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Driving Techniques and Fuel Economy
Automated Transmissions and Highway Safety

Fuel Economy in Highway Transportation

Road Fuel Consumption Profile

International 9200i, Cummins ISX435ST, 10-speed Eaton UltraShift automated transmission


During 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.


The fuel consumption data for hilly terrain and prairies in summer (April through October), and winter (November through March) months are presented in Table 1.
Mountains in Summer Mountains in Winter Prairies in Summer Prairies in Winter G.V.W. (Kg)
38.1 43.6 35.5 Not Available <25000
40.4 45.4 41.8 46.9 25000-34000
44.2 50.4 45.5 47.0 34000-45000
Table 1 - Mean fuel consumption in litres/100Km for assembly of vehicles of various G.V.W.

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. 

Fig.1 - The fuel consumption measured during trips through hilly terrain in summer. Cummins ISX435ST engine with the 10-speed Eaton UltraShift automated transmission. International 9200i trucks driven in team, line haul operations.

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.
Fig.2 - The fuel consumption measured on trips through prairie-like flat terrain in summer. The same equipment as reported in Fig.1.

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 hilly terrain.
Fig.3 - The effect of terrain. The fuel consumption measured on units over G.V.W. of 30000 Kg was consistently lower in mountains than in flat prairies.


The 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 fuel management technique had more effect on our fuel consumption than the gear shifting timing would have.