Comparing Enhanced Natural Thermal Stratification Against Retarded Combustion Phasing for Smoothing of HCCI Heat-Release Rates
Comparing Enhanced Natural Thermal Stratification Against Retarded Combustion Phasing for Smoothing of HCCI Heat-Release Rates
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DOI：
10.4271/2004-01-2994
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发表时间：
2004-10
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影响因子：
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通讯作者：
Magnus Sjöberg;J. Dec;A. Babajimopoulos;D. Assanis
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文献类型：
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作者：
Magnus Sjöberg;J. Dec;A. Babajimopoulos;D. Assanis
在这项实验/CFD联合工作中，研究并比较了两种降低不可接受的高HCCI放热率的方法。研究发现，通过降低进气温度来延缓燃烧相位对平滑热释放率和减少发动机爆震具有良好的潜力。这至少有三个原因：1)较低的燃烧温度，2)在膨胀行程中燃烧时压力上升较小，3)在上止点附近自然热分层增加。然而，过度延迟燃烧会导致部分燃烧循环的不稳定运行，从而导致高IMEPg变化和排放增加。通过将冷却剂温度从100°C降低到50°C，可以通过增加传热速率来增强自然热分层。在一定条件下，该策略大大降低了放热速率，降低了爆震强度。为了进一步利用这一效应，通过增加缸内空气涡流来进一步提高传热速率。这导致了更长的燃烧持续时间。不幸的是，较高的热损失与高空气旋流降低了IMEP g。当增加燃料速率来补偿时，热释放率的大部分改进都失去了。总的来说，发现燃烧相位延迟比通过本工作中考虑的方法增强热分层具有更好的平滑热释放率的潜力。然而，高度迟滞燃烧的操作需要精确控制点火时间。随着当量比的增大，进气可接受温度范围迅速缩小。在一定的加油速率以上，不能通过将进气温度设置为固定值来建立稳态工作点。这个问题是由壁面加热和壁面温度与燃烧相位的耦合引起的。
Two methods for mitigating unacceptably high HCCI heat-release rates are investigated and compared in this combined experimental/CFD work. Retarding the combustion phasing by decreasing the intake temperature is found to have good potential for smoothing heat-release rates and reducing engine knock. There are at least three reasons for this: 1) lower combustion temperatures, 2) less pressure rise when the combustion is occurring during the expansion stroke, and 3) the natural thermal stratification increases around TDC. However, overly retarded combustion leads to unstable operation with partial-burn cycles resulting in high IMEPg variations and increased emissions. Enhanced natural thermal stratification by increased heat-transfer rates was explored by lowering the coolant temperature from 100 to 50°C. This strategy substantially decreased the heat-release rates and lowered the knocking intensity under certain conditions. To further exploit the effect, the heat-transfer rates were further enhanced by increasing the in-cylinder air swirl. This led to even longer combustion durations. Unfortunately, the higher heat losses associated with high air swirl decreased the IMEP g . When the fueling rate was increased to compensate, most of the improvements on the heat-release rates were lost. Overall, combustion phasing retard was found to have better potential for smoothing heat-release rates than enhancing the thermal stratification by the means considered in this work. However, operation with highly retarded combustion requires precise control of the ignition timing. Furthermore, it is found that the acceptable intake temperature range narrows rapidly with increasing equivalence ratio. Above a certain fueling rate a steady state operating point cannot be established by setting the intake temperature to a fixed value. This problem is caused by wall heating and the coupling between wall temperature and combustion phasing.