1993/02/15 Caterpillar

Caterpillar Truck Engines

Fuel, driver, truck payment, insurance and general maintenance are all known "costs of operating a truck". These are the expenses the most efficient fleets and owner/operators try to minimize and keep under their control. Another "cost" that is often not addressed or given little emphasis is low idling operating expense.

"Low idle operating expense" can be divided into several categories. This article reviews the three primary categories:

* Direct Cost (More obvious-Fuel Consumed)

* Indirect Cost (Less obvious-Engine Wear)

* The affect of idling on tank mileage

Direct Cost

Fuel consumed at idle in Caterpillar Truck Engines (3306, 3176 and 3406) varies from .35 gph to .42 gph in lab conditions. In-the-truck idle fuel consumption has been documented from .55 to .60 gph at 700 to 750 rpm. If the "low" idle rpm is raised to 1000, the in-the-truck fuel consumption will increase 50% (from .60 to .90 gph). Accessories will also add to the fuel consumption. See the following chart. Even though the actual fuel consumption is low, the cost is real.

It is difficult and/or expensive to accurately measure exactly how much a specific mechanically governed truck idles but with the electronics systems available today, truck owners can tell exactly the idle time on each truck. Currently, typical coast-to-coast operations with 2-man driving teams, have indicated that during the time the engine is running, 10 to 30 percent is idle time. For single drivers, 25 to 50 percent is idle time.

For example, if a truck accumulates 100,000 miles per year and idles 8 hours per day, 5 days per week, 50 weeks per year and consumes 1.10 gph, this truck's engine will consume 2,200 gallons of fuel per year. At $1.20 per gallon, thats an annual cost of $2,640. The direct fuel consumed from operating 100,000 miles per year at 6.5 mpg is 15,384 gallons. At $1.20 per gallon, the annual fuel cost is $18,462. If you add in the $2640 cost of idling, fuel consumption cost increases almost 15 percent per year. In this example, the fuel consumption cost increase to $21,102 per year. In many fleet records, this 15 percent fuel consumption increase, will not be subtracted from the "over-the-road" fuel consumption calculations. In this example, if the truck normally gets 7 mpg "over-the-road", the actual fuel consumption could easily be lowered 0.8 mile per gallon. (see Tank Mileage Fuel Consumption)

Indirect Costs

Although studies have been done to determine how much wear results when a truck idles, the bottom line is hard to quantify. Caterpillar studies indicate low engine operating temperatures at idle will allow water vapor to form and condense in the crankcase. The water in the crankcase will combine chemically with sulfur oxides (a by-product of combustion). This combination will result in sulfuric acid. The acid can damage bearings, cylinders, piston rings and valve stems, just to name a few. This damage will shorten engine life.

The cooling system on a typical diesel engine is designed to operate at 175 to 190 °F. Excessive cooling, which can result from idling, can decrease the running temperature by 10 °F. This can lead to soot and carbon deposits forming on piston rings, valve stems and injectors. This can result in increased oil consumption, low power and accelerated engine wear. This is difficult to quantify but very real.

As a general rule, a Caterpillar 3406C Truck Engine will consume 100,000 gallons of fuel before requiring an overhaul. At 7 mpg, that equates to 700,000 road miles or at 100,000 miles per year, 7 years to overhaul. If the truck idles 2,000 hours per year, consuming 1.1 gph, (see the earlier example) this engine will consume 15,400 gallons at idle in seven years. Subtract 15,400 gallons from the 100,000 gallon "overhaul target".

As a result, the engine will burn 84,600 gallons "over-the-road" and 15,400 idling before overhaul. This means that the engine may now require an overhaul in less that 6 years, not 7 years without idling. In other words, a direct result of idling could be 15 percent reduction in engine life.

Tank Mileage Fuel Consumption

Vehicle fuel consumption "tank mileage" can be adversely affected by excessive engine idling. Many fuel consumption complaints are the result of fleets or owner/operators that may not even consider quantifying fuel consumed at idle before calculating tank mileage. The included graphs will help show the variability that idling can have on fuel efficiency/consumption.

The first graph, titled "PERCENT IDLE TIME" can be used to determine what percent of the total trip time that a vehicle is at idle. The horizontal axis represents the total accumulated engine hours for a given trip (Total Trip Time). The vertical axis represents the total time that the engine is running but the vehicle is not moving (Total Idle Time). When these values are know, the graph can be used to determine the percent idle time. Idle times from 30 to 50 percent are not uncommon, even for coast-to-coast operations.

The second graph, titled "TANK MILEAGE CALCULATION" addresses two specific but different idle conditions. The horizontal axis represents Idle Time Percentage (obtained from the first graph). The vertical axis represents Tank Mileage (mpg). One over-the-road example averages 7.0 mpg while the other averages 6.0 mpg. Both conditions graphically display two different idle fuel rates. One rate is .75 gph (700 to 750 rpm low idle, no accessories) and the other higher fuel rate is 1.1 gph (1,000 rpm low idle, with driver comfort accessories and vehicle marker lights on). The graphs dramatically show that even small percentage changes in idle time can cause significant reductions in tank mileage.

Summary

Today's electronic engines can accurately measure fuel consumed in both idle and over the road conditions. Unnecessary idling adds both direct cost (fuel consumed) and indirect cost (engine wear).

Caterpillar electronic engines offer idle limiting features which can help monitor/manage unwanted idle time. Other devices offered by other manufacturers, such as auxiliary generator sets or heaters/air conditioners, offer alternatives to engine idling to provide the same desired results - driver comfort. Caterpillar does not endorse any of these alternative devices but we would expect they can have value in some applications/operations.

For some operators, extended idle times may have a "short term" direct cost benefit when compared to motel room expenses. But, for the "long term" be sure to consider both the direct cost (fuel consumed) and the indirect cost (engine wear) when deciding how long to idle.