GB2555116A

GB2555116A - Piston for gas engine

Info

Publication number: GB2555116A
Application number: GB1617649.7A
Authority: GB
Inventors: Snuis Hans-Bernhard
Original assignee: Caterpillar Energy Solutions GmbH
Current assignee: Caterpillar Energy Solutions GmbH
Priority date: 2016-10-18
Filing date: 2016-10-18
Publication date: 2018-04-25
Anticipated expiration: 2036-10-18
Also published as: GB2555116B; GB201617649D0

Abstract

A piston for a spark ignited gas engine is configured to define a bowl 27 with cascaded, first and second radially inwardly facing recesses 25, 26 defined by re-entrant surfaces 23, 24 which extend radially inwardly from the respective recess to respective, first and second throats 30, 33. The first throat defines the inner periphery of an end surface 29 which may generate a first squish flow while the second throat defines the inner periphery of a radially wider, intermediate surface 32 which may generate a second, larger squish flow. The end surface and intermediate surface may extent at a plane normal to the axis of the piston or slope either towards or away from the periphery of the piston. A spark ignition engine using such a piston is also disclosed as well as a method of reducing peak pressure in an engine.

Description

(54) Title of the Invention: Piston for gas engine

Abstract Title: Double re-entrant piston for gas engine (57) A piston for a spark ignited gas engine is configured to define a bowl 27 with cascaded, first and second radially inwardly facing recesses 25, 26 defined by re-entrant surfaces 23, 24 which extend radially inwardly from the respective recess to respective, first and second throats 30, 33. The first throat defines the inner periphery of an end surface 29 which may generate a first squish flow while the second throat defines the inner periphery of a radially wider, intermediate surface 32 which may generate a second, larger squish flow.

The end surface and intermediate surface may extent at a plane normal to the axis of the piston or slope either towards or away from the periphery of the piston. A spark ignition engine using such a piston is also disclosed as well as a method of reducing peak pressure in an engine.

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Piston for gas engine

Technical Field

This invention relates to pistons for gas engines.

Background

In this specification, a gas engine means a spark ignited internal combustion piston engine which is designed to be powered by a gaseous fuel, which is to say, a fuel which enters the combustion chamber in gaseous form and which remains in gaseous form but not in liquid form at ambient temperature and pressure.

A gas engine is thus distinct from a petrol engine or diesel engine, in which the fuel is stored at ambient temperature and pressure in liquid form and enters the combustion chamber as a vapour or as a suspension of atomised liquid droplets, although it is possible to power such engines using gaseous fuels as a replacement for, or (in diesel engines) in admixture with, the liquid fuel for which the engine was designed.

The gaseous fuel for a gas engine may be stored and handled in gaseous form (conveniently, under pressure) or may be stored in liquid form before supplying it as a gas to the engine. Gaseous fuels used in gas engines include for example: natural gas (methane), biogas, syngas, and liquefied petroleum gas (LPG). Usually the fuel gas is mixed with air in a mixing device, e.g. incorporating a Venturi, before the mixture enters the combustion space, which is to say, the space within which combustion occurs. Typically a gas engine is designed to run exclusively on gaseous fuel without admixture with other fuels.

Gas engines are commonly used in static electrical generators, which typically run at a constant speed, but also in mobile applications, e.g. in marine propulsion units, buses and forklift trucks.

Gas engines have different combustion characteristics from other internal combustion engines such as diesel engines, which is reflected inter alia in the configuration of the piston. For example, unlike a diesel engine, the piston of a gas engine does not require a shaped bowl to deflect and redirect a jet of atomised fuel from an injector. Similarly, since combustion of the gaseous fuel in the outer region of the combustion space is not inhibited by the proximity of the cylinder wall as is often the case in diesel engines, the piston of a gas engine does not require a raised annular rim which approaches close to the cylinder head at top dead centre to generate a localised high velocity flow or squish effect immediately proximate the cylinder wall. This feature is often found in diesel engines to scavenge from this region the fuel which would otherwise be incompletely combusted.

