ARB study compares in-use NOx emissions from diesel, hydraulic hybrid diesel and LNG refuse trucks

ARB study compares in-use NOx emissions from diesel, hydraulic hybrid diesel and LNG refuse trucks

12 June 2017

A team from the California Air Resources Board (ARB), in partnership with the City of Sacramento, has characterized the in-use emissions from model year (MY) 2010 or newer diesel, liquefied natural gas (LNG) and hydraulic hybrid diesel engines during real-world refuse truck operation. A paper on their study is published in the ACS journal Environmental Science & Technology.

The team used a portable emissions measurement system (PEMS) to quantify the emissions from five trucks: two diesels equipped with selective catalytic reduction (SCR), two LNG’s equipped with three-way catalyst (TWC) and one hydraulic hybrid diesel equipped with SCR. The engines were certified to the MY 2010 (0.2 g NOx/bhp-hr) or interim MY 2010 (0.5 g NOx/bhp-hr) standard over typical refuse operation.

By some accounts, 80% of the refuse truck operation is spent in low speed regime. Based on our previous work on evaluating SCR functionality on Class
8 heavy-duty diesel engines, the refuse truck operation could present a challenge to the exhaust aftertreatment system due to low average speeds and frequent stops which might not be adequate to maintain the exhaust temperature for optimal SCR performance without special attention to exhaust temperature management. Hence there’s a need to evaluate aftertreatment performance from this vocation to assess if the air quality benefits envisioned under various regulations are being achieved by this sector.

Results from this study comparing in-use emissions of diesel, LNG and hydraulic hybrid diesel refuse trucks with emissions standards for NOx and GHG, could be used to refine emissions inventories used for air quality planning and regulatory policy development, and to guide potential approaches for further NOx control across a wider engine operational envelope.

—Misra et al.

The results showed that the brake-specific NOx emissions for the LNG trucks equipped with the TWC catalyst were lowest of all the technologies tested.

The ARB team also showed that the brake-specific NOx emissions from the conventional diesel engines were significantly higher despite the exhaust temperature being high enough for proper SCR function.

  • Diesel 1, which was certified at 0.18 gNOx/bhp-hr, exhibited average brake-specific NOx of 1.17 gNOx/bhp-hr—exceeding the certification value by a factor of 6-7.

  • Diesel 2 averaged NOx emissions of 0.66 gNOx/bhp-hr.

Like the straight diesels, the hydraulic hybrid diesel also exceeded certification levels; the average NOx emissions from all four tests were 0.46 gNOx/bhp-hr—exceeding the certification value. However, the researchers noted that the hybrid engine was not driven on highway and therefore those emissions (which tend to be lower) were not recorded as a result. The researchers suggested that it was highly likely that if the truck was driven on highway and the emissions data was collected, the results would have been more in line with the FTP work-specific NOx emission rate.

The data for hydraulic engine also shows that maintaining a temperature for SCR functionality needs to be explored in future technology development. While the hydraulic component can have substantial effects on fuel cost savings (with concomitant effect on cutting down GHG emissions), the diesel engine and the hydraulic components in a hybrid system should also work in tandem to control NOx emissions during all the modes of driving.

—Misra et al.


  • Chandan Misra, Chris Ruehl, John Collins, Don Chernich, and Jorn Herner (2017) “In-Use NOx Emissions from Diesel and Liquefied Natural Gas Refuse Trucks Equipped with SCR and TWC, Respectively” Environmental Science & Technology doi: 10.1021/acs.est.6b03218


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