An engine system employs a pre-assembled and/or removable cartridge. In another aspect, an ignitor, a fuel injector and an air inlet valve are all accessible from a top of a cartridge even after assembly of the cartridge to an engine cylinder head. A further aspect positions centerlines of an ignitor, a fuel injector and an air inlet valve angularly offset from each other and also angularly offset from a vertical centerline of a cartridge to which they are mounted.