GB2469939A

GB2469939A - Split-cycle engines

Info

Publication number: GB2469939A
Application number: GB1007415A
Authority: GB
Inventors: Keith Gordon Hall
Original assignee: Individual
Current assignee: Individual
Priority date: 2009-05-01
Filing date: 2010-05-04
Publication date: 2010-11-03
Also published as: GB2470808A; GB0907496D0; GB201007415D0; GB201007416D0

Abstract

A split-cycle engine has a combustion chamber 6 which is supplied with compressed air from a compression cylinder 1. Fuel is injected into the air by an injector 7 and is ignited by spark plug 8. The combusted product passes via a valve 10 into an expansion cylinder 11 of larger volume than cylinder 1. Valve 10 opens around TDC of cylinder 11 and closes around BDC, whereas exhaust valves 12,13 of cylinder 11 open around BDC and close around TDC. Each crankshaft revolution comprises two compressor-piston strokes and two expander-piston strokes. The combustion chamber valves 4,10 open sequentially separated by the combustion phase. The engine may operate in compressed-air recovery modes in which compressed air from cylinder 1 or from cylinders 1 and 11 is stored and admitted via a valve 16 to pressurise the combustion chamber 6. The engine-cycle may include drive by recovered compressed air without ignition, and cylinder banks, each comprising a compression cylinder and an expansion cylinder with an interposed combustion chamber, may be intercoupled by disc drive.

Description

Split-Cycle Engines The present invention relates to split-cycle engines.

According to the present invention there is provided a split-cycle engine wherein a valved combustion chamber is interposed between a compressor cylinder and an expansion cylinder, for introducing a dedicated combustion phase to the engine cycle.

The dedicated combustion phase of the split cycle of the engine of the present invention, may follow induction and compression phases, and be followed by expansion and exhaust phases.

Sequential operation of the combustion-chamber valves may trap the charge without it being displaced, thus eliminating additional compressor pumping work that is normally associated with split-cycle engines. Phase lag of the expander piston behind the compressor piston determines the time envelope available for combustion initiation within which the fuel admission timing and spark ignition timing may be optimised.

Combustion may continue into the expansion phase and the expander may be of larger capacity than the compressor in order to achieve the more efficient Atkinson cycle.

The transfer passage between the compressor and expander cylinders of the split-cycle engine of the invention may be optimised (for example, as a compact lean-burn combustion chamber), and being external to the compressor and expander cylinders enables the introduction into the engine cycle of a fifth phase dedicated to combustion. The short flame travel in a compact combustion chamber offers the opportunity to employ larger cylinder volumes, and thus to reduce the number of cylinder pairs. This scale effect enables greatly reduced engine package size, complexity and cost whilst accommodating the large expander volumes required for Atkinson cycles. The scale effect also reduces piston sealing area, combustion chamber surface area and combustion crevice volumes so as further to increase engine efficiency.

The split-cycle engine of the invention may involve compressed air recovery in the deceleration modes because the clearance volume is external to the cylinder volumes, thus providing the capability for efficient compressed air generation in hybrid vehicles.

An engine cycle per revolution offers improved smoothness and reduced mechanical losses compared with the conventional 4-stroke Otto cycle that runs at double the speed for the same power strokes. The split Atkinson cycle provides four strokes across dedicated compressor piston/expander piston pairs, working in unison with the motion of the pistons to provide an engine cycle per revolution. This halving of engine speed per engine cycle gives increased efficiency and power density.

Engines of the 4-stroke kind are also compromised because the mono-piston system precludes piston phase differences between compression and expansion. The piston top dead centre (TDC) position is common to both compression and expansion so combustion has to be initiated before compression is completed in order to be in time for the expansion. This results in combustion-pressure rise before TDC and so creates negative work.

With the engine of the present invention, combustion (also encompassing fuel/air mixing) forms a fifth engine phase in its own right.

Split-cycle engines benefit from a cooler compression cylinder but traditionally suffer increased handling losses due to the transfer process of the working fluid from compressor to expander. The split-cycle engine of the present invention on the other hand, may switch the clearance volume between compressor cylinder, combustion chamber and expansion cylinder in a way that avoids compressor discharge losses. The compressor discharge valve (CDV) and expander inlet valve (Ely) may in this respect be operated sequentially and separated in time by the combustion phase so as to trap the charge without it being displaced, and thus eliminate additional pumping work normally associated with split-cycle engines. The phase duration is determined by the lag of the expander piston behind the compressor piston. More efficient quiescent combustion would require a longer combustion phase.

