Engine’s Fuel Consumption
A gasoline-powered engine generates an average power output of 50 kW. Neglecting any inefficiency that may be present, calculate the volume of fuel consumed each hour. Express your result in the dimensions of liters and gallons.
Approach
To find the volumetric use of fuel we use the heating value of gasoline, 45 MJ/kg, from Table 7.3.
Table 7.3
Heating Values for Certain Fuels*
| Heating Value, H | |||
| Type | Fuel | MJ/kg | Btu/lbm |
| Gas | Natural gas
Propane |
47
46 |
20.2×10^{3} 19.8×10^{3} |
| Liquid | Gasoline
Diesel Fuel oil |
45
43 42 |
19.3×10^{3} 18.5×10^{3} 18.0×10^{3} |
| Solid | Coal
Wood |
30
20 |
12.9×10^{3} 8.6×10^{3} |
| *The numerical values are representative, and values for specific fuels could vary with their chemical composition. | |||
Therefore, following Equation (7.6),
Q = mH (7.6)
45 MJ of heat are released for each kilogram of gasoline burned. To convert the fuel usage into a volume measure, Table 6.1
Table 6.1 Density and Viscosity Values for Several Gases and Liquids at Room Temperature and Pressure
| Density,ρ | Viscosity, μ | |||
| Fluid | kg/m^3 | slug/ft^3 | kg/(m . s) | slug/(ft . s) |
| Air | 1.20 | 2.33×10^{-3} | 1.8×10^{-5} | 3.8×10^{-7} |
| Helium | 0.182 | 5.53×10^{-4} | 1.9×10^{-5} | 4.1×10^{-7} |
| Freshwater | 1000 | 1.94 | 1.0×10^{-3} | 2.1×10^{-5} |
| Seawater | 1026 | 1.99 | 1.2×10^{-3} | 2.5×10^{-5} |
| Gasoline | 680 | 1.32 | 2.9×10^{-4} | 6.1×10^{-6} |
| SAE 30 oil | 917 | 1.78 | 0.26 | 5.4×10^{-3} |
lists the density of gasoline as 680 kg/m^3. The volume can be converted between the SI and USCS by using the factor 1 L = 0.2642 gal from Table 3.6
Table 3.6 Conversion Factors between Certain Quantities in the USCS and SI
| Quantity | Conversion |
| Length | 1 in. = 25.4 mm |
| 1 in. = 0.0254 m | |
| 1 ft = 0.3048 m | |
| 1 mi = 1.609 km | |
| 1 mm = 3.9370 ×10^{-2} in. | |
| 1 m = 39.37 in. | |
| 1 m = 3.2808 ft | |
| 1 km = 0.6214 mi | |
| Area | 1 in^2 = 645.16 mm^2 |
| 1 ft^2 = 9.2903 × 10^{-2} m^2 | |
| 1 mm^2 = 1.5500 × 10^{-3 }in^2 | |
| 1 m^2 = 10.7639 ft^2 | |
| Volume | 1 ft^3 = 2.832 × 10^{-2} m^3 |
| 1 ft^3 = 28.32 L | |
| 1 gal = 3.7854 × 10^{-3} m^3 | |
| 1 gal = 3.7854 L | |
| 1 m^3 = 35.32 ft^3 | |
| 1 L = 3.532 × 10^{-2} ft^3 | |
| 1 m^3 = 264.2 gal | |
| 1 L = 0.2642 gal | |
| Mass | 1 slug = 14.5939 kg |
| 1 lbm = 0.45359 kg | |
| 1 kg = 6.8522 × 10^{-2} slugs | |
| 1 kg = 2.2046 lbm | |
| Force | 1 lb = 4.4482 N |
| 1 N = 0.22481 lb | |
| Pressure or stress | 1 psi = 6895 Pa |
| 1 psi = 6.895 kPa | |
| 1 Pa = 1.450 × 10^{-4 }psi | |
| 1 kPa = 0.1450 psi | |
| Work, energy, or heat | 1 ft · lb = 1.356 J |
| 1 Btu = 1055 J | |
| 1 J = 0.7376 ft · lb | |
| 1 J = 9.478 × 10^{-4} Btu | |
| Power | 1 (ft · lb)/s = 1.356 W |
| 1 hp = 0.7457 kW | |
| 1 W = 0.7376 (ft · lb)/s | |
| 1 kW = 1.341 hp |
