Abstract:
Presented is a technique that utilizes ion current to determine the concentration of nitrogen oxides (NO x ) produced in the combustion chamber(s) of diesel engines, on a cycle by cycle basis during the combustion of conventional petroleum-based fuels, other alternate fuels, and renewable fuels. The technique uses an ion current measuring means, a calibration means and a signal processing means connected to the engine control unit (ECU). The ion current sensing means is positioned in the chamber(s) of the engine, to measure the ion current produced during the combustion process. The calibration means utilizes NO x  values measured in the exhaust port or manifold of the engine to calibrate the ion current signal. The calibrated ion current signal is fed into a processor that is connected to the ECU to adjust various operating parameters to improve the trade-off between NO x  and other emissions, fuel economy, and power output.

Description:
BACKGROUND 
       [0001]    Diesel engines and other compression ignition engines are used to power light and heavy duty vehicles, locomotives, off-highway equipment, marine vessels and many industrial applications. Government regulations require the engines to meet certain standards for the exhaust emissions in each of these applications. Currently, the emission standards are for the nitrogen oxides NO x , hydrocarbons (HC), carbon monoxide (CO), and particulate matter (PM). Government agencies and industry standard setting groups are reducing the amount of allowed emissions in diesel engines in an effort to reduce pollutants in the environment. The environmental emissions regulations for these engines are becoming more stringent and difficult to meet, particularly for NO x  and PM emissions. To meet this challenge, industry has developed many techniques to control the in-cylinder combustion process in addition to the application of after treatment devices to treat the engine-out exhaust gases and reduce the tail-pipe emissions. The emissions targets for the new production engines are even lower than the regulated emissions standards to account for the anticipated deterioration of the equipment during the life time of the engine after long periods of operation in the field. For example, proposed regulations for new heavy duty engines require additional NO x  and diesel particulate emission reductions of over seventy percent from existing emission limits. These emission reductions represent a continuing challenge to engine design due to the NO x -diesel particulate emission and fuel economy tradeoffs associated with most emission reduction strategies. Emission reductions are also desired for the on and off-highway in-use fleets. 
         [0002]    Traditionally, there have been two primary forms of reciprocating piston or rotary internal combustion engines. These forms are diesel and spark ignition engines. While these engine types have similar architecture and mechanical workings, each has distinct operating properties that are vastly different from each other. The diesel engine controls the start of combustion (SOC) by the timing of fuel injection. A spark ignited engine controls the SOC by the spark timing. As a result, there are important differences in the advantages and disadvantages of diesel and spark-ignited engines. The major advantage that a pre-mixed charge spark-ignited natural gas, or gasoline, engine (such as passenger car gasoline engines and lean burn natural gas engines) has over a diesel engine is the ability to achieve low NO x  and particulate emissions levels. The major advantage that diesel engines have over premixed charge spark ignited engines is higher thermal efficiency. 
         [0003]    One reason for the higher efficiency of diesel engines is the ability to use higher compression ratios than spark ignited engines because the compression ratio in spark ignited engines has to be kept relatively low to avoid knock. Typical diesel engines, however, cannot achieve the very low NO x  and particulate emissions levels that are possible with premixed charge spark ignited engines. Due to the mixing controlled nature of diesel combustion, a large fraction of the fuel exists at a very fuel rich equivalence ratio, which is known to lead to particulate emissions. A second factor is that the combustion in diesel engines occurs when the fuel and air exist at a near stoichiometric equivalence ratio which leads to high temperatures. The high temperatures, in turn, cause higher NO x  emissions. As a result, there is an urgent need to control the combustion process, not only to reduce the engine-out emissions, but also to produce the exhaust gas composition and temperature that would enhance the operation of the after treatment devices and improve their effectiveness. 
         [0004]    The control of the in-cylinder combustion process can be achieved by optimizing the engine design and operating parameters. The engine design parameters include, but are not limited to engine compression ratio, stroke to bore ratio, injection system design, combustion chamber design (e.g., bowl design, reentrance geometry, squish area), intake and exhaust ports design, number of intake and exhaust valves, valve timing, and turbocharger geometry. For any specific engine design, the operating variables can also to be optimized. These variables include, but are not limited to, injection pressure, injection timing, number of injection events, (pilot, main, split-main, post injections or their combinations), injection rate in each event, duration of each event, dwell between the injection events, EGR (exhaust gas recirculation) ratio, EGR cooling, swirl ratio and turbocharger operating parameters. 