Known split-cycle engines (for example that described in US Patent Specification No 7,353,786), lack a dedicated combustion phase, and so require the expander piston to lead the compressor piston and for both CDV and EIV valves to be open simultaneously in order that the expander is pressurised from the compressor.

The expander intake valve closes prior to combustion starting, so the transfer passage is always pressurised but not involved in the combustion.

Energy recovery during engine braking is a means of increasing overall efficiency and the split-cycle engine lends itself to compressed air recovery (CAR) because of the large cylinder volumes employed and the minimal clearance volumes in the compressor and expander cylinders. Moreover, a heavy electrical starter motor is not required, since the recovered compressed air may be stored in a reservoir for use when starting and for other drive functions. The reservoir may be supported by a back-up battery-driven compressor.

The stored compressed air may be used to create a compressed air drive (CAD) and the storage pressure increased to improve energy density. For example, if raised to engine peak combustion pressure, then depending on the cut-off ratio, CAD performance could be equivalent to the performance of the engine of the invention in its internal-combustion (ICE) drive mode.

If fuel is added and combusted in the CAD mode then a compressed-air boost mode (CAB) is created that may temporarily provide increased power.

Split-cycle engines in accordance with the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic representation of the cylinder arrangement of a first split-cycle engine of the present invention together with indication of phases of its engine cycle; Figure 2 is a chart showing details of actuation of valves of the engine of Figure 1 during alternative modes of operation; Figures 3(a) and 3(b) are illustrative of a typical valve of the split-cycle engine of Figure 1 in its open and closed states respectively; Figure 4 is illustrative of a simplified split-cycle engine according to the present invention; and Figure 5 shows the simplified split-cycle engine layout of Figure 4 in a compact flat-4 arrangement with disc-drive intercoupling.

Referring to Figure 1, air is induced into compression cylinder 1 of the split-cycle engine through inlet valves 2 and 3, and is discharged through compressor discharge valve (CDV) 4 to discharge tract (CDT) 5 feeding combustion chamber 6. Fuel injector 7 introduces the fuel to create a combustible mixture in the combustion chamber 6 which is ignited by a spark plug 8.

Optionally, the combustion air may be seeded upstream with fuel, preferably to a mixture level below self-ignition, to assist lean combustion or stratified charge performance. Combusted products pass out of the combustion chamber 6 along expander inlet tract (El?) 9 and when expander inlet valve (Ely) 10 is open, enter an expander cylinder 11.

Valves CDV 4 and EIV 10 function as the inlet and outlet valves respectively to the combustion chamber 6. Valve EIV 10 opens around TDC of the expander cylinder 11 and closes around bottom dead-centre (BDC) . Expander exhaust valves EEV 12 and 13 open around BDC and closes around TDC.

The principal phases of the engine cycle are indicated in Figure 1, each cycle comprising two compressor-piston strokes and two expander-piston strokes per crankshaft revolution. That is to say, there are four piston strokes plus the crank angle phase difference between compressor piston and expandor piston in each cycle.

A novel feature of the split-cycle engine of the invention is the switching of the clearance volume by valve means from the compressor to the expander. The clearance volume comprises CDT 5, combustion chamber 6 and EIT 9. When CDV 4 is open and EIV 10 closed this combustion chamber volume becomes the compressor clearance volume, and when CDV 4 is closed and EIV 10 open it becomes the expander clearance volume. Because the trapped working medium has not moved, no work has been expended moving it from the compressor to the expander. Unlike the known split-cycle engine of US Patent Specification No 7,353,768 where valves corresponding to valves CDV 4 and EIV 10 are open simultaneously with the working fluid flowing from the compressor cylinder to the expander cylinder, the valves CDV 4 and EIV 10 of the split-cycle engine of the present invention open sequentially, separated by the additional combustion phase.

Compressed air recovery valves are also shown in Figure 1 for a range of compressed air recovery modes detailed in the chart of Figure 2. Use of only the compressor cylinder 1 for compressed air recovery and storage is termed "CAR 1", whereas recovery of compressed air using the compressor and expander cylinders 1 and 11 is termed "CAR 2". The compressed air drive mode is termed "CAD" and the fuel boosted compressed air drive mode is termed "CAB".

