Abstract: A system for determining properties of fuel supplied to an internal combustion (IC) engine and for adjusting operations of the IC engine based on the determined properties. The system includes an air-flow throttle configured to control the air supplied to the IC engine; a fuel-flow throttle configured to control the fuel supplied to the IC engine; and an engine control module (ECM) configured to receive readings from and control operation of the throttles. The ECM is configured to perform a fuel-air determination program where the ECM determines a percent-error air-to-fuel ratio (AF) based on a true AF ratio compared to an ideal AF ratio. The ECM is configured to perform a fuel property determination and adjustment program in which the ECM is configured to adjust operations of the IC engine based on a fuel property value determined using the percent-error AF ratio.