afr — air to fuel ratio . the air to fuel ratio may be the mass of air divided by the mass of fuel . stoichiometric combustion — an ideal combustion in which the fuel is completely burned . in a stoichiometric combustion , all the carbon is converted to co2 , all hydrogen is converted to h 2 o , and all the sulfur is converted to so2 . the improved system and method of controlling a direct injection spark ignition engine is described . the improved system and method may use sensors that are found on existing disi engines to indirectly measure the fuel level in the evaporated fuel vapors . the improved system and method allows the evaporated fuel vapors to be purged into the intake manifold when the engine is run lean . thus , the engine operates longer in the more efficient stratified mode resulting in a lower fuel consumption . the following description of the preferred embodiments of the invention is not intended to limit the scope of the invention to these preferred embodiments , but rather to enable any person skilled in the art to make and use the invention . [ 0043 ] fig1 illustrates an embodiment of a direct injection spark ignition (“ disi ”) engine 100 . the disi engine 100 may include such components as an accelerator pedal and sensor assembly 102 , a crank angle sensor 104 , a cam position sensor 106 , an engine coolant sensor 108 or cylinder head temperature sensor 108 , a powertrain control module 110 , an engine control module 112 , a throttle control module 114 , an injector driver module 116 , a carbon canister 118 , a temperature / mass air flow meter 120 , a map sensor 122 , a vapor management valve 124 , an ignition control 126 , a fuel pump 128 , a fuel injector 130 , a fuel tank 132 , a spark plug 134 , an exhaust gas re - circulation (“ egr ”) valve 136 , a high compression engine 138 , a universal exhaust gas oxygen (“ uego ”) sensor 140 , a three - way catalytic converter (“ twc ”) 142 , a nox trap / catalyst 144 , a catalyst temperature sensor (“ cts ”) 146 , a heated exhaust gas oxygen (“ hego ”) sensor 148 , a throttle 150 , an intake manifold 152 , and an exhaust manifold 154 . the evaporative fuel vapor system captures evaporative fuel vapors from the gasoline tank 132 , stores the evaporative fuel vapor in the canister 118 , and releases the vapors into the intake manifold 152 via the vapor management valve 124 . the vapor management valve 124 may be controlled by the engine controller 112 . the evaporative fuel vapors tend to be a homogeneous mixture of fuel vapors and air . the fuel level in the evaporative fuel vapors varies over time . the vapor management valve 124 may release the evaporative fuel vapors into the intake manifold 152 . then the evaporative fuel vapors enter the combustion chamber via an intake valve . the fuel injector 130 injects fuel directly into the combustion chamber . the injected fuel and the evaporative fuel vapors mix before combustion . the exhaust gases resulting from the combustion are exhausted via an exhaust valve into the exhaust manifold 154 . the cts 146 is located near the lean nox trap / catalyst 144 and measures the temperature of the lean nox trap / catalyst 144 during the cleansing of the wash coat of sox to control poisoning . the engine 100 is controlled by various engine controllers including the power train control module 110 . the power train control module 110 may in include such controllers as an engine control 112 , a throttle control 114 , and an injector driver 116 . for example , the power train control module 110 may include a canister purge valve controller for a direct injection spark ignited internal combustion engine that includes a temperature sensor interface that receives a temperature of a catalyst in the engine &# 39 ; s exhaust system , an engine mode controller that switches the engine between a stratified mode and a homogeneous mode , and a canister purge valve interface that controls a flow rate of evaporated fuel vapors into the engine &# 39 ; s intake manifold via a canister purge valve . the canister purge valve controller may control the flow rate of evaporated fuel vapors as a function of the temperature received by the temperature sensor interface and the engine &# 39 ; s mode . the components of the disl engine 100 may include various variations of the devices described above . the fuel pump 128 may include a high pressure fuel pump , an electronically controlled pressure regulator , and a pressure sensor assembly . the fuel injector 130 may include a high pressure fuel rail assembly . the fuel tank 132 may include a feed pump . the egr valve 136 may include an electric egr valve . the engine 138 may include multiple cylinders , each with a direct injection fuel injector . the twc 142 may include a quick light - off twc . the nox trap / catalyst 144 may include a lean nox trap , a lean nox catalyst , or other device to reduce nox in the exhaust gas . the throttle 150 may include an electronic controlled throttle . other variations in the components of the disi engine 100 will also be apparent and within the scope of the invention . [ 0047 ] fig2 illustrates a flow diagram of a method of controlling the disi engine 100 of