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Heavy Truck Transmission and PTO Questions


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  #31  
Old 10-11-2017, 11:53:24 PM
Troll Troll is offline
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Default Re: Heavy Truck Transmission and PTO Questions

Long ago I worked on an MCI motorcoach belonging to none other than Michael Jordan the basketball player. That coach had a 10kw Kohler set buried in a compartment just in front of the left front drive wheels. When this generator was running you could barely hear it under the floor of the coach when right above it.

I think an Onan 10kw "Quiet Diesel" set would do you right fine myself if you had side access panels or were willing to make access for mounting one of these.
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  #32  
Old 10-12-2017, 09:40:19 AM
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Default Re: Heavy Truck Transmission and PTO Questions

Pto's are also rated by percent of engine rpm desired. In other words if you want a pto to run at 1800 rpm to direct couple to a two bearing four pole generator and have the engine run at say an economical 1200rpm, you would need a pto spec'd at 150% of engine speed. This would drive the ouput shaft of the pto at 1800rpm if the engine were kept at a steady 1200rpm. That Cummins engine will have more than enough rotating mass to drive any generator you'd like to apply. A simple AMBAC, or Bosch governor installed on the engine and you'd have portable genset at your fingertip selection.

If you were going this route I'd install at least a 25kw set to supply auxiliary power to anything you'd like in an emergency as the available horsepower will be there and available.
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  #33  
Old 10-12-2017, 07:55:08 PM
Heins Heins is offline
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Default Re: Heavy Truck Transmission and PTO Questions

We had a gas v-8 engine, five speed transmission with a two speed rear end in a truck and when it was loaded, I would split the gears with the rear end and each gear would drop the engine rpm's 500.
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  #34  
Old 10-12-2017, 08:01:25 PM
Vanman Vanman is offline
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Default Re: Heavy Truck Transmission and PTO Questions

Quote:
Originally Posted by Gary Barber View Post
You must have an attachment to these Crowns
Haha, that would be an understatement! I sought out both of our Crowns for a number of reasons that appeal to the motorhead in me. Making it into a motor coach appeals to the builder in me. Making it unconventional is just how I roll. Without question, if my goal was merely to convert a bus, the MCI would be the top of the list. With a twin turbo 12V71 Detroit in the back and probably a 13 speed trans. Whoops- that motorhead part is slipping out again.

Those modern oil cooled automotive generators are no doubt very good, but they cannot be motored. With my set up, I will actually be able to move my bus with the engine off by driving into the PTO with the trans in gear and the engine clutch disengaged. No, it won't be fast, but it'll be a cool feature that may come in handy some day. Indeed the generator could be motored into the PTO with the trans in neutral and the engine clutch engaged to start the engine... Cool stuff, but I recognize that these are things that appeal to the mad scientist / engineer in me, and would be of little interest to the majority.

I forgot, to add, I will be using a Trace Engineering SW4024 inverter. I'm not one that usually cares too much for modern stuff, but this is a beautiful machine, once you study how it works.

The auxiliary generator set currently under consideration is a 6-1/2 kW, L600 powered Kohler. Bigger than I need, but I already have it. I'll mill the head to cut down on the propane use (no road tax). And waste heat will be used for space heating, water heating, and even to warm up the old Cummins, if desired.

Last edited by Vanman; 10-12-2017 at 08:41:50 PM.
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  #35  
Old 10-12-2017, 11:20:27 PM
Gary Barber
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Default Re: Heavy Truck Transmission and PTO Questions

I can sure relate to using the waste heat I was going to tie my 10 kw kubota powered genset into my water circulation circuit in my GMC4104 It has a diesel fired webasto boiler with hotwater baseboard heat and is also tied to engine water jacket My water heater also has a marine heat exchanger that is tied in If you were going to do a 12V71 in an MCI find on old MC6 they had 12V's in them from the factory then get a personalized license plate reading "buzndoz" Best of luck to ya keep us posted with progress
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  #36  
Old 10-13-2017, 09:37:19 AM
AngrySailor AngrySailor is offline
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Default Re: Heavy Truck Transmission and PTO Questions

Quote:
Originally Posted by CharlieB View Post
I get that this is a budget project.
Regarding the generator step-up drive...........
I don't know any reason why a small automotive manual transmission couldn't be run "backwards" off of whatever PTO is available with the Roadranger. First gear in these little guys is right around the 2.75/1 that you're looking for.
The helical cut on the gears when running in the "engine driving" direction causes the input and out put shafts to push away from each other and the trans case takes the thrust, when engine braking, the tiny thrust washer riding against the needle bearings between the input/output shaft now takes this thrust. It probably wouldn't be an issue driving a small generator, but I have seen those thrust washers fail from hard downshifts with big tires and angry big blocks...


