Enclosed CHP diesel.
https://www.youtube.com/watch?v=kiYXCLXwlv4 (https://www.youtube.com/watch?v=kiYXCLXwlv4)
Here is the translated description:
"I finally managed to make a video of my self-made CHP running. The CHP generates enough heat to heat a 200m² building from the 1970s in winter and the attached workshop as well. 3KW of electricity are generated from one liter of heating oil. The smooth running is amazing, although a China single cylinder for 289 € from ebay."
Why that thing can't be efficient! it has a V belt drive ;)