Scaling Horsepower

[Andrew Pugh]

Hello all, this may be my first post in the Riding Scale area...

I have been interested for some time in building riding scale trains for use on 7.5" gauge track (1.5" or 1.6" scale), and have done plenty of reading on the subject.

One thing I have noted in particular for the gas/hydraulic gas/electric locomotives is that the prime mover horsepower rating is typically grossly over what the scale would call for.

Take a GP38-2 for example.

Per http://www.thedieselshop.us/Data%20EMD%20GP38-2.HTML, continuous TE is 54,700lb at 11.1mph. This works out to 1619HP, which is a tad off from the 2000hp rating of the prime mover, but that's simply an interesting aside.

If you divide that 54,700lb by scale cubed (421 for 1.6") you get ~130lb.

Divide 11.1mph by scale yields ~1.5mph.

130lb * 1.5mi/hr * 5280ft/mi * hr/3600s = 286ft-lb/s * s/550ft-lb = 0.52hp.

It seems that barely more than 1/2hp to the wheels is proper scale horsepower for a 1.6" scale GP38. And yet, we've got locomotives rolling around with 16hp gas engines.

Granted, our efficiency is probably terrible, especially with hydraulic drives, but even at 50% efficient transmission we're talking 1hp for the prime mover.

OK, so slogging up grade at 1.5mph is probably a little slow for the club track...say we need 6mph. That would be 4hp at 50% transmission efficiency.

What is the purpose of using 10+ HP engines to drive these things? Is it a matter of availability of suitable engines? I suppose no one wants to hear a weed-wacker engine running flat out...

Gross weights for riding cars compare favorably with loaded fright cars too. Figure an unloaded steel car weighs in the neighborhood of 150lb, that's 63150lb full scale. Add three 170lb riders and the gross weight comes up to 278000lb full scale. Nice coincidence.

The floor is open

Andrew

[Pontiacguy1]

I'll tell you why most people use the big V-twin engines... They are smoother with a lot less vibration, and also, they can deliver basically all the power you can put to the track just above their idle speed, so they are much more quiet.

You could use a 5 HP motor on there, but then you're going to have to rev it up and run it at 3000 RPM to do the work you want. The 16 HP Briggs and Stratton Vanguard V-twins that most of these locomotives seem to be using can do the same work at about 2000 RPM, which is much quieter and with less vibration.

[steamin10]

I detune Briggs 2.5 & 3.5 engines by taking about .010 off the valve stem, and adding a big flywheel slug to the shaft, for more smooth and slower idle. It absorbs the vibration too. The engines will not hiccup at low speed with this done, and will perform most handily. I take off the governor too, as running up against it on a downhill will make it gallop, on - off that is annoying. While it may be argued that it is possible to over-speed the little powerhouse, my gearing is about 12 MPH ground speed, and running is about half throttle, so it is not a problem with engine screaming or high speed.

[6491]

I have to agree with what has been said. My loco is only small but I run 6.5hp. With the motor working at a high idle it can pull a two of carriages of passengers all day and only use a couple of litres of petrol.
Another advantage is the cost of these motors. about $140 new or if you want up market like mine that's $180 with electric start.
Being a plant operator for 50 years I have always liked working with more power than I needed. a lot less stress.

[cbrew]

Good Morning Andrew,
what you came up with is in line with what i have found to be true,
this video is testing one of my box cabs locos,
they have a single 500 (~2/3 hp) watt PM motor and has zero issue pulling any load the weight of the (~325lbs) loco will handle
this motor still had more then enough power to spin the wheels on dry rail

and what everyone else as said about the lawn mowers is true.
just one thing i learned about the smaller engines that have a oil slinger for lubrication is they are designed to run at the higher RPMs and tend to wear out faster when run at lower rpms for extended run times. this is another reason you will see the larger v-twins, they have pressure oil systems and can run at the lower rpms all day long



my days of running/owning gas locos are long gone.

these are the last two I and a friend built, sporting a 500 watt pm motor on each axles and weights in at 902 and 875 lbs
the power form the four 500 motors is amazing and they are dead quite when i mute the sound effects.

it was nice cruising the north side of TM with the sound off, a

[6491]

To use a Caterpillar term "high idle" is around 2000 revs.

[Andrew Pugh]

Wow cbrew, that thing is hauling the mail...do you know the approximate speed? Also it looks like your post may have been cut short?


