Day Tank Sizing
Rule of Thumb for tank size with 25% reserve
0.056 × Ave. BHP demand × Hours between refills × 1.25 = _____ gal.
0.27 × Ave. BKW demand × Hours between refills × 1.25 = _____ liters
NOTE: Additional tank capacity required for cooling of recirculated fuel in unit injected engines. Tank should be located below level of injectors or nozzles.
Fuel Supply Line Maximum Restriction*:
3600 ... -38.8 kPa (11.6 in Hg)(Vacuum)
3400, 3500 ... -30 kPa (9 in. Hg) (Vacuum)
3300 ... -20 kPa (6 in. Hg) (Vacuum)
3208 ... -27 kPa (8 in. Hg) (Vacuum)
Fuel Return Line Maximum Restriction:*
3600 ... 350 kPa (51 psi)
3300 ... 20 kPa (3 psi)
3208, 3400, 3500 ... 27 kPa (4 psi)
*Locate day tank and design piping to meet these requirements.
Blended (Heavy) fuels are usually described by their viscosity, expressed either in "centistokes" (cSt) or "Seconds Redwood". The Redwood scale at 100°F is being phased out and replaced by the centistokes scale at 50°C. The centistoke viscosity may be preceded by the letters IF for "intermediate fuel" or IBF for "intermediate bunker fuel". For example, IF 180 fuel has a viscosity of 180 cSt at 50°C. The following table gives the approximate relationship between the two scales.
Density and Specific Gravity
Fuel API Correction Chart - API Gravity Corrected to 60°F
Tooling: Fuel Thermo-hydrometer 1P7408
Test Breaker 1P7438
Distillate Fuel Temperature
Maximum Fuel Supply Temperature:
- Without Power Reduction: 85°F (29°C)
Power is reduced 1% for each 10°F (5.6°C) above 100°F (38°C) if engine is running against fuel stop.
- Without Injector Damage: 150°F (65°C)
Performance Analysis Rules of Thumb
Performance curves represent typical values obtained under normal operating conditions. Ambient air conditions and fuel used will affect these values. Each of the values may vary in accordance with the following tolerances:
Exhaust Stack Temperature ±42 DEG C
±75 DEG F
Intake Manifold Pressure-Gage ±10 kPa
±3 in Hg
Power ±3 Percent
Fuel Consumption ±6 g/kW-hr
Fuel Rate ±5 Percent
Ratings are based on SAE J1349 standard conditions of 100 kPa (29.61 in Hg) and 25°C (77°F). These ratings also apply at ISO 3046/1, DIN 6271 and BS 5514 standard conditions of 100 kPa (29.61 in Hg), 27°C (81°F) and 60% relative humidity.
Fuel Rates are based on fuel oil of 35° API [16°C (60°F)] gravity having an LHV of 42 780 kJ/kg (18,390 Btu/lb) when used at 29°C (85°F) and weighing 838.9 g/liter (7.001 lbs/U.S. gal).
Additional Formulas Used to Develop Marine Par Curves
For Torque Check GPH proceed as follows:
Torque Check GPH = TQ COR. Fuel Rate (G/MIN) ÷ 454 × 60 = LBS/HR
LBS/HR ÷ 7.076 = GPH
For BSFC proceed as follows:
BSFC = Adjusted CSFC (G/kW HR) ÷ 454 = LBS/kW HR
LBS/kW HR ÷ 1.34 = BSFC (LBS/HP HR)
Recommended "Guide Line" Gas Supply Pressures for AVSpare Gas Engines - All Values in PSIG (kPag) Natural Gas
Gas Regulator Capacity Chart
Fuel Gas Methane Numbers
Physical Constants of Gases:
Air-Fuel Ratio Adjustment for Initial Start-up:
Fuel Consumption Calculation
Published fuel consumption values are for 905 BTU/FT3 LHV. To calculate fuel consumption for other fuel gas, the following equation can be used:
Special Lube Oil Information for Gas Engines:
Alternate Oil Analysis (additional test procedures for more data)
Differential infrared analysis of used oil must not exceed the following absorbance/CM:
Scheduled Oil Sampling
If analysis of the used oil at the recommended oil change interval exceeds the condemning limits, the following courses of action can be taken:
1. Modify the jacket water temperature and/or adjust the air-to-fuel to minimize the oil degradation rate.
2. Shorten the oil change interval.
3. Work with your oil supplier to arrive at an oil that will not exceed the limits.
Maximum PPM (Parts Per Million) of Wear Metals Detected at 750 Hours to Achieve Projected Overhaul Interval
Causes and Avoidance
Symptoms of oil failure are stuck piston rings, heavy piston deposits, sludged oil, plugged oil filters, rapid ring and liner wear, and high copper concentrations in oil analysis.
It is important that oil analysis measure oxidation and nitration since they result in corrosive wear and accelerate oil degradation. Oxidation and nitration cause oil to thicken and form lacquer and maroon-colored deposits.
The nitration rate of an oil is associated with the air-to-fuel ratio for the engine. There is evidence that suggests operating the engine with an air-to-fuel ratio between 10:1 and 11:1 promotes rapid nitration of the oil. This is the normal air-to-fuel ratio range for most AVSpare Natural Gas Engines and permits the optimum fuel consumption at rated power. In this range, nitrous oxide (NOx) as measured in the exhaust stream is at its highest level. This range may cause the oil to degrade at an unacceptable rate.
If nitration is determined to be the principal reason for oil degradation, it may be necessary to adjust the air-to-fuel ratio either higher or lower to minimize the nitration rate.
If the air-to-fuel ratio is changed, it must be done with care because it may have a negative effect on the power of the engine or result in excessive exhaust temperature which could affect the service life of the engine.
Engine Data Sheet 195.0 Form LEKQ2364, of the AVSpare Technical Manual shows the effects of different air-to-fuel ratio settings on various engine functions. The graphs included in that publication are for 6.25 inch bore natural gas engines, but the general shapes of the curves are similar for all natural gas engines.
The chart on page 52 illustrates how fuel consumption, exhaust temperature, exhaust emissions, and engine power vary with air-fuel ratio. The air-fuel ratio which produces the lowest fuel consumption generally produces the maximum oxides of nitrogen (NOx). This level of NOx can cause rapid deterioration of lube oil through nitration.
At air-fuel ratios within the nitration range, engine jacket water temperature becomes very important. Cooler temperature allows moisture to condense within the crankcase creating acids which result in corrosive wear to piston rings and liners. The jacket water temperature must be kept as warm as possible to improve oil life and reduce wear. Radiator-cooled systems should be maintained between 93°C to 99°C (200°F to 210°F). Ebullient-cooled engines have high jacket water temperatures and usually deteriorate the oil at a slower rate.