Field of the Invention

The invention relates to methods of operating a two-stroke engine and improvements to two-stroke engines.

Prior art known to the applicant

The invention proposes detailed improvements to two-stroke engines and their methods of operation, specifically to increase the recoverable exhaust energy.

Summary of the invention

In a first broad independent aspect, the invention provides a method of operating a two stroke engine comprising the steps of providing at least one cylinder; an intake system; an exhaust system; and a variable area valve; communicating between intake and exhaust systems; and varying the area of said valve to facilitate the variation of recoverable exhaust energy from a power cylinder with inlet and exhaust port areas. In preferred embodiments, the inlet and exhaust port areas meet a minimum size requirement- incorporating within the engine structure a variable area valve linking the intake and exhaust systems so that a proportion of the engine air mass flow travelling from intake to exhaust does not need to traverse the power cylinder; and providing a controller to modulate exhaust flow and/or intake flow externally from said cylinder, dependent upon the mode of operation of said engine. In a subsidiary aspect, the method further comprises the step of pressure-charging said engine.

In a further subsidiary aspect, the method further comprises the step of providing an intake port area which occupies from 60% to 80% of the cylinder's circumference and is a minimum of 44% of the bore area. This offers a vastly enhanced intake port area in contrast to the developing art in two-stroke engines. On a time area basis such porting could deliver an intake time area of 57χ10 ^Λ -5 sec.cm ^A 2/cm ^A 3, or greater, as compared to a conventional two stroke which would be 8~ 10χ10 ^Λ -5 sec.cm ^A 2/cm ^A 3. In a further subsidiary aspect, the method further comprises the step of providing a valve external to said cylinder; wherein a controller is configured to control intake flow by selectively actuating the valve. This configuration is particularly advantageous since it may allow the de-coupling of charge induction timing from the geometry of the cylinder ports. This approach also permits the use of variable intake timing, which improves engine operational efficiency. This configuration is particularly advantageous in terms of adaptability to engine requirements and conditions of operation.

In a further subsidiary aspect, the method comprises the step of providing an exhaust port area, which occupies from 80% to 100% of the cylinder's circumference and is a minimum of 40% of the bore area. This approach achieves vastly improved air flow factors and is in contrast to the current developing art in two-stroke engine design. On a time area basis such porting could deliver an exhaust time area of 40χ10 ^Λ -5 sec.cm ^A 2/cm ^A 3, or greater, as compared to a conventional two stroke engine which would be 14~ 15x10 ^A -5 sec.cm ^A 2/cm ^A 3.

In a further subsidiary aspect, the engine comprises an exhaust valve as part of the cylinder and the controller is configured to modulate its operation. In certain embodiments the engine comprises a secondary exhaust valve downstream from the cylinder, used to permit larger overall exhaust flow areas, and the controller is configured to modulate its operation.

In a further subsidiary aspect, the method further comprises the step of providing a single variable area valve.

In a further subsidiary aspect, the method further comprises the step of facilitating the flow of a proportion of intake charge through a variable area valve to said exhaust system in addition to the charge flowing through the or each combustion cylinder.

In a further subsidiary aspect, the method further comprises the step of providing a plurality of variable area valves, configuring them to provide an aggregate flow, thus increasing the recoverable exhaust energy.

In a further subsidiary aspect, the variable area valve is configured to have an average aggregate flow area of at least 75% of the average reduced flow area for the engine. In a further subsidiary aspect, the variable area valve has an average aggregate flow area which is at least 4.3% of the total engine bore area.

In a further subsidiary aspect, the variable area valves used in the engine will have the ability during each crankshaft revolution to either; vary between zero flow area and an intermediate flow area up to its maximum flow area, or to stay at a constant flow area, or to remain at zero flow area.

Preferably, said variable area valve is configured to respond to external control which is part of an engine performance control system.

In a further subsidiary aspect, the method comprises the further step of the valve adopting a pre-determined flow area and remaining constant during a mode of operation. In a further subsidiary aspect, the method comprises the step of phasing the variation in flow restriction between the variable area valve and the cylinder. This is particularly advantageous in terms of ensuring that the disruption to the mass flow through the engine cylinders is minimised.

In a subsidiary aspect, the method comprises the steps of disabling and re-enabling the variable area valve dependent upon the conditions of operation. This further optimises the operation for a variety of operational requirements.

In certain embodiments the variable area valve comprises a rotary disk valve for controlling an orifice size and the timing of its opening.

In a further subsidiary aspect, the variable area valve may be a poppet valve. These may in certain embodiments be an electro-magnetically driven valve or an electro-hydraulically driven valve in order to provide improved actuatable control for an optimisation of the engine operation.

In a further subsidiary aspect, the method comprises the further step of allowing the variable area valve to respond to external control which is part of an engine performance control system.

