An affordable classic hit and miss engine. So called hit and miss engines were popular between the 1890s to 1940s, the originals are often seen in the UK at steam/vintage fairs. They typically ran of paraffin or petrol and were used as an early type of generator or for providing mechanical power. The name comes from the speed control. They fire ("hit") only when operating at or below a set speed, and then free wheel without firing ("miss") when they exceed their set speed.
The sound these types of engines make is very distinctive. Basically an irregular "POP whoosh whoosh whoosh whoosh POP" as the engine seeks to maintain the perfect average speed.