Serpollet
(Simple, Single and Double Acting, Poppet Valve Steam Engine)
Leon Serpollet, an early automobile developer, built flash boilers which raised steam rapidly while his poppet valve
engines were very successfully run with superheated steam. He briefly held the Land Speed Record in 1902 at
75.06 mph, with a 4 cylinder, single acting steam engine of about 105 horsepower and 75 x 90 mm bore x stroke. At
a time when saturated steam and slide or piston valves predominated, Serpollet was pushing the envelope towards
the future.
The Serpollet flat-4 cylinder engine (top, left hand corner of page) is
illustrated above. Lacking crosshead or piston rod, it is a single
acting engine similar to modern internal combustion engines and
uncannily resembles the VW Beetle flat 4 and similar light aircraft
engines. Much like a small block Chevy V-8, a central camshaft
operates the poppet valves via pushrods; the camshaft is turned by
counter-rotating gears driven from the crankshaft. A roller following
the cam surface pushes a tappet inside a bushing, which in turn
activates the valves, the tappet having two screws to adjust the ‘play’
in the valve train as it rocks the lever. The valve stem is held against
the lever by the valve spring, which also holds the poppet valve head
tight against the seat; when the lever pushes the stem, the valve lifts
from the seat, admitting steam to the cylinder
Clearance volume is not just the space between the piston at TDC and the cylinder head; it is all volume in free
communication with the cylinder that is not swept by the piston during the stroke. Other factors equal, efficiency
improves as clearance volume diminishes. The passages to and from the valve are clearance volume, thus we can
see that “D” and piston valves have more clearance than poppet valves and DA engines often have similarly greater
clearance, as in the Serpollet example above.
The Serpollet engines also use labyrinth packing on the
piston rod, as illustrated in the blowup to the right. Sealing
is improved by injecting oil into the seal immediately next
to the cylinder end, the steam having to force the oil
through the labyrinth before escaping. Since oil is more
viscous than steam, it flows through the labyrinth much less
readily, correspondingly diminishing the leakage. The
escaping oil serves to lubricate the crosshead and then vents
out of the engine.
The narrowed gap at the top of the groove restricts steam flow at full pressure. Upon entering the downstream
groove, the flow becomes turbulent, which dissipates the flow velocity. The flow is further restricted by the
narrow gap between the next groove and further turbulence occurs when the steam reaches that groove. And so
on. Labyrinth seals cannot fully stop leakage but they can reduce it to an acceptable value. Since packing is
compressed against the rod, it has the potential for a more complete seal, but this compression also leads to
friction that requires lubrication, entails wear and increases mechanical losses. Labyrinth seal images
Admission and exhaust valves for the Serpollet DA engine are
shown at right, the paired admission and exhaust valves for each
cylinder are stacked atop each other, the engine itself is at the top
right side of the page. Incoming steam pressure forces the
admission poppet valves (upper drawing) against the seats while
cylinder pressure likewise forces the exhaust valves closed. These
forces result in tight seals until the camshaft forcibly lifts the valves
from their seats. Because the sealing surface lifts away from the
seat rather than sliding along it, there is minimal friction, wear and
the need for lubrication is much reduced. The valves are light,
moving rapidly with greater ease, and a better choice for very
short cutoff engines.
Clearance volume is not just the space between the piston at TDC and the cylinder head; it is all volume in free
communication with the cylinder that is not swept by the piston during the stroke. Other factors equal, efficiency
improves as clearance volume diminishes. The passages to and from the valve are clearance volume, thus we can see
that “D” and piston valves have more clearance than poppet valves and DA engines often have similarly greater
clearance, as in the Serpollet example above.
