Abstract:
The speed of a turbocharger may be estimated using data from sensors that are readily available in most engine management systems. In some cases, a pressure measurement from a MAP sensor may be used, in combination with one or more computational models, to provide an efficient, lower cost estimate of turbo speed that can be used to control operation of the engine and/or the turbocharger.

Description:
TECHNICAL FIELD 
       [0001]    The present disclosure pertains generally to automotive engine controls, and more particularly pertains to estimating the rotation speed of a turbocharger of the engine. 
       BACKGROUND 
       [0002]    The speed at which a turbocharger rotates is an important parameter in controlling operation of turbocharged engines. The maximum turbo speed may define a maximum achievable air flow, which in turn defines a maximum achievable torque of the engine. Exceeding the maximum turbo speed can lead to mechanical damage to the turbocharger and can ultimately lead to engine failure. A goal in controlling operation of some turbocharged engines, therefore, is the ability to achieve a high turbo speed without exceeding the maximum allowed speed. 
         [0003]    In some cases, the turbo speed may be directly measured by a dedicated speed sensor that may, for example, utilize blade counting or the like. This tends to be an expensive solution. In some cases, a less expensive alternative is to estimate the turbo speed using inputs from other sensors utilized in engine management. For example, the turbo speed may be estimated using mass air flow, compressor inlet pressure, compressor outlet pressure and compressor inlet temperature. In some cases, these values may be related via a compressor map in which the turbo speed may simply be looked up once the other values are measured and known. 
         [0004]    A shortcoming of this estimation method is that a compressor outlet pressure sensor must be placed at the output of the turbo&#39;s compressor. Most contemporary engine designs do not include a compressor outlet pressure sensor. Thus, the addition of the compressor outlet pressure sensor may increase the cost of the engine management system, and may even reduce the reliability of the system since there is an additional failure mechanism. 
       SUMMARY 
       [0005]    Modern vehicles often include a Manifold Absolute Pressure (MAP) sensor for sensing the pressure at the intake manifold of the engine. The MAP sensor is often used to control the operation of the engine. The MAP sensor is typically located at the intake manifold of the engine, and for engines with a turbocharger, the MAP sensor is typically located downstream of the turbocharger&#39;s compressor outlet, and in most cases downstream of the engine&#39;s throttle valve. In some cases, a charge air cooler is disposed between the compressor outlet and the throttle valve. The charge air cooler, when provided, may cool the compressed air provided by the compressor in order to provide a more dense intake charge into the engine, which can increase the power output of the engine. The present disclosure relates to techniques for estimating the turbo speed of a turbocharger using data from a pressure sensor that is located downstream of the throttle valve, such as the MAP sensor, instead of a compressor outlet pressure sensor that is located at the output of the compressor. This approach may reduce the cost and increase the reliability of the engine management system. 
         [0006]    In a particular example of the present disclosure, a controller may be used to estimate an operating speed of a turbocharger that has a compressor with an air inlet and an air outlet. The compressor feeds compressed air to a downstream throttle that provides a throttled air flow to an air intake manifold of an engine. The controller may include an inlet port, an outlet port and a processor that is operatively coupled to the inlet port and the outlet port. In some cases, the inlet port may receive one or more of a compressor air flow signal that represents a measure of air flow through the compressor, a compressor air inlet pressure signal that represents a measure of pressure at the air inlet of the compressor, an intake manifold pressure signal that represents a measure of pressure at the air intake manifold of the engine, a temperature signal that represents the temperature of the air at the throttle, and a throttle signal that represents the throttle position of the throttle. In some cases, the controller may include a memory that stores instructions that are executable by the processor to estimate the operating speed of the turbocharger based at least in part on the values received by the inlet port. In some cases, the processor may provide one or more control signals via the output port to control an operation of the turbocharger and/or the engine in response to the estimated operating speed of the turbocharger. 
         [0007]    In another example of the present disclosure, a controller may be used to estimate an operating speed of a turbocharger that has a compressor with an air inlet and an air outlet. The compressor feeds compressed air to a downstream throttle that provides a throttled air flow to an air intake manifold of an engine. The controller may include an inlet port, an outlet port and a processor that is operatively coupled to the inlet port and the outlet port. In some cases, the inlet port may receive one or more of a compressor air flow signal that represents a measure of air flow through the compressor, a compressor air inlet pressure signal that represents a measure of pressure at the air inlet of the compressor, an intake manifold pressure signal that represents a measure of pressure at the air intake manifold of the engine and a throttle signal that represents the throttle position of the throttle. In some cases, the controller may include a memory that stores instructions that are executable by the processor to estimate an air pressure at the air outlet of the compressor using a throttle model that references at least the throttle signal and the measure of pressure at the air intake manifold of the engine. The controller may further estimate the operating speed of the turbocharger using a turbocharger model that references at least the measure of air flow through the compressor, the measure of pressure at the air inlet of the compressor and the estimated air pressure at the air outlet of the compressor. In some cases, the processor may provide one or more control signals via the output port to control an operation of the turbocharger and/or the engine in response to the estimated operating speed of the turbocharger. 
         [0008]    The above summary is not intended to describe each disclosed embodiment or every implementation of the present disclosure. The Figures, and Detailed Description, which follow, more particularly exemplify some of these embodiments. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0009]    The disclosure may be more completely understood in consideration of the following detailed description in connection with the accompanying drawings, in which: 
           [0010]      FIG. 1  is a schematic diagram of an air inlet system of an engine encompassing a turbocharger in accordance with an illustrative embodiment of the present disclosure; 
           [0011]      FIG. 2  is a schematic diagram of the air inlet system of  FIG. 1 , with the addition of particular sensors; 
           [0012]      FIG. 3  is a schematic diagram of the air inlet system of  FIG. 1 , showing particular measured and estimated parameters; 
           [0013]      FIG. 4  is a schematic diagram of a controller that may be used in combination with the air inlet system of  FIG. 1  to estimate the turbocharger speed of the turbocharger; 
           [0014]      FIG. 5  is a schematic diagram of the controller of  FIG. 4 , showing illustrative inputs and outputs of the controller; and 
           [0015]      FIG. 6  is a flow diagram showing an illustrative method of operating an engine and/or turbocharger. 
       
