Patent Description:
Air-cooled, four-stroke engines are popular choices for powering airplanes, helicopters, UAVs (unmanned aerial vehicles), and the like. A typical air-cooled engine may include an aluminum cylinder head with heat fins exposed to ambient air. Heat from combustion flows into the aluminum cylinder head and out to the surrounding air via conduction through heat fins.

A typical air-cooled engine uses an open combustion chamber with widely spaced spark plugs. Two spark plugs are used per cylinder to allow for redundant operation, such that the engine can continue working even if one spark plug fails.

Unfortunately, the above-described air-cooled aviation engine has certain deficiencies. The open combustion chamber can require flame fronts, initiated by spark plugs, to travel long distances to traverse the entire volume of the chamber. To accommodate longer durations required to traverse these distances, spark-plug timing is often advanced considerably, with spark plugs firing <NUM> degrees or more prior to top dead center (TDC) of the pistons within their respective cylinders. Such advanced firing of spark plugs results in negative work that subtracts from the positive mechanical work that the engine performs. Further, the relatively long distance needed for flame fronts to travel toward an exhaust valve in the combustion chamber means that uncombusted fuel-air mixtures remain in contact with hot exhaust components longer than desired. This can cause the hot fuel-air mixture near the exhaust components to ignite prematurely, resulting in knock. Although knock can be remedied at least in part through the use of higher-octane fuel, such fuel is more expensive than lower-octane fuel and is difficult to obtain in some parts of the world. Prior art document <CIT> relates to an internal-combustion engine comprising a combustion chamber with an elliptical shape and provided with a single intake valve, a single exhaust valve, two plugs and a fuel injector.

To address the above deficiencies at least in part, an improved air-cooled aviation engine includes a compact combustion chamber having a generally elliptical shape. The combustion chamber is formed by a cylinder head and a piston when the piston is disposed at or near an uppermost position within a cylinder bore. The cylinder head includes an intake valve opening and an exhaust valve opening. The elliptical shape has an area smaller than that of the cylinder bore and has a major axis that intersects both the intake valve opening and the exhaust valve opening.

The compact combustion chamber brings several advantages. Flame fronts initiated by spark-plug firing can traverse the entire volume of the combustion chamber in little time, completing combustion quickly and reducing the degree to which spark-plug timing may be advanced (e.g., from <NUM> to <NUM> degrees before TDC). The engine thus performs less negative work than in prior designs, resulting in higher efficiency. Further, the compactness of the combustion chamber means that the hot exhaust components have little time to heat the fuel-air mixture, reducing heat rejection and preventing knocking. Prevention of knocking allows less expensive, lower octane fuel to be used, substantially reducing costs of operation and increasing availability. Many other improvements and advantages are contemplated.

The invention is directed to an air-cooled aviation engine. The engine includes a cylinder head having an intake valve opening, an exhaust valve opening, a plurality of spark plugs having electrodes, and an interior surface. The engine further includes a cylinder bore having a cross-sectional area and containing a piston configured to move reciprocally within the cylinder bore. The engine still further includes a combustion chamber having an upper region defined by the interior surface of the cylinder head and a lower region defined by the piston when the piston is in an uppermost position within the cylinder bore. The combustion chamber has an elliptical shape that extends over a cross-sectional area smaller than the cross-sectional area of the cylinder bore, and the elliptical shape has a major axis that intersects the intake valve opening and the exhaust valve opening.

In some examples, the cylinder head has a flat shoulder region that abuts a flat outer region of the piston when the piston is in the uppermost position within the cylinder bore, and the flat shoulder region and the flat outer region constrain the combustion chamber to the elliptical shape.

In some examples, the electrodes of the plurality of spark plugs extend below the interior surface of the combustion chamber.

According to the invention, the electrodes of the plurality of spark plugs are disposed closer to the exhaust valve opening than to the intake valve opening.

In some examples, the piston includes a depression beneath the electrodes of each of the plurality of spark plugs, and at least a portion of the piston is elevated relative to the depression.

In some examples, the cylinder bore has a center line and a radius, and the electrodes of the plurality of spark plugs are disposed at respective distances from the center line that are less than <NUM>% of the radius of the cylinder bore.

In some examples, the cylinder head further includes an intake valve seat that fits within the intake opening and an exhaust valve seat that fits within the exhaust opening.