In a further subsidiary aspect, the method comprises the step of phasing the variation in flow restriction between the variable area valve and the cylinder, by controlling the instantaneous variable area valve flow area and the phasing of this flow area to the flow area of the power cylinder. This is particularly advantageous in terms of ensuring that the disruption to the mass flow through the engine cylinders is minimised.

In a subsidiary aspect, the method comprises the steps of disabling and re-enabling the variable area valve dependent upon the conditions of operation. This further optimises the operation for a variety of operational requirements. In a further subsidiary aspect, the method comprises the further step of allowing said variable area valve to respond to external control which is separate from an engine performance control system. In a further subsidiary aspect, the method further comprises the step of providing one or more sensors to detect one or more values representative of engine operating conditions and modulating exhaust and/or intake flow dependent upon the value detected by the sensors. This configuration further improves the control and adaptation of the operation of a two-stroke engine in order to optimise energy recovery.

In a further subsidiary aspect, the method comprises the steps of providing a compressor and controlling the supply of air to the engine to significantly exceed the required flow for combustion. In a subsidiary aspect, the engine further comprises means for varying the supply pressure of the air; wherein a controller is configured to match a predetermined level of mass flow through the engine with a predetermined engine speed.

In a further subsidiary aspect, the method further provides an exhaust stream and the step of powering at least in part the compressor, either directly or indirectly, from the exhaust stream. This configuration is also particularly advantageous in terms of effective recovery from the exhaust stream.

In certain embodiments the method comprises the step of providing one or more turbines for the conversion of exhaust engine to mechanical or electrical energy. This configuration improves the achievable recovery of energy.

In a further subsidiary aspect, the method further provides an exhaust stream and the step of generating thrust, either directly or indirectly, from the exhaust stream. This configuration is also particularly advantageous in terms of effective recovery from the exhaust stream.

In certain embodiments the method comprises the steps of providing at least one thermal storage device and at least one exhaust recovery device; and providing a thermal storage device upstream from the exhaust recovery device. This configuration is particularly advantageous in order to potentially buffer heat. In certain embodiments, it may be employed to reduce the variation in temperature at the inlet of the exhaust energy recovery device.

In certain embodiments, the method further comprises the steps of providing an exhaust stream and one or more bypass arrangements in the exhaust stream; and selectively opening the bypass arrangements to bypass one or more devices provided in the exhaust stream. This is particularly advantageous in terms of load balancing between devices, such as in certain embodiments, air compressors or other means for regulating the mass flows and pressures during operation. In further embodiments, bypass arrangements may be replaced by means for regulating the operation to achieve load balancing between exhaust and recovery devices.

In a further subsidiary aspect, the method comprises the step of providing an exhaust valve as part of the cylinder and modulating its operation. This will further improve the operation of the engine by permitting the use of variable exhaust timing. This

configuration is particularly advantageous in terms of adaptability to engine requirements and conditions of operation.

In a further subsidiary aspect, the method comprises the step of providing an exhaust valve downstream from the cylinder and modulating its operation. This configuration will also improve the exhaust timing to improve engine operation efficiency. This configuration is particularly advantageous in terms of adaptability to engine requirements and conditions of operation.

Brief description of the figures

Figure 1 shows a graph for the port areas for sleeve valves in a two-stroke engine.

Figure 2 is an illustration of Rolls Royce 'Crecy' type power-cylinder construction.

Figure 3 shows a generic illustration showing one embodiment of rotating valves performing function of variable area valve.

Figure 4 shows a generic illustration showing a combination of embodiments.

Detailed description of embodiments of the invention

In one embodiment, the invention seeks to increase the available exhaust stream energy to supplement energy recovery methods.

Considering a pressure charger, the power to dri be stated as:

Wc = Ma x Cpa x Ta

Where:

Wc = compressor power

Ma = mass of charge air per unit time

Cpa = Specific heat at constant pressure (at average compressor temperature)

Ta = inlet air temperature

Pre = Pressure ratio across the compressor

ya = Cpa/Specific heat at constant volume

EffC = compressor efficiency

If those terms which are either temperature, or functions of temperature, are excluded to reflect their dependency on upstream conditions, and the efficiency is excluded because it is a design variable, the compressor power is seen to be a function of mass flow and pressure ratio. Wc~Fn(Ma, Pre)

Similarly the power developed by a turbine can be stated as:

Wt = me x Cpe x Tp x ( 1 - (Pre) r ^e J x EffT Wt = turbine power

Me = mass of exhaust flow per unit time

Cpe = Specific heat at constant pressure (at average turbine temperature) Tp = inlet to turbine temperature

Pre = Pressure ratio across the turbine

ye = Cpa/Specific heat at constant volume

EffT = turbine efficiency

If those terms which are either temperature, or functions of temperature, are excluded to reflect their dependency on combustion/fuelling conditions, and the efficiency is excluded because it is a design variable, the turbine power is seen to be a function of exhaust mass flow and pressure ratio.

