Increasing the Effective AKI of Fuels Using Port Water Injection (Part I)

Document Type

Technical Report

Publication Date



Department of Mechanical Engineering-Engineering Mechanics


Anti-knock index (AKI) is a metric that can be used to quantify the anti-knock performance of a fuel and is the metric used in the United States. AKI is the average of Research Octane Number (RON) and Motor Octane Number (MON), which are calculated for every fuel on a Cooperative Fuel Research (CFR) engine under controlled conditions according to ASTM test procedures. Fuels with higher AKI have better knock mitigating properties and can be run with a combustion phasing closer to MBT in the knock limited operating region of a gasoline engine. However, fuels with higher AKI tend to be costlier and less environmentally friendly to produce. As an alternative, the anti-knock characteristics of lower AKI fuels can be improved with water injection. In this sense, the water injection increases the ‘effective AKI' of the fuel. The injected water absorbs heat, which lowers the in-cylinder temperatures and mitigates knock, which permits advancing the knock limited CA50 (KLCA50) and reducing fuel consumption for a given speed-load condition. This project focused on quantifying the amount of water required, both as a ratio with fuel, and absolute, to achieve an increase in ‘effective AKI'. This was accomplished by comparing the anti-knock performance of a high AKI reference fuel to lower AKI fuels supplemented with water injection. Tests were conducted at varying speeds and loads to investigate trends in the relationship between effective AKI per unit water injected. The results showed that engine speed does not have a profound effect on the required water-fuel ratio, but as the load is increased, less water is required to increase the effective AKI by a specific amount. The effect of water injection on COV of IMEP was also observed.

Publication Title

SAE Technical Papers