fig1 . in block 202 , the cts 146 ( fig1 ) is tested to determine if it is operating within calibration limits without evaporated fuel vapors being introduced into the combustion chamber . the engine 100 ( fig1 ) may be in the homogeneous mode or the stratified mode during the calibration . the calibration includes comparing the catalyst temperature as measured by the cts to an expected range . if the engine 100 is outside the calibration range , the engine 100 may be operated in the stratified mode with the evaporative fuel vapors turned off and in the homogeneous mode with the evaporative fuel vapors on or off . the activities in block 204 to 220 are not executed when the engine 100 is operating outside the calibration range . in block 204 , if the engine 100 is in the homogeneous mode , the engine 100 is switched to the stratified mode . the engine control 112 ( fig1 ) may control the mode of operation of the engine 100 . in block 206 , the evaporated fuel vapors from the carbon canister 118 ( fig1 ) are allowed to enter the intake manifold 152 ( fig1 ). the flow rate of the evaporated fuel vapors may be controlled by the vapor management valve 124 ( fig1 ) and the engine control 112 . in block 208 , the temperature of the catalyst is measured when the evaporated fuel vapors are being introduced into the combustion chamber . the temperature of the catalyst may be measured by the cts 146 . the catalyst temperature should rise after the evaporated fuel vapors are introduced due to the oxidation of the unburned fuel in the exhaust gas . the catalyst temperature change can be determined as the temperature difference between the temperature measured in block 208 and the temperature measured in block 202 or as the temperature difference between the temperature measured in block 208 and an expected temperature based on a temperature model . in block 210 , if the catalyst temperature change from block 208 is within an acceptable range , the engine can remain in the stratified mode with the evaporated fuel vapors being introduced , and the fuel level of the evaporated fuel vapors can be determined as a function of the temperature change measured in block 208 . the acceptable range may have different upper and lower thresholds depending on various factors . in block 214 , if the catalyst temperature change from block 208 is above the acceptable range , the evaporated fuel vapors contain too much fuel . the flow rate of the evaporated fuel vapors may be adjusted , normally reduced , to allow some of the evaporated fuel vapors to be purged while maintaining the engine 100 in the stratified mode . the flow rate can be repeatedly reduced in block 214 until the catalyst temperature is within an acceptable range . alternatively , the engine 100 could be switched to the homogeneous mode with the evaporated fuel vapors being purged at the same rate until the fuel level in the evaporated fuel vapors is reduced to an acceptable level for operation in the stratified mode . in block 212 , if the catalyst temperature change from block 208 is below the acceptable range , the fuel level in the evaporated fuel vapors is either low or very high . if the fuel level in the evaporated fuel vapors is very high , the entire stratified charge becomes combustible . to determine whether the fuel level in the evaporated fuel vapors is low or very high , the engine 100 is switched to the homogeneous mode , then block 218 is executed . in block 218 , after the engine 100 is switched to the homogeneous mode in block 212 , the fuel level of the evaporated fuel vapors can be accurately measured via an ego sensor , for example an uego . then either block 216 or 218 is executed based on whether the ego sensor indicates a high or low level of fuel in the evaporative fuel vapors . in block 216 , if the fuel level of the evaporated fuel vapors measured in block 218 is below a fuel level threshold , the engine is switched back to the stratified mode . then , the purge fuel level is determined as a function of the catalyst temperature in block 210 . in block 220 , if the fuel level of the evaporated fuel vapors measured in block 218 is above the fuel level threshold , the engine is switched back to the stratified mode . then , the flow rate of the evaporated fuel vapors is reduced in block 214 . alternatively , the engine 100 could be operated in the homogeneous mode with the evaporated fuel vapors being purged at the same rate until the fuel level in the evaporated fuel vapors is reduced to an acceptable level for operation in the stratified mode . the process or portions of the process described in blocks 202 to 220 may be repeated a number of times . further , the process described in blocks 202 to 220 may be a component of a larger process that cycles the engine between the stratified mode and the homogeneous mode based on other factors , such as engine load , acceleration , and other factors . as a person skilled in the art will recognize from the previous description and from the figures and claims , modifications and changes can be made to the preferred embodiments of the invention without departing from the scope of the invention defined in the following claims .