GB2511652A - reciprocating heat engine - Google Patents

Abstract

A Brayton Cycle Engine 1 (BCE) comprises a reciprocating piston engine operating on the Brayton cycle. The engine has at least one working four-stroke cylinder in which the piston power stroke is powered by expansion of previously compressed and heated intake gas admitted via a valve 27 mounted on the cylinder head 31. The intake gas may be air, heated via an external heat exchanger or solar energy collector 15. Alternatively or in addition the intake gas may be the combustion product of an external combustor 4. The power stroke may be aided by combustion of small amounts of fuel directly injected into the cylinder via a port 29. Moving parts of the piston and/or relative movement between the piston and cylinder may be lubricated using air bearings (19, 20, fig. 3). The valves may be lubricated by air bearings (21, 22, fig. 2). The cylinder may be cooled by circulating fluid, e.g. air, flowing through an annulus (18, figure 3) formed between the cylinder and the engine block (17, fig. 3).

Description

RECIPROCATING HEAT ENGINE

The reciprocatingl heat engine described in this invention operates on the Brayton or Joule thermodynamic cycle. The Brayton Cycle Engine ( BCE) operates with higher efficiency than either Spark Ignition (SI) or Compression Ignition (Cl) engines and with significantly lower atmospheric emissions including NOx, nanoparticulates and carcinogens. It can replace these engines in a wide range of applications covering most of the internal combustion engine market including transportation and power generation -It also has application in markets eg solar power generation, which SI and Cl engines cannot address. The BCE offers wide ranging fuel and operational flexibility and aligns itself very well with the's Intelligent Engine's electronic control concept. The Intelligent Engine concept means that a camless valve train using electronic control means is the preferred system at higher engine outputs. An appropriately designed BCE can accept a wide range of gaseous and liquid fuels without mechanical modification and whilst operating simultaneously in fuel powered or solar powered modes.

The reciprocating piston engine described here can be built and operated either as a single cylinder or as a multicylinder engine based on either the simple Brayton cycle or on a more advanced and powerful hybrid version of that cycle.

The current Brayton Cycle Engine ( BCE) design, in either form, derives from the reciprocating, two stroke, naturally aspirated, double ended, Brayton cycle engine described in US Patent 6012280. As described here the current engine is, preferably, a multi-cylinder, turbocharged, four stroke design which retains and extends the low friction aerostatic bearing concept pioneered in US 601 and first described in US 5560714 but which is now a single ended single piston design. The valvetrain and any additional fuel admission means are now built into the cylinder head.

It should be noted that two stroke and six stroke versions of this BCE design are also entirely feasible but detailed engineering and materials technology ( superalloys, coatings, ceramics etc) need further careful analysis and are not examined further here. However the essential principles and advantages of two, four or six stroke BCEs over known engine designs remain substantially similar..

It should be recognised,however, that the four stroke cycle commonly used in the BCE is not the conventional four stroke cycle used in SI and Cl engines. In the simple BCE, three of the four cycle strokes (induction, compression and exhaust) reflect those used in a conventional SI or Cl four stroke engine, but the fourth, power, stroke Goes not involve the fuel combustion process within the piston I cylinder envelope as used in those two designs.

Rather, it involves the admission into the cylinder of a charge of high pressure, high temperature gas ( most usually air or combustion products) -generated externally and continuously, either in a separate combustor or in a separate heat exchanger -or both of these, operating in parallel -and it's expansion to low pressure and temperature during the engine working (power) stroke. The single ended Brayton cycle engine has separate valve means controlling each of these four strokes, each valve being mounted in the cylinder head The simple means of actuation of these four valves is not described here but follows best current reciprocating engine design practice Valvetrain operation conventional, pneumatic, hydraulic or electric, can be extended advantageously in the intelligent BCE to VVT or camless valve actuation. Similarly bottom end engine construction follows conventional design practice, modified as necessary to accommodate different cylinder numbers and configurations. For multicylinder BCE designs, monobloc engine construction is preferred to give optimum overall stiffness and in four cylinder engines a five bearing,oil lubricated, crankshaft is preferably used Additionally, and when the BCE is configured in the hybrid mode, an additional fifth fuel admission port can be built into the BCE cylinder head.

