最近推出的北美數(shù)據(jù)庫可以讓用戶在線對比不同車型的溫室氣體排放情況，和實際工況下的燃油經(jīng)濟性表現(xiàn)。
Emission Analytics 公司的 EQUA 指數(shù) (http://usa.equaindex.com) 是世界最大的汽車行業(yè)真實工況測試結(jié)果數(shù)據(jù)庫。這家公司將使用同一套測試標準對各款車型進行測試，即在車輛尾氣管處安裝一款便攜式尾氣檢測系統(tǒng) (PEMS)。
英國 Emissions Analytics 公司創(chuàng)始人兼首席執(zhí)行官 NickMolden 表示，“實驗室非常適合進行一些預(yù)先設(shè)計好，且具有高度可重復(fù)性的測試。然而，實驗室測試并無法評估車輛的真實道路行駛表現(xiàn)，而真實世界測試則可以提供車輛在真實駕駛環(huán)境下的關(guān)鍵性能參數(shù)。”
2017 年 9 月，PEMS 作為一種測試方法, 取得里了程碑式的進展，成為了歐洲車輛認證測試程序的基石。全球 PEMS 系統(tǒng)供應(yīng)商，位于密歇根州Saline市的 Sensors Inc 公司首席技術(shù)官 David Booker 博士表示，“任何輕型汽車在正式銷售之前都必須進行實驗室測試和真實工況下的道路行駛排放測試。”
公司發(fā)言人 Nancy Grantham 表示，美國環(huán)保署（EPA）將使用 PEMS 等工具提取車輛行駛過程中的排放特性數(shù)據(jù)，并借此監(jiān)測任何可能存在的作弊裝置或其他合規(guī)性問題。任何新車在獲得合格證之前均必須同時通過基于PEMS 等工具的真實工況測試和一系列實驗室測試。
Grantham 補充道，美國環(huán)保署并未針對 PEMS 測試結(jié)果專門制定一個新的指標，而是主要將 PEMS 等真實工況測試數(shù)據(jù)與實驗室測試結(jié)果進行對比，驗證是否一致。
可重復(fù)性和可驗證性
過去 6 年中，Emissions Analytics 公司已經(jīng)在歐洲地區(qū)測試了超過1600 輛輕型乘用車。這家私營公司自 2013 年開始進行北美測試，一支由 4 名駕駛員組成的團隊已經(jīng)在洛杉磯和最近加入的底特律地區(qū)，測試了超過 520 輛汽車，其中既有小型汽車，也有大型皮卡。
Molden 表示，“我們的駕駛員經(jīng)過專門訓練，他們的駕駛風格可以反映絕大多數(shù)人的‘平均’水平。此外，我們還會收集每一秒的數(shù)據(jù)，進而判斷駕駛員是否的確以平均速度按照指定的路線行駛，并在規(guī)定的時間或地點進行制動和加速操作。”
在 Emission Analytics 公司的整體測試機制下，保證駕駛路線的可重復(fù)性和可驗證性非常重要。“我們會使用 PEMS 工具測試結(jié)果制定一個真實世界性能標簽，”Molden 解釋說，“EQUA 指數(shù)不是一種認證，并不屬于監(jiān)管規(guī)定范圍內(nèi)。該指數(shù)的主要作用在于不同車型之間的比較，并不存在真正意義上的‘合格’或‘不合格’。”
EQUA指數(shù)正逐漸被視為提供給消費者、立法者和汽車制造商的指南，更重要的是，它還可以讓優(yōu)秀的工程設(shè)計脫穎而出，為市場所認可。
Molden 建議說：“汽車制造商應(yīng)進行測試的目的應(yīng)該是促進環(huán)保和降低燃耗，而不是單純?yōu)榱藵M足監(jiān)管認證的最低要求。”
Emissions Analytics自 2011 年成立以來，一直采用 PEMS 系統(tǒng)進行車輛測試。具體來說，為了完成測試，每輛汽車都需要在 5°C 到 25°C 的特定溫度范圍內(nèi)行駛 100 英里（161 公里），其中市區(qū)道路駕駛占 55％，公路駕駛占 45％。近年來，Sensors 公司供應(yīng)的 PEMS 設(shè)備在減重和小型化方面取得了很大進展，目前重量僅為大約 220 磅（100 公斤）。
Molden 表示：“你可以盡力在實驗室中模擬真實駕駛工況，但絕不可能做到和真實情況完全一樣。任何會給發(fā)動機帶來負擔的駕駛場景，都會影響車輛的燃耗和排放數(shù)據(jù)，而在底盤測功機上模擬各種道路條件非常困難，特別是爬坡的情況。
