Wei Tongchuan, Frey H Christopher
Department of Civil, Construction, and Environmental Engineering, North Carolina State University, Raleigh, NC, USA.
J Air Waste Manag Assoc. 2021 Sep;71(9):1127-1147. doi: 10.1080/10962247.2021.1923586. Epub 2021 Jun 8.
Light-duty gasoline vehicle (LDGV) tailpipe emission rates can be quantified based on pollutant concentrations measured using portable emission measurement systems (PEMS). Emission rates depend on exhaust flow. For simplified and micro-PEMS, exhaust flow is inferred from engine mass air flow (MAF) and air-to-fuel ratio. For many LDGVs, MAF is broadcast via the on-board diagnostic (OBD) interface. For some vehicles, only indirect indicators of MAF are broadcast. In such cases, MAF can be estimated using the speed-density method (SDM). The SDM requires an estimate of the engine volumetric efficiency (VE), which is the ratio of actual to theoretical MAF. VE is affected by intra-vehicle variability in the engine load and inter-vehicle variability in engine characteristics (e.g., the type of valvetrain). The suitability of SDM-based estimates of MAF in conjunction with simplified and micro-PEMS has not been adequately evaluated. Therefore, the objectives are to: (1) quantify VE accounting for intra- and inter-vehicle variability; and (2) evaluate the accuracy of SDM-based vehicle emission rate estimation approaches. Seventy-seven naturally-aspirated LDGVs were measured using PEMS. For each vehicle, VE was estimated using three approaches: (1) constant VE calibrated to actual fuel use; (2) average estimates of VE for Vehicle Specific Power modes imputed from OBD data; and (3) modeled VE using multilinear regression (MLR). The MLR models were developed based on engine load and engine characteristics. The best model was selected based on various statistical diagnostics. When engines were under load, variability in VE was most sensitive to variations in engine load. During idling, VE differed between engines depending on engine characteristics. The constant and modeled VE estimation approaches enable the accurate estimation of microscale and mesoscale emission rates, with errors typically within ±10% compared to values imputed from OBD data. Thus, accurate emission rates can be obtained from simplified and micro-PEMS. : Simplified and micro portable emission measurement systems (PEMS) enable widespread measurement of vehicle exhaust emission. As a cost saving measure, they estimate exhaust flow indirectly rather than via measurement, typically based on engine mass air flow (MAF). For some vehicles, MAF is not reported by the on-board diagnostic (OBD) system but can be inferred from other reported variables and volumetric efficiency (VE). However, VE is typically proprietary. Methods demonstrated here for estimating VE enable accurate quantification of emission rates, thereby enabling use of these PEMS for policy-relevant applications such as technology assessments, trends analysis, and emissions inventories.
轻型汽油车（LDGV）的尾气排放率可根据使用便携式排放测量系统（PEMS）测得的污染物浓度进行量化。排放率取决于排气流量。对于简化型和微型PEMS，排气流量是根据发动机质量空气流量（MAF）和空燃比推断得出的。对于许多轻型汽油车，MAF通过车载诊断（OBD）接口进行传输。对于一些车辆，仅传输MAF的间接指标。在这种情况下，可使用速度密度法（SDM）估算MAF。SDM需要估算发动机容积效率（VE），即实际MAF与理论MAF之比。VE受发动机负荷的车内变异性和发动机特性（如气门机构类型）的车际变异性影响。基于SDM估算MAF并结合简化型和微型PEMS的适用性尚未得到充分评估。因此，目标是：（1）量化考虑车内和车际变异性的VE；（2）评估基于SDM的车辆排放率估算方法的准确性。使用PEMS对77辆自然吸气轻型汽油车进行了测量。对于每辆车，使用三种方法估算VE：（1）根据实际燃油使用量校准的恒定VE；（2）根据OBD数据推算的车辆特定功率模式下VE的平均估算值；（3）使用多元线性回归（MLR）建模的VE。MLR模型是基于发动机负荷和发动机特性开发的。根据各种统计诊断方法选择最佳模型。当发动机处于负荷状态时，VE的变异性对发动机负荷的变化最为敏感。在怠速期间，不同发动机的VE因发动机特性而异。恒定VE和建模VE估算方法能够准确估算微观和中观尺度的排放率，与根据OBD数据推算的值相比，误差通常在±10%以内。因此，可通过简化型和微型PEMS获得准确的排放率。简化型和微型便携式排放测量系统（PEMS）能够广泛测量车辆尾气排放。作为一种成本节约措施，它们通常基于发动机质量空气流量（MAF）间接估算排气流量，而非通过测量。对于一些车辆，车载诊断（OBD）系统未报告MAF，但可从其他报告变量和容积效率（VE）推断得出。然而，VE通常是专有的。此处展示的估算VE的方法能够准确量化排放率，从而使这些PEMS可用于技术评估、趋势分析和排放清单等与政策相关的应用。
