DMU's and number of engines required

[truckman060473]

I was just wondering if, say you had a 3 coach DMU like a 175, why it is not sufficient to have engines in just the two outer coaches? If there are two driven axles in each power car, aren't a total of 4 powered axles enough for a 3 coach unit?

 

[starrymarkb]

You'll find that UK units are quite underspecced in terms of engines.

The Danish IC3 has 4 engines per 3 coach train.

The French go one better, there is a 2 car DMU with 6 engines (2 traction and 1 Aux per car!)

 

[truckman060473]

Well the 100mph capable 175's apparently use 400hp engines. So 800hp not enough for 3 units? a class 20 loco is 1000hp which is only bit more, and that could shift a reasonable amount of stock. Maybe it needs to be 1200hp for running at 100mph.

 

[mumrar]

A 4 car Voyager has 3000hp at it's disposal, I'd hardly consider that under-specced. The rules for first gen DMUs used to be that an engine was only permitted on a vehicle with a cab, presumably for safety reasons. It's all about faster acceleration between stops, after all, if the linespeed remains the same, then acceleration, and braking are the only possible ways to make services quicker. It also means they can climb gradients better. For example, a turbostar 2 or 3 car will top the Lickey bank at 43mph if you are doing 80 or so at the bottom. A Voyager however will maintain it's speed with some drivers finding it necessary to throttle down as soon as you reach the top at Blackwell.

 

[royaloak]

Its to maintain the power to weight ratio, if you only had power in the outer coaches then a 2 coach one (both powered) would be more powerful than a 3 coach one with a trailer coach in the middle.

 

[truckman060473]

i guess so. but when locos haul trains it could be 3 coach up to more than 10. that must give big differences in getting up to speed etc.

 

[royaloak]

Yes that is why there is a maximum timing load that can be hauled, once that is exceeded the train gets late.

 

[generalvegitable]

I think that it is as said about power to weight ratio, and it could also be about redundancy, if in a 3 car train with 1 engine in each car you lose an engine, you still have 2 to keep the train going, if you only had 2 engines for a 3 car train and one fails you only have one engine left, so if that fails you are screwed, and with only 1 engine per train if that fails you are definately screwed, which is why most modern locos have 2 engines (I think)

 

[royaloak]

Which modern locos have 2 engines?

 

[generalvegitable]

I thought that they all did, but I'm not an expert so my opinion is most likely useless

 

[truckman060473]

Hmmmm! well the Deltic had two engines but hardly a modern loco!

 

[mumrar]

The only locos to run here with two engines are Class 42+43 Warships, Class 52s and Class 55s, and that's since we've had diesel locos. However, I'm bound to end up looking an arse by missing one (and HSTs don't count, they're two seperate power cars). Technically Class 73s too I suppose. What a shame we never got the alleged Class 51 'Super Deltic' - (All off Wikipedia so maybe total fiction)

A 4,400 bhp (3,300 kW) Co-Co "Super Deltic" was proposed but not built, which would have weighed 119 long tons (121 t) with a maximum axle load of 19 long tons (19 t). This would have been designated Class 51 and would have had two 18-cylinder turbocharged engines, based on the 9- cylinder turbocharged engine used in the Britsh Rail Class 23 "Baby Deltic". The Class 55's engines were not turbocharged, although they did have centrifugal scavenging blowers

Also, an edit - Class 17 Claytons had either a pair of Paxmans of Rolls Royce engines, and the Class 53, which I believe was a prototype for the Class 47 had an identical pair to the Westerns.
--- old post above --- --- new post below ---
With Class 158s having operated over the Transpennine routes and a lot of similar routes throughout the UK, they could be considered an upgrade to the Class 124 (obviously locos came between the two). The Class 124s were 4-car sets with two engines providing a total of 460hp and top speed of 70mph. A pair of 158s making 4 cars has 1400hp and a top speed of 90mph. I'd hate to have seen a 124 going up Ais Gill (don't think they ever did), or any other long distance climb.

 

[Turbostar]

I thought that they all did, but I'm not an expert so my opinion is most likely useless

Er, ok...

 

[truckman060473]

see how easy it is for an original thread to go off topic? DMU engines, not locomotive engines!

 

[Turbostar]

Try reading post #8 then! Take it up with generalvegitable, not me... :roll:

 

[truckman060473]

yes, I know, mentioning two engines in a locomotive. threads are often side tracked when it goes into too much info about that. my reply about loco hauled was just in comparison to the "all engined" DMU's. When a thread is started it should be issues surrounding that really, not side tracked into another discussion. (ok i admit to saying the Deltic had two engines and maybe i am partly to blame for sidetracking)!

 

[jopsuk]

It is quite a contrast with electric units that often have only one or two motor coaches in a four car train (the 317s for example have a single motor coach)

 

[No Fault Found]

Is it not to do with the underframe space available for a decent output engine, designed to be low in height?

