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Advanced Uniflow Engine - Dan Gelbart
Dan
Gelbart's
Advanced
Uniflow
Steam
Engine
Working
model
An Advanced Uniflow Steam Engine has been built to overcome some of
the limitations of traditional steam engines. In the history of the
steam engine, development was prematurely stopped after the
introduction of the more efficient diesel engine and steam turbine in
the early 1900s. A uniflow steam engine is an alternative to the more
common compound steam engine.
The Advanced Uniflow Steam Engine has increased efficiency, no valve
gear, no lubricating oil mixed with the steam, no vacuum condenser,
and electronic control of power and speed. Modern materials,
manufacturing techniques, and electromechanical valve actuation make
these advances possible.
(Note: Click on a picture to see a larger view and then use your
browser to enlarge it further.)
Contents
1. History
2. Description
2.1 Design Objectives
2.2 Engine Operation
2.3 Major Components
2.4 Intake and Exhaust Valves
2.5 Electronic Controls
3. Design Details
3.1 Constant Diameter Cylinder & Piston
3.2 No Lubrication
3.3 Test Results
3.4 Scalability
4. References
5. External links
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History:
Carnot's theorem (thermodynamics) established the theoretical limit
to Rankine cycle heat engine efficiency in 1824; hotter input
temperatures increase efficiency. Typical steam engines operate at
10% to 25% efficiency. To maximize efficiency, steam engines are
designed to discharge the steam at the lowest possible temperature
and pressure - about 100 degrees C and at atmospheric pressure
(unless a vacuum condenser is used). In a conventional steam engine,
the steam enters and is discharged through the same valve. This
creates two problems. The first is that discharged steam needs a
large port, which requires significant energy to operate against the
pressure of the intake steam. The second problem is that discharged
steam cools the intake valve and steam passages, leading to a
significant energy loss.
The first of these problems was addressed in 1804 by the compound
steam engine, expanding the steam in stages through several
cylinders, each cylinder having larger diameter and larger ports.
Both problems were addressed in 1827 by the uniflow steam engine in
which the steam enters the cylinder via a small high pressure port
and leaves the cylinder by a large port in the cylinder wall,
uncovered by the piston towards the end of its stroke. The inlet port
always stays hot while the exhaust port remains at approximately 100
degrees C. The uniflow engine was popularized by in the early 1900s
(Stumpf, Johann 1905). Unfortunately this was also a time when
interest in reciprocating steam engines was falling rapidly, since
the recently invented (1893) diesel engine achieved efficiencies of
about 40%, which is much higher than any reciprocating steam engine
can achieve even today (however steam turbines can).
Even with the uniflow engine, several problems remain. If oil is
added to the steam to lubricate the piston, it later forms a solid
layer inside the boiler that reduces heat transfer. Another problem
is poor sealing of the piston in the cylinder over a wide temperature
range along the length of the cylinder. Finally, the complex
mechanical linkage of the intake valve gear consumes significant
power due to frictional losses and steam leaks (Cozby 2012). Some of
the intake valve problems were addresses by uniflow engines with
piston operated valves (Curtis 1901; Divine 1975; Harvey 1968; Kaneff
1979). The most advanced steam engine design (Sturtevant 1968) had
electric control of steam cutoff but did not have a simple solution
to allow the steam cutoff to be reduced to zero, or a method to keep
the electromagnet from being overheated by the steam. All these
problems are addressed in the Advanced Uniflow Steam Engine,
developed by Dan Gelbart in Vancouver, BC, Canada in 2010.
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Description:
Design Objectives:
1. No Valve Gear - Eliminate camshaft, steam leaks and mechanical
losses.
2. Electromechanical Intake Valve - Electronic control of steam
cutoff (steam engine).
3. Zero minimum cutoff - Minimize steam consumption at very light
loads.
4. Elimination of Spent Steam - No vacuum condenser (surface
condenser) is needed to purge used steam from the cylinder.
5. No Lubrication - No oil (or other lubricants) injected into the
steam.
6. Better Sealing - No steam leaks from valves or between the piston
and cylinder (better piston guidance).
7. High Efficiency - Reduce energy loss as result above
improvements.
Back to Top
Engine Operation:Unilfow Engine operation animation
Advanced Uniflow Steam Engine. Color code: Red = high-pressure steam;
Orange = expanding steam; Yellow = spent steam at atmospheric
pressure.
1. Intake: Steam enters the two-stroke engine at the red header
chamber which has a spring-loaded intake valve, held closed by
high-pressure steam. When the piston bumps the intake valve open,
a solenoid coil is energized to hold it open. The intake valve
open period is electronically controlled; when electric power to
the solenoid is stopped (steam cutoff), the intake valve closes.
2. Power: The exhaust valve at the top of the piston is forced
closed during the power stroke. As with all uniflow engines, the
engine extracts all the power in the expanding steam in one
piston stroke [4].
