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