Wt~Fn(Me, Pre)

For a given engine operating condition the maximisation of exhaust energy requires focus on the mass-flows in the compressor and turbine, and the pressure ratios across each device. Simplistically, the excess of turbine power over compressor power represents the potential exhaust energy that can be recovered.

A two stroke engine can be considered as a fixed area orifice and for a given pressure drop effectively the same air flow will result regardless of whether the engine is stationary or rotating at maximum rpm. By varying the supply pressure of the air the mass flow through the engine may be matched to the desired flow for any given engine speed.

As long as sufficient pressure drop exists across the two-stroke power cylinder to promote adequate scavenging, while not choking the flow, the engine fundamentally does not care about exhaust back pressure and this parameter can be managed for exhaust energy recovery. The two-stroke engine also lends itself to alternative power-cylinder valving arrangements, such as sleeve-valves, which can provide massively greater flow areas than the corresponding four stroke engine for the same cylinder bore size. In these two key respects the two stroke engine lends itself to exhaust energy recovery.

In certain embodiments, a sleeve-valved two-stroke engine is configured to utilise a specific size and form of cylinder porting and a variable area valve to simultaneously permit higher exhaust back pressures and vastly increased air mass flows thereby raising the recoverable exhaust energy. The benefits of certain embodiments include, but are not limited to, greatly improved thermal efficiency, reduced exhaust stream temperatures, and greatly improved energy density.

In certain embodiments, where variable area valves are employed on an two stroke engine the 'average aggregate flow area' is the average value, over a single crank revolution, of the instantaneous summations of the open flow areas for all the variable area valves employed.

The 'reduced area' for a two stroke cylinder is defined as:

1

Reduced Area(at crank angle &) =

Where;

ΑΕ(Θ) = Exhaust port area at crank angle Θ

ΑΙ(Θ) = Intake port area at crank angle Θ

Reduced Area(6) = reduced area at crank angle Θ

An example of the above calculation for a two stroke engine which achieves the required minimum value for 'reduced area' is shown in figure 1.

The 'reduced area' is zero when either the intake or exhaust flow areas are zero and varies as the described function while the intake and exhaust areas are both greater than zero.

The 'average reduced flow area' of a two stroke engine is the average value, over a single crank revolution, of the instantaneous summations of the reduced flow areas of all the engine cylinders. It has been demonstrated on a 12 cylinder two stroke engine that the 'average reduced flow area' can be considerably greater than zero for the entire engine revolution, because the reduced areas for individual cylinder overlap in time, and hence a short circuit path from intake to exhaust is always open and charge is always being lost to the exhaust system. Such an engine, the Rolls Royce 'Crecy', ran extensively in the UK in the 19 0's.

Figure 2 Illustrates the Rolls Royce 'Crecy' type power-cylinder construction.

1. Intake flow to cylinder

2. Exhaust flow from cylinder

3. Cylinder

4. Cylinder head

5. Sleeve valve

6. Piston

7. Combustion chamber

8. Spark plug access to combustion chamber

9. Fuel injector access to combustion chamber

10, Upper edge of sleeve-valve, controlling exhaust port timing

1 1. Intake port

As mass flow rates increase through a two stroke cylinder the parasitic losses will increase until ultimately the flow reaches a sonic limit and the flow 'chokes'. To maximise the total flow through the engine within these limits the total flow area needs to be increased above that of the cylinders alone.

In a further subsidiary aspect, the use of a variable area valve provides the two stroke engine, of any given number of power cylinders, a proportionately larger 'average reduced flow area' permitting greater flow capacity for the engine than would be possible using the power cylinders alone.

In further embodiments, an engine may use a variable orifice (variable area valve), external to the engine cylinders, which allows the flow of charge between the compressor and exhaust energy recovery systems without such flow having to traverse a cylinder. The variable area valve may advantageously permit total mass flow to be increased and also permit control of exhaust stream flow conditions. In a further subsidiary aspect, the aggregate maximum flow area of the variable area valve, or valves, used in the engine will be greater than 75% of the 'average reduced flow area' of the power cylinders of the engine, and no less than 4.3% of the aggregate bore area of the engine.

In a further subsidiary aspect, increases in the 'average reduced flow area' of a two stroke engine will allow beneficial increases in the achievable 'aggregate maximum flow area' of the variable area valves used in the engine. Consequently changes to intake and exhaust port geometry which promote increases in 'average reduced flow area' for the engine may be sought provided; adequate blowdown period is achieved; adequate supercharge period is achieved; and adequate cylinder scavenging is achieved.

In certain embodiments, a cylinder ported two-stroke engine may utilise asymmetrical exhaust timing which may or may not be variable.

