A Compact Pollution-Free
External Combustion Engine
with High Part-Load Efficiency
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7. Features of the Cycle
The somewhat unusual features of this Cycle, particularly with respect
to earlier engines, warrant some discussion and justification. This is
There are low expansion ratios per stage, about 2.3 to one.
This has three advantages:
- High Expander adiabatic efficiency. Losses tend to come from inlet
and exhaust turbulence, which is minimized because of more relaxed valve
operation, and from thermal losses, which are much less due to reduced
temperature swings. The actual expansion is low velocity and essentially
- High ratio of Mean Effective Pressure to Peak Pressure. This reduces
loads on bearings and other drivetrain components, and should be smoother.
In general, it allows for higher Expander speeds and smaller bearings,
cranks, and connecting rods.
- High pressure is confined to small sizes. The Expander cylinder size
depends on the volume of its exhaust steam. Smaller size means
lighter weight and greater safety.
Note that total expansion ratio across all four stages is very high,
about 26 to one.
Therefore the total work per unit mass extracted from the steam is also
The Expander exhaust steam preheats air to the Burner.
Less fuel is therefore needed to reach a given combustion temperature.
But there are other advantages of the very hot Burner inlet air and the
- It makes it easier to burn difficult fuels. This engine is intended
to be truly multifuel.
- It preheats the fuel so that it can ignite more readily.
- As mentioned, the steam entering the Condenser is wet, reducing Condenser
- Net efficiency is high even though the condensing temperature is quite
- Sufficient Regenerator and Steam Generator driving temperature differences
arise automatically from the relative air and steam mass flows.
There is no feedwater preheating. It is replaced by the burner
air preheating, which is more effective. The exhaust gas is therefore
at the minimum possible temperature giving the minimum heat loss out the
tailpipe. Also, at very low load settings, water vapor in the combustion
products condenses as it passes through the Boiler, allowing use of the
fuel higher heating value. If the feedwater were preheated, an exhaust
gas recuperator would be needed or there would be insignificant, if any,
There is a simple Throttle Valve between the Steam Generator
and the Expander. Many steam engines use variable cutoff for
control, to maximize expansion ratio at part-load. But the advantages
of a simple Throttle Valve are:
- All downstream components are at reduced pressure during normal operation.
- The lower pressure carries all the way to the Condenser, resulting
in lower heat rejection temperature. This increases efficiency at part
- Steam Generator pressure is constant. There is therefore a reserve
of pressure just behind the Expander for rapid response. Also, the Steam
Generator is not subject to pressure cycling that could cause fatigue
- Heat addition in the Reheats increases, which also increases efficiency.
The increased reheat temperatures are tolerable because the reduced
pressure at part load reduces metal stresses. This stress reduction
is greater than the reduction in metal strength due to higher temperature.
Note that the only energy loss due to throttling is compressing the
liquid phase feedwater. This is very small. It is a unique advantage of
the Rankine Cycle.
There is a two-stage Combustor. This is not part of
the invention but is a worthwhile feature. As discussed later, it has
been shown to improve combustion. The fuel first mixes with a small amount
of the preheated air, giving a very rich air/fuel ratio. There it is heated,
vaporized, and partially burned. Then it is mixed with about 100% excess
air for very complete combustion at moderate temperature.
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