Improving engine efficiency and reducing pollutant emissions are extremely important. Here, we report our fuel injection technology based on the new physics principle that proper application of electrorheology can reduce the viscosity of petroleum fuels. A small device is thus introduced just before the fuel injection for the engine, producing a strong electric field to reduce the fuel viscosity, resulting in much smaller fuel droplets in atomization. Because combustion starts at the droplet surface, smaller droplets lead to cleaner and more efficient combustion. Both laboratory tests and road tests confirm our theory and indicate that such a device improves fuel mileage significantly. The technology is expected to have broad applications, applicable to current internal combustion engines and future engines as well.