However, in common with other internal combustion piston engines, combustion of the fuel in a gas engine is enhanced by turbulence within the combustion space. This may be achieved by configuring the piston of the gas engine to define a bowl surrounded by a raised annular region and by configuring the piston, cylinder head and valve inlets to generate swirl or tumble within the bowl.

It will be understood that swirl means a flow that circulates about an axis generally aligned with the axis of the piston, hence a flow that moves around the wall of the bowl if the piston is considered with its axis in a vertical orientation. Tumble means a flow that circulates about an axis that lies in a plane generally normal to the axis of the piston, hence a rolling or overturning motion in the same orientation.

Generally swirl is generated in a bowl which has a small diameter relative to the outer diameter of the piston, while tumble is generated in a relatively larger bowl. Advantageously, in a piston having a small bowl and relying on swirl, the relatively wide annular surface extending between the bowl and the periphery of the piston may be arranged to approach near to the opposing surface of the cylinder head (which may be generally flat) as the piston nears top dead centre, expelling a sufficient volume of the mixture in that region as a squish flow to generate swirl in the bowl. However, it is found that the swirl generated in this way tends to be stable rather than turbulent, whereas a turbulent flow is more effective in propagating the flame front throughout the combustion space. In a large bowl relying on tumble the squish effect will often be weak or absent due to the relatively much narrower annular region surrounding the bowl, with tumble being generated instead by other features of configuration including the shape and position of the valve inlet.

In use, it is found that gas engines can sometimes exhibit excessively high pressures within the combustion space, resulting in damage to the engine.

Summary

Disclosed herein is a piston for a spark ignited gas engine powered by a gaseous fuel, the piston including a first end region and an opposite, second end region, the first end region defining in use a wall of a combustion space when the piston is mounted for reciprocal motion along an axis of the piston. The first end region of the piston includes:

- a first throat, the first throat including a transition surface, the transition surface being radiused when considered in a plane containing the axis;

- an end surface extending radially inwardly with respect to the axis from an outer periphery of the piston to the first throat;

- a first re-entrant surface extending radially outwardly with respect to the axis from the first throat to define a first recess facing the axis;

- an intermediate surface extending radially inwardly with respect to the axis from the first recess to a second throat, the second throat being arranged radially inwardly of the first throat with respect to the axis; and

- a second re-entrant surface extending radially outwardly with respect to the axis from the second throat to define a second recess facing the axis.

Further disclosed is a gas engine incorporating the piston, and a method of configuring the first end region of the piston to reduce peak pressure in a combustion space of the engine.

Brief Description of the Drawings

Further features and advantages will be appreciated from the various illustrative embodiments which will now be described, purely by way of example and without limitation to the scope of the claims, and with reference to the accompanying drawings, in which:

Fig. 1 is a longitudinal section through a first end region of a piston for use in a gas engine in accordance with a first embodiment;

Fig. 2 is an enlarged view of part of Fig. 1;

Figs. 3 - 6 are longitudinal sections corresponding to that of Fig. 1 through the first end region of a piston, respectively in accordance with second, third, fourth and fifth alternative embodiments; and

Fig. 7 shows schematically part of a gas engine including the piston of Fig. 1.

Reference numerals and letters appearing in more than one of the figures indicate the same or corresponding features in each of them.

Detailed Description

Referring to Fig. 7, a spark ignited gas engine 1 includes at least one piston 2 mounted in a chamber (generally referred to herein as a cylinder) 3 in an engine block 4 and connected via a conventional linkage 5 to a crankshaft 6 haviing an oil lubrication system (not shown). The cylinder is closed at its (conventionally upper) open end by a cylinder head 7 having inlet and outlet ports 8, 9 closed by inlet and outlet valves 10, 11 through which the gaseous mixture 12 is drawn into the combustion space 13 and the combustion product 14 is expelled. The mixture comprises a gaseous fuel 15 as described above, e.g. natural gas (methane), biogas, syngas, or liquefied petroleum gas (LPG) which is combined with air 16 in a mixing device 17 before passing into the cylinder where it is ignited by a spark ignition means 18, typically including a spark gap arranged within the combustion space and a spark generating circuit for inducing a spark across the gap.