In some examples, the intake valve seat and the exhaust valve seat are each composed at least in part of copper-infiltrated powdered metal and/or solid alloy that includes copper.

In some examples, the intake valve seat and the exhaust valve seat each have a respective inner region, each inner region having multiple discretely angled surfaces.

In some examples, the cylinder head further includes an intake valve arranged to selectively admit intake air into the combustion chamber via the intake valve opening. The intake valve has an intake valve stem and an intake valve head, and the intake valve stem has a diameter of less than <NUM>. In such examples, the cylinder head further includes an exhaust valve arranged to selectively allow exhaust gases to be expelled from the combustion chamber via the exhaust valve opening. The exhaust valve has an exhaust valve stem and an exhaust valve head, and the exhaust valve stem has a diameter of less than <NUM>.

In some examples, the intake valve head and the exhaust valve head have multiple discretely-angled surfaces adjacent to contact regions with the respective valve seats.

In some examples, the cylinder head includes an exhaust valve-stem guide around the exhaust valve stem. The exhaust valve-stem guide is composed at least in part of copper and produced as a powdered metal and/or solid alloy.

In some examples, each of the intake valve stem and the exhaust valve stem includes a triple-bead attachment to a respective valve retainer, each triple-bead attachment constructed and arranged to facilitate rotation of the respective valve.

In some examples, the engine further includes a high-tumble intake port coextensive with the intake valve opening of the cylinder head. The high-tumble intake port is constructed and arranged to induce vertical currents within the combustion chamber.

Other embodiments are directed to an air-cooled aviation engine that includes multiple cylinders, where each cylinder includes a cylinder head having an intake valve opening, an exhaust valve opening, a plurality of spark plugs, and an interior surface. The engine further includes a cylinder bore having a cross-sectional area and containing a piston configured to move reciprocally within the cylinder bore. The engine still further includes a combustion chamber having an upper region defined by the interior surface of the cylinder head and a lower region defined by the piston when the piston is in a substantially uppermost position within the cylinder bore. The combustion chamber has an elliptical shape that extends over a cross-sectional area smaller than the cross-sectional area of the cylinder bore, and the elliptical shape has a major axis that intersects the intake valve opening and the exhaust valve opening.

Still other embodiments are directed to an aircraft that includes an air-cooled aviation engine in accordance with any of the examples above.

The engine may include any number of chambers of the kind described (e.g., any number of cylinders), arranged in any manner (e.g., straight, opposed, V-shaped, W-shaped, radially-shaped, etc.).

The foregoing summary is presented for illustrative purposes to assist the reader in readily grasping example features presented herein; however, this summary is not intended to set forth required elements or to limit embodiments hereof in any way.

The foregoing and other features and advantages will be apparent from the following description of particular embodiments, as illustrated in the accompanying drawings, in which like reference characters refer to the same or similar parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of various embodiments.

Embodiments of the improved technique will now be described. One should appreciate that such embodiments are provided by way of example to illustrate certain features and principles but are not intended to be limiting.

An improved technique provides an air-cooled aviation engine that includes a compact combustion chamber having a generally elliptical shape. The combustion chamber is formed by a cylinder head and a piston when the piston is disposed at or near an uppermost position within a cylinder bore. The cylinder head includes an intake valve opening and an exhaust valve opening. The elliptical shape has an area smaller than that of the cylinder bore and has a major axis that intersects both the intake valve opening and the exhaust valve opening.

In some examples, the engine includes spark plugs, and the spark plugs have electrodes that are placed closer to the exhaust valve than to the intake valve. Such placement means that there is little time for hot exhaust components to heat the fuel-air mixture, reducing heat rejection and preventing knocking.

In some examples, the spark plugs are disposed parallel to a minor axis of the elliptical region and are placed closer to the exhaust valve than to the intake valve.

In some examples, the piston has a top surface in which one or more depressions are formed. In some examples, the depressions have a shape that substantially mirrors a shape of the inside of the cylinder head.

In some examples, the cylinder head includes valve seats composed at least in part of copper-infiltrated powdered metal. Such copper-infiltrated powdered metal has higher thermal conductivity than stainless steel (e.g., <NUM>% higher) but provides similar wear resistance. The valve seats may be thermal interference-fit into the cylinder head, contacting the cylinder head over a large area and forming a low thermal-resistance attachment. The valve seats may thus provide a higher thermal-conductivity path between valve heads and the surrounding cylinder head.

