Patent Description:
In classic internal combustion engines, gasoline burns best when it is mixed with air in proportions of around <NUM>:<NUM> (lambda = <NUM>) depending on the particular type of fuel. Most modern gasoline engines used in vehicles tend to operate at or near this so-called stoichiometric point for most of the time. Ideally, when burning fuel in an engine, only carbon dioxide (CO2) and water (H2O) are produced. In practice, the exhaust gas of an internal combustion engine also comprises significant amounts of carbon monoxide (CO), nitrogen oxides (NOx) and unburned hydrocarbons.

It is desirable to increase fuel efficiency and reduce unwanted emissions. One possible route for increasing fuel efficiency is to burn the fuel with an excess of air. Burning fuel in such an oxygen-rich environment is usually called lean-burning. Typical lean-burn engines may mix air and fuel in proportions of, for example, <NUM>:<NUM> (lambda > <NUM>) or even <NUM>:<NUM> (lambda > <NUM>).

Advantages of lean-burn engines include, for example, that they produce lower levels of CO2 and hydrocarbon emissions by better combustion control and more complete fuel burning inside the engine cylinders. The engines designed for lean burning can employ higher compression ratios and thus provide better performance, more efficient fuel use and lower exhaust hydrocarbon emissions than conventional gasoline engines. Additionally, lean-burn modes help to reduce throttling losses, which originate from the extra work that is required for pumping air through a partially closed throttle. When using more air to burn the fuel, the throttle can be kept more open when the demand for engine power is reduced.

Lean burning of fuel does, however, also come with some technical challenges that have to be overcome for providing an engine that is suitable and optimised for efficiently burning hydrocarbons in an oxygen-rich environment. For example, if the mixture is too lean, the engine may fail to combust. Especially at low loads and engine speeds, reduced flammability may affect the stability of the combustion process and introduce problems with engine knock. Further, a lower fuel concentration leads to less output. Because of such disadvantages, lean burn is currently only used for part of the engine map and most lean-burning modern engines, for example, tend to cruise and coast at or near the stoichiometric point.

In order to enable the lean burning of fuel over a larger portion of the engine map, the engine needs to be designed in such a way to enable a large airflow into the combustion chamber and to ensure a reliable combustion process that will effectively burn all fuel, despite the oxygen rich conditions.

It is an aim of the present invention to provide an improved lean-burn gasoline engine.

<CIT> dislcoses a method of machining a cylinder head.

<CIT> discloses a cylinder head with several pairs of facets.

Aspects and embodiments of the invention provide a cylinder head for an engine, an engine, and a vehicle with such an engine. The engine may be suitable for use with fuels including gasoline, diesel, hydrogen, LPG or any other suitable combustible fuel. The engine may be a lean-burn engine.

According to an aspect of the present invention there is provided a method of machining a combustion chamber roof surface in a cylinder head for an engine, wherein the machined cylinder head comprises:.

This method is advantageous as the second and third pairs of machined facets are machined by the same cutter in the manufacturing process. This reduces the complexity of the manufacturing process and improves manufacturing efficiency.

Optionally the first pair of facets are machined before the second pair of facets.

The second pair of facets are optionally machined before the third pair of facets.

The method may comprise using the second cutter to machine a fourth pair of facets of the combustion chamber roof surface, wherein a first one of the fourth pair of facets is located between the first one of the air inlet openings and the first one of the exhaust outlet openings, and a second one of the fourth pair of facets is located between the second one of the air inlet openings and the second one of the exhaust outlet openings.

Again, this method is advantageous as the fourth pair of machined facets are machined by the same cutter used for the second and third pairs of facets which reduces the complexity of the manufacturing process and improves efficiency.

In one example the method comprises using the second cutter to machine a fifth pair of facets of the combustion chamber roof surface, wherein a first one of the fifth pair of facets is located between the first one of the air inlet openings and the first one of the exhaust outlet openings, and a second one of the fifth pair of facets is located between the second one of the air inlet openings and the second one of the exhaust outlet openings. As above, this method is advantageous as the fifth pair of machined facets are machined by the same cutter used for the second to fourth pairs of cuts to reduce complexity and improve efficiency.