The piston is mounted for reciprocal motion in the cylinder along the central longitudinal axis X of the piston which is contained in the plane of its longitudinal section as shown more clearly in Fig. 1. The piston includes a first end region 21 and an opposite, second end region 22. The first end region 21 defines in use a moving wall of the combustion space 13, with the opposite, fixed wall being defined by the opposed, inwardly (conventionally, downwardly) facing surface 7' of the cylinder head.

Referring also to Figs. 1 and 2, the first end region 21 of the piston defines an axial array of cascaded, first and second re-entrant surfaces 23, 24, each extending radially outwardly with respect to the axis X to define a wall, respectively of a first 25 and second 26 recess. The recesses 25, 26 face (i.e. open towards) the axis X to surround a generally central recess or bowl 27 which extends axially into the piston to form a major portion of the combustion space 13 and terminates at a base surface 28. In the illustrated embodiments, the recesses 25, 26 are generally annular, extending continuously around the axis, although in alternative embodiments either or both of the recesses might be interrupted.

The end surface 29 of the piston may also be generally annular as shown and extends radially inwardly with respect to the axis X from the outer periphery 20 of the piston, defined in the illustrated example by its cylindrical outer wall, to a first throat 30 which forms an orifice through which the bowl 27 is in fluid communication with the interior of the cylinder and the inlet and outlet ports of the cylinder head. The first throat 30 includes a transition surface 31 which is radiused to define a radius of curvature r (Fig.

2) when considered in a plane containing the axis X, as shown.

The first re-entrant surface 23 extends from the first throat 30, radially outwardly with respect to the axis X, and axially divergently from the axis X towards the second end region 22 of the piston, to define a wall of the first annular recess 25, which optionally as shown in the illustrated examples may also be radiused.

The first annular recess 25 blends into an intermediate surface 32 which extends radially inwardly with respect to the axis X from the first annular recess 25 to a second throat 33, which defines a second orifice through which the lower portion of the bowl fluidly communicates with the interior of the cylinder via the orifice defined by the first throat. The second throat 33 is arranged radially inwardly of the first throat 30 with respect to the axis X, and optionally as best seen in the embodiments of Figs. 1, 3 and 4 may be axially spaced apart from the first throat and closer to the second end region 22 of the piston.

It will be noted that the intermediate surface 32 thus forms a generally annular shelf which is exposed through the first throat 30 when considered in the downward direction (towards the second end region 22 of the piston) as illustrated. In the alternative embodiments of Figs. 5 and 6 it can be seen that the first and second throats may be aligned or nearly aligned in the axial direction, so that they both approach close to the cylinder head 7 when the piston reaches top dead centre (i.e. its closest position to the cylinder head).

When considered in a plane containing the axis X, the second re-entrant surface 24 extends from the second throat 33, radially outwardly and axially towards the second end region 22 with a divergent slope angle ε with respect to the axis X, to define a wall of the second annular recess 26, which in the illustrated embodiment is radiused to blend into the flat base surface 28 of the bowl.

As shown in the illustrated embodiments, the first end region 21 of the piston including the central bowl 27 may generally be somewhat wider (in the radial R direction) than it is deep (in the axial direction), although alternative configurations are possible. For example, as shown in the illustrated embodiments, the diameter (= 2R) of the piston at the first end region 21 may be more than twice the axial length of the first end region from the axial extremity of the end surface 29 to the opposite axial extremity of the base surface 28 of the bowl.

The end surface 29 of the piston may be relatively radially narrow (i.e. a relatively small radial dimension B) so that it does not substantially influence the flow pattern within the central bowl, but is preferably arranged to approach the opposing surface 7' of the cylinder head sufficiently closely to generate a high velocity flow immediately proximate the cylinder wall 3', sufficient to scavenge lubricating oil from the cylinder wall 3'. For example, at the top dead centre position of the piston, the end surface 29 may be spaced apart from the opposing surface 7' of the cylinder head at its closest point by a distance D from about 5 mm to 10 mm. The radial width B of the end surface 29 between the outer periphery 20 and the point where it meets the transition surface 31 may be for example from about 10 % to 20 % of the overall radial distance R from the outer periphery 20 of the piston to its axis X.

