Why are Multiple Units Better Accelerating than Locomotives?

[al.currie93]

I know that this is a fact, but, out of curiosity, can anyone actually give a good explanation as to why?? I have also heard that they are mor energy efficient, so any explanations to that would be great as well!

Thanks guys

 

[starrymarkb]

MUs have more driven axles so more traction and less chance of wheel slip.

There is a video online of a light OBB Taurus doing 0-75 faster then most cars.

 

[12CSVT]

I remember this phenomenon demonstrated very well coming out of York on a class 104 DMU in the 1970s. A deltic which departed simultaneously was about 100 yards behind by the time it diverged at Chaloners Whin Jct.

 

[JamesRowden]

I know that this is a fact, but, out of curiosity, can anyone actually give a good explanation as to why?? I have also heard that they are mor energy efficient, so any explanations to that would be great as well!

Thanks guys

The force that can be produced by friction is roughly directly proportional to the reaction force (in terms of trains this is the weight upen the driven wheels). The force required to accelerate the train is equal to the mass of the train multiplied by the acceleration. Therefore, the maximum acceleration is limited to the proportion of the trains weight that is across driven wheels (when grip is limiting maximum acceleration rather than something else [e.g. power]). In a locamotive hauled service the mass of the carriages does not add any extra grip but it does mean that extra force is required to acclerate the train at a certain rate.

 

[D365]

MUs have more driven axles so more traction and less chance of wheel slip.

There is a video online of a light OBB Taurus doing 0-75 faster then most cars.

This is this video:

[youtube]cdy3G_an65s[/youtube]

 

[edwin_m]

In terms of energy efficiency there are probably two factors.

Firstly a MU is probably lighter than a loco-hauled set of equivalent capacity, so needs less energy to move it around.

Secondly all modern and some older EMUs can return power to the electric supply when braking, by making the electric motors act as generators. Loco-hauled sets could also do this but as the EMU has a greater proportion of axles with motors it can recover a greater proportion of the energy.

 

[John Webb]

The OP doesn't make clear if they are talking about Diesel or Electric MUs. DMUs will be limited in acceleration since the diesel engines have a finite output, whereas EMUs can draw heavily on the electrical supply as they accelerate and for a short period have a much higher power output than the continuous rating of the motors might suggest.

But for either type of MU, the application of power through many more wheels than available on a loco, as several previous posters have pointed out, allows a higher acceleration rate for the MUs.

 

[al.currie93]

The OP doesn't make clear if they are talking about Diesel or Electric MUs. DMUs will be limited in acceleration since the diesel engines have a finite output, whereas EMUs can draw heavily on the electrical supply as they accelerate and for a short period have a much higher power output than the continuous rating of the motors might suggest.

But for either type of MU, the application of power through many more wheels than available on a loco, as several previous posters have pointed out, allows a higher acceleration rate for the MUs.

Apologies, meant any MU regardless of method of traction

 

[TDK]

A class 67 with 3 coaches and a DVT has an acceleration curve almost the same as a Voyager. It will certainly beat a 4 car 158 or any other sprinter. It is infinitely faster than a 168 also.

 

[HSTEd]

A class 67 with 3 coaches and a DVT has an acceleration curve almost the same as a Voyager. It will certainly beat a 4 car 158 or any other sprinter. It is infinitely faster than a 168 also.

And it weighs in at about 220t, 80 of which is in the locomotive.

Which means it has 36% of its weight on driven axles, compared to 50% for the Voyager, so it will tend to be comparable at low speeds.

A Sprinter/Turbostar/Networker Turbo has a relatively low performance (and very low cost) hydraulic transmission that simply can't compete with the more expensive modern electronically controlled electric transmission.

You can get really silly though when you get to things like 500 Series and N700 Series Shinkansen sets that can do 0-170mph in 3 minutes or something thanks to have 75-100% of all axles motored.
(The 500 sets had 100% of all axles motored but were rather expensive and advances in electronics mean the N700s can beat them now).

Also an MU has an advantage in wet weather because a loco at the front will tend to lose tractive effort due to wet rail, but after the first couple of vehicles the rail is effectively dry no matter how wet it started.

