Editing Steam engine (section)

==== Lead in the valve timing====
The above effects are further enhanced by providing ''lead'': as was later discovered with the [[internal combustion engine]], it has been found advantageous since the late 1830s to advance the admission phase, giving the valve ''lead'' so that admission occurs a little before the end of the exhaust stroke in order to fill the ''clearance volume'' comprising the ports and the cylinder ends (not part of the piston-swept volume) before the steam begins to exert effort on the piston.<ref name="Bel, Locomotives">{{cite book|last=Bell|first=A.M.|title=Locomotives|publisher=Virtue and Company|year=1950|location=London|pages=61–63}}</ref>