The intermediate surface 32 is relatively radially wider than the end suface 29 and advantageously may be arranged to approach sufficiently closely to the cylinder head to generate a substantial squish effect. For example, the radial width C of the intermediate surface 32 (taken as C = (R - (B + Rt)) where Rt is the radial dimension of the second throat 33) may be from about 45 % to 50 % of the overall radial distance R from the outer periphery 20 of the piston to its axis X.

The relatively more voluminous squish flow generated by the intermediate surface 32 is destabilised by the second re-entrant surface 24 and the second annular recess 26, resulting in a turbulent swirling flow pattern in the bowl 27 which aids combustion of the fuel.

It is found that if the radiused transition surface 31 at the first throat 30 is replaced by a vertical flat surface, turbulence in this region is reduced so that the oil scavenged from the cylinder wall is less effectively entrained by the more voluminous turbulent flow generated by the intermediate surface 32 as it approaches the cylinder head.

For this reason the transition surface 31 at the first throat 30 preferably has a radius r of at least 2mm. The radiused transition surface 31 in combination with the first re-entrant surface 23 and first annular recess 25 is found to generate sufficient turbulence to entrain in the squish flow generated by the intermediate surface 32 the oil transported by the much more localised and less voluminous squish flow generated by the end surface 29, which may have a substantially higher velocity. In this way the oil is effectively distributed throughout the volume of the bowl 27 by the more voluminous, optionally lower velocity squish flow generated by the intermediate surface 32.

The velocity and other flow characteristics of the squish flows generated by the end surface 29 and the intermediate surface 32 may be adjusted by changing the shape and angular arrangement of those surfaces, the relative dimensions A, B and C, and the radii of curvature of the transition surface 31 and (where as shown the first annular recess 25 is curved) also of the first annular recess 25, taking into account the diameter of the piston 2 (diameter = 2R) and of the bowl 27 and the frequency of the engine. Those skilled in the art will appreciate that the optimal squish flow characteristics will be determined by reference to the behaviour of the mixture under the operating conditions of the engine, and so for example, the faster the engine speed, the more quickly the squish flow may be required to dissipate into the bowl.

The overall depth of the bowl 27 in the axial direction may be selected to obtain the required combustion space volume and flow characteristics. Although the bowl 27 is shown with a flat base surface 28 normal to the axis X, other contoured features may be provided. In addition, the position of the inlet and outlet ports 8, 9, particularly the inlet port 8 of the cylinder head may be selected to impinge the inflowing gaseous mixture on the intermediate surface 32 and other exposed surfaces of the piston to generate the required flow pattern.

In the examples shown, both the first and second annular recesses 25, 26 are radiused, and the second throat 33 exhibits a relatively sharper transition (smaller radius) between the intermediate surface 32 and the second re-entrant surface 24 when compared with the transition surface 31 at the first throat 30. These features may be adapted as required to generate the required flow pattern.

The squish flow characteristics of the end surface 29 and intermediate surface 32 are particularly dependent on their respective slope angles α, β, y and δ relative to a plane normal to the axis when considered in a plane containing the axis. These slope angles may be selected depending inter alia on the diameter of the piston 2 (diameter = 2R) and of the bowl 27. The angles α, β, y and δ prefably lie within a range from 0° to 45°. The angle ε of the second re-entrant surface 24 is preferably not more than 45°.

To optimise turbulence in the squish flow generated by the end surface 29, the axial dimension A between the end surface 29 at the point where it meets the transition surface 31 at the first throat 30 and the intermediate surface 32 at the point where it meets the first annular recess 25 may be within the range from 10 mm to 20mm.

In the example of Fig. 1 the end surface 29 extends radially inwardly with respect to the axis from the outer periphery 20 of the piston to the first throat 30 in a plane normal to the axis X. The intermediate surface 32 also extends radially inwardly with respect to the axis X from the first annular recess 25 to the second throat 33 in a plane normal to the axis X.

In alternative embodiments (not shown) only one of the end surface 29 and intermediate surface 32 may extend in a plane normal to the axis X as shown in Fig. 1, with the other respective surface 29 or 32 extending axially with a slope angle a, β, y, δ as illustrated in Figs. 3 - 6.

