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

Abstract

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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Abbreviations

aBDC:

After bottom dead center

aTDC:

After top dead center

bBDC:

Before bottom dead center

bTDC:

Before top dead center

CA:

Crank angle

CA50:

Crank angle at which 50% of fuel is combusted

CAD:

Crank angle degree

CFD:

Computational fluid dynamics

CI:

Compression ignition

CNG:

Compressed natural gas

CO:

Carbon monoxide

CO2 :

Carbon dioxide

EGR:

Exhaust gas recirculation

ER:

Equivalence ratio

EVO:

Exhaust valve opening

HC:

Hydrocarbon

HCCI:

Homogeneous charge compression ignition

HRR:

Heat release rate

IC:

Internal combustion

IVC:

Inlet valve closing

IMEP:

Indicated mean effective pressure

ISFC:

Indicated specific fuel consumption

LHV:

Lower heating value

LTC:

Low temperature combustion

NG:

Natural gas

NO:

Nitrogen monoxide

NOx :

Nitrogen oxides

P :

Pressure

PM:

Particulate matter

PPCI:

Partially premixed compression ignition

RCCI:

Reactivity controlled compression ignition

RI:

Ringing intensity

RPM:

Revolution per minute

SI:

Spark ignition

SOC:

Start of combustion

SOI:

Start of injection

T :

Temperature

TDC:

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):

Mass

P (kPa):

Pressure

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

Chemical reaction parameter

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

Spray parameter

U (m s−1):

Velocity

Ym (–):

Mth species mass fraction

δml (–):

Dirac delta function

ε (kJ):

Energy dissipation

λ (–):

Methane energy ratio

ρ (kg m−3):

density

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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Correspondence to Omid Jahanian.

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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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Keywords

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