    
    
       [0016]    While the disclosure is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the disclosure to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the disclosure. 
       DESCRIPTION 
       [0017]    For the following defined terms, these definitions shall be applied, unless a different definition is given in the claims or elsewhere in this specification. 
         [0018]    All numeric values are herein assumed to be modified by the term “about,” whether or not explicitly indicated. The term “about” generally refers to a range of numbers that one of skill in the art would consider equivalent to the recited value (i.e., having the same function or result). In many instances, the terms “about” may include numbers that are rounded to the nearest significant figure. 
         [0019]    The recitation of numerical ranges by endpoints includes all numbers within that range (e.g. 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4, and 5). 
         [0020]    As used in this specification and the appended claims, the singular forms “a”, “an”, and “the” include plural referents unless the content clearly dictates otherwise. As used in this specification and the appended claims, the term “or” is generally employed in its sense including “and/or” unless the content clearly dictates otherwise. 
         [0021]    It is noted that references in the specification to “an embodiment”, “some embodiments”, “other embodiments”, etc., indicate that the embodiment described may include one or more particular features, structures, and/or characteristics. However, such recitations do not necessarily mean that all embodiments include the particular features, structures, and/or characteristics. Additionally, when particular features, structures, and/or characteristics are described in connection with one embodiment, it should be understood that such features, structures, and/or characteristics may also be used connection with other embodiments whether or not explicitly described unless clearly stated to the contrary. 
         [0022]    The following description should be read with reference to the drawings in which similar structures in different drawings are numbered the same. The drawings, which are not necessarily to scale, depict illustrative embodiments and are not intended to limit the scope of the disclosure.  FIG. 1  is a schematic illustration of an air inlet system  10  that may, for example, be used in combination with any suitable gasoline and/or diesel-powered engine. It will be appreciated that in some cases the air inlet system  10  may include additional components not shown in  FIG. 1 . As illustrated, the air inlet system  10  may include a throttle valve  12  that is positioned upstream of an air intake manifold  14 . The throttle valve has an air inlet  11  and an air outlet  13 . Air entering the air intake manifold  14  can be mixed with fuel and fed into the combustion chambers via one or more intake valves disposed atop each engine cylinder. In some engines, known as direct injection engines, fuel may be directly injected into each engine cylinder. In these engines, fuel is not mixed with air within the air intake manifold  14  itself. 
         [0023]    In some cases, the throttle valve  12  may be opened further, to provide additional air, or at least partially closed, to provide less air, in response to a throttle command signal emanating from an engine management system (not shown). The engine management system may provide a command signal to change the position of the throttle valve  12  in response to the driver pushing harder on the accelerator pedal, or perhaps taking their foot off the accelerator pedal. In some cases, the engine management system may provide a throttle command signal to change the position of the throttle valve  12  in order to maintain a desired speed in response to a cruise control system (not shown). While the engine management system is generally referenced herein as a unitary control structure, it will be appreciated that in actuality, the engine management system may include a number of distinct computers, controllers, processors, sensors and the like. 
         [0024]    Moving upstream in the illustrative air inlet system  10 , air entering the air inlet system  10  is provided to a compressor  16  of a turbocharger. The compressor  16  includes an air inlet  15  and an air outlet  17 . It will be appreciated that compressor  16  is disposed within the air intake flow. The turbocharger will typically also include a drive turbine (not shown) that is disposed within an exhaust gas flow of the engine. Gases within the exhaust flow cause the drive turbine to rotate. The drive turbine typically drives the compressor  16  via a compressor drive shaft. The compressor  16 , which is disposed within the air intake flow, rotates via the aforementioned shaft. As the compressor  16  rotates, it draws air through the air inlet  15 , compresses the air, and provides the compressed air to the air outlet  17 . Air entering