Patent Publication Number: US-10760480-B2

Title: System and method for reducing NOx emissions for natural gas engines

Description:
TECHNICAL FIELD 
     The present disclosure relates to emission reduction for natural gas engines and in particular to reducing NOx emissions. 
     BACKGROUND 
     To reduce the emissions of NOx (NO and NO 2 ) and to use the natural gas fuel with high efficiency, engine manufacturers developed spark ignited reciprocating internal combustion engines (SI-RICE) that operated with very lean air to fuel ratios. In a lean mixture of air and fuel, there is more air present than necessary to fully combust the natural gas fuel. The presence of the excess air reduces the maximum temperature during the combustion period, which reduces the formation rate for nitrogen oxides (NOx). Nitrogen oxides in the atmosphere, under many conditions, cause the formation of ozone, which is harmful to plant and animal health. As a consequence, NOx emissions, or emission rates, are limited by regulation in many jurisdictions. As air-fuel mixtures become leaner (more excess air) NOx emissions are reduced. However, as the mixtures become leaner, the standard sparkplug is unable to reliably ignite the air-fuel mixtures. 
     To overcome this limitation a small pre-combustion chamber (PCC) is placed in or adjacent to the combustion zone in the engine. The air-fuel mixture in the PCC is controlled separately from the main combustion chamber, such that the mixture is made richer than the main combustion zone mixture. This enables a standard sparkplug to reliably ignite the mixture inside the PCC. As the mixture in the PCC expands, the hot, burning gases enter the main combustion chamber through several holes in the PCC. These burning gases are then able to more readily ignite the lean mixture in the main chamber than the standard sparkplug by itself. These gases expelled from the PCC have a relatively higher NOx concentration compared to the combustion products from the main chamber. The resulting NOx concentration in the exhaust gases comes from the sum of those produced in the PCC and the main combustion chamber. Since the PCC volume is only a few percent of the volume of the main chamber, the resulting NOx concentration is lower than can be achieved without a PCC. 
     Additional apparatus, including a gas admission valve and a separate fuel delivery apparatus such as the check valve and fuel regulator for the PCC as shown in  FIG. 1 , is required to ensure a richer air-fuel mixture in the PCC. The gas admission valve can fail to operate correctly and the PCC fuel delivery control can go out of adjustment, which requires engine adjustment or shutdown and gas admission valve replacement. To eliminate the need for a separate fuel system, PCCs using a standard sparkplug were designed to operate with the main chamber air-fuel mixture. 
     Some manufacturers have increased the compression ratio of engines with PCCs, as the very lean mixtures enable a more-efficient higher compression ratio to be used, compared with engines operated with a richer air-fuel mixture. 
     There are a number of natural gas engines currently in service having PCCs with the separate gas admission valves. In these engines, the additional apparatus required for the PCC design makes the engine more complex to optimize and more subject to malfunction than the open chamber engine with a standard spark plug. Hence, there is a need for a trouble-free implementation for engines with PCCs. For existing engines with PCCs and the associated apparatus, the innovations described in the following description can provide benefits to the users and owners of such engines. 
     Accordingly, systems and methods that enable emission reduction in natural gas engines remain highly desirable. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features and advantages of the present disclosure will become apparent from the following detailed description, taken in combination with the appended drawings, in which: 
         FIG. 1  shows a representation of an engine fuel system with PCC; 
         FIG. 2  shows a representation of an engine head with PCC; 
         FIG. 3  shows a modified PCC engine head with a chambered spark plug; 
         FIG. 4  shows a plug carrier and modified extension; 
         FIG. 5  shows a sectioned view of the plug carrier and modified extension; 
         FIG. 6  shows a representation of an engine management system; 
         FIG. 7  shows a method of ignition timing; 
         FIG. 8  a representation of determining advance or retard determination; and 
         FIG. 9  shows a comparison of NOx emission between a PCC and converted PCC engine. 
     
    
    
     It will be noted that throughout the appended drawings, like features are identified by like reference numerals. 
