Rankine engines, usually in the form of steam engines, have the following inherent advantages a) lack of noise and vibration, b) good torque characteristics at low speed, c) continuous combustion, enabling the use of a variety of fuels and easy pollution control.

Such engines essentially consist of a vapour generator, an expander and a condenser. A working fluid flows through each of these units in turn so that work may be produced at the expander according to the Rankine thermodynamic cycle.

The following discussion outlines some disadvantages of conventional Rankine engines, and the reasons for their lack of popular favour over internal-combustion engines. The maximum efficiency of any heat engine is restricted by the maximum temperature of the working fluid in the expander. In a conventional Rankine engine, heat enters the working fluid by passing through a physical barrier in the vapour generator. On one side of this barrier is the heating fluid, usually the hot gases of combustion, while on the other side is the working fluid, usually water or steam. The problems imposed by such a barrier are a) The temperature gradient across the barrier restricts the maximum temperature of the working fluid to being lower than that of the heating fluid, whereas the internal-combustion engine has its highest temperatures within its working fluid. b) An efficient Rankine engine must maintain a constant high temperature at the barrier causing stress on the barrier material. The internal-combustion engine, however, achieves the high temperatures of its working fluid intermittently,

allowing adjacent surfaces to cool during each cycle, thus allowing higher maximum temperatures for a given level of material stress. c) The need for heat to flow through a barrier retards the response of the engine to changes in the temperature of the heating fluid, necessitating an engine control system which throttles from a pressurised reservoir of working fluid to cope with sudden changes in load. Such systems incur penalties of weight and complexity. d) For the effective transfer of heat, the barrier needs to have a large surface area able to withstand high temperatures and pressures. Such barriers are usually arrangements of metal tubes which incur penalties of weight, bulk, expense and resistance to the flow of fluids due to friction and turbulence.

It is the object of the invention to eliminate the vapour generator and its barrier from a Rankine engine, by providing an expander in which the heat transfer from the heating fluid to the working fluid occurs as the two fluids physically mix. The vapour generator is thus integrated with the expansion phase. The two fluids may then be separated in the condenser.

The simplest form of such an expander unit is shown in schematic cross-section in FIG 1. It closely resembles a normal reciprocating diesel engine, with an enclosed piston 1 connected to a crank 2, an inlet valve 3, an exhaust valve 4, and an injector nozzle 5. There is insulation 6 where a cooling jacket would normally be expected.

The four simplified diagrams of FIG 2 illustrate the operating cycle of the expander according to the following description. On the inlet stroke 7, the inlet valve opens and the downward-moving piston allows the heating fluid, a charge of pressurised hot gas, into the cylinder. As the piston reaches the bottom of its stroke, the inlet valve closes and the upward-moving piston compresses the gas 8, during this compression stroke. As the piston reaches the top of its stroke, a fine spray of the working fluid, water, begins to be injected into the chamber. The heat of the gas turns the water into steam 9. This change of state generates extra pressure which now works against the downward-moving piston, causing a power

stroke. After a suitable time the water ceases to be injected, and the gases continue to expand until the piston reaches the bottom of its stroke. The exhaust stroke 10 begins as the exhaust valve opens and the upward-moving piston expels the gases.

The expansion chamber is insulated to prevent heat loss and allow adiabatic expansion of the gases. A number of such expander units may be connected together in the manner of a multi-cylinder engine.

A possible arrangement of the main parts of a complete engine system is shown in FIG 3 and is described in the following. A compressor 11 forces air into a combustion chamber 12 where it is mixed with a fuel 13 and burns to form the heating fluid 14. This enters the expander 15 and at an appropriate time is mixed with the injected water from a timed injection pump 16 which draws on a reservoir 17. The exhaust gases 18 travel from the expander to the condenser 19, where the condensate is returned to the reservoir to be recycled, and the remaining combustion by-products 20 are exhausted to the atmosphere.

Apart from the aforementioned advantages of Rankine engines and the elimination of the vapour generator, the invention displays the following advantages over some other heat engines. a) The only major wasteful heat loss is at the condenser, and is therefore more easily controlled. b) The heating fluid need not be at a very high temperature for the engine to be efficient, as its temperature is raised by compression before the power stroke, enabling the use of low-grade heat sources such as solar, geothermal and industrial waste heat. c) There is no large thermal mass to be heated prior to the operation of the engine, saving time, energy losses and the dangers of enclosing large volumes at high pressure. d) The invention may be substantially made with the technology and tooling currently used in internal-combustion engine manufacture.

The invention need not be of the four-stroke form described herein, but may be realised in a two-stroke form in the similar manner of internal-combustion engines. The invention need not be of a reciprocating form but of any number of rotary or other forms where the necessary two or four-stroke cycles may be obtained.

The heating fluid need not be the gases of combustion, but any compressible fluid which is at a suitably higher temperature than either the outside environment or the heat sink around the condensing element.

The working fluid need not be water but any fluid able to change between the liquid and vapour phases at the working temperature.