         [0005]    Many types of after treatment devices have been developed, or are still under development to reduce the engine-out emissions such as NO x  and PM in diesel engines. The effectiveness of each of the after treatment devices depends primarily on exhaust gas properties such as temperature and composition including the ratio between the different species such as NO x , hydrocarbons and carbon (soot). Here, also, the properties of the exhaust gases depend primarily on the combustion process. 
         [0006]    The precise control of the combustion process in diesel engines requires a feed back signal indicative of the combustion process. Currently, the most commonly considered signal is the cylinder gas pressure, measured by a quartz crystal pressure transducer, or other types of pressure transducers. The use of the cylinder pressure transducers is limited to laboratory settings and can not be used in the production engine because of its high cost and limited durability under actual operating conditions. 
       BRIEF SUMMARY 
       [0007]    Described herein is, among other things, an inexpensive direct indicator of NO x  in the cylinder of compression ignition engines during the combustion process, which requires no or just minor modifications in the cylinder head and gives a signal that can be used to control the combustion process and engine-out exhaust gases, particularly NO x , in diesel engines and the like. 
         [0008]    In an embodiment, NO x  emissions formed in a combustion chamber of a compression ignition engine is determined by receiving an ion current signal indicating the concentration of ions in the combustion chamber and determining the NO x  emissions based upon a derived relationship between the ion current signal and the NO x  emissions. The engine may be controlled based in part upon the derived NO x  emissions. 
         [0009]    The relationship is derived by receiving an ion current signal from an ion current sensor and NO x  exhaust emissions data obtained from NO x  emissions measuring equipment, comparing the ion current signal to the NO x  emissions data, and fitting a function through the NO x  emissions data and ion current data. This may be accomplished by creating a plot of the NO x  emissions versus ion current magnitude and fitting a function through the plot. In one embodiment, the function is a volume fraction of NO x  per unit of ion current. 
         [0010]    The relationship between the NO x  emissions and ion current is derived for each chamber of the compression ignition engine in one embodiment. This is accomplished by receiving an ion current signal indicating the concentration of ions in each of the cylinders and NO x  emissions data and deriving the relationship that is, in one embodiment, a volume fraction of NO x  per unit of ion current flowing in the one of the plurality of cylinders. Other functions may be derived for the relationship. For each cylinder, parameters for fuel injection, EGR (exhaust gas recirculation) rate and others are adjusted based upon the derived NO x  emissions in the cylinder indicated by the ion current. 
         [0011]    Additional features and advantages will be made apparent from the following detailed description of illustrative embodiments, which proceeds with reference to the accompanying figures. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The accompanying drawings incorporated in and forming a part of the specification illustrate several aspects of the technologies described herein, and together with the description serve to explain the principles of the technologies. In the drawings: 
           [0013]      FIG. 1  is a schematic view of a representative environment in which the techniques may operate; 
           [0014]      FIG. 2  is a block diagram view of an ionization module in which the techniques may be incorporated within; 
           [0015]      FIG. 3  is a graphical illustration of combustion pressure and ionization current versus engine piston crank angle; 
           [0016]      FIG. 4  is a graph illustrating an example of a plot of the relationship between NO x  emissions, plotted as volume fraction in parts per million, and ion current; 
           [0017]      FIG. 5  is a flowchart illustrating the steps performed to derive the relationship between NO x  emissions and ion current; 
           [0018]      FIG. 6  is a block diagram schematic illustrating an embodiment of the components used to derive the relationship between NO x  emissions and ion current; 
           [0019]      FIG. 7  is a flowchart illustrating the steps performed to determine NO x  emissions based upon an ion signal during engine operation; 
           [0020]      FIG. 8  is a block diagram schematic illustrating an embodiment of components used to control an engine based upon ion current and engine operating parameters; and 
           [0021]      FIG. 9  is a block diagram schematic illustrating an embodiment of components used to calibrate ion current versus NO x  emissions independently in each cylinder and control each cylinder independently. 
       
    
    
       [0022]    While the techniques will be described in connection with certain embodiments, there is no intent to limit it to those embodiments. On the contrary, the intent is to cover all alternatives, modifications and equivalents as included within the spirit and scope of the invention as defined by the appended claims. 
       DETAILED DESCRIPTION 
       [0023]    The apparatus and method described herein determines NO x  emissions based upon the ion current produced during the compression process in compression ignition engines of different designs while running on conventional, alternate, or renewable diesel fuel without requiring the use of an in-cylinder NO x  sensor or NO x  measurement in the exhaust. 