     DETAILED DESCRIPTION 
     In accordance with an aspect of the present disclosure there is provided a method of modifying a pre-combustion chamber engine to reduce NOx emissions, the method comprising: removing pre-combustion chamber cylinder heads; disabling an admission valve assembly of the pre-combustion chamber engine; inserting a modified cylinder head for receiving a pre-chambered sparkplug in an extension tube; connecting the sparkplug to ignition terminals; connecting a detonation detection system to the engine; and modifying ignition timing of the pre-combustion chamber engine based upon signals received from the detonation detection system. 
     In accordance with an aspect of the method the ignition timing is determined by calculating Output %=I r (FDen)+I r (F s1 )+Max [I r (AMP), I r (AMT), I r (Fuel), I r (Load), I r (RPM), I r (Det)]*[100−I r (FDen)+I r (F s1 )] where I r (AMP) is a calculated ignition retard % for an air manifold pressure; I r (AMT) is a calculated ignition retard % for an air manifold temperature; I r (Fuel) is a calculated ignition retard % for fuel flow; I r (Load) is a calculated ignition retard % for load; I r (RPM) is a calculated ignition retard % for RPM; I r (FDen) is a calculated ignition retard % for density; I r (F s1 ) is a calculated ignition retard % for supplemental gas flow; and I r (Det) is a calculated ignition retard % for detonation. 
     In accordance with an aspect of the method the Output % is a difference between a desired Output % and the Output % according to actual timing value. 
     In accordance with an aspect of the method determining an Output % error wherein the Output % error is a difference between an actual output % and a desired output %. 
     In accordance with an aspect of the method an Output % error is compared to a specified minimum error for retard and a maximum error for advance. 
     In accordance with an aspect of the method if the Output % error becomes too large, then the Output % is retarded at a specified rate until the Output % error drops below a minimum error for retard. 
     In accordance with an aspect of the method the Output % error becomes less than the maximum % error for advance the Output % is advanced. 
     In accordance with an aspect of the method the ignition timing is modified until a stable condition is reached where neither advance nor retard occurs unless one of an operational parameter changes sufficiently. 
     In accordance with an aspect of the method a Waukesha™ VHP GL engine cylinder heads are replaced by a non-PCC cylinder heads from a Waukesha™ VHP GSI engine. 
     In accordance with an aspect of the present disclosure there is provided an engine management system comprising: an input/output (I/O) interface coupled to one or more sensors of a natural gas engine having chambered sparkplugs; a processor coupled to the I/O interface; a memory containing instructions which when executed by the processor for: receiving sensor data from the engine through the I/O interface; receiving a detonation signal from a detonation detection system from the engine; determining an output % from the sensor data; and generating a timing signal to advance or retard ignition timing to adjust the detonation signal to achieve a desired output %. 
     In accordance with an aspect of the engine management system the ignition timing is determined by calculating Output %=I r (FDen)+I r (F s1 )+Max [I r (AMP), I r (AMT), I r (Fuel), I r (Load), I r (RPM), I r (Det)]*[100−I r (FDen)+I r (F s1 )] where I r (AMP) is a calculated ignition retard % for an air manifold pressure; I r (AMT) is a calculated ignition retard % for an air manifold temperature; I r (Fuel) is a calculated ignition retard % for fuel flow; I r (Load) is a calculated ignition retard % for load; I r (RPM) is a calculated ignition retard % for RPM; I r (FDen) is a calculated ignition retard % for density; I r (F s1 ) is a calculated ignition retard % for supplemental gas flow; and I r (Det) is a calculated ignition retard % for detonation. 
     In accordance with an aspect of the engine management system the Output % is a difference between the desired Output % and the Output % according to actual timing value. 
     In accordance with an aspect of the engine management system the determining an Output % error wherein the Output % error is a difference between an actual output % and the desired output %. 
     In accordance with an aspect of the engine management system an Output % error is compared to a specified minimum error for retard and a maximum error for advance. 
     In accordance with an aspect of the engine management system if the Output % error becomes too large, then the Output % is retarded at a specified rate until the Output % error drops below a minimum error for retard. 
     In accordance with an aspect of the engine management system the Output % error becomes less than the maximum % error for advance the Output % is advanced. 
