An Editorial by dieselmann
Volume 10; February 01: Fuel Economy Concerns
The past summer's fuel price fiasco, and the likelyhood of increased prices in the coming months has increased the concerns of poor economy. Most people purchase diesel vehicles for their reputation of being economical and ability to haul heavier loads than gasoline -powered vehicles. Unfortunatley, the customer also wants a vehicle that will perform as well as a gasoline vehicle, and this is the source of many poor fuel mileage complaints.
If the PowerStroke was left in its original, International configuration, top RPM would be limited to 2600 RPM as was the case with the older IDI engines. The engine would have plenty of horsepower and torque, but you would not have the acceleration which is present in the Ford version of this engine. However, fuel consumption would be less.
Fuel consumption is directly related to the demand placed on the engine. Driving at a steady speed of 55 unloaded is going to require less fuel than if you were to travel at the same speed with a trailer attached because the engine is going to require more fuel to maintain that speed. Similarly, in town driving unloaded is going to use less fuel than if the truck were loaded because the engine is going to need more fuel to accelerate from a stop.
Those are both examples of normal increased fuel consumption. Abnormal increases are usually due to something which affects the performance of the engine. A plugged fuel filter or weak lift pump, since this would limit fuel to the engine, would naturally make you think that fuel economy should increase, but as you would compensate for the lack of performance with increased presure on the accelerator, the engine is going to use more fuel to perform the task. This would be the same result if the air filter were resticted, or the tire pressures low. Of course, there is also the case of lost fuel which is unrelated to the demand on the engine, such as internal or external leakage.
Recently, I had contacted Ford's technical assistance HOTLINE to report a customer complaint of poor economy which I was unable to verify--he reported 12 MPG, I was able to get 16.5 MPG. Here are the main points they touched on:
- For every 5 MPH of road speed above 55 MPH, you can expect a loss of approximately 2 MPG.
- This relates to the load placed on the engine. Higher road speed results in increased drag on the vehicle. This drag effectively increases the weight the engine is attempting to push down the road. Add to this the surface area of a camper or trailer, and fuel consumption increases further. In addition, the higher RPM's of the engine requires an increased amount of fuel.
- Driving at speeds below 50 MPH in overdrive requires more fuel.
- This again is a load-related increase in fuel consumption. You would think that the lower RPM of overdrive would result in better economy, however, there is less torque available to the rear wheels in higher gears. This is why automatics downshift on hills, or why you would have to shift to 4th gear to maintain speed on a hill. I have checked the mass fuel desired (MFDES) reading on the scan tool, and fuel demand in overdrive is higher than in the next lowest gear when maintaing a steady speed below 50 MPH:
Speed
55
60
65
50
45
45 | Gear
OD
OD
OD
OD
OD
Drive | MFDES
22 MG +/- 2
26 MG +/- 2
31 MG +/- 3
20 MG +/- 2
13-29 MG*
15 MG |
*at 45 MPH in overdrive the MFDES fluctuates widely as you try to maintain speed and adjust for traffic. These readings are average for several different trucks and vary depending on weight, axle ratio and model/year. If you try to accelerate when below 50 MPH, fuel demand is even higher and it takes longer to reach the desired speed in overdrive than if you downshifted. Select the appropriate gear for the driving situation. Don't over-rev, or in this case, lug the engine.
- Every hour of idling consumes as much fuel as 30-50 miles of driving.
- Trucks that are used for commercial or industrial purposes many times spend more time idling than they do on the road. Any time the engine is running, fuel is being consumed, but without compensating for this time when calculating your economy will provide a false result. These vehicles should have an hour meter which is active whenever the engine is running because these extended idle times are reducing the service life of the oil--every 100 hours equals 6000 miles of driving. Personal vehicles don't need to be idled as much, but sometimes if they are being used for recreation or work, the engine is allowed to idle to provide comfort for the owner (heat; A/C). Asside from the dangers of engine damage from extended idling, your fuel consumption is also suffering.
Now, in situations were fuel economy has inexplicably decreased suddenly, or degraded over time, the vehicle is going to have to be diagnosed or inspected to determine what has changed.
Mechanical or system problems that affect performance are going to decrease economy. Usually these problems are going to involve fuel delivery--fuel pressure and volume, filter; air flow--restricted air filter or exhaust, turbo performance from damage or exhaust leaks; or injection control--HP oil system, sensor problems. The engine and controls need to be diagnosed to determine if there is a fault. It is also going to have to be determined if something has been installed which may be affecting how the engine runs--aftermarket fuel filter which may have reduced fuel flow; bug shield which may restrict the air cleaner inlet when the hood is closed; defective oil filter; auxillary fuel tank.
Also, the shop is going to have to look at the entire vehicle to determine if there is a mechnical problem or if something has been done to alter the load placed on the engine. Things that can affect load would be larger diameter tires or lower gear ratio which reduce the torque at the rear wheels, as well as change the speedometer calibration; wider or aftermarket tires which have a higher rolling resistance than the original tires, or low air pressure; lower stall speed on the torque converter, or no torque converter unlock under load; dragging brakes; extra weight added to the vehicle, such as tool box or auxiliary fuel tank; binding drive axle components or locked 4X4 hubs.
Also to consider would be alterations to the outside of the truck which could increase air resistance. Air resistance on the vehicle can increase if the vehicle's ride height is increased by installing larger diameter tires, a lift kit or any of the following: running boards, bug shield, windshield visor, light bar winch bumper, or camper shell. Of course, too, a properly designed camper shell, or a toneau cover, can decrease the air resistance caused by the pickup bed and actually improve your fuel mileage.
Assuming there is no problems with the vehicle, there are two ways to improve your fuel economy. The first is the easiest (for some) and the least expensive. You need to modify your driving habits to prevent excessive fuel consumption. This means being lighter on the throttle in traffic, using the appropriate gear for the driving situation, reducing speed during highway driving, especially when towing, and avoiding unnecessary idling.
The other method is to improve the efficiency of the engine by installing performance enhancing devises, beit use of a chip, propane enrichment, or increasing air flow through the engine. Keep in mind, though, high performance also means high fuel consumption. If you hotrod an unmodified truck, chances are your economy is not going to improve if do the modifications.
Changing the axle ratio can be a way of improving your economy. But if you decrease the ratio too much, the engine can't produce the required torque, and your mileage is not going to improve. There auxiliary over/underdrive transmissions that serve to double the available gear ratios, so there is not as large of a drop in RPM and torque as the engine upshifts.
It is also possible to improve your economy with synthetic lubricants which reduce the friction of parts in the powertrain, and/or fuel additive that improve the combustion of the fuel while preventing potential fuel system problems which could affect performance.
Charles David Ledger; dieselmann©1999
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