Basically it all comes down to a few questions when you think about really flying any time soon:
- Is if suitable for our needs (e.g. weight to power ratio, noise, fuel consumption, responsiveness, …)?
- Can you actually buy it (if your are not the DARPA or the US military, etc)?
- Is it affordable?
- Is it reliable?
- Can you get spare parts without too much of a delay and in several years as well (without a major support contract)?
While the projects from Liquid Piston look great (+1 for investing in thermal engine innovation!) and might be a fit for PPG, the answers to all other questions are either “no” or “unknown” on the first glance. Hopefully that will change some day.