     In accordance with an aspect of the engine management system the ignition timing is modified until a stable condition is reached where neither advance nor retard occurs unless one of an operational parameter changes sufficiently. 
     In accordance with an aspect of the engine management system an engine air-fuel ratio and ignition timing is different compared to that of an original equipment manufacturers recommendations. 
     In accordance with an aspect of the engine management system a Waukesha™ VHP GL engine cylinder heads are replaced by non-PCC cylinder heads from a Waukesha™ VHP GSI engine. 
     In accordance with an aspect of the present disclosure there is provided a method of engine management in a natural gas engine having a chambered sparkplug, the method comprising: receiving sensor data from the engine; receiving a detonation signal from the engine from a detonation detection system; determining an output % from the sensor data and the detonation signal; and generating a timing signal to advance or retard ignition timing to adjust the detonation signal to achieve a desired output %. 
     In accordance with an aspect of the method the ignition timing is determined by calculating Output %=I r (FDen)+I r (F s1 )+Max [I r (AMP), I r (AMT), I r (Fuel), I r (Load), I r (RPM), I r (Det)]*[100−I r (FDen)+I r (F s1 )] where I r (AMP) is a calculated ignition retard % for an air manifold pressure; I r (AMT) is a calculated ignition retard % for an air manifold temperature; I r (Fuel) is a calculated ignition retard % for fuel flow; I r (Load) is a calculated ignition retard % for load; I r (RPM) is a calculated ignition retard % for RPM; I r (FDen) is a calculated ignition retard % for density; I r (F s1 ) is a calculated ignition retard % for supplemental gas flow; and I r (Det) is a calculated ignition retard % for detonation. 
     In accordance with an aspect of the method the Output % is a difference between the desired Output % and the Output % according to actual timing value. 
     In accordance with an aspect of the method determining an Output % error wherein the Output % error is a difference between an actual output % and the desired output %. 
     In accordance with an aspect of the method an Output % error is compared to a specified minimum error for retard and a maximum error for advance. 
     In accordance with an aspect of the method the Output % error becomes too large, then the Output % is retarded at a specified rate until the Output % error drops below a minimum error for retard. 
     In accordance with an aspect of the method the Output % error becomes less than the maximum % error for advance the Output % is advanced. 
     In accordance with an aspect of the method the ignition timing is modified until a stable condition is reached where neither advance nor retard occurs unless one of an operational parameter changes sufficiently. 
     In accordance with an aspect of the method an engine air-fuel ratio and ignition timing is different compared to that of an original equipment manufacturers recommendations. 
     In accordance with an aspect of the method a Waukesha™ VHP GL engine cylinder heads are replaced by non-PCC cylinder heads from a Waukesha™ VHP GSI engine. 
     In accordance with an aspect of the present disclosure there is provided a chambered sparkplug carrier for a natural gas engine, the chambered sparkplug carrier comprising: an circular shaft for receiving a chambered sparkplug within a first end of the shaft, the chambered sparkplug engaged with the shaft by threads at a tip of the first end wherein a combustion chamber of the chambered sparkplug extends beyond the tip of the first end; an internal extension coupling the chambered sparkplug to an ignition system at a second end of the shaft; and wherein the chambered sparkplug carrier is used in a pre-combustion chamber engine to replace the pre-combustion chamber with the chamber sparkplug carrier. 
     In accordance with an aspect of the chambered sparkplug the ignition timing of the engine is modified to be advanced or retarded to achieve a desired output % of the engine. 
     In accordance with an aspect of the chambered sparkplug the ignition timing of the is modified by engine management system by receiving a detonation signal from the natural gas engine. 
     In accordance with an aspect of the chambered sparkplug further comprises outer threads on the circular shaft for engaging the cylinder head. 
     In accordance with an aspect of the chambered sparkplug further comprises bolts for engaging the cylinder head. 
     In accordance with an aspect of the chambered sparkplug wherein a Waukesha™ VHP GL engine cylinder heads are replaced by non-PCC cylinder heads from a Waukesha™ VHP GSI engine. 
