ORIGINAL: digital_trucker
Those are 'hot bulb' engines. The main features of them are low compression (3:1 is about the average I hear) and no complicated ignition system.
The 'hot bulb' engine with 3:1 and up to 7:1 was the early construction with
glowing "hot bulb" while the engine ran.. Later the 'hot bulb' engine with
hot "hotbulb" had the compression ratio 7:1 and up to 10:1.