Internal-combustion engines – Combustion chamber means having fuel injection only – Using multiple injectors or injections
Reexamination Certificate
2002-08-19
2004-11-23
Argenbright, Tony M. (Department: 3747)
Internal-combustion engines
Combustion chamber means having fuel injection only
Using multiple injectors or injections
C123S294000, C123S301000, C123S568110, C123S661000
Reexamination Certificate
active
06820587
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for controlling the combustion process in a combustion engine. The invention especially relates to such a method for reducing soot emissions and nitrogen oxide emissions (NOx) formed in combustion engines in which the fuel/cylinder gas mixture is ignited by compression heat generated in the cylinder.
BACKGROUND AND PRIOR ART
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.
Soot particles are a product which, during combustion, can both be formed and subsequently oxidized into carbon dioxide (CO
2
). 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-heavy, i.e. 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 (O), oxygen molecules (O
2
), 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. Such regions are, in particular, zones with intense cooling located close to the cylinder wall. Another example is cavities between piston and cylinder lining.
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.
Conventional combustion engines which work according to the diesel process exhibit relatively high values in terms of discharged emissions, such as nitrogen oxides and soot particles.
It is previously known to reduce the soot particle formation by injecting the fuel early in or prior to the expansion stroke, whilst, at the same time, an ignition delay is sought, so that the fuel has time to be vaporized and be mixed before the fuel is ignited with gases present in the cylinder. There are therefore methods for reducing the content of emissions which are given off from a conventional engine.
In order further to reduce the soot emissions specifically, a known method has been proposed, which entails the fuel being injected directly into the combustion chamber under relatively high injection pressure (up to 2000 bar has been tested) by means of injection devices disposed in the combustion chamber. The high injection pressure results in a high flow rate for the fuel relative to the injection device and the cylinder gas. The high flow rate of the fuel supplies energy to the mixing process between fuel and cylinder gas, which leads to a high mixing rate between these. When the mixing rate is sufficiently high, the chemical reactions between fuel and the cylinder gas which lead to combustion do not have time to occur, so that the combustion occurs farther into the combustion chamber. A large so-called “lift-off” is obtained, i.e. a relatively large distance between the mouth of the injection device and the place downstream in the spray at which the fuel/cylinder gas mixture reacts. The large distance offers the opportunity for more cylinder gas and hence oxygen to be sucked in to the central parts of the spray. The result of the combustion occurring farther into the combustion chamber is that fuel and cylinder gas are mixed to a higher degree prior to combustion. When the mixing rate and the degree of mixing are sufficient, the combustion of the fuel/cylinder gas mixture is realized with a sufficient quantity of oxygen to reduce soot particle formation in the spray. At lower flow rates of the fuel, the oxygen in the cylinder gas is not mixed sufficiently well with the fuel prior to combustion, with the result that much of the combustion is realized with considerable oxygen deficiency. This creates large quantities of soot particles. According to this method, the mixing process between the fuel and the cylinder gas is primarily realized locally in the spray. The drawback with this method is principally that the nitrogen oxide emissions are not sufficiently low, but also the soot emissions can be somewhat too high. Without Exhaust Gas Recirculation, there is a lower limit for the minimum level of nitrogen oxide emissions which are obtainable. Lower nitrogen oxide emissions are most commonly achieved by postponing the time for the start of the injection during a combustion cycle. The later after the upper dead centre of the piston the combustion occurs, the lower the compression temperature. In such a case, combustion is realized at a lower temperature, thereby producing lower nitrogen oxide emissions. However, the postponed start of the injection influences the time available for completion of the combustion. Time available for soot oxidation is reduced for the same reasons. The result is that the soot emissions from the engine increase more and more the later that combustion occurs. This therefore limits the minimum level of nitrogen oxide emissions which can practically be reached without Exhaust Gas Recirculation.
The majority of measures which reduce soot emissions increase the nitrogen oxide emissions. A “trade-off” is talked of, which is typical of the diesel engine, between soot emissions and nitrogen oxide emissions, which “trade-off” is difficult to influence.
In order further to lower the emissions, a known method has been proposed which involves the fuel being injected at low pressure into the charge-air system of the engine during a certain time window, for example during the early part of the induction stroke. During the induction stroke, a large quantity of exhaust gases is also recirculated to the cylinder, this in order to cool down the combustion process to prevent nitrogen oxides from being formed. The formation of nitrogen oxides occurs at high combustion temperatures. The recirculated exhaust gases reduce the concentration of oxygen in the combustion chamber, due to the fact that a considerable proportion of the combustion chamber is taken up by re-introduced exhaust gases. A lesser quantity of oxygen leads to a cooler combustion process, but, at the same time, also to the formation of more soot emissions due to greater risk of local oxygen deficiency. The remaining quantity of oxygen must therefore be more effectively utilized in the reactions with the fuel. The solution to this is to supply more mixing energy, so that the oxygen “hits upon” the fuel. This is done by making the recirculated exhaust gases and newly supplied air flow in a vortex motion, which is generally referred to as swirl, so that an essentially homogeneous mixture of fuel and cylinder gas is formed in the cylinder. When the piston then approaches the upper dead centre position, the homogeneous fu
Eismark Jan
Hoglund Anders
Magnusson Ingemar
Sarnbratt Ulla
AB Volvo
Argenbright Tony M.
Young & Thompson
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