ABSTRACT

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.

Implications: 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.

摘要

轻型汽油车 (LDGV) 尾气排放率可以根据使用便携式排放测量系统 (PEMS) 测量的污染物浓度进行量化。排放率取决于排气流量。对于简化和微型 PEMS，排气流量是从发动机质量空气流量 (MAF) 和空燃比推断出来的。对于许多 LDGV，MAF 是通过车载诊断 (OBD) 接口广播的。对于某些车辆，仅广播 MAF 的间接指标。在这种情况下，可以使用速度密度方法 (SDM) 估计 MAF。SDM 需要估计发动机容积效率 (VE)，即实际 MAF 与理论 MAF 的比率。VE 受发动机负载的车辆内部可变性和发动机特性（例如，气门机构的类型）的车辆之间可变性的影响。尚未充分评估基于 SDM 的 MAF 估计与简化和微型 PEMS 相结合的适用性。因此，目标是： (1) 量化 VE 考虑车辆内部和车辆间的可变性；(2) 评估基于 SDM 的车辆排放率估计方法的准确性。使用 PEMS 测量了 77 辆自然吸气 LDGV。对于每辆车，使用三种方法估计 VE：(1) 校准到实际燃料使用的常数 VE；(2) 根据 OBD 数据估算的车辆特定功率模式的 VE 平均估计值；(3) 使用多元线性回归 (MLR) 对 VE 建模。MLR 模型是根据发动机负载和发动机特性开发的。根据各种统计诊断选择最佳模型。当发动机处于负荷状态时，VE 的变化对发动机负载的变化最为敏感。在怠速期间，VE 因发动机特性而异。常数和建模 VE 估计方法能够准确估计微尺度和中尺度排放率，与从 OBD 数据推算的值相比，误差通常在 ±10% 以内。因此，可以从简化的微型 PEMS 中获得准确的排放率。

影响： 简化的微型便携式排放测量系统 (PEMS) 能够广泛测量车辆尾气排放。作为一种节省成本的措施，他们间接地而不是通过测量来估计排气流量，通常基于发动机质量空气流量 (MAF)。对于某些车辆，车载诊断 (OBD) 系统不会报告 MAF，但可以从其他报告的变量和容积效率 (VE) 中推断出。但是，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，从而能够将这些 PEMS 用于与政策相关的应用，例如技术评估、趋势分析和排放清单。