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The basic principles of determining how to conserve fuel have been around a long time. Only when the cost of fuel increased dramatically was there a scramble by component manufacturers and fleet operators to use these principles.
Fuel accounts for 35% to 50% of the cost of operating a truck today. There is no magical wand that will alter this situation-only knowledge.
However, top management, in many cases, believes it is just a matter of their vehicle maintenance director or shop supervisor turning some mystical engine screw to save 10% on fuel costs. Maybe this theory has surfaced because engine fuel consumption is going down. Let's examine the major components of fuel consumption, beyond the engine.
Rolling resistance, air drag, and driver operation are the three sources of fuel demand. Their interrelationship is getting more attention today. The related problems have been there all along.
Looking at data for straight truck (van body) operation, it may be of interest to note that when driving at 35 mph, 50% of the power demand is needed to overcome rolling resistance, and 50% to overcome air drag. This same straight truck at 55 mph needs 30% of its power to overcome rolling resistance and 70% for air drag.
A tractor-trailer traveling at 60 mph has four times the aerodynamic resistance (air drag) as the identical unit traveling at 30 mph. The configuration of the tractor and trailer influences these factors. A cattle trailer has the most air drag, caused by the air coming in and out of the trailer's ventilating holes. The least aerodynamic resistance is found in a flat bed trailer without sides.
Cargo on a flat bed trailer, by its shape, can trap air. Plastic or iron pipe, loaded lengthwise on the trailer bed, traps a lot of air. Putting tarps over the forward ends of the pipe will force the air over or under the trailer, thus avoiding the problem of air being caught within the individual pipes.
Using 100% as the base for aerodynamic resistance or air drag, tests have shown a cabover tractor and reefer trailer have the following drag:
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- ... 80% air drag.
- ... 5% skin friction of the vehicles.
- ... 10% interference drag-mirrors, antenna, and protruding components.
- ... 5% internal drag-engine cooling, cab ventilation.
- ... 80% air drag.
The configuration or contour of the tractor and trailer has a great influence on the aerodynamics of the vehicle. Rounded leading edges of the cab and trailer can reduce air drag 17% when compared to square corners. Tractors alone with rounded corners contribute 13% of the 17% reduction. The use of air shields and gap panels on the tractor can reduce air drag another 15%.
The overall theory of aerodynamics of a tractor-trailer or straight truck is to streamline the surface of the vehicle. A smooth flow of air moving along the outer surfaces produces less drag than air making abrupt changes in direction. Little insignificant things, such as open cab windows, catch moving air and add to the overall air drag or aerodynamic resistance. Protruding attachments like mirrors, air cleaners, and even large hood ornaments disturb the smooth movement of air over the vehicle surface.
The reduction of air drag by streamlining the vehicle and the use of air shields, gap panels, trailer nose cones, air dams, etc. will affect your fuel economy. For every 2% reduction in air drag or aerodynamic resistance, you can expect 1% improvement in fuel savings. This is true, at least in theory; however the driver is a major consideration in the final result.
Rolling resistance is equal to 30% to 50% of the resistance in moving the vehicle and cargo. Increase speed and the rolling resistance increases as is true in mechanical functions of this type.
Radial tires compared to bias ply tires have reduced rolling resistance in the area of 15% to 22%. Every 7% reduction in rolling resistance equals 1% improved fuel economy. But again, we have not evaluated the driver's influence.
The reduction of rolling resistance and aerodynamic resistance at 55 mph has reduced the engine power demand. Power demand equates to fuel consumption. Lower demand usually means less fuel consumed. Greater power demand increases fuel consumption.
Today's vehicles-that include the most efficient engines and aerodynamic resistant reduction devices-canget 30% better mileage than older vehicles. This savings is available, but many fleet operators are not realizing the improvement. Why? In our discussion so far, we have not included the negative effects of the driver's operation of the vehicle.
In theory, by reducing air drag and rolling resistance, we are using less engine horsepower at 55 mph. But the driver may use the power saved to increase his road speed. This is really no surprise. Why then do we continue to allow this practice to continue?
Regardless of what most fleet managers give as the reason, it should really be titled "Confrontation with the driver." This underlying problem has been approached by mechanical/electronic vehicle controls such as TRW, ETEC, and others; passive driver control methods where his/her activities are recorded and later reviewed; and the direct approach of asking the driver for help.
In my travels around the country, I have been convinced that a combination of approaches is a practical solution. Always start with the direct appeal to the driver for help. Please note that I said HELP, not reprisal or punishment for every infraction. Secondly, mechanical/electrical controls in the vehicle with the capability of recording driver activities could be used to control as well as instruct the driver.
I am familiar with a fleet where management did not believe they had an engine idling problem until the driver's activities were recorded. Even the driver did not fully comprehend the fuel being wasted by his idling practice. Most of the drivers (reviewing their recorded activities for the day) were honestly surprised that they had idled the tractor an average of 2 3/4 hours per day. After showing the drivers what was happening and asking for help, the idling average was reduced 89%. Calculating 1/2 gallon of diesel fuel per hour used at idle, many gallons were saved by controlling this practice.
Speeding was also a serious problem within this fleet. Management was not dedicated, at the highest level, to controlling speed. This attitude filtered down to the drivers who exceeded speed limits 51% of the time. The speeding did not deliver the cargo any faster, but it provided more time at the cafe for the driver. Fuel consumption goes up 1% for each mile per hour over 55 mph. The fleet's potential fuel economy was 6.3 mpg. The actual ranged between 5.3 to 5.6 mpg.
The most important element in any fuel economy program is the driver. Even with old style engines and vehicles, fuel consumption can be reduced with the help of the driver. Fuel prices are stable now, but there is every indication that if world conditions change because of wars in the oil-rich areas, the price could again rise.
The survival of trucking fleets can rest on fuel consumption. Isn't it time we all got together, management and drivers, to solve this problem? The solution is common knowledge, but we have to do something instead of just talking about it.
Reprinted with permission of GO WEST, August 1985.