Spark-ignition
Spark-ignition means relating to a gasoline-fueled engine or any other type of engine with a spark plug (or other sparking device) and with operating characteristics significantly similar to the theoretical Otto combustion cycle. Spark-ignition engines usually use a throttle to regulate intake air flow to control power during normal operation.
Source
40 CFR § 91.3
Scoping language
The following definitions apply to this part 91. All terms not defined herein have the meaning given them in the Act.