18 February 2012

Thomas Built Buses released results of fuel consumption tests that compared a Thomas C2 school bus powered by a Cummins ISB 6.7 L engine with an IC Bus CE Series powered by a Navistar MaxxForce DT 7.6 L engine. The tests showed that the Cummins engine with urea-SCR technology could provide a 7-27% fuel economy (mpg) advantage, depending on the test conditions, over the non-SCR MaxxForce engine.

In tests over an urban route, the fuel economy advantage of the SCR bus was 7-10%, with bus transmissions in Performance and Economy modes, respectively. When urea (DEF) consumption and cost were considered, the overall DEF+fuel economy advantage of the SCR engine was 4-7%, with an annual cost advantage estimated at about $600 per bus.

In tests over a highway route, the fuel economy advantage of the SCR bus was 27% in both transmission modes. With the DEF consumption, the overall DEF+fuel economy advantage of the SCR engine was 24-25%, with an annual cost advantage of about $1400 per bus.

The test was commissioned by Thomas and conducted by Bosch Automotive Proving Grounds in New Carlisle, IN. The fuel economy performance was determined according to the SAE J1526, Type III methodology, based on six valid runs for each engine/transmission configuration.

Thomas selected the MaxxForce DT 7.6 L engine because it has the same in-line six cylinder configuration as the ISB engine, and because it is the most commonly used Navistar engine for the school bus application. However, according to Navistar, the MaxxForce 7 6.4 L V8 engine is their most fuel efficient model.

According to Navistar’s internal test report released in May 2011, the MaxxForce 7 6.4 L showed a 0.5% advantage in the overall DEF+fuel economy compared to the Cummins ISB in a Thomas C2 bus, and a 1.5% advantage over the ISB in a Blue Bird Vision bus. The tests were carried out over a school bus route (0-50 mph with frequent stops) and fuel economy was determined according to the TMC Type IV methodology. Fuel economy results (not including the DEF consumption) were not revealed.

Source: Thomas Built Buses