Nowadays a lot of efforts are devoted on reducing the hazardous emissions and improving the engine efficiency due to the energy issue and the environmental pollution. Besides, the depletion of fossil fuels has led to the investigation on alternative fuels covering from production to the application. Di-methyl-ether (DME) as an alternative fuel for the diesel was used to investigate its effects on the direct injection compression ignition engine combustion. The final objective was to investigate the performance and emission of an Atkinson cycled engine which has a higher expansion ratio than the compression ratio with DME fuel. The Atkinson cycle was realized through reducing the effective compression ratio by means of late intake valve closing (LIVC) strategy.
Prior to the implementation of DME fuel, the preliminary experiments of LIVC strategy with conventional diesel fuel were carried out. The engine speed was fixed at 1200 rpm, while the injection quantity was varied from 8.2 mg to 25 mg per stroke. The intake valve closing (IVC) timing was swept from base IVC timing of 28 CAD ABDC (crank angle degrees after bottom dead center) to 88 CAD ABDC, in general. The IVC timing was retarded up to 120 CAD ABDC for an extreme case where the limitation of LIVC strategy was examined.
The IMEP was maintained equal with respect to IVC timing retardation only at the injection quantity of 8.2 mg/stroke, which was low load condition. The IMEP started to decrease as the IVC timing was retarded at higher engine load condition of 12.0 and 19.8 mg/stroke. The deterioration of IMEP was attributed to several reasons, such as decreased combustion efficiency, sub-optimized combustion phasing etc. However, the reduction of effective compression ratio with respect to IVC timing retardation was considered to be the main cause of such IMEP deterioration.
The $NO_x$ emission was able to be reduced with either decreasing the $O_2$ concentration by introducing the exhaust gas recirculation (EGR) or retarding the IVC timings. The $NO_x$ emission showed significant dependency according to the $O_2$ concentration, since it directly controlled the adiabatic flame temperature of the spray. The IVC timing retardation also yielded in NOx emission reduction; however, the order of reduction with LIVC strategy alone was not as significant as decreasing the $O_2$ concentration. Combination of both EGR and LIVC strategy showed potential for further $NO_x$ emission reduction. The smoke emission was also reduced with IVC timing retardation as far as the absolute amount of oxygen was secured inside the combustion chamber. The prolonged ignition delay caused by the IVC timing retardation contributed in smoke reduction. However, the smoke emission started to deteriorate when the absolute amount of oxygen decreased. In summary, the trade-off relation between $NO_x$ -smoke was able to be improved only when the absolute amount of oxygen trapped inside the cylinder were abundant.
The decrease of the IMEP with respect to IVC timing retardation required intake boosting to maintain the original efficiency of the engine. In addition, the increased portion of repelled charge with respected to IVC timing retardation also demanded intake boosting in order to maintain the maximum power output of the engine as well. The increase of the boost pressure, in general, required high backpressure not only for the turbine, but also for the high-pressure loop EGR (HP-EGR). The pumping work increased as the boost pressure increased, due to faster increase rate of backpressure than that of intake pressure. An alternative configuration, which composed of internal EGR plus low-pressure loop EGR (LP-EGR) was proposed to mitigate such pumping loss without deteriorating the response of HP-EGR. The IMEP was able to be maintained at an equal level, while the relation of NOx-smoke was able to be improved at an injection quantity 25.0 mg/stroke condition.
Eventually the implementation of DME fuel to LIVC was conducted. The $NO_x$ -smoke relation was significantly improved due to its intrinsic property of the fuel. Such improvement might bring expectation of implementing greater amount of EGR to mitigate $NO_x$ emission, since the smoke emission would not be deteriorated. However, implementation of LIVC strategy with EGR yielded negative effect on the combustion efficiency due to the poor oxidation and incomplete combustion of the fuel. The reduced in-cylinder temperature by IVC timing retardation brought the DME fuel to react in the low temperature reaction (LTR) temperature and negative temperature coefficient (NTC) range. Prolonged ignition delay due to such chemical property resulted in decreased combustion efficiency. Therefore, a careful selection of LIVC and EGR strategy was required in order to maintain the efficiency of the engine fueled with DME.
