1. Overview
Engine horsepower re-rating is the process of determining an engine's actual available power output when operating conditions differ from the manufacturer's standard rating conditions. Natural gas compressor engines in midstream operations frequently require derating due to site elevation, ambient temperature extremes, and emissions compliance modifications mandated by federal and state regulatory agencies.
The standard rating basis for most industrial natural gas engines is sea level elevation, 60°F ambient temperature, and 29.92 inHg barometric pressure. Any deviation from these conditions reduces the mass of air available for combustion, directly impacting the engine's ability to produce rated power.
Altitude
Air Density Reduction
Less oxygen per cycle at higher elevation
Temperature
Intake Air Heating
Warmer air is less dense, reduces charge mass
Emissions
Combustion Modification
Lean burn, timing retard, catalytic backpressure
Fuel Quality
BTU Content Variation
Lean gas reduces power per combustion event
Regulatory Drivers
| Regulation | Pollutant | Typical Requirement | Engine Impact |
|---|
| EPA NSPS JJJJ | NOx, CO, VOC | NOx ≤ 1-2 g/bhp-hr | Lean burn or catalytic conversion |
| EPA NESHAP ZZZZ | HAPs, formaldehyde | Formaldehyde ≤ 2.7 ppmvd @ 15% O2 | Oxidation catalyst required |
| State permits | NOx, CO, VOC | Varies by state and air basin | May exceed federal requirements |
| Nonattainment areas | Ozone precursors | BACT/LAER determinations | Most stringent controls required |
Critical consideration: Engine re-rating must account for all derating factors simultaneously. Altitude, temperature, and emissions effects are cumulative and multiplicative. An engine rated at 1,000 HP at standard conditions may only deliver 700-800 HP at a high-altitude site with emissions controls installed.
2. Altitude & Temperature Derating
Altitude and temperature derating are fundamentally linked through their effect on air density. As altitude increases or temperature rises, the mass of air entering the engine per intake stroke decreases, reducing the amount of fuel that can be burned and the resulting power output.
Atmospheric Pressure vs Altitude
Standard atmosphere model:
P_atm = 14.696 * (1 - 6.8753e-6 * h)^5.2559
Where:
P_atm = Atmospheric pressure (psia)
h = Elevation above sea level (ft)
Simplified altitude derating (naturally aspirated):
F_alt = 1 - 0.03 * (h / 1,000)
Where:
F_alt = Altitude derating factor (dimensionless)
h = Elevation above sea level (ft)
0.03 = 3% loss per 1,000 ft
Example values:
Sea level: F_alt = 1.000 (no derating)
2,000 ft: F_alt = 0.940
4,000 ft: F_alt = 0.880
5,280 ft: F_alt = 0.842 (Denver, CO)
7,500 ft: F_alt = 0.775
Turbocharged Engine Altitude Correction
Turbocharged engines compensate for reduced atmospheric pressure by increasing boost pressure, but this compensation has limits. Above the turbocharger's design altitude (typically 3,000-5,000 ft), the turbo reaches its maximum speed or pressure ratio and can no longer fully compensate.
| Engine Type | Derate per 1,000 ft | Compensation Range | Notes |
|---|
| Naturally aspirated | 3.0% | None | Linear derating from sea level |
| Turbocharged | 1.0-2.0% | 0-3,000 ft (no derate) | Derate above compensation limit |
| Turbocharged & aftercooled | 0.5-1.5% | 0-5,000 ft (no derate) | Best altitude performance |
Temperature Derating
Temperature derating factor:
F_temp = 1 - 0.001 * (T_amb - T_rated) for T_amb > T_rated
Where:
F_temp = Temperature derating factor
T_amb = Actual ambient temperature (deg F)
T_rated = Standard rating temperature (typically 60 deg F)
0.001 = 0.1% per deg F (or 1% per 10 deg F)
For T_amb <= T_rated: F_temp = 1.0 (no credit for cold weather)
Air density ratio method (more precise):
F_density = (T_rated + 460) / (T_amb + 460)
Example at 100 deg F ambient:
Simple: F_temp = 1 - 0.001 * (100 - 60) = 0.960 (4% loss)
Density: F_density = 520/560 = 0.929 (7.1% loss)
Note: The simple 1%/10 deg F rule is conservative for modest
temperature exceedances. Use air density ratio for precise work.