     Embodiments are described below, by way of example only, with reference to  FIGS. 1-9 . The schematic for an engine  100  with a pre-combustion chamber (PCC) is shown in  FIG. 1 . Intake air to the engine passes through an air cleaner  102 , a turbo charger  104  and intercooler  106  before entering a carburetor  108 . An engine cylinder  120  receives air-fuel mixture from carburetor  108  where the fuel gas is supplied through a biased fuel pressure regulator  110 . The PCC  122 , shown by a circle at the top of the main combustion chamber  124  of an engine cylinder  120  has small holes to permit the flow of gases into and out of the main combustion chamber  124 . In the operation of a four-stroke cycle engine during the downward intake stroke while a very lean air-fuel mixture enters the main chamber  124  through the open intake valve, fuel to the PCC is supplied through a check valve  114  and a biased fuel pressure regulator  112 . During the compression stroke the very lean air-fuel mixture from the main chamber  124  flows into the PCC  122 . The check valve  114  prevents the previously added fuel from being expelled. A spark plug (not shown) in the PCC ignites the richer air-fuel mixture in the PCC  122 . The ensuing combustion causes the gases to reach a higher pressure than is present in the main chamber  124 . These hot burning gases consequently flow out into the main chamber  124  igniting the very lean air-fuel mixture in the main chamber  124 . In the power stroke force is transmitted to the engine crankshaft (not shown). In the exhaust stroke the exhaust gases from the PCC  122  and the main chamber  124  are expelled to the engine exhaust system through the main chamber  124  exhaust valve. The whole sequence of operation is then repeated.  FIG. 1  is shown for illustrative purposes, it should be understood that there are a variety of possible PCC fuel or air-fuel delivery arrangements. In addition supplementary gas source may be utilized to power the engine. The supplementary gas can be provided by captured vent gas in oil production and storage and used by the engine to reduce vent gas emissions by combusting excess gas. The supplemental gas flow displaces a portion of the main fuel flow requirement of the engine and may be of a different composition than the main fuel source. 
     A cross-section of an engine head showing the PCC and the gas admission valve is shown in  FIG. 2 . The PCC  122  is positioned in the main chamber  124  of the cylinder head  220 . During the intake stroke, when the lean air-fuel mixture is drawn into the main chamber  124 , the PCC admission valve  230  opens to enable some fuel to enter the PCC  122 . During the compression stroke, the PCC admission valve  230  closes and some of the lean air-fuel mixture from the main chamber  124  enters the PCC  122  to mix with the fuel previously admitted. A few degrees before top-dead center (TDC); the spark discharge from the spark plug  222  ignites the mixture in the PCC  122 . The hot, burning gases expand into the main chamber  124 , igniting the lean mixture through openings  226 . During the expansion stroke and subsequent exhaust stroke the gas admission valve  230  remains closed to prevent the hot combustion gases from flowing back to the PCC  122  air-fuel mixing apparatus. 
     Referring to  FIG. 3 , a modified PCC engine cylinder is shown where the PCC is replaced with a chambered sparkplug. To extend the lean limit the chambered sparkplug can be utilized having a small chamber around the normal sparkplug electrodes which serves as a region where conditions for igniting a lean air-fuel mixture are optimized. Such chambered sparkplugs are available commercially from Altronic™ and Multitorch™. Use of these chambered sparkplugs in combination with leaner operation can be effective in reducing NOx emissions from SI-RICEs. A sparkplug carrier can be used to replace the original PCC and spark plug carrier items with a single plug carrier that allows the chambered spark plug to extend into the combustion chamber. In  FIG. 3  the PCC is replaced with the chambered spark plug  322  provided inside the carrier  320  inserted in the PCC cylinder head  220 . The modified spark plug carrier  320  is used to hold the chambered spark plug  322  in place and position the ignition chamber  324  in the main chamber  124 . The chamber  324  of the chamber spark plug  322  is filled through the orifices  326  during the compression stroke with the same lean mixture as that in the main chamber  124 . The volume of the chamber  324  is a much smaller fraction of the combustion chamber than the typical PCCs  122 . The PCC admission valve  230  in the igniter sleeve  210  is bypassed with the installation of the chambered spark plug  322  and spark plug carrier  320 . 
