The design clues are the key: excellent fuel economy, high thermal efficiency, and relatively high combustion noise all point to one class of diesel combustion chamber.

Diesel chambers split into two families, undivided (open, direct-injection) chambers and divided chambers such as pre-combustion, swirl and air-cell types.

A direct-injection chamber sprays the fuel straight into a single open combustion space above the piston, so there is no connecting throat to throttle the gases and no large extra surface bleeding heat away, which gives high thermal efficiency and low fuel consumption.

Because ignition and burning start rapidly in that open space, cylinder pressure rises steeply, and this sharp pressure rise is heard as the characteristic diesel knock, so the design is comparatively noisy.

Pre-combustion and swirl chambers burn the fuel first in a small connected chamber, which smooths the pressure rise and quietens combustion but loses efficiency through throttling and extra heat loss, so they do not match the high-economy, noisy description.

An air-cell chamber similarly stores and releases air from a separate cavity to soften combustion, again trading efficiency for quietness, so it too fails the clues.

Hence, the answer is Direct injection chamber with fuel injected into the main chamber.