Climate change is one of the most important issues we are seeking to address in the United States today.  The transportation industry contributes the most (28%) to U.S. greenhouse gas (GHG) emissions followed by power generation (25%)[1]. Public and private sector fleets are rapidly moving away from diesel and are adopting cleaner alternate sources of energy including propane to reduce GHG emissions and meet their sustainability goals. Deployment of electric vehicles is gaining momentum in the on-road light-duty transportation sector, but many heavy-duty and off road sectors remain dominated by diesel powered equipment.

 Ports are one sector where diesel dominates. A wide variety of diesel fueled heavy machinery and vehicles, from small forklifts to yard tractors, create a tremendous amount of pollution. This makes ports prime targets for air quality and emissions reductions efforts. Going electric is one option, but the operational feasibility and financial sustainability of electrically powered equipment, including vehicles, have not been fully established. Ports need equipment that ensures uninterrupted workflow to avoid loss of revenue. Hence, there is a need for cleaner fuel options to run the internal combustion engine powered equipment and vehicles in this space today.

Propane is a clean, low carbon, low cost, and widely available alternative fuel which has been serving the needs of millions of Americans in a wide variety of applications for over 100 years. Adopting propane-based near-zero emission technologies such as yard tractors in ports will reduce criteria pollutants faster and more economically, all without compromising port workflow.

To help port operators make informed strategic decisions, it is important to measure and compare the emissions either in controlled or real-world operating conditions. The Propane Education & Research Council (PERC) contracted Energy Environmental Analytics (E2A) to evaluate and compare the real-world tailpipe emissions performance of one diesel and one propane yard tractor at the Port of Newark Container Terminal (PNCT) while each was engaged in normal revenue service. The diesel tractor has a model year (MY) 2014 engine certified to EPA Tier 4 Interim standards. The propane tractor has a MY2022 engine which has the CARB ultra-low NOx (0.02 g/bhp-hr) on-road technology for this off-road application. Gaseous and particulate phase emissions were quantified using a Portable Emissions Measurement System (PEMS) and the collected data were binned according to vehicle speed. Exhaust emissions data, including nitrogen oxides (NOx), particulate matter (PM), carbon dioxide (CO2), total hydrocarbons (THC) and carbon monoxide (CO) for a diesel yard tractor and a propane yard tractor, both in idling and urban operation, are presented and compared in the present study.

Summarized emission results are presented in Table 1-1.

  • Total cycle-averaged idle and brake specific emissions rates showed over 99% lower NOx emissions for the propane-fueled tractor in comparison to the diesel-fueled tractor. More importantly, propane fueled yard tractor demonstrated near-zero NOx emissions under real-world operating conditions. Cycle averaged NOx emissions were approximately 0.0046 g/bhp-hr and 3.838 g/bhp-hr for the propane-fueled and diesel-fueled tractors, respectively.
  • Likewise, propane tractor showed near zero PM emissions. Measured PM rates resulted in averages of 0.0086 g/bhp-hr and 0.0021 g/bhp-hr for diesel and propane, respectively.
  • For idle operation, the propane-fueled tractor produced higher time-specific CO2 emissions, when compared to the diesel-fueled tractor. However, for urban operation, brake-specific CO2 emissions from the propane-fueled tractor were lower than those of the diesel-fueled tractor by approximately 14%. This would correlate to approximately a 14% reduction in lifecycle CO2 emissions as well considering upstream emissions for both fuels using Argonne National Laboratory’s GREET 2021 model. 
  • THC emission rates were lower by about 77% and 51% for the propane tractor in comparison to the diesel-fueled tractor for idle and urban operation, respectively.
  • On a cycle average basis, diesel tractor showed lower idle and brake-specific CO emissions compared to the propane tractor.

Comparison to Electric Port Tractor Grid Based Emissions

Although electric port tractor lifecycle CO2 emissions were not measured in this study, EPA’s AVoided Emissions and geneRation Tool (AVERT) (2021 power sector data)[1] reports Mid-Atlantic (mostly PJM Interconnection) marginal emissions of 1604 lb/MWh (542.5 g/hp-hr) CO2 power plant emissions. As per GREET 2022, 10.5% of the total lifecycle CO2 emissions from U.S. power generation are attributable to upstream emissions. In addition, it was assumed that the powertrain inefficiencies were balanced by regenerative braking. Thus, the estimated lifecycle CO2 emission for charging the electric port tractor is 606 g/hp-hr (542.5/0.895).

Comparatively, propane port tractor’s measured urban tailpipe average CO2 emissions were 500.5 g/bhp-hr. Correcting for upstream emissions (for propane production, storage and transportation) using GREET 2022, we expect propane port tractor’s lifecycle CO2 emissions to be 607 g/bhp-hr. The difference between the electric port tractor and propane port tractor lifecycle CO2 emissions is negligible. In addition, this analysis does not include for electric battery manufacturing CO2 emissions.

Conversely, diesel port tractor’s measured urban tailpipe average CO2 emissions were 585 g/bhp-hr. Correcting for upstream emissions using GREET 2022, we expect diesel port tractor’s lifecycle CO2 emissions to be 706 g/bhp-hr.


Hence, at least for Mid-Atlantic ports, near-zero emission propane port tractors seem to be the best solution for GHG reduction along with criteria pollutant reduction in a financially sustainable manner without hindering day-to-day operations compared to electric and diesel port tractors.

Call to Action

We are all on a path to zero emissions and near-zero emission propane technology provides a pathway today to significantly reduce criteria pollutants and GHG emissions at the ports. Propane powered port tractors are commercially available, operationally feasible and economically workable with a readily available infrastructure.  With significantly lower emissions than diesel, propane provides sustainable and economical solutions for the ports today.

Terminal tractors are used in more than just the ports for cargo handling.  With applications in warehousing and the railroads, there are numerous spaces where propane can make a difference with emissions reductions at a lower cost with ease of service.  Propane terminal tractors are eligible for port funding through MARAD, off-road DERA grants from the EPA, and local grant opportunities including some air quality management districts in California. PERC is well positioned to support agencies and allied industries applying for these grants.

The reduction of emissions is about justice; environmental and social justice for disenfranchised communities that surround ports, warehouses, rail sidings and power plants. When considering clean alternatives, propane powered equipment in the off-road market and terminal tractors is the right choice for cleaner air, reduced GHG emissions, and financial sustainability.

For a complete summary of the data, download the Real-World Propane Terminal Tractor emissions study or visit to learn more.

Download the Port Tractor Emissions One Pager