         [0024]    Referring initially to  FIG. 1 , a exemplary system  100  in which the present apparatus and method operates is shown. The system includes an ionization module  102 , a driver  104 , an engine electronic control unit (ECU)  106 , and a diesel engine. The ionization module  102  communicates with the ECU  106  and other modules via, for example, the CAN (Controller Area Network) bus  108 . While the ionization module  102 , the driver  104  and the engine control unit  106  are shown separately, it is recognized that the components  102 ,  104 ,  106  may be combined into a single module or be part of an engine controller having other inputs and outputs. The components  102  and  106  typically include a variety of computer readable media. Computer readable media can be any available media that can be accessed by the components  102 ,  106  and includes both volatile and nonvolatile media, removable and non-removable media. The diesel engine includes engine cylinders  110 , each of which has a piston, an intake valve and an exhaust valve (not shown). An intake manifold is in communication with the cylinder  110  through the intake valve. An exhaust manifold receives exhaust gases from the cylinder via an exhaust valve. The intake valve and exhaust valve may be electronically, mechanically, hydraulically, or pneumatically controlled or controlled via a camshaft. A fuel injector  112  injects fuel  116  into the cylinder  110  via nozzle  114 . The fuel may be conventional petroleum based fuel, petroleum based alternate fuels, renewable fuels, or any combination of the above fuels. An ion sensing apparatus  118  is used to sense ion current and may also be used to ignite the air/fuel mixture in the combustion chamber  120  of the cylinder  110  during cold starts. Alternatively, a glow plug can be used to warm up the cylinder to improve the cold start characteristics of the engine and sense ion current. 
         [0025]    The ion sensing apparatus  118  has two electrodes, electrically insulated, spaced apart and exposed to the combustion products inside the cylinder of diesel engines. It can be in the form of a spark plug with a central electrode and one or more side electrodes that are spaced apart, a glow plug insulated from the engine body where each of the glow plug and engine body acts as an electrode, a combined plasma generator and ion sensor, etc. The ion sensing apparatus  118  receives an electric voltage provided by driver  104  between the two electrodes, which causes a current to flow between the two electrodes in the presence of nitrogen oxides and other combustion products that are between the two electrodes. The driver  104  provides power to the ion sensing apparatus  118 . The driver  104  may also provide a high energy discharge to keep the ion sensing detection area of the ion sensing apparatus clean from fuel contamination and carbon buildup. While shown separate from the fuel injector  112 , the ion sensing apparatus  118  may be integrated with the fuel injector  112 . 
         [0026]    The ionization module contains circuitry for detecting and analyzing the ionization signal. In the illustrated embodiment, as shown in  FIG. 2 , the ionization module  102  includes an ionization signal detection module  130 , an ionization signal analyzer  132 , and an ionization signal control module  134 . In order to detect concentration of ions in a cylinder, the ionization module  102  supplies power to the ion sensing apparatus  118  and measures ionization current from ion sensing apparatus  118  via ionization signal detection module  130 . Ionization signal analyzer  132  receives the ionization signal from ionization signal detection module  130  and determines the different combustion parameters such as start of combustion and combustion duration. The ionization signal control module  134  controls ionization signal analyzer  132  and ionization signal detection module  130 . The ionization signal control module  134  provides an indication to the engine ECU  106  as described below. In one embodiment, the ionization module  102  sends the indication to other modules in the engine system. While the ionization signal detection module  130 , the ionization signal analyzer  132 , and the ionization signal control module  134  are shown separately, it is recognized that they may be combined into a single module and/or be part of an engine controller having other inputs and outputs. Returning now to  FIG. 1 , the ECU  106  receives feedback from the ionization module and controls fuel injection  112 , and may control other systems such as the air delivery system and EGR system, to achieve improved engine performance, better fuel economy, and/or low exhaust emissions. 
         [0027]    The ion current signal can be correlated to the level of NO x  emission and in-cylinder pressure produced during combustion. Turning now to  FIG. 3 , a sample of the ion current and the gas pressure measured in one of the cylinders of a 4-cylinder, 2L, direct injection turbocharged diesel engine is shown. The operating conditions are 75 Nm torque, 1600 rpm, 40% EGR, and a dialed injection timing of 13° bTDC (before top dead center). The ion current trace  140  shows two peaks that cannot be explained by the findings in spark ignition engines, where the first peak is caused by chemi-ionization in the flame front, which is not the case in diesel engines, and the second peak is caused by thermal ionization. The gas pressure trace  142  shows clearly that autoignition started with a cool flame that caused a slight increase in the cylinder gas pressure. The energy released by the cool flame is known to be fairly small and causes a slight increase in the combustion gas temperature. The ions generated during this period are expected to be fairly low in concentration. At the end of the cool flame, the ion current starts to increase sharply at approximately a half crank angle degree bTDC (point  144 ). 