환경 및 배기 규제로 인해 엔진 효율 향상 및 배기 배출 저감에 많은 노력이 진행되고 있다. 화석 연료의 고갈 또한 대체 연료 연구에 대한 관심을 가져오게 하였다. DME (Di-methyl ether)는 디젤 연료의 대표적인 대체연료 중 하나이기 때문에 이를 디젤 엔진에 적용하여 연구를 수행하였다. 최종 목적은 Atkinson cycle이 DME를 적용한 압축 착화 엔진에 미치는 영향을 연구한 것이다. Atkinson cycle의 구현은 LIVC (late intake valve closing) 전략을 통해 팽창비를 압축비보다 높게 설정하는 방법을 사용하였다.
DME 연료의 적용에 앞서 디젤 연료와의 비교를 위해, 디젤 연료를 사용하여 LIVC의 영향에 대한 고찰을 우선 수행하였다. 통상적인 경우, IVC (intake valve closing) timing을 28 CAD ABDC (crank angle degrees after bottom dead center) 부터 88 CAD ABDC 까지 고찰하였고, IVC timing 영향의 한계를 파악하기 위해 최대 120 CAD ABDC까지 지각하였다.
IMEP (indicated mean effective pressure)의 경우, 저부하 조건인 8.2 mg/stroke 조건에서 IVC timing 지각에 따라 큰 차이를 보이지 않았다. 하지만 엔진 부하를 증가하여 연료량을 12.0 또는 19.8 mg/stroke로 증가할 경우, IVC timing 지각에 따라 IMEP가 감소하는 경향을 보였다. IMEP의 저하는 연소 효율의 저하 또는 최적 연소상의 이탈 등의 영향도 있지만, 유효 압축비 감소가 가장 큰 이유인 것으로 판단된다.
NOx 배출의 경우, 흡기 $O_2$ 농도를 변화시킬 수 있는 EGR (배기가스 재순환) 사용 또는 IVC timing 지각 등의 방법으로 감소가 가능하지만, $NO_x$ 의 생성은 $O_2$ 농도에 크게 지배적이기 때문에 EGR를 사용하는 것이 더욱 효과적이었다. LIVC와 EGR를 전략을 동시 사용할 경우, NOx의 추가적인 감소가 가능하였다. smoke 배출의 경우, 실린더 내부 산소의 절대량이 충분할 경우 IVC timing 지각에 따라 감소하였다. 하지만 IVC timing 지각에 따라 체적효율이 감소하게 되고, 충분한 산소량을 확보하지 못할 경우, smoke의 배출은 다시 증가하였다. $NO_x$ -smoke의 trade-off 관계 개선은 실린더 내부에 산소량이 충분할 경우에만 가능하다.
IVC timing 지각에 따른 IMEP의 감소는 흡기 과급을 통해 효율 보상이 필요하다. 하지만, 흡기 압력 상승은 터빈과 HP-EGR (high-pressure loop EGR) 구조로 인해 배압 상승을 야기한다. 흡기 압력 상승율보다 배압 상승율이 빠르기 때문에, 과급량 증가에 따라 펌핑 손실이 증가하게 된다. 그렇기 때문에 펌핑을 유발하는 HP-EGR를 제거하는 대신, HP-EGR의 응답성을 대체할 수 있는 internal EGR과 LP-EGR (low-pressure loop EGR)를 사용하여 엔진 성능 및 배기에 미치는 영향을 분석하였다. 엔진 부하 25.0 mg/stroke 조건에서, IMEP는 유지하면서 NOx-smoke의 trade-off가 개선될 수 있다는 것을 관찰하였다.
마지막으로 LIVC 전략이 DME 엔진 연소에 미치는 영향을 고찰하였다. NOx-smoke의 trade-off 관계를 연료의 smokeless combustion 특성으로 인해 개선될 수 있었다. 그렇기 때문에 EGR의 증가와 LIVC를 통해 NOx를 감소하고자 하는 전략을 예상할 수 있는데, 예상과 달리 LIVC와 EGR를 동시 사용 시 연소효율 저하로 인해 효율 저하가 발생하는 부정적인 효과가 발생하였다. 이는 DME 연료가 LTR (low temperature reaction)와 HTR (high temperature reaction) 사이에 나타나는 NTC (negative temperature coefficient) 영역에 진입함에 따라 점화 지연이 길어지고 lean mixture 비중이 증가하였기 때문이라 판단된다. 그렇기 때문에 DME 연료 사용 시, LIVC와 EGR 전략 간의 최적화 작업이 필요하게 된다.