 

[Beveridges]

A Class 90 light engine will out accelerate any Unit with ease

 

[ainsworth74]

A Class 90 light engine will out accelerate any Unit with ease

Not much room for passengers though

 

[D365]

A Class 90 light engine will out accelerate any Unit with ease

Hmm, any videos anywhere?

 

[jopsuk]

barring power transmission limits, MUs don't lose acceleration as you make the train longer, of course

 

[Crossover]

barring power transmission limits, MUs don't lose acceleration as you make the train longer, of course

Unless you add trailers

 

[TOCDriver]

I think you can compare this to 4 wheel drive (i.e the Multiple Unit) compared to 2 wheel drive (i.e the loco) A 4 wheel drive machine of same power and torque output, will easily out accelerate a 2 wheel drive machine any day of the week. It's all about getting the power and thus the traction down on to the road. By spreading the power out over the vehicle, you will get better traction and better acceleration.

 

[cjmillsnun]

I think you can compare this to 4 wheel drive (i.e the Multiple Unit) compared to 2 wheel drive (i.e the loco) A 4 wheel drive machine of same power and torque output, will easily out accelerate a 2 wheel drive machine any day of the week. It's all about getting the power and thus the traction down on to the road. By spreading the power out over the vehicle, you will get better traction and better acceleration.

Sadly that analogy doesn't work. 2wd vehicles would generally out accelerate 4wd due to transmission losses being greater on 4x4s

 

[edwin_m]

At low speeds the maximum acceleration depends on adhesion and the proportion of weight that is on powered axles, so the 4WD should be quicker away from a stop. Transmission losses will become more important at higher speeds so the 2WD will probably start to catch up as they both accelerate.

 

[Beveridges]

Hmm, any videos anywhere?

I don't have any videos, but when I've seen them go, they can do 0-60mph in about 5 seconds.

In comparison, a fast EMU takes about 25 seconds to reach 60mph.


*Note* - These are rough estimates based on my memories, I haven't been on a light engine 90 and recorded it with a stopwatch!



I'd like to see how fast a light engine 90 can reach 110mph, the only problem would be trying to stop!

 

[edwin_m]

The acceleration of a class 90 running light was a factor in the Bletchley derailment.

http://www.raib.gov.uk/latest_news/news_archive/news_archive_2012/121121_pn_bletchley_junction.cfm

 

[Beveridges]

Plus, that 90 would not have even been taking full power.

 

[Metrailway]

A Virgin Trains driver told me many years ago that a Class 87 + 9 Mk3 + DVT consist would easily out-accelerate a Class 390 until around 80-90 mph.

 

[87015]

90s and short sets quite happily kept to EMU timings on the North Berwicks IIRC, and anything they can do an 87 will do far better...

 

[Aictos]

A Class 90 light engine will out accelerate any Unit with ease

And a Class 91 would probably leave the Class 90 standing

 

[D6975]

Way back when the 91s were being built I saw one at Crewe come out of the works into the station before reversing for a test run up the WCML. I was absolutely staggered by how quickly it shot off. Like a stabbed rat as James May would say.

 

[HSTEd]

I would like to see a Shinkansen set against a light engine.

Since a 500 Series or a N700 can do stupid acceleration.

 

[TOCDriver]

Sadly that analogy doesn't work. 2wd vehicles would generally out accelerate 4wd due to transmission losses being greater on 4x4s

Not with the physics I know of they wouldn't.

 

[theageofthetra]

You mean like that slouch of a 4wd the Bugatti Veyron?

 

[TDK]

A Class 90 light engine will out accelerate any Unit with ease

All light engines will out accelerate any unit including a class 350
--- old post above --- --- new post below ---

barring power transmission limits, MUs don't lose acceleration as you make the train longer, of course

The more coaches on a DMU the better the performance as there is less wind drag per weight etc.

 

[Dieseldriver]

Can vouch for that on the DMU score, I regularly drive anything from a 2 car to a 10 car and the difference can be very noticable!