the compressor  16  at the air inlet  15  is generally at or close to ambient pressure, while air exiting the compressor  16  at the air outlet  17  is at an increased pressure relative to ambient pressure. The air passing through the compressor  16  may be heated as a result of being compressed, as well through some engine heating. Accordingly, in some cases, while not required, a charge air cooler  18  may be disposed downstream of the compressor  16  in order to cool the air before it reaches the throttle valve  12  and the air intake manifold  14  of the engine. The charge air cooler  18 , when provided, may cool the compressed air provided by the compressor  16  in order to provide a more dense intake charge into the engine, which can increase the power output of the engine. 
         [0025]      FIG. 2  provides another view of the illustrative air inlet system  10  of  FIG. 1 , with several sensors added as part of the engine management system. Many engines include a mass airflow sensor  20  that provides a mass flow rate for air entering the air inlet system  10 , which may be used by an engine management system to determine a proper amount of fuel to add for proper combustion. An inlet pressure sensor  22  provides an indication of the air pressure upstream of the compressor  16 . In some cases, this may provide an indication of ambient air pressure, but it will be recognized that ambient air pressure can vary with weather systems, altitude, temperature and other factors. An inlet temperature sensor  24  provides an indication of the air temperature of the air entering the compressor  16 . In the example shown, a throttle inlet temperature sensor  26  provides an indication of the temperature of the air within the air inlet system  10  downstream of the compressor  16 , near the air inlet  11  of the throttle valve  12 . A Manifold Absolute Pressure (MAP) sensor  28  provides an indication of air pressure downstream of the throttle valve  12 , such as at or in the intake manifold  14  of the engine. The MAP sensor  28  is commonly used on many modern engines for controlling the operation of the engine, such as controlling the operation of the fuel injectors (if present), engine timing, turbo speed, and/or other engine parameters. 
         [0026]      FIG. 3  provides a view of the air inlet system  10 , identifying some of the parameters useful in estimating turbocharger speed. The parameters shown in  FIG. 3  may be measured using the sensors identified in  FIG. 2 .  FIG. 3  also identifies parameters useful in estimating turbocharger speed that are not directly measured, but rather are calculated using the measured parameters. Moving left to right in  FIG. 3 , the parameter {dot over (m)} C  represents an uncorrected mass flow rate, and may be measured via, for example, the mass airflow sensor  20  (see  FIG. 2 ). The parameter T C,in  represents a temperature of the air entering the air inlet  15  of the compressor  16 , and may be measured using, for example, the inlet temperature sensor  24  (see  FIG. 2 ). The parameter p C,in  represents a pressure of the air entering the air inlet  15  of the compressor  16 , and may be measured using, for example, the inlet pressure sensor  22  (see,  FIG. 2 ). A corrected mass flow rate {dot over (m)} C,Cor  may be calculated using the uncorrected mass flow rate {dot over (m)} C , p C,in , and T C,in . Downstream of the charge air cooler  18  (if present), the parameter {dot over (m)} T  is the mass flow rate through the throttle valve  12 . In some cases, the parameter {dot over (m)} T  is not measured, but rather is set equal to the mass flow rate {dot over (m)} C  discussed above. 
         [0027]    The parameter p T,out  represents the pressure at the outlet of the throttle valve  12  and may be measured by the MAP sensor  28 . The parameter u T  represents a throttle command signal, which may provide an indication of a commanded throttle position, or perhaps an actual throttle position if different from the commanded throttle position. The throttle command signal u T  may be provided by an engine management system. 
         [0028]    These measured parameters, along with several reference values as will be further detailed below, may be used to calculate additional parameters. For example, the parameter p T,in , which represents the pressure at the inlet of the throttle valve  12 , may be calculated. The parameter p C,out , which represents the pressure at the air outlet  17  of the compressor  16 , may in some cases be set equal to the parameter p T,in . In some cases, these calculated parameters, and ultimately the turbocharger speed, may be determined by simultaneously or sequentially solving several equations. 
         [0029]    For example, in some cases the following turbocharger model expressed as a continuous function may be solved to calculate the turbocharger operating speed: 
         [0000]        {dot over (N)}   T,Cor =SOLVE{Ψ P   2 (φ, M )− P   1 (Φ, M )=0; P   2 (Φ, M )&gt;0}  (Equation 1)
 
