最近推出的北美数据库可以让用户在线对比不同车型的温室气体排放情况，和实际工况下的燃油经济性表现。

Emission Analytics 公司的 EQUA 指数 (http://usa.equaindex.com) 是世界最大的汽车行业真实工况测试结果数据库。这家公司将使用同一套测试标准对各款车型进行测试，即在车辆尾气管处安装一款便携式尾气检测系统 (PEMS)。

英国 Emissions Analytics 公司创始人兼首席执行官 NickMolden 表示，“实验室非常适合进行一些预先设计好，且具有高度可重复性的测试。然而，实验室测试并无法评估车辆的真实道路行驶表现，而真实世界测试则可以提供车辆在真实驾驶环境下的关键性能参数。”

2017 年 9 月，PEMS 作为一种测试方法, 取得里了程碑式的进展，成为了欧洲车辆认证测试程序的基石。全球 PEMS 系统供应商，位于密歇根州Saline市的 Sensors Inc 公司首席技术官 David Booker 博士表示，“任何轻型汽车在正式销售之前都必须进行实验室测试和真实工况下的道路行驶排放测试。”

公司发言人 Nancy Grantham 表示，美国环保署（EPA）将使用 PEMS 等工具提取车辆行驶过程中的排放特性数据，并借此监测任何可能存在的作弊装置或其他合规性问题。任何新车在获得合格证之前均必须同时通过基于PEMS 等工具的真实工况测试和一系列实验室测试。

Grantham 补充道，美国环保署并未针对 PEMS 测试结果专门制定一个新的指标，而是主要将 PEMS 等真实工况测试数据与实验室测试结果进行对比，验证是否一致。

可重复性和可验证性

过去 6 年中，Emissions Analytics 公司已经在欧洲地区测试了超过1600 辆轻型乘用车。这家私营公司自 2013 年开始进行北美测试，一支由 4 名驾驶员组成的团队已经在洛杉矶和最近加入的底特律地区，测试了超过 520 辆汽车，其中既有小型汽车，也有大型皮卡。

Molden 表示，“我们的驾驶员经过专门训练，他们的驾驶风格可以反映绝大多数人的‘平均’水平。此外，我们还会收集每一秒的数据，进而判断驾驶员是否的确以平均速度按照指定的路线行驶，并在规定的时间或地点进行制动和加速操作。”

在 Emission Analytics 公司的整体测试机制下，保证驾驶路线的可重复性和可验证性非常重要。“我们会使用 PEMS 工具测试结果制定一个真实世界性能标签，”Molden 解释说，“EQUA 指数不是一种认证，并不属于监管规定范围内。该指数的主要作用在于不同车型之间的比较，并不存在真正意义上的‘合格’或‘不合格’。”

EQUA指数正逐渐被视为提供给消费者、立法者和汽车制造商的指南，更重要的是，它还可以让优秀的工程设计脱颖而出，为市场所认可。

Molden 建议说：“汽车制造商应进行测试的目的应该是促进环保和降低燃耗，而不是单纯为了满足监管认证的最低要求。”

Emissions Analytics自 2011 年成立以来，一直采用 PEMS 系统进行车辆测试。具体来说，为了完成测试，每辆汽车都需要在 5°C 到 25°C 的特定温度范围内行驶 100 英里（161 公里），其中市区道路驾驶占 55％，公路驾驶占 45％。近年来，Sensors 公司供应的 PEMS 设备在减重和小型化方面取得了很大进展，目前重量仅为大约 220 磅（100 公斤）。

Molden 表示：“你可以尽力在实验室中模拟真实驾驶工况，但绝不可能做到和真实情况完全一样。任何会给发动机带来负担的驾驶场景，都会影响车辆的燃耗和排放数据，而在底盘测功机上模拟各种道路条件非常困难，特别是爬坡的情况。

Molden 表示，欧洲实验室测试中的滚动阻力普遍小于真实道路测试条件，这种情况非常常见。Molden 介绍说，较低的滚动阻力可以降低车辆的燃耗和排放数据，“这种做法在技术上并不算违规，但滚动阻力误差的确是实验室测试的致命弱点。”

