Fuel vapor enrichment for exhaust exothermic catalyst light-off

A method of operating a combustion engine with a secondary air source, comprising:

• supplying liquid fuel having a first A/F ratio to the engine;
• receiving an indication of an operational state of the secondary air source;
• supplying liquid fuel at a second A/F ratio and vapor fuel at a third A/F ratio to the engine based on said indication to produce an exhaust A/F ratio;
• receiving a second exhaust A/F ratio from an exhaust A/F ratio sensor
• comparing said second exhaust A/F ratio to a target A/F ratio;
• and
• adjusting flow of said liquid fuel and said vapor fuel when said second exhaust A/F ratio is not equal to said target A/F ratio.

A method of operating a combustion engine with a secondary air source comprising:

• receiving an indication of an operational state of the secondary air source;
• determining a first A/F ratio of a first supply of liquid fuel;
• determining a second A/F ratio of a second supply of vapor fuel based on said first A/F ratio and said indication;
• supplying said first supply of liquid fuel and said second supply of vapor fuel to the engine;
• supplying air to exhaust gas from the engine while the secondary air source is operational;
• comparing an exhaust A/F ratio to a target A/F ratio;
• and
• adjusting said first supply and second supply when said exhaust A/F ratio is not equal to said target A/F ratio.