For a given pressure and cutoff, a steam cylinder performs a fixed amount of work per stroke, the total amount of
work being done depends upon the number of strokes completed. Power is defined as the amount of work done in a
fixed period of time, an engine running at 200 strokes each minute (rpm) produces twice the power it would at 100
rpm because it has done twice the work. An engine can be made more powerful by increasing the pressure (which
includes lengthening the cutoff), making the cylinder larger and running the engine at higher speed, or any
combination of all three. A number of factors come into play when making these choices, higher rpm is more difficult
to achieve because the valves must move faster, the steam has less time to enter the cylinder and more there is more
harmful inertia in the reciprocating components. On the other hand, higher rpm gives steam less time to work past
the piston rings and reduces loss of steam to blow-by; it also allows a smaller and lighter powerplant to achieve the
same power. DA engines are effectively twice as large because they work in both directions, but the high
reciprocating forces developed by the crosshead, piston and rod serve to make higher rpm operation difficult.
A notable difference between Serpollet and the Stanley and White engines is the sealing of piston rods and valve
stems. As you will recall, a packing gland compressed the lubricant soaked packing against the shaft and stuffing
box walls to minimize leakage along the shaft. Serpollet skipped the packing in favor of a series of grooves cut into
the valve stem, called a “labyrinth seal.”
The Serpollet engines also use labyrinth packing on the
piston rod, as illustrated in the blowup to the right. Sealing
is improved by injecting oil into the seal immediately next
to the cylinder end, the steam having to force the oil
through the labyrinth before escaping. Since oil is more
viscous than steam, it flows through the labyrinth much less
readily, correspondingly diminishing the leakage. The
escaping oil serves to lubricate the crosshead and then vents
out of the engine.
The narrowed gap at the top of the groove restricts steam flow at full pressure. Upon entering the downstream
groove, the flow becomes turbulent, which dissipates the flow velocity. The flow is further restricted by the
narrow gap between the next groove and further turbulence occurs when the steam reaches that groove. And so
on. Labyrinth seals cannot fully stop leakage but they can reduce it to an acceptable value. Since packing is
compressed against the rod, it has the potential for a more complete seal, but this compression also leads to
friction that requires lubrication, entails wear and increases mechanical losses. Labyrinth seal images
Admission and exhaust valves for the Serpollet DA engine are
shown at right, the paired admission and exhaust valves for each
cylinder are stacked atop each other, the engine itself is at the top
right side of the page. Incoming steam pressure forces the
admission poppet valves (upper drawing) against the seats while
cylinder pressure likewise forces the exhaust valves closed. These
forces result in tight seals until the camshaft forcibly lifts the valves
from their seats. Because the sealing surface lifts away from the
seat rather than sliding along it, there is minimal friction, wear and
the need for lubrication is much reduced. The valves are light,
moving rapidly with greater ease, and a better choice for very
short cutoff engines.
Clearance volume is not just the space between the piston at TDC and the cylinder head; it is all volume in free
communication with the cylinder that is not swept by the piston during the stroke. Other factors equal, efficiency
improves as clearance volume diminishes. The passages to and from the valve are clearance volume, thus we can see
that “D” and piston valves have more clearance than poppet valves and DA engines often have similarly greater
clearance, as in the Serpollet example above.
For a given pressure and cutoff, a steam cylinder performs a fixed amount of work per stroke, the total amount of
work being done depends upon the number of strokes completed. Power is defined as the amount of work done in a
fixed period of time, an engine running at 200 strokes each minute (rpm) produces twice the power it would at 100
rpm because it has done twice the work. An engine can be made more powerful by increasing the pressure (which
includes lengthening the cutoff), making the cylinder larger and running the engine at higher speed, or any
combination of all three. A number of factors come into play when making these choices, higher rpm is more difficult
to achieve because the valves must move faster, the steam has less time to enter the cylinder and more there is more
harmful inertia in the reciprocating components. On the other hand, higher rpm gives steam less time to work past
the piston rings and reduces loss of steam to blow-by; it also allows a smaller and lighter powerplant to achieve the
same power. DA engines are effectively twice as large because they work in both directions, but the high
reciprocating forces developed by the crosshead, piston and rod serve to make higher rpm operation difficult.
A notable difference between Serpollet and the Stanley and White engines is the sealing of piston rods and valve
stems. As you will recall, a packing gland compressed the lubricant soaked packing against the shaft and stuffing
box walls to minimize leakage along the shaft. Serpollet skipped the packing in favor of a series of grooves cut into
the valve stem, called a “labyrinth seal.”