Combined Altitude and Temperature
Combined derating factor:
F_combined = F_alt * F_temp
Or using density ratio directly:
F_combined = (P_atm / P_std) * (T_std + 460) / (T_amb + 460)
Where:
P_std = 14.696 psia (sea level)
T_std = 60 deg F (standard rating)
Derated horsepower:
HP_derated = HP_rated * F_combined
Design practice: Always use site maximum ambient temperature for derating calculations, not average temperature. The engine must be capable of delivering required compressor power on the hottest day of the year. Many operators use the 1% design temperature from historical weather data.
3. Emissions Modifications
Emissions compliance modifications alter the combustion process or exhaust treatment to reduce pollutant formation. Each modification carries a power penalty that must be quantified during the re-rating process. The magnitude of the penalty depends on the technology, the engine model, and the required emissions level.
Emissions Control Technologies
| Technology | Target Pollutant | HP Impact | Mechanism |
|---|
| Lean burn conversion | NOx | 5-10% loss | Excess air dilutes charge, lowers peak temperature |
| Prechamber retrofit | NOx | 3-8% loss | Stratified charge enables leaner main chamber |
| Ignition timing retard | NOx | 5-12% loss | Later ignition reduces peak pressure and temperature |
| Oxidation catalyst | CO, VOC, formaldehyde | 1-3% loss | Exhaust backpressure from catalyst bed |
| SCR system | NOx | 2-4% loss | Backpressure from catalyst + reagent system parasitic load |
| EGR (exhaust gas recirculation) | NOx | 3-7% loss | Inert exhaust gas dilutes intake charge |
| Air-fuel ratio controller | All | 0-2% loss | Tighter AFR control may limit peak power operation |
Lean Burn Conversion Details
Lean burn operation runs the engine at air-fuel ratios significantly above stoichiometric (typically 1.6-2.0 times stoichiometric). This excess air absorbs combustion heat, reducing peak flame temperature and NOx formation. The power penalty comes from displacing fuel-air mixture with excess air, reducing the energy released per cycle.
Lean burn power relationship:
The power output is approximately proportional to the fuel energy
input per cycle. As the mixture is leaned:
HP_lean / HP_rich = eta_lean / eta_rich * (AFR_stoich / AFR_actual)
Where eta accounts for thermal efficiency changes.
Typical air-fuel ratios:
Rich burn (standard): lambda = 0.97-1.03 (near stoichiometric)
Lean burn: lambda = 1.4-1.8 (40-80% excess air)
Ultra-lean burn: lambda = 1.8-2.2 (80-120% excess air)
NOx reduction vs power loss tradeoff:
lambda = 1.4: ~60% NOx reduction, ~5% power loss
lambda = 1.6: ~80% NOx reduction, ~8% power loss
lambda = 1.8: ~90% NOx reduction, ~12% power loss
Ignition Timing Retard
Retarding ignition timing shifts the peak combustion pressure later in the expansion stroke. This reduces peak cylinder pressure and temperature (lowering NOx) but decreases the work extracted from the combustion gases. Each degree of timing retard typically costs 0.5-1.0% of rated power.
Timing retard derating:
F_timing = 1 - 0.0075 * degrees_retarded
Where:
degrees_retarded = Timing retard from standard setting (deg BTDC)
0.0075 = Typical 0.75% loss per degree (range 0.5-1.0%)
Example:
Standard timing: 22 deg BTDC
Emissions timing: 14 deg BTDC
Retard: 8 degrees
F_timing = 1 - 0.0075 * 8 = 0.94 (6% power loss)
Exhaust Backpressure Effects
Backpressure derating:
F_bp = 1 - 0.004 * delta_P_exhaust
Where:
delta_P_exhaust = Added backpressure from catalyst/SCR (inH2O)
0.004 = ~0.4% loss per inH2O above baseline
Typical added backpressure:
Oxidation catalyst: 2-6 inH2O
SCR system: 4-10 inH2O
Combined: 6-16 inH2O
Maximum allowable backpressure:
Most engines: 27-40 inH2O total (consult manufacturer data)
Cumulative emissions derating: When multiple emissions technologies are applied, their effects are multiplicative. F_emissions = F_lean * F_timing * F_backpressure. A lean burn conversion with timing retard and oxidation catalyst may result in 15-20% total power loss.