         [0028]    In the sample shown, the ion current reaches a peak (point  146 ) after 3 CAD (crank angle degree) from its starting point. Up to this point, combustion occurs in the premixed combustion fraction of the charge. The amount of the charge that is burnt during this period and the corresponding rise in temperature depend on many factors including the total lengths of the ignition delay and the cool flame periods, the rate of fuel injection, and the rates of fuel evaporation and mixing with the fresh oxygen in the charge. The ion current reaches a fairly high peak in about three crank angle degrees, or about 0.3 ms, after which it dropped, reached a bottom value (point  148 ), started to increase again at a slower rate and reached a second peak (point  150 ) at 10° aTDC (after top dead center). This indicates that the rate of formation of the ions leading to the second peak is much slower than that for the first peak. The slower rate of formation of ions leading to the second peak can be attributed to the slower rate of mixing of the unburned fuel with the rest of the charge, the drop in temperature of the combustion products caused by the piston motion in the expansion stroke, and to the increase in the cooling losses to the cylinder walls. Since the ionization in the second peak follows the same characteristics as the mixing-controlled and diffusion-controlled combustion fractions, it is reasonable to consider that it is caused by this combustion regime. Here the ionization is caused by a combination of the chemi-ionization and the thermal ionization. Following the second peak, the ionization signal decreases at a slow rate, caused by the gradual drop in the gas temperature during the expansion stroke. In this figure, the ionization was detected during about 30 to 40 crank angle degrees. 
         [0029]    The rates of formation of both the ions and NO x  depend on many engine design parameters and the properties of the fuel used to run the engine. The design parameters may vary from one engine to another and include, but are not limited to, the following: compression ratio, bore to stroke ratio, surface to volume ratio of the combustion chamber, inlet and exhaust ports and valves design, valve timing, combustion chamber design, injection system design parameters and cooling system design parameters. The injection systems parameters include, but are not limited to, injection pressure, nozzle geometry, intrusion in the combustion chamber, number of nozzle holes, their size, and shape and included spray angle. The important fuel properties that affect the combustion process, NO x  formation and ion current include hydrogen to carbon ratio, distillation range, volatility and cetane number. As a result, variations in the design parameters from one engine to another and in the fuel properties affect the cylinder gas temperature and pressure, mixture formation, and the distribution of the equivalence ratio in the combustion chamber, all of which affect the formation of ions and NO x . 
         [0030]    From the foregoing, it can be seen that ion current can be used to determine NO x . It can also be seen that the ion current signal should be calibrated with respect to NO x  emissions in each engine make and type and for each of the fuel types used. Turning now to  FIG. 4 , a sample of the calibration of an ion current signal in a multi-cylinder engine is shown.  FIG. 4  is a plot of NO x  engine-out emissions (volume fraction in parts per million) versus the summation of the peaks of the ion currents measured in the four cylinders at 1600 rpm, under a wide range of operating conditions: EGR: 40%, 45%, 50% and 55%; Torque: 25 Nm, 50 Nm and 75 Nm; and injection timing that was varied between 11° bTDC and 25° bTDC, depending on the load and EGR percentage. It can be clearly seen from the plot that there is a relationship between the magnitude of the ion current peaks and the level of NO x  emissions. 
         [0031]    Turning now to  FIG. 5 , the steps to determine the relationship between the magnitude of the ion current peaks and the level of NO x  emissions is shown. The ion current signal is received from an ion current sensor (step  160 ). The NO x  engine out emissions is received from NO x  standard emissions measuring equipment (step  162 ). The NO x  emissions data and ion current signal are compared (step  164 ) and the relationship between NO x  emissions and ion current is derived (step  166 ). The relationship can be derived by plotting the NO x  emissions versus ion current magnitude and fitting a function through the data. The function may be a linear line, a piecewise linear line, a polynomial function, an exponential function, etc. The relationship is transmitted to the appropriate control modules (step  168 ), such as the ionization module  104 , the ECU  106 , etc. 
