Fuel Economy Test Summary
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Fuel Economy Test Summary
Page 1
FUEL ECONOMY TEST SUMMARY
An independent, third-party evaluation of a Thomas Built Buses Saf-T-Liner® C2,
powered by a Cummins ISB 6.7 L engine, tested against an IC BusTM CE Series,
powered by a MaxxForce® DT 7.6 L engine. Both engines are 2010 diesel emissions-
compliant.
December 2011
TBB/MC-M-0112 Thomas Built Buses is registered to ISO 9001:2008 and ISO 14001:2004. Copyright © Daimler Trucks North
America LLC. All rights reserved. Thomas Built Buses Inc. is a subsidiary of Daimler Trucks North America LLC, a Daimler Co.Fuel Economy Test Summary
Page 2
SUMMARY
The purpose of this test is to provide a third-party comparison of the fuel economy differences
between a Thomas Built Buses Saf-T-Liner® C2, powered by a Cummins ISB 6.7 L engine, and
an IC BusTM CE Series, powered by a MaxxForce® DT 7.6 L engine. The test distinguishes
between diesel fuel and diesel exhaust fluid (DEF) consumption, and provides a cost analysis
incorporating total diesel fuel and DEF used in a vehicle’s operation.
The Thomas Built C2 with selective catalytic reduction (SCR) technology results in a 7%
to 27% fuel economy advantage over the IC CE Series with in-cylinder exhaust gas
recirculation (EGR), depending on the route (urban or highway) and transmission mode
(Economy or Performance). While there are competing claims in the market, this test,
conducted by Bosch Automotive Proving Grounds using specific test methodology, provides
evidence that SCR technology, including the cost of diesel exhaust fluid, delivers better fuel
economy and is the more cost-effective choice for customers.
BACKGROUND
Both of the engines tested were 2010 emissions-compliant, but the Thomas Built bus was
equipped with SCR technology and the IC bus was equipped with EGR technology. SCR
technology removes harmful nitrogen oxide (NOx) emissions from the exhaust stream after it is
formed during the combustion process, while EGR technology prevents the formation of NOx
during the combustion process. Both are successful in reducing NOx levels; however, each has
a different effect on the engine efficiency and fuel economy.
SCR reduces tailpipe NOx by treating the exhaust stream with a spray of DEF. DEF, combined
with exhaust heat and a catalyst, converts NOx into nitrogen and water vapor. Treating the
exhaust after the combustion process allows the engine to function at optimal combustion
temperatures, increasing fuel efficiency and reliability.
EGR prevents the formation of NOx by introducing cooled exhaust gas into the intake system.
This recirculated exhaust gas contains less oxygen than the usual diesel-air mixture, reducing
the combustion temperature and producing less NOx. The lower combustion temperature,
however, translates into less efficient combustion, reduced engine performance and often an
increase in particulate matter and regenerations. To compensate for the reduced efficiency,
more diesel fuel is consumed to achieve proper engine performance.
While DEF is required with SCR technology and not with EGR technology, the benefit of SCR
and the costs associated with using DEF outweigh the loss in fuel economy associated with
EGR technology.Fuel Economy Test Summary
Page 3
METHODOLOGY
The test was commissioned by Thomas Built Buses, but was conducted entirely by an
independent third-party, Bosch Automotive Proving Grounds, New Carlisle, IN.
Test runs simulated both a stop and go (urban) route and a steady (highway) route.
Separate test runs were performed with bus transmissions in Economy Mode and in
Performance Mode.
Industry-recognized methodology, SAE J1526, Type III, was used for calculating fuel
consumption differences between vehicles.
For each route/transmission configuration, data were gathered from six valid runs, with
three runs by driver A and three runs by driver B to eliminate any effect the driver might
have on fuel economy.
A run was considered valid when it was compared to two other valid runs and the results
were within 2% of each other.
Prior to testing, OEM dealers verified that the vehicles were operating properly, with no
stored fault codes and no diagnostic messages.
Test vehicle specifications were closely matched to ensure accurate fuel economy
comparison results. (See page five for details.)
Vehicles were serviced prior to the test, including oil and filter change, air filter change,
fuel filter change and vehicle alignment adjusted to specs. Cold tire pressure and fluids
were checked daily throughout testing.
