A simulation study on the effect of hydrogen and ethanol addition as supplementary fuel for gasoline engine at lean mixture (equivalence ratio  = 0.8) was carried out to reduce the gasoline share in the mixture, thus reducing fuel consumption and harmful emissions. The effect on engine performance, emissions, and availability was conducted. This was done by changing the ratio between gasoline and the supplementary fuels in the fuel mixture to achieve the required equivalence ratio. The first part of the simulation consisting of the performance and emissions calculated using the first law, was conducted for all engine speeds. The second part consisting of availability analysis was conducted at the rated speed of 2750 rpm. The simulation study was conducted using the data obtained from measurements of Ricardo E6/T engine parameters (variable compression ratio engine). The data was also used to verify the models. The study shows that hydrogen addition reduced carbon monoxide (CO) and nitrogen oxides (NOx) share at the lean mixture. Hydrogen addition significantly improved the heat release rate compared with pure gasoline; however, the heat released was close to the top dead center due to its fast burning speed. Ethanol addition improved the engine's first law performance, e.g., power and efficiency; however, at the cost of increased heat loss. It also improved the indicated work availability in comparison with the addition of hydrogen.