It is widely understood that the period of time between when the engine begins to turn at startup and the time when fresh oil from the sump reaches moving parts causes up to 90% of the wear an engine sees during its lifetime. A prelube pump puts the oil where it is needed before the engine begins to move, thus reducing this high wear period.
Historically operators of large engines would avoid shutting them down. This was done to reduce startup engine wear in addition to other operational advantages. In colder climates for instance, letting an engine cool all the way down can cause significant problems when trying to restart it. Fuel “gels” and very cold engines can be nearly impossible to start. Until recent years the simplest and most widely used solution to this problem was to just leave the engine running. Large engines might only be shut down for periodic maintenance. Fuel was inexpensive and environmental concerns were not considered. As the price of fuel climbs ever higher and idle emissions are no longer acceptable, operators are shutting down engines more than ever before.
Fuel costs as well as the EPA and other regulatory agencies are increasingly requiring engines to go into “auto start” mode rather than just idle. In auto start mode the engine starts its self and runes only long enough to warm the fuel, coolant, oil and the engine block back up to operating temperatures. The engine then shut itself back down to save fuel and emissions. As a result the number of times some engines need to be started in their lifetime may go up from only 80 or 100 to as many as 86,000 times. It’s easy to see how wear rates that were considered acceptable with only 80 lifetime starts would have a significant impact on engine life with 86,000 lifetime starts.
In the end the decision as to whether a prelube system is appropriate for a given installation comes down to the cost benefit analysis of three factors. The number of times the engine will need to be started in its lifetime, and the cost of replacing/rebuilding the engine, against the cost of adding the prelube system.
As an example a good prelube system may run in the range of $1500 by the time it is installed. So if the engine is small and only costs $10,000 a prelube may be impractical. As the engine size goes up in the calculation though, a $1500 investment could save many times its price in future repairs and downtime. Likewise if an engine were only going to be shut down every three months for maintenance, the cost of a prelube system might be unjustified but for a system that is restarted multiple times a day it would be a necessity.
Wednesday, December 5, 2007
Considerations in wiring power to a prelube pump
A prelube pump is operated prior to cranking the engine. Its purpose is to fill the oil passages with oil prior to startup. The power to operate the prelube pump comes from the starting battery or from some other auxiliary power source.
The simplest wiring system consists of a pushbutton switch to operate the prelube pump. The operator pushes the switch for the appropriate length of time and then starts the engine. Most prelube pumps draw too much current for an ordinary pushbutton switch, so an electric relay is usually added to the circuit, as shown in the diagram below.
In automating the engine prelube, there needs to be some measurable condition that indicates that the prelube function is complete. One possibility is to check the oil pressure. This is the fastest way to safely start the engine. If the oil pressure is not zero, then you can start the engine. But, most prelube pumps are too small to significantly raise the oil pressure in an engine of any size, especially if the engine has been run recently and the oil is still hot. While these pumps may fill the passages with oil they cannot raise much pressure within the engine because of the many places the oil must go, such as spraying cooling oil to the underside of the pistons, etc. Therefore, unless a large prelube pump is used, checking oil pressure as a condition to start the engine is problematic.
If an oil pressure switch is used to signal prelube completion, make sure that the prelube circuit is disabled after the engine starts. This is to avoid the situation where the oil pressure gets low enough to restart the prelube pump, such as when a hot, well used engine is running at idle. At this point the prelube pump could cycle endlessly as it adds enough oil to bring the oil pressure up only to be shut down again.
Another measurable condition is time. One could run the prelube for, say 5 seconds, and then start the engine. The actual time would depend upon the pump, its plumbing, and the engine. There are several options here.
1. Prelube for longer than anyone would think is enough. Advantage, for sure the passages have oil in them. Disadvantages, longer time until start, and extra drain on the battery.
