"Compression ignition" refers to the process in an engine where fuel and air are compressed so tightly that they automatically ignite without the need for a spark plug. Full definition
In other words, because a spark plug is now being used at all times, the engine can switch seamlessly between combustion using compression ignition and combustion using spark ignition. (greencarcongress.com)
That is to say, the potential application of compression ignition has now dramatically expanded, allowing this technology to be used in almost all driving conditions. (greencarcongress.com)
Because a spark plug is used all the time, the system is able to switch seamlessly to spark ignition combustion in rpm or load ranges where compression ignition would be difficult. (greencarcongress.com)