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Research Article

Exergetic and exergoeconomic analyzes of compressed natural gas as an alternative fuel for a diesel engine

ORCID Icon, ORCID Icon, ORCID Icon & ORCID Icon
Pages 3722-3741 | Received 22 Feb 2020, Accepted 10 Aug 2020, Published online: 03 Sep 2020
 

ABSTRACT

The goal of this study is to investigate the use of compressed natural gas in a diesel engine compared to diesel fuel, according to exergetic and exergoeconomic analyzes. For this purpose, compressed natural gas (CNG) injector has been mounted to intake manifold and small, single cylinder diesel engine was converted to dual mode. Experiments have been carried out according to ISO 8178 E2 test cycle, at maximum power and four different load as 25%, 50%, 75%, and 100%. CNG ratios were chosen as 0%, 20%, 40%, and 60% percentages as energy content. Exergetic and exergoeconomic analyses were executed to results considering capital, operation-maintenance, fuel and environmental damage costs. According to results, diesel fuel has superiority than CNG fuels except full load. In these loads, with using CNG, exergetic efficiency and exergoeconomic factor reduces as up to 38.3% and 32.8%, and also exergy destruction percentage, lost exergy rate and specific cost of shaft work exergy unit increases as up to 10.8%, 74.8%, and 41.7%, respectively. Besides CNG fuels need to improve lower load conditions performance, CNG20 has the favorable exergetic results at full load. According to environment related cost results, remarkable cost of UHC affects negatively to CNG fuel results at 25% and 50% loads. Last of all, CNG20 is preferable at 75% load and all CNG fuels are preferable at full load conditions according to total cost flow rates against diesel.

Nomenclature

c=

Specific cost of exergy

CNG=

Compressed natural gas

CNG20=

20% CNG – 80% diesel ratio (acc. energy)

CNG40=

40% CNG – 60% diesel ratio (acc. energy)

CNG60=

60% CNG – 40% diesel ratio (acc. energy)

CH4=

Methane

CO2=

Carbon dioxides

C=

Carbon

CO=

Carbon monoxide

ECU=

Electronic control unit

E˙=

Energy rate (kW)

E˙x=

Exergy rate (kW)

ε=

Chemical exergy (kj/mol)

ēkCH=

Standard molar chemical exergy (kj/mol)

f=

Exergoeconomic factor

H=

Hydrogen

K=

Unit environmental damage cost

kW=

Kilowatt

Gj=

Gigajoule

Hu=

Lower heating value (kj/kg)

m˙=

Mass flow rate (kg/s)

NOx=

Nitrogen dioxides

N2=

Dinitrogen

N2O=

Nitrous oxide

O2=

Oxygen

ϕ=

Chemical exergy factor

n˙=

Molar flow rate (mol/s)

Pr=

Specific price ($/kg)

Rˉ=

Molar gas constant (kj/molK)

sˉ=

Molar entropy (kj/mol)

Q˙=

Heat (kW)

T=

Torque (Nm)

W˙=

Work rate (kW)

T=

Time (hour)

Z=

Engine capital and operation maintenance cost ($)

Z˙=

Engine capital and operation maintenance cost rate ($/h)

Ψ=

Exergetic efficiency

SUBSCRIPT=
chem=

Chemical

cw=

Cooling water

CI=

Capital investment

dest=

Destruction

exh=

Exhaust

in=

Inlet

OM=

Operation-maintenance

tm=

Thermomechanical

W=

Work

0=

Reference state

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