In the CAR 1 mode, CDV 4, EIV 10 and fuel injector 7 remain closed so as to generate compressed air in cylinder 1 in the usual manner, and compressor recovery valve 15 opens to admit the generated compressed air to the reservoir (not shown) Combustion chamber recovery valve (CCRV) 16 is open in order to pressurise the combustion chamber ready for the next drive cycle and to assist in holding CDV 4 and EIV 10 closed. Hence, compressed air recovery occurs only in the compressor cylinder 1, the expander cylinder 11 preferably being vented at this time.

Venting may be achieved by unseating EEVs 12 and 13.

In the CAR 2 mode both the compressor and expander cylinders 1 and 11 are switched to the compressed air recovery function in which the compressor cylinder 1 and CCRV 16 function as described in the previous paragraph. Induction in the expander cylinder 11 is via expander recovery inlet valves (ERIV) 17 and 18 or by retarding EEVs 12 and 13 half a revolution. Discharge is via expander recovery outlet valve (EROV) 19 to the reservoir or by unseating EIV 10 against its spring with the tappet oil unloaded.

The CAR 2 mode is suited to twin pressure compressed air recovery, medium pressure from the compressor cylinders and high pressure from the expander cylinders.

In the CAD mode combustion chamber recovery valve CCRV 16 admits stored compressed air to the combustion chamber on a timed basis, no fuel is admitted and expansion follows as per the ICE mode.

Figure 2 shows the compressor cylinders generating compressed air during the CAD mode but because the expander cylinder volume is larger than the compressor volume for Atkinson-cycle purposes, positive drive power is still achieved. A further option, not shown, is to vent the compressor cylinders in order to increase the CAD power at the expense of CAD range.

In the CAB mode fuel is added to the recovered compressed air to provide a temporary high performance mode.

The poppet valves of a cylinder conventionally open into the cylinder in order to improve sealing by utilising the high pressure within the cylinder to increase the valve seat sealing pressure. In the split-cycle engine of the present invention the hIgh pressure is generated externally of the cylinders 1 and 11, namely in combustion chamber 6, so it is preferred that valves CDV 4 and EIV 10 are orientated to open outwardly in order that combustion pressure is used to increase valve sealing pressure.

The valve opening motion away from the cylinder also avoids need for valve cut-outs in the pistons (thereby eliminating hydrocarbon forming crevice volumes in the expander piston) and leaves the valve port unobstructed by the valve stem and valve head.

A typical construction of CDV 4, which may also be used for EIV 10, is shown in Figures 3(a) and 3(b), Figure 3(a) is representative of the valve in its lifted, open state, and Figure 3(b) is representative of it in its closed state. Component parts of the valve are identified by references in Figure 3(a) only. (It would also be possible to operate CDV 4 by differential pressure actuation, commonly seen in compressor applications, provided the valve is strong enough to withstand combustion pressures.) Referring to Figure 3(a) , rotation of a cam 20 of camshaft 21 is effective to displace a bucket tappet 22 of CDV 4 against a coil spring 24 of the valve, from the state represented in Figure 3 (b) . The tappet 22 is located within a valve guide 23 of cylinder head 26, with the bore for the valve guide 23 of sufficient diameter to allow assembly of the valve. The guide 23 is located in the bore flush with the cylinder head 26 by clamp-plate 28.

A valve cap seal 25 within the bucket tappet 22 together with other seals 29 to 31 are effective to retain oil within a cavity 32 of the tappet 22. The cavity 32 is filled with oil via a passage 33 and unloading valve 34. For normal valve lift operation as represented in Figure 3 (a) , valve 34 prevents backf low of oil but for operating modes requiring valves CDV 4 and EIV 10 to remain closed, unloading valve 34 is unseated and the tappet displaces the oil without effecting valve lift. The valve spring cavity 35 is air filled and may act as an air spring or be vented.

An alternative compressed air recovery route in the CAR mode is via CDV 4 and EIV 10 to the combustion chamber and back to the reservoir via CCRV 16. Separate compressor and expander recovery valves 15 and 19 respectively are then not required. The tappet cavity oil is unloaded so that the cylinder compression pressures lift CDV 4 and EIV 10 off their seats against their respective springs, the valves then operating on a differential pressure basis regardless of cam position.