     By replacing the PCC  122  and its sparkplug  222  with a modified sparkplug carrier  320  and a chambered sparkplug  322 , the need for a separate PCC, gas admission valve  230 , and apparatus for the PCC can be eliminated. The sparkplug carrier  320  is shown in  FIG. 4 .  FIG. 5  shows a sectioned view of the plug carrier  320 . The chambered spark plug  322  is engaged to the sparkplug carrier  320  by threads  502 . An extender  530  inside the spark plug carrier  320  connects to the coil ignition terminals or leads. O-ring grooves  510  may be provided on the outside of the plug carrier  320  to create a seal with the cylinder head  220  and the sleeve  210 . Cylinder head threads  512  are provided to retain the sparkplug carrier  320  within the cylinder head  220  is a modified igniter sleeve  210  is utilized with receiving threads. Alternatively the sparkplug carrier  320  may be a bolt-in to replace the PCC and not require cylinder thread heads  512 . 
     Referring to  FIG. 6 , the engine  650  is controlled by an engine managment system (EMS)  610 . The change in combustion conditions between the original engine and the modified engine with the single plug carrier and chamber spark plug requires a modified ignition timing advance and retard algorithm that includes an input from a detonation detection system  660  in addition engine sensors from speed, temperature etc. The detonation detection system  660  can be added as part of the retrofit application of PCC engine to chamber sparkplug carrier system. The EMS  610  changes provides a modified ignition timing advance and retard algorithm. The EMS  610  comprises a processor  612  and memory  614  containing instructions which when executed by the processor are for controlling the ignition of the engine. Input/Output (I/O) interface  616  can receive sensor data or signals from the engine such as detonation detection, air manifold pressure, air manifold temperature, engine load, rotation per minute (RPM), fuel flow rate, and fuel density and provide modified ignition timing to control the modified engine  650  with chambered sparkplugs. 
     For the high compression engine where detonation is more likely compared to the lower compression engine the inclusion of detonation with the other items which may control engine timing is important to avoid premature cylinder failure. Due to the need to have ignition advance and lean operation under load without excessive intake temperatures, the ignition control was integrated with the air control to optimize engine operation with the available turbocharger output. The modified engine ignition control enables starting and warm-up with retarded ignition timing and then slowly advances the ignition timing to a maximum depending on the availability of sufficient cool air. 
     Normally the spark ignition timing is described in degrees before top-dead-centre (TDC). A timing advance means that the spark ignition occurs earlier than the previous reference timing, while a timing retard means the spark ignition occurs later than the previous reference timing. 
     With reference to the method shown in  FIG. 7 , ignition advance operates by receiving sensor data and providing ignition timing for the natural gas engine. The desired ignition retard from the maximum allowed advance is expressed as an output percentage of the difference in degrees between the advance and retard angles. Each parameter for which the timing can be adjusted is included. The formula for the contribution of each parameter to the ignition retard effect is specific to each engine type. The final output percentage is calculated according to the formula below. The output percentage is then converted to an electrical or software value appropriate to control the spark ignition timing.
 
Output %= I   r ( F Den)+ I   r ( F   s1 )+Max[ I   r (AMP), I   r (AMT), I   r (Fuel), I   r (Load), I   r (RPM)]*[100− I   r ( F Den)+ I   r ( F   s1 )]
 
Where I r (AMP) is the calculated ignition retard % for the air manifold pressure ( 720 )
 
     I r (AMT) is the calculated ignition retard % for the air manifold temperature ( 722 ) 
     I r (Fuel) is the calculated ignition retard % for fuel flow ( 724 ) 
     I r (Load) is the calculated ignition retard % for load ( 726 ) 
     I r (RPM) is the calculated ignition retard % for RPM ( 728 ) 
     I r (FDen) is the calculated ignition retard % for density ( 732 ) 
     I r (F s1 ) is the calculated ignition retard % for supplemental gas flow ( 734 ). 
