On modern four-stroke engines the topmost part of the cylinder liner, in the region of the top ring groove and at the top dead centre where rings reverse direction, is fitted with a "protection" (or chrome/flame) ring, often a separate steel or specially hardened ring pressed into a recess at the top of the liner. Its function is to protect the cylinder bore at the point of maximum thermal and mechanical loading. At TDC the rings are momentarily stationary and the gas pressure is highest, so the top ring cannot wipe away combustion products entering the clearance above it, leading to rapid localised wear, ring groove hammering and carbon build-up. The protection ring provides a hardened, corrosion-resistant wearing surface which preserves the integrity of the liner throat, reduces vibration and fretting of the liner top, prevents erosion by the flame, and prevents the top liner material from being worn away. It also gives a consistent sealing surface for the top compression ring, improving oil control and reducing the risk of bore polishing.

In a conventional four-stroke engine the fuel injection pump is driven by a cam (or, on some, by the low-speed camshaft) with a spring-return plunger and fixed injection timing set by cam profile. In modern medium-speed four-stroke engines the fuel injection drive has been modified by the introduction of electronic unit injectors and/or the replacement of the mechanical camshaft drive by electronically controlled individual pumps. There are two broad trends: (1) Common rail injection, where fuel is stored at high pressure and each cylinder has an injector opened by a solenoid or hydraulic valve, with timing, duration and (on some) pressure controlled electronically; (2) Camshaft-less, electronically controlled injection (as on some four-strokes) where each unit has a high-pressure pump and the injection timing is controlled by an electronic control unit rather than by cams. The modification removes the need for precise cam timing, allows variable injection timing (VIT) and flexible control of start/end of injection to improve combustion and reduce emissions, and reduces wear of driving gear.

In a multi-hole injection nozzle the holes are arranged so that the fuel jets from adjacent holes enter the combustion chamber at slightly different angles. Staggering refers to arranging the nozzle tip holes so that the spray from each hole does not impinge symmetrically on the piston bowl lip or collide with the spray from the neighbouring hole, and to give a uniform distribution around the bowl. The staggered (offset) layout also means that the spray axes are not all radial/equal, which, together with the swirl, ensures that the fuel is spread evenly and no two jets coincide, improving atomization and mixing, reducing wall wetting on the piston crown and liner, and giving more even heat release and lower smoke and emissions.

Swirl is a rotary motion of the air charge about the cylinder axis. It is generated primarily by a helically/tangentially vaned inlet port which imparts angular momentum to the incoming air during the suction stroke. Swirl gives high relative velocity between fuel spray and air, improving mixing, shortening the ignition delay, giving faster and more complete combustion and a more even temperature field, reducing smoke and increasing efficiency.

Squish is the radial inward movement of the air from the outer edge into the piston-bowl at the end of the compression stroke, generated by the piston crown design - when the piston approaches TDC the air in the squish band (the narrow gap between the piston crown edge and the cylinder head) is forced radially into the bowl. Squish adds turbulence close to the fuel injection point, promoting mixture formation and combustion, and helps delay knock by mixing the burning and unburned gases. Both swirl and squish together produce a turbulent flow which promotes cleaner, faster combustion.