Robert F. Bourque, Ph. D., P.E.
Bourque Engineering LLC
Los Alamos, New Mexico USA

The Bourque Steam Engine







Motivations For This Engine


Requirements and Constraints




Prototype Development


Description of the Bourque Cycle


Features of the Cycle


The Complexity Issue


Fuel Requirements


First Example Engine in a Vehicle


Description of the Expander


Expander Hot Cylinder Lubrication


Expander Piston Structural Analysis


Two More Engine and Vehicle Examples


Other Engine Components






Water Freezing


Control System


Starting Time






Some Unit Conversions


Notes and References

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 presented below.

There are low expansion ratios per stage, about 2.3 to one. This has three advantages:

  1. 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 isentropic.
  2. 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.
  3. 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 high.

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 desuperheated steam:

  1. It makes it easier to burn difficult fuels. This engine is intended to be truly multifuel.
  2. It preheats the fuel so that it can ignite more readily.
  3. As mentioned, the steam entering the Condenser is wet, reducing Condenser core size.
  4. Net efficiency is high even though the condensing temperature is quite high.
  5. 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, efficiency gain.

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:

  1. All downstream components are at reduced pressure during normal operation.
  2. The lower pressure carries all the way to the Condenser, resulting in lower heat rejection temperature. This increases efficiency at part load.
  3. 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 failure.
  4. 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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