         [0000]    where P 1  and P 2  are bi-variate polynomials obtained by fitting compressor map data (often provided by the compressor manufacturer). In this equation, Φ, Ψ, and M represent the following functions: 
         [0000]    
       
         
           
             Φ 
             = 
             
               
                 4 
                  
                 
                     
                 
                  
                 
                   
                     m 
                     . 
                   
                   
                     C 
                     , 
                     Cor 
                   
                 
                  
                 
                   RT 
                   
                     C 
                     , 
                     
                       i 
                        
                       
                           
                       
                        
                       n 
                     
                   
                   reference 
                 
               
               
                 
                   p 
                   
                     C 
                     , 
                     
                       i 
                        
                       
                           
                       
                        
                       n 
                     
                   
                   reference 
                 
                  
                 π 
                  
                 
                     
                 
                  
                 
                   d 
                   C 
                   2 
                 
                  
                 
                   U 
                   C 
                 
               
             
           
         
       
       
         
           
             Ψ 
             = 
             
               
                 
                   c 
                   p 
                 
                  
                 
                   
                     T 
                     
                       C 
                       , 
                       
                         i 
                          
                         
                             
                         
                          
                         n 
                       
                     
                     reference 
                   
                    
                   
                     ( 
                     
                       
                         
                           ( 
                           
                             
                               p 
                               
                                 C 
                                 , 
                                 out 
                               
                             
                             
                               p 
                               
                                 C 
                                 , 
                                 
                                   i 
                                    
                                   
                                       
                                   
                                    
                                   n 
                                 
                               
                             
                           
                           ) 
                         
