Why nozzle geometry determines spray quality

Diesel combustion depends on getting fuel and air mixed thoroughly in a very short time. The nozzle is where that mixing begins, and its geometry determines whether the spray atomises cleanly or merely squirts.

Atomisation is a geometry problem

Each spray hole is a precisely formed passage, often only a few tens of microns across, with a controlled entry radius, taper and surface finish. Fuel forced through it at rail pressure emerges as a cone of fine droplets rather than a jet, and the quality of that cone is set by the hole geometry.

Small deviations matter disproportionately. A hole that is slightly out of round, or whose entry edge is not correctly radiused, will produce an asymmetric spray with larger droplets.

What poor atomisation produces

• Larger droplets that do not fully evaporate before combustion completes — visible as black smoke.
• Uneven fuel distribution across the cylinder, causing local hot spots and higher NOx.
• Incomplete combustion, which means fuel is being paid for and not used.
• Progressive coking of the nozzle, which degrades the spray further.

How the holes are produced

Spray holes are formed by electrical discharge machining, then finished to the required surface condition. The entry radius, in particular, is a controlled feature — a sharp entry edge promotes cavitation inside the hole, which distorts the spray.

This is also why nozzle condition is not recoverable by cleaning alone. Once the entry edge has eroded, the spray pattern has changed, and no amount of ultrasonic cleaning restores geometry that is no longer there.

Why this drives our equipment choices

Holding this geometry is a machine accuracy problem, not a skill problem. It is why our grinding and honing operations run on equipment specified for sub-micron work, and why every finished injector is flow-tested rather than sampled: spray quality is the final arbiter of whether the geometry was achieved.