Editing Steam engine (section)

=== Uniflow (or unaflow) engine ===
{{Main|Uniflow steam engine}}
[[File:Uniflow steam engine.gif|thumb|upright=1.15|Animation of a [[uniflow steam engine]].<br />The [[poppet valves]] are controlled by the rotating [[camshaft]] at the top. High-pressure steam enters, red, and exhausts, yellow.]]

Uniflow engines attempt to remedy the difficulties arising from the usual counterflow cycle where, during each stroke, the port and the cylinder walls will be cooled by the passing exhaust steam, whilst the hotter incoming admission steam will waste some of its energy in restoring the working temperature. The aim of the uniflow is to remedy this defect and improve efficiency by providing an additional port uncovered by the piston at the end of each stroke making the steam flow only in one direction. By this means, the simple-expansion uniflow engine gives efficiency equivalent to that of classic compound systems with the added advantage of superior part-load performance, and comparable efficiency to turbines for smaller engines below one thousand horsepower. However, the thermal expansion gradient uniflow engines produce along the cylinder wall gives practical difficulties.{{citation needed|date=January 2013}}.