Extending oil drain intervals in on-highway applications is often possible with the right quality engine oil. These engines typically operate in light-duty service and under constant operating conditions in cruise control. The major issue is the long idle time that accelerates oil contamination. These contaminants result in the used oil becoming aggressive to lead contained in bearing and bushing materials and overlay.

Extending oil drain intervals in off-highway engines can be more challenging. Dirt ingress and coolant leaks occur more frequently. At times, oil drain intervals are established to minimize damage from such problems.

Off-highway engines' operating severity varies greatly because a particular type of engine can be installed in a range of equipment with different operating parameters and severity. For example, dozers may operate in light- or severe-duty applications. Fuel consumption rates on a particular piece of equipment can vary by 30% depending on the work being done. This can have a significant impact on oil drain intervals.

Properly defined optimized oil drain intervals have proven successful in mining applications. Field tests show that engines achieved acceptable life without reliability problems. Inspections of these high-hour engines showed normal wear and deposits.

Engine oil performance Engine oil performance is usually measured on-highway in terms of miles to engine overhaul. With any oil, it is a matter of time, gallons of fuel consumed, before the additives are depleted and the oil becomes saturated with contamination from combustion byproducts. Better performing oils are capable of neutralizing and suspending more contamination. This permits more gallons of fuel to be consumed prior to an oil change. Better performing oils are capable of providing longer oil drain intervals.

Fleet oils are generally designed based on cost targets. These oils meet all the API service classification requirements and give engine design life and reliability when used at conservative oil drain intervals (Figure 1). Extended service interval (ESI) oils can produce equivalent engine life and reliability at almost twice the oil drain intervals possible with fleet oils (Figure 2).

The used oil condition in an engine depends on combustion conditions, oil volume, gallons of fuel consumed, and the oil consumption rate of the engine. Any oil, regardless of quality, will need to be changed after the engine has consumed the maximum gallons of fuel.

Optimized oil change intervals In establishing optimized oil drain intervals in off-highway applications, the cost reduction associated with longer oil and filter change intervals must be balanced against the risk of shortened engine life and the cost associated with less reliability if oil drain intervals are extended too far.

The process of establishing extended oil drain intervals is the same as that used for on-highway engines.

* construct a fleet profile;

* estimate oil drain intervals and potential cost savings associated with the optimum oil drain intervals;

* assess the risk associated with the optimum oil drain intervals;

* establish a testing plan for a small segment of the equipment that mirrors that in the fleet to determine if the estimated oil drains are acceptable; and

* after the testing, implement the optimized oil drain intervals.

Fleet profiles describe the equipment, service severity, operating conditions, and maintenance practices. Different engine models contaminate engine oil at different rates. The same engine may have a different oil volume in different equipment. Fuel and oil consumption rates differ for different equipment and different service severity. Idle time varies.

The oil drain capability of the oil and the engines must be considered to properly estimate oil drain intervals for the equipment.

A gallon of fleet oil typically possesses the ability to neutralize and suspend the contamination generated by burning about 300 gallons of fuel. Premium oils have a cap at about 400 gallons of fuel, and extended service interval oil about 500 gallons of fuel.

Based on limited testing, today's oils that meet the requirements of the API service classification CH-4 appear to provide a 10% to 20% improvement over the prior quality API CG-4 engine oils. The oil drain capability of the engine depends on the rate at which the engine contaminates the oil, total engine oil volume (sump plus filters), fuel and oil consumption, and idle time.

Fuel consumption rates have a significant impact on oil change intervals. Reasonable estimates of engines' fuel consumption rates are necessary to estimate oil drain intervals.

Oil consumption rates of engines are usually not known. For engines with large oil volumes, oil consumption rates do not greatly change the oil drain intervals. The small volume on new make-up oil does not greatly change the properties of the large volume of oil in the engine sump. For engines with small oil volumes, the change in oil properties with oil consumption rate is more significant. If unknown, low oil consumption rates for engines result in safe estimated oil change intervals.

Estimated oil drain intervals indicate that haul trucks with large oil capacity and that operate at lower load factor have longer oil drain intervals. All the other equipment with small oil sumps and operating at different service severity have a shorter oil drain interval.

Estimated economics for the potential service interval alternatives (see Table 1) indicate that a significant cost savings is possible with minimal risk of decreased engine durability or other problems associated with extended oil drain intervals. However, these estimated drain intervals must be properly established based on the fleet operating conditions and variation in these conditions.

Because the operator often estimates fuel consumption, oil consumption or operating conditions, testing the estimated oil drain intervals is essential. Because of variation in the operating conditions, the estimated oil drain intervals can be as much as 30% too long or too short.