GB2308864A - Mechanically coupled heat engine and compressor arrangement - Google Patents

Abstract

The piston 6 of the compressor 5 is mechanically coupled to the piston 2 of the engine 1. The output from the compressor 5 passes through a heat exchanger 9, where the gas is heated by the engine exhaust gases, before reaching the heat input section 10 where fuel, eg methanol/water or sunflower oil, is burnt. The heat exchanger 9 tends to keep heat recirculating within the engine until it is output as mechanical energy. Four cylinders may be used, in a 90-degree layout. Vane-type or turbine compressors and motors may be used instead. A diaphragm engine using a low-grade heat source is disclosed.

Description

HEAT ENGINE This invention relates to an engine that converts heat energy into mechanical energy.

Heat engines are widely used in both mobile form for transport and stationary form for powering machinery and electricity generators. Much work has been invested in reducing the pollution and noise that Internal Combustion Engines produce, and also into raising their efficiency from the current level of between 15* and 25%. The causes of the inefficiency are that the engines require cooling in order to reduce the temperatures in the combustion chamber to values that can be withstood by normal engineering materials and that the exhaust gases are very hot.Pollution levels are high because the fuel is burnt in barely sufficient air in order to increase the power output from the same size of engine and also because gas flow at high speed requires considerable overlap of input and output gasflows causing unhurnt fuel to be exhausted.

According to the present invention the heat previously wasted by cooling the engine is saved by ensuring that the gas temperature does not rise beyond that which may be withstood by current materials. The heat that was wasted in the exhaust gases is now recirculated back into the engine.

Since there is now no avoidable wastage of heat, most of the heat ptit into the engine must appear as mechanical output.

The unavoidable wastage is in the exhaust gases. Since these are cooled by the action of the engine, there is a minimum temperatllre differential between these and the input gases (normally atmospheric air) that depends upon the internal pressure of the engine.

Preferring to Fig 1, the engine consists of: 1: Gas compressor (5).

2: Motor driven by compressed gas (1).

3: Heat-exchanger (9).

4: Heat input section (10! i.e. fuel burner or other heat source.

The compressor is mechanically coupled to the motor so that the motor will use more gas per unit time than the compressor will supply.

The compressor output is passed to the heat-exchanger where it picks up heat from the exhaust gases. It then passes to the heat input section where extra heat is added either by burning fuel or by some other means. The gas, being heated at constant pressure, expands by the ratio of its absolute temperature (temperature Celsius plus 273) before and after heating. The heated and compressed gas is now of sufficient volume to run the motor. The motor input valve timing is such that the output gas when released has no excess pressllre, having hen expanded back to atmospheric pressure within the motor.

Output gas from the motor is passed through the heatexchanger, losing some of its heat to the compressor output before being vented. The output power of the engine is the motor output power minus the power needed for the compressor and the kinetic energy imparted to the exhaust gases.

The efficiency of the engine depends upon a sufficiently effective heat-exchanger keeping heat energy recirculating within the engine until it is output as mechanical energy.

The engine is insulated to stop unwanted heat loss. With an ideally efficient heat exchanger, no frictional losses and perfect insulation the only energy lost is in the exhaust air which will be at the output temperature of the compressor air and will have kinetic energy. Frictional losses may be reduced by using suitable materials since the engine stresses are much lower than in Internal Combustion Engines.

By insulating the hot parts of the engine system and by running the engine at relatively low pressures the efficiency of the engine in converting heat energy to mechanical energy may be raised to about 70%. Raising the running temperature of the motor will improve the efficiency of the engine to over 80* but will make manufacture more difficult. Use may also be made of the latent heat of water by adding water into the fuel or injecting it after the burn.

This will give a larger volume of hot gas to run a bigger motor and also will release its latent heat whilst condensing in the heat-exchanger. This may add another 10 efficiency to the engine. Considerations of size and expense will obviously reduce practical examples to less than maximum efficiency but 50% efficincy should be easily achievable. This is twice that of the current best Internal Combustion Engines.

The fuel is burnt inside the engine in an excess of air and at elevated temperature, thus giving very low pollution levels. For this application any fluid fuel may be used, giving identical power outputs for the same calorific value of fuel burnt. Good results may be achieved with a methanol/ water mixture if a larger heat-exchanger is used so that condensed water does not clog it. Heat may also be applied to the engine via a second heat exchanger so that the heat from solid fuels or sunlight may be utilised for conversion into mechanical energy.

