What is the advantage of more than 2 cylinders

[John LluchParticipant @johnlluch21161]

Is having 3 or 4 cylinders worth in general cases as opposed to only 2

[John LluchParticipant @johnlluch21161]

Hi,

A loco with internal cylinders is harder to build due to more parts involved and so on. But does having internal cylinders really pay the extra building effort?

Considering a locomotive with 2 sideway external cylinders, and another one with 3 cylinders, can both be considered to perform the same provided that the total volume displacement is the same (i.e cylinders on the first loco are 50% bigger to compensate for the lack of the internal cylinder)?.

OR, what is the ultimate reason why locos would have more than 2 cylinders. Is it a mater of compactness (constraints on loco total width) or is there a more subtle reasons, such as smother running? Is this really significant on a model locomotive?

Thanks.

[Tony MartyrParticipant @tonymartyr14488]

John,

I think the answer to your last question is No.

The marginal thermal efficiencies important in full sized designs including degrees of super-heat and compounding through 3 or 4 cylinders is not significant to most model engineers. I think that even those involved with efficiency competitions find that good driving technique is far more important than scaled down multi-cylinder design.

Your first question has no real meaning for me; not that it is a bad question; because it is the building process that involves my interest rather than the long-term or fuel efficient performance of the finished engine. I can't see that two cylinder, double acting multi-drive-wheel engines are any less smooth, in the delivery of power to the wheels than 3 cylinder engines smoother take off at start perhaps?). Most complications in loco design have been induced by the drive for high pulling capacity and the efficient use of fuel in a well balanced boiler/engine combination – smoothness perceived by drivers and passengers is probably well down the list.

Tony

[Tim StevensParticipant @timstevens64731]

If you are making a 'true' model of a 3 cylinder engine I am sure some people would expect it to make a 3 cylinder noise, which is significantly different. But as the BBC usually get it wrong (along with ignoring the revs-chuff relationship, and adding music), you will not be alone if you ignore this aspect.

Two cylinders is enough for reliable starting, as long as they are about 90 degrees apart, of course.

Cheers, Tim

[John LluchParticipant @johnlluch21161]

Thanks Tim, I am not for a 'true' model, so what you posted is just what I wanted to hear. Of course, I will take care of the correct phasing between left and right pistons, though I suppose it is not the first time someone arranges them to fire in parallel, LOL!. If you think on it, a 2 cylinders loco is like a 8 cylinder internal combustion engine as far as pulses per cycle are concerned !. Should be more than enough for smooth running I guess !

[Neil WyattModerator @neilwyatt]

On holiday in France, many years ago, I bought a model boat magazine. In it was a design for a simple 2-cylinder steam engine. The two cranks were at 90-degrees to each other – but the engine was single acting!

Neil

<Spill chicken failure>

Edited By Neil Wyatt on 29/09/2014 20:24:12

[Howard LewisParticipant @howardlewis46836]

I am not a loco man, but my understanding is this.

1) A two cylinder engine, whether single or double acting has the cranks set at ninety degrees to ensure self starting.

2) Full scale locos, intended to run at speed had three, or possibly, more often, four cylinders, to improve the balancing. Weights added to provide primary balance, result in a secondary out of balance force, at right angles to the primary.

In a loco, this results in hammer blow, which increases the loads, and therefore, wear on the track.

This does not endear them to the civil engineers, who reduce the maximum allowed axle loads to compensate!

Reputedly, a type of loco built and used on (I think) the LBSC, had greatly reduced primary balance, to minimise hammerblow, but they gave the passengers in the front carriage a very rough ride, shaking them to and fro!

By using more cylinders, the balance weights become smaller and lighter, decreasing the secondary out of balance forces.

As a digression, the Brough Superior motorcycle (Known, with R-R agreement, as the Rolls Royce of Motorcycles) always used two cylinder ninety degree twins. With each cylinder subjected to primary balancing, the secondary out of balance matched the forces from the other cylinder.

Unfortunately, it is not really practicable to build locos with the cylinders, (as opposed to crankpins) disposed at ninety degrees to each other.

Ninety degree V4 and V8 engines are smooth, rotationally, and well balanced, vertically and horizontally, but tend to squirm because the various forces become distanced from those that they are supposed to counteract.

In the model world, hammerblow may be of less consequence, because, although the rails are smaller, lighter, and much less strong . ( Beam strength is related to the cube of the beam depth) The out of balance forces are related to the square of the speed, and the components involved are so much smaller (and since volume is effectively a square law effect) much lighter, so drastically reducing the forces.

The Romney Hythe and Dymchurch Railway, at one third scale, had two locos, designed by Henry Greenly, with three cylinders. But with the additional complication, and the difficulty of the boiler being able to feed three cylinders adequately, they were converted to two cylinders. To this day, they acquit themselves well.