GPS 非常关键

在 Emissions Analytic 公司的测试中，车辆将配备一款连接至排气管的 PEMS 系统、一个 GPS系统及一个微型气象站（用来捕捉空气温度、压力和湿度）。Molden 表示，“我们会使用 GPS 系统，因此完全不需要任何车载定位系统，甚至连车速和加速度数据都不需要。另外，GPS 系统还可以提供海拔数据，因此我们还可以测量道路的坡度。”

道路测试可以在一定程度上避免排放测量结果作弊的影响。Molden 表示，“大众汽车的作弊设备就是一个很好的例子，它可以在很大程度上影响测试结果，这在实验室测试中难以避免，但却很难‘骗过’道路测试。”

此外，Emissions Analytic 公司还会在测试流程中综合考虑多方因素，包括空调的使用情况等，进而测量最准确的燃油经济性及CO2、CO、NO、NO2等物质的排放水平。值得说明是，考虑到当地使用习惯，与欧洲测试相比，Emissions Analytic 公司在北美车辆测试中专门将空调的使用时长延长了 50％。

为了能更好地进行横向比对，EQUA 指数的基本测试方法完全相同。不过，Emissions Analytics 还会进行一些特定研究，增加一些额外的考量因素，比如寒冷天气测试和陡坡测试等。此外，PEMS 测试还可以生成定制报告。

瞬时排放、燃油消耗、车辆速度、行车坡度和行车高度等信息可以让工程师更好地模拟车辆的动态特性，并绘制更贴近现实情况的“排放图”。例如通过用 X 轴表示速度，Y 轴表示加速度，用户即可在排放图中找到峰值排放或峰值燃耗对应的特定操作情况。

车辆燃油经济性和温室气体排放之间的联系不可否认。通过测量排气管中的碳氧化物气体排放，我们只要只要知道车辆所有燃料的化学成分组成，即可以利用“碳平衡守则”，准确计算出车辆的燃耗。Molden 表示：“我们就是这样计算车辆燃耗的，完全不需要在车辆的燃油管路安装任何设备，也完全用不到 CAN 总线。”

在真实世界中，车辆尾气排放分析测试也可以防止一些别有用心的装置影响测试结果，臭名昭着的“大众柴油作弊门”就是一个例子。如果没有加装作弊设施，大众汽车 (VW) 的 MPG 数据将增加 10％，二氧化碳排放量虽然有所降低，但氮氧化物的排放数据将会非常高。

Molden 解释说：“鉴于这种情况的存在，你必须在同一天，使用同一种测试方法，把同一辆车的所有数据全部测完，然后把这些数据放在一起对比，否则你的测试系统就可能很容易遭到篡改。”

A recently launched North American database provides online users with vehicle-to-vehicle comparisons of greenhouse gas emissions and real-world fuel economy.

As the largest industry database of real-world vehicle emissions and fuel consumption, Emission Analytics’ EQUA Index (http://usa.equaindex.com) is based on a standardized test regime for vehicles fitted with a Portable Emissions Measurement System (PEMS) at the tailpipe.

“The lab is great for doing defined, very repeatable tests. But lab-based tests don’t measure the vehicle’s real road experience and real world driving provides vital performance metrics,” Nick Molden, Founder and CEO of the U.K.-based Emissions Analytics, said during an Automotive Engineering interview.

PEMS achieved a milestone in September 2017 as the testing method became a cornerstone of Europe’s enhanced vehicle certification process. “In order to sell a new light-duty vehicle, you need to do the test cell measurements, which has always been the case, as well as perform real world on-road emissions tests,” said David Booker, Ph.D, Chief Technical Officer of Saline, MI-based Sensors, Inc., a global PEMS supplier.

The U.S. EPA uses PEMS and other tools to characterize in-use emissions and to screen for high off-cycle emissions that could indicate a defeat device or other compliance concern, according to agency spokesperson Nancy Grantham. PEMS and real-world test tools are used with lab data for certification testing prior to issuing certificates of conformity.

The EPA isn’t creating an in-use conformity factor for PEMS testing, Grantham added. It uses PEMS and other real world test data to confirm that real world emissions results are consistent with lab test results.