In some examples, the valve seats have a narrow profile, exposing less surface area to combustion and experiencing less direct heating.

One of the functions of the valve seats is to perform heat-sinking that cools the valve heads. In some examples, the back sides of the valve heads includes a flat region that matches a correspondingly flat region of the valve seats. This contact regions enable heat to flow from valve heads into valve seats, and then into the engine block, where the heat is radiated into ambient air through heat fins. Additional angles added adjacent to the contact area (multi-angled design) provides a Venturi shape for more efficient air flow.

In some examples, the valves include narrow valve stems. Narrower valve stems reduce mass of the valves and enable them to be actuated more aggressively by the camshaft, such that the valves can be opened and closed sharply (at high rates). This feature helps to ensure that valves are not kept open or closed longer than necessary. For example, valves remain seated longer during each combustion cycle, providing more time for heat to conduct into the valve seat and cylinder head. Having the valves seated longer also increases the effective compression and expansion ratios, improving efficiency. Further, narrower stems expand less with increases in temperature, helping them to maintain alignment and cylindricity, and reducing valve distortion, leakage, and wear. They also block less space in intake and exhaust ports and thus promote more effective breathing.

In some examples, one or both valves have valve stems and high thermal-conductivity valve-stem guides, such as those made from copper-infiltrated powdered metal and/or high thermal-conductivity solid copper alloy, such as brass billet. In some examples, a valve stem may have an elongated, hollow internal region that contains sodium. When the valve stem heats, the sodium melts and sloshes within the hollow region, picking up heat from the valve head. The heat is transferred up the valve stem and laterally to the high thermal conductivity valve guides and then into the cylinder head.

In some examples, the valve stems are actuated by a camshaft, which opens and closes the respective valves through respective cams and levers. Springs bias the valves closed, and the rotating cam shaft forces them open through the levers.

In some examples, the valve stems are free to rotate, and repetitive compression and expansion of the valve springs gradually rotates the valves, e.g., at a rate of between <NUM> and <NUM> rotations per minute. Rotation of valves in this manner tends to distribute heat more evenly within valve heads and tends to wipe clean any deposits that form in the sealing area between valve heads and valve seats.

In some examples, the valve stems are coupled to respective springs via a triple-bead design. The end of each valve stem has three grooves for engaging a key that attaches the valve stem to an upper spring seat, reducing stress in the valve stem and increasing valve rotation as compared with single-bead designs.

In some examples, valve stems are isolated from oil that bathes an upper valvetrain via molded elastomer rings or other seals. For example, each valve stem guide has a respective seal. The seal prevents low-octane oil from leaking past the valves into the combustion chamber.

In some examples, an intake port into the combustion chamber creates high-tumble, vertically-rolling currents within the combustion chamber. Such rolling currents tend to cause fuel within the fuel-air mixture to migrate close to the spark plugs (by centrifuging action), promoting effective ignition.

In some examples, the spark plugs have electrodes that extend into the combustion chamber, promoting direct propagation of flame fronts. For example, ends of the plugs may be flush with the combustion chamber with electrodes extending into the main chamber volume.

In some examples, pistons are provided with dual compression rings (e.g., composed of steel). Such compression rings prevent gases from entering the space within the bores alongside the pistons. They also avoid blow-by of combustion gases around the pistons. In some examples, each piston includes a separate oil-control ring (e.g., composed of stainless steel). The oil-control ring may be placed below the compression rings on each piston and serves to prevent oil in the crankcase from seeping up and around the piston and into the combustion chamber.

In some arrangements, the valves are arranged at an angle relative to each other. In other arrangements, the valves are arranged in parallel to each other and to the direction of movement of the piston.

The above and additional features will now be described with reference to the accompanying figures.

<FIG> shows an example cylinder head <NUM> of an engine in accordance with certain embodiments. The depicted cylinder head <NUM> is shown from the bottom. Thus, the part facing the reader is the part that normally faces toward a cylinder bore <NUM> (<FIG>). An intake valve <NUM> is shown to the left, and an exhaust valve <NUM> is shown to the right. The intake valve <NUM> sits within an intake valve seat <NUM>, which is disposed within an intake valve opening <NUM>. Likewise, the exhaust valve <NUM> sits within an exhaust valve seat <NUM>, which is disposed within an exhaust valve opening <NUM>. In an example, the valve seats <NUM> and <NUM> are thermally interference fit into the respective valve openings <NUM> and <NUM>, although other attachment modalities may be used.

