The Measure of Muscle: UIC Leaflet 622 Power Definitions

Introduction to UIC Leaflet 622

In the marketing brochures of locomotive manufacturers, a diesel engine might be rated at 4,000 HP. But can it deliver that power in a hot tunnel in summer? Or at high altitude? UIC Leaflet 622, titled “Definition of the nominal power of locomotives and railcars fitted with internal combustion engines,” is the standard that cuts through the ambiguity.

It establishes a rigorous method for defining exactly what “Nominal Power” means for a railway vehicle, ensuring that operators can compare apples to apples when purchasing diesel fleets. It prevents the overestimation of performance by defining strict reference conditions for temperature and pressure.

Snippet Definition: What is UIC 622?

UIC Leaflet 622 is a technical regulation that defines the “Nominal Output” ($P_{UIC}$) of internal combustion engines (primarily diesel) used in railway traction. It specifies the standard atmospheric reference conditions (temperature, pressure, humidity) under which the engine’s power must be measured and defines the methodology for deducting the power consumed by essential auxiliaries to determine the useful power available for traction.

The Concept of UIC Nominal Power

The leaflet clarifies the difference between the engine’s theoretical maximum and its useful reality.

• UIC Nominal Output: The maximum power the engine can deliver continuously at the output flange (crankshaft) under the defined reference conditions.
• Useful Power: The standard often requires the calculation of power after the losses from essential engine-mounted accessories (cooling pumps, fuel pumps) are subtracted.

Standard Reference Conditions

A diesel engine “breathes” air. If the air is thin (altitude) or hot, power drops. UIC 622 mandates that the Nominal Power be stated relative to specific environmental baselines, often aligning with ISO 3046:

• Total Barometric Pressure: Typically 100 kPa (1 bar).
• Air Temperature: Typically 25°C (298 K) at the inlet.
• Relative Humidity: Typically 30% or 60%.

If a train is to operate in conditions worse than these (e.g., a mountain railway at 2000m altitude), UIC 622 provides the formulas for De-rating (calculating the mandatory power reduction) to prevent engine overheating.

Comparison: UIC 622 vs. UIC 623

These two leaflets work in tandem but serve different stages of the procurement process.

Operational Relevance

For a railway undertaking, UIC 622 is critical for Timetable Planning. If a locomotive is rated at 3000 kW according to UIC 622, the operations team knows exactly how much tonnage it can haul up a 2% gradient. Without this standardization, a “3000 kW” engine from one supplier might stall where another succeeds, simply because they used different temperature baselines for their rating.