This fifth fuel admission port is typically installed on the engine centre line and is arranged to admit a controlled flow of fuel -liquid or gaseous -into the cylinder during the engine power stroke. Admission of additional fuel and it's rapid and clean combustion under relatively high excess air conditions, maintains engine output at a high level during the greater part of the BCE power stroke. Any increase in exhaust gas temperature resulting from this process can be recovered through an engine turbocharger. The normal operating air/fuel ratio of the BCE ( between 10 and 100:1) comfortably meets the stoichiometry demands of any additional fuel. Admission of this controlled fuel quantity can easily double engine power 2.

output, offering significantly increased engine performance at critical high demand points in the overall engine duty envelope eq if in transportation mode, when moving away from vehicle standstill, when accelerating or on uphill gradients. Variation of this fuel flow rate and the admission period may follow conventional reciprocating engine practice but, depending on engine load profile, additional fuel admission may also frequently be dispensed with for the greater proportion of a transportation BCE duty cycle -road, off-road, rail or marine. The BCE can thus be most closely sized to meet average traction power demand and therefore be designed to provide the highest possible Brayton cycle efficiency and lowest emissions over the widest possible engine operating range. In forced induction mode a turbocharger may also be mapped by the smart system to support the drive for highest overall efficiency. A further significant power boost can be obtained -as with Brayton cycle based gas turbines -by water injection (fogging) into the engine high temperature gas inlet circuitry.

Decisions on which of these operating regimes to adopt can be handled by the intelligent engine's smart control system operating to a pre-determined operating programme -all subject to operator intervention as appropriate.

Because the simple Brayton cycle engine can be equipped with either a separate combustor or a separate heat exchanger ( or both) devices, it offers the major advantage that it is almost completely non-selective as far as gaseous or liquid fuel types and qualities are concerned, in fuel terms a given engine is effectively omnivorous. Any BCE can operate on any of the fuels used commonly in SI, Cl and gas turbine ( GT) engines. However, the BCE does not demand specially tailored fuels such as gasoline ( petrol), diesel or aviation kerosene. It will handle such conventional tailored fuels with ease but with equal facility will handle a significantly more extensive range of gaseous and bio-derived fuels as well as heavy lkiuid fuels ( Bunker I C! ,crude oil etc) plus emulsified fuels and any other fuel capable of controlled combustion -a unique polyvalent combustion capability and one of obviously increasing importance as world fuel markets tighten and as opportunity fuels increasingly penetrate those markets. With appropriate engine layout plus a smart control system, such polyvalent fuel flexibility can be built into a single intelligent engine -an important advantage which conventional engines cannot offer.

The optimum use of air bearings, first described in the US 6012280 Patent * is retained as a fundamental design principle in the current BCE but is significantly extended by the use of such bearings to offer specifically targetted and very low intemal friction losses over a number of high pressure and high temperature surfaces within the engine. A practical form of this air bearing was first described in US Patent 5560714 and two such bearings were built into the US 6012280 engine,This basic air bearing design offers the twin virtues of axial stiffness -providing the requisite precise piston movement support -plus appropriate radial resilience to facilitate automatic compensation for differential thermal expansion between the piston and the bearing shell (cylinder wall I liner) over the BCE working temperature range.

This radial resilience is provided by a series of longitudinal slots cut into the bearing shell. An improved engine cooling function can be provided by by reprofiling the longitudinal slots described in US 5560714 to form extended cooling surface fins or pin fins. To further improve engine cooling this extended surface may form the inner face of an annular cylindrical cooling jacket formed between the air bearing shell and the engine outer shell ( engine block) and through which a fluid coolant may be arranged to flow. The fluid coolant may be either a suitable liquid or the engine charge air itself. In the latter configuration the charge air can be arranged to flow from the forced induction means via the cooling jacket and into the engine cylinder. In higher output BCEs -above a nominal 500 kW -conventional liquid cooling ( not shown) becomes the preferred cooling means.