Spark plugs <NUM> are disposed between the two valves <NUM> and <NUM>. Each spark plug <NUM> includes electrodes <NUM>, e.g., a pair of electrodes for each spark plug. As shown, the spark plugs <NUM> may be placed such that the electrodes <NUM> are closer to the exhaust valve <NUM> than they are to the intake valve <NUM>. Although two spark plugs <NUM> are shown, alternative embodiments may include greater than two spark plugs.

The cylinder head <NUM> participates in defining a combustion chamber having an elliptical shape <NUM>. The elliptical shape <NUM> has a major axis <NUM> as well as a minor axis (not shown). As shown, the major axis <NUM> intersects the valves <NUM> and <NUM>. In some examples, the major axis <NUM> has a length that substantially matches a diameter of the cylinder bore <NUM>. In other examples, the major axis <NUM> is smaller than the diameter of the cylinder bore <NUM>. One should appreciate that the elliptical shape <NUM> need not be a perfect ellipse. Rather, all that is required is that the shape <NUM> be generally the shape of an ellipse, i.e., curved and longer in one direction (the major axis) than along a perpendicular direction (the minor axis).

As further shown in <FIG>, a center line <NUM> (shown as a point) of the cylinder bore <NUM> is located along the major axis <NUM> near the intake valve <NUM>. The electrodes <NUM> of top and bottom spark plugs <NUM> are disposed at respective radial distances <NUM> and <NUM> from the center line <NUM>. Also shown is a radius <NUM> of the cylinder bore <NUM>, relative to the same center line <NUM>.

In some examples, the cylinder head <NUM> is distinguished from prior designs in its placement of electrodes <NUM>. For example, the radii <NUM> and <NUM> may be less than <NUM>% of the radius <NUM> of the cylinder bore <NUM>. In some examples, the radii <NUM> and <NUM> may be less than <NUM>% of the radius <NUM> of the cylinder bore <NUM>. Electrodes <NUM> are thus more centrally located within the cylinder head <NUM> than is typical, and this more central location promotes more uniform and efficient combustion.

The cylinder head <NUM> has an interior surface <NUM> within the bounds of the elliptical shape <NUM>, as well as a flat shoulder region <NUM> outside the bounds of the elliptical shape <NUM>. Typically, combustion occurs within the region defined by the interior surface <NUM> (within the ellipse) but not within the region <NUM> (outside the ellipse). Thus, combustion is normally constrained to the area of the ellipse <NUM>, which is less than the area of the cylinder bore <NUM> (as defined by radius <NUM>). Confining the area in which combustion occurs further promotes efficiency, e.g., by reducing path lengths of combustion fronts.

<FIG> show respective cross-sectional views of the cylinder head <NUM> of <FIG>, with the section in each figure taken through the spark plugs <NUM>. <FIG> is a magnified view of portions of <FIG>, with contour lines and other details removed.

As shown, a combustion chamber <NUM> is formed between the cylinder head <NUM> and a piston <NUM>. The combustion chamber <NUM> has an upper region 220a, defined by the above-described interior surface <NUM>, as well as a lower region 220b, defined by the piston <NUM>.

The piston <NUM> is configured to move reciprocally within the cylinder bore <NUM>, which is defined by a cylinder barrel <NUM>. In the figures, the piston <NUM> is shown in its uppermost position, which corresponds to top dead center (TDC).

With the piston <NUM> in its uppermost position, the above-described flat shoulder region <NUM> of the cylinder head <NUM> directly opposes a flat outer region <NUM> of the piston <NUM>, effectively closing the space between the cylinder head <NUM> and the piston <NUM> and constraining combustion to more radially central locations. It is noted that the elliptical shape <NUM> (<FIG>) is not visible from the perspective of <FIG>. However, the width of the combustion chamber <NUM> as shown in <FIG> corresponds approximately to the minor axis of the ellipse.

Also evident from <FIG> is that the electrodes <NUM> of the spark plugs <NUM> extend into the combustion chamber <NUM>. For example, a main body of the spark plugs <NUM> may be flush with the interior surface <NUM>, with the electrodes <NUM> extending into a main volume of the combustion chamber <NUM>. The depicted arrangement promotes rapid propagation of flame fronts and differs from many prior designs, where electrodes are recessed (placed higher), resulting in slower combustion.