4. BMEP & Torque Analysis
Brake Mean Effective Pressure (BMEP) is the fundamental measure of engine loading intensity. It represents the average pressure that would need to act on the piston throughout the power stroke to produce the observed brake output. BMEP is independent of engine size and speed, making it the best metric for comparing loading across different engines.
BMEP calculation (4-stroke):
BMEP = (HP * 33,000 * 2) / (L * A * N * n_cyl)
Or in terms of total displacement:
BMEP = (HP * 792,000) / (V_d * RPM)
V_d = (pi/4) * D^2 * S * n_cyl [displacement, in^3]
Where:
HP = Brake horsepower
L = Stroke length (ft)
A = Piston area (in^2) = (pi/4) * D^2
N = Engine speed (RPM)
n_cyl = Number of cylinders
D = Bore diameter (in)
S = Stroke (in)
V_d = Total swept displacement (in^3)
(2-stroke: replace 792,000 with 396,000 — power stroke every revolution.)
Alternative using torque:
BMEP = (150.8 * Torque) / V_d [4-stroke]
Where Torque is in ft-lbf and V_d in in^3
Typical BMEP ranges for gas engines:
Naturally aspirated: 60-90 psi
Turbocharged: 100-160 psi
High-performance turbo: 160-220 psi
Torque and Speed Relationships
| Parameter | Formula | Unit | Notes |
|---|
| Power from torque | HP = Torque * RPM / 5,252 | HP | Torque in ft-lbs |
| Torque from power | Torque = HP * 5,252 / RPM | ft-lbs | At constant speed |
| Power from BMEP | HP = BMEP * V_d * RPM / 792,000 | HP | 4-stroke; V_d = (pi/4)*D^2*S*n_cyl |
| BMEP from torque | BMEP = T * 150.8 / V_d | psi | T in ft-lbf, V_d in in^3 |
Speed-Load Envelope
After re-rating, the engine's operating envelope is defined by the intersection of maximum BMEP (torque limit), maximum speed (mechanical limit), and derated power (thermal/emissions limit). The available operating region shrinks with each derating factor applied.
Maximum torque at derated power:
Torque_max = HP_derated * 5,252 / RPM_rated
BMEP at derated condition:
BMEP_derated = BMEP_rated * (HP_derated / HP_rated)
Speed-load limit check:
At any operating point, verify:
1. BMEP <= BMEP_max (manufacturer limit)
2. RPM <= RPM_max (mechanical limit)
3. HP <= HP_derated (thermal limit)
4. Exhaust temperature <= T_exhaust_max
BMEP limits: Never exceed the manufacturer's maximum BMEP regardless of the re-rating. While derated power reduces average BMEP, transient loading or speed changes can produce instantaneous BMEP spikes. Maintain a 5-10% margin below the BMEP limit for reliable operation.
5. Operational Considerations
Engine re-rating has cascading effects on the entire compression system. Reduced engine power means reduced compressor capacity, which affects gathering system pressures, throughput, and potentially upstream well performance.
Compressor Matching After Derate
| Impact Area | Effect of Derating | Mitigation Strategy |
|---|
| Throughput capacity | Reduced proportionally to HP loss | Add compression, optimize ratio |
| Compression ratio | May need to reduce ratio to stay within HP | Stage compression, reduce discharge P |
| Suction pressure | Higher suction P needed if ratio limited | Adjust system backpressure |
| Speed control | May need to reduce RPM to match load | Variable speed drive, governor adjust |
| Cylinder loading | Unloaders may need adjustment | Reconfigure clearance pockets |
Fuel Consumption Changes
Brake Specific Fuel Consumption (BSFC):
BSFC = Fuel_rate / HP_output (BTU/hp-hr or SCF/hp-hr)
Typical BSFC values:
Rich burn engine: 7,500-8,500 BTU/hp-hr
Lean burn engine: 7,000-8,000 BTU/hp-hr
Turbocharged lean burn: 6,500-7,500 BTU/hp-hr
Fuel rate after derate:
If BSFC remains constant:
Fuel_new = Fuel_rated * (HP_derated / HP_rated)
If BSFC changes (common with emissions mods):
Fuel_new = BSFC_new * HP_derated
Lean burn efficiency note:
Lean burn engines often have better thermal efficiency than
rich burn, partially offsetting the power loss with lower BSFC.