Optionally the method comprises using a third cutter to machine a sixth pair of facets of the combustion chamber roof surface, wherein a first one of the sixth pair of facets is located at least partially between the pair of air inlet openings, and a second one of the sixth pair of facets is located at least partially between the pair of exhaust outlet openings, wherein each one of the sixth pair of facets intersect the gasket interface surface to define a first pair of opposing sections of the combustion chamber opening.

The fourth pair of facets are optionally machined after the third pair of facets.

The fifth pair of facets may be machined after the fourth pair of facets.

In one example the sixth pair of facets are machined after the fifth pair of facets.

Optionally the third pair of facets comprise a pair of flat surfaces. This helps to open up the combustion chamber and remove material between the inlet side and the outlet side which may otherwise hinder flow from one side of the combustion chamber to the other.

The first one of the third pair of facets is optionally substantially parallel to the plane of the first one of the exhaust outlet openings, and the second one of the third pair of facets is substantially parallel to the plane of the second one of the exhaust outlet openings. This configuration is beneficial for efficient flow across the combustion chamber from one side to the other.

Each one of the third pair of facets may be located between curved surfaces which boarder the air inlet openings and the exhaust outlet openings respectively.

In one example the second pair of facets comprise opposing curved surfaces to help encourage "omega swirl" of the inflowing air as it moves from the air inlet openings towards the exhaust outlet openings and then back towards the air inlet openings; and from the centre of the combustion chamber towards the edges of the combustion chamber.

Optionally the intersection of the first pair of facets and the gasket interface surface define a second pair of opposing sections of the combustion chamber opening.

The intersection of the sixth pair of facets and the gasket interface surface optionally define a second pair of opposing sections of the combustion chamber opening.

In another aspect the present invention provides a cylinder head comprising a combustion chamber roof surface machined as described above.

In a further aspect the present invention provides an engine comprising a cylinder head as described above.

In a still further aspect the present invention provides a vehicle comprising an engine as described above.

<FIG> shows a vehicle <NUM> in which the invention may be used. In this example, the vehicle <NUM> is a car, but the invention is equally applicable to other vehicles driven by a lean-burn gasoline engine <NUM>. In this vehicle <NUM>, the lean-burn gasoline engine <NUM> is positioned in the front and coupled to a drivetrain to drive the front and/or rear wheels of the vehicle <NUM>. The energy needed for driving the vehicle <NUM> is provided by burning fuel in the engine's cylinders and let the cylinder pistons drive a crankshaft that is mechanically connected to the vehicle's drivetrain.

Compared to classic internal combustion engines, the lean-burn engine <NUM> of this vehicle <NUM> burns the fuel with an excess of air in the air-fuel mixture. Typical lean-burn engines may mix air and fuel in proportions of, for example, <NUM>:<NUM> (lambda > <NUM>) or even <NUM>:<NUM> (lambda > <NUM>). Advantages of lean-burn engines include more efficient fuel use and lower exhaust hydrocarbon emissions than conventional gasoline engines.

In order to enable the lean burning of fuel over a large portion of the engine map, the engine <NUM> is designed in such a way to enable a large air flow into the combustion chamber and a good mixing with the relatively small amount of fuel that is to be burnt to ensure a reliable combustion process that will effectively burn all fuel, despite the oxygen rich conditions.

<FIG> shows a cross section of a portion of an engine block <NUM> and a cylinder head <NUM> of the lean burn engine <NUM>. The engine block <NUM> comprises a cylinder <NUM> which houses a piston <NUM> shown near bottom dead centre (BDC) in <FIG>. The cylinder head <NUM> comprises a combustion chamber <NUM> which extends into the cylinder head <NUM> away from a gasket interface surface <NUM>, which is substantially planar. A head gasket <NUM> is located between the engine block <NUM> and cylinder head <NUM>.

Referring additionally to <FIG>, a pair of air inlets 49a, 49b are located on an air inlet side <NUM> of the combustion chamber <NUM>. The air inlets 49a, 49b provide a path for a flow of air to the combustion chamber <NUM> in use. A pair of exhaust outlets 56a, 56b are located on an exhaust outlet side <NUM> of the combustion chamber <NUM>. The exhaust outlets 56a, 56b provide an exhaust path for the combustion products exiting the combustion chamber <NUM> in use. The air inlets 49a, 49b connect to respective air inlet openings 91a, 91b located in the roof surface <NUM> on the air inlet side <NUM> of the combustion chamber <NUM>, and the exhaust outlets 56a, 56b connect to respective exhaust outlet openings 92a, 92b located in the roof surface <NUM> on the exhaust outlet side <NUM> of the combustion chamber <NUM>. The first air inlet opening 91a and the first exhaust outlet opening 92a are located on a first side 93a of the combustion chamber <NUM>, and the second air inlet opening 91b and the second exhaust outlet opening 92b are located on a second side 93b of the combustion chamber <NUM>. The first 93a and second 93b sides of the combustion chamber <NUM> are located on either side of a plane of symmetry <NUM> of the combustion chamber <NUM>. The cross section of <FIG> is taken along section A-A which passes through the first air inlet opening 91a and the first exhaust outlet opening 92a on the first side 93a of the combustion chamber <NUM>.