     For a high compression engine detonation, the uncontrolled initiation of combustion ignition, is a known problem. The original detonation system acts on the ignition timing control to retard the ignition timing in a step change. This is incompatible with the previously developed ignition control strategy. For the incorporation of the detonation signal, rates of timing advance and retard were used and incorporated into the above expression for ignition output. A detonation input is added to the above expression as shown to calculate the maximum output percentage ( 702 ). The output % can then be calculated ( 702 ) according to:
 
Output %= I   r ( F Den)+ I   r ( F   s1 )+Max[ I   r (AMP), I   r (AMT), I   r (Fuel), I   r (Load), I   r (RPM), I   r (Det)]*[100− I   r ( F Den)+ I   r ( F   s1 )]
 
     where I r (Det) is the calculated ignition retard % for detonation ( 730 )
     The ignition timing angle is then Ign−I ret *Output %/100   Where Ign is the ignition timing angle BTDC
       I ret  is the maximum ignition retard amount   
       

     Also with reference to  FIG. 8 , an Output % error is calculated  802 , which is the difference between the desired Output % and the Output % according to actual timing value. The Output % error signal  800  is compared to the specified minimum error  804  for retard and a maximum error  806  for advance ( 706 ). If the error signal becomes too large (YES at  708 ), then the Output % is retarded at the specified rate ( 712 ) of retard until the error drops below the minimum error for retard. Similarly, if the error signal becomes less than the maximum error percentage (YES at  710 ) for advance the Output % is advanced ( 714 ). Typically the rate of retard is greater than the rate of advance. Eventually the ignition timing reaches a stable condition where neither advance nor retard occurs unless one of the operational parameters changes sufficiently. 
     Alternatively, a cylinder head design with a PCC can be replaced by a non-PCC head design for the same engine block but with a chambered sparkplug in place of the normal spark plug if the replacement head is suitable for the higher pressure associated with the higher compression ratio. This can remove the need for a modified sparkplug carrier however the ignition timing method of  FIG. 7  is still required. 
     Both designs enable NOx emission concentration limits achievable with the engine heads with PCCs with or without separate fuel delivery systems to be met by the chambered sparkplug either with a modified sparkplug carrier or with a non-PCC head. It is recognized that, to achieve the desired NOx emission limits, the engine air-fuel ratio and ignition timing is different compared to that of the original equipment manufacturers&#39; recommendations. 
       FIG. 9  is a graph  900  showing comparison of NOx emissions where the cylinder heads for a Waukesha™ VHP GL engine (GL NOx emissions), which has a PCC as shown in  FIG. 2 , were replaced by the non-PCC cylinder heads from a Waukesha™ VHP GSI engine and chambered sparkplugs (converted GL NOx emissions). The Waukesha™ VHP GSI heads do not have a PCC or a gas admission valve. For this engine type, carbon moNOxide (CO) and total hydrocarbon (THC) emissions reductions can also be expected because there are no high CO concentrations from the PCC and there are fewer crevice volumes and quenching surfaces with the GSI cylinder head and the chambered sparkplug, compared to the GL cylinder head. 
     The result of the innovative plug carrier and ignition control is that regulated exhaust emissions are either reduced or not increased with a simpler arrangement that is less prone to mal-adjustment or malfunction. Furthermore, if the original engine with the heads modified for PCCs has a modification for a higher compression ratio, the higher compression feature can be retained with the innovation. 
     Each element in the embodiments of the present disclosure may be implemented as hardware, software/program, or any combination thereof. Software codes, either in its entirety or a part thereof, may be stored in a computer readable medium or memory (e.g., as a ROM, for example a non-volatile memory such as flash memory, CD ROM, DVD ROM, Blu-ray™, a semiconductor ROM, USB, or a magnetic recording medium, for example a hard disk). The program may be in the form of source code, object code, a code intermediate source and object code such as partially compiled form, or in any other form. 
     It would be appreciated by one of ordinary skill in the art that the system and components shown in  FIGS. 1-9  may include components not shown in the drawings. For simplicity and clarity of the illustration, elements in the figures are not necessarily to scale, are only schematic and are non-limiting of the elements structures. It will be apparent to persons skilled in the art that a number of variations and modifications can be made without departing from the scope of the invention as defined in the claims.