Why is the BMW i8 engine so efficient?

I've heard that the BMW i8 has a highly efficient engine, what makes it so efficient and how does it compare to other engines on the market?

The BMW i8's engine is designed with a system called BMW TwinPower Turbo technology, which combines both a turbocharger and an electric motor. This allows the car to achieve a higher level of power output while also reducing fuel consumption.

Compared to other engines, the BMW i8's engine is able to achieve a higher fuel economy due to its efficient hybrid system. This technology allows the car to run on electricity alone for short distances, reducing the need for constant gas refills.