It seems for a 1.6" GP38 120W traction motors would yield realistic performance, but that assumes that the motors can deliver 120W throughout their speed range, which is a poor (and incorrect) assumption.

I suppose you really have to look at the motor torque curve and do math to ensure it will perform as one wishes, and than limit the power applied to them. I imagine the traction motors would end up much bigger than 120W each.


As far as prime movers go, it seems agreed that oversized engines are selected to keep RPM and noise down.

[Steggy]

One thing I have noted in particular for the gas/hydraulic gas/electric locomotives is that the prime mover horsepower rating is typically grossly over what the scale would call for.

I use a 16 HP Briggs V-twin in my F7, with "full throttle" being 2400 RPM, and idle at 1000 RPM. According to the SAE power curves for this engine, the V-twin would produce a gross output around 11.5 HP at 2400 RPM. SAE gross, however, is a rating that assumes that there are no parasitic loads on the engine, such as imposed by the built-in alternator, cooling blower and exhaust system backpressure. When those items are factored into the picture the result is the SAE net rating, which is typically about 65-70 percent of gross. So the V-twin's net output at 2400 RPM would be about 8 HP at best.

In my unit's case, additional parasitic loading comes from the air compressor, amounting to about 3/4 HP at 2400 RPM (engine speed) and an external alternator, approximately 3/4 HP as well. So I would have 6.5 flywheel HP available for traction. However, there's more to it.

Hydrostatic propulsion of any kind has a best-case efficiency of around 72 percent, based upon the rated efficiencies of the pump and motor(s). In practice, 60 percent is more commonly achieved, due to, among other things, pumping losses in the valves and plumbing, as well as blowby in the motors and pump. Using the 60 percent number, the 6.5 flywheel horsepower is seen as approximately 3.9 HP output at the traction motors, amounting to about 1645 HP when scaled up to full size, which is close to the F7's rated 1500 traction horsepower.

Incidentally, the scale 1645 HP is optimistic in my unit, as another source of loss exists in the propulsion system over and above hydrostatic losses. I will be describing the F7's propulsion system design in a topic that I have started about my project.

As others have noted, the V-twin produces less mechanical and torsional vibration, and has a more robust torque curve, making it better suited to low RPM operation as compared to a one-lunger of the same horsepower rating. Also, the exhaust note in the RPM range I am using doesn't sound like that of a lawn mower engine spinning at 3600 RPM—it is a low pitched rumble more akin to what a full sized locomotive might emit.

[Andrew Pugh]

Good to see you in here, er...BigDumbDinosaur.

I had a look at your thread, and saw some pictures in one of the other threads, I think it might have been your first post on the site even. Looks great! I find it interesting and like the fact that you are trying to replicate to scale the performance aspects of your F7.

How are you calculating 1645HP from 3.9HP in the scale locomotive? My understanding is that power should be to the 4th power of the scale, 7.5^4 for 1.6" scale or 8^4 for 1.5". I had calculated ~1hp input to a hydraulic pump as appropriate for a 2000HP model.

I definitely agree with avoiding the lawn mower running flat out sound. A well muffled V-twin is probably about as good as it gets. Until someone makes a small displacement multi-cylinder model with a big flywheel for smooth performance at low RPM...

[Steggy]

How are you calculating 1645HP from 3.9HP in the scale locomotive? My understanding is that power should be to the 4th power of the scale, 7.5^4 for 1.6" scale or 8^4 for 1.5". I had calculated ~1hp input to a hydraulic pump as appropriate for a 2000HP model.

I've long used the mass ratio n^3 (where n is the scale) in computing scale horsepower. Based upon how my F-unit performs under heavy loading, n^3 seems to be a good approximation.

I definitely agree with avoiding the lawn mower running flat out sound. A well muffled V-twin is probably about as good as it gets. Until someone makes a small displacement multi-cylinder model with a big flywheel for smooth performance at low RPM...

The ideal, of course, would be to have a 16 cylinder, 45 degree Vee two-stroke Diesel engine like the original, just a little smaller. That's a bit more machining than I care to do.

[Andrew Pugh]

Hmmm...I think if you dig deeper into the math, you will find the scale for power is n^4, but like I mentioned in my first post, it would be very slow on grades under load for most people. Few would tolerate a diesel model struggling upgrade at 2mph. Heck even the shays run faster then that.

This model https://www.youtube.com/watch?v=E4-fOw6vr34 gives me hope that we will one day see something with more cylinders in 1/8 and 1/7.5 scale diesels.