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ROUSH CleanTech develops near-zero NOx propane autogas engine; CARB-certified at 0.02 g/bhp-hr

Alternative-fuel technology company ROUSH CleanTech has developed the first available propane autogas engine certified to California Air Resources Board’s (CARB’s) optional low oxides of nitrogen emissions standard for heavy-duty engines with 0.02 grams per brake horsepower-hour (g/bhp-hr). (Earlier post.)

The engine is 90% cleaner than the current Environmental Protection Agency’s most stringent 0.2 g/bhp-hr heavy-duty engine standard.

Last year, we introduced a propane autogas engine that was 75% cleaner than the EPA’s emissions standard. But, we knew our next challenge was to meet CARB’s lowest NOx standard at 0.02 g/bhp-hr. Our newest propane autogas engine reinforces ROUSH CleanTech’s commitment to provide vehicle solutions that reduce the impact to the environment while leveraging an abundant, domestically produced fuel that costs less than diesel.

—Todd Mouw, president of ROUSH CleanTech

NOx emissions are known to be harmful to human health and to the environment, contributing to regional ozone attainment challenges, smog and other air-quality issues. Heavy-duty diesel trucks are the single largest source of NOx emissions, contributing to smog in a majority of the nation’s most populated urban regions. According to the EPA, operating vehicles with ultra-low emission engines can make significant improvements to regional air quality and reduce a wide variety of human health impacts.

In Southern California, the development and deployment of near-zero emission vehicle technologies are critical to meeting clean air standards. We congratulate ROUSH CleanTech on this achievement.

—Wayne Nastri, executive officer for the South Coast Air Quality Management District

Cummins offers its own line of Near-Zero NOx engines running on natural gas.

This new certification covers all of ROUSH CleanTech’s 6.8L V10 3V propane engines for Class 4-7 vehicles. Installation of the new optional ultra-low NOx engines has begun with some 2018 Ford commercial vehicles and Blue Bird Vision propane school buses.

ROUSH CleanTech, which has deployed more than 18,000 propane autogas vehicles, will manufacture both the low-NOx 0.05 and ultra-low NOx 0.02 g/bhp-hr propane engines.

School, shuttle and transit buses and Class 4-7 medium-duty trucks equipped with ultra-low NOx engines are expected to be very competitive in seeking funding from the Volkswagen Environmental Mitigation Trust settlement.

The certification of this ultra-clean propane engine could not have come at a more opportune time as private and public fleets can now begin applying for a piece of the $2.9 billion Volkswagen settlement funds. The settlement is meant to fund projects that cost-effectively reduce NOx emissions, so vehicles powered by the new ultra-low NOx ROUSH propane autogas engine are an ideal fit for accomplishing this goal.

—Tucker Perkins, president of the Propane Education & Research Council



Good alternative for trucks-buses-trains and ships.

Could COAL be cleanly transformed into clean propane for the same purpose?


Coal can be transformed to DME which can be used like propane.

Nick Lyons

@SJC: Agree DME is a great practical option for trucks/heavy equipment. Natural gas -> methanol -> DME is probably more likely than converting coal.


But again, from an air quality perspective, lower NOx emissions at the expense of higher HC, CO, PM/PN, and NH3 emissions is potentially counter-productive.

Comparing the CARB certified emissions of the ROUSH "near-zero" propane engine and a competitor diesel engine (Detroit Diesel DD8)...

ROUSH - 0.04 g/hp-hr (NMHC); 0.01 g/hp-hr (NOx); 5.0 g/hp-hr (CO); 0.002 g/hp-hr (PM)

DD8 - 0.000 g/hp-hr (NMHC); 0.05 g/hp-hr (NOx); 0.4 g/hp-hr (CO); 0.000 g/hp-hr (PM).

https://www.arb.ca.gov/msprog/onroad/cert/mdehdehdv/2018/roush_hdoe_a3440086_6d8_0d02_lpg.pdf (ROUSH)

https://www.arb.ca.gov/msprog/onroad/cert/mdehdehdv/2018/detroitdiesel_mhdd_a2900163_5d1-7d7_0d20-0d01.pdf (Detroit Diesel)

The ROUSH engine also has 18% higher GHG emissions than the DD8 Diesel per cert.


This is a move to cleaner transportation by taking small steps. Any new ICEV powered by fossil fuels will still be around polluting for decades until replaced by a zero emissions vehicle. Going directly to battery electric is a leap and is preferred in all cases. Having said that, any thing that replaces diesel is preferred, especially replacing the huge gross polluting diesel ship engines. A much cleaner alternative and more space efficient alternative to shipboard diesel is LNG turbines.


These engines can have cleaner exhaust than ambient air. For example, PM is "zero" in the diesel case. NMHC is also certified as "zero" for the diesel engine. Running this engine will clean the air from PM emissions. This is something electric vehicles never can do.

I recognize your point that GHG emissions are higher for the ROUSH propane engine. This is usually the case when you compare engines using "otto" and "diesel" cycles, even if the fuel for the otto engine (propane) contain less carbon per unit of energy. Moreover, 90% cleaner than the standard does not necessarily mean 90% cleaner than the corresponding diesel engine, as the data you cite show. We know that recent advance in diesel aftertreatment very soon could enable meeting the future 0,02 g/bhp-hr limit (you need a certain margin to the limit value, so this means certifying at ~0.01 g/bhp-hr). However, I would not anticipate that engine manufacturers will apply such technology in the near future on diesel engines; not even on selected engine families, simply because this would put pressure on all engines to fulfil stricter standards. Consequently, on paper, it looks that the propane engine for the moment would have a significant advantage regarding NOx emissions (but disadvantages for all other emission compounds...) in the near future but this advantage is smaller than the "true" advantage from a technical viewpoint. If the best available technology would be applied in both cases, as a technology-neutral approach would require, the difference would diminish. Finally, both engines are so clean that all talk about the need for electrified vehicles can be dismissed.


I do not encourage coal to DME, but Harvey asked.
Use renewable carbon not fossil carbon.

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