In the ICE mode requiring valve lift, oil (being virtually non-compressible) is displaced by the cylindrical area At of the tappet 22 and acts on the valve cap area Ac less the valve stem area As. The valve cap 25 is displaced upwards the same distance the tappet is displaced downwards for equal areas, so (At-Ac) = (Ac-As) . To return the valve 36 to its seat 37, valve spring 24 holds the tappet against the cam 20 and the valve cap 25 against the trapped oil. Valve lift may be designed to exceed tappet displacement by decreasing area (Ac-As) relative to area (At-Ac) Figure 4 shows a simple split Atkinson cycle engine valve layout with induction in compressor cylinder 41 through inlet valve 42, and delivery through valve 44 to combustion chamber 46. Fuel is introduced via injector 47, mixed in the chamber 46 and ignited with spark plug 48. Burnt products are expelled through expander inlet valve 50 into expander cylinder 51 and exhausted through expander exhaust valve 52 on the return stroke. The shape of the combustion chamber illustrated gives a high compression ratio.

In order to retain close valve-centres additional chamber volume may be added outboard of the valves. This addition is preferably to one side only and of spherical form.

Figure 5 shows the simplified split Atkinson cycle valve layout in a flat 4 disc-drive engine arrangement. This arrangement simplifies engine construction and reduces engine size and weight. The disc-drive intercoupling retains complete dynamic balance whilst accommodating the expander piston lag required for the combustion phase. There are two cylinder banks (compressor and expander cylinders with combustion chamber) each of which has a construction of the form described above with reference to Figure 4, and the following description of the engine is confined to one of these duplicate banks.

Referring to Figure 5, induction in compressor cylinder 41 is through inlet valve 42 and delivery is through valve 44 to combustion chamber 46. Fuel is injected into the chamber 46 for mixing and the mixture is ignited by spark. Burnt products are expelled from the chamber 46 through expander inlet valve 50 into expander cylinder 51 and exhausted through expander exhaust valve 52.

Atkinson cycle engines, by the nature of their larger expansion stroke, open the exhaust valve at a lower cylinder pressure.

Hence the potential energy recovery mechanism exploited by engine turbochargers to boost engine performance is not available to a similar degree for Atkinson cycle engines. In any case, the turbo response time is often slow enough to impair the driving experience. If exhaust energy is to be extracted it would be more beneficial to feed the recovered energy to the vehicle energy storage systems and retain a lively engine response characteristic. An impulse turbine exploits the exhaust gas velocity and would better suit the Atkinson engine turbocharger than conventional reaction turbines that exploit a pressure differential. By way of example, the turbocharger could drive an alternator as both are high speed devices. Examples of impulse turbines are the Turgo turbine and the Pelton wheel.

Claims

Claims: 1. A split-cycle engine wherein a valved combustion chamber is interposed between a compressor cylinder and an expansion cylinder, for introducing a dedicated combustion phase to the engine cycle.
2. A split-cycle engine according to Claim 1 wherein the dedicated combustion phase of the split cycle of the engine cycle follows induction and compression phases, and is followed by expansion and exhaust phases.
3. A split-cycle engine according to Claim 2 wherein sequential operation of the combustion-chamber valves traps the charge without it being displaced.
4. A spilt-cycle engine according to any one of Claims 1 to 3 wherein combustion in the engine cycle continues into the expansion phase.
5. A spilt-cycle engine according to any one of Claims 1 to 4 wherein the expansion cylinder is of larger capacity than the compression cylinder.
6. A split-cycle engine according to any one of Claims 1 to 5 wherein the engine cycle includes compressed air recovery.
7. A split-cycle engine according to Claim 6 wherein compressed air is transferred from the compression cylinder to a reservoir for admission to the combustion chamber for pressurising the combustion chamber in advance of the dedicated combustion phase.
B. A split-cycle engine according to Claim 6 or Claim 7 wherein air compressed in the expansion cylinder is transferred to pressurise the combustion chamber.
9. A split-cycle engine according to any one of Claims 6 to 8 wherein the engine cycle includes drive by recovered compressed air without fuel injection.
10. A split-cycle engine according to any one of Claims 1 to 9 comprising a plurality of cylinder banks, each cylinder bank comprising compression and expansion cylinders with interposed valved combustion chamber.
11. A split-cycle engine according to Claim 10 including disc-drive inter-coupling of the cylinder banks.
12. A split-cycle engine substantially as hereinbefore described with reference to Figure 1 of the accompanying drawings.
13. A split-cycle engine substantially as hereinbefore described with reference to Figure 4 of the accompanying drawings.
14. A split-cycle engine substantially as hereinbefore described with reference to Figure 5 of the accompanying drawings.
15. A split-cycle engine according to any one of Claims 1 to 14 including one or more valves substantially as hereinbefore described with reference to Figures 3(a) and 3(b) of the accompanying drawings.