One clear benefit of the use of an air bearing at the piston / cylinder interface is that piston rings -used universally in known SI and Cl engine designs and a major source of engine internal friction -can be eliminated. The high pressure gas sealing function normally provided by the piston ring pack is now provided by the airbearing sealing air flow which may have a substantial and variable positive differential (typically 20% ) over the BCE charge air pressure. The engine benefits further from the recognised advantages of airbearings over conventional oil based lubricating systems in that not only are sliding surfaces within the air bearing lubricated, supported and sealed against gas leakage but the airflow through the bearing can also provide an important and closely targetted flow of cooling air across such bearing surfaces. Surface cooling is thus a fourth important potential benefit -largely unrecognised in known air bearing applications -but of considerable importance in accommodating the higher working fluid temperatures ( up to 1500 deg. C and beyond) and pressures ( up to 500 bar) required in the advanced, high performance BCE. These advanced operating parameters in turn require careful selection of engine materials including an appropriate choice of superalloys, coatings, ceramics and hybrid materials particularly for -3.

high temperature gas train duty in the combustor, heat exchanger, interconnecting ductwork and the piston /cylinder envelope itself.

Despite the well recognised low friction advantages of air bearings in the lubrication of sliding surfaces, it is known that their overall performance is partially compromised by their limited ability to absorb lateral forces such as those generated in a reciprocating engine. Such lateral forces are generated principally by crankshaft throw. Fortunately the disposition of operating valve gear and any additional fuel admission porting within the BCE cylinder head means that to retain engine breathing efficiency, an increased total valve flow area -and hence cylinder head plan area -is required to accommodate the BCE four valve pattern.

Additionally, two of the four valves are hot gas valves. The requisite valve flow area increase means that, for a given engine displacement, engine cylinder bore is increased and piston stroke proportionately decreased. The decrease in piston stroke has the beneficial effect of limiting crankshaft throw and thus reducing the associated lateral forces acting on the air bearing. Most BCE's are of oversquare and therefore of short stroke or ultra-short stroke design. Short stroke engine design offers the additional benefit that, for any given engine displacement, piston and cylinder surface-to-volume ratios are maximised.This fact in turn impinges favourably on detailed upper cylinder cooling design. In order to further limit the lateral forces acting on the cylinder wall air bearing, the BCE is preferably built as a desaxe design with the cylinder bore axis offset from the nominal crankshaft centreline.

For most engines above 5kW output a further, crosshead air bearing can be mounted on an extended piston rod such that additional lateral load bearing support is provided for the BCE trunk piston itself. The crosshead bearing is typically positioned to offer overlapping piston support to that provided by the cylinder liner air bearing As already indicated, the fourfold benefits of air bearing technology can be applied elsewhere within the engine -not only as piston/cylinder and crosshead air bearings collectively supporting the piston -but also in the support, lubrication, sealing and -most important in the BCE context -cooling of the engine hot gas admission and exhaust valves.

Cooling of these hot valves involves recognition and response to the similarly severe cooling challenge faced by their associated valve seats. Whilst air cooled seats can be used in low kW output engines the transition from air cooling to liquid cooled seats will occur at an early point in the BCE output range and wherever very high temperature engine operation -for highest engine efficiency -is employed. A hybrid liquid / air, engine cooling system may therefore be preferred as overall thermal duty is increased. Evaporative seat and valve cooling is a further option.

Typically, therefore; a total of four airbearings can be built into the BCE,reducing internal friction losses to a practical minimum These air bearings are: 1. Cylinder! Piston Air Bearing 2. Crosshead Air Bearing 3. Hot Gas Admission Valve Air Bearing 4. Exhaust Valve Air Bearing Use of air bearings at strategic points within the engine envelope can also reduce particulate and particularly nanoparticulate emissions by the elimination of oil lubrication from the highest temperature internal surfaces of the engine. Continuous seal air flow through the piston / cylinder air bearing also ensures that crevice generated emissions are reduced to a practical minimum.

The engine also offers the virtues of smoother operation, quieter operation and lower vibration levels due to the elimination of the discontinuous combustion processes inherent in both SI and Cl reciprocating engines.