Referring once again to <FIG>, an inlet valve <NUM> controls the opening and closing of the first air inlet opening 91a, and an exhaust valve <NUM> controls the opening and closing of the first exhaust outlet opening 92a. An equivalent inlet valve (not shown) controls the opening and closing of the second air inlet opening 91b, and an equivalent exhaust valve (not shown) controls the opening and closing of the second exhaust outlet opening 92b. The inlet valve <NUM> and the exhaust valve <NUM> are shown in the closed position in <FIG>.

A dotted line provides a simplified 2D representation of the preferred air flow path <NUM> into and through the combustion chamber <NUM> and cylinder <NUM> during the intake stroke. As noted above, the inlet valve <NUM> is shown in the closed position in <FIG>. The air flow path <NUM> is not possible with the inlet valve <NUM> in the closed position as shown. Nonetheless, the preferred air flow path <NUM> is shown for the purpose of illustration.

With the design of this embodiment, it is possible to create a tumble motion of the incoming air, first along the roof <NUM> of the combustion chamber <NUM> towards the opposite wall of the cylinder <NUM>, under the outlet valves <NUM> that close off the exhaust outlet openings 92a, 92b, and then down along that opposite wall of the cylinder <NUM>, back over the top surface of the piston <NUM> and up along the other wall of the cylinder <NUM> in the direction of the inlet valves <NUM> again. This tumble is preferably kept in motion during the full intake stroke and at least a portion of the compression stroke of the piston <NUM> moving through the cylinder <NUM>. The thus produced tumble helps to obtain an optimal distribution of air and fuel inside the cylinder <NUM> and combustion chamber <NUM> that can then break down in the latter stages of the compression stroke into turbulence to facilitate the subsequent combustion process. Wherein, in this context, "turbulence" refers to a flow state having chaotic changes in velocity and pressure and no necessarily clear flow directions as is well known in the art.

The piston <NUM> comprises a working surface <NUM> which has a central scooped portion <NUM> and outer sloped portions <NUM>, <NUM>. The outer sloped portions <NUM>, <NUM> of the working surface <NUM> conform to the shape of sloped surface portions <NUM>, <NUM> of the combustion chamber roof surface <NUM>.

<FIG> shows a plan view of the underside of the cylinder head <NUM> and head gasket <NUM> which shows the machined roof surface <NUM> of the combustion chamber <NUM>. The combustion chamber roof surface <NUM> extends into the cylinder head <NUM> away from the gasket interface surface <NUM>. The intersection between the combustion chamber roof surface <NUM> and the gasket interface surface <NUM> comprises a combustion chamber opening <NUM> in the gasket interface surface <NUM>. The pair of air inlet openings 91a, 91b, and the pair of exhaust outlet openings 92a, 92b are formed in the combustion chamber roof surface <NUM>. For the avoidance of doubt, the internal surfaces of the air inlets 49a, 49b, and exhaust outlets 56a, 56b seen in <FIG> do not form part of the combustion chamber roof surface <NUM>.

As best shown in <FIG>, a central domed surface portion <NUM> of the combustion chamber roof surface <NUM> defines a central domed portion <NUM> of the combustion chamber <NUM>. In this embodiment the central domed surface portion <NUM> is elongate such that it extends from one side of the combustion chamber <NUM> to the other in a direction substantially perpendicular to the plane of symmetry <NUM>. That is to say, the central domed surface portion <NUM> extends between the first side 93a and the second side 93b of the combustion chamber <NUM>. Note that the central domed surface portion <NUM> of the combustion chamber roof surface <NUM> is not a single smooth surface, but rather is a surface made up of a plurality of facets made by different machine cutters during manufacture as will be described in detail below.

