Fuel Economy in Highway Transportation
Road Fuel Consumption Profile
International 9200i, Cummins ISX435ST, 10-speed Eaton UltraShift automated transmission
IntroductionDuring two years of line haul operations, using a number of different units which were driven by different drivers, we had been meticulously collecting fuel consumption measurements on all our trips. As a result we obtained a complete fuel consumption profile for an International 9200i equipped with the 15 litre, 435 Hp Cummins ISX435ST engine and the 10-speed Eaton UltraShift automated transmission. We were primarily interested in finding out what effect would the terrain and various loads have on our fuel consumption, and how this information could be used to improve our future mileage. Finally, we wanted to know how significant would the fuel economy improvement resulting from the use of an automated transmission be.
ResultsThe fuel consumption data for hilly terrain and prairies in summer (April through October), and winter (November through March) months are presented in Table 1.
As expected, the table shows an increase of
fuel consumption during cold winter months and a proportionate consumption
increase with an increase of gross vehicle weight. No surprises there.
Ambient temperature and humidity directly affect the engine performance
and thus the fuel consumption as the optimal temperatures within the engine
combustion chamber, or its filling may not be achieved at low temperatures
or high humidity.
We consistently saw a high scatter of data about the mean (red line) as the environmental conditions fluctuated. Figures 1 and 2 show this as a scatter plot of fuel consumption measurements collected during the summer months and plotted as a function of G.V.W. in mountains and prairies respectively. The scatter of data points about the mean was somewhat higher in prairies than in the hills suggesting greater environmental fluctuations (temperature, humidity, wind) affecting trucks in the prairies. Notice however, that in both, the mountains and the prairies the fuel consumption did not go up linearly with the increasing loads. Instead, it peaked around G.V.W. of 37000 Kg in the mountains and around G.V.W of 33000 Kg in the prairies, and then it remained almost constant. This means that any increase of load above these values is transported at little extra cost.
Polynomial regression analysis of fuel consumption for team, line haul trips yielded the following mathematical relationship for mountainous terrain in summer (Fig. 1):
Y = 14.818+0.0012X-1.2x10-8X2
and for the prairies in summer (Fig. 2):
Y = 11.743+0.0014X-1.6x10-8X2
where Y is fuel consumption in litres per
100 Km, and X is G.V.W. of both units in Kg.
Another interesting relationship was revealed
when we plotted the mean summer fuel consumption for the hills and prairies
(Table 1). At low G.V.W., the fuel consumption was lower in the
flat prairies, but this relationship changed as the load was increased
(Fig. 3). This phenomenon can be well explained by the engine torque
curve characteristics, vehicle(s) mass, and the nature of the terrain.
In prairie-like flat terrain, the engine was subject to steady load which
had to be counteracted by a steady power output. The heavier the load,
the higher the output required. This was advantageous only with lighter
loads as the engine worked in a relatively fuel efficient area of its torque
curve. As the load increased under the conditions of steady engine output
however, the engine was forced into the less fuel economical area of its
torque curve and its fuel consumption increased. In the hills on the other
hand, the engine had to develop more power while climbing the hills and
that became more fuel costly. However, as the vehicles' G.V.W. increased,
the increased fuel demand to climb hills became more than compensated for
by the increase of vehicle(s) inertia which helped to propel both units
down the slope at much lower engine power. It follows, that for weights
over 30000 Kg G.V.W. it becomes advantageous dispatching trucks through
ConclusionThe data presented here is limited to International 9200i trucks with the 10-speed Eaton UltraShift automated transmission. Many trucks with Cummins ISX435ST engine might show similar fuel consumption characteristics, however the transaxle gear ratios are an important determinant in which portion of the torque curve the engine is operated at any given instant.
We did not find any support for the claim
that the automated transmission would contribute significantly to an increase
in fuel economy in line haul operations. The construction of the Eaton
UltraShift automated transmission is similar to a manual gear box except
that the gears are shifted automatically. Any potential fuel economy could
be realized only through the shift timing, and on our trucks it was identical
to the shift timing we would normally use with a manual gear box. Besides,
for the most frequently encountered G.V.W. between 25000 and 34000 Kg the
average fuel consumption of 41.0 l/100 Km (5.7 MPG) was not all that impressive.
The International ProSTAR equipped with an identical engine and the 12-speed
UltraShift automated transmission fared a little better showing somewhat
improved fuel economy. However, from what we have seen so far, neither
of the trucks with automatic transmission beat the fuel economy of trucks
with the 13-sp manual trannie. During this set of empirical measurements
we realized that the operator's
management technique had more effect on our fuel consumption than the
gear shifting timing would have.