The format shown in the drawing is not the only way of building the engine. Using a vane compressor and motor would give the constant gas-stream required with less problems of valve timing than a piston compressor and engine. For low temperature differentials, where the heat source cannot achieve the several hundred degrees Celsius required for the format drawn here, a largediameter diaphragm compressor and motor may be used as this will give low frictional losses. For high-speed work turbines may be used as compressor and motor, and in this case high temperatures may be used giving very high efficiencies.

Operation of engine in double-acting piston format: Refer to Fig 1 for schematic drawing of this format.

The same sequence of events will happen to the gas in any format of the engine.

The Pistons (2) and (6) start at the right of the cylinders.

The right motor inlet valve (4) is opened and compressed gas from the system forces the piston (2) to the left. The left exhaust valve (3) is opened and exhaust gases are forced out of the left motor chamber to the heat exchanger (9). The compressor piston (6) thus moves to the left, and when the air pressure exceeds the system pressure the spring valve (7) is forced open allowing compressed gas into the system. As the motor piston has about twice the area of the compressor piston, the system pressure gives a net leftward force.

When the motor has enough compressed gas in order that the cylinder would be just filled after expansion to atmospheric pressure, the input valve (4) shuts off. For the 1 bar pressure this is 52 of piston travel. Exhaust gases from the motor pass through the heat exchanger (9), releasing heat to the compressor output air.

Compressor output temperature is fairly close to air input temperature, since we are using pressures of the order of 1 bar. The compressor output air is heated in the heatexchanger by the exhaust gases, thus increasing in volume at the same pressure.

The compressed air, having passed through the heat-exchanger and gained most of its heat, is now further heated in the extra heat section (10) by burning fuel in it. It is now at about 300 degrees Celsius and has enough volume to run the motor.

As the piston reaches the left of the cylinder the valves all shut. The cycle runs again, but this time reversing right and left.

The system will work best with a constant flow. This means using multiple cylinders to get a smoother flow or using a vane pump and motor to get the same effect.

When starting the motor, the compressor will not generate enough air as the heat-exchanger will not be heated by exhaust gases. In this case, the vacuum release valve (8) operates allowing extra air in. Heat at this time will be totally supplied by the burning fuel, but will be supplemented within a few cycles by the heat-exchanger.

Fuel regulation that senses system pressure and allows extra fuel in when the pressure is below the set point will cope adequately with starting. The accelerator on a normal engine would be replaced by a mechanism to adjust the pressure set point.

Heat exchanger calculations The heat exchanger must provide minimal resistance to gas flow in either chamber whilst giving a large surface area for heat transference from one side to the other. The input and output feeds from either side must be divided up into a large number of small cross-sectional areas with maximum areal contact with the other chamber. Since the temperature on both sides will tend to equalise, the gases should run in opposite directions in either chamber such that the input from the compressor (as cool as the engine gets) is exchanging heat with the exhaust to air and the output to the motor (now warmed hy passage through the exchanger) is exchanging heat with the output gases from the motor (maximum temperature available).

The thermal resistance of the exchanger will depend mostly upon the area of surface common to both chambers. This overall thermal resistance sets the maximum efficiency of the engine. Since the exchanger scavenges the residual heat of the motor output gases, it needs to transfer more than the motor outpllt (as the motor is less than 25 efficient on its own). As a starting point, it seems reasonable to assume about 3 times the motor power at a 5C differential.

Assume air at 20C and 1 bar pressure. Also assume minimal frictional losses. Assume constant pressure in the compressor (approximation) and the motor - this gives a higher calculated power output but does not change the efficiency rating.

For explanations of approximations see page 6.

The motor as drawn has a motor volume of about 130cc, with a compressor of about 73cc. With a working pressure of 1 Kgf/sqcm (about 1 bar) then the compressor output will be about 34 degrees Celsius (316K) and the motor will run at about 300 degrees Celsius (57so).

The net downward force on the piston on the power stroke will be 9Kgf or 90N for a stroke length of 5 cm. At 3000rpm this gives 50 double strokes of a total length of 5m per second. The power delivered is thus 450 NM/s or 450 watts.

A good starting point for the heat-exchanger would be 1KW for a differential temperature of SC. Output temperature of exhaust gases would then be 39C, giving 19C above ambient.

Input air is 7.3 litres per second, and this has a thermal capacity of 0.832 J/1K or 6 J/K. This gives a wasted thermal energy of 114W. We will ignore here the kinetic energy of the exhaust as this depends upon exhaust diameter and will be fairly small; it may also be largely recovered by use of a gas turbine attached to the exhaust.

