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

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.

  • LNG 1 averaged 0.12 gNOx/bhp-hr—lower than its certification value of 0.13 gNOx/bhp-hr.

  • LNG 2 averaged 0.09 gNOx/bhp-hr.

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



So we should switch to LNG trucks equipped with the TWC catalysts. This probably applies to all urban delivery trucks.

If it is 5-10x better than diesel, it seems like a no brainer.


It would be nice to know what emission rates of other criteria pollutants were.

Another recent report extolling the virtues of CNG trucks in Southern California (Johnson et al. "Ultra-Low NOx Natural Gas Vehicle Evaluation ISL G NZ") showed very low emissions of NOx emissions relative to diesel trucks with SCR, but higher emissions of all other tested pollutants (CO, NMHC, PN, NH3).

The trade-off of lower NOx emissions for higher emissions of all other pollutants is questionable.


CNG trucks and buses have lowered emissions.
Reform ethanol for PEMs to make emissions bio CO2 with no NOX.


FYI, the inventors of Hythane claim a 50% reduction in engine-out NOx over straight NG fuel.

Hydrogen is still a gas when methane freezes, so Hythane requires either a compressed-gas fuel system or separate fuel storage with mixing in the vehicle.


Just for my own edification, 20% hydrogen by volume in methane is 1/33 hydrogen by mass.  5 kg of hydrogen (a typical FCEV load) would mix with 160 kg of methane to make Hythane.  It takes about 2.9 kg of methane to equal the energy of a gallon of diesel, so 160 kg of methane is roughly equivalent to 55 gallons (especially after you add the hydrogen).

Seems like a combined LNG/CH2 vehicle isn't infeasible, though working the bugs out of such a fuel system is always going to be a slog.

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