Anaylsis And Optimization Of Combustion Process Of High Dilution Premixed Natural Gas Engines

In recent years, restriction of emission regulations, the environmental awareness, and the increasing price of conventional fuels led to a promotion in using alternative fuels. Among the alternative fuels, natural gas(NG) is very promising and highly attractive for its clean nature of combustion, high octane number, abundant resources, and low encouraging prices. Currently the main application forms of natural gas in internal combustion engines are SI(spark ignition) natural gas engines and diesel PI(pilot ignition) natural gas engines. In order to use TWC(Three Way Catalysts) to reduce emissions of THC, CO and NOx simultaneously, SI natural gas engines need to run stoichiometric operations. However, stoichiometric operations are faced with higher thermal loads, larger pumping losses and heat losses, resulting in lower thermal efficiencies. To improve this situation, scholars around the world began to focus on natural gas engines fueled with dilution mixtures since the 1990 s. This is because the dilution mixtures have higher heat capacities, resulting in reduced engine heat loads. In addition, using dilution mixtures can reduce the pumping loss and heat loss of the engine, which can improve the engine thermal efficiency. Dilution mixtures can be obtained by adding excess air into the intake charge(i.e. air dilution), using EGR(Exhaust gas recirculation)(i.e. EGR dilution), or combining air dilution and EGR dilution(i.e. EGR-air double dilution). High dilution mixtures were used to keep NOx emissions at low levels. However, the engine using high dilution mixtures always was faced with serious combustion deteriorations, high combustion cyclic variations and even misfires, losing the advantage of relatively high thermal efficiencies. This indicates that there is a “trade-off” relationship between the engine fuel economy and NOx emissions, which limits the application of high dilution SI NG engines in engineering. Therefore, how to use high dilution mixtures to improve both of the engine fuel economy and NOx emissions has become one of the key researches in the field of SI natural gas engines. PI NG engines were also faced with the same problem. This is because PI NG engine evolved from Diesel cycle engines, and the dilution degree of mixtures was very high. PI NG engine has been faced with this problem from the date of its birth. The phenomenon in engineering related to this problem was that the thermal efficiency of PI NG engine at low loads was very low and THC emissions were very high due to the too high dilution mixtures.In view of the theories and engineering problems mentioned above, the combination of theoretical analysis and experimental study was adopted in this research, which was supported by the National Basic Research Program of China(973 Program), the National Natural Science Foundation of China, the Province Technology Development Project and University Graduate Innovation Fund. The experiments were arranged based on a High dilution-Premixed SI NG engine with and a High-dilution Premixed PI NG engine. The various effects of intake dilution(e.g. air dilution, EGR dilution) were investigated, experimentally and theoretically. The combustion processes of High dilution-Premixed SI and PI NG engines were analyzed and the combustion optimization control strategies improving both of the engine fuel economy and emissions were proposed in this research. The main research work and conclusions are summarized as follows:1. In order to analyze and quantify the contributions of the diluent and thermal effects of EGR dilution on NOx emissions, effects of Ar(Argon), N2(Nitrogen) and CO2(Carbon dioxide) on the in-cylinder O2(Oxygen) concentration, the mixture heat capacity and NOx emissions were studied. During the test, Ar, N2 and CO2 were separately added into the intake charge in the SI NG engine under 1450 rpm and 50% load, with λ(Excess air ratio) being kept at 1.40. The results show that:(a) NOx emissions decrease significantly with increasing DR regardless of the dilution gas species. NOx emissions decrease by 30%, 58% and 64% for Ar, N2 and CO2 respectively when DC(Dilution coefficient) changes from 0% to about 130%.(b) The mechanism of the dilution gas in controlling NOx emissions consists of two parts: the diluent effect and the thermal effect. The Cdiluent effect(Contribution of the diluent effect on reducing NOx emissions) is about 100%, 57% and 51% for Ar、N2 and CO2, respectively. The relevant Cthermal effect(Contribution of the thermal effect on reducing NOx emissions) is 0%, 43% and 49% for Ar、N2 and CO2, respectively. Generally, the thermal effect of the dilution gas(e.g. CO2) with a relatively high specific heat capacity will contribute a larger proportion than the diluent effect on reducing NOx emissions. But NOx emissions can still be reduced significantly by the diluent effect of the dilution gas(e.g. Ar) even if the heat capacity of the mixture is unchanged with increasing DC.2. In order to analyze and quantify the various effects of air dilution on the combustion and NOx emissions of NG engines, seven different kinds of effects were defined as well as their quantitative indicators, based on Arrhenius Law and Zeldovich Mechanism. Based on this, the effects of N2, O2 and Ar in excess air on combustion and NOx emissions of NG engines were investigated, experimentally and theoretically. During the test, excess air and N2 were separately added into the intake charge with fuel being unchanged in the SI NG engine at 1450 rpm. The results show that:(a) There are mainly seven different kinds of effects of excess air on combustion of NG-air mixture and NOx emissions: the diluent effect, the thermal effect, the chemical effect, the fuel compensation effect, the oxidation or reduction effect, the oxidative or reduction compensation effect, and the inertia effect.