Cylinder in a two-stroke uniflow scavenged crosshead engine, and a method for reduction of NOX The present invention relates to a cylinder in a two-stroke uniflow crosshead engine having a combustion chamber which is supplied with oxygen- containing charging air in a swirl in the combustion chamber through scavenge air ports when a piston is in the lower section of the combustion chamber, and in which the upper section of the combustion chamber has an exhaust valve and at least two fuel injectors each having a number of nozzle holes, of which a backward-facing nozzle hole of a fuel injector is directed at least to some degree against the swirl direction and a forward-facing nozzle hole of another fuel injector is directed at least to some degree in the direction of the swirl.

Such a cylinder with two fuel injectors is known from the diesel engine type VTBF of the make B&W Diesel, in which the injectors have three forward- facing nozzle holes and a single backward-facing nozzle hole. The four nozzle holes extend from the same level in the nozzle bore and open and close simultaneously. The backward-facing nozzle hole is used to generate fuel combustion in the charging air swirl upstream of the fuel injector in order to generate so-called internal recirculation of combustion gas to the combustion areas associated with the forward-facing nozzle holes on the same fuel injector.

For many years, recirculation of combustion gas has been used as a means of reducing the formation of NOX in the combustion process, and the recirculation is based on the assumption that the recirculated combustion gas has a cooling effect on the combustion area, and that the formation of NOX is mainly caused by combustion at an excessive temperature. The internal recirculation obtained by directing one of

the nozzle holes backwards so that the charging air swirl contains combustion gas from the combustion of fuel from this nozzle hole when it reaches the areas for combustion of fuel from the forward-facing nozzle holes results in slower combustion of the fuel at a lower temperature. The lower temperature results in reduced formation of NO, on combustion.

It is a problem of the prior-art cylinder with fuel injectors that the formation of NOX is not sufficiently suppressed.

The object of the present invention is to provide a cylinder for a diesel engine yielding a further reduction of the formation of NOX during the combustion.

In view of this object, the present invention is characterized in that the backward-facing and forward-facing nozzle holes located on different injectors are directed setwise towards and can inject fuel into a joint mixing area located in the combustion chamber between the nozzle holes of the set, and that the distance between the injectors providing the set of nozzle holes is 0.9 x B at the most, B being the cylinder bore.

When the injectors are opened, the fuel is atomized jet-like at high speed into the combustion chamber. The nozzle holes belonging in a set are located on at least two different fuel injectors, and as the fuel is injected at the high speed into the joint mixing area, the two or more fuel jets meet at a very early stage of the engine cycle. This time of mixing is noticeably prior to the time of the engine cycle when the charging air swirl of the prior-art cylinder has had time to mix combustion gas from the combustion of a backward-facing jet into the area of a forward-facing jet from the same fuel injector. The early mixing in the cylinder according to the invention renders it possible to apply a mechanism, so far non-applied, for avoiding the formation of NOX,

which will now be discussed in more detail.

The formation of NOX in a combustion process depends on a number of factors. At high temperatures during the combustion process, NOX may form by oxidation of the nitrogen of the air. With the holding times for fuel and gases occurring in a diesel engine, the temperature has to exceed 2000°K for a major quantity of NOX to form. At a low oxygen content, the oxidation is diminished, and the formation of NOx is lowered. At a large excess of oxygen, the temperature is reduced, which also reduces the formation of NOX. In the cylinder according to the invention, the fuel jets rapidly reach the joint mixing area in which the oxygen content is low as a consequence of the initial combustion and resulting production of combustion gas so that rising temperatures do not result in any major increase in the formation of NOX. During the initial combustion the temperature is lower than the 2000°K which is the lower threshold temperature for the formation of NOX. In other words, the invention is based on a rapid initial, oxygen-consuming combustion at a suitably low temperature and a subsequent more extended combustion at a high temperature in a mixing area in which the oxygen content is low. If the distance between the injectors with the set of nozzle holes is larger than 0.9 x B, the risk may arise that the oxygen content will not become sufficiently low in the mixing area for sufficient suppression of the formation of NOx.

The invention limits or prevents combustion in the manner previously applied with a higher oxygen content, at which the formation of NOX was mainly controlled by the current temperature level of the combustion, which cannot anyway be kept below the 2000°K during the entire combustion.

