Determination of Qin from the Fuel Heating Value

The higher heating value(HHV) of the fuel will be assumed. This means that all the water products will be liquid. The HHV for each fuel is summarized in Table II, along with the air/fuel ratio for stoichiometric combustion.

Stoichiometric combustion occurs when all the oxygen atoms in the oxidizer react chemically to appear in the products. The oxidizer in this case is the air mixing with the fuel in the piston cylinder. The chemical reaction is given by:

Where:

a₁,b₁,c₁,d₁ = molar coefficients

a,b,c,d = fuel components

The stoichiometric fuel to air ratio can be calculated with:

FAR_s = 1 mole of fuel / a₁(O₂ + 3.76N₂)

Table II. Fuel Properties

Fuel	Chemical Formula	HHV [kJ/kg] [kg air/kg fuel]	Air/Fuel Ratio Stoichiometric	Fuel/Air Stoichiometric
Methane	CH₄	55,500	17.2	0.05814
Propane	C₃H₈	50,300	15.6	0.06410
Octane	C₈H₁₈	47,900	15.1	0.06623
Methanol	CH₃OH	22,700	6.5	0.15385
Ethanol	C₂H₅OH	29,700	8.99	0.11123
Hydrogen	H₂	141,600	27.2	0.03677
Gasoline	C₈H₁₅	47,300	14.6	0.06849
Diesel	C_12.5H_22.2	44,800	14.5	0.06897

In order to determine the heat input, Qin, in [J], the mass of the fuel entering the piston has to be calculated.

The equivalence ratio can be expressed as:

If the equivalence ratio is given the above relation can be solved for the actual mass of the fuel.

The mass of the air, m_a,actual, can be estimated from the displacement volume and the density.

Where:

b = bore

s = stroke

density of air = 1.2 kg/m³

The enrgy input, Qin, can be defined as:

Qin = HHV_fuel * m_f,actual

Two separate fuels can be tested to observe the changes in engine performance with the Engine Performance Applet with Fuel Type. This applet also includes the effect of heat transfer using Woschni's correlation.

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