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FUELS DATA

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Air Fuel Ratio comparison

  •   E85 fuel requires a richer air fuel mixture than gasoline for best results.

  • Successful conversions generally require 27% to 30% more fuel flow than when the engine burns 100% gasoline.  

  •  Flexible fuel vehicles additionally impose a wider range of air fuel ratios that must be achieved than what is required for vehicles that operate only on gasoline or ethanol.

  • his is because a wider range of air fuel ratios is required to use all the varying percentages of ethanol and gasoline efficiently that may be present in the fuel tank at any given time.

  • The nominal, chemically correct air fuel ratio is 14.64:1 by mass ‘’not volume’’ for burning 100% gasoline, but in practice the nominal air fuel ratio for most 100% gasoline fuel injection systems ranges from about 14.6 to 14.7 for a typical nominal value, depending on manufacturer, with the ratio of 14.7 being slightly preferred for increasing fuel economy under light load conditions.

  • The following table shows the range of air fuel ratios typically used for burning gasoline, E85, and pure ethanol (E100) under an assortment of assumed operating conditions:

Fuel

AFRst

FARst

Equivalence
Ratio

Lambda(λ)

Gasoline stoichiometric

14.7

0.068

1

1

Gasoline max power rich

12.5

0.08

1.176

0.8503

Gasoline max power lean

13.23

0.0755

1.111

0.900

E85 stoichiometric

9.765

0.10235

1

1

E85 max power rich

6.975

0.1434

1.40

0.7143

E85 max power lean

8.4687

0.118

1.153

0.8673

E100 stoichiometric

9.0078

0.111

1

1

E100 max power rich

6.429

0.155

1.4

0.714

E100 max power lean

7.8

0.128

1.15

0.870

      • AFRst: Alcohol Fuel ratio Stoichiometric
      • FARst: Fuel Alcohol Ratio Stoichiometric
    • The term AFRst refers to the air fuel ratio under stoichiometric or ideal air fuel ratio mixture conditions. (See stoichiometry.) FARst refers to the fuel air ratio under stoichiometric conditions, and is simply the reciprocal of AFRst.
  • Equivalence ratio is the ratio of actual fuel air ratio to stoichiometric fuel air ratio; it provides an intuitive way to express richer mixtures. Lambda (λ) is the ratio of actual air fuel ratio to stoichiometric air fuel ratio; it provides an intuitive way to express leanness conditions (i.e., less fuel, less rich) mixtures of fuel and air.
  • Air fuel ratio is always computed on the basis of ratios of mass (not volume). The following is a computation of the theoretical E100 (100% ethanol, C2H6O) air fuel ratio, based on stoichiometric (perfect combustion) principles:
      • C2H6O + 3 O2 = 2 CO2 + 3 H2O
  • Adding up the molar mass for ethanol:
    • (6 x 1.00794) + (2 x 12.0107) + (1 x 15.9994) = 46.0684 grams per mole of ethanol
    • 1 mol x 46.0684 g/mol ethanol : 3 mol x 2 x 15.9994 g/mol oxygen
    • 46.0684 : 95.9964 = 1:2.0838 for the fuel:oxygen ratio for perfect (i.e., stoichiometric) combustion.
    • Now, oxygen is 20.9% of air by volume, or equivalently, 23.1% of air by mass, assuming that atmospheric gases behave as ideal gases. (See Earth's atmosphere.)
  • Hence, the theoretical air fuel ratio for E100 (100% ethanol) is:
      • (2.0838/0.23133) : 1 = 9.0078 : 1
  • So, for E85 (summer blend), the required air fuel ratio can be estimated as:
      • 0.85 x 9.0078 + 0.15 x 14.64 = 9.8526
  • Likewise, for E85 (winter blend), the required air fuel ratio can be estimated as:
      • 0.70 x 9.0078 + 0.30 x 14.64 = 10.6975, which is closer to the gasoline air fuel ratio.
  • The estimated required E85 summer blend air fuel ratio compares very closely to the value of 9.765 given in the table. In practice, though, the exact stoichiometric air fuel ratio for gasoline varies as a function of the exact blend of gasoline, which, in turn, is varied by time of year by refineries to increase or decrease volatility, prevent vapor locking, etc., for better matching seasonal climatic changes.
  • Deviations from stoichiometric combustion computed values are required during non-standard operating conditions such as heavy load, or cold weather operation, in which case the mixture ratio can range from 10:1 to 18:1 for burning 100% gasoline. Slightly wider ranges than even this on the low end of the air fuel ratio, dropping to below 8:1, are required for burning all possible blends of E85 and gasoline efficiently under all conditions of engine loads and inlet air temperatures.
  • At inlet air temperatures below 15 C (59 F), it is likewise not possible to start the typical internal combustion engine on pure ethanol (E100); for cold engine starts, starting the engine on gasoline and then shifting to E100 can be done. Similarly, for starting a vehicle on E85 summer blend in extremely cold weather, it is likewise required to add additional gasoline during at least the starting of the engine, before switching to burning the E85 summer blend. In practice, it is easier simply to add more pure gasoline to the fuel tank when extremely cold weather is expected, prior to the arrival of the cold weather, to avoid cold engine start difficulties.
  • Fortunately for those converting non-FFVs to operate on E85, the wide range of inherent air fuel control required for burning pure gasoline is very nearly the same range required for burning many blends of E85 with gasoline up to approximately 60% E85, at least for non-extreme engine loads and non-extreme weather conditions. Hence, the common success seen in practice for burning many blends of E85 with gasoline even in non-FFVs at blends in excess of 50% E85, especially under light engine loads cruising under benign weather conditions.
  • All of these theoretical stoichiometric combustion estimated values should be taken only as approximations to what may really be required for achieving perfect combustion. The lambda sensor is what ultimately confirms whether stoichiometric combustion is taking place in practice.
  • Additionally, the ideal stoichiometric mixture typically burns too hot for any situation other than light load cruise. This is the target mixture that the ECU attempts to achieve in closed-loop fueling to get the best possible emissions and fuel mileage at light load cruise conditions. This mixture typically can give approximately 95% of the engine's best power, provided the fuel has sufficient octane to prevent damaging detonation (i.e., knock).
  • The "max power rich" condition is the richest air fuel mixture (more fuel than best power) that gives both good drivability and power levels, within approximately 1% of the absolute best power on that fuel.
  • he "max power lean" condition is the leanest air fuel mixture (less fuel than best power) that gives good drivability, acceptable exhaust gas temperatures to prevent engine damage, and power levels within approximately 1% of the absolute best power on that fuel.
  • Lambda, typically used for referring to lean versus rich air fuel mixtures, is normally measured by the lambda sensor] (also known as an oxygen sensor.)
  • Depending on seasonal blend variations E85 will weigh approximately 6.5 pounds per U.S. Gallon, having a liquid density of approximately 0.77 - 0.79 compared to gasoline which has typical values of 6.0 - 6.5 pounds per U.S. gallon and a density of 0.72 - 0.78.

2006 Gilles Desormeaux