Molden 表示，歐洲實驗室測試中的滾動阻力普遍小于真實道路測試條件，這種情況非常常見。Molden 介紹說，較低的滾動阻力可以降低車輛的燃耗和排放數(shù)據(jù)，“這種做法在技術(shù)上并不算違規(guī)，但滾動阻力誤差的確是實驗室測試的致命弱點。”
GPS 非常關(guān)鍵
在 Emissions Analytic 公司的測試中，車輛將配備一款連接至排氣管的 PEMS 系統(tǒng)、一個 GPS系統(tǒng)及一個微型氣象站（用來捕捉空氣溫度、壓力和濕度）。Molden 表示，“我們會使用 GPS 系統(tǒng)，因此完全不需要任何車載定位系統(tǒng)，甚至連車速和加速度數(shù)據(jù)都不需要。另外，GPS 系統(tǒng)還可以提供海拔數(shù)據(jù)，因此我們還可以測量道路的坡度。”
道路測試可以在一定程度上避免排放測量結(jié)果作弊的影響。Molden 表示，“大眾汽車的作弊設(shè)備就是一個很好的例子，它可以在很大程度上影響測試結(jié)果，這在實驗室測試中難以避免，但卻很難‘騙過’道路測試。”
此外，Emissions Analytic 公司還會在測試流程中綜合考慮多方因素，包括空調(diào)的使用情況等，進而測量最準確的燃油經(jīng)濟性及CO2、CO、NO、NO2等物質(zhì)的排放水平。值得說明是，考慮到當?shù)厥褂昧晳T，與歐洲測試相比，Emissions Analytic 公司在北美車輛測試中專門將空調(diào)的使用時長延長了 50％。
為了能更好地進行橫向比對，EQUA 指數(shù)的基本測試方法完全相同。不過，Emissions Analytics 還會進行一些特定研究，增加一些額外的考量因素，比如寒冷天氣測試和陡坡測試等。此外，PEMS 測試還可以生成定制報告。
瞬時排放、燃油消耗、車輛速度、行車坡度和行車高度等信息可以讓工程師更好地模擬車輛的動態(tài)特性，并繪制更貼近現(xiàn)實情況的“排放圖”。例如通過用 X 軸表示速度，Y 軸表示加速度，用戶即可在排放圖中找到峰值排放或峰值燃耗對應(yīng)的特定操作情況。
車輛燃油經(jīng)濟性和溫室氣體排放之間的聯(lián)系不可否認。通過測量排氣管中的碳氧化物氣體排放，我們只要只要知道車輛所有燃料的化學成分組成，即可以利用“碳平衡守則”，準確計算出車輛的燃耗。Molden 表示：“我們就是這樣計算車輛燃耗的，完全不需要在車輛的燃油管路安裝任何設(shè)備，也完全用不到 CAN 總線。”
在真實世界中，車輛尾氣排放分析測試也可以防止一些別有用心的裝置影響測試結(jié)果，臭名昭著的“大眾柴油作弊門”就是一個例子。如果沒有加裝作弊設(shè)施，大眾汽車 (VW) 的 MPG 數(shù)據(jù)將增加 10％，二氧化碳排放量雖然有所降低，但氮氧化物的排放數(shù)據(jù)將會非常高。
Molden 解釋說：“鑒于這種情況的存在，你必須在同一天，使用同一種測試方法，把同一輛車的所有數(shù)據(jù)全部測完，然后把這些數(shù)據(jù)放在一起對比，否則你的測試系統(tǒng)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
Emission Analytics 公司已經(jīng)通過在車輛尾氣管道上安裝 PEMS 系統(tǒng)（車載尾氣檢測設(shè)備，如圖所示），在全美測試了超過 500 輛汽車。現(xiàn)階段，Emission Analytics 公司的測試主要以內(nèi)燃機車型為主，但也開始測試越來越多的電動車乃至燃料電池汽車，從而更好地對比不同車型的電效率。
根據(jù) EQUA 指數(shù)標簽，我們可以看到這輛2017 款豐田凱美瑞汽油小轎車的二氧化碳排放級別為 C級。（圖片來源：Emissions Analytics）
安裝在車輛后座上的數(shù)據(jù)采集設(shè)備。
Emissions Analytics 公司測試車輛安裝了一個“迷你氣象站”，可以測量道路測試期間的空氣溫度、壓力和濕度。
2013 年以來幾乎沒有變化；氮氧化物的平均排放量為 43 mg/mi，比美國環(huán)保署 Tier 2 Bin 5 標準低 14%。Molden 表示，如果將平均排量降低至 1.5 L，發(fā)動機的 CO2排放則可以降低 23％。