In certain embodiments, where sleeve-valves are used, to further increase the available flow area, the control of the inlet ports can be further improved by increasing the height of the intake ports and introducing an external, to the cylinder, valve to control the intake flow. In doing this the control of intake timing is no longer controlled by the piston. This permits a further advantage that the exhaust timing may be varied, by modifying the sleeve drive geometry, independently of the intake timing so the performance of the engine may be optimised.

In certain embodiments, to achieve adequate cylinder intake area an additional modulatable intake valve may be used upstream of the cylinder to decouple charge induction timing from the geometry of the cylinder ports. This embodiment also permits the use of variable intake timing to improve engine operational efficiency.

In certain embodiments, to achieve adequate cylinder exhaust port area a modulatable exhaust valve may be used as part of the cylinder construction, or downstream of the cylinder, to permit the use of variable exhaust timing to improve engine operational efficiency. In further embodiments, in the two-stroke sleeve-valve cylinder construction the exhaust timing may be effected by suitable phasing of the sleeve drive to crankshaft. Downstream exhaust valves may also be provided. In certain further embodiments, the engine may operate with a compressor, or multiples thereof depending on engine operational requirements, for provision of adequate air mass flows and pressures, which may be driven directly by a turbine, or multiples thereof, as part of a turbo-charger, or mechanically by the engine, or electrically or a combination of these approaches.

In certain further embodiments, the engine may operate with a turbine, or multiples thereof, or similar devices for the conversion of exhaust energy to mechanical or electrical energy, this turbine may or may not be part of a turbo-charger(s), or could be used in conjunction with a turbo-charger, or may be a standalone device.

In certain further embodiments, multiple stages of such devices may be required to avoid choking conditions in the power cylinders, variable area valves, or exhaust stream.

In certain further embodiments, the engine may use, as a function of desired engine operating conditions, thermal storage devices, such as catalyst assemblies, to buffer heat and reduce the variation in temperature at the inlet of the exhaust energy recovery device(s).

In certain further embodiments, the engine may use, as shown necessary as a function of desired engine operating conditions, one or more bypass arrangements to allow load balancing between air compressor devices, or other means for regulating their mass flows and pressures during operation.

In certain further embodiments, the engine may use, as shown necessary as a function of desired engine operating conditions, one or more bypass arrangements to allow load balancing between exhaust energy recovery devices, or other means for regulating their operation. In certain embodiments, the engine may operate in conjunction with a control system to balance the needs of the base engine and the load generation system which can, in addition to the normal engine and required after-treatment management functions can simultaneously in one or more embodiments:

• Modulate the variable area valve depending on the requirements of the engine operating condition;

• Modulate the exhaust temperature and mass flow for the benefit of the required after-treatment efficiency;

• Modulate the operation of the compressor and exhaust energy recovery devices to maximise the total engine system thermal efficiency;

• Modulate the intake and exhaust valve timing depending on the requirements of the engine operating condition.

Embodiments of the invention may include one or more of the following:

• Stationary and non-stationary engine applications;

• Variable and constant speed applications;

• A wide range of combustion systems, permitting almost any contemporary system to be used;

• Two-stroke engines with internal or external fuel combustion;

• Two-stroke engines with single power cylinders, or multiples thereof

• Two stroke engines of widely varying scavenge requirements because of the

freedom of combustion chamber shape;

• Two stroke engines for use with external exhaust gas recirculation systems;

• Two stroke engines utilising increased mass air-flow to create exhaust efflux thrust.

Figure 3 shows a generic illustration showing one embodiment of rotating valves performing the function of a variable area valve. In view 13, the disk number 1 is effectively static and rotated to vary the flow area. In view 13A, the valve number 1 is fully open. Arrow 14 shows the adjustment. Numerical reference 15 illustrates the port. In view 13B, the valve number 1 is fully closed. In view 13C, the valve number 1 is part open. In view 16, the disk number 2 rotates at the crank speed to control the port timing. Port 17 is illustrated. Numerical reference 18 is the timing edge whilst arrow 19 shows the direction of rotation at the crank speed. View 21 shows disk number 2 as numerical reference 22. In this view, disk number two has been adjusted to be in the fully open position. Numerical reference 23 shows disk number 1 rotating at the crank speed. The disk number 1 is shown as open.

Figure 4 shows a generic illustration showing a combination of embodiments. The following numerical references are shown: 24 is a first stage compressor; 25 is a second stage compressor; 26 is the direction of rotation of the compressor drive; 27 is an optional bypass valve; 28 is an optional external intake valve; 29 is a power cylinder; 30 is a sleeve drive; 31 is a crank; 32 is a variator valve; 33 is an optional external exhaust valve; 34 is an optional bypass valve; 35 is an energy recovery means; 36 is an optional turbine second stage; 37 is the exhaust stream.