Repeatable and verifiable

Emissions Analytics has tested more than 1600 light-duty passenger vehicles over the past six years in Europe. The privately owned company began its North American testing in 2013. A team of four drivers have tested more than 520 vehicles in the U.S., ranging from sub-compact cars to one-ton pickup trucks, in Los Angeles and more recently in Detroit.

“The drivers are trained to drive in a manner that mimics ‘average’ driving. Because we’re collecting second-by-second data, we can audit to determine if the drivers did in fact drive at average speeds, follow the designated route, and perform typical braking and acceleration,” said Molden.

Repeatable and verifiable driving routes are important to Emission Analytics’ overall testing methodology. “We’re using PEMS to create a real-world label," Molden explained. "The EQUA Index is not a certification. It’s not policing of regulations. It’s testing for grades of comparison rather than a pass or fail."

The index is being touted as a guide for consumers, legislators, and automakers. And on an over-arching basis, the index recognizes good engineering work.

“Vehicle makers should look at emissions and mpg testing as an environmental and fuel saving benefit that goes beyond doing just the minimum required to achieve regulatory certification,” Molden suggested.

Since its inception in 2011, Emissions Analytics has relied on PEMS for its on-road vehicle testing. Each vehicle test covers 100 miles (161 km) of city (55%) and highway (45%) driving in a specific temperature range between 5°C and 25ºC. The Sensors, Inc.-supplied PEMS equipment has reduced in package size and weight, now at 220 lb (100 kg), in recent years.

“You can do your best to simulate real world driving in a lab, but it isn’t the same as real world driving,” said Molden. Any driving scenario that puts a load on an engine impacts mpg and emissions numbers, artificially recreating various road conditions on a chassis dynamometer, is a difficult undertaking, especially for simulating hill climbing.

There have been widespread instances in Europe of the rolling resistance in lab testing being less than what it would be on a road, according to Molden. Less rolling resistance means better mpg and lower emissions. "What was being done was technically legal, but roller settings are the Achilles heel of lab testing,” he noted.

GPS is a key

For Emissions Analytic tests, a vehicle is fitted with a PEMS that connects to the tailpipe, a GPS system, and a mini-weather station to capture the air temperature, pressure, and humidity. “We’re not relying on any of the onboard vehicle systems, not even for vehicle speed and acceleration as we’re using the GPS. The GPS also provides the altitude, so we can measure the gradient of the road,” said Molden.

On-road testing adds a layer of protection from emissions cheating. “The VW defeat device is a good example of what can be done to ‘game’ predictable, known lab testing. It’s much harder to defeat on-road testing,” he said.

Fuel economy as well as CO2, CO, NO, and NO2 numbers are addressed via a testing process that covers many aspects, including the use of air conditioning. (North America’s index has a 50% longer A/C load during testing compared to European-tested vehicles.)

The EQUA Index uses the same base methodology for comparative reasons. Emissions Analytics also does specific studies where the base methodology has an additional consideration, such as cold weather testing and steep gradient testing. Customized reports are possible with the PEMS-focused testing.

Knowing the instantaneous emissions and fuel burn as well as the vehicle speed, road grade and altitude enables engineers to model the dynamic characteristics of the vehicle. Those characteristics can be used to create emission maps. For instance, by showing speed on an x-axis and acceleration on a y-axis, a user can see the specific operation combinations that elicit emission peaks or fuel burn peaks.

The connection between fuel economy and GHG emissions is undeniable. By measuring the carbon gases at the tailpipe and knowing the chemical composition of the fuel in the tank, you can very accurately calculate the gallons burned using the carbon balance method. "That’s how in our testing we calculate mpg without cutting into the fuel line or using the CAN bus,” Molden said.

A tailpipe emissions analysis of a vehicle driven in the real world can also dissuade tampering attempts. Vehicles tested by VW during the infamous "Dieselgate" scandal had 10% higher mpg than if they’d been in compliance, with low CO2 emissions, but at the price of very high NOx emissions.

“So unless you have all the numbers lined up side-by-side from the same test, on the same car, on the same day, you have a fragmented system that is weak for manipulation,” Molden explained.

Author: Kami Buchholz
Source: SAE Automotive Engineering Magazine