3. Exhaust: Near the end of power stroke, pressure drop in the
cylinder allows a spring to open the exhaust valve at the top of
the piston. Spent steam exits through side ports in the cylinder
wall. During the return stroke, the exhaust valve in the piston
remains open and the hollow piston pumps the remaining spent
steam out of the cylinder (eliminating the need for a vacuum
condenser).
Back to Top
Major Components:
Uniflow Engine General Layout
System Components of the Advanced Uniflow Steam Engine
The complete system includes a boiler, atmospheric pressure
condenser, steam engine, and electronic controls. The model steam
engine is a small (~9 cubic cm engine displacement) but produces .5
HP at 6000 RPM.
Steam is produced in a heavily insulated vertical fire-tube boiler
with a superheater (made from 316L stainless steel, TIG welded). A
recuperator (not shown) preheats intake air by heat exchanging with
the burner exhaust gas. The heat source for the boiler is a small
camping stove burner fueled by butane or propane. A vertical
electrode (that measures conductivity) senses water level in the
boiler. Feed water for the boiler is pre-heated in the condenser
reservoir by exhaust steam from the engine. A water pump powered by
the steam engine supplies feed water to the boiler. An
electromagnetic valve cuts off the water supply when boiler is full.
A steam pressure sensor controls gas flow to burner; the flame is
reduced to a pilot when boiler pressure reaches 40 bar (580 PSI). A
diaphragm servo valve controls gas flow. The fire-tube boiler
requires 4 minutes to reach operating pressure.
Photo of Advanced Uniflow Steam Engine Model. For
Gelbart Advanced Uniflow Steam Engine a video of the operating engine see external
links below.
Back to Top
Intake and Exhaust Valves:
Uniflow Engine Section Detail
Section Drawing of Advanced Uniflow Steam Engine
Steam from the boiler enters the cylinder at one end and exits via
exhaust ports at the other end, based on standard uniflow engine
practice. This allows the intake valve to stay hot at all times. The
valves are completely sealed within the cylinder.
1. Intake Valve: A key feature of the advanced uniflow steam engine
is electromagnetic cutoff for the intake valve, which eliminates
losses from mechanical valve gear and allows precise control of
speed and power. When the piston reaches the leftmost end of the
cylinder (TDC - top dead center) the tip of the exhaust valve
impacts the intake valve, forcing it open against the pressure of
the inlet steam (the tip of the exhaust valve momentarily blocks
the intake orifice, allowing zero cutoff). Once the intake valve
opens, a small external solenoid coil is energized to keep it
open (the solenoid does not have enough power to open the intake
valve by itself). The magnetic field from the coil penetrates
into the cylinder via a ferromagnetic insert, while the cylinder
head is completely sealed. There are 0.1 mm air gaps between the
coil and the cylinder - and also between the coil and the
ferromagnetic insert - to reduce heat transfer to solenoid coil.
A ferromagnetic disc attached to the back of the intake valve
allows the electromagnet solenoid to keep the valve open for a
variable period (cutoff). A small spring helps close the intake
valve when the magnetic field stops. The travel of the intake
valve is small, about 0.8 mm in this model. The intake valve is
held open 5 to 10 percent of the time, depending on power
required. The intake valve is made from silicon nitride, as steel
will develop pitting from the high-speed steam flow in the small
gap.
2. Exhaust Valve: A classic uniflow engine requires a vacuum
condenser to remove spent steam from the cylinder after the power
stroke. In the advanced uniflow engine, the exhaust valve is in
the end of the hollow piston. It is forced into the closed
position when it strikes the intake valve (TDC) and is held
closed by steam pressure during the power stroke. At the end of
power stroke, when steam pressure in the cylinder drops as the
side ports in the cylinder wall are exposed, a spring opens the
exhaust valve. The exhaust valve remains open during the return
stroke and the hollow piston pumps the remaining spent steam out
of the cylinder side ports; the returning piston is not resisted
by re-compression of spent steam (providing smoother power and
reducing the required mass of the flywheel). The travel of the
exhaust valve in the piston is about 2 mm. It is open about 60
percent of the time. The exhaust valve is made from 440C
stainless steel, hardened to Rc60.
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Electronic Controls:
An absolute shaft encoder measures the angle of crankshaft rotation;
a control circuit (logic circuit or a microprocessor) derives the
position of the piston in the cylinder, and also the engine RPM. When
the piston reaches TDC, the intake valve is forced open by the
piston; immediately thereafter, the control circuit activates the
solenoid to keep the intake valve open to allow steam to enter the
cylinder. When the control circuit switches off the current to the
solenoid coil, the intake valve closes (steam cutoff). For example,
as the load increases on the steam engine, the control circuit can
hold the intake valve open for a longer period during the power
stroke (letting in more steam), and thus maintain constant RPM. Other
control modes for variable speed or power are possible.
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Design Details:
Dan Gelbart's-Uniflow Constant Diameter Cylinder
Constant Diameter Cylinder and Piston (The diameter change with
temperature is highly exaggerated for clarity.)
