Fuel-Air Injection (FAI) Engine is:
1. A two-stroke engine with fuel and air injections.
2. A hybrid engine with an air motor built in the cylinder.
3. A high thermal efficiency pulsating rocket engine with piston and crankshaft as its output.
4. A clean combustion machine with gas expansion precisely controlled by digital fuel and air injections.

Conventional piston engines are subject to energy inefficiency and oxygen deficient combustion. Hence their fundamentals are being challenged:
1. Thermal Inefficiency - Thermal efficiency is compromised mainly due to time sharing of the cylinder. Air compression and gas expansion are two distinct processes; each has its own efficiency requirement. Since these two processes sequentially time sharing the same cylinder, both efficiencies are limited for optimization.
2. Frictional losses – Severe frictional losses associated with piston rings, valves and valve-train.
3. Oxygen deficient combustion - Insufficient oxygen supply and improper fuel-air ratio cause incomplete combustion which results in poor fuel efficiency and pollutant emission.
Cylinder sharing also causes other drawbacks such as, heat to be concentrated in the cylinder and complicated engine structure. For more efficient gas expansion and more complete combustion, it’s essential to remove the air compression function out of the gas expander in a piston engine.

FAI engine has simplified structure, it implemented dedicated gas expander and air compressor to optimize efficiency for both function blocks.