 

[TDK]

Well the 100mph capable 175's apparently use 400hp engines. So 800hp not enough for 3 units? a class 20 loco is 1000hp which is only bit more, and that could shift a reasonable amount of stock. Maybe it needs to be 1200hp for running at 100mph.

a 3 car 175 performs quite well on 2 engines and it is also overpowered if you take power to weight ratio into consideration, a 3 car 175 out performs a 2 car by far.

 

[Murph]

What a shame we never got the alleged Class 51 'Super Deltic'

Mmm, one of the great lost opportunities. We did, however, manage to prove the viability of twin 2200hp Deltic T18s as prime movers (also a Deltic T9 for aux power): http://en.wikipedia.org/wiki/Hunt_class_mine_countermeasure_vessel

At the risk of sending the thread further off on a tangent, does anyone know if there would be any fundamental problem adapting the basic Deltic engine design to modern emissions standards?

 

[ChrisCooper]

Power to weight ratio isn't an issue, as you could just as easily put higher powered engines under less coaches. The problem is the size of the engines to get that amount of power. The 750hp lumps on the 22Xs etc take up a lot of space, and require a higher floor to fit them under. They are needed on the 22Xs for 125mph running, and on the 185s for performance on the hilly bits, so it's worth it, but there would be no advantage to having a 3 car unit with only the outer coaches powered. You would also then have the unpowered trailer with all that wasted space underneath. Axle weight is an issue too. A more powerful engine means more weight, look at the weight of the 185s compared to the Turbostars. Overall weight might be the same, but the axle weight of the motor coaches will be higher, and that means higher track access charges and possible speed restrictions (185s can not use higher speeds for Sprinters due to weight) or even route restrictions. The unpowered trailer is lighter, but for what? Another thing is flexability. With engines under each coach you get roughly the same power-weight ratio and performance regardless of length, wheras if only some coaches are powered then changing the length will alter the power-weight ratio and performance. Adheasion is a factor to, especially for mechanical transmission as generally one engine powers one bogie, so whilst each coach on a DMU is powered, generally only half the bogies are powered. The only exceptions are the 22X DEMUs where all bogies are powered, but I think only the inner axle on each is powered.

When it comes to engines per coach in the UK, the twin power Class 101s had 2 engines per coach, giving 4 in total on a 2 car unit, and 600hp. That was pretty good power-weight ratio too, considering they were pretty lightweight, better than the Sprinters and Pacers, and they did move. Most 1st generation units had either 1 or 1.5 engines per coach, i.e a 2 car motor-trailer set with 2 engines in the motor coach, or a 3 car motor-trailer-motor set with 2 engines in each motor. Some were 4 car, again with 1 engine per coach, motor-trailer-trailer-motor, 2 engines per motor. None powered on all axles though, as the engines only drove the inner axle of the bogie.

 

[starrymarkb]

One interesting concept is that of the Stadler GTW, which has 2 400hp engines in a separate module (think mini locomotive)

 

[HSTfan!!!]

a 3 car 175 performs quite well on 2 engines and it is also overpowered if you take power to weight ratio into consideration, a 3 car 175 out performs a 2 car by far.

I watched a 3 car 175 pull away with just the front engine the other day, middle and rear engines just idling... I dread to think how far that got lol
besides surely it's not all about hp to consider here with the weight being shifted, surely torque and gearing would be a better consideration?

 

[mumrar]

You're right, hp is all about speed against resisting factors such as weight, rail friction, aerodynamic forces etc. Torque is all about getting load shifted. Hence why quite often it's possible to see a Class 37 with a couple of vehicles move off, just by the driver selecting forward or reverse. Even at tick over they generate plenty.

 

[90019]

You're right, hp is all about speed against resisting factors such as weight, rail friction, aerodynamic forces etc. Torque is all about getting load shifted. Hence why quite often it's possible to see a Class 37 with a couple of vehicles move off, just by the driver selecting forward or reverse. Even at tick over they generate plenty.

And why my friend's ZX 1.9D could almost get into 3rd without you having to touch the accelerator

 

[HSTfan!!!]

my old diesel corsa would propel itself without gas, quite freaky seeing it lol, miss the diesel!

 

[merlodlliw]

Just got back from picking up a colleague at Ruabon Station.

It was due in from Cardiff at around 1350, I think it was the 1120 CDF to HDD, anyhow it was 20 late, 3 car 175, last coach had the engine dead (175 115)
and it was stinking of diesel, lost 30 minutes between Aber & Hereford, dont know if that was the reason. M

 

[royaloak]

my old diesel corsa would propel itself without gas, quite freaky seeing it lol, miss the diesel!

Miss mine as well, 62mpg and comfortable for a 75 mile commute.