Depending on detailed engine design and operating parameters -and recognising the significant reduction in internal friction losses resulting from the strategic use of air bearing technology -overall engine thermal efficiencies in excess of 60 % may be achieved. A further increase in efficiency can also be achieved if the BCE is operated in combined cycle or CHP mode. In comparison, the overall delivered efficiency of grid electric power in the UK is approximately 36 % and is associated with unacceptably high NOx, particulate and other emissions. The long term impact of such emissions on worldwide climate change is now recognised as being beyond reasonable doubt. As discussed below, these facts are also strongly supportive of the concept of energy independence and distributed generation in the UK and other power generation markets.

Continuous combustion -external to the working cylinder -rather than discontinuous combustion within the cylinder itself, as with SI and Cl engines -means that BCE emission levels ( NOx, CO, particulate matter (including nanoparticulates), carcinogens, unbumt hydrocarbons and other greenhouse gases ( GHGs)] are minimised and are in fact closely derivative of those achievable in the Brayton cycle based gas turbine ( GT) . This is in marked contrast to achievable emission levels from SI and Cl engines where not only are gaseous and particulate emissions unacceptably high, but unsought for emissions of the closely associated carcinogens and nanoparticulates is increasingly seen as severely compromising CI engine environmental acceptability. The World Health Organisation llntemational Agency for Research on Cancer ( IARC)J classified diesel fuel exhaust in Group "Diesel exhaust is carcinogenic to humans "in June 2012. The IARC lists amongst a number of diesel exhaust carcinogenic chemicals: 3-nitrobenzanthrone -one of the strongest carcinogens known The fledgling science of nanotoxicology is therefore unlikely to bring much comfort into the Diesel engine world over the next few years and in the meantime annual global diesel fuel demand has reached 1,300 million tonnes (2010) with an annual growth rate in excess of 4% -creating a steadily more pressing health issue which, sooner or later, the world will have to confront. Population growth projections suggest that from about 7 million today, world population is set to exceed 9 million by 2050 and may peak at around 10 million between 2050 and 2100. To put these population growth figures into a world health context, a recent report in "The Lancet" indicated that in 2010 more than 3.2 million people worldwide died from the effects of air pollution -mainly from diesel exhaust. That death toll had increased by 300 % between 2000 and 2010. Air pollution is now one of the world's top 10 killers and absent dramatic worldwide action these figures can only increase. Recently published data show that large marine diesels also make a massive and totally disproportionate contribution to global NOx emissions. It is claimed that the largest 100 marine diesel powered ships afloat today emit more NOx than the world's 700 million diesel engined land vehicles combined -a truly appalling statistic. The BCE offers an efficient means to dramatically reduce these totally unacceptable Cl engine emissions.

Tuming from transportation to power generation, and the world search for an increased contribution from renewables, it is at first sight surprising that solar power has not yet taken a central role in this global search despite the sun being recognised as the world's ultimate green energy source. Solar power is clean,free, abundant, inexhaustible -and totally renewable. One possible reason for the minimal progress made to date in harnessing solar energy worldwide is that limitations to the size, shape and performance capabilities of available solar energy conversion technologies may have had an inhibiting effect on market development. The BCE now offers a comprehensive and advanced technical solution, independent of individual solar power generator size, and suitable for all solar generation markets. The likely engine output range across such markets is of course considerable and whilst certain solar markets eg Southwestern USA, are looking at large solar power facilities -megawatt rated -feeding power into the US national grid, it is clear that the significantly more abundant solar opportunities lie across the developing world in the solar microgeneration or microCHP fields where individual power outputs will typically lie in the kilowatt range and where RAMS considerations may predominate. The BCE is thus of unique strategic importance in these solar markets since it can operate in combination with any of the commonly used solar collector I receiver technologies including Fresnel mirrors, trough collectors, power towers or parabolic reflectors. Single or multicylinder BCE designs can accommodate the wide range of outputs likely to be of interest in solar markets. Additionally, the BCE's power generation efficiency, depending on configuration, may be more than double that of currently available solar energy conversion technologies over an output range of 1kW to 5MW -and beyond.

