POWER UN­DER PRES­SURE

Steam en­gines have huge power and torque from ef­fec­tively zero revs, but how does that trans­late into re­al­ity? Who bet­ter to ex­plain than Cay­man owner Peter May­nard, who has ex­pe­ri­ence of many dif­fer­ent types, from diminu­tive 0-6-0s to mas­sive 2-10-0 frei

911 Porsche World - - Big-bore Engines -

Most road ve­hi­cles have in­ter­nal com­bus­tion en­gines of one form or an­other, but the steam lo­co­mo­tive has at least two ex­ter­nal com­bus­tion en­gines, fed with steam from a boiler. In­stead of gen­er­at­ing en­ergy within each cylin­der, the steam lo­co­mo­tive cre­ates the en­ergy re­quired to move it by heat­ing wa­ter by fire, most of­ten us­ing coal, but some­times oil.

Each ‘engine’ on the lo­co­mo­tive typ­i­cally takes the form of a cast-iron cylin­der block with an in­te­gral valve chest lo­cated above the cylin­der. Some en­gines have two cylin­ders, some larger ones three. The valve em­ployed may be of the slide type or, on more pow­er­ful lo­co­mo­tives, a pis­ton valve slid­ing to and fro, ad­mit­ting and ex­haust­ing steam to and from each end of the cylin­der in turn. (The steam engine scores a point over its in­ter­nal-com­bus­tion ri­vals by be­ing dou­ble-act­ing. Ev­ery pis­ton stroke counts.) Steam en­gi­neers flirted with pop­pet valves – as in mod­ern in­ter­nal­com­bus­tion en­gines – but seemed al­ways to re­turn to the trusty pis­ton valve.

Steam lo­co­mo­tives don’t have a gearbox but they do have a ‘re­verser’, which not only con­trols the di­rec­tion of travel (in the­ory the engine can travel as fast in ‘back gear’ as it can in for­ward gear) but also the amount of steam ad­mit­ted to the cylin­der dur­ing each pis­ton stroke. This can be as much as 75 per cent (steam is ad­mit­ted for the first three-quar­ters of the pis­ton travel, and then ‘cut off’ for the re­main­ing quar­ter) to as lit­tle as none, in which case the loco is in ‘mid-gear’, ie not in for­ward or back gear.

The work done by ex­pand­ing steam in the cylin­der is con­verted to mo­tion along the track by a con­nect­ing-rod and a crank at­tached to one of the driv­ing wheels (or driv­ing axles in the case of a cylin­der in­side the lo­co­mo­tive’s frames). Power – typ­i­cally ex­pressed in pounds of ‘trac­tive ef­fort’ – is a func­tion of boiler pres­sure, cylin­der di­am­e­ter and driv­ing-wheel size. Higher boiler pres­sure: more force to drive the pis­tons. Big cylin­ders: able to ac­com­mo­date more steam. Small driv­ing wheels: the work car­ried out dur­ing one ro­ta­tion moves the train a smaller dis­tance than would be the case with a big driv­ing wheel. Freight en­gines that needed to move heavy trains at low speeds had small driv­ers, whereas the ‘race­horses’ like Fly­ing Scots­man, de­signed to work faster and lighter pas­sen­ger ex­presses, had much larger ones – ef­fec­tively a higher fi­nal drive. So-called ‘mixed traf­fic’ lo­co­mo­tives had a compromise some­where in be­tween.

As­sum­ing full boiler pres­sure – on a large, mod­ern lo­co­mo­tive over 200 pounds per square inch (say around 1400kpa or 14 bar) – the driver has at his dis­posal max­i­mum power and torque at max­i­mum (ie 75 per cent) cut-off. Thus when start­ing from rest ju­di­cious ap­pli­ca­tion of the reg­u­la­tor, which con­trols the flow of steam from the boiler to the cylin­ders, is called for in or­der to avoid wheel­slip. ‘Trac­tion con­trol’ is the driver’s hand grip­ping the reg­u­la­tor han­dle, deftly re­duc­ing the flow of steam through the reg­u­la­tor valve. Large reg­u­la­tor open­ings and long cut-offs, though, are hugely waste­ful: in car terms it would be like cruis­ing at 60mph in sec­ond gear. The cor­rect ap­proach, once nicely on the move, is to re­duce the cut-off so that steam is ad­mit­ted to the cylin­ders for a shorter length of the pis­ton stroke; it takes only a rel­a­tive puff of steam to keep the train mov­ing. And at the same time the reg­u­la­tor can be opened more widely; the re­duc­tion in torque due to the shorter cut-off re­duces the like­li­hood of wheel­slip.

As­sum­ing suf­fi­cient trac­tion is avail­able (and you are deal­ing with nar­row steel ‘tyres’ on pos­si­bly wet and greasy steel rails, re­mem­ber), max­i­mum ac­cel­er­a­tion is with full reg­u­la­tor and 75 per cent cut-off; the car equiv­a­lent is a wide-open throt­tle in first gear. Once up to speed, full reg­u­la­tor de­liv­ers max­i­mum power but at a short cut-off a lower amount of torque. Think of it as be­ing akin to your foot flat on the floor in sixth or sev­enth gear. And, just as you change down (for more torque) to climb a hill, so the engine driver length­ens the cut-off to achieve the same ef­fect.

With steam lo­co­mo­tives, then, there is no ‘rev-drop ef­fect’. They ac­tu­ally have a con­tin­u­ously vari­able trans­mis­sion, ex­cept that the vari­abil­ity comes from the valves that con­trol the ad­mis­sion of steam to the cylin­ders. And steam lo­co­mo­tives are cer­tainly not ‘au­to­mat­ics’.

The au­thor of this piece, Peter May­nard, now owns a 2015 Cay­man GTS in place of this 2013 ‘S’ model (be­low left), pho­tographed at the her­itage Great Cen­tral Rail­way in Lough­bor­ough, Le­ices­ter­shire, where he both fires and drives all kinds of clas­sic Bri­tish steam en­gines. That’s Peter – lucky chap – at the con­trols of 92220Evening Star, as it was badged in 2015, as a trib­ute to the last such lo­co­mo­tive built by Bri­tish Rail­ways in 1960, al­though it has since been re­turned to its ‘cor­rect’ guise, 92214 (be­low). Key to any such engine’s ef­fi­cient op­er­a­tion is the so-called re­verser (far left, bot­tom), which de­ter­mines the per­cent­age of each pis­ton stroke dur­ing which steam is ad­mit­ted to the cylin­ders – hence the num­bers you can see on the drum. Think of it as a cross be­tween a Porsche engine’s vari­able valve tim­ing and its gearbox

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