1. A heat engine converts thermal energy into mechanical energy through various processes. Common heat engines include steam engines and internal combustion engines. 2. Internal combustion engines burn fuel inside the engine, which heats the air and causes it to expand quickly, transferring energy to move the pistons. Common types are diesel, 2-stroke, and 4-stroke engines. 3. Refrigerators and air conditioners work similarly by absorbing heat inside and releasing it outside using a coolant. The coolant absorbs heat inside, is compressed to release heat outside, then cools and repeats the cycle.