This efficiency and energy input flexibility cannot be offered by SI or Cl engines which will not run on sunshine and fresh air only -.they are intrinsically non-solarizable and are thus excluded from the potentially massive world solar market. The contrasting capability of the BCE means that it is ideally suited to act as the energy conversion means, for example, in the Concentrated Solar Power ( CSP) field..The BCE also offers 24/7 power generation security since a given engine can operate simultaneously in solar power or conventional fuel powered modes. In solar power mode the intelligent BCE can operate with a smart control system and appropriate sensors responding to changes in solar energy input to the collector I receiver device. It is a simple matter, therefore, to base load solar energy input and to vary heat flow to the engine from the liquid or gaseous fuel fired combustor and thus to match power demand. Power output can thus be continuously maintained at the desired level during cloudy periods or at night. The solarized BCE will therefore find a ready market opportunity throughout the developing world. Market development is expected to further accelerate as the logic of Distributed Generation ( DG) in those widely distributed markets is increasingly recognized Sub-Saharan Africa, for instance, has a rapidly growing population of 600 million people in more than 400,000 communities spread over a land area in excess of 11 million square kilometres -most without access to electric power today. India also offers a broadly proportionate challenge -Simple economics suggest that DG is the logical and possibly the only way forward in such markets Additionally, and as storage battery technology develops, a further logical scenario can be visualised in which, when the BCE is running in solar power mode, any excess engine output may be diverted to battery charging duty -further reducing overall fossil fuel consumption and emissions. Depending on demand level, battery power may meet a large proportion of nighttime power demand. Additionally, the well proven Reverse Brayton cycle Bell-Coleman cycle) means that air conditioning or other cooling duty can also be provided within the overall BCE system by integration of an appropriately configured BCE, running in reverse, and at minimal operating cost. Air conditioning may not be a near term priority for individual consumers in the developing world solar market but it is clearly of immediate and obvious value in hospitals and other public buildings.

When configured in solar power generation mode the BCE can also be built as a double ended or single ended airbearing supported and cushioned ( using a fifth airbearing) free piston engine and as such can operate using simple reciprocating motion by using a linear power generator mounted on the engine's axis. Such configurations avoid the need for conversion of axial to rotary motion within the engine. The avoidance of crankshaft and connecting rod bearing friction losses, churning losses etc which can be significant in any engine, makes a further measurable contribution to engine efficiency as well as lowering maintenance requirements. If, therefore, such a BCE is built into a CSP system, it forms an omnivorous, low friction, high efficiency, green machine which runs on an inexhaustible energy source. Water can be injected into the BCE to deliver a useful power boost.

In summary, therefore, the BCE offers meaningful performance improvements and advantages over existing SI and Cl engines in the following applications and markets: 1. Low emissions, high efficiency transportation -road, off-road rail and marine to 5MW output and beyond as a direct replacement for SI and Cl engines.

2. Distributed Power Generation ( OG) -1 kW to 5MW and beyond 3. Small and medium scale (1 -100 kW) microgeneration and microCi-IP.

4. Solar Power Generation and more specifically Concentrated Solar Power ( CSP) -preferably using a parabolic dish collectorlreceiver system. The massive worldwide solar power generation market is estimated by the World Energy Council to total between $3,000 billion ( $3 trillion) and $10,000 billion ($10 trillion) to 2050 -The inherent flexibility of the intelligent BCE design in both simple and hybrid forms means that, for a given power output, the system designer has available to him an extensive range of options covering engine configurations, engine displacements ( bore, stroke, number and arrangement of cylinders), engine speed range -slow, medium and high, plus system components, operating parameters and fuels. The different markets described above may demand a variety of technically different solutions Nevertheless, all engines adhere to the same basic Brayton cycle design principle.

As already discussed, and based on the following drawings and accompanying explanation, the present invention can be seen to be a unique improvement over conventional SI and Cl engines.The BCE can be built in a wide range of configurations and outputs depending on the target market. In fact, and as described here, more than 10,000 permutations of the basic BCE concept are feasible in practice. Realistically,these permutations cannot all be described. Instead -and since there is a significant and intentional design overlap between the simple BCE and the hybrid BCE -a single cylinder preferred exemplary embodiment of the BCE which defines the principal construction and operational features of an engine of either design, is now described. It is not intended that this embodiment should describe all possible forms of the invention. The words used in the specification should be taken, therefore, as words of description rather than limitation. It can similarly be seen that many and various changes may be made to the intelligent BCE basic design without departing from spirit and scope of the invention. Additionally, certain implementing features of the embodiment described in this specification may be combined to form further embodiments of the engine. The three figures used here to illustrate this embodiment are not necessarily to scale and some features may be exaggerated or minimised to show the arrangement and details of particular system components. Therefore, specific structural and functional details of the engine disclosed herein are not to be interpreted as limiting but merely as a representative basis for teaching one skilled in the art to C;,.

variously build and employ the BCE in irs simple or hybrid forms and whilst targetting the significantly different markets listed above.

