I've always been a fan of 4-axle locomotives, whether they be Geeps, U-boats or ALCOs. It is sad to see them go by the wayside to the bigger 6's and "toasters." I've performed many calculations with head to head performance between 4-axle and 6-axle locos. Certainly on grades and with heavier unit trains, the 6's own. But why are they perferred on the lighter intermodal and auto trains where they have little if any advantage over 4-axle units. Is it hp alone or starting tractive effort? I know 4-axle can be slippery but certainly not on flat level track. By the way, is there a calculation to find train resistance on flat track? I can't seem to find one.
MLG
The threebiggies are utilization, range and a shift toward tonnage dispatching/reduced HP/ton.
The less specialization in the fleet, the lower the dwell between assignments can be, on the average, and the smaller the fleet can be. A six can aways do what a 4 can do, but not vise versa.
You can cram a 5000 gallon fuel tank under a 6 axle and still make a reasonable axle weight. It's always a struggle to get a decent sized tank under a 4 and make decent axle loading.
The lower the HP/ton, the better the fuel economy. The trend lately has been toward drag ratings and away from HP/ton ratings for most trains. This reduces the places where 4 axles are the best fit.
-Don (Random stuff, mostly about trains - what else? http://blerfblog.blogspot.com/)
Look for 4 Motors to return at least on the BNSF in a fashion. They are done ordering DC ES44's from GE. Instead they are ordering ES44C4's instead were the Trucks are 6 axles but only have 2 Traction Motors on them. The Center Axle is basically an Idler like an E Units were but can be lifted at low speed for more TE. Last I heard they had close to 60 on Roster with another 100 to come.
MLG481/2 I've always been a fan of 4-axle locomotives, whether they be Geeps, U-boats or ALCOs. It is sad to see them go by the wayside to the bigger 6's and "toasters." I've performed many calculations with head to head performance between 4-axle and 6-axle locos. Certainly on grades and with heavier unit trains, the 6's own. But why are they perferred on the lighter intermodal and auto trains where they have little if any advantage over 4-axle units. Is it hp alone or starting tractive effort? I know 4-axle can be slippery but certainly not on flat level track. By the way, is there a calculation to find train resistance on flat track? I can't seem to find one. MLG
Weight is the biggest issue, look at the trouble AT&SF had to keep the GP60M and Dash8-40BWs within acceptable axle loadings, they had to sacrifice fuel and hence range. With the increased cooling capacity required to meet Tier 2 emissions standards they likely would have needed to sacrifice even more fuel. The very heavy weight also has made them hard riding.
edbenton Look for 4 Motors to return at least on the BNSF in a fashion. They are done ordering DC ES44's from GE. Instead they are ordering ES44C4's instead were the Trucks are 6 axles but only have 2 Traction Motors on them. The Center Axle is basically an Idler like an E Units were but can be lifted at low speed for more TE. Last I heard they had close to 60 on Roster with another 100 to come.
These units are DC six axle equivalents, so, for BNSF at least, they are are good fit in their current fleet, too.
They are DC 6 axle equivalents, but they are AC units so there's also a cost reduction vs a full ES44AC.
I'd be curious to know how the raised and lowered center axle really affected traction expressed as a percentage.
For example as compared to a standard ES44AC with simply 2 fewer traction motors, how much of a performance boost do you get?
There are quite a few spots where 6 axle power is not allowed, either due to curvature or track condition. Track should also be beefed up to handle them as well. On the mainlines, 6 axle is fine.
One place where I am not taking a 6 axle again.
Most of the track is buried in concrete, so it won't move much. If it wasn't, well...
Mike WSOR engineer | HO scale since 1988 | Visit our club www.WCGandyDancers.com
IIRC the lifting of the Center axle transfers up to 15% of the axle weight to the other 2 axles for added TE at speeds below 15MPH were you would need it. Beyond that it will not raise the axle. So basically it gives 7.5% of the weight that that axles carries to each powered axle and the EVO44C4 is rated for more TE than a EVO44DC IIRC so it must be a good system. All this for 2 air Pistons on each side a bell crank and a few pieces opf chain on each truck.
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