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Nice to See This
It has been a while since I have seen my Detroit Diesel monitor over 8 MPG average. When I first bought the bus in 2008, the average was Always well above 8. I did not pull a toad the first 6 months and I was too shaky behind the wheel to drive above 62. As I became used to driving the bus, my average speeds increased and I routinely set my cruise at 70 on wide open interstate. That is about the same time I started pulling my first toad and even though it was less than 5000 lbs, the extra drag and increased speed knocked the average to the high 7’s and occasionally 8.
This includes idling and I am not afraid to let it idle when we are stopped for and hour or so. I assume generator usage affects the average too but I would appreciate clarification on that.
In 2011 I pulled a 9000 lb trailer part of the year and since early 2012 I have pulled a 16,000 lb trailer part of the year. Even though I drive no more than 65 while pulling the trailer, the average on the Detroit Diesel monitor went south and stayed south. Even with at least half my annual miles being without the trailer I have pretty much stayed in the high 6.8 – 7.3 for a yearly average. I reset in every new year.
This year I reset it in Texas and have driven in east Texas, Louisiana, Mississippi and Alabama so far this year. I have been in no mountains, driven on the interstate very little and pulled only my 2400 lb car. The Detroit Diesel monitor quickly went over 8 and has stayed there. I have driven less than 2000 miles and I know that is not near enough miles to be an extremely accurate sample but it is still nice to see it.
Unfortunately I will need to hook up to the tent trailer and pull it at least 900 miles toward the end of next month. That will change things.
Davy
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6 Replies
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Combine great mileage with low diesel prices and everytime you fill the coach you end up feeling like you are stealing fuel.
It would be great if we all got that mileage and if the price of diesel got below $20. When we got our first bus fuel cost $.63. Of course that was some time just after the wheel was invented.
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Not wanting to thread drift here but being interested in comments this might engender,…keying off of the statement that the diesel is allowed to idle for an “hour” or so… if you go back through some of the Detroit releases relating to this practice, you may find that they recommend that this not be done …beyond 5 minutes or so if idling can be reasonably avoided….not including while in traffic.
The diesel engine is inefficient while idling and unburned fuel can easily “pass by” the rings and enter into the oil in the system. It is my understanding that this dilution, when left to sit as we sometimes do with our coaches, promotes the formation of sulphuric acid which attacks the bearings of the engine.
I know that truckers do it all of the time and it is my understanding that they pay a handsome price for doing so… if the idea is to keep the engine warm in adverse conditions, it has been shown that even in freezing temperatures, after a 30 minute shutdown, the main crank is still very, very hot …basically unchanged from the moment of shutdown… don’t know but I have read the studies… and I don’t travel anywhere in cold weather as a practical matter…
The recommendation I follow when stopping for a period of time is to idle down for 2 to 5 minutes allowing the turbo to wind down before shutting off the engine. This also helps to cool down the engine in hot temperatures.As I have said many times, I am not the expert…..
Just some thoughts….
Pres
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I would be interested in reading studies related to idling especially concerning the issues you raise. Most opinions I have read against idling are coming more from the stand point of saving fuel or cutting noise polution and emissions issues.
My thinking on this issue was framed by entertainer coach owners. They were my only Prevost source before Prevost Community. They told me any stops less than two hours, they let it idle. My cut off time on that has been an hour.
They also let it idle 15-20 minutes after a several hour hard run. I generally do the same after a long day run. Usually by the time I have pulled into a church parking lot, unhooked the car, looked over the situation and manuvered into the place I need to be, that 15-20 minute span is long gone so I shut down when parked.
I have been known on Really cold nights, like say below 20 with a lot of wind, to let it idle while I slept 4-5 hours. No worries in the morning so I sleep better the few hours I am there.
Again, my perspective was shaped by entertainer coach owners and they are admittedly not experts on Detroit engines. Although they are very sensitive to the costs of doing business, both in short term fuel cost and long term maintenance and equipment costs. They can tell you exactly what it costs to roll each wheel down the road per mile.
I am open to learn.
Davy
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I would bet money Davy was on high idle.
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Yes, Tom, I never idle on low idle except for the first few minutes and the last minute or so.
Davy
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Let’s separate the apples from the oranges.
When we have been running down the interstate the turbo is working. Think of it as a pump and that pump takes the energy from the flow of exhaust and uses it to compress the air going into the engine.
The harder it runs, such as when trying to maintain interstate speeds, the hotter the exhaust driving it, and the greater the heat of compression from the work it is doing. That heats up the entire assembly. Oil is pumped by the engine past the bearings. The temperatures of the case and bearings are quite hot, in excess of what the oil can tolerate, but since the oil is flowing it is not getting as hot as it takes to damage or cook the oil.
But when you stop in a rest area for example if you shut down the engine you also stop the oil flow. But if the turbo has not had a chance to cool down the oil that is now sitting at the bearings literally burns. It ends up being a black charred deposit that is cooked into the area of the turbo that is in dire need of constant lubrication. Over time if that happens often enough you get to buy another turbo. So an idle at low RPM allows the heat in the turbo and its bearings to dissipate and you do not get that “coking” of the oil. The turbo stays in good condition. How long you have to idle depends on a lot of things. If you have been driving slow and not been generating turbo boost you might be able to just shut down. But if the rest area is at the top of an eight mile hill that required you to peg the boost gauge the entire 8 miles that turbo needs a long cool down period.
Now let’s talk about the other reason for idling. If you want to idle all night long so you can use bus heat or AC that is fine as long as you are on high idle. Pres talked about low idle not being good. That is correct. But high idle should generate the heat and conditions required to keep the engine healthy and not subject to the damage of cold engine temps, raw fuel unburned, or discharge of batteries due to current draw from the OTR system and coach lights.
For our purposes I could not justify running the engine for extended periods because we all have generators and they are ideal for providing us with the power needed to stay comfortable or keep our batteries charged. But that is only my personal stance on engine idling.
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