The cylinder of a uniflow steam engine has large temperature gradient
along its length, since - in order to maintain efficiency - it cannot
be cooled (unlike a water-cooled IC engine). The hot end of the
cylinder will have a larger diameter than the cooler exhaust end.
However, an "athermalized" cylinder design (ID not effected by heat)
can maintain good sealing of the piston at all cylinder temperatures.
A constant diameter cylinder has an inner liner made of a material
with a higher coefficient of thermal expansion (CTE) and a lower
Young's modulus than outer cylinder material. When heated, the liner
tries to expand faster than the cylinder body but it is constrained
from doing so by the smaller CTE of the cylinder. Since the outer
cylinder expands less, the liner material is compressed and the
inside diameter (ID) remains constant. Thus, a temperature gradient
along the cylinder will not change the ID. The liner material (carbon
filled polyimide, DuPont Vespel SP-21) was selected for
self-lubrication and very low wear against the hard materials used
for the piston. The Vespel liner wall thickness should be about 7% of
the liner OD (inserted in a 440 stainless cylinder) to maintain
constant ID over a wide temperature range. The liner is cooled before
insertion into the outer cylinder.
The piston is fabricated from Invar alloy, which has a near-zero CTE.
The piston is plated with electroless nickel (EN) hard coating to
achieve low wear. Depending on the materials selected for the
cylinder liner and piston plating, Teflon piston rings may - or may
not - be required.
No Lubrication
Superheated steam (250-300 degrees C) provides little lubrication for
a piston moving in a cylinder. By carefully choosing compatible
materials for the cylinder liner and the piston coating, an engine
requiring no lubrication can be achieved. When the piston has a very
good fit to the cylinder (at all temperatures) and has a large
contact area with the cylinder, hydrodynamic lubrication can be
achieved when the piston is in motion. Under such conditions, wear is
minimal - allowing the use of steam with no oil or other lubrication.
Test Results
A scale model of the engine was built with a cylinder ID of 18 mm,
stroke of 36 mm, working pressure of about 30 bar (435 PSI), and
speed of up to 6000 RPM. The model was coupled to a small generator
and was tested with an electrically heated boiler and atmospheric
pressure condenser. Efficiency was measured as the ratio of the
electrical power produced by the generator, corrected for generator
efficiency, divided by the boiler net heating power (about 1KW, which
was the limit of the test boiler). While it is difficult to achieve
high efficiencies in a small-scale model, the demonstration engine
achieved an efficiency of about 10%. At 40 bar (580 PSI) pressure, in
burst mode with steam stored in the boiler, the engine produced over
300W.
Scalability
The advanced uniflow steam engine can be scaled to larger sizes using
modern materials and designs. The electromagnetic cutoff for the
intake valve (no valve gear) is a major improvement. A constant
diameter cylinder can be made from cast iron with a low-friction
carbon-polymer liner (which is also a good insulator). An Invar
piston with a diamond-like coating (DLC) may be able to achieve low
wear and a good seal without piston rings. The front surface of the
exhaust valve in the piston should be plasma coated with zirconia to
insulate it from the inlet steam. With a constant diameter cylinder
and a precise fit for the piston, a bulky crosshead mechanism may not
be needed.
For a large stationary engine, other designs may work best, while
still using electromagnetic cutoff for the intake valve. The cylinder
could be machined with a tapered bore, to approximate constant
diameter when heated at one end. Both the cylinder and piston may be
plasma coated with zirconia. Piston rings of polymer/graphite or pure
graphite can provide lubrication and a good seal. A crosshead will
eliminate side forces of the piston on the cylinder, reducing wear.
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References:
1. Curtis, N. (1901) "Engine" US Patent 671,394 Uniflow Steam Engine
with Exhaust Valve in Piston
2. Divine, W.J. (1975) "Uniflow Steam Engine" US Patent 3,910,160
Uniflow Steam Engine with Inlet and Exhaust Valves Operated by
Piston
3. Harvey, R. (1968) "Steam Engine with Self-Contained Valvular
Mechanism" US Patent 3,361,036 Uniflow Steam Engine with Piston
Operated Intake Valve and Exhaust Valves in Piston
4. Kaneff, S. (1979-89) "The White Cliffs Solar Steam Engine"
Australian Government Report on White Cliffs Solar Project
5. Schoell, H. (2006) "Heat Regenerative Engine" US Patent 7,080,512
Uniflow Steam Engine that Uses Water as Both the Working Fluid
and as the Lubricant
6. Stumpf, Johann (1905) The Una-flow Steam Engine (Johann Stumpf
(engineer))
7. Sturtevant, H.V. (1968) "Steam Engine with Inlet Valve Mechanism"
US Patent 3,397,619
Back to Top
External links:
1. Dan's Uniflow Steam Engine (YouTube)
2. Cyclone Uniflow Steam Engine
3. Uniflow Steam Engine with Piston Operated Intake Valves
Tom says these two patents look vaguely familiar:
1934 E. L. Fickett Patent - simular uniflow engine
1902 R. O. Hood Valve Gear For Engines
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