There are HCCI engine prototypes, but they are finicky, because the mixture (mostly regarding initial temperature and pressure) has to be in a narrow flammabili

There are HCCI engine prototypes, but they are finicky, because the mixture (mostly regarding initial temperature and pressure) has to be in a narrow flammability band, so that it ignites precisely at Top Dead Centre from adiabatic compression heat. If it ignites earlier, the pressures are far higher than normal. If it doesn't trigger, it won't for the complete cycle, as the piston expands and the temperature drops. Free-piston engines have an advantage there, as they don't force the compression ratio with a crankshaft. They would decelerate the piston somewhat earlier and harder than necessary, but there are no bearings that have to handle this load. Their issue has more to do with extracting energy from the oscillation, and keeping a stable operating point that's neither prone to stalling nor prone to runaway.