Figs. 3 - 6 show further alternative embodiments in which the end surface 29 and the intermediate surface 32 extend with a slope angle α, β, γ, δ in the same axial direction (Figs. 4 and 5) and in opposite axial directions (Figs. 3 and 6). This is to optimize the turbulence for different Squish/Swirl ratios.

As shown in Figs. 3 and 5, the end surface 29 may extend radially inwardly with respect to the axis X and axially at a slope angle y away from the second end region 22 from the outer periphery 20 of the piston to the first throat 30.

Ceteris paribus, compared with the arrangement of Fig. 1, this arrangement increases the resistance to the squish flow induced by the end surface 29 at top dead centre and so increases turbulence, more effectively entraining the scavenged oil into the squish flow induced by the intermediate surface 32. It also increases the time required for the squish flow generated by the end surface 29 to enter the central region of the bowl 27.

As shown in Figs. 4 and 6, the end surface 29 may extend radially inwardly with respect to the axis X and axially at a slope angle δ towards the second end region 22 from the outer periphery 20 of the piston to the first throat 30.

Ceteris paribus, compared with the arrangement of Fig. 1, when the piston 2 is at top dead centre (i.e. when it approaches most closely to the cylinder head 7), this configuration reduces the resistance to the squish flow induced by the end surface 29 and so reduces the flow velocity.

As shown in Figs. 5 and 6, the intermediate surface 32 may extend radially inwardly with respect to the axis X and axially at a slope angle β away from the second end region 22 from the first annular recess 25 to the second throat 33.

Ceteris paribus, compared with the arrangement of Fig. 1, this arrangement increases the resistance to the squish flow induced by the intermediate surface 32 at top dead centre and so increases turbulence. It also increases the time required for the squish flow generated by the intermediate surface 32 to enter the central region of the bowl

27.

As shown in Figs. 3 and 4, the intermediate surface 32 may extend radially inwardly with respect to the axis X and axially at a slope angle a towards the second end region 22 from the first annular recess 25 to the second throat 33.

Ceteris paribus, compared with the arrangement of Fig. 1, when the piston 2 is at top dead centre, this configuration reduces the resistance to the squish flow induced by the intermediate surface 32 and so reduces the flow velocity.

In the illustrated examples, the end surface 29, intermediate surface 32, and second reentrant surface 24 are generally straight when considered in longitudinal section, hence defining generally flat or conical surfaces of revolution about the axis X, but any of those surfaces could alternatively be curved in longitudinal section, in which case the slope angle α, β, γ, δ, ε of each surface may be considered as the mean slope angle of the surface.

In summary,a preferred piston for a spark ignited gas engine may be configured to define a bowl 27 with cascaded, first and second radially inwardly facing recesses 25, 26 defined by re-entrant surfaces 23, 24 which extend radially inwardly from the respective recess to respective, first and second throats 30, 33. The first throat 30 defines the inner periphery of an end surface 29 which may generate a first squish flow while the second throat 33 defines the inner periphery of a radially wider, intermediate surface 32 which may generate a second, larger squish flow.

The inwardly facing surface 7' of the cylinder head Ί, defining the fixed (conventionally, upper) wall of the combustion space 13 which faces the first end region 21 of the piston, may be flat as shown in the illustrated embodiment, or alternatively may be contoured in a manner complementary to the configuration of the piston.

Of course, the engine may include several pistons 2 as shown. Conveniently and conventionally each piston 2 may be generally cylindrical as shown in each of the illustrated embodiments, so that its outer periphery 20 is circular when considered in transverse section normal to the axis X, although it is possible to envisage a non-circular section. The term annular is construed accordingly to mean extending generally around the axis X, and optionally, extending in a circular configuration when considered in transverse section. The term diameter is construed mutatis mutandis in a noncircular section as the mean dimension normal to the axis X. In the illustrated embodiments, all of the surfaces of the piston facing the combustion space 13 are surfaces of revolution about its axis X and so define a circular section in any plane normal to the axis X and passing through the combustion space. Of course, each and any of said surfaces may instead be formed with an asymmetric configuration so as to obtain a desired turbulent flow pattern.