Net fuel consumption change may be less than expected.
Maintenance and Reliability
Emissions modifications can affect engine maintenance intervals and component life. Lean burn operation produces lower exhaust temperatures but may increase misfiring at very lean ratios. Catalytic converters require periodic inspection and may need catalyst replacement every 3-5 years depending on operating conditions and fuel quality.
Spark Plugs
Reduced Life
Lean burn requires higher ignition energy; plug life may decrease 20-40%
Exhaust Valves
Improved Life
Lower exhaust temps from lean burn extend valve life
Catalyst
3-5 Year Life
Depends on fuel contaminants (sulfur, siloxanes)
Oil Life
Variable Impact
Lean burn may extend oil life; timing retard may reduce it
System-level impact: When evaluating an engine re-rate, always assess the full system impact including compressor capacity, pipeline throughput, and upstream well performance. A 10% engine derate may cause a disproportionate throughput loss if the system is already constrained.
6. Worked Examples
Example 1: Field Compressor at Altitude with Lean Burn
Given:
Engine rated HP: 1,350 HP at sea level, 60 deg F
Site elevation: 4,500 ft
Design ambient temperature: 95 deg F
Emissions modification: Lean burn conversion (8% power loss)
Oxidation catalyst installed (3 inH2O backpressure)
Engine type: Naturally aspirated
Step 1: Altitude derating
F_alt = 1 - 0.03 * (4,500 / 1,000)
F_alt = 1 - 0.135 = 0.865
Step 2: Temperature derating
F_temp = 1 - 0.001 * (95 - 60)
F_temp = 1 - 0.035 = 0.965
Step 3: Emissions derating (lean burn)
F_lean = 1 - 0.08 = 0.920
Step 4: Backpressure derating (catalyst)
F_bp = 1 - 0.004 * 3 = 0.988
Step 5: Combined derating
F_total = 0.865 * 0.965 * 0.920 * 0.988
F_total = 0.759
Step 6: Derated horsepower
HP_derated = 1,350 * 0.759 = 1,025 HP
Result: 325 HP reduction (24.1% total derating)
Step 7: Verify BMEP
BMEP_derated = BMEP_rated * 0.759
If rated BMEP = 85 psi:
BMEP_derated = 85 * 0.759 = 64.5 psi (well within limits)
Example 2: Turbocharged Engine with SCR
Given:
Engine rated HP: 2,000 HP (turbocharged, aftercooled)
Site elevation: 6,000 ft
Turbo compensation limit: 5,000 ft
Design ambient temperature: 105 deg F
Emissions: SCR system (8 inH2O backpressure)
Timing retard: 6 degrees from standard
Step 1: Altitude derating (turbocharged)
Below 5,000 ft: no derating
Above 5,000 ft: 1.5% per 1,000 ft
Effective altitude above compensation: 6,000 - 5,000 = 1,000 ft
F_alt = 1 - 0.015 * (1,000 / 1,000) = 0.985
Step 2: Temperature derating
F_temp = 1 - 0.001 * (105 - 60) = 1 - 0.045 = 0.955
Step 3: Timing retard derating
F_timing = 1 - 0.0075 * 6 = 0.955
Step 4: Backpressure derating (SCR)
F_bp = 1 - 0.004 * 8 = 0.968
Step 5: Combined derating
F_total = 0.985 * 0.955 * 0.955 * 0.968
F_total = 0.870
Step 6: Derated horsepower
HP_derated = 2,000 * 0.870 = 1,740 HP
Result: 260 HP reduction (13.0% total derating)
Note: Turbocharged engine retains more power at altitude
than naturally aspirated equivalent.