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Optimization of Piston Geometry and Operating Conditions of Diesel Engines Using Genetic Algorithm

Title
Optimization of Piston Geometry and Operating Conditions of Diesel Engines Using Genetic Algorithm
Other Titles
유전 알고리즘을 이용한 디젤 엔진의 연소실 형상 및 엔진 운전 조건의 최적화 연구
Author
Lee, Seung pil
Alternative Author(s)
이승필
Advisor(s)
박성욱
Issue Date
2019-02
Publisher
한양대학교
Degree
Doctor
Abstract
본 연구의 목적은 최적화 알고리즘과 해석 모델을 결합하여, 서로 다른 엔진 사이즈에서의 엔진 성능과 배기 성능을 개선할 수 있는 엔진 운전 조건 및 연소실 형상의 최적화 결과를 도출하는 것이다. 엔진 제어 변수에는 흡기압력, 흡기온도, 분사시기, 분사압력, EGR율등의 존재하며 각 변수에 대한 영향과 상관관계에 대해서도 연구를 진행하였다. 또한 다양한 연소실 형상의 생성을 위해 Bezier curve 생성법을 도입하여 11개의 디자인 변수를 이용해 연소실 형상을 자유롭게 생성하였고 형상에 따른 연소특성 또한 연구하였다. 정확한 최적화 결과의 도출을 위해 해석 모델의 검증이 먼저 진행되었으며, 검증은 다양한 실험 조건에서 격자의존성, 분무, 연소 및 배기 모델의 검증으로 이루어졌다. 이상해석은 격자크기에 해석결과가 의존적이기에 각 엔진 사이즈에 따른 적절한 격자 크기의 선택이 중요하다. 소형엔진과 대형엔진의 이상적인 격자 크기는 달랐으며, 이는 계산의 효율성과 결과의 정확성을 바탕으로 정해졌다. 분무 모델은 다양한 분사압력, 분위기압력에서, 공기항력에 의한 분열을 예측하는 KH-RT 분열 모델을 기반으로, 분무 도달거리와 가시화 분무 패턴과의 비교를 통해 분무 모델을 검증하였다. 다양한 분사시기와 EGR율에서의 엔진 시험 조건에서 화학반응기구를 이용한 연소 해석 검증은 착화시기와 연소상등을 비교하였고, 높은 정확성을 보였다. 최적화 알고리즘은 마이크로 유전 알고리즘을 사용하였으며, 이는 적은 모집단에 서도 사용이 가능한 알고리즘으로 제한된 해석 자원에 유용하다. 선택, 교배, 변이등의 유전 방법으로 이루어져있으며, merit 함수 개념을 적용하여, 연료소비율과 배기 배출물을 동시에 저감이 가능한 최적화 결과를 도출하였다. 압축비가 고정된 소형엔진의 최적화에서는 형상과 운전조건의 변화가 크게 발생하였다. 대형엔진과는 다른 분사 전략을 가지며, 파일럿, 주, 후분사의 총 3회의 분사로 이루어져 있다. 파일럿 분사시기가 지각했음에도 분사압력의 증가로 연로-공기의 원활한 혼합과 주분사의 착화지연을 감소시켰다. 후분사량이 증가하여 soot의 산화와 연소압의 저감을 방지하여 gISFC의 저감을 유도하였다. 또한 분사압력과 EGR의 상관관계를 확인하였다. 분사압력의 증가는 연소기간을 단축시키며, 분무의 미립화를 촉진해 soot의 저감을 촉진하나 연소의 활성화를 통한 NOX 촉진 가능성을 초래한다. 이를 EGR 율의 급격한 증가를 통해 연소 온도를 저감시켜 NOX의 저감을 도모했다. 또한 연소실 형상은 re-entrant 타입에서 hat 타입의 연소실 형상을 도출하였다. 동일하게 압축비가 고정된 대형 엔진의 최적화에서는 소형엔진에서와는 다르게 형상의 변경은 존재하나, 유사한 타입의 형태를 유지하며 merit 함수가 최대화되며 gISFC와 배기의 동시 저감을 만족하는 결과를 도출하였다. 흡기압력의 증가와 흡기온도의 저감은 충전 효율을 상승시키며, EGR율 상승과 함께 연소 온도를 저감시켜 NOX의 저감을 유도하였다. 파일럿과 주 분사로만 이루어진 대형엔진에서 파일럿 분사량과 분사시기는 소형엔진에서의 최적 분사 전략과는 다른 형태를 보였다. 엔진 사이즈의 차이로 인한 파일럿 분사량의 증가와 분사시기의 진각을 보였으며, 이는 넓은 체적의 연소실의 온도를 증가시키기 위해 충분한 시간과 양을 확보하기 위한 결과이다. 분사압력과 EGR율을 기존의 경우에 비해 2배이상 증가하며 소형엔진의 최적화와 유사한 결과를 보였다. 연소실 형상은 기존의 re-entrant 타입을 유지하며 이는 보울이 넓고 분사량이 많은 대형엔진에서 연료의 원활한 흐름과 스퀴시 영역과 보울 영역으로의 적절한 분배를 통해 농후한 영역을 감소시켰다. 마지막으로 압축비를 최적화 변수로 포함하며, 흡기 조건을 고정시켜 제어인자의 변수를 줄이고 제어인자의 최적화 범위를 좁혀 분사전력과 EGR율만의 변화를 통해 최적 결과를 도출하였다. 다른 최적화 결과에 비해 최적화 변수의 수가 많지 않아 merit함수의 변화가 크지 않았다. 마찬가지로 분사압력이 증가하며 EGR율이 증가하는 결과가 도출되었으며, 이를 통해 분사압력과 EGR율이 정확한 상관관계를 이루고 있는 것을 확인하였다. 최적 압축비는 기존의 17.2에서 20.2로 증가하며 엔진의 연소 성능을 증가시켰다.