(b) The essential reason why NOx emissions change with increasing λ is that the oxidation effect and the fuel compensation effect of O2 change with the in-cylinder temperature, which is controlled by the thermal effect of N2, O2 and Ar. The contribution rate of N2, O2 and Ar on thermal effect is 82.6-86.1%, 13.3-16.7% and 0.6-0.7% respectively. The chemical effect of O2 is fully offset by the diluent effect of N2 and Ar. The contributions of N2 and Ar on the diluent effect are 98.31% and 1.69%, respectively.(c) The increase of NOx emissions caused by O2 in excess air increases first and then decreases. Form the view of the composition of air, the comparatively high level of N2 in air(N2 is about 3.3 times the mass fraction of O2 in air) is the main reason why NOx emissions decrease sharply with increasing excess air.3. In order to establish the mapping relationship between the dilution gas type and the engine performance, effects of atomic, diatomic and polyatomic inert dilution gases in EGR and air on combustion, thermal efficiency and NOx emissions of NG engines were investigated. Ar, N2 and CO2, present in exhaust gas recirculation(EGR) and air, are common atomic, diatomic and polyatomic inert gases, separately. During the test, Ar, N2 and CO2 were separately added into the intake charge to dilute the mixture in the SI and PI NG engines. The results show that:(a) The same dilution gas type has similar effects on the combustion process, thermal efficiency and NOx emissions. Adding CO2 into the intake charge has the biggest impact on the combustion process of the engine, followed by N2, and then Ar at the same DC. NOx emissions decrease significantly with increasing DC regardless of the dilution gas species. This trend is most obvious for CO2 dilution, followed by N2 and then Ar. The engine using Ar as the dilution gas produces the highest thermal efficiency, followed by N2, and then CO2 at the same DC.(b) The engine using Ar as the dilution gas produces the highest thermal efficiency, followed by N2, and then CO2 at the same levels of NOx emissions. This indicates that Ar is most suitable in the three types of dilution gases to improve the “trade-off” relationship between the engine fuel economic and NOx emissions.4. In order to discover the main cause of the low thermal efficiency and high THC emissions of High dilution-Premixed-PI NG engine, from the macro and micro perspectives the datum point was analyzed by heat balance analysis and CFD analysis(using Star-cd), respectively. The condition of 1335 rpm and 90% PES(without intake throttling and without EGR) was chosen as the datum point, at which the total energy of the fuel was equal to that of the original diesel engine at 1335 rpm and 25% load. The results show that:(a) The percentage of the energy converted into the effective work in the total energy is just 18.41% at the datum point. Among all the losses, the percentage of the incomplete combustion loss is the largest, followed by the mechanical loss, and then the exhaust loss and the cooling loss. The primary problem to be resolved in High dilution-Premixed-PI NG engines is reducing the incomplete combustion loss.(b) It can be concluded from CFD simulation that the main factor that causes incomplete combustion is burning interruptions in PI NG engines. The main factor that causes serious burning interruptions is: the stratified combustion of diesel that results in limited distribution of diesel ignition zone, leading to that only the NG in and around the ignition zone is ignited; the high dilution degree of NG-air mixture that unable to provide condition of fast flame propagations; the low temperature of the NG-air mixture leading to slow chemical reaction rates; the low turbulent kinetic energy in the combustion chamber and near the cylinder axis, which cannot guide NG-air mixtures to carry out turbulent flame propagations. Burning interrupt coefficient can effectively characterize the spatial distribution of the burning interruption of High dilution-Premixed-PI NG engines.5. In order to establish the mapping relationship between important combustion boundaries and the engine performance, effects of combustion boundaries in larger range on the combustion process, thermal efficiency and emissions of PI NG engines were comprehensively investigated using single variable methods. During the test the conditions of A25(low speed and low load) and C25(high speed and low load) were separately used as datum points. The results show that:(a) The total excess air ratio, the ignition-zone excess air ratio and the premixed excess air ratio can evaluate the air dilution degree of NG-air mixtures in the overall cylinder, the ignition-zone and the flame propagation-zone, separately. In this research, the premixed excess air ratio is about 2.0 under different combustion boundaries except excess air ratio and EGR rate.(b) Increasing the NG PES(Percentage energy substitution), advancing SOI(Start of diesel injection), increasing Pinj(Diesel injection pressure), intake throttling, increasing EGR rate, increasing Tin(Intake air temperature) and so on can improve the engine thermal efficiency and reduce THC emissions. However, increasing PES, advancing SOI in a small range, increasing Pinj, intake throttling and increasing the intake air temperature result in higher NOx emissions. In addition, increasing PES is contrary to the original intention of reducing the use of fossil fuels; using high Pinj results in unstable phenomenon of the diesel injection system, due to the small amount of diesel; intake throttling causes higher pumping loss and higher heat stress; using EGR can improve the engine fuel economy and emissions of THC, CO and NOx simultaneously, but the optimization results are not very significant; the responsiveness of increasing the intake air temperature is poor.