Synthetic diesel benefits, drawbacks still unclear, engine makers say

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Updated Dec 29, 2015

Cummins ISX15As fleets continue to add renewable diesel to their alternative fuel line-up, major diesel engine manufacturer Cummins says it’s exercising caution as it continues to test the biofuel.

“Cummins is still evaluating the use of renewable diesel and is not yet in a position to approve or disapprove the use of renewable diesel fuels,” reads a statement the company provided last month to CCJ sister site Hard Working Trucks.

“The environmental benefits look promising and we support the efforts to reduce the GHG (greenhouse gases) and overall emission footprint of our products.”

Renewable diesel is produced from various plant and animal matter. In earlier tests, the synthetic fuel produced impressive emissions results that beat conventional diesel. Piquing even more interest is the fact that the alternative fuel can be used as a direct diesel substitute, requiring no engine modifications, according to the Truck and Engine Manufacturers Association (EMA; the T is omitted).

The biofuel’s compatibility and its reduced emissions have made it a popular alternative fuel among a growing number of fleets, including global package delivery giant UPS. (Refer to HWT article “Renewable diesel attracting more fleets.”)

In a July press release, UPS reported that renewable diesel “offers superior performance and benefits, much like how synthetic lubricants are used in cars instead of petroleum-based lubricants.”

However, other than lowering emissions, Cummins does not expect renewable diesel to have an impact on engine durability.

“Provided a fuel meets the various global and OEM specifications, the fuel should not have any impact on engine durability, negative or positive,” the company statement reports.

In an October bulletin, EMA states that engine manufacturers face challenges in testing the performance characteristics of renewable diesel.

“Renewable hydrocarbon diesel may be produced through different processes using any one of several feedstocks,” the report states. “It is difficult for engine manufacturers to evaluate and approve a fuel defined as renewable hydrocarbon diesel without first determining the feedstock and process utilized for its production.”

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The largest provider of renewable diesel fuel in the U.S., Neste, favors using feedstocks that do not come directly from the food chain, such as animal fats and waste material. Neste will be supplying supplying UPS with 46 million gallons of its NEXBTL renewable diesel over the next three years.

EMA says some engine manufacturers have recommended renewable diesel, while others haven’t been as enthusiastic.

“Some (manufacturers) are concerned, however, that neat renewable hydrocarbon fuel or fuel blends having a high percentage of renewable hydrocarbon fuel may cause engine malfunction,” EMA reports. “Engine manufacturers continue to evaluate renewable hydrocarbon diesel fuel to determine potential concerns and consider additional standard recommendations.”

EMA did not provide the names of engine manufacturers that have concerns with the fuel.

Cummins expects to be done with its initial testing in January. The Indiana-based company will be examining the fuel’s impact on emissions, maintenance intervals, performance, reliability and durability.

Fuel density, cetane number, energy content and other characteristics of the fuel will be evaluated.

Cummins is also hoping to get feedback from consumers who have been using renewable diesel in Cummins engines. They can contribute to the evaluation by contacting their Cummins account manager or calling 1-800-DIESELS.

“We welcome any actual field data that customers have experienced in Cummins engines and renewable diesel fuel to assist in our evaluation,” the company states.

EMA reports that renewable diesel’s density is lower which thus makes its energy content lower as well. Still, EMA advises that not enough data is available yet to determine the overall impact on fuel consumption rates.

While EMA recognizes the lower emissions of renewable diesel, it cautions that more recent data is needed.

“The influence of renewable hydrocarbon diesel fuel’s properties on 2010 and later on-highway and Tier 4 non-road engines has not been evaluated sufficiently to make similar comparisons (to earlier tests made against conventional diesel),” the report reads.

Though it wouldn’t mention names, Cummins says it has been in communication with various renewable diesel manufacturers and that it may launch a larger scale study following the conclusion of its initial evaluation next month.

“Our current evaluation stems from changes in fuel properties that have been shown in the past to have an impact on emissions, both negative and positive,” reports the company. “We believe it is important to understand if the use of this fuel would create higher tailpipe emissions in all possible applications and drive cycles.”