Vanman, my vote it for the RTO, I'm putting a 6v53 into a pickup right now and looked at aux trans, two speed rears and the hassle for lousy ratios just didn't seem worth it. The RTO has a nice split, same rpm drop every gear and a .74 over final ratio. Even with my short 31" tall tires and 3.23 final I get reasonable speed/rpm and lots of gears to downshift to!
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  #37  
Old 10-13-2017, 12:24:07 PM
CharlieB CharlieB is offline
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Default Re: Heavy Truck Transmission and PTO Questions

Good point about reversal of the torque increasing the load on the thrust bearings.....
The caveat is that the transmission will be working way below its design parameters.
There are better engineering solutions, but this is a budget project.
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  #38  
Old 10-13-2017, 02:07:51 PM
Vanman Vanman is offline
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Default Re: Heavy Truck Transmission and PTO Questions

In the meantime, I found the ratios for the RT-910 here:

http://www.roadranger.com/rr/Product...ons/PCT_486870

Assuming that mine is the same.

Between that and looking at the different axle ratios on the link provided earlier, I've been doing a little bit of fooling around, plugging in 2.64, as that was one of the highest ones I could find.

As it happens, the overall ratio in 8th gear would wind up being 4.11, which happens to be what the vast majority of the Crown School Coaches were equipped with, from the ~'70's on. This results in a maximum speed of around 65 mph. Pretty slow, while the engine is racing at full speed with very little load on it. Very inefficient. This was done as they're limited to a stupid-slow 55 mph if children are on board.

I had one long ago with 4.88 gears! With the engine blasting away at it's maximum no-load speed of 2250, it would make all of 58 mph! And would have to slow down considerably on a grade to get the engine to full load at 2100 rpm. Ugh!!

Meanwhile, moving on with this thought experiment, 9th gear turns out to be 3.27. This gives a top speed in that gear of ~80. That's pretty good, though, on level ground with no headwind, I'm quite confident that the engine would still be running at a pretty light load.

So, high gear is of course 2.64. That works out to right about 100 mph. That would be pretty cool, if the rest of the vehicle, tires and so forth were up to the task. Say, in Germany. Here where traffic laws are 99% revenue oriented, and driving is not taken seriously by the driving public... Well, that old Cummins isn't going to be able to make that much HP anyway, and, if it could, the fuel consumption would be unsavory.

HP required to overcome aerodynamic drag is related to the cube function of speed.

But, looking back at the top of 9th gear, 80 mph, in high gear, that would be somewhere around 1670 rpm. Probably quite nice, again on level ground and so on. Want to cruise at 75? 1565. 70? 1460. Remember, as speed drops, HP required drops by the cube of the difference.

And at any speed up to 80, 9th gear is right there waiting for when you need some more snot.

What's really cool is that someone has already done exactly this, with a Crown, except he had the double overdrive RTOO-910 (direct in 8th) and the 4.11 axle gears. He reported the same speeds, except he chickened out 92 mph. Purportedly still accelerating though. As I recall he had a Detroit (671), and I don't recall if it was turbo or not.

Having driven practically identical Crowns with both the Cummins NH and the Detroit 671 (both non-turbo), I would say that you could not detect the difference. You, know, if you couldn't hear the Detroit's music, or the Cummins's rattle and rumble (both nice sounds ). People not used to the Detroit's music complain of them having poor low speed torque, but this is simply not true. Instead they're running them considerably Slower than they would the Cummins because they are driving by ear, not the tach!
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  #39  
Old 10-13-2017, 07:16:46 PM
Vanman Vanman is offline
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Default Re: Heavy Truck Transmission and PTO Questions

Two other considerations with such a tall top gear are of course, minimum speed and maximum gradeability.

A nice thing about the RT-910 is that it is able to retain a nice, deep low gear of 8.14 since it has enough gears to cover a wide range without too much step between gears.

This works out to around 4 mph at idle, and about 12 mph maximum in low gear. (with that aforementioned 2.64 final drive ratio).

If we just throw a number out there of 500 ft lbs of torque available from the engine at idle for starting on a grade, and we say we loose around 20% to gearing, the resulting tractive effort is in the neighborhood of 4900 lbs.

If we figure that the bus's gross weight is 30k, and the most I can tow with a class C as an RV or with my class B is 10K in both cases, the absolute highest gross would be 40K. That's actually only a 12% grade.

In reality, I'd like to think the bus would be less than 30k when fully outfitted and loaded. And towing something that's 10k would be unlikely, 1/2 that much maybe. So let's say 28 and 5. At 33k, maximum gradeability would be ~15%.

These are obviously not grades you see on ordinary roads, but I've seen some pretty steep ones, off the beaten path. Well, maybe I'll call this one good enough. This is to be a budget conscious project after all.

My other Crown is the First three axle, 40 foot bus that Crown built. THAT one is going to get the special treatment. If I live so long lol.

Last edited by Vanman; 10-13-2017 at 08:12:23 PM.
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  #40  
Old 10-13-2017, 08:36:12 PM
CharlieB CharlieB is offline
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Default Re: Heavy Truck Transmission and PTO Questions

It sounds like your gear ratios are starting to come into focus. It does seem like the 2.64 axle ratio might be a little too tall, though. Google can show you a torque curve for your engine. That might help you decide the most efficient engine RPM for the speed that the bus will spend most of its time at.
There is also a school bus conversion forum that might be of help. I've opened it to a thread about a Crown bus modification similar to what you're doing:

http://www.skoolie.net/forums/f33/re...tios-5829.html
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