         [0032]      FIG. 6  shows one implementation of calibrating the ion current signal. During operation of the engine  200 , the NO x  emission measuring instrument  202  draws a sample of the exhaust gases from exhaust manifold  204  through a sampling probe  206  and determines the NO x  emissions and displays it on optional display unit  208 . In one embodiment, the NO x  emissions are determined in volume fraction in ppm (parts per million). The NO x  emissions measuring instrument  202  sends the NO x  data to the calibration module  210 . For purposes of illustration, the calibration module  210  is shown as a separate component. The calibration module may be an independent module, part of the ionization module  102 , or part of the ECU  106 . The ion current signal  212  is produced by the ion probe, with its electrodes exposed to the combustion products in the combustion chamber  120  of the engine. The calibration module  210  receives the ion current signal  212  and a signal from the emissions measuring unit that measure the volume fraction of NO x  in the exhaust of the cylinder. The calibration module  210  calibrates the ion current signal  212  with respect to the NO x . Once the ion signal is calibrated at one operating condition, it can be used over the whole range of engine speeds, loads, and operating modes. The output of the calibration module  210  gives the relationship between NO x  and ion current (e.g., volume fraction of NO x  in ppm per unit and ion current), which is fed into the ECU  106  and is used in the control of the engine. The calibration module may also feed the output to other modules within the operating environment. 
         [0033]    Turning now to  FIGS. 7 and 8 , during operation, the ECU  106  receives the ion current signal (step  220 ), analyzes the ion current signal and determines the key combustion parameters such as the start of combustion, rate of heat release, maximum rate of heat release due the premixed combustion fraction, the minimum rate of heat release between the premixed combustion fraction and the mixing and diffusion controlled combustion fraction, the maximum rate of heat release due the mixing and diffusion controlled combustion fraction, and the rate of decay of the heat release during the expansion stroke. Based on this information, the ECU  106  is programmed to develop signals to the different actuators and control all the systems in the engine. The ECU  106 , via the calibration module  210 , determines the NO x  emissions based upon the derived relationship (step  222 ), and in conjunction with engine operating parameters  220 , controls operation of the engine  200  (step  224 ). The ECU  106  may control the engine to minimize NO x  emissions, improve the trade-off between NO x  and other emissions such as particulate matter, carbon monoxide, hydrocarbons, and aldehydes The ECU  106  may also use the calibrated signal to control the engine parameters and increase the engine power output and improve its efficiency. The ion current signal  212  can be from one cylinder or, alternatively, from the sum of the ion currents from all the cylinders in a multi-cylinder engine. In one embodiment, an exhaust sampling probe  206  is placed in the manifold of one of the cylinders or, alternatively, in the location where all the exhaust gases from the cylinders meet. The calibration module  210  can be used to update the NO x  emissions—ion current relationship as the engine changes over time, as new components are added, etc. 
         [0034]    Turning now to  FIG. 9 , the ECU  106  may control each cylinder of an engine  200  separately. The ion signal  212   x  from each cylinder is calibrated by calibration module  210   x  (where x indicates the cylinder number) and fed into the ECU  106  that controls the parameters for each of the cylinders independently of the other cylinders. The ECU  106  uses the calibration module output to determine the NO x  in the corresponding engine cylinder (e.g., cylinder  1 , cylinder  2 , etc.) and in conjunction with each cylinder&#39;s operating parameters  240   x , controls operation of the specific cylinder. While x number of calibration modules are shown for clarity, the calibration modules may be in a single calibration module, part of the ionization module, part of the ECU  106 , etc. The ECU  106  may control each cylinder to minimize NO x  emissions, improve the trade-off between NO x  and other emissions such as particulate matter, carbon monoxide, hydrocarbons, and aldehydes for each cylinder. The ECU  106  may control the whole engine to minimize NO x  emissions, improve the trade-off between NO x  and other emissions such as particulate matter, carbon monoxide, hydrocarbons, and aldehydes of the whole engine. For example, the output of the cylinders in a multi-cylinder diesel engine can be balanced by adjusting the fuel injection parameters in each cylinder. Such balancing improves the load distribution among the cylinders and improves the operation, fuel economy and engine emissions of the whole engine. 
         [0035]    From the foregoing, it can be seen that a relationship between NO x  emissions and ion current magnitudes can be determined and used in the control of diesel engines. The ion current is compared to measured NO x  emissions to determine the relationship. The relationship is then used during operation by determining NO x  emissions from the measured ion current. 
         [0036]    The use of the terms “a” and “an” and “the” and similar referents in the context of describing the invention (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention. 
         [0037]    Preferred embodiments of this invention are described herein, including the best mode known to the inventor for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventor expects skilled artisans to employ such variations as appropriate, and the inventor intends for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.