I use an electronic starter from an old barbecue, powered by one AA cell. It sparks my spark plug about twice per second. Once a flame gets going the spark is turned off. The large hole is for the spark plug to reach in. It must be, by definition, because air moves from high pressure to low pressure.
Air has mass.
The air and fuel are the mass, and the compressor and turbine make it flow. The intake of the compressor. On the right is my home made tachometer sensor made of 2 tubes, one with a light, the other with a photo-transistor. At the very top center you can see part of the brass fitting that feeds oil to the hydrodynamic bearings. The next closer tube goes directly into the hot-wheel housing at a steep angle to help start the engine.