The joint mixing area may be located substantially at the same radius from the longitudinal axis of the

cylinder as the injectors with the set of nozzle holes. Thus, the backward-facing and forward-facing nozzle holes of the set are oriented substantially directly against and substantially directly with the swirl motion of the charging air in the area at the fuel injectors, which counteracts spreading of, particularly, the fuel jet from the forward-facing nozzle hole during its travel to the mixing area.

In one embodiment which provides particularly rapid mixing, the joint mixing area is located closest to the backward-facing nozzle hole of the set, wherefrom the fuel jet is sprayed out in counter-current against the motion of the charging air and therefore has a smaller speed of propagation in the combustion chamber than the fuel jet from the other nozzle hole of the set.

In a further embodiment aiming at obtaining a high cylinder effect with a suitable distribution of the temperature load on cylinder liner, cover and piston while a substantial proportion of the fuel is combusted in areas with reduced formation of NOX, each fuel injector has a backward-facing nozzle hole forming a set with a forward-facing nozzle hole on a fuel injector located behind the said each fuel injector, and a forward-facing nozzle hole forming a set with a backward-facing nozzle hole on a fuel injector located in front of the said each fuel injector, and at least one further nozzle hole oriented more inwards into the cylinder than the forward-facing and backward-facing nozzle holes. The principle applied here is that a substantial part of the fuel is injected into mixing areas with a low oxygen content in the radially outermost part of the cylinder, in which the largest quantity of air is available during the period of the combustion sequence, while the remaining part of the fuel is atomized into the central area of the cylinder, in

which the available quantity of air is smaller, for which reason the oxygen content here is reduced relatively rapidly to a suitably low level. It is possible to design the cylinder with only two fuel injectors, but preferably it has at least three fuel injectors to reduce the distance from the individual injector to the associated mixing areas.

It is possible to design the injector with a nozzle hole directed backwards by only a small angle, such as 25°, but preferably the backward-facing nozzle hole extends in a direction which, projected on a plane at right angles to the longitudinal axis of the cylinder, forms an angle of at least 35° with the radius from the longitudinal axis of the cylinder. This provides a more uniform distribution in the cylinder of the fuel combustion. In yet another embodiment, the said angle between the backward-facing nozzle hole and the radius is at least 48°, the result of which is that the associated mixing area is displaced in relation to the central area of the cylinder, and that the mixing area is gradually supplied with more oxygen from the charging air swirl so that the combustion will not be incomplete. In a further preferred embodiment, the said angle is at least 60°, which, with three injectors evenly spaced along the circumference of the cylinder, results in a mixing area located radially outside a straight connecting line between the two injectors each carrying a nozzle hole of one of the sets of nozzle holes.

In another, independent aspect, the present invention relates to a method of reducing the formation of NOX at combustion of fuel in a cylinder of a diesel engine which has a number of fuel injectors in the cylinder in which charging air in the cylinder is whirled around the longitudinal axis of the cylinder before and during the combustion, and in which the fuel is injected from nozzle holes in

the injectors in directions at least partially with and at least partially against the swirl motion of the charging air. Such a method is known in connection with the above internal combustion engine of the B&W Diesel make.

In order to reduce the emission of NOX by means of primary methods, that is, by reducing the formation of NOX in the actual combustion process in the cylinder, the method is characterized in that fuel is injected from backward-facing and forward-facing nozzle holes associated in sets from at least two injectors so that the combustion areas associated with the set are mixed in at least one joint mixing area located between the two fuel injectors each having a nozzle hole of the set, and in which the combustion of the fuel from the at least two fuel injectors keeps the oxygen content low for reduction of the formation of NO,,.

The individual fuel injectors in the cylinder may suitably be actuated for injection simultaneously, which simplifies the control of the injectors and thus reduces the risk of unintended formation of NOX due to a delay in or lack of injection from one injector.

It is possible to open for fuel injection through the backward-facing nozzle holes before opening for fuel injection through the forward-facing nozzle holes in order to get the mixing area located at a larger distance from the fuel injector with the backward-facing nozzle hole so that this injector is less exposed to the heat action from the combustion process.

Embodiments and examples of the invention will now be described in more detail below with reference to the highly schematic drawing, in which Fig. 1 is a cross-sectional view of a prior-art cylinder, Fig. 2 is a diagram showing the correlation

between the formation of NOx and the oxygen concentration at the combustion, Fig.. 3 is a longitudinal sectional view through a cylinder according to the invention, Fig. 4 is a cross-sectional view through an example of an embodiment of the invention, Fig. 5 is a diagram showing the concentration of fuel vapours and oxygen in the cylinder, and Fig. 6 is a diagram showing the combustion rate in a cylinder of a prior-art type and a cylinder according to the invention.