A spark plug seat <NUM> and a fuel injector seat <NUM> are located in the cylinder head <NUM>. Both the spark plug seat <NUM> and fuel injector seat <NUM> open into the domed surface portion <NUM> of the combustion chamber roof surface <NUM>. The spark plug seat <NUM> opens into roof surface <NUM> at the approximate centre of the combustion chamber <NUM>, and the fuel injector seat <NUM> opens into the roof surface <NUM> substantially adjacent to the spark plug seat opening further towards the air inlet openings 91a, 91b than the spark plug seat opening. Both the spark plug seat opening and the fuel injector seat opening are located substantially on the plane of symmetry <NUM> of the combustion chamber <NUM>.

The spark plug seat <NUM> is configured so that the tip of the spark plug <NUM> is supported towards the centre of the central domed portion <NUM> substantially on the plane of symmetry <NUM> of the combustion chamber <NUM>. The fuel injector seat <NUM> is configured to support the tip of the fuel injector <NUM> proximate the combustion chamber roof surface <NUM> substantially in line with the tip of the spark plug <NUM>.

Referring once again to <FIG>, broadly speaking the combustion chamber <NUM> has two zones, a central domed portion <NUM> and an outer sloped portion <NUM>. The central domed portion <NUM> is bounded by the central domed roof surface portion <NUM> of the combustion chamber roof surface <NUM>, and the outer sloped portion <NUM> is bounded by two sloped surface portions <NUM>, <NUM> of the combustion chamber roof surface <NUM>. In this embodiment the sloped surface portions <NUM>, <NUM> each have a shape which conforms to the surface of a single cone. That is to say, the sloped surface portions <NUM>, <NUM> each form part of the surface of the same conical shape. Thus, the cross sections of the sloped surface portions <NUM>, <NUM> are straight along the plane of symmetry <NUM> of the combustion chamber <NUM>. As best shown in <FIG>, the combustion chamber roof surface <NUM> between the sloped surface portions <NUM>, <NUM> and the combustion chamber opening <NUM> comprises curved portions <NUM> which extend from the sloped surface portions <NUM>, <NUM> to the combustion chamber opening <NUM>.

The machining steps taken to achieve the finished profile of the combustion chamber roof surface <NUM> will now be described with reference to <FIG>. <FIG> shows a plan view of the underside of the cylinder head <NUM> in the "as cast" condition. That is to say that the profile of the underside of the cylinder head is entirely determined by the casting process and no machining process has yet been undertaken. As shown in <FIG>, holes to form the air inlets 49a, 49b and exhaust outlets 56a, 56b are formed in the casting process. In addition, holes to form the spark plug seat <NUM> and fuel injector seat <NUM> are formed during the casting process. A recess <NUM> is formed in the underside of the cylinder head <NUM> in the casting process. However, the recess <NUM> does not comprise any of the features of the finished combustion chamber roof surface <NUM>.

<FIG> shows a plan view of the underside of the cylinder head <NUM> after a first machining process has taken place. In the first machining process material is removed from the cylinder head <NUM> by a ball nose cutter to form the first cuts of the central domed portion <NUM> of the combustion chamber roof surface <NUM>. The first machining process cuts surfaces <NUM>, <NUM>. <FIG> shows the envelope <NUM> traced by the ball nose cutter during the first machining process. The shadow of the envelope <NUM> is also shown in plan in <FIG> and in side view in <FIG>.

<FIG> shows a plan view of the completed combustion chamber roof surface <NUM>. Here it can be seen that after all of the machining processes have been completed, only small portions of the first machining process cut surfaces <NUM>, <NUM> remain between the air inlet openings 91a, 91b and the exhaust outlet openings 92a, 92b. These remaining portions <NUM>, <NUM> of the first machining process cuts form part of the central domed surface portion <NUM> of the combustion chamber roof <NUM>. The surfaces <NUM>, <NUM> may be described as a first pair of machined facets comprising curved surfaces.

<FIG> shows a plan view of the underside of the cylinder head <NUM> after a second machining process has taken place. In the second machining process material is removed from the cylinder head <NUM> by a radiused cutter to form the second cuts of the central domed portion <NUM> of the combustion chamber roof surface <NUM>. The second machining process cuts surfaces 185a, 185b. <FIG> shows the envelope <NUM> traced by the radiused cutter during the second machining process. The shadow of the envelope <NUM> is also shown in plan in <FIG> and in side view in <FIG>.

