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MEMS as Fuel Injectors
Patrick FlahertyME 2082
Why use MEMS as fuel injectors?
• Quicker response time than conventional solenoid system
• More accurate control of fuel, higher efficiency
• Easier to obtain cleaner emissions, comply with emissions legislation
Emissions Control Legislation
MEMS as Fuel Injectors
MEMS technology with potential as fuel injection systems:
• Thermal Actuation• Electrostatic Actuation• Piezoelectric Actuation
Thermal Actuation
• Technique is very similar to that utilized by thermal inkjet printer, well understood
• Uses thermal energy to form a bubble inside a chamber, which acts as a pump to eject a droplet
• Nozzle area atomizes the jet of liquid, increasing efficiency of combustion
Thermal Actuation
Principles of operation:Driving Force
Velocity:
sstv PtPPtP +−−= 5.0)exp()()(τ
∫∫ =+ 21 )(21 mVdAdttPmV v
∫= dttPd
V vd
)(4
32 ρ
Thermal ActuationPenetration distance
in air:
Temperature profile of diesel fuel:
Clausius-ClayperonEquation:
5.12
1195.0 dVv
xaa
d
ρρ=
−−+= ∞ )()exp(1 2 ζζξπρ
erfccktqTT
dpd tx
dαζ
2=
fgv
satsat hR
TTTρσ2+=
Thermal Actuation – Diesel Activiation Curve
Thermal Actuation - Application
• Low penetration distance in air has not yet been improved. May be improved by airflow-assisted injection.
• Diesel flashpoint of 71°C may cause combustion in actuator
Electrostatic Actuation
• Uses double-acting diaphragm pump to inject fuel into the combustion chamber
• Originally researched as a potential injection system for PDE, which requires a high degree of atomization
• Requires less energy than thermal actuation, and is less temperature sensitive than piezoelectric actuation
Electrostatic Actuation
Electrostatic Actuation – Principles of Operation
Electrostatic Actuation – Principles of Operation
2
12h
LvP µ=∆ 2
8
hRLvP µ=∆
Driving Force:
(Hagen-Poiseuille)
Total Force on Diaphragm:
Electrostatic Force:
Spring Force:
ke FFF +=
[ ]2))((
22__
__
)(21
)(dielectricfuelrfueldielectricr GtyG
dielectricrfuelroe
tVtF
εε
εεε+−
=
)()( tyktF sk ==
Electrostatic Actuation - Application
• Analytical models have been qualified using an inkjet type of test setup
• Prototypes have been built, but experimental data has not yet been obtained
• Fabrication may prove to be challenging
Piezoelectric Actuation
• The only method that is currently being used in production engines (Denso, Siemens)
• Simple, easily manufactured
Piezoelectric Actuation – Principles of Operation
Piezoelectric material exhibits strain in the presence of an electric field:
Piezoelectric Actuation
Piezoelectric Actuation -Application
• Rapid switch action, friction-free• More precise dosing than solenoid
system• Improved atomization• Emissions reduced up to 20 percent
Summary
• MEMS techniques (such as thermal and electrostatic actuation) continue to be researched as options for fuel injection
• Piezoelectric actuation has been adapted successfully in diesel fuel injection systems
• MEMS fuel injection, specifically piezoelectric, will probably be implemented in gasoline engines in the near future