Fig. 1 shows a cross-sectional view through a cylinder of a two-stroke crosshead engine known as the VTBF type. A cylinder 1 comprises a cylinder liner 2 having scavenge air ports 3 in its lower part, which ports 3 are inclined in relation to the cylinder radius in order to impart a swirling motion to the charging air in the cylinder by means of the inflowing air when the piston moves past its bottom dead centre position. The cylinder has two fuel injectors 4 each injecting fuel a through a single backward-facing nozzle hole and simultaneously injecting fuel b through forward-facing nozzle holes. The swirling motion of the charging air around the longitudinal axis 5 of the cylinder is indicated by A. In connection with the combustion, the swirling motion causes combustion gas from the combustion of the fuel a to be passed in the circumferential direction of the cylinder and to be mixed with the fuel b injected by the same injector.

The thermal formation of NOX at the combustion of the fuel is dominated by two different factors that depend on the current oxygen content at the combustion. Fig. 2 shows a parabolic graph with an upper maximum of the tendency to form NOX as a function of the oxygen content in the combustion zone. If the oxygen content in the combustion zone is lower than approximately 5 per cent by volume marked

by E, the formation of NOxis repressed because only a small quantity of oxygen is available for the oxidation, and although further combustion will result in a rising temperature, the oxygen content and thus the formation of NOX will fall. At an oxygen content above approximately 5 per cent, the thermal capacity of the excess oxygen starts reducing the combustion temperature, but in this case further combustion will result in a rising temperature and a falling oxygen content and thus vigorous formation of NOX. It should be noted that the percentage E of approximately 5 per cent depends to some extent on the size of the engine, and hence it may vary.

The graph C marks a threshold value for the allowable maximum formation of NOX. In the prior-art cylinder illustrated in Fig. 1, the mixing of combustion gas from the area a to the areas b takes place suitably slowly and with the presence of so much oxygen that the course of the graph on the right hand side of Fig. 2 under the line C is relevant.

Fig. 3 shows an embodiment of a cylinder according to the invention, which cylinder 10 comprises a cylinder liner 12 with scavenge air ports 13 and a cylinder cover 11 with an exhaust valve 17 and fuel injectors 14 supplied with fuel from a fuel system 15 when an actuator unit 16 actuates the fuel system.

The fuel system may, for example, comprise a fuel pump actuated by a cam on a camshaft or by a hydraulic actuator on the basis of a control signal from an electronic control unit when an injection sequence is to be carried out. Alternatively, the fuel system may comprise a source of fuel at a pressure at least as high as the injection pressure, and an electronically or mechanically actuated control valve that can open for the connection between the fuel source and the injector when an injection sequence is to be carried out. The fuel

injectors may be connected individually to the fuel system with a possibility of individual actuation, but preferably all injectors of the cylinder 10 are connected to a joint fuel system that actuates the injectors simultaneously. The fuel is typically heavy fuel oil, but any ordinary type of fuel oil can be used. The different fuel systems are well-known in the art.

When the upper surface of a piston 18 in the cylinder is moved down past the scavenge air ports 13, oxygen-containing charging air flows from a pressurized scavenge air chamber 19 into the combustion chamber 20, the exhaust valve 17 being kept open at the same time. The scavenge air ports 13 are inclined so that the charging air in the combustion chamber performs a strong swirling motion around the longitudinal axis of the cylinder as illustrated by the arrows D in Fig. 4. The charging air swirl continues its motion during the upward compression stroke of the piston and the compression while the exhaust valve is kept closed.

When the piston is near its top dead centre (TDC), the fuel system is actuated to perform an injection sequence. The exact timing of the actuation and the duration expressed in degrees of angular rotation of the crankshaft depend on the current operating mode and load of the engine. As an example, the injection can be started 2 degrees before TDC and continue until 15 degrees after TDC.