To further promote efficient combustion, the piston <NUM> may include one or more depressions <NUM> located beneath the electrodes <NUM> of the spark plugs <NUM>. In some examples, the depression(s) <NUM> may be a single depression that extends completely around the top surface of the piston <NUM>, e.g., in an annular-type manner. In other examples, two independent depressions <NUM> may be formed, one beneath each electrode <NUM>. In some examples, a central elevated region <NUM> may be formed in the piston <NUM> between the electrodes. The elevated region <NUM> limits the volume of the combustion chamber <NUM> while still allowing flame fronts to develop by virtue of the depression(s) <NUM>.

<FIG> shows a cross-sectional view of the engine, with the section taken through the valves <NUM> and <NUM>. Visible in <FIG> is an intake port <NUM>, for supplying intake air to the combustion chamber <NUM> via the intake valve <NUM>, and an exhaust port <NUM>, for conveying exhaust gases out of the combustion chamber <NUM> via the exhaust valve <NUM>. In some examples, the intake port <NUM> is a high-tumble port constructed and arranged to induce vertical currents of fuel and air within the combustion chamber <NUM>.

<FIG> further shows additional details of the valves <NUM> and <NUM>. Here, the intake valve <NUM> is closed against the intake valve seat <NUM>. The intake valve <NUM> includes an intake valve head 110a and an intake valve stem 110b. The intake valve stem 110b is surrounded, over at least a portion of its length, by an intake valve-stem guide 110c. Likewise, the exhaust valve <NUM> is seen to include an exhaust valve head 120a and an exhaust valve stem 120b. The exhaust valve stem 120b is surrounded, over at least a portion of its length, by an exhaust valve-stem guide 120c. In an example, the exhaust valve stem 120b includes a hollow region 120d which contains sodium. When the exhaust valve stem 120b heats, the sodium melts and sloshes within the hollow region 120d, picking up heat from the exhaust valve head 120a. The heat is transferred up the valve stem 120b and laterally to the valve guides 120c and then into the cylinder head <NUM>.

In an example, the valve stems 110b and 120b each have a diameter of less than <NUM> and, in some cases, less than <NUM>. In an example, the intake valve <NUM> has a mass less than <NUM> grams (e.g., <NUM> grams) and the exhaust valve <NUM> has a mass less than <NUM> grams (e.g., <NUM> grams).

In some examples, the intake valve stem guide 110c is composed of at least <NUM>% copper. Likewise, the exhaust valve stem guide 120c is composed of at least <NUM>% copper. The materials could be provided as a uniform solid alloy or as a powdered metal, for example.

In some examples, the valve seats <NUM> and <NUM> are each composed of a minimum of <NUM>% copper and can be produced as a powdered metal or a uniform solid alloy.

<FIG> shows another cross-sectional view of the cylinder head <NUM>. As with <FIG>, the section is taken through the valves <NUM> and <NUM>. As shown at the upper-right of <FIG>, a top of the exhaust valve stem 120b includes a triple-bead geometry <NUM>, such as three concentric rounded grooves. A triple-bead key <NUM> engages the triple-bead geometry of <NUM>. The triple-bead key <NUM> is fixedly attached to (e.g., wedged into) a upper spring seat <NUM>. A helical spring (not shown) is held in compression between the upper spring seat <NUM> and a lower spring seat <NUM>. The spring may be concentric with the valve stem 120b and acts to push the valve retainer <NUM> up, ensuring that the valve head 120a is biased upwardly against the valve seat <NUM>. A rocker <NUM> is configured to push down repetitively on the valve stem 120b as the rocker pivots up and down in response to rotation of a camshaft (not shown). When the rocker <NUM> pushes down, the retainer <NUM> compresses the spring and pushes the valve <NUM> open. When the rocker <NUM> stops pushing down, the spring pushes the retainer <NUM> back up, causing the valve <NUM> to close.

The triple-bead key <NUM> holds the triple-bead geometry <NUM> of the valve stem 120b without concentrating stress in a single area, as would be the case with a single-bead or a rotator valve construction. The triple-bead construction is thus well suited for the narrow and light valve stem 120b, which might otherwise wear prematurely or break during normal operation.

