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ISO 3046 engine power rating, BSFC, BMEP, torque, and thermal efficiency. Diesel vs gas engine comparison with derating support.
Results
Engine Power from Fuel
The brake power output is derived from fuel energy input multiplied by thermal efficiency:
- ṁ_f — fuel mass flow rate (kg/s)
- CV — calorific value of fuel (kJ/kg)
- η — brake thermal efficiency (decimal)
BSFC — Brake Specific Fuel Consumption
BSFC indicates how efficiently the engine converts fuel to work. Lower is better. Typical diesel: 190–230 g/kWh.
BMEP — Brake Mean Effective Pressure
- n_R — revolutions per power stroke (2 for 4-stroke, 1 for 2-stroke)
- V_d — total displacement (m³)
- n — engine speed (rev/s)
Torque
ISO 3046 Power Rating Types
| Rating | Code | Factor | Description |
|---|---|---|---|
| Continuous | COP | 1.0× | Unlimited hours at constant load |
| Prime | PRP | 1.1× | Unlimited hours, variable load, avg ≤ 80% of PRP |
| Limited Time | LTP | 1.2× | Max 500 h/year at 100% LTP |
| Emergency Standby | ESP | 1.3× | Max 200 h/year, no overload allowed |
Note: The multiplication factors above are indicative industry rules of thumb for comparing rating categories. ISO 3046-1 / ISO 8528-1 define the categories (COP, PRP, LTP, ESP) but do not define universal conversion factors between them — always use the manufacturer’s declared rating for each category.
Practical Example
Given: Fuel consumption = 50 kg/h, CV = 42,700 kJ/kg, η = 38%, Displacement = 12 L, Speed = 1500 RPM, 4-stroke
Q_in = 50 × 42,700 / 3600 = 593.1 kW (thermal)
P = 593.1 × 0.38 = 225.4 kW
BSFC = 50,000 / 225.4 = 221.8 g/kWh
Torque = 225,400 / (2π × 25) = 1435 N·m
BMEP = 225,400 × 2 × 60 / (0.012 × 1500) = 15.0 bar
Diesel vs Gas Engine Comparison
| Parameter | Diesel | Natural Gas |
|---|---|---|
| Thermal Efficiency | 35–52% | 28–42% |
| BSFC (equivalent) | 190–230 g/kWh | 230–300 g/kWh |
| BMEP (turbocharged) | 15–25 bar | 12–20 bar |
| A/F Ratio (stoich.) | 14.5:1 | 17.2:1 |
| Compression Ratio | 15–22:1 | 10–14:1 |
| CO₂ per kWh | ~650 g | ~490 g |
| NOx | Higher | Lower (lean burn) |
Typical Engine BSFC Values
| Engine Type | BSFC (g/kWh) | Efficiency |
|---|---|---|
| Small high-speed diesel (<100 kW) | 230–270 | 30–36% |
| Medium-speed diesel (100–1000 kW) | 195–230 | 35–42% |
| Large medium-speed (1–10 MW) | 180–200 | 40–46% |
| Slow-speed marine diesel (10+ MW) | 160–175 | 48–52% |
| Gas engine (lean burn) | 230–280 (eq.) | 35–42% |
⚠️ Note: ISO 3046 reference conditions: 25 °C, 100 kPa, 30% relative humidity. Derate approximately 3.5% per 300 m above 300 m altitude and ~2% per 5.5 °C above 25 °C.
Brake Specific Fuel Consumption is the fuel consumed per unit power per hour, in g/kWh. It directly indicates engine efficiency:
- Small high-speed diesel: 230–270 g/kWh
- Medium-speed diesel: 195–230 g/kWh
- Large slow-speed marine: 160–175 g/kWh
- Gas engine (lean burn): 230–280 g/kWh equivalent
Lower BSFC = higher efficiency = less fuel per kW.
BMEP (Brake Mean Effective Pressure) is the hypothetical constant pressure acting on the piston that would produce the same net work per cycle. It normalizes performance independent of engine size.
- Naturally aspirated diesel: 7–10 bar
- Turbocharged diesel: 15–25 bar
- High-performance racing: 25–30+ bar
Higher BMEP means more power from a given displacement — a measure of specific output.
ISO 3046-1 defines four standard power ratings for reciprocating IC engines:
- COP: Continuous Power — unlimited running at constant load
- PRP: Prime Power — unlimited hours at variable load (avg ≤ 80%)
- LTP: Limited Time Power — max 500 h/year at 100%
- ESP: Emergency Standby — max 200 h/year
Reference conditions: 25°C, 100 kPa, 30% relative humidity.
Per ISO 3046-1, engine power must be derated when operating above reference conditions:
- Altitude: ~3.5% per 300 m above 300 m elevation
- Temperature: ~2% per 5.5°C above 25°C ambient
- Humidity: minor effect, usually neglected
Example: at 1500 m altitude and 40°C, derating ≈ 3.5%×4 + 2%×2.7 ≈ 19.4%.
Diesel engines are inherently more efficient (35–52%) due to higher compression ratios (15–22:1) and lean combustion. Gas engines (28–42%) use lower compression (10–14:1) but produce less NOx.
For the same power output, gas engines consume more fuel (by energy) but produce ~25% less CO₂ per kWh due to the hydrogen content of methane.
The procedures described on this page were developed and field-tested by the Vibromera team using the Balanset-1A, a portable dual-channel balancer and vibration analyzer for on-site rotor balancing.
Professional field balancing instruments and software. Used in 50+ countries.
Nikolai Shelkovenko
Nikolai Shelkovenko is a vibration analysis engineer and the founder and CEO of Vibromera. For more than 15 years he has balanced rotating equipment in the field rather than on a test bench: mulchers, industrial fans, crushers, centrifuges, shafts and spindles. That work is what the Balanset instruments grew out of — they were designed as a tool a specialist can carry to the machine and use alone, on site, not as laboratory equipment. Vibromera was founded in 2017 and has been based in Porto, Portugal, since 2023. Development, assembly and support of the Balanset line all happen here. The flagship instrument is the Balanset-1A, a portable analyser for single- and two-plane balancing and for vibration diagnostics. Nikolai is personally involved in customer support, in working through difficult balancing cases and in the development of the software. He works with customers worldwide, in any language.