<FIG> shows a plan view of the completed combustion chamber roof surface <NUM>. Here it can be seen that after all of the machining processes have been completed, only the end portions of the second machining process cut surfaces 185a, 185b remain at either end of the central domed portion <NUM> of the combustion chamber roof surface <NUM>. These end portions 185a, 185b of the second machining process cuts form part of the central domed surface portion <NUM> of the combustion chamber roof <NUM>. The end portions 185a, 185b may be described as a second pair of machined facets comprising opposing curved surfaces.

The cuts made by the radiused cutter in the second machining process are beneficial as they form curved wall portions of the central domed portion <NUM> of the combustion chamber roof <NUM>. This encourages an "omega swirl" flow path as the inflowing air moves from the air inlet openings 91a ,91b towards the exhaust outlet openings 92a, 92b and then back towards the air inlet openings 91a, 91b; and from the centre of the chamber <NUM> towards the edges of the chamber <NUM>. The "omega swirl" flow path is illustrated in <FIG> by the dotted lines <NUM>. It should be noted that the "omega swirl" flow pattern <NUM> is superimposed with the "tumble" flow pattern (illustrated by dotted line <NUM> in <FIG>) in the operating engine <NUM>.

The cut surfaces 185a, 185b made by the radiused cutter in the second machining process form the characteristic curved portions 176a, 176b of the combustion chamber opening <NUM> located between the air inlet openings 91a, 91b and the exhaust outlet openings 92a, 92b respectively. These curved portions 176a, 176b may be described as a second pair of opposed curved sections of the combustion chamber opening <NUM>.

<FIG> shows a plan view of the underside of the cylinder head <NUM> after a third machining process has taken place. In the third machining process material is removed from the cylinder head <NUM> by the same radiused cutter as that used to form the second cuts 185a, 185b. The third cuts form part of the central domed portion <NUM> of the combustion chamber roof surface <NUM>. The third machining process cuts surfaces 186a, 186b. <FIG> shows the envelope <NUM> traced by the radiused cutter during the third machining process. The shadow of the envelope <NUM> is also shown in plan in <FIG> and in side view in <FIG>.

<FIG> shows a plan view of the completed combustion chamber roof surface <NUM>. Here it can be seen that after all of the machining processes have been completed, portions of the third machining process cut surfaces 186a, 186b remain between the air inlet openings and the exhaust outlet openings. These remaining portions 186a, 186b of the third machining process cuts form part of the central domed surface portion <NUM> of the combustion chamber roof <NUM>. The surfaces 186a, 186b may be described as a third pair of machined facets comprising substantially flat surfaces.

With particular reference to <FIG>, it can be seen that the envelope <NUM> traced by the radiused cutter during the third machining process has a substantially flat top profile that is substantially parallel to the plane of the exhaust outlet opening 92a. As a result of this, the cut surfaces 186a, 186b formed by the third machining process are substantially flat in the region of the third cut surfaces 186a, 186b located between the air inlet openings and the exhaust outlet openings. Only the air inlet opening 91a and the exhaust outlet opening 92a are shown in <FIG>. The substantially flat cut made by the radiused cutter in the third machining process are beneficial as they remove material between the air inlet openings 91a, 91b and the exhaust outlet openings 92a, 92b so that inflowing air may flow across the top of the combustion chamber <NUM> from the air inlet openings 91a ,91b towards the exhaust outlet openings 92a, 92b without interference. By substantially matching the flat portions of the machined surfaces 186a, 186b located between the air inlet openings and the exhaust outlet openings to the plane of the exhaust outlet openings 92a, 92b, uninterrupted flow between the air inlet openings and the exhaust outlet openings can be maximised. This is beneficial to the creation of a tumble flow pattern in the cylinder <NUM> during the intake stroke of the piston <NUM>.

<FIG> shows a plan view of the underside of the cylinder head <NUM> after a fourth machining process has taken place. In the fourth machining process material is removed from the cylinder head <NUM> by the same radiused cutter as that used to form the second cuts 185a, 185b and the third cuts 186a, 186b. The fourth cuts form part of the central domed portion <NUM> of the combustion chamber roof surface <NUM>. The fourth machining process cuts surfaces 188a, 188b. <FIG> shows the envelope <NUM> traced by the radiused cutter during the fourth machining process. The shadow of the envelope <NUM> is also shown in plan in <FIG> and in side view in <FIG>.