Preferably, the triple-bead key <NUM> does not fit tightly onto the triple-bead lock <NUM> but rather maintains a small amount of radial and axial clearance. For example, a clearance of <NUM> may be maintained between the triple-bead geometry <NUM> and the triple-bead key <NUM>. The clearance enables the valve stem 120b, and thus the entire valve <NUM>, to rotate within the retainer <NUM>. Rotation may be achieved incrementally on successive compressions of the spring. For example, the helical spring provides slight rotation each time it is compressed. Some of the rotation is imparted to the valve stem 120b, which may rotate, e.g., at a rate of between <NUM> and <NUM> rotations per minute. Rotation of the valve <NUM> in this manner tends to distribute heat more evenly within the valve head 120a and tends to wipe clean any deposits that form in the sealing area between the valve head 120a and the valve seat <NUM>.

Although the triple-bead arrangement has been described in connection with the exhaust valve <NUM>, a similar or identical arrangement may be used with the intake valve <NUM>. Indeed, <FIG> shows the same features on both valves. Thus, both the intake valve <NUM> and the exhaust valve <NUM> may benefit from the durability and valve-rotation afforded by the triple-bead construction.

Although a triple-bead construction is specifically shown, some embodiments may employ greater than three beads. For example, a quadruple-bead construction may be used. It should be appreciated, though, that any multi-bead arrangement having greater than two beads necessarily includes a triple-bead construction.

<FIG> is a cross-sectional view of the intake valve <NUM> closed against the intake valve seat <NUM>. As shown in <FIG>, the intake valve seat <NUM> is a ring that extends completely around the intake valve opening <NUM> and lines that opening.

As shown to the left of <FIG>, the valve head 110a has multiple discretely angled surfaces <NUM>, with each surface extending completely around the outside of the intake valve head 110a. In a complementary manner, the valve seat <NUM> has multiple discretely angled surfaces <NUM>, which extend completely around the inside of the valve seat <NUM>. One of the surfaces <NUM> maintains contact with a corresponding one the surfaces <NUM> to effectively seal the valve head 110a against the valve seat <NUM> when the valve <NUM> is closed. As temperatures rise, the radial contact area between the two surfaces may change but the surfaces' tangential contact remains.

The multi-angled surfaces <NUM> and <NUM> promote smooth and efficient airflow between the intake port <NUM> and the combustion chamber <NUM> when the valve <NUM> is open. For example, air is efficiently funneled through a gap that forms between the surfaces <NUM> and <NUM>. The surfaces <NUM> and <NUM> thus facilitate the introduction of air into the combustion chamber <NUM>, promoting efficient combustion.

Although the example of <FIG> relates to the intake valve <NUM> and the intake valve seat <NUM>, the exhaust valve head 120a and exhaust valve seat <NUM> may be arranged in a similar or identical manner. Thus, the exhaust valve head 120a and exhaust valve seat <NUM> may each include multiple discretely angled surfaces as described above. Exhaust gases may thus follow a similarly smooth path from the combustion chamber <NUM> to the exhaust port <NUM> (<FIG>).

Those skilled in the art will therefore understand that various changes in form and detail may be made to the embodiments disclosed herein without departing from the scope of the following claims.

Claim 1:
An air-cooled aviation engine, comprising:
a cylinder head (<NUM>) having an intake valve opening (<NUM>), an exhaust valve opening (<NUM>), a plurality of spark plugs (<NUM>) having electrodes (<NUM>), and an interior surface (<NUM>);
a cylinder bore (<NUM>) having a cross-sectional area and containing a piston (<NUM>) configured to move reciprocally within the cylinder bore (<NUM>); and
a combustion chamber (<NUM>) having an upper region (220a) defined by the interior surface (<NUM>) of the cylinder head (<NUM>) and a lower region (220b) defined by the piston (<NUM>) when the piston (<NUM>) is in an uppermost position within the cylinder bore (<NUM>),
wherein the combustion chamber (<NUM>) has an elliptical shape (<NUM>) that extends over a cross-sectional area smaller than the cross-sectional area of the cylinder bore (<NUM>), the elliptical shape (<NUM>) having a major axis (<NUM>) that intersects the intake valve opening (<NUM>) and the exhaust valve opening (<NUM>),
characterised by the electrodes (<NUM>) of the plurality of spark plugs (<NUM>) being disposed closer to the exhaust valve opening (<NUM>) than to the intake valve opening (<NUM>).