Locomotive Performance and Tractive Effort Discussion

Use a buckeye coupling???

 

Eric Langridge gives quite a bit of detail about dynamometer cars and road testing in 'Under 10 CMEs Vol 2' (2011) The Oakwood Press, Usk ISBN 978 085361 716 7. The equipment, including the drawbar, would be zeroed after the loco was hooked on. The post-war Haslar equipment, and probably that from the Big Four also, would not only record pull but also push exerted through the buffers, so could be used in braking trials as well.

 

This one?
http://www.heritage-railways.com/locosdb/locosdb2.php?RefNo=3372
And it's only narrow gauge.

 

The South African Class GL. Weight is given as 211 tons 1cwt or 214 tons 2 cwt and it was built to Cape Gauge (3' 6") and the design dates from the 1920s. What is the design's power output? Remember this is a freight locomotive with 48" driving wheels and it is very difficult to obtain a high horsepower from a steam locomotive at low speed. To achieve 5,600 hp you are looking to a much larger compound design with a total weight close to 430 tons, though this is a little unfair since this articulated is not a Garratt and carries more water and fuel. It is also standard gauge and a newer design.

So what about the GL, tractive effort is little guide to horse power, could they produce in excess of 3,000 hp? If so how much excess?

 

I prefer this one...

Union Pacific Big Boy - Wikipedia

 

I seem to recall that the only Cape Gauge Garratt that I ever rode behind had a grate area of about 70 square feet. Presuming reasonable design of cylinders and valves, the right sort of coal, and running at an appropriate speed, the maximum output power (dbhp, edbhp or indicated, take your pick) should be roughly proportional to grate area. That suggests that the big Cape Gauge Garratts should be able to produce more hp than anything that ever ran in Britain.

 

Wouldn't a comparison with an Australian version be more appropriate?
6029 NSW standard gauge.

 

High speed would not be a feature of trains in what I would consider typical“Garratt country” in South Africa. Steep climbs and tight curves were the order of the day on my runs behind Garratts in SA.

 

Many years ago I worked with a bloke who drove Garratts in what was then Rhodesia.

 

Talking about Garratts again, specifically the LNER one, no one who worked on it liked it. The eastern men at Worsborough didnt, double work for the fireman, it was always short of steam, and the length of it made buffering up difficult. The LMS crews at Bromsgrove hated it, describing it as a useless thing which wouldn't steam properly.

 

Mentioned before, but as with the LMS garratts, when railway's D.Os deferred to the expertise of Beyer Peacock, there tended to be no problems. Just saying!

 

Correct. The LMS insisted on using the short travel valves and undersized 4F axleboxes which weren't really big enough for a 4F. On the LNER one, Gresley added two unnecessary extra cylinders, which made it permanently short of steam. On a test trip from Gorton it only made it as far as Mottram yard before running short. If Beyer Peacock had done the whole job they would both have been ok.

 

The NSWGR AD60 Garratts were of similar overall size to the South African example but had a smaller grate area of 65 sq ft. This grate area was matched by the non-articulated D57 4-8-2, which had a much higher axle-load of 23 tons. The D57 had similarities to LNER designs - 3 cylinders and Gresley conjugated valve gear.

https://en.wikipedia.org/wiki/New_South_Wales_D57_class_locomotive

All of these were slow-speed freight designs with small driving wheels. If we wish to compare the P2 with other 8-coupled passenger designs, there were a number of 2-8-2, 2-8-4, 4-8-2 & 4-8-4 types in Continental Europe, but the Post-Chapelon French compounds outclassed every other steam type for thermal efficiency & power-weight ratio.

 

Apologies in advance for mangling imperial and metric units ...

Isn't there a generally accepted figure that maximum combustion rates are of the order of 100lb coal per sq foot of grate area per hour for a narrow firebox engine, and about 80lb / sq ft / hour for a wide firebox engine?

Either way, that gives a combustion rate of about 4,000lb / hr for British locos (a 40sq ft King or a 50 sq ft Duchess as about the largest of either type).

(Switches to metric units).

Taking 30MJ/kg as a reasonable energy content of bituminous coal, 4,000lb = 1,800 kg, so the energy release is 30MJ * 1,800 = 54GJ.

That is the energy release in 1 hour, so the power release = 54GJ / 3600 seconds = 15MW.

(Going back to imperial ...)

1hp = 750W, so 15MW ~ 20,000hp generated on the grate.

If the loco is ~ 10% overall efficiency, that suggests the limit of cylinder horse power in UK usage, given the largest grate areas in use, is about 2,000hp sustained (providing the fireman can keep up with that rate of firing of ~ 4,000lb per hour!). Higher outputs are no doubt possible for shorter periods but would imply some degree of running down the water level or depth of firebed or both.

That's a fag packet calculation; there's a few assumptions in there that would nuance the numbers in either direction (heat content of the coal, grate limit, overall efficiency of boiler + cylinders) but I suspect it is reasonably close. I doubt continuous power outputs much in excess of about 2,000hp were possible in Britain.