This gives an efficiency of 79% in converting input heat energy to mechanical energy for this approximately 0.4 HP engine. The fuel burnt would need to supply 564W to keep the pressure at 1 bar. With a perfect heat exchanger 84W would still be wasted, giving an efficiency of 84% - this would be the maximum achievable without recovery of latent heat of condensation of the water vapour produced by burning fuel or recycling exhaust kinetic energy.

If the engine was run at 0.5 bar, the output from the compressor would be at about 25.5C, the heat exchanger would be twice as efficient (it has half the heat to exchange) thus the exhaust gases would be at about 28C. Wasted energy would be about 48W with an engine output of 224W. In this case the efficiency would be 82% for approximately 0.22 HP, and the fuel burnt would need to supply 272W.

With a perfect heat exchanger waste energy would he 33W giving 87% efficiency.

The test engine used for proving the theory will have an electric heating element as the extra heat source. In this way input power may be easily measured. Output power may be measured using a constant-magnet alternator with variable resistive load.

Effects of approximations on calculations: 1: The work done in compressing the air is not constant with either linear displacement of the piston or angular displacement of the flywheel. It does in fact increase exponentially with piston displacement until the pressure in the cylinder reaches system pressure, after which it will be substantially constant. For the 0.5bar sytem pressure it will be constant after 33 travel, for 1 bar after 50 travel. The volume of compressed air produced is therefore not only later but of much lower volume than assumed in the simple explanation. The work done to produce it is also less.

2: The work produced in the motor is also less than assumed in the simple explanation. This is because the input valve shuts early, allowing the pressurised gas to release more of its energy albeit at an exponentially decreasing pressure.

The valve-closing time should be governed by the system pressure such that the volume of air used is equal to the motor volume when fully expanded to atmospheric pressure.

Therefore for 0.5 bar it shuts at 68 of the travel and at 1 bar it shuts at 52 of travel, for example. If extra temperature may be withstood in the motor then the valve may be closed later, but this will waste pressure to the exhaust as the pressure in the motor when the outlet valve opens will be above ambient. Again, higher output power would he gained at the expense of efficiency.

3: If the valve timing is for maximum efficiency as above, then the work graph of the compressor and the motor will be mirror images with the motor graph being 130/73 times that of the compressor (when drawn on a linear piston displacement), or purely the ratio of the piston area since the stroke is the same. The flywheel stores the extra energy at the start of the stroke (when motor pressure is maximum and compressor pressure is 0 above ambient) and delivers it back at the end of the stroke when the motor delivers no power and the compressor is still doing work on the gas. The net result is that the energy halance remains the same and efficiency calculations remain the same, but the total energy input and output will drop.

For 0.5 bar system pressure the output will be about 82% of that calculated above, and for 1 bar the output will be about 71% of that calculated above (the flywheel stores energy for 50% of travel and releases it for the last 33% of travel).

The system volume between the compressor valves and the motor valves should be at least 10 times the compressor volume. This evens out the system pressure for this twocylinder format. A four-cylinder 90 degree format would need less evening out as even at 1 bar there would always be one compressor at full pressure. A multi-vane pump and motor would also need little smoothing, but the porting would need to alter with varying system pressure.

Turbine compressors and motors are inherently smooth and would need only the volume of the heat-exchanger.

Use in transport: Compressor throughput is about 15 litres per second for 1 kilowatt at 1 bar and 70% efficiency. An average family car needs about 40KW of power giving 0.6 cubic metre of air per second in the compressor side and 1.2 in the motor. A vane pump running at 12000rpm would need 3 litres per revolution for this power output and the motor would need 6 litres per revolution, both compressor and motor running at the same speed. Pressure-control led input ports on the motor would keep the efficiency at maximum for whatever system pressure was used, and similarly separate spring-valved outputs from the compressor would keep compressor efficiency as high as possible. This control is needed as for most of the time the engine will be running under 10KW and 0.25bar thus giving an efficiency of over 90% theoretically.

Current low-drag passenger cars at 90 kilometers per hour use 4.5 litres of petrol per hour, which is a total heat input of about 139 megajoules or 38.6KW. This corresponds to less than 9KW needed to propel the car at this speed. If an engine of 70R efficiency were used, then in the same time at the same speed the car would only use 1.5 litres of petrol or equivalent 46 megajoules of another fuel.

The heat-exchanger would need to be of the order of 100KW and would probably be the most expensive component. Since maximum power is rarely used, the heat-exchanger could be scaled down somewhat to about 50KW for lower manufacturing costs whilst still giving reasonable results.