                         
                           
                             γ 
                             - 
                             1 
                           
                           γ 
                         
                       
                       - 
                       1 
                     
                     ) 
                   
                 
               
               
                 
                   1 
                   / 
                   2 
                 
                  
                 
                     
                 
                  
                 
                   U 
                   C 
                   2 
                 
               
             
           
         
       
       
         
           
             M 
             = 
             
               
                 U 
                 C 
               
               
                 
                   γ 
                    
                   
                       
                   
                    
                   
                     RT 
                     
                       C 
                       , 
                       
                         i 
                          
                         
                             
                         
                          
                         n 
                       
                     
                     reference 
                   
                 
               
             
           
         
       
     
         [0030]    In these equations, the following variables are defined, in addition to those discussed above:
       {dot over (m)} C,Cor  is the corrected compressor mass flow rate, which is based on an uncorrected mass flow rate {dot over (m)} C  that can be provided by a mass flow sensor positioned at the inlet of the compressor:       
 
         [0000]    
       
         
           
             
               
                 m 
                 . 
               
               
                 C 
                 , 
                 Cor 
               
             
             = 
             
               
                 
                   m 
                   . 
                 
                 C 
               
                
               
                 
                   p 
                   
                     C 
                     , 
                     
                       i 
                        
                       
                           
                       
                        
                       n 
                     
                   
                   reference 
                 
                 
                   p 
                   
                     C 
                     , 
                     
                       i 
                        
                       
                           
                       
                        
                       n 
                     
                   
                 
               
                
               
                 
                   
                     
                       T 
                       
                         C 
                         , 
                         
                           i 
                            
                           
                               
                           
                            
                           n 
                         
                       
                     
                     
                       T 
                       
                         C 
                         , 
                         
                           i 
                            
                           
                               
                           
                            
                           n 
                         
                       
                       reference 
                     
                   
                 
                 . 
               
             
           
         
       
       
         
           
             T C,in   reference  is a reference compressor inlet temperature that corresponds to the compressor map, and is often provided by the compressor manufacturer along with the compressor map. 
             p C,in   reference  is a reference compressor inlet pressure that corresponds to the compressor map, and is often provided by the compressor manufacturer along with the compressor map. 
             U C  is (corrected) blade tip speed πd C {dot over (N)} T,Cor /60, where d C  is the rotor wheel diameter. 
             c p  is the specific heat of dry throttle command air, which is approximately 1006 J/(kg K). 
             γ is the heat capacity ratio of the air, which is approximately 1.4. 
             R is the specific gas constant, which is approximately 287.058 J/(kg K) for the dry air. 
           
         
       
     
         [0038]    In some cases, there may not be a pressure sensor that is positioned at the air outlet  17  of the compressor  16  to provide a value for p C,out . Rather, in some cases, the parameter p C,out  may be calculated using a throttle model, which solves for the parameter p C,out . The throttle model solves for the compressor outlet pressure p C,out , which is then used to determine the compressor speed as discussed above using the turbocharger model expressed in Equation 1. The following throttle model, expressed as a continuous function, may be solved to calculate the throttle input pressure p T.in : 
         [0000]    
       
         
           
             
               
                 
                   
                     p 
                     
                       T 
                       , 
                       
                         i 
                          
                         
                             
                         
                          
                         n 
                       
                     
                   
                   = 
                   
                     Solve 
                      
                     
                         
                     
                      
                     
                       { 
                       
                         
                           
                             
                               m 
                               . 
                             