Tom

 

242A1 had a grate area of 53.81 sq ft and could sustain 5,500 ihp and over 4,200 tender dbhp. So you are saying that the Cape Gauge Garratt should achieve what exactly? 5463 dbhp?

All things are not equal. Railways are forced to use larger grate areas in order to make use of the locally available fuel - see lignite. Steam locomotives are poor at producing horse power at low speeds. You can do it but you need something like a certain 6 cylinder 2-12-0 to do it.

There is no doubt that Beyer Peacock could produce a locomotive to meet a customer's requirement. Tell them what you needed to achieve and they would produce what you needed but if you decided to meddle with the process things could go very wrong. The Garratt offered a flexible locomotive with high tractive effort and modest axle loading. Apart from the Cossart valve gear 4-6-2 +2-6-4 type produced by Franco-Belge for the Algerian railways Garratt were low speed sloggers, built to haul freight over difficult routes. If you do not have a high speed requirement then you can manoeuvre substantial loads with a very modest horse power, the 08 shunter only mustered 350 hp and look what they could achieve.

Concerning the U1 Garratt, it was yet another design which lacked full or at least adequate development not only of the locomotive itself but also the support infrastructure.

 

You are being too conservative here. 40.5 sq ft of grate is adequate for 4,000 ihp continuous output. The UK loading gauge might present some limitations but 2,000 hp worth?

 

So where does it get that extra power? Doubling the power means doubling the firing rate - 8,000lb an hour, or nearly four tons per hour - or doubling the efficiency (up to 20% - has any steam locomotive ever got remotely close to that figure?) or some combination of both.

So which is it? Where's the fault in my maths or assumptions? You can assert 4,000hp from 40.5 sq foot of grate area, but where is the evidence and how was it achieved? (I'm talking continuously sustained power output, not peaks delivered for a few minutes).

Tom

 

The crews at Bromsgrove did not care for either the LMS or the LNER Garratt's when used for banking. Due to their size buffering up to the end of a train requiring banking was said to be difficult to judge particularly in the dark. Another factor was said to have been that they would replace more than one locomotive and therefore jobs of footplatemen were at risk.

The LMS Garratt's were not total write offs. They all survived for at least a quarter of century ( better than some early diesel classes and costing a lot less...) with their major job moving long slow coal trains from Toton to Brent and were scrapped when needing major overhauls at which point 9F's were coming into service and there were also plenty of 8F's around. The minimum mileage recorded for one of the LMS Garratt's was 443,871 and three just made it to over 600,000 ( 606,777) which isn't too shoddy at about 20mph!

The LMS were fully aware of their defects from an early stage and in 1937 approached Beyer Peacock who proposed new frames, axles and axleboxes and converting them to 2-6-2+2+6+2. It was decided not to proceed however.

 

Perhaps a comparison with a UK operating loco might offer some insight. Since this
discussion was initiated by the query as to whether 3252 is the most powerful loco
resident in the UK a comparison with a LMS Coronation 4-6-2 may be apposite as
arguably the most powerful UK loco.

3252 : Duchess
Fire grate 75 : 50 sq.ft
Tubes 3049 : 2577 sq.ft
firebox 347 : 250 sq.ft
S/H 835 : 856
psi 200 : 250
Whilst the BG firebox is 50% greater in area than the Duchess the tube
area is only 19% greater. Suggesting the designers were allowing for lower
calorific value fuels ?
The superheater area is actually two percent less in the BG than with
the Duchess ( c. 4% less for the last two, Nos. 6256/7 )
The Duchesses were one of the few UK locos indicated at greater than 3,000
IHP ( i.e. cylinder horse power, ) c. 2600 at the Drawbar ( DBHP ) which
will equal the equivalent ( EDHP ) on the absolute level.
The higher pressure of the Duchess ( i.e. saturation temperature ) aligned
to a higher ratio of superheater area suggests the maximum Duchess
superheat temperature would be significantly higher.
However all other variables being equal the Garrett might have a slight
advantage in maximum power at the cylinders.
But we are comparing a 1929 front end design of simple traditional robust
characteristic with the fruit of Stanier and his team’s experience derived
from Swindon and the initial tribulations with the Princess Royal Pacifics.
The Duchess boiler ( and as an aside certainly the MN ) I am sure was a
more efficient steam raiser.

I think 3252 is unlikely to have achieved 3,000IHP, and in practice with
lower quality fuels some way short. But as noted previously with a high TE,
which whilst based on an arbitrary formula is indicative of the torque at the
actual point of movement I.e. an ability to start and move very heavy consists.

In addition comparing an express 4-6-2 designed for standard gauge
express duties with a Cape gauge maid of all work might be ‘apples and
pears’ but….

Michael Rowe

 

Here is the preserved D57, 5711, at Valley Heights Museum in 2009 showing the exposed but very robust conjugated valve gear:-

Peter