(c) The engine with EGR-air double dilution can obtain higher thermal efficiency and lower NOx emissions compared with other dilution methods including air dilution, no intake throttling + EGR dilution and stoichiometry combustion + EGR dilution.(d) Combustion boundaries are arranged by the control range on the engine thermal efficiency from strong to weak as follows: PES, SOI, λ, low-pressure EGR rate, high-pressure EGR rate, Pinj and Tin. In particular, the BTE increases significantly and emissions of THC, CO and NOx decrease simultaneously as SOI changes from 0°BTDC to 40°BTDC. Combustion boundaries are arranged by NOx emissions from low to high at the same BTE: SOI, high-pressure EGR rate, low-pressure EGR rate, Tin, λ and Pinj. SOI, λ and high-pressure EGR rate are the primary combustion boundaries when BTE, NOx emissions and changes of the original structure are considered.(e) The incomplete combustion loss decreases with advancing SOI properly, decreasing λ or increasing high-pressure EGR rate, but the improvement is different. Decreasing λ is most effective in reducing the incomplete combustion loss, followed by advancing SOI, and then increasing high-pressure EGR rate.(f) As the engine speed increases, the BTE decreases, THC and CO emissions increase, but NOx emissions decrease with the amount of fuel in each cycle and the PES being kept unchanged. Changing the amount of NG and diesel can both control the engine load. As the engine load increases, the BTE increases, THC and CO emissions decrease, but NOx emissions increase.6. To construct an effective method to analyze the combustion processes of High dilution-Premixed-PI NG engines and filter out the critical combustion boundaries, the combustion heat release processes were summarized and analyzed under different combustion boundaries at 1335 rpm and 25% load(A25 condition). The results show that:(a) There are three typical combustion modes of the High dilution-Premixed-PI NG engines at low loads, including h, m and n, based on the shape of the heat release rate curve. There were two peaks of the heat release rate curve with h combustion mode, with the first peak higher than the seconde; there also were two peaks with m combustion mode, but their value were similar; there was only one peak of the heat release rate curve with n combustion mode. The time-sequenced characteristic and the HRR(heat release rate)-imbalanced characteristic were found in the combustion process of dual fuel engines, due to the differences of the reaction activities between diesel and NG. To quantify these characteristics, two quantitative indicators, including TSC(Time-sequenced coefficient) and HBC(HRR-balanced coefficient) were defined, according to the combustion phase and heat release rate.(b) The three typical combustion modes of the High dilution-Premixed-PI NG engine can be effectively distinguished by TSC and HBC. Increasing the TSC and the HBC of the heat release rate can decrease HC emissions and improve the thermal efficiency significantly. Changing the diesel injection timing and the excess air ratio control the TSC and HBC most effectively, followed by low-pressure EGR rate and PES, and then Pinj.(c) The n combustion mode is often accompanied by the feature of two-stage combustion mode. Largely advancing the diesel injection timing and largely decreasing intake throttle opening can both realize two-stage combustion mode, but these two ways are essentially different: The former belongs to low temperature combustion, the latter belongs to high temperature combustion. Therefore, the former has an obvious advantage of low NOx emissions, and the latter has a clear advantage of low THC emissions when the engine obtains the same thermal efficiency.7. To optimize the combustion process and increase the BTE of High dilution-Premixed-PI engines at low loads, 3 combustion optimization control strategies were proposed based on the mechanism of combustion interruptions, critical combustion boundaries and two-stage combustion mode(i.e. n combustion mode), in view of ignition, re-burning and flame propagation. The test was carried out under the conditions of A25, B25 and C25 at about 90%PES. The results show that:(a) Strategy 1 is advancing SOI significantly(35°BTDC) to extend the desel ignition delay substantially and realize two-stage low temperature combustion mode(i.e. n combustion mode), providing wider ignition zone for NG-air mixtures; Stragety 2 is increasing high-pressure EGR rate base on Strategy 1, making more unburned fuel in the exhaust gas reburn; Strategy 3 is reducing the throttle opening based on Strategy 2, to provide the condition of fast flame propagation for NG-air mixtures and allow more exhaust gases to recirculate into the cylinder.(b) Engine brake thermal efficiency and levels of emissions are something different with the 3 strategies. The engine using Strategy 3 produces the highest thermal efficiency, followed by Strategy 2, and then Strategy 1. However, NOx emissions using Strategy 3 are the highest in all the strategies. In addition, the throttle valve needs to be installed with Strategy 3, which changes the original structure of the engine. Strategy 1 and especially Strategy 2 can improve the engine fuel economy and emissions, which avoids the disadvantage(i.e. the “trade-off” relationship between engine fuel economy and NOx emissions) of traditional combustion optimization strategies, without any changes of the original structure of the engine. The engine brake thermal efficiency with Strategy 1 compared with the condition before optimization increases by 8.7%, and THC, CO, NOx and Soot emissions decrease by 75.4%, 59.4%, 62.8% and 45.4%, respectively; The engine brake thermal efficiency with Strategy 2 increases by 10.7%, and THC, CO, NOx and Soot emissions decrease by 84.4%, 74.0%, 41.9% and 50.9%, respectively.