Instruments used included fuel flow meters (to determine fuel consumption), GPS
(speed and distance), thermocouple at flow meter (fuel temperature), thermocouple at
exhaust outlet (exhaust temperature) and a DEF measurement device (pre-weighted
container for exact DEF usage).
Vehicles were loaded with equivalent payloads of a target of 77 passengers at 125 lbs
per passenger.
Urban route: 27 mile run with steady state speeds of 25, 30 and 40 mph, with 2.33
stops/mile and targeted deceleration at 8 feet per second per second.
Highway route: three mile drive to simulate getting to the highway, followed by 54 miles
at a steady speed of 58 mph, followed by another three mile drive to simulate returning
to the bus yard.Fuel Economy Test Summary
Page 4
CONCLUSION
When performing fuel economy testing, it is important to ensure that both buses are in
the same transmission mode for each run, either Economy or Performance. The
Thomas Built Buses C2 with SCR technology results in a 7% to 27% fuel economy
advantage over the IC CE Series with EGR, depending on the route and transmission
mode driven. When considering the usage and cost of DEF in an SCR vehicle, the
results still favor the Thomas Built C2, and can provide an average annual savings of
$600 - $1,400 per bus, based on the assumptions provided*.
URBAN ROUTE
Diesel Fuel MPG Diesel Fuel + DEF MPG
ECONOMY PERFORMANCE ECONOMY PERFORMANCE
IC Bus 6.76 6.61 6.76 6.61
Thomas Built 7.43 7.06 7.24 6.88
Thomas Built
Efficiency Advantage
10% 7% 7% 4%
Thomas Built Annual
Cost Advantage per Bus*
$768 $603
HIGHWAY ROUTE
Diesel Fuel MPG Diesel Fuel + DEF MPG
ECONOMY PERFORMANCE ECONOMY PERFORMANCE
IC Bus 7.61 7.57 7.61 7.57
Thomas Built 9.65 9.63 9.41 9.44
Thomas Built
Efficiency Advantage
27% 27% 24% 25%
Thomas Built Annual
Cost Advantage per Bus*
$1,416 $1,417
*Assumptions: Average DEF price per gallon is $2.73; average diesel price per gallon is $3.25; average annual miles driven are
14,500.Fuel Economy Test Summary
Page 5
VEHICLE STATISTICS
Vehicle Stats
Thomas Built Buses
Vehicle IC Bus™ CE Series
Saf-T-Liner® C2
Wheelbase 23'2" 23'
Length=40'1" Length=39'9"
Bus Measurements Width=96" Width=96"
Height=10' Height=10'6"
Engine Cummins ISB 6.7 L MaxxForce DT 7.6 L
Rated HP 220 HP @ 2300 RPM 215HP @ 2200 RPM
Emission Technology SCR In-cylinder EGR
Front Axle GVW 10,000 10,000
Rear Axle GVW 21,000 19,800
GVWR 31,000 29,800
Payload (77 passenger at
9,610 lb 9,600 lb
125lb per passenger)
Engine Belt Driven Alternator, Engine Fan, Water Alternator, Engine Fan, Water
Accessories Pump Pump
Engine Gear Driven Air compressor, Power Steering Air compressor, Power Steering
Accessories Pump Pump
Fan Drive on Engine Viscous Viscous
Allison 2500 PTS w/ Economy Allison 2500 PTS w/ Economy
Transmission
and Performance Modes and Performance Modes
Rear End Ratio 5.71 5.86
Tires Goodyear G662RSA 11R22.5 G Goodyear G662RSA 11R22.5 G
Tire Pressure 105psi 105psi
Tread Depth on Tires 22/32" 22/32"
Note: Throughout the duration of the test, four regenerations occurred in the EGR-equipped
bus, and one regeneration occurred in the SCR-equipped bus. Runs where a regeneration took
place were not used. Increased frequency of regenerations is created by the greater production
of particulate matter with the EGR system due to the reduced combustion temperature and the
resulting increased amount of unburned diesel fuel.
This document was written by Thomas Built Buses. Bosch Automotive Proving Grounds
performed the testing and provided the test data in a detailed report. The data was analyzed
and the conclusions were drawn by Thomas Built Buses. The detailed test reports are available
upon request through your local Thomas Built Dealership.You can also read