                             T 
                           
                           - 
                           
                             
                               
                                 
                                   C 
                                   d 
                                 
                                  
                                 
                                   ( 
                                   
                                     u 
                                     T 
                                   
                                   ) 
                                 
                               
                               
                                 R 
                               
                             
                              
                             
                               
                                 Ψ 
                                 T 
                               
                                
                               
                                 ( 
                                 
                                   
                                     p 
                                     
                                       T 
                                       , 
                                       
                                         i 
                                          
                                         
                                             
                                         
                                          
                                         n 
                                       
                                     
                                   
                                   
                                     p 
                                     
                                       T 
                                       , 
                                       out 
                                     
                                   
                                 
                                 ) 
                               
                             
                              
                             
                               
                                 p 
                                 
                                   T 
                                   , 
                                   
                                     i 
                                      
                                     
                                         
                                     
                                      
                                     n 
                                   
                                 
                               
                               
                                 
                                   T 
                                   
                                     T 
                                     , 
                                     
                                       i 
                                        
                                       
                                           
                                       
                                        
                                       n 
                                     
                                   
                                 
                               
                             
                           
                         
                         = 
                         0 
                       
                       } 
                     
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                      
                     
                         
                     
                      
                     2 
                   
                   ) 
                 
               
             
           
         
       
     
         [0000]    where C d (u) is the throttle discharge coefficient which is modeled as a rational polynomial function of the throttle command signal u T . Ψ T  represents the flow function valid for isoentropic expansion of the fluid as follows: 
         [0000]    
       
         
           
             
               Ψ 
               T 
             
             = 
             
               
                 
                   ( 
                   
                     
                       p 
                       
                         T 
                         , 
                         
                           i 
                            
                           
                               
                           
                            
                           n 
                         
                       
                     
                     
                       p 
                       
                         T 
                         , 
                         out 
                       
                     
                   
                   ) 
                 
                 
                   - 
                   
                     1 
                     γ 
                   
                 
               
                
               
                 
                   
                     
                       2 
                        
                       
                           
                       
                        
                       γ 
                     
                     
                       γ 
                       - 
                       1 
                     
                   
                    
                   
                     ( 
                     
                       1 
                       - 
                       
                         
                           ( 
                           
                             
                               p 
                               
                                 T 
                                 , 
                                 
                                   i 
                                    
                                   
                                       
                                   
                                    
                                   n 
                                 
                               
                             
                             
                               p 
                               
                                 T 
                                 , 
                                 out 
                               
                             
                           
                           ) 
                         
                         
                           
                             1 
                             - 
                             γ 
                           
                           γ 
                         
                       
                     
                     ) 
                   
                 
               
             
           
         
       
     
         [0000]    In these equations, the following variables are utilized:
       {dot over (m)} T  is the mass flow rate through the throttle valve, which is set equal to the mass flow rate through the compressor {dot over (m)} C . As detailed above, {dot over (m)} C  is based on the output of a mass airflow sensor  20  positioned at the inlet of the compressor  16 .   p T,in  is the pressure at the air inlet  11  of the throttle valve  12 , which is calculated by the throttle model (e.g. Equation 2).
 
The above flow function Ψ T  saturates (becomes constant) for supersonic flows, i.e. when:
       
 
         [0000]    
       
         
           
             
               
                 p 
                 
                   T 
                   , 
                   
                     i 
                      
                     
                         
                     
                      
                     n 
                   
                 
               
               
                 p 
                 
                   T 
                   , 
                   out 
                 
               
             
             &gt; 
             
               
                 
                   ( 
                   
                     2 
                     
                       γ 
                       + 
                       1 
                     
                   
                   ) 
                 
                 
                   γ 
                   
                     1 
                     - 
                     γ 
                   
                 
               
               . 
             
           
         
       
     
