The present invention relates to a device for controlling the combustion process in a combustion engine. The invention relates to such a device for reducing especially soot emissions but also carbon monoxide and hydrocarbon in combustion engines in which the fuel/cylinder gas mixture is ignited by compression heat generated in the cylinder.
Soot particles (or particulates) are a product which, during combustion, can both be formed and subsequently oxidized into carbon dioxide (C02). The quantity of soot particles measured in the exhaust gases is the net difference between formed soot and oxidized soot. The process is very complicated. Combustion with fuel-rich, fuel/air mixture with poor mixing at high temperature produces high soot formation. If the formed soot particles can be brought together with oxidizing substances such as oxygen atoms (0), oxygen molecules (02), hydroxide (OH) at sufficiently high temperature for a good oxidation rate, then a greater part of the soot particles can be oxidized. In a diesel engine, the oxidation process is considered to be in the same order of magnitude as the formation, which means that net soot production is the difference between formed quantity of soot and oxidized quantity of soot. The net emission of soot can therefore be influenced firstly by reducing the formation of soot and secondly by increasing the oxidation of soot. Carbon monoxide emissions (CO) and hydrocarbon emissions (HC) are normally very low from a diesel engine. Yet the percentages can rise if unburnt fuel ends up in relatively cool regions or stagnation regions. Regions with intense cooling can be located close to the cylinder wall. An example of where a stagnation zone can arise is where two progressing flame plumes collide or where one progressing flame plume collides with a piston wall.
Nitrogen oxides (NOx) are formed from the nitrogen content in the air in a thermal process which has a strong temperature dependency and depends on the size of the heated-up volume and the duration of the process.
A combustion process in which the fuel is injected directly into the cylinder and is ignited by increased temperature and pressure in the cylinder is generally referred to as the diesel process. When the fuel is ignited in the cylinder, combustion gases present in the cylinder undergo turbulent mixing with the burning fuel, so that a mixture-controlled diffusion flame is formed. The combustion of the fuel/gas mixture in the cylinder gives rise to heat generation, which causes the gas in the cylinder to expand and which hence causes the piston to move in the cylinder. Depending on a number of parameters, such as the injection pressure of the fuel, the quantity of exhaust gases recirculated to the cylinder, the time of injection of the fuel and the turbulence prevailing in the cylinder, different efficiency and engine emission values are obtained.
Below follows an example of state of the art arrangement attempting to lower both soot and NOx-emissions by controlling the flame, and trying to brake the well known “trade off” between soot emissions and nitrogen oxide emissions, which is typical of the diesel engine, and which “trade-off” is difficult to influence. The majority of measures which reduce soot emissions increase the nitrogen oxide emissions.
WO2009/058055 discloses as combustion engine with a combustion chamber comprising a piston, an injector with plurality of orifices arranged to inject spray/flame plumes, which impinge on a piston outer bowl section during, most of the injection. Between spray/flame plume impingement areas and in a plane substantially perpendicular to reciprocal piston movement are arranged a first type of protrusions protruding into the combustion chamber, having a smooth form for preserving kinetic energy in the flame and for redirecting circumferential flame progress mainly towards a center axis of the piston with minimal flame-to-flame interaction. A second type of protrusions are arranged in the impingement areas, being adapted for redirecting flame progress into a circumferential flame progress direction in a plane substantially perpendicular to said reciprocal piston movement and with minimal flame-to-piston wall interaction and minimal kinetic energy loss.
Said first type of protrusions work well regarding minimizing kinetic energy loss in the flame plume, but the form of the first type of protrusions according to prior art is not optimal when it comes to controlling the progress of the flame plume.
Thus, there is a need to achieve a more optimal form of said first type of protrusions (below called only “protrusion”).
It is, therefore, desirable to overcome the deficiencies of the prior art and to provide an internal combustion engine containing a combustion chamber arrangement designed to further optimize the progress of the flame plume. This is done by identifying a stagnation zone where flow losses occur at the top of a protrusion and by giving the protrusion a shape that minimizes said stagnation zone.
It is also desirable to increase the robustness in the control of the flame plume main flow. The increased robustness is due to decreased sensitivity of the position where the main flow of the flame plume separates from the protrusion when said flame plume continues its progress towards the center of the combustion chamber.
It is also desirable to pr mote further after oxidation of remaining soot. The soot reduction is especially important for fuels such as for example diesel. The invention further contributes, according to an aspect thereof, to the reduction of carbon monoxide (CO) emissions and hydrocarbon (HC) emissions. The reduction of CO and HC becomes especially important for fuels such as for example DME (dimethyl ether). A bigger amount of the available kinetic energy in the flame plume can be used in a useful way for increased oxidation of remaining fuel. In this way the duration of the combustion process will be shorter, which results in a decreased fuel consumption.
It is also desirable to increase efficiency. The design of the protrusion in the combustion chamber according to an aspect of the present invention contributes to a faster combustion process and thereby resulting in increased efficiency.
Known effects when using, for example, an increased amount of Exhaust Gas Recirculation can at least partly be compensated for by aspects of the present invention.
According to an aspect of the invention, a piston is arranged for reciprocal movement in a combustion engine cylinder between a bottom dead center position and a top dead center position. Said piston including a piston crown comprising an upper surface facing a combustion chamber. Said piston crown containing a piston bowl formed by an outwardly opening cavity, and where said piston bowl comprising an outwardly flared outer bowl section having a concave curvilinear shape in cross section, at least one intake port arranged to provide the combustion chamber with intake air, an injector arranged to inject fuel into the combustion chamber from a position adjacent a geometrical centre of said combustion chamber and having an impingement area of a progressing flame plume in said outer bowl section, and where substantially half way between said impingement areas and in a plane substantially perpendicular to said reciprocal movement are arranged protrusions protruding into the combustion chamber and having a smooth form adapted for preserving kinetic energy in a flame plume and where each of said protrusions has a shape of a longitudinal ridge that extends only in the outer bowl area in a plane substantially parallel to or dose to being parallel to said reciprocal movement, and where said ridge comprising a left side flank, a top section and a right side flank when seen in a plane perpendicular to said reciprocal movement, and where a transition section is arranged between each of said side flanks and the top section. Said piston is characterized in that said transition section comprising a deflection edge in order to minimize flow losses.
According to one embodiment of the invention each of said side flanks having a side surface and said top section having a top surface, and where an separation angle between a side surface tangent and a top surface tangent in said transition section, where each of said side surfaces meet said top surface and forming said deflection edge, is between 90 and 160 degrees. In a further embodiment said separation angle is between 120 and 150 degrees.
According to a further embodiment of the invention a deflection angle is defined as the angle between a line parallel to a centerline of the protrusion and said side surface tangent. In a further embodiment said deflection angle (88) is between −30 and +30 degrees. Different values of the deflection angle can be combined with different values of the separation angle.
In another embodiment of the invention each of said side flanks having a concave curvilinear shape with a first radius and said top section having a convex curvilinear shape with a second radius when seen in a plane perpendicular to said reciprocal movement.
In a further embodiment of the invention said deflection edge is arranged along the whole extension of the ridge.
In another embodiment of the invention said protrusion are arranged for redirecting circumferential flame progress mainly towards a center axis of the piston with minimal flame-to-flame interaction.