PEMS takes a bigger testing role in U.S., Europe
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 VWdefeat 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.
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This article was written for SAE International. You can view the original post here.