The uniflow engineJohann Stumpf, of the Berlin Technical High School · 1908
The piston engine's last word. Stumpf sent the steam one way only: in through valves at either end — kept always hot — and out through a belt of ports at the barrel's centre, uncovered by the long piston itself at the stroke's end. No part of the cylinder is asked to be both hot and cold, so a single superheated cylinder matches the compound's economy with half the machinery. Strengths: simplicity per horsepower, and thermal manners a compound needed two cylinders to learn. Weaknesses: the ports close early on the return and the trapped exhaust is squeezed nearly back to boiler pressure — harmless cushioning with a condenser, punishing without one — and its timing was fatal: Parsons' turbine was already spinning. Throw off the condenser and feel the compression trap; then look about this museum, for after 1908 the story moves to blades.
Key to the plate
a. — from the boiler house. Superheated steam at a hundred and fifty pounds — dried past saturation in the boiler-house flues, because a uniflow’s economy leans on steam that will not condense at first touch of the walls.
b. — governor. Working poppet valves at the hot ends through the shortest of cutoffs. At a hundred and ten turns a minute for the dynamos, the droop band is a few revolutions wide — the lamps in town flicker at what the governor forgives.
c. — uniflow cylinder & centre ports. The last idea in piston steam: steam enters hot at each end and leaves cold at the middle, through a belt of ports the piston itself uncovers at the end of its travel. Steam moves one way only — uniflow — so no cold exhaust ever retraces the hot inlet’s path, and each end of the cylinder keeps its temperature for good. One cylinder, compound economy.
d. — crosshead & guides. The piston is a long trunk — it must be, to blanket the centre ports at each end of the stroke — and the trapped cushion of compressed exhaust it rides on at each reversal replaces half the work of the crosshead’s bearings. Run without the condenser and that cushion turns punitive: the trap recompresses against you.
e. — fly-wheel. Belted or coupled straight to the dynamo, and balanced for speeds the Corliss men never dreamt of. Past it, history has only one more card to play, and Parsons is already playing it: the turbine.