Project-based learning can be broken down to two types: (1) inquiry-based is when students seek out to answer an essential question, and (2) design-based is when students seek to solve an essential problem. Both models are effective and have a rich research and practice history. In my work I often focus on the design-based model and use the Explore-Create-Share framework to scaffold and drive student learning.
If you are new to design-based learning or are looking for some new resources, check out Curiosity Machine. It is a STEM website dedicated to connecting students with real world engineers and scientist as they solve design challenges. They have a collection of design projects and some nice frameworks to organize student learning. I also love their brand--super colorful and fun.