Note that 0.6 cubic metre per second needs a wide-bore exhaust pipe. If a 100sqcm exhaust pipe (11cam diameter) pipe was used then the output air speed would be 60 metres per second which is quite fast. Kinetic energy in this gas is 2.66KW, which is quite a lot to waste. It is therefore reasonable to choose an 18cm diameter (250 sqcm) exhaust path thus losing only 0.4KW or 1% in efficiency.

This exhaust air may be used to drive a turbine to drive the generator for automobile electrical power. In this case an electric heating element in the heat input section may be used to absorb excess power not used for running the vehicle.

The electricity used here would cut the fuel required, increasing the overall efficiency of the system by a few more points. Output gases will be at about 20C to 40C above ambient. A heat-exchanger could be used on this to recover heat for the vehicle.

By using these two energy-recovery processes a large percentage of otherwise waste energy may be made useful.

Discussions so far have centred around a peak temperature of 300C as this is achievable using available technology. The problems in the physics of moving large volumes of gas may be made easier and the engine may be made smaller if this restriction is lifted. This means that the motor would have to be made of a ceramic material to withstand the higher temperatures. Current techniques in ceramics production coupled with the simplicity of a vane pump mean that this option could give both increased efficiency and higher power output for the same volume of input air.

It is envisaged in any case that this engine will run with no lubrication, using either ceramics or PTFE as the bearing and sealing surfaces. If this is done, then the engine needs no checks for water or oil; all it needs is some heat input to function. It will thus have no problems in arctic conditions and in fact the colder the input air the more efficient it gets providing that the heat-exchanger does not clog with ice.

Fuelling with a mixture of alcohol and water gives extra power output for the same input air, though the same effect could be achieved by spraying water into the combustion chamber. Exhaust gas would be 100% humid, but the condensate could be recycled. If the water ran out, then the engine would still run at a reduced power and efficiency. In this case, the 300C maximum temperature would not be a problem as anything over about 200C would work reasonably. Higher running temperatures reduce the requirements for the heatexchanger whilst increasing those for the motor.

Diaphragm engine for low temperature differential exploitation.

If a temperature differential of 30C is available, for example from sunlight, hot springs or waste heat from industrial processes, then an engine to use this must have a volume ratio compressor to motor of 1:1.1 in order to work. Only 9% of the total motor power will be available for external use, the other 91% driving the compressor. For an 0.3bar pressure and a 30cm diameter motor piston (700 sqcm), this gives a total pressure of 2300N and a useable pressure of about 210N.

With a 10cm stroke, as would be reasonable in a diaphragm of this size, then at 1000rpm (16.6 strokes per second each of a total 20cm) we would have 3.33M of work-length for a double-acting motor. This gives an output of 660W of useable energy. The compressor output temperature would be in the order of 2.4C up from ambient, and the heat-exchanger would have to pass 0.24 cubic metres of air per second and transmit 298J/K each second making 7150J. We would aim at 25C exhaust, giving a 2.6C differential so we need a 7.2KW heat-exchanger at 2.6C or about 2.75KW/C to get to 30% efficiency.

Maximum efficiency for this low heat differential is 47%, since the compressor output cannot cool the exhaust gases below 2.4C above ambient, so we have 232 litres per second of air with a thermal energy of 716 Joules per second.

This heat-exchanger could be built from vacuum-formed plastic since temperatures and pressures are low enough for these materials.

With this size of engine, a large volume of air is being moved and it becomes far more productive to add a turbine to the exhaust output to recover the kinetic energy. This could be used as the main output of the engine by using a small-bore exhaust, thus saving the problems of a drive mechanism such as belts or gears. In this case the engine speed would not be regulated but the pressure would be, as this would determine the exhaust gas kinetic energy once equilibrium was established.

Despite the low efficiency of the engine in this format, it may be the only way of recovering mechanical energy from low-grade heat sources. If a higher temperature is used, then the efficiency will rise accordingly. Adjustment for the temperature available may be achieved by changing the gearing between the compressor and the motor, thus achieving the highest efficiency available from that heat source.

Advantages of this engine design 1: The efficiency of the engine is very high since no energy is wasted except where unavoidable.

2: The efficiency may be easily tuned to achieve the best compromise between engine size, output power and cost.

3: In the format shown here it gives virtually zero monoxide and unburnt hydrocarbons. Since it can also use methanol, sunflower oil, or any fluid fuel in this format and still give the sam results it reduces the technical require ments of road fuels. No lead or other additive is required.

