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Internal Combustion Engine Group. The effect of compression ratio on exhaust emissions from a PCCI Diesel engine. ECOS 2006 12-14 July 2006. Laguitton, Crua, Cowell, Heikal, Gold. Content. Introduction Experimental set-up Validation of single cylinder design

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Internal combustion engine group

Internal Combustion Engine Group

The effect of compression ratio on exhaust emissions from a PCCI Diesel engine

ECOS 2006

12-14 July 2006

Laguitton, Crua, Cowell, Heikal, Gold


Content

Content

  • Introduction

  • Experimental set-up

  • Validation of single cylinder design

  • Strategy for low NOx, soot and FC

  • Conclusions


Highly pre mixed and cool combustion

+

REDUCE OXYGEN

CONCENTRATION

INCREASE EFFICIENCY

ADVANCED AIR/EGR SYSTEMS

INCREASED EGR RATES AND TEMPERATURE MANAGEMENT

REDUCED COMPRESSION RATIO COMBUSTION SYSTEM DESIGN

IMPROVED AIR SYSTEM EFFICIENCY

COLD START

TECHNOLOGY

IMPROVED AIR/FUEL MIXING

ADVANCED FIE TECHNOLOGY

INCREASED IGNITION

DELAY

ADVANCED COMBUSTION & AIR PATH CONTROL

ROBUSTNESS CONTROL

Highly pre-mixed and cool combustion


Oxygen concentration

10

Soot formation area

9

8

7

6

5

Local Equivalence Ratio

4

3

2

1

1000

1400

1800

2200

2600

3000

Temperature /(K)

Combustion trend to more pre-mixed and lower temperature

NOx formation area

Source: MTZ 11/2002: Toyota

Oxygen concentration

Euro 4 – O2 Concentration Map

Approach is to reduce the oxygen concentration characteristics over the engine speed and load operating area:

  • Oxygen concentration in the flame is reduced

  • Less NOx are formed

70 %

85 %

100 %

Level 3 – O2 Concentration Map

70 %

85 %

100 %


Low nox strategy

Low NOx strategy

Level of premixed fuel

Improved air/fuel mixing to achieve low soot and good combustion efficiency

Euro 4

Level 2

Level 3

Increasing Load

Trend is clear:

  • Injection durations reduced by increased injection pressure and nozzle flow

  • Ignition delay increased by changes to air/fuel, CR, intake temperature and EGR

SOC – Real SOI

Injection

period


Single cylinder engine facility

Single cylinder engine facility

Single cylinder – Ricardo HYDRA:

  • 500cc swept volume (86mmx86mm)

  • 2.0L high-flow head

  • Variable swirl (1.0-3.5 Rs)

  • Compression Ratio 18.4:1 and 16.0:1

  • Off-engine HP pump + common rail

  • Delphi injector

  • Delphi nozzle library

  • EmTronix FIE controller

  • Reference ultra low sulphur diesel fuel

    Test bed:

  • Horiba gas analyser MEXA 7100DEGR

  • AVL733 dynamic fuel meter

  • AVL415 variable sampling smoke meter

  • High speed data logger

  • Custom-built low speed data logger

  • TDM post processing

Piston-bowl cross-sections


Validation of single cylinder design

Validation of single cylinder design

Full Load Results

Part Load Results


Effect of compression ratio on nox emissions

Effect of compression ratio on NOx emissions

Fixed calibration

2000 rev/min 7.7 bar GIMEP

LEVEL 2: CR 18.4 and CR 16.0:1

2000 rev/min 10.8 bar GIMEP

2000 rev/min 7.7 bar GIMEP

1500 rev/min 3.0 bar GIMEP

Reduced CR decreases NOx emissions especially at high loads. At low loads (1500 rev/min 3.0 bar GIMEP), slight improvements but combustion is already fully premixed, hence reduced benefits


Effect of compression ratio on auto ignition delay

Effect of compression ratio on auto-ignition delay

2000 rev/min 10.8 bar GIMEP

2000 rev/min 7.7 bar GIMEP

1500 rev/min 3.0 bar GIMEP

Reduced CR decreases in-cylinder pressures. Combustions occur later, increasing the level of premixed leading to higher maximum pressure variations but lower NOx


Late injection strategy for low nox and soot

Late injection strategy for low NOx and soot

Summary of single injection timing responses at 1500 rev/min 6.6 bar GIMEP

(43% EGR rate, 1000 bar rail pressure)

High FC penalty with very

retarded single injections

19.0:1 AFR

17.0:1 AFR


Doe modelling

DOE model: NOx (g/h)

DOE Model: Soot (g/h)

Test data for FC (kg/h)

DOE modelling

  • NOx reduced by high EGR and low AFR

  • Low soot and good fuel consumption is achieved through improved air/fuel mixing

    • Low CR, swirl and rail pressure enhancement is critical

  • Good fuel consumption is achieved by optimising 50% burn after TDC. Late combustion is avoided by shortening combustion duration


Combustion phasing for optimum fuel consumption

Test data for FC (kg/h)

Combustion phasing for optimum fuel consumption

  • Good combustion efficiency:

  • Rapid combustion

  • Centred between 0 and 10 °CA ATDC

  • This is a conflicting requirement with low NOx combustion strategies, which require slow and late combustion

  • A compromise to minimise impact on combustion efficiency is to operate:

  • Slow combustion

  • Well phased combustion


Conclusions

Conclusions

  • A good comparison with multi cylinder baseline results was achieved

  • Ultra low NOx has been achieved through highly pre-mixed and cool combustion

  • At 1500 rev/min, 3.0 bar GIMEP - a twin early injection strategy achieved improved HC and CO results compared to a pilot + “late” main strategy

  • At 1500 rev/min, 6.6 bar GIMEP - testing showed that a late injection strategy was essential for low NOx. A single late injection with high EGR achieved the best overall results

  • With 16:1 CR, an early injection strategy was only beneficial below 3.0 bar GIMEP. Late, high pressure injection combined with EGR is recommended

  • With the combustion bowl geometry tested, 10 and 12 hole nozzles did not offer an advantage at rated power. Reduced spray penetration, bowl interaction and air utilisation was detrimental at the higher loads and speed


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