<FIG> shows a plan view of the completed combustion chamber roof surface <NUM>. Here it can be seen that the fourth machining process cut surfaces 188a, 188b are located adjacent the air inlet openings 91a, 91b. The fourth machining process cuts 188a, 188b form part of the central domed surface portion <NUM> of the combustion chamber roof <NUM>. The surfaces 188a, 188b may be described as a fourth pair of machined facets comprising curved surfaces.

The cuts made by the radiused cutter in the fourth machining process are beneficial as they remove material between the air inlet openings 91a, 91b and the exhaust outlet openings 92a, 92b so that inflowing air may flow across the top of the combustion chamber <NUM> from the air inlet openings 91a ,91b towards the exhaust outlet openings 92a, 92b without interference. In particular, the cuts 188a, 188b made in the fourth machining process help to open up the roof <NUM> of the combustion chamber to promote tumble flow.

<FIG> shows a plan view of the underside of the cylinder head <NUM> after a fifth machining process has taken place. In the fifth machining process material is removed from the cylinder head <NUM> by the same radiused cutter as that used to form the second cuts 185a, 185b, the third cuts 186a, 186b and the fourth cuts 188a, 188b. The fifth cuts form part of the central domed portion <NUM> of the combustion chamber roof surface <NUM>. The fifth machining process cuts surfaces 190a, 190b. <FIG> shows the envelope <NUM> traced by the radiused cutter during the fifth machining process. The shadow of the envelope <NUM> is also shown in plan in <FIG> and in side view in <FIG>.

<FIG> shows a plan view of the completed combustion chamber roof surface <NUM>. Here it can be seen that the fifth machining process cut surfaces 190a, 190b are located adjacent the exhaust outlet openings 92a, 92b. The fifth machining process cuts 190a, 190b form part of the central domed surface portion <NUM> of the combustion chamber roof <NUM>. The surfaces 190a, 190b may be described as a fifth pair of machined facets comprising curved surfaces.

The cuts made by the radiused cutter in the fifth machining process are beneficial as they remove material between the air inlet openings 91a, 91b and the exhaust outlet openings 92a, 92b so that inflowing air may flow across the top of the combustion chamber <NUM> from the air inlet openings 91a ,91b towards the exhaust outlet openings 92a, 92b without interference. In particular, the cuts 190a, 190b made in the fifth machining process help to open up the roof <NUM> of the combustion chamber to promote tumble flow.

<FIG> shows a plan view of the underside of the cylinder head <NUM> after a sixth machining process has taken place. The sixth machining process forms the outer sloped portion <NUM> of the combustion chamber <NUM> by cutting sloped surface portions <NUM>, <NUM>. <FIG> shows the envelope <NUM> traced by the cutter during the sixth machining process.

<FIG> shows a plan view of the completed combustion chamber roof surface <NUM>. Here it can be seen that the sloped surfaces <NUM>, <NUM> are located between the air inlet openings 91a, 91b and the exhaust outlet openings 92a, 92b respectively. The sloped surfaces <NUM>, <NUM> may be described as a sixth pair of machined facets comprising opposing curved surfaces.

<FIG> shows a sectional view of the cylinder head <NUM> along the plane of symmetry <NUM> of the combustion chamber. Here it can be seen that the sloped surfaces <NUM>, <NUM> are directed towards the tip of the spark plug <NUM>. This orientation of the sloped surfaces <NUM>, <NUM> is beneficial as the slope of the sloped surface <NUM>, <NUM> conform to the sloped surfaces <NUM>, <NUM> of the piston <NUM> such that as the piston <NUM> approaches top dead centre, the air fuel mixture is squeezed out of the sides of the combustion chamber <NUM> towards the tip of the spark plug <NUM>. This helps to facilitate a more complete combustion of the air fuel mixture.

The outermost edges of the sloped surfaces <NUM>, <NUM> made by the sixth machining process form curved portions of the combustion chamber opening <NUM> located between the air inlet openings 91a, 91b and the exhaust outlet openings 92a, 92b respectively. These curved portions may be described as a pair of opposed curved sections of the combustion chamber opening <NUM>.