In the present study, the optimization of engine operating conditions and piston bowl shape on light-duty and heavy-duty diesel compression ignition engine has been investigated. Especially, present optimization study has the goal of manufacturing real piston that derived from optimization process. Thus, machinability of piston is considered as one of the important factor as well. The optimizations were conducted with the combination of optimization algorithm and CFD code. The KIVA code coupled with CHEMKIN chemistry solver was used as CFD tool and micro-genetic algorithm is used. Micro-genetic algorithm is sort of genetic algorithm and it use the small population so that the optimization can be conducted in restricted computational resources. Before optimization process, in order to validate the numerical models and to increase the accuracy and reliability, the validation of the spray models, combustion models, and emissions models were conducted for various test conditions. The spray model validations on the reference injectors used in the respective engine were conducted by comparing the experimental spray tip penetration and spray pattern from visualization for various ambient pressure and injection pressure. Since the reference injectors used in each engine have totally different specifications, the spray models constants were set differently. And computational grid size dependency is also tested, the proper grid size was selected for each engine size considering accuracy and time/cost efficiency. As a results, the spray models agree well with the experiments. And ignition and combustion model validation is conducted. The ignition and combustion phenomena were predicted by the chemical reaction mechanism. Since the diesel fuel consists of numerous hydrocarbon, it has the difficulty to use directly in CFD code, thus, the surrogate fuel that has similar thermal-chemical characteristics is used. The normal-heptane mechanism was used as surrogate fuel. The combustion model validation was also conducted for various injection timing and EGR rates on the light-duty engine and heavy-duty engine. The combustion validation results also agreed well with the experiment results. The optimizations were conducted in 3cases. In the light-duty engine, the engine operating conditions and piston bowl shape optimization with fixed compression ratio was conducted, and in the heavy-duty engine, the same optimization in the light-duty engine is conducted and other optimization reduced the operating condition variables and added the compression ratio as a variable. In fixed compression ratio optimization, the operating condition variables are 9, and design variables are 11. And in non-fixed compression ratio optimization, in order to find the optimum operating condition near the baseline conditions, the variables (e.g. initial pressure, initial temperature, dwell time) are removed from optimization variables. And optimization goal is maximizing the merit function value consists of NOX, soot, and gISFC. The light-duty engine optimization was progressed about 150 generation and merit value increased from 200 to 320 which is an optimized case. The gISFC and NOx and Soot emissions decreased simultaneously. The gISFC decreased by about 9.3%, Soot decreased 69% and NOX decreased about 50%. The optimized case had different operating conditions and piston geometry with baseline. The light-duty engine had one pilot and one post injection. The pilot injection timing was retarded, however, the main injection timing was nearly the same as baseline. And post injection timing was also retarded. And pilot injection quantity was same, post injection quantity slightly increased. And the EGR rate increased and injection pressure also increased. The piston geometry was changed from re-entrant type to wide hat bowl type. And the second optimization that conducted in the heavy-duty engine which had a similar process with light-duty engine optimization shows a similar trend of optimization results with the light-duty engine. The optimization was progressed to 150 generation and merit function value increased from 200 which is baseline to 270 which is optimized case. The optimization results compared with baseline and selected 5 cases that merit function value was increased dramatically. Each case shows the lower gISFC and NOX and soot emissions. And optimized case shows that gISFC decreased by 7.5%, NOX decreased 45%, and Soot decreased 35% compared to the baseline. Since the compression ratio is fixed, the piston bowl volume was also maintained to keep the compression ratio. The intake air temperature increased and pressure increased to improve charge efficiency. And because heavy-duty engine consisted of one pilot injection and main injection, the injection strategies was slightly different with light-duty engine. The pilot injection timing was advanced and quantity was also increased. It caused the increase of in-cylinder temperature and shortened the ignition delay of the main combustion. And the EGR rate increased and it doubled compared to the baseline. The injection pressure was also increased to the 190 MPa to improve the atomization of spray and reduce the injection duration. And piston bowl geometry was maintained the re-entrant type, however, the piston center depth is shallowed and curve from center to bowl was rapidly steeped. And 3rd optimization was carried out in the heavy-duty engine. However, the 3rd optimization variables were reduced in operating conditions and increased in design parameters. The operating condition parameters were reduced to 4 variables compared to the previous optimization that had 8 variables and the one design parameter, compression ratio, was added to the optimization process. The present optimization was progressed to 200 generation and the merit function value increased to about 250. The operating conditions were constrained compared to the other two optimizations, thus the injection strategies which are pilot injection timing and injection pressure and EGR rate could be changed in the narrow range. However, the optimization results also show that the gISFC and emissions were reduced. And the EGR rate increased and injection pressure also increased. The compression ratio increased to 20.2 compared to the 17.2 of baseline. The increased compression ratio led to an increase in engine performance.
URI
https://repository.hanyang.ac.kr/handle/20.500.11754/99492http://hanyang.dcollection.net/common/orgView/200000434656
Appears in Collections:
GRADUATE SCHOOL[S](대학원) > MECHANICAL CONVERGENCE ENGINEERING(융합기계공학과) > Theses (Ph.D.)
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