In the embodiment shown in Fig. 4, three fuel injectors 14 are mounted evenly spaced in the cylinder cover 11 at a mutual distance of 0.6 x B, B being the cylinder bore. Each fuel injector has four nozzle holes, one hole 23 being directed backwards towards an outer mixing area 21 located behind the injector, the second hole 24 being directed backwards at an angle a towards an inner mixing area 22 located behind the injector, the third hole 25 being directed

forwards towards the outer mixing area 21 located in front of the injector, and the fourth hole 26 being directed forwards at an angle S towards the inner mixing area 22 located in front of the injector.

Forwards means a location on the downstream side of the injector, as seen in relation to the direction of flow of the swirl D, while backwards means a location on the upstream side of the injector. Preferably the three injectors are of the same design.

For a cylinder of a two-stroke crosshead engine having a bore of 90 cm of the type 90MC and the make MAN B&W Diesel, Fig. 5 illustrates a sectional view through the cylinder at the nozzle hole level at four different positions of the engine cycle, viz. 4.9, 6.2,7.4 and 8.9 degrees after TDC. Each fuel injector here has three nozzle holes. Expressed by the angles H, V, H being the horizontal angle between the course of the nozzle hole and the cylinder radius through the nozzle, calculated positively in the direction of the swirl, and V being the vertical angle between the nozzle hole and a plane at right angles to the longitudinal axis of the cylinder, calculated positively downwards, a first backward- facing nozzle hole extends at an angle of H, V =- 66°, 9°, a second forward-facing nozzle hole extends at the angle H, V = 6°, 12°, and a third forward- facing nozzle hole extends at the angle H, V = 49°, 7°. The third forward-facing nozzle hole forms a set with the first backward-facing nozzle hole on the injector located behind the first injector. The second nozzle hole is oriented more inwards towards the centre of the cylinder.

The cross-sections to the left side in Fig. 5 show the fuel vapours of the combustion areas and thus their propagation in the cylinder, and the cross- sections to the right side in the Figure show the oxygen content in the cylinder. The dark tones show higher concentrations of fuel vapour and higher

oxygen content, respectively, and the white tone shows a low concentration of fuel vapours and a low oxygen content, respectively. It is apparent that the fuel jets are mixed already at 8 degrees after TDC and that large parts of the combustion take place at a low oxygen content where the formation of NOX is repressed.

Fig. 6 illustrates the combustion rate, that is, the speed at which the combustion occurs. The solid line shows the combustion rate in a reference cylinder with the known fuel injectors, and the dashed line shows the combustion rate in a cylinder according to the invention, in which the fuel from the injectors 14 is injected from the backward-facing and forward-facing nozzle holes associated in sets into the joint mixing areas 21 with the low oxygen content. It is apparent that the combustion of the fuel takes longer in the cylinder according to the invention because the low oxygen content in the combustion areas has a limiting effect on the combustion rate.

The engine with the cylinder according to the invention may be a two-stroke engine, for example a two-stroke crosshead engine of the applicant's type ME or MC, which are well-known to the worker skilled in the art. The engine may, for example, have a bore in the range from 30 to 120 cm, a number of cylinders in the range from 5 to 20 and as concerns the crosshead engines it may have an rpm in the range from 60 to 300 rpm. The output of the engine may, for example, be in the range from 3,000 to 130, 000 kW.

The location of the mixing areas in the cylinder is controlled by means of the direction of and possibly also in part the diameter ratios between the nozzle holes in the sets. In the design of a concrete engine cylinder, it is completely usual to adapt the number, direction and diameter of the nozzle holes according to the current output required and the

design of the combustion chamber geometry and the location of the fuel injectors. The injectors are typically mounted in the cylinder cover, but may, instead, be mounted in the cylinder liner. There can be, for example, from 2 to 8, typically from 3 to 6 nozzle holes per injector. Conventionally, the direction of the nozzle holes is determined by the need to obtain an advantageous distribution of the temperature load on the cylinder members. With the present invention it is also necessary to take into account that backward-facing and forward-facing nozzle holes have to be directed setwise towards mixing areas.

Setwise means that at least one backward-facing nozzle hole of at least one fuel injector and at least one forward-facing nozzle hole of at least one other injector are included in the set, but the set can also comprise more nozzle holes directed at the same joint mixing area, such as two forward-facing nozzle holes on the second injector, the nozzle holes pointing in almost the same direction, or nozzle holes placed on more than two injectors.

It is possible to open for the nozzle holes with different timing, for example by using fuel injectors with two spindles and by controlling the spindles for opening at different times, which is in principle well-known from dual-fuel injectors.

Details from the various embodiments can be combined into new embodiments.