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The Ultra-Lean Partially Stratified Charge Approach to Reducing Emissions in Natural Gas Spark-Ignited Engines

  • L. Bartolucci
  • E. C. Chan
  • S. Cordiner
  • R. L. Evans
  • V. Mulone
Chapter
Part of the Energy, Environment, and Sustainability book series (ENENSU)

Abstract

Lean-burn natural gas engines can be used to reduce exhaust emissions significantly. However, as the mixture is leaned out, the occurrence of extinction and incomplete combustion increases, resulting in poor performance and stability, as well as elevated levels of unburned hydrocarbon (UHC) and nitrogen oxides (NOx) emissions. The partially stratified charge (PSC) method can be used to mitigate these issues, while extending the lean misfire limit (LML) beyond its equivalent, homogeneous level. In this chapter, the PSC ignition and combustion processes are examined following a comprehensive experimental and numerical approach. Experiments are conducted in an idealized PSC configuration, using a constant volume combustion chamber (CVCC), to identify the principle enabling mechanisms of the PSC methodology. Engine tests conducted in a single-cylinder research engine (SCRE) demonstrate the feasibility of various PSC implementations in improving performance and emission characteristics in real-world settings. Complementary numerical analyses for the CVCC are obtained through large eddy simulations (LES), while Reynolds-averaged Navier–Stokes (RANS) simulations are conducted for SCRE with reduced chemical kinetics. The corresponding simulated results provide additional insights in characterizing the effect of fuel stratification on flame kernel maturation and flame propagation, the interplay between chemistry and turbulence at different overall air–fuel ratios, as well as formation of major pollutant species.

Keywords

Natural gas combustion Spark ignition (SI) engine Lean combustion Stratified charge High efficiency Low emissions 

Roman Symbols

Do

Injector nozzle diameter (mm)

E

Energy (kJ)

f

Arbitrary function

H

Energy content of air–fuel mixture (kJ)

k

Turbulent kinetic energy (m2/s2)

Ke

Jet entrainment constant (–)

Le

Turbulent integral length scale (mm)

me

Entrained jet mass (mg)

mfuel

Mass of fuel (in air–fuel mixture) (mg)

mo

Injected jet mass (mg)

p

Pressure (bar)

p

Power in Lp combination (–)

SL

Laminar flame speed (m/s)

t

Time (s)

t0

Time at start of injection (s)

T

Temperature (K)

uʹ

Turbulent velocity fluctuation (m/s)

Uo

Jet velocity at nozzle (m/s)

V

Volume (cc)

z

Jet penetration distance (mm)

Z

Normalized energy release (–)

Greek Symbols

γ

Ratio of specific heats (–)

Γ

Jet penetration constant (–)

δL

Laminar flame brush thickness (m)

ε

Dissipation rate of turbulent kinetic energy (m2/s3)

ϕ

Fuel–air ratio relative to stoichiometric level (–)

λ

Air–fuel ratio relative to stoichiometric level (–)

μ

Mean value of an observable

μj

Mixing ratio between entrained and injected mass (–)

ρ

Density of ambient gas (kg/m3)

ρ0

Density of injected gas (kg/m3)

ρb

Density of burned gas (kg/m3)

ρu

Density of unburned gas (kg/m3)

σ

Standard deviation of an observable

τ

Normalized time (–)

Acronyms and Abbreviations

abs

Absolute (pressure level)

AMR

Adaptive mesh refinement

AS

After spark onset

ASOI

After start of injection (ms)

BFSC

Brake-specific fuel consumption (g/kWh)

BMEP

Brake-specific mean effective pressure (bar)

B/ATDC

Before/after top dead center

CAD

Crank angle degree (°)

CFD

Computational fluid dynamics

(C)NG

(Compressed) natural gas

CoV

Coefficient of variation (%)

CVCC

Constant volume combustion chamber

DNS

Direct numerical simulation

HR(R)

Heat release (Rate) (kJ (/s))

I/EVC

Intake/exhaust valve closed

I/EVO

Intake/exhaust valve open

IMEP

Indicated mean effective pressure (bar)

LES

Large eddy simulation

LML

Lean misfire limit (–)

LPG

Liquefied petroleum gas

MBT

Mean best torque

MFB

Mass fraction burned (%)

NOx

Nitrogen oxides (i.e., NO + NO2)

PaSR

Partially stirred reactor

PM

Particulate matter

PSC

Partially stratified charge

RANS

Reynolds-averaged Navier–Stokes

RNG

Renormalization group

RPM

Revolutions per minute

S/EOI

Start/end of injection (ms)

SCRE

Single-cylinder research engine

T/BDC

Top/bottom dead center

TCI

Turbulence chemistry interaction

TKE

Turbulent kinetic energy (m2/s2)

(U)HC

(Unburned) hydrocarbon

WOT

Wide-open throttle

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Copyright information

© Springer Nature Singapore Pte Ltd. 2019

Authors and Affiliations

  1. 1.Department of Industrial EngineeringUniversity of Rome “Tor Vergata”RomeItaly
  2. 2.Institute for Advanced Sustainability StudiesPotsdamGermany
  3. 3.Department of Mechanical EngineeringThe University of British ColumbiaVancouverCanada

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