Howard

[Neil WyattModerator @neilwyatt]

> 1) A two cylinder engine, whether single or double acting has the cranks set at ninety degrees to ensure self starting.

A 90-degree single acting arrangement won't self a start in all positions, each cylinder has a lot less than 180-degrees of push, so overlapping them guarantees a big dead spot. Look how the admission periods in this diagram:

These valve event diagrams show how two engines have admission much less than 180 degrees, and bear in mind that self starting can only happen for the part of the admission cycle between just after TDC and just before full valve closure. This means a 90-degree single-acting is actually less likely to self start than a single double acting cylinder. Such an engine also produces almost as uneven torque as a single-cylinder single acting eninge.

The simplest self starter (other than elaborate non-dead centre designs) is a three cylinder single acting at 120-degrees or a double cylinder double acting at 90 degrees.

I don't dispute your other points, but as your comment was presumably a reaction to mine about the french engine…

Neil

Edited By Neil Wyatt on 30/09/2014 17:46:31

[WearyParticipant @weary]

Steam railway locomotives can be, and have been, built with cylinders at 90 degrees to each other as 'V' twin.

Just 'google' Heisler locomotive.

Regards,

Phil.

[Neil WyattModerator @neilwyatt]

But always double acting?

Neil

[WearyParticipant @weary]

Neil,

Was in response to Howard's statement (ninth paragraph) at 16:51:57:

quote: "Unfortunately, it is not really practicable to build locos with the cylinders, (as opposed to crankpins) disposed at ninety degrees to each other".

Regards,

Phil

Edited By Weary on 30/09/2014 21:06:54

[julian atkinsParticipant @julianatkins58923]

as a bit of an aside to Howard's excellent summary,

the LBSCR (Stroudley) wheel balancing was a pet thing of Stroudley's to reduce strain on his inside cylindered loco crank axles and reduce axlebox wear.

in the UK the loading gauge imposes a limit on the size of outside cylindered locos for big locos to make use of the adhesive weight of such 'big' locos, limiting tractive effort other than increasing the boiler pressure. hence the use of more than 2 cylinders. most UK 4 cylinder locos had divided drive (ie the 2 inside cylinders working on the front coupled axle, and the 2 outside working on the middle coupled axle). this reduced hammerblow, as also happens on a 3 cylinder loco. then we have the SR Lord Nelsons and in miniature the Holcroft LBSC design giving further variations.

Churchward imported 3 French compounds for comparison with his standard 2 (outside) cylinder locos, and was greatly impressed by the smooth running of same and so settled on this arrangement of 4 cylinders with divided drive for his famous Stars, Castles, and Kings.

cheers,

julian

[Martin KyteParticipant @martinkyte99762]

It occurs to me that with smaller cylinders in a 3 cylinder design over the same overall displacement with 2 there is less steam to shift each intake stroke and it may be that this would lead to better steam transfer characteristics if designed right and subsequently higher cylinder pressures when working hard. This is obviously in addition to the issues already quoted. I suppose in practice loading gauge limits the size of outside cylinders and to get more power you have to have more cylinders which have to go somewhere.

Martin

[John LluchParticipant @johnlluch21161]

Thanks for this fully steamed discussion . So basically to answer my original question: since a model locomotive will rarely have any loading gauge constraints, provided everything is well designed I can just go for 2 sideways, double acting cylinders of the appropriate size.

John

[HopperParticipant @hopper]

Posted by Howard Lewis on 30/09/2014 16:51:57:…

As a digression, the Brough Superior motorcycle (Known, with R-R agreement, as the Rolls Royce of Motorcycles) always used two cylinder ninety degree twins. With each cylinder subjected to primary balancing, the secondary out of balance matched the forces from the other cylinder.

Howard

To digress just a tad further, the Broughs used a 50-degree V-twin engine, with associated shake rattle and roll.

90 degree V-twins with the perfect primary balance include the Ducati and various Japanese "copies" there-of. And they do run beautiful and smooth. But different type of engine altogetehr with one vertical and one horizontal cylinder.

And even further from the OP but worth a chuckle is the story that when Rolls Royce sent a couple of inspectors over to the Brough works to see what all these claims about "Rolls Royce of Motorcycles" were all about, they just happened to be shown a special model being hand-assembled for the annual Earls Court bike show. All nuts were covered in tape before assembly to prevent spanner marks and so on. They were so impressed with the care that Broughs were (apparently) assembled with, they agreed to let Brough continue to use the RR name in their advertising.

[Nick HughesParticipant @nickhughes97026]

As a bit of a "Curve ball", how about 8 single acting cylinders with rotary sleeve valves:- **LINK**

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