by 2.0°under the same condition.The test results also indicated that,since shortening the combustion duration was helpful for improving the engine combustion stability,the addition of hydrogen was able to reduce the cyclic variation and extend the lean burn limit for the methanol,ethanol and butanol engines. |
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