The protection ring is a separate hardened ring seated in a groove at the top of the cylinder liner. Its function is to protect the upper liner bore where the piston rings reverse direction at top dead centre, where combustion gas pressure is maximum and the top ring is practically stationary. At this position the ring cannot wipe away the products of combustion, so the liner throat would otherwise wear rapidly and crack face erosion by the flame. The protection ring provides a hard, corrosion- and wear-resistant surface which: preserves the liner bore diameter, gives a consistent sealing surface and prevents bore polishing, stops the flame eroding the liner rim, and protects against ring groove hammering and fretting. It is usually made of a special hardened steel, and is a replaceable part which can be renewed when worn, so the liner itself lasts longer.

In a conventional four-stroke engine the fuel pump plunger is driven by a cam on the low-speed camshaft through a follower and roller, with a fixed lift profile and a return spring; injection timing is fixed by the cam angle and the timing of the fuel delivery is adjusted by mechanical means (nozzle/rack). In a modern engine the fuel injection drive has been modified by one or both of two approaches; (1) common rail injection, where a high-pressure rail is charged by a power-cylinder-driven or dedicated pump and each injector is opened electronically (solenoid/hydraulically triggered) so timing, duration and pressure are variable, decoupling injection timing from the mechanical cam; (2) electronic unit injection (or single camshaft-less pumps) where the pump and injector are mounted in one unit and the injection is controlled by an electronic control unit (ECU) using signals of engine speed, load and temperature. The modification removes fixed cam timing, allowing variable injection timing (VIT) and flexible injection control for better combustion, lower smoke/emissions and improved fuel economy, and reduces wear on drive gear/cams.

Multi-hole nozzle tips have a series of injection holes. Staggering means the holes are arranged so that the spray axes of adjacent holes are offset by a small angle in relation to the valve centreline or to each other. This means that no two sprays issue from diametrically opposite/equal angles, so the sprays divide into the combustion chamber space evenly and are less likely to hit the piston crown/bowl lip or liner (wall wetting). The staggered arrangement, together with swirl, gives a more even spatial distribution of fuel, better mixing, avoids overlapping of adjacent sprays that would shield each other, improves atomization, and produces more uniform heat release, reducing smoke and unburnt fuel and increasing efficiency.

Swirl is the rotary motion of the air charge about the cylinder axis generated by a tangential/helical inlet port during the induction stroke: the port vanes impart angular momentum to the air. Effect: high relative velocity between the fuel spray and the air improves atomization and mixing, shortens the ignition delay, promotes fast and complete combustion, and gives more uniform gas temperature and lower smoke and emissions.

Squish is the rapid radial inward movement of the air from the outer piston-cylinder clearance (squish band) into the piston bowl as the piston approaches TDC, generated by the piston crown geometry. Effect: it induces turbulence in the combustion chamber just before and during injection, which thoroughly mixes fuel and air, accelerates the flame front, improves combustion and prevents knock, while reducing unburnt HC. Combined, swirl and squish create the turbulence that yields efficient, clean, rapid combustion.