MBE 900 – Section 15.17 Poor Fuel Economy

Section 15.17
Poor Fuel Economy

Customer expectations of fuel economy are usually based on past experiences, recommendations from colleagues or sales staff, and wishful thinking. It is paramount that the end user attain reasonable expectations, based on sound predictions. Fuel economy management can be accomplished with the aid of Detroit Diesel's Spec Manager® computer program. The program takes into consideration a number of factors such as vehicle design, duty cycle, and route profile as it predicts real world fuel economy. Contact the local Detroit Diesel distributor for Spec Manager support, prior to contacting the factory. Make sure that there is sound basis for the poor fuel economy complaint. These precautions applies to both new and in-service vehicles.‪

There are many variables which may affect fuel economy. Included are:‪

  • Vehicle Frontal Area
  • Vehicle Weight
  • Rolling Resistance (Static and rolling)
  • Air Resistance (Excessive add-ons increase air resistance; trailer gap too large)
  • Engine Speed
  • Tire Size (Revs/mile)
  • Axle Ratio
  • Transmission Gear Ratios
  • Road Surface (Cement, blacktop, gravel)
  • Duty Schedule (Highway, stop-and-go, excessive idling)
  • Vehicle Maintenance (Plugged air filter, low tire pressure, poor wheel alignment, brakes dragging)
  • Vehicle Cruise Speed
  • Driver Habits
  • Weather Conditions

Note: If any of these variables is not optimized, a serious penalty in fuel economy will result. Many of these items are out of the operator's control. However, there are factors that can be controlled and have a major impact on fuel economy.

Section 15.17.1
Troubleshooting Guidelines for Vehicle Cruise Speed

One of the most common factors that reduces fuel economy is vehicle cruise speed.‪

Maintain as slow a cruising speed as possible. Listed in Table "Fuel Economy Reduction based on Cruising Speed" are the approximate percentages of fuel economy reduction based on vehicle cruising speed.‪

Vehicle Cruise Speed (mph)‪

Approximate Reduction in Fuel Economy (%)‪















Table 1. Fuel Economy Reduction based on Cruising Speed

Section 15.17.2
Troubleshooting Guidelines for Excessive Engine Idling

Another factor to consider is excessive engine idling. Aside from the fact that the operator is getting 0 mpg during idling, combustion is less than ideal. Poor combustion can compromise the efficiency of the turbocharger. Regarding the notion that idling maintains cab heat in the winter, some tests have shown that during the first hour after full load operation, the engine actually cools down at a faster rate when idling than when turned off. Therefore, to increase fuel economy minimize engine idling.‪

Section 15.17.3
Troubleshooting Guidelines for Fuel Economy in Cold Weather

It is not uncommon to experience a loss in fuel economy during the winter. Cold ambient conditions bring on more engine idling and increased resistance in moving parts such as engine, transmission, and axle, due to fluid viscosity increase. There is also the issue of winter-blended fuel. Blended fuel typically does not contain as much energy as summer fuel. Therefore, it takes more fuel to move the equipment. Tests have shown that there can be as much as a 7% penalty in fuel economy due to blended fuels. Therefore, to save on fuel economy, comparisons must be made for the same yearly time frame.‪

MBE 900 Service Manual - 6SE414
Generated on 10-13-2008

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