Usage:
Fuel consumption complaints are related to engine owners expectations. They may often times be related to the engine itself, causes other than the engine, and in some cases the fuel consumption may be normal for the application. Only a good discussion with the owner/operator, as described in the Owner/Operator Input section, will give guidance as to a correct repair or to prevent unnecessary repairs.
The following are some of the questions which should be asked before beginning any diagnosis or repair for an engine performance complaint. There Are No Hard And Fast Answers For These Questions. There are many factors that can cause poor fuel mileage or make users believe they are getting fuel-poor-mileage.
There are also a variety of customer expectations which are acceptable. The answers to these questions will give you a better understanding and perspective on the complaint and may identify characteristics which will help pinpoint the cause of the complaint quickly.
1. Are miles measured accurately?
A most common problem in determining mpg is errors in recording the number of miles traveled.
A. Is this vehicle hub or cab odometer accurate?
The easiest way to check an odometer is to install a hub odometer known to be accurate and for the tire size on the truck. Run the truck over several hundred miles and compare the reading with the original odometer.
Odometers may also be checked by comparing them to interstate mile posts or by running over a course of known length-a 50-mile run is required to get a good check.
B. Are "book miles" or "driver-paid miles" correct?
Another way in which miles for mpg calculations are obtained by a fleet is the use of "book miles" or "driver-paid miles." The use of this system can short mileage accumulation by 10 to 15%; thus, mpg is low by 10 to 15%.
C. Is the vehicle used for pick-up and delivery operation?
Another problem affecting mileage accumulation in many fleets is the use of linehaul equipment for pickup and delivery operations. In the "book mile" system, the truck rarely gets credit for any miles run in pick-up and delivery.
2. Is fuel measurement accurate?
There are a number of ways in which fuel measurement can be the source of mpg problems.
A. Are fuel pumps calibrated?
If fuel tickets come from company-owned fuel pumps, there can be errors because nonrevenue fuel pumps do not have calibration requirements in many states.
B. Are road fuel tickets accurate?
The only way to verify fuel additions when road fuel tickets are used is a laborious ticket-by-ticket audit ensuring that the correct amount of fuel has been entered for the vehicles in question and that there are no indications of incorrect entries.
C. Are tank-full mileage checks correctly done?
The big problem in "tank-full" checks is getting the tank filled to the same level before and after the user "checked the mileage." With two 100 gallon tanks, errors of 5 to 10 gallons are very common in topping off tanks because of the way truck tanks crossfeed with a relatively small common line and the sensitivity to the truck being level.
3. Does the Cat Engine have the comparable rating-hp and rpm?
Obviously, if a competitive engine is rated at 1800 rpm and the Cat engine is rated at 1900 rpm, the Cat engine is at a disadvantage. If a competitive engine is rated at 300 hp and the Cat engine is rated at 350 hp, the Cat engine is again at a disadvantage.
A. Is the wheel horsepower comparable?
When checking wheel horsepower using PAR Level II, to compare Caterpillar and competitive engines, if the Caterpillar Engine has more wheel horsepower or power at higher rpm, the competitive engine has an advantage.
B. Is the maximum vehicle speed comparable?
When you give the driver of the vehicle higher rpm and more power, it gives the vehicle the potential to go faster. The faster the truck goes, the more fuel it will burn.
C. Is wheel horsepower and vehicle speed higher than unit with better mpg?
If the Cat Engine is set to specifications and this does not equalize the wheel horsepower and vehicle speed, the use of an alternate lower horsepower rating-when available for the Caterpillar Engine should be considered.
4. Are the tractor specifications comparable?
Often, a general discussion or questioning of a tractor's specifications will uncover a significant difference which leads to differences in mpg results.
A. Tires?
The difference in fuel efficiency between radial and bias-ply tires is well known. A vehicle or a fleet of vehicles that are on bias-ply tires will have worse fuel consumption than those on radial tires. Also, tire size changes have the same effect as rear end ratio changes.
B. Rear end ratio?
One objective in choosing a rear end ratio for optimum fuel consumption is to limit the engine rpm at the user's desired road speed. Normally, a higher ratio (lower numerical number) will yield better fuel consumption at a given speed. However in some situations, the higher ratio can give additional vehicle speed which will hurt the fuel consumption if the higher potential vehicle speed is used.
C. Transmission ratios?
The transmission ratio difference which has the greatest effect is an overdrive transmission versus a direct transmission with the same rear end ratio. Obviously, the overdrive ratio allows the vehicle to go faster which can hurt mpg; but overdrives can be used to reduce average engine rpm at a low vehicle speed which helps mpg. Therefore, the same situation exists as with rear end ratios. What can be good in one application can be bad in another.
The number of gears in the transmissions can also be significant. The effect of the number of gears depends on the skill and motivation of the driver. Again, general rules do not always apply but, it would be expected that less skillful drivers would get better mpg results with 7- or 9-speed transmissions than with 13-speeds. However, a very skillful driver may be able to get better mpg with a 13-speed.
D. Temperature-controlled fan?
A malfunctioning or poorly engineered temperature-controlled fan can be a very significant contributor to an mpg complaint. An appropriate question for all mpg complaints is "does the temperature controlled fan run often?" If the answer to that question is "yes," normally there is something wrong with the way the temperature-controlled fan is installed or engineered or there has been a system malfunction.
E. Cab aerodynamics or cab style?
There can be significant differences in aerodynamics, and therefore, mpg between two cab designs. The effects are not always predictable.
When cab designs of two vehicles are different, it is difficult to make comparisons of or prove that engines are the source of mpg complaints.
F. Air deflector and air deflector setting?
Use of wind screens or air deflectors is very common today. Obviously, different brands of air deflectors may have different performance. Also, some deflectors may be adjusted to various settings which affect their performance.
G. Gap between back of cab and trailer?
The performance of air deflectors and the fuel consumption of tractors without deflectors are greatly influenced by the distance between the back of the cab and the front of the trailer. The wheelbase of the tractor, and therefore distance between the back of the cab and the front of the trailer, will significantly affect mpg. The closer the trailer is to the tractor, the better the mpg will be.
5. Is the operation the same for all units?
For dump trucks, mixers, garbage trucks, etc., variations in the operation that can be very difficult to find may have very significant effects on mpg.
A. Assigned or slip seat drivers?
With assigned drivers, the driver's driving habits are applied to the vehicle directly. The assigned driver can be the total problem. In a slip seat operation where different drivers drive the truck every trip, the effect of the driver on fuel consumption is essentially eliminated.
B. Routes?
If vehicles consistently run different routes, there is an effect on mpg.
C. Equal loads?
If one vehicle is consistently at a significantly higher gross weight than another vehicle, it will have poorer fuel consumption than the lighter unit.
D. Assigned trailers, trailer aerodynamics, and trailer tires?
If one tractor always pulls a vertical rib trailer and another tractor always pulls a smooth-sided trailer, the tractor pulling the smooth-sided trailer has an advantage as far as fuel consumption is concerned. The same is true if one trailer has radials and another trailer has bias-ply tires; if one trailer is properly aligned and another is not; or if one trailer is higher than another.
E. Operational changes and weather changes?
Mpg complaints can result from operational changes. Moving trucks from one location to another can have a dramatic effect on fuel consumption.
Changes in the weather will also dramatically change fuel mileage. An industry rule of thumb of 10 to 15 % loss in fuel mileage from summer to winter is a close approximation of actual results for fleets that run throughout the country.