That little spark is what ignites the mixture and pushes the piston back up. The other type of combustion engine is a compression-ignition engine.

Both spark ignition and compression ignition engines can work on a two- or four-stroke cycle. The more common of the two for automobiles, and certainly pickups, is the four-stroke.

Well that’s exactly what Mazda has done. In the ranges where compression-ignition isn’t possible, the SkyActiv-X engine runs like any other gasoline engine.

Richard R. Burgett, Joseph M. Leptich, Karma V. S. Sangwan, Measuring the Effect of Spark Plug and Ignition System Design on Engine Performance, SAE Transactions, Vol. 81, SECTION 1: Papers ...

Torbjorn Dillstrom, A High-Power Spark-Ignition Fuel-Injection Engine, SAE Transactions, Vol. 29 (1934), pp. 431-438 ...