4: Other formats may be used for different requirements providing that the compressor side has a lower volume output than the motor uses in the same time period. For high-speed use therefore a turbine compressor and motor may be used giving very high output power for size and weight. For medium speed a vane-type compressor and motor could be used; this may be the best solution for road transport use because extremely quiet, vibration-free and pollution-free engines may be fabricated.

5: For conversion of low-grade heat sources such as hot springs or sunlight large-volume diaphragm pumps may be used as these will have lower frictional losses than sliding surfaces. The heat-exchanger in this case could be built out of vacuum-formed plastic, and plastic mouldings could be used extensively in the engine const ruction giving a low-cost product. Such an engine could utilise temperature differentials of 30C or so if the motor volume per second was 10* more than the compressor volume per second.

6: The engine produces very little thermal pollution. For electricity generation either locally or nationally a large amount of energy is delivered to the atmosphere along with the other pollutants. By using this high efficiency engine in power-stations and in factory gener ators thermal pollution from these sources will be cut to less than a fifth of current output. Apart from giving cheaper electricity there is also a side benefit in that temporary overloads may be coped with at the cost of a small loss of efficiency. The engine could initially be used to work on the low-grade heat wasted by the primary motors currently used. The simplicity of the engine design makes large-scale engines much cheaper than gas turbines with much more flexibility in power output, also the efficiency rises under low load situations.

Claims (1)

1: An engine consisting of a gas compressor, gaspowered motor, heat-exchanger and heat input section which achieves high efficiency in converting heat energy to mechanical energy by recovering the heat from the motor output gases and applying this heat to the compressor output gases so that energy is recycled within the engine.

2: An engine as claimed in Claim 1 which will use any commonly available liquid fuel with no adjustments to the fuel metering device, the mechanical energy output being the same for an equivalent heat energy input.

3: An engine as claimed in Claim 1 or Claim 2 which after modification to the fuel metering device will use any commonly available gaseous fuel, thereafter needing no further change to run on a different gaseous fuel and giving the same mechanical energy output for an equivalent heat energy input.

4: An engine as claimed in Claim 1 which will use any solid fuel after modification to the heat input section.

5: An engine as claimed in Claim 1 which will use any available source of heat after modification to the heat input section.

6: An engine as claimed in Claim 1, Claim 2, Claim 3, Claim 4 or Claim 5 where alternative methods of implementing the gas compressor and motor may be used to fit the engine to its required use.

7: An engine as claimed in Claim 1, Claim 2, Claim 3, Claim 4, Claim 5 or Claim 6 where energy is retrieved from the final exhaust stream by using a further heat exchanger or a turbine or both, and where such recovered energy is used either externally for other purposes or recycled back into the engine.

8: An engine as claimed in Claim 1, Claim 2, Claim 3, Claim 4, Claim 5 or Claim 6 where mechanical energy is obtained in the form of an airstream which drives a turbine or fan. The turbine or fan mechanical energy is then used to drive further machinery.

9: An engine substantially as described herein.

Amendments to the claims have been filed as follows CLAIMS 1: An engine consisting of a gas compressor, gaspowered motor, heat-exchanger and heat input section which achieves high efficiency in converting heat energy to mechanical energy by recovering the heat from the motor output gases and applying this heat to the compressor output gases so that energy is recycled within the engine.

The heat input section is external to the compressor and motor in order to achieve continuous heat input. No cooling is required since the working temperature is controlled by the heat energy input. This arrangement allows complete combustion of fluid fuels and also allows input from other heat sources.

2: An engine as claimed in Claim 1 which will use any commonly available liquid or gaseous fuel after adjustments to the fuel metering device, the mechanical energy output being the same for an equivalent heat energy input.

3: An engine as claimed in Claim I which will use any solid fuel or other source of heat after modification to the heat input section.

4: An engine as claimed in Claim 1, Claim 2, or Claim 3 where alternative methods of implementing the gas compressor and motor may be used to fit the engine to its required use.

5: An engine as claimed in Claim 1, Claim 2, Claim 3 or Claim 4 where energy is retrieved from the final exhaust stream by using a further heat exchanger or a turbine or both, and where such recovered energy is used either extemally for other purposes or recycled back into the engine.

6: An engine as claimed in Claim 1, Claim 2, Claim 3 or Claim 4 where mechanical energy is obtained in the form of an airstream which drives a turbine or fan. The turbine or fan mechanical energy is then used to drive further machinery.

7: An engine substantially as described herein.