         [0041]    In some cases, once the throttle input pressure p T.in  is calculated using the throttle model (e.g. via Equation 2), the throttle input pressure p T.in  is used as the compressor outlet pressure p C,out  when determining the compressor speed using the illustrative turbocharger model expressed in Equation 1. 
         [0042]      FIG. 4  is a schematic diagram of a controller  30  that may be used in combination with the air inlet system of  FIG. 1  to process the above-referenced parameters to calculate the turbocharger speed. In some cases, the controller  30  may be a standalone controller or computer. In some cases, the controller  30  may be manifested as a component of the aforementioned engine management system. The controller  30  may include an input port  32  and an output port  34 . A processor  36  may be operatively coupled to the input port  32  and to the output port  34 . A memory  38  may be coupled to the processor  36  and may, for example, store instructions that are executable by the processor  36  to estimate the speed of the compressor  16 . In some cases, the processor  36  may estimate the turbocharger speed within the confines of a unitary model. In some cases, the processor  36  may estimate an air pressure at an outlet of the compressor  16  using a throttle model, and estimate the turbocharger speed using the air pressure provided by the throttle model in combination with a turbocharger model. 
         [0043]    In  FIG. 5 , it can be seen that there are several inputs to the input port  32 . For example, the inputs to the input port  32  may include a compressor air flow signal  40  representing a measure of air flow through the compressor  16 , a compressor air inlet pressure signal  42  representing a measure of pressure at the air inlet  15  of the compressor  16 , an intake manifold pressure signal  44  representing a measure of pressure at the air intake manifold  14 , and a throttle position signal  46  that represents the throttle position of the throttle valve  12 . In some cases, processor  36 , utilizing the throttle model (e.g. Equation 2), estimates an air pressure at the air outlet  17  of the compressor  16  referencing at least the throttle signal and the measure of pressure at the air intake manifold  14  of the engine. In some cases, the throttle model also references a measure of temperature of the air at the throttle valve  12 . In some cases, the throttle model also references a measure of air flow through the compressor  16 . In some instances, the throttle model models the effects of the charge air cooler  18 , when present. In some cases, the throttle model further references a measure of a temperature of the air in the throttle valve  12 , where the measure is taken downstream of the charge air cooler  18  and upstream of the throttle valve  12 . 
         [0044]    In some cases, the processor  36  of the controller  30 , utilizing the turbocharger model (e.g. Equation 1), estimates the operating speed of the turbocharger referencing at least the measure of air flow through the compressor  16 , the measure of pressure at the air inlet  15  of the compressor  16  and the estimated air pressure at the air outlet  17  of the compressor  16  (e.g. as estimated by the throttle model). In some cases, the turbocharger model also references a measure of temperature of the air at the air inlet  15  of the compressor  16 . In some instances, the turbocharger model further references one or more of the specific heat of air, the heat capacity of air and/or the specific gas constant of air. In some cases, the processor  36  provides, via the output port  34 , one or more control signals  48  that may be used to control an operation of the turbocharger and/or the engine in response to the estimated operating speed of the turbocharger. 
         [0045]      FIG. 6  is a flow diagram of an illustrative method for estimating an operating speed of a turbocharger that has a compressor  16  with an air inlet  15  and an air outlet  17 . The compressor  16  feeds compressed air to a downstream throttle, such as the throttle valve  12 , which provides a throttled air flow to an air intake manifold of an engine, such as the air intake manifold  14 . As generally referenced at block  50 , the operating speed of the turbocharger may be estimated. In some cases, the estimated operating speed of the turbocharger may be based, at least in part, upon one or more of a measure of air flow through the compressor  16 , a measure of pressure at the air inlet  15  of the compressor  16 , a measure of pressure at the air intake manifold  14  of the engine, and/or a throttle signal that represents the throttle position of the throttle. As generally referenced at block  52 , operation of the turbocharger and/or the engine may be controlled in response to the estimated operating speed of the turbocharger. 
         [0046]    In some cases, estimating the operating speed of the turbocharger as referenced at block  50  may also be based on a measure of temperature of the air at the air inlet  15  of the compressor  16 . In some cases, estimating the operating speed of the turbocharger as referenced at block  50  may also be based on a measure of temperature of the air at the throttle. In some cases, estimating the operating speed of the turbocharger as referenced at block  50  may also be based on a measure of temperature of the air at the air inlet of the compressor and a measure of temperature of the air at the throttle. 
         [0047]    It should be understood that this disclosure is, in many respects, only illustrative. Changes may be made in details, particularly in matters of shape, size, and arrangement of steps without exceeding the scope of the disclosure. This may include, to the extent that it is appropriate, the use of any of the features of one example embodiment being used in other embodiments.