This preferred embodiment is described with reference to the following drawings: FIGURE 1 -is a key sketch showing a typical BCE system. The components external to the cloud perimeter do not form part of the current invention but are preferred components within the generalised BCE system.

FIGURE 2 -shows a plan view of the engine cylinder head showing the four inlet and outlet valves plus the fuel admission port which is used in the hybrid BCE design FIGURE 3 -shows a side elevation of the simple four valve BCE.

As shown in Figure 1 a typical Brayton engine system comprises the following components and interconnections: 1. A Brayton cycle engine 1.

2. A turbocharger or supercharger 2.

3 An aftercooler -often referred to as an intercooler 3.

4. An externally mounted combustor 4 5. A seal air compressor 13, receiver 43 and header 44 6. An externally mounted heat exchanger or solar power collector / receiver 15 7 A smart system controller 33.

Patent coverage is not sought for any of these components other than the Brayton Cycle Engine ( BCE) I itself. This delineation is indicated by the cloud perimeter shown surrounding the engine. In addition, certain engine builds eg naturally aspirated designs, can clearly employ simplified system circuitry. Irrespective of the number of cylinders in a given engine a single combustor 4 is used for a group of cylinders to provide smooth combustion and smoothed hot gas flow into the engine. Equivalently a multi-cylinder engine would preferably be equipped with a single turbocharger 2.

As shown in Figure 2 the BCE cylinder head 31 has a total of four valves mounted in it, a charge air inlet valve 27, a compressed charge air outlet valve 28 a hot gas inlet valve 25 and a an exhaust valve 26..An aeditinal fuel admission port 29 can be mounted in the BCE cylinder head to allow controlled admission of fuel into the BCE cylinder during the engine power stroke. The main engine fuel being bumt in the comOustor, Fuel A, is admitted through port 41 and may be of different specification from the additional, Fuel B, 42 admitted to the cylinder through port 29 when the engine operates in hybrid mode. However whilst heavier fuels may be used in Fuel A duty, higher volatile or gaseous fuels (depending on engine speed) are typically used in Fuel B duty to obtain maximum overall engine performance benefit As shown in Figure 3, a trunk piston 5 reciprocates within a cylinder liner 32.

Piston axial movement is lubricated and supported by an airbearing 19 forming said cylinder liner. Additional support and lubrication for said piston can be provided by a crosshead air bearing 20 mounted on support posts 11. This air bearing lubricates and supports an axially disposed piston rod 6. Conventional little end 7 and big end 8 bearings are mounted on a connecting rod 40 driving an engine crankshaft ( not shown). Multiple channels 14 supply seal air to the cylinder wall air bearing. Seal air for the crosshead air bearing bearing 20 flows via one or more channels 12 through the bearing support posts 11. The BCE cylinder liner is fluid cooled by means of coolant flowing through an annulus 18 formed between said cylinder liner and the outer engine shell ( engine block) 11. In charge air cooled mode, coolant flows from the inlet header 35 via said cooling jacket annulus to the outlet header 36 and thence to the engine charge air inlet valve 27. When operating in charge air cooling mode, air bearing 19 is configured with extended external surface 16 to improve cylinder wall (cylinder liner) cooling performance. The hot as inlet valve 25 and engine exhaust valve 26 may be provided with lubrication and cooling via air bearings 21 and 22. Seal air for said hot gas inlet and exhaust valves flows from the seal air compressor 13 via distribution header 44 7..

and receiver 43 to said bearings via channels 24.The engine normally employs conventional air start up but an electric starter motor can also be used for low output engines.

It is re-emphasised that the components listed in 1 -7 above represent available building blocks which can be variously employed to produce a wide range of market responsive engine builds -but all firmly based on the Brayton cycle design principles set out in

this specification