<FIG> shows a plan view of the underside of the cylinder head <NUM> after seventh and eighth machining processes have taken place. The seventh machining process forms surface <NUM> in the vicinity of the fuel injector seat <NUM>, and the eighth machining process forms surface <NUM> in the vicinity of the spark plug seat <NUM>. Both the seventh and the eighth cuts help to open up the roof <NUM> of the combustion chamber <NUM> to allow for better flow and more complete combustion.

Although the spark plug <NUM> and fuel injector <NUM> are shown in line along the plane of symmetry <NUM> of the combustion chamber <NUM>, it will be appreciated that the spark plug <NUM> and fuel injector <NUM> may in other embodiments be located sided by side in a plane perpendicular to the plane of symmetry <NUM> or in any other suitable position.

<FIG> shows the volume envelopes traversed by the cutters in the first to the eighth machining processes superimposed on one another, and <FIG> shows the volume envelopes traversed by the cutters in the first to the sixth machining processes in a flow chart.

It is envisaged that the order in which the cuts are made may vary from that described above and shown in <FIG>. For example, the cuts <NUM>, <NUM> made by the sloped surface cutter moving through envelope <NUM> may be made at the beginning of the machining process rather than near the end. Similarly, the cuts <NUM>, <NUM> made by the ball nose cutter moving through the envelope <NUM> may be made towards the end of the machining process rather than at the beginning. In fact, any of the cuts described above may be made in any order. However, it is preferred that the machining processes be ordered as illustrated in <FIG> as this sequence provides an optimal balance between the amount of material to be removed in any given machining process, and the amount of "fresh air" that a cutter moves through. It is well known in the art that it is desirable for a cutter to continuously remove a small amount of material in a cutting process without biting too deeply into the material as this can cause imperfections in the cut surface and too much load on the cutter. It is also desirable to avoid the cutter repeatedly lifting off and re-contacting the material.

With exception of the "sixth" machining process described above with reference to <FIG> - and represented by envelope <NUM> - the sequence of machining processes from the "first" to the "fifth" is preferably as described above. The "sixth" machining process is preferably made after the "fifth" machining process as described above, or it may preferably be made at the beginning of the machining processes. The "seventh" and "eighth" cuts described above with reference to <FIG>, are preferably the last cuts to be made. However, they may optionally be made at any other suitable position in the machining sequence.

It is beneficial to make all cuts made by the same tool in sequence. For example, the sequence of cuts made by the radiused cutter moving through the envelopes <NUM>, <NUM>, <NUM>, <NUM>. These machining processes may optionally be made in any suitable order. However, the order shown in <FIG>, and described above with reference to <FIG>, is preferred because it provides the above described optimal balance between the amount of material to be removed in any given machining process, and the amount of "fresh air" that the cutter moves through.

In the description above, the completed roof surface <NUM> of the combustion chamber <NUM> comprises only machined surfaces. In an alternative arrangement, some of the roof surface <NUM> of the combustion chamber <NUM> may be "as cast" such that no material is removed from certain areas of the roof of the recess <NUM>.

Claim 1:
A method of machining a combustion chamber roof surface in a cylinder head for a lean-burn gasoline engine, wherein the machined cylinder head comprises:
a substantially planar gasket interface surface; and
a combustion chamber extending into the cylinder head away from the gasket interface surface, wherein the combustion chamber roof surface intersects the gasket interface surface to define a combustion chamber opening,
the method comprising:
using a first cutter to machine a first pair of facets (<NUM>, <NUM>) of the combustion chamber roof surface, wherein a first one of the first pair of facets is located at least partially between a pair of air inlet openings in the combustion chamber roof located on an air inlet side of the combustion chamber, and a second one of the first pair of facets is located at least partially between a pair of exhaust outlet openings in the combustion chamber roof located on an exhaust outlet side of the combustion chamber;
using a second cutter to machine a second pair of facets (185a, 185b) of the combustion chamber roof surface, wherein the second pair of facets are located between the pair of air inlet openings and the pair of exhaust outlet openings such that each one of the second pair of facets extends between the air inlet side of the combustion chamber and the exhaust outlet side of the combustion chamber; and
using the second cutter to machine a third pair of facets (186a, 186b) of the combustion chamber roof surface, wherein a first one of the third pair of facets is located between a first one of the air inlet openings and a first one of the exhaust outlet openings, and a second one of the third pair of facets is located between a second one of the air inlet openings and a second one of the exhaust outlet openings.