Many further possible adaptations within the scope of the claims will be evident to those skilled in the art.

Industrial Applicability

A gas engine incorporating at least one piston as disclosed herein may advantageously be employed to replace a conventional gas engine in any application as known in the art.

It has been found that the excessive peak cylinder pressures sometimes observed in gas engines can be generated by the presence of lubricating oil which enters the combustion chamber between the piston and the cylinder wall. The oil forms droplets on the wall of the combustion chamber which are found to autoignite due to the elevated pressure in the combustion chamber after the spark has ignited the fuel to create a first flame front, but before the first flame front has propagated through the combustion chamber to reach the cylinder wall. The burning oil droplets create a second flame front which travels towards the first flame front so that the observed peak in cylinder pressure is generated by the convergence of the two flame fronts.

Advantageously, embodiments may suppress or avoid the formation of a second flame front so that the formerly observed excessive cylinder pressures do not arise. Whilst the disclosure is not bound by theory, it is believed that by configuring the piston as described above, the localised high velocity squish flow induced by the annular end surface as it closely approaches the cylinder head may scavenge oil droplets from the cylinder wall by evaporating or entraining them into the flow. The localised squish flow carries the oil in the form of vapour or very small suspended droplets into the more voluminous squish flow generated by the intermediate surface. The turbulent flow distributes the oil throughout the central volume of the combustion space where it burns together with the fuel in a common flame front.

In the claims, reference numerals and letters in parentheses are included for information only and should not be construed as limiting.

Claims

1. A piston (2) for a spark ignited gas engine (1) powered by a gaseous fuel (15), the piston including a first end region (21) and an opposite, second end region (22), the first end region (21) defining in use a wall of a combustion space (13) when the piston is mounted for reciprocal motion along an axis (X) of the piston and including :

- a first throat (30), the first throat including a transition surface (31), the transition surface being radiused when considered in a plane containing the axis;

- an end surface (29) extending radially inwardly with respect to the axis from an outer periphery (20) of the piston to the first throat (30);

- a first re-entrant surface (23) extending radially outwardly with respect to the axis from the first throat(30) to define a first recess (25) facing the axis;

- an intermediate surface (32) extending radially inwardly with respect to the axis from the first recess (25) to a second throat (33), the second throat (33) being arranged radially inwardly of the first throat (30) with respect to the axis; and

- a second re-entrant surface (24) extending radially outwardly with respect to the axis from the second throat (33) to define a second recess (26) facing the axis.

2. A piston according to claim 1, wherein the end surface (29) extends radially inwardly with respect to the axis from the outer periphery (20) of the piston to the first throat (30) in a plane normal to the axis.

3. A piston according to claim 1, wherein the intermediate surface (32) extends radially inwardly with respect to the axis from the first recess (25) to the second throat (33) in a plane normal to the axis.

4. A piston according to claim 1, wherein the end surface (29) extends radially inwardly with respect to the axis and axially away from the second end region (22) from the outer periphery (20) of the piston to the first throat (30).

5. A piston according to claim 1, wherein the end surface (29) extends radially inwardly with respect to the axis and axially towards the second end region (22) from the outer periphery (20) of the piston to the first throat (30).

6. A piston according to claim 1, wherein the intermediate surface (32) extends radially inwardly with respect to the axis and axially away from the second end region (22) from the first recess (25) to the second throat (33).

7. A piston according to claim 1, wherein the intermediate surface (32) extends radially inwardly with respect to the axis and axially towards the second end region (22) from the first recess (25) to the second throat (33).

8. A spark ignited gas engine (1) including at least one piston (2) as defined in claim

1.

9. A method of reducing peak pressure in a combustion space (13) of a spark ignited gas engine (1) powered by a gaseous fuel (15), the gas engine including a piston (2) mounted for reciprocal motion along an axis (X) of the piston, the piston (2) including a first end region (21) and an opposite, second end region (22), the first end region (21) defining in use a wall of the combustion space, wherein the first end region (21) is configured to define:

- a first re-entrant surface (23) extending radially outwardly with respect to the axis from the first throat (30) to define a first recess (25) facing the axis;

Intellectual

Property

Office

Application No: GB 1617649.7