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Split Injection for CNG Engines

Patrik Soltic, Hannes Biffiger Empa, Automotive Powertrain Technologies Laboratory

Motivation

CNG engines are gaining on importance in the stationary and in the mobility sector (availability/costs of natural gas, CO2 advantage versus crude oil product, low pollutant emissions with comparably simple exhaust gas treatment, full compatibility with alternative methane such as biogas or synthetic NG)

Methane is a very good fuel for internal combustion engines because of its knock resistance

Methane is hard to ignite (→ high ignition energy/voltage needed for high BMEP operation, especially for lean or EGR concepts) and the early flame development phase is slow

It is known that the addition of small amounts of hydrogen massively enhances inflammability, combustion stability and increases engine efficiency

Up to now: port fuel injected concepts are dominating → nearly perfect premixed air/fuel (& EGR)

Main question: is there an advantage for stratified concepts (with or without hydrogen addition)?

State of Research

cyl1+4 cyl2+3

Pure methane DI λ=1 (no EGR) operation on a Volkswagen EA111 engine

DI in direction of the spark plug shows a massive acceleration of combustion (1)

This is mainly a turbulence (not a stratification) effect (2)

Pure DI is problematic because large gas volumes have to be injected which needs very special injectors

(1) Soltic P, Egli R, Mauke D, Wright Y M, Bach C, “Strömung , Gemischbildung und Verbrennung bei Methandirekteinblasung im homogenen λ = 1 Betrieb : Simulationen und Versuchsergebnisse,” in 6- Tagung Gasfahrzeuge, 26.-27.10.2011, Stuttgart, 2011.

(2) Schmitt M, Hu R, Wright Y M, Soltic P, Boulouchos K, “Multiple Cycle LES Simulations of a Direct Injection Natural Gas Engine,” Flow, Turbul. Combust., 2015.

Question: is there an advantage if only a small amount is directly injected (into a premixed environment)?

methane methane/hydrogen

PFI

PFI M100 PFI M85H15 PFI M75H25

8bar

Basi

s

methane

PFI

Hydrogen Direct fuel injection

Massflow meter

DFI H100 PFI M100

30bar

8bar

H2

DI

Druckregler PFI

PFI

methane DFI M100 PFI M100

8bar

30bar

CH4

DI

Experimental Approach

Swissauto 0.25 l single cylinder engine on test bench (bore 75mm, stroke 56.5mm, compression ratio 12.5, operated at 3500 rpm and WOT)

Experimental Approach

PFI DI

air

Experimental Approach

Swissauto 0.25 l single cylinder engine on test bench (bore 75mm, stroke 56.5mm, compression ratio 12.5, operated at 3500 rpm and WOT)

Single-hole DI injector with bent injection direction

Side-mounted DI injector (between intake and exhaust valves) with four injection angles experimentally covered

8°

Fuel Properties and Test Cases PFI DI total total

CH4 [vol%]

H2 [vol%]

CH4 [vol%]

H2 [vol%]

CH4 [mass%]

H2 [mass%]

CH4 [energy%]

H2 [energy%]

100 0 0 0 100 0 100 0

92.5 0 0 7.5 99 1 98 2

85 15 0 0 98 2 95 5

80 0 0 20 97 3 93 7

75 25 0 0 96 4 91 9

91 0 9 0 100 0 100 0 same energy split PFI/DI

The addition of H2 to methane leads to only very minor reductions of the expected power output

Theory: Injection of Gases What do we expect from DI of H2 and CH4 in terms of penetration

The H2 jets penetrates considerably less then the CH4 jet

(2) Birch, A. D., Brown, D. R., Dodson, M. G., and Swaffield, F. The structure and concentration decay of high pressure jets of natural gas. Comb.Sc. and techn., 36(5-6), 249-261 (1984) (1) Müller, F., Schmitt, M., Wright, Y., and Boulouchos, K., "Determination of Supersonic Inlet Boundaries for Gaseous Engines Based on Detailed RANS and LES Simulations," SAE 2013-24-0004

(1)

(2)

If the pressure-ratio is critical and above, an “underexpanded jet” is produced

(3) Bonelli, F., Viggiano, A., Magi, V., A Numerical Analysis of Hydrogen Underexpanded Jets Under Real Gas Assumption, Journal of Fluids Engineering, 135, 1-11 (2013)

(3)

Direct injection species / amount

Injection duration

H2 / 7.5 vol% (1 mass% or 2.4 energy%) 0.36 ms (72%) H2 / 20 vol% (3 mass% or 7 energy%) 0.70 ms (140%) CH4 / 9 vol% (9 mass% or 9 energy%) 0.50 ms (100%)

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Jet penetration (for same backpressure)

(4)

(4) Biffiger, H., Soltic, P., Effects of split port/direct injection of methane and hydrogen in a spark ignition engine, International Journal of Hydrogen Energy 40 , 1994-2003 (2015)

For our case

Results: Combustion Stability For injection direction towards spark plug (CWR0)

• DI of H2 led to better combustion stability when injected early, DI of CH4 led to better combustion stability when injected late

• DI of H2 does not show an advantage compared to premixed CH4/H2 fuel • Late DI of CH4 allows much leaner global combustion than for pure PFI

Results: Early Flame Phase For the initial combustion phase (ignition to 5% fuel mass burned)

• Especially at lean conditions, the early phase of combustion is strongly accelerated by late DI toward the spark plug (CWR0)

• Very late injection (EOI 50°CA bTDCF) is optimal for CH4, but not for H2 -> the directly injected H2 seems not to reach the spark plug

Results: Early Flame Phase Ignition setting (for the example 7.5 vol% H2 and CWR0)

• MBT ignition settings vary considerably for different injection settings

Results: Later Flame Phases

• The later combustion phases depend also on the injection settings, but not that pronounced as the early flame phases

Results: Efficiency Ignition always MBT, Variation of Lambda

• H2 addition to the fuel shows the best efficiencies, DI does not show an advantage • Split injection of CH4 leads to the highest output, the efficiency behavior does not show

an advantage • But: all this was done on an engine, not optimized for DI, engine adaptions, accompanied

by CFD simulations, would be necessary to unveil the real potential

Results: NOx Emissions Ignition always MBT, Variation of Lambda

• DI in direction of the spark plug leads to fuel-richer zones which increses NOx at globally lean conditions (But: all this was done on an engine, not optimized for DI)

Results: HC (Methane) emissions Ignition always MBT, Variation of Lambda

• DI in direction of the spark plug shows disadvantages regarding HC emissions (most probably due to flame quenching effects)

• But: all this was done on an engine, not optimized for DI

Conclusions

Split PFI/DI injection gives new degrees of freedom for mixture formation of gaseous fuels

Small amounts of DI are possible with comparably small injectors To exploit the full potential, the engine would have to be laid out

accordingly CFD investigations would help to understand and optimize the

processes involved (e.g. swirl/tumble flow)

Acknowledgments

Competence Center Energy and Mobility for financial support Empa Nanoscale Materials Science Laboratory and Oerlikon Balzers

for coating of the DI injectors