Engine formulas


Cylinder Swept Volume (Vc):

where:
            Vc= cylinder swept volume [cm3 (cc) or L]
            Ac = cylinder area [cm2 or cm2/100]
            dc = cylinder diameter [cm or cm/10]
            L = stroke length (the distance between the TDC and BDC) [cm or cm/10]

BDC = Bottom Dead Center
            TDC = Top Dead Center

* Increase the diameter or the stroke length will increase the cylinder volume, the ratio between the cylinder diameter/cylinder stroke called “bore/stroke” ratio.
- “bore/stroke” >1 is called over square engine, and is used in automotive engines
- “bore/stroke” =1 is called square engine
- “bore/stoke” <1 is called= under square engine, and is used in tractor engine


Engine Swept Volume (Ve):

where:
            Ve = engine swept volume [cm3 (cc) or L]
            n = number of cylinders
            Vc = cylinder swept volume [cm3 (cc) or L]
Ac = cylinder area [cm2 or cm2/100]
dc =  cylinder diameter [cm or cm/10]

* The units of cylinder swept volume is measured in (cm3, cubic centimeter (cc), or liter)
- Ve  for small engines, 4 cylinder engines is (750 cc:1300 cc)
- Ve for big engine, 8 cylinder engines is (1600 cc:2500 cc)


Compression Ratio (r):

where:
            r = compression ratio
            Vs = cylinder swept volume (combustion chamber volume) [cc, L, or m3]
            Vc = cylinder volume     [cc, L, or m3]

* Increase the compression ratio increase engine power
- r (gasoline engine) = 7:12, the upper limit is engine pre ignition
- r (diesel engine) = 10:18, the upper limit is the stresses on engine parts


Engine Volumetric Efficiency (hv):

where:
            hV  = volumetric efficiency
            Vair = volume of air taken into cylinder [cc, L, or m3]
            Vc = cylinder swept volume [cc, L, or m3]

* Increase the engine volumetric efficiency increase engine power
- Engine of normal aspiration has a volumetric efficiency of 80% to 90%
- Engine volumetric efficiency can be increased by using:
(turbo and supper charger can increase the volumetric efficiency by 50%)



Engine Indicated Torque (Ti):

where:
            Ti = engine indicated torque [Nm]
            imep = indicated mean effective pressure [N/m2]
            Ac = cylinder area [m2]
                L = stroke length [m]
            z = 1 (for 2 stroke engines), 2 (for 4 stroke engines)
n = number of cylinders
θ = crank shaft angle [1/s]


Engine Indicated Power (Pi):

,                       
where:
            imep = is the indicated mean effective pressure [N/m2]
            Ac = cylinder area [m2]
            L = stroke length [m]
            n = number of cylinders
            N = engine speed [rpm]
            z = 1 (for 2 stroke engines), 2 (for 4 stroke engines)
            Vc = cylinder swept volume [m3]
            Ve = engine swept volume [m3]
            Ti = engine indicated torque [Nm]
            ω = engine angular speed [1/s]


Engine Mechanical Efficiency (hm):

 
where:
            hm = mechanical efficiency
            Pb = engine brake power [kW]
            Pi = engine indicated power [kW]
            Pf = engine friction power [kW]


Engine Specific Fuel Consumption (SFC):

where:
            SFC = specific fuel consumption [(kg/h)/kW, kg/(3600 s x kW), kg/(3600 kJ)]
            FC = fuel consumption [kg/h]
            Pb = brake power [kW]


Engine Thermal Efficiency (hth):

 
where:
            hth = thermal efficiency
            Pb = brake power [kW]
            FC = fuel consumption [kg/h = (fuel consumption in L/h) x (ρ in kg/L)]
            CV = calorific value of kilogram fuel [kJ/kg]
            ρ = relative density of fuel [kg/L]

Disclaimer:
The above article doesn't give you any guarantee and the sole purpose of this article is to share my learning in the way I understood. Any comments to refine this article are welcome with great pleasure. Please report any breaking link by commenting below.

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