Low heat rejection engines

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In most conventional Internal Combustion Engine's (Otto and Diesel cycle) only about 1/3 of the fuel consumed is actually converted to usable power, Hence about 30% efficient. The remaining 2/3 of the fuel is excess heat which can not be used by the engine so it rejected to the environment through the exhaust and cooling system. Gas turbines (Brayton Cycle) which have recuperators / heat exchangers installed apply heat from the exhaust to the compressor discharge are also about 30% efficient. Combined Cycle Brayton (gas turbine) and Rankin (steam) turbines have an overall efficiency of about 40%. Another consideration is that wasted heat emitted from combustion engines could be a significant contributor to global and local warming conditions. In addition to probable environmental benefits, it is known that combustion engines capable of Lower Heat Rejection utilize fuel more efficiently. Much work has been done on the concept of Low Heat Rejection engines. Most experiments with LHR engines attempt to use the Otto Or Diesel cycle. LHR Diesel engines are generally engines that have been modified with ceramics and ceramic coatings for high temperature operation and fitted with exhaust driven turbines in an attempt for improved efficiency. In the Otto and Diesel cycle the air is compressed and expanded in the same cylinder. While heat is good for expansion it is bad for compression. Maybe a four cycle engine not ideal for a LHR engine concept. One good candidate for LHR operation is the Brayton cycle. It is easily recouperated (returning unused heat back to the engine) this is because the Brayton cycle uses three dedicated zones of operation, air is compressed by a cold compressor, heated by a burner and then expanded in an expander. When recouperated latent heat (normally wasted and discharged to the environment) of the exhaust is applied to the air between compressor discharge and the burner using a heat exchanger AKA recouperator. The end result is the recouperator lowers the exhaust temperature and raises the temperature of the compressed air, thus less fuel is required to operate the engine. One important point to make about Gas turbines is that the they require large quantities of air to move through them to keep the turbine in operation. (turbine blades are mini airfoils and need airflow to prevent stalling). Gas Turbines are also inefficient at partial loads for this reason. For a practical Low Heat Rejection (LHR) engine it is this authors opinion that three zones are needed... one for compression, one to apply heat and another for expansion (the Brayton cycle) with latent exhaust heat applied to the air in between compression and prior to heat addition. As already mentioned, one problem with turbines is that they require large quantities of air, so an efficient recouperator needs to be very large in relation to the size and power of the turbine engine.... What about positive displacement? Using pistons instead of turbine blades. Braytons first engines (the Ready Motor 1865) were positive displacement. They used two pistons.... one to compress air, the air was heated with a burner, then timed valves admitted the hot pressurized air to a second larger piston / expander where the expansion was used to produce work. Both pistons were connected via a common rod and work was applied to a crankshaft thus producing usable power. But at the time high temperature materials were not available so cooling of the expander piston and cylinder was necessary thus cooling the fluid while in the expander which reduced the efficiency of the engines. It was stated by Clerk (an engine builder of the day that "the cycle is a good one and it should be tested on an engine having a hot expansion cylinder". It is not known if recouperators were ever tested or employed on the positive displacement Brayton engines. With 100% heat transfer the exhaust temperature of a recouperated Brayton engine could be decreased to that of the exit temperature of the high pressure side of the compressor.... One significant disadvantage of the Brayton cycle is that the compressor must always pump against the expansion created by the continuously heated gas / fluid, this is referred to as Back work. One way to reduce back work is to precompress or raise the density of the working fluid. This can be done by creating a closed loop of operation for the Brayton cycle but positive displacement is better for this since it requires significantly less volume of working fluid than the turbine Brayton engine. To close the loop the exhaust is cooled close to ambient after the recouperator and then returned to the compressor intake. One problem is how to heat the working fluid? It could be accomplished with a flame applied to a heat exchanger but direct contact of the flame and working fluid is the most efficient way. To sustain the flame O2 and fuel must be present.... so a burner capable of sustaining a flame under these conditions was developed. What evolved is a swirling or cyclone burner with the working fluid and O2 mixture added at the peripheral of a tube in which the flame propagates through the center. the engine is described as a Semi Closed Loop Positive Displacement Brayton Engine with Hot Expander. after testing the engine it is apparent it has some advantages that may hold some real promise for a practical LHR engine. Here is a description of the Cycle: The cycle begins at the intake of the compressor where the fluid is drawn into the compressor cylinder through a one way valve by a descending piston. At this stage the working fluid is at the lowest pressure and temperature in the system.... the fluid is then compressed by the piston as it returns to the top of the cylinder thus raising the pressure and temperature and density of the fluid. As the displaced fluid exits through another one way valve it enters the high pressure side of the gas circuit. Pressure is created later in the high pressure side of the gas circuit when the fluid is heated by the recouperator and burner. The compressed fluid travels to the recouperator where latent heat from the exhaust is absorbed by the fluid resulting in expansion....(it's important to mention that our testing determined that percentage of back work is decreases when the loop pressure is increased. This is due to increased expansion from higher volumetric density. Thus the compressor discharge temperature is also decreased since less volume is required for the same amount of ultimate expansion. Due to cooler compressor discharge temperatures the compressed fluid is able to absorb more of the latent heat from the exhaust through the recouperator prior to entering the burner...also it's important to mention that gasses absorb heat better as the pressure of the gas is increased).... fluid then enters the burner where hydrocarbon fuel and O2 are added and continuous combustion is taking place thus converting the hydrocarbon fuel to H2O and CO2 heat from the combustion is absorbed by the compressed fluid which results in further expansion. At this point the fluid is at about 500-600C and about 3 times the original volume. The admission valve times the entry of the increased volume into the expander cylinder where the pressure created during the expansion performs work on the expander piston....... as the piston descends in the expander cylinder volume is increased pressure is decreased and some cooling takes place.....the exhaust valve opens at the bottom of the stroke... exhaust is pushed out as the expander piston rises in the cylinder..... As the piston nears the top of the stroke the exhaust valve closes. After being expelled the exhausted fluid is now at the low pressure side of the loop.....the exhausted fluid enters the recouperator where latent heat is passed to the fluid on the high pressure side of the cycle. This reduces the temperature of the exhausted fluid to within a few degrees of the compressor discharge temperature ( about 100C).... next the fluid passes through a final finned heat exchanger for final cooling. Here a fan circulates ambient air around the finned tubes further decreasing the temperature of the working fluid until the temperature is close to ambient. During the final cooling process steam that was present in the exhaust is condensed to water. As the fluid exits the finned cooler a water / gas separator with a pressure relief valve (set to maintain the loop pressure) expels the H2O and excess Co2 created by the fuel and O2. Pressure exiting the relief valve is used to power a positive displacement pump which aids in the injection of fuel and O2 into the burner thus reducing the work required to inject the fuel and O2). The now cooled working fluid (Co2) is returned to the intake port of the compressor thus completing the cycle. Water and CO2 are the only thing that exits the engine. At this stage the CO2 can be sequestered or released to the atmosphere.