The Propane Education & Research Council (PERC) says propane delivers higher energy efficiencies than compressed natural gas and gasoline.
In a review of forklift emission studies, released this month, PERC also found propane forklifts fitted with certain closed-loop controls and exhaust catalysts produced very low emissions, meeting or exceeding the strict California Air Resource Board's large spark ignition standards.
PERC president Roy Willis said while 80 percent of class four and five IC forklifts were propane-fuelled, the forklift market was "plagued" by misleading or conflicting emissions data.
"We believe this review sets the record straight. When forklift emissions data is compared accurately, propane delivers the highest efficiencies and remains one of the cleanest fuels available for industrial lift trucks," he said.
Conducted by two independent experts in alternative fuel vehicles, the "Industrial Truck Emission Data Compared by Fuel" study examines research findings from seven forklift emissions studies performed between 1990 and 2002.
Author William McGlinchey said some previous studies "often did not use the same fuel delivery system and engine, elements that are crucial".
The review examined two studies that used life cycle analysis to assess the emissions impact of a given fuel.
Regardless of the propane source, either petroleum or natural gas, propane delivered the highest total efficiency rating at 89.3 percent, compared to compressed natural gas (87.2 percent), conventional diesel (85.6 percent), reformulated diesel (83.6 percent), reformulated gasoline (82.6 percent), and conventional gasoline (81.6 percent).
The PERC review emphasised the role regular maintenance played in fuel control system performance and emissions reduction, Mr Willis said.
"Our next step is to support the training and maintenance needs of propane-powered forklift operators," Mr Willis said. "PERC has funded programs to ensure propane-powered forklift operators receive the support they need to comply with emissions standards and achieve greater return on their investments."