Effects of equivalence and fuel ratios on combustion characteristics of an RCCI engine fueled with methane/n-heptane blend


Rising fuel costs and efforts for reducing greenhouse gases have led researchers to propose optimized models of combustion which have high efficiency and low emissions. Reactivity controlled compression ignition (RCCI) engines are attractive due to their high efficiency and low NOx and soot emissions over a wide range of operating conditions. In this study, methane and n-heptane are used as low and high reactive fuels, respectively, to create suitable fuel stratification within the cylinder. Modeling is carried out by AVL FIRE coupled with a chemical kinetics solver to investigate the effects of fuel ratio, initial temperature and equivalence ratio on the combustion performance and emission characteristics. Methane/n-heptane ratios are varied according to the energy ratio of each fuel while total input energy and total equivalence ratios are fixed. By increasing methane energy ratio from 65% to 85% in the constant intake temperature and pressure, the mixture Octane number increases, which would lead to an increase in ignition delay up to 5 crank angles. As a result, IMEP would be enhanced and also NOx emission decreases because of lower combustion temperature. By increasing intake temperature, the maximum in-cylinder pressure, heat release rate and NOx emission would increase significantly while soot emission decreases, and also ringing intensity increases up to 10%. On the other hand, increasing intake temperature reduces volumetric efficiency; as a result, IMEP is reduced by 11%. Also by increasing equivalence ratio from 0.35 to 0.55 in a constant energy ratio, noticeable growth in the maximum amount of pressure and temperature could be achieved; consequently, NOx emission would increase significantly, IMEP increases by 43%, and ISFC decreases by 30%. The results indicate that these parameters have significant effects on the heavy-duty RCCI engine performance and emissions.

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After bottom dead center


After top dead center


Before bottom dead center


Before top dead center


Crank angle


Crank angle at which 50% of fuel is combusted


Crank angle degree


Computational fluid dynamics


Compression ignition


Compressed natural gas


Carbon monoxide

CO2 :

Carbon dioxide


Exhaust gas recirculation


Equivalence ratio


Exhaust valve opening




Homogeneous charge compression ignition


Heat release rate


Internal combustion


Inlet valve closing


Indicated mean effective pressure


Indicated specific fuel consumption


Lower heating value


Low temperature combustion


Natural gas


Nitrogen monoxide

NOx :

Nitrogen oxides

P :



Particulate matter


Partially premixed compression ignition


Reactivity controlled compression ignition


Ringing intensity


Revolution per minute


Spark ignition


Start of combustion


Start of injection

T :



Top dead center

A (–):

Numerical coefficient

A0 (kJ):

Turbulence kinetic energy

D (m2 s−1):

Diffusion factor

Fs (N):

Spray force

I (kJ):

Internal energy

J (kW m−2):

Heat flux

K (–):

Turbulence tensor

M (kg):


P (kPa):


\(\dot{Q}^{\text{c}}\) (kW):

Chemical reaction parameter

\(\dot{Q}^{\text{s}}\) (kW):

Spray parameter

U (m s−1):


Ym (–):

Mth species mass fraction

δml (–):

Dirac delta function

ε (kJ):

Energy dissipation

λ (–):

Methane energy ratio

ρ (kg m−3):


ρm (kg m−3):

Mth species density

\(\dot{\rho }_{\text{m }}^{{ {\text{c}}}}\) (kg m−3 s−1):

Combustion parameter

\(\dot{\rho }^{\text{s}}\) (kg m−3 s−1):

Spray parameter

σ (N m−2):

Stress tensor


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Taqizadeh, A., Jahanian, O. & Kani, S.I.P. Effects of equivalence and fuel ratios on combustion characteristics of an RCCI engine fueled with methane/n-heptane blend. J Therm Anal Calorim 139, 2541–2551 (2020). https://doi.org/10.1007/s10973-019-08669-9

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  • RCCI
  • Equivalence ratio
  • Fuel ratio
  • Methane/n-heptane blend