2. Find out experimentally how long it takes for the oil passages to have sufficient oil. Prelubing a cold engine will probably cause some pressure increase when the oil passages become full. At that point an oil pressure gauge may indicate a little pressure or the sound of the pump may change a little. It is a reasonable guess that a warm engine with hot oil will take about the same time to fill the passages. Advantage, the time to start is shorter and the drain on the battery from the prelube pump is less. Disadvantages, it is just a guess and certain situations may require longer prelube, such as when the engine oil has been changed.
3. One might also remove an oil port at the furthest point possible from the oil filter and see how long it takes for the oil to start flowing out. Advantages and disadvantages, the same as #2 above.
4. If the engine has a glow plug cycle, the prelube pump circuit can be connected there to provide a prelube cycle the same length as the glow plug cycle. Advantage, easy to do and requires little extra circuitry. Disadvantage, the glow plug duration may not be optimal for prelubing.
The diagram below shows one example for wiring a prelube pump using a timer.
When all is said and done, how accurate does the length prelube cycle need to be? What happens when the prelube cycle is too short? One must realize that any prelube at all is better than none. So, if the prelube time is a bit short, the oil passages will still have gained additional oil. As the engine starts, the passages will fill faster than if the prelube was not done at all. There will always be some benefit.
The simplest wiring system consists of a pushbutton switch to operate the prelube pump. The operator pushes the switch for the appropriate length of time and then starts the engine. Most prelube pumps draw too much current for an ordinary pushbutton switch, so an electric relay is usually added to the circuit, as shown in the diagram below.
In automating the engine prelube, there needs to be some measurable condition that indicates that the prelube function is complete. One possibility is to check the oil pressure. This is the fastest way to safely start the engine. If the oil pressure is not zero, then you can start the engine. But, most prelube pumps are too small to significantly raise the oil pressure in an engine of any size, especially if the engine has been run recently and the oil is still hot. While these pumps may fill the passages with oil they cannot raise much pressure within the engine because of the many places the oil must go, such as spraying cooling oil to the underside of the pistons, etc. Therefore, unless a large prelube pump is used, checking oil pressure as a condition to start the engine is problematic.
If an oil pressure switch is used to signal prelube completion, make sure that the prelube circuit is disabled after the engine starts. This is to avoid the situation where the oil pressure gets low enough to restart the prelube pump, such as when a hot, well used engine is running at idle. At this point the prelube pump could cycle endlessly as it adds enough oil to bring the oil pressure up only to be shut down again.
Another measurable condition is time. One could run the prelube for, say 5 seconds, and then start the engine. The actual time would depend upon the pump, its plumbing, and the engine. There are several options here.
1. Prelube for longer than anyone would think is enough. Advantage, for sure the passages have oil in them. Disadvantages, longer time until start, and extra drain on the battery.
2. Find out experimentally how long it takes for the oil passages to have sufficient oil. Prelubing a cold engine will probably cause some pressure increase when the oil passages become full. At that point an oil pressure gauge may indicate a little pressure or the sound of the pump may change a little. It is a reasonable guess that a warm engine with hot oil will take about the same time to fill the passages. Advantage, the time to start is shorter and the drain on the battery from the prelube pump is less. Disadvantages, it is just a guess and certain situations may require longer prelube, such as when the engine oil has been changed.
3. One might also remove an oil port at the furthest point possible from the oil filter and see how long it takes for the oil to start flowing out. Advantages and disadvantages, the same as #2 above.
4. If the engine has a glow plug cycle, the prelube pump circuit can be connected there to provide a prelube cycle the same length as the glow plug cycle. Advantage, easy to do and requires little extra circuitry. Disadvantage, the glow plug duration may not be optimal for prelubing.
The diagram below shows one example for wiring a prelube pump using a timer.
When all is said and done, how accurate does the length prelube cycle need to be? What happens when the prelube cycle is too short? One must realize that any prelube at all is better than none. So, if the prelube time is a bit short, the oil passages will still have gained additional oil. As the engine starts, the passages will fill faster than if the prelube was not done at all. There will always be some benefit.
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