Newly Released Test Shows SCR Could Result in Up to 27-Percent Increase in Fuel Economy


Thomas Built Buses says a third-party test it commissioned last year claims the fuel economy of Selective Catalyst Reduction (SCR) technology of Cummins diesel engines could outperform that of Exhaust Gas Regeneration (EGR) used by Navistar by 7 to 27 percent, depending on the route and transmission mode.

IC Bus, meanwhile, called the results an “apples to oranges” comparison because it said Thomas tested a Cummins ISB 6.7L against a Navistar MaxxForce DT 7.6L engine rather than a MaxxForce 7 6.4L V8, which the company said is its fuel economy leader. Thomas said it tested the Cummins ISB against the MaxxForce DT because both engines have in-line six cylinders. Earlier in February, Cummins announced that the ISB 6.7L was certified by EPA and CARB for 0.20g Oxides of Nitrogen (NOx) per brake horsepower hour without the use of emissions credits.

The results from the Thomas test were distributed to media on Feb. 8 prior to an event that featured an environmental stewardship award from the North Carolina Department of Environment and Natural Resources at Thomas’ Saf-T-Liner C2 plant in High Point for running a zero-waste-to-landfill facility.

The different responses to EPA 2010 Clean Air mandates to reduce emissions, notably limiting nitrogen oxide (NOx) to levels of 0.2 grams per hour, have resulted in a battle for market acceptance during the past several years. Both Thomas and Blue Bird Corporation chose the SCR technology embraced by Cummins, while Navistar went the path of EGR. The technologies differ in that EGR uses regeneration to burn off excess NOx in the engine, whereas SCR treats NOx downline in the tailpipe.

Thomas’ test, conducted at the Bosch Automotive Proving Grounds in New Carlisle, Ind., and finalized in December, compared the Cummins ISB 6.7L, six-cylinder diesel engine with SCR in a Saf-T-Liner C2 against the the MaxxForce DT 7.6L, six-cylinder engine with EGR in an IC Bus CE Series. The Bosch test methodology included SAE J1526 Type III for calculating fuel consumption while meeting EPA diesel emission requirements, and data was gathered from six valid runs performed by two different drivers.

Jed Routh, product planning manager at Thomas, said bias was removed by rotating drivers between buses and routes. Bosch ensured both buses were serviced properly, instrumentation was measured using flow meters and the vehicles carried identical weights, with the DEF tanks required of SCR factored into the test.

The test used assumptions of diesel priced at $3.25 per gallon for both buses and included a DEF price of $2.73 per gallon. Both buses were equipped with the same Allison 2500 PTS transmission. Vehicle specs were also closely matched, and both school buses weighed approximately the same — 31,000 pounds GVWR for Thomas versus 29,800 pounds GVWR for the IC CE Series.

Each bus also was operated at speeds that mimicked both urban, home-to-school routes and highway activity trips. Routh said the Thomas C2’s stop-and-go routes resulted in a 7- to 10-percent fuel economy increase on urban routes in economy and performance modes, respectively, compared to IC and a 27-percent increase in both modes on the highway routes at sustained speeds of 58 mph.

In terms of cost to bus operators, Routh said regular school routes using SCR buses could result in more than $600 in annual savings compared to EGR. That figure increases to more than $1,400 for activity and athletic trips.

“Test results exceeded our expectations,” added Tom Hodek, general manager of Cummins’ Bus Business. “Since day one, we’ve been confident that our SCR technology offers a fuel economy advantage over alternate emissions reduction technology. This test further defines the advantages of a cleaner, cooler-running SCR engine.”


The MaxxForce DT engine with EGR technology.

Navistar published its own findings last May that showed its EGR solution resulted in better overall fluid economy, or fuel mileage benefits combined with the advantage of operators not needing to purchase, store and maintain the Diesel Exhaust Fluid that is required by SCR to treat NOx emissions.

Those results showed that EGR resulted in as much as a 4.5 percent increase in the total cost of diesel fuel consumed, plus no expenses for the Diesel Exhaust Fluid (DEF) that is required of SCR to reduce emissions. IC Bus said its CE Series with MaxxForce 7 V8 got a 1.5-percent increase in fluid economy over a Blue Bird Vision with the ISB 6.7L, in-line six. IC Bus said the CE Series held a 0.5-percent fluid economy increase over a Thomas C2 also with a Cummins ISB.

In addition to the up to a 4.5 percent advantage in fluid economy in typical school bus conditions in the best-case scenario against the Blue Bird, the test commissioned by Navistar determined that the MaxxForce 7 engine could offer a fluid-economy advantage of between 0.5 and 1.5 percent over the Thomas Saf-T-Liner C2 by using TMC Type IV testing methods. IC said these methods are considered to most closely replicate real-world fuel economy because they factor in vehicle aerodynamics, power train and tires.

An IC Bus spokesperson said the study of the Cummins ISB 6.7L versus the MaxxForce DT engine “is like comparing apples to oranges” and, had the Bosch test compared the Cummins engine against the MaxxForce 7 V8 with a six-speed, which the spokesperson said is Navistar’s fuel economy leader, “the result would have been different.”

In addition to wanting to test two in line-six cylinder engines, a Thomas representative added that the company tested the MaxxForce DT against the Cummins ISB because Polk automotive data shows the Navistar’s engine is the most commonly purchased in the global engine sales marketplace. An IC Bus rep said the the MaxxForce DT is the Navistar’s global sales leader but added that the company sees “a shift to the Maxxforce 7 as economy becomes are larger part of the decision process.”