FREE-PISTON ENGINE

Terry Johnson Sandia National Laboratories

Tuesday, May 14, 2013

Project ID: ACE008 This presentation does not contain any proprietary, confidential, or otherwise restricted information

Overview

Timeline • Project provides fundamental

research that supports DOE/ industry advanced engine development projects.

• Project currently scheduled for completion this fiscal year

Budget • Project currently funded by DOE/VT • FY12: $50K (ACE) and $50K (FT) • FY13: $100K (ACE) (Carryover funds supplemented FY12 and FY13

and were expended.)

Barriers • Increased thermal efficiency (>50%)

via high compression ratio • Petroleum displacement : multi-fuel

capability via variable compression ratio (hydrogen, ethanol, biofuels, natural gas, propane)

• Reduced emissions: lean, premixed combustion (LTC/HCCI)

• Low cost and durability: port fuel injection, uniflow port scavenging

Partners • General Motors/Univ. of Michigan

CRL • Ronald Moses (LANL)

RELEVANCE

Goals of the Free-Piston Engine Research Prototype

• Study the effects of continuous operation (i.e. gas exchange) on indicated thermal efficiency and emissions of an opposed free-piston engine utilizing HCCI combustion at high compression ratios (~20-40:1)

• Concept validation of passively synchronizing the opposed free pistons via the linear alternators, providing a low cost and durable design

• Proof of principle of electronic variable compression ratio control, allowing optimized combustion timing and fuel flexibility, by means of mechanical control of bounce chamber air pressure

• Provide a research tool to explore the free-piston engine operating envelope across multiple inputs, such as boost level, equivalence ratio, and alternative fuels

• Explore the potential thermal efficiency of an HCCI engine at high compression ratio by removing constraints typically imposed due to mechanical concerns (e.g. limiting compression ratio and heat release rate)

APPROACH

Unique opposed piston design enables high efficiency with low complexity

Opposed Free-Piston Engine • Mechanically-simple control of compression ratio • Port fuel injection, two stroke configuration minimize cost • Uniflow scavenging and potential for boosted intake to improve

power density Piston Synchronization through Passive Coupling of Linear Alternators • Loads also act to synchronize pistons, reducing complexity and cost • Stators on either side of center are tied to a common load, providing

a stabilizing force Bounce chambers Combustion

chamber

Linear alternators

FY12 and FY13 Milestones Time Milestone or Go/No-Go Decision

Jan 2012 Magnets bonded onto backirons. Completed pistons installed in engine.

Feb-Mar 2012 He start tests performed with magnets. Analysis of electrical current and piston position data showed evidence of passive synchronization.

May 2012 New bounce chamber vent manifolds and stop system installed

June 2012 N2 supply system and new bounce chamber heads installed

July-Aug 2012 Tests reveal limitation in air drive supply; new tubing and fittings installed

Sept 2012 Motoring for greater than 1 sec (~30 cycles) achieved

Oct 2012 New lubrication system and valve plates installed

Dec 2012 Motoring for > 4 sec; initial single-shot combustion experiments completed

Jan-Feb 2013 Numerous motoring tests at ~10sec duration demonstrate passive piston synchronization; first combined motoring/combustion experiments conducted

Mar-Apr 2013 New pistons installed. Go/No-Go: Reduced friction demonstrated?

May-June 2013 Combustion experiments over a range of equivalence ratios and compression ratios to assess efficiency

July-Aug 2013 Expanded range of operating conditions and extended run times to demonstrate efficiency and low emissions. Complete the project and document the results.

TECHNICAL ACCOMPLISHMENTS

Bounce chamber gas supply and vent systems redesigned to enable FPLA performance evaluation

Original manifolds were too small; didn’t allow for full venting New design has ~2L volume and much larger flow area to vent tanks

Solenoids

Solenoid system designed to bring pistons to a safe, controlled stop

New Manifold

BC head modified for extended travel

Pin length increased from 1.1” to 1.35”

1.0” tubing added

When stop system is triggered, He is injected from high pressure reservoir into vent manifold

Valve plates: • Larger sealing surface • Tighter radial fit • Higher strength material • Thicker

New Head Supply Tubing He Injection

Original tubing diameter limited flow into bounce chamber New design has larger volume and much larger flow area

BC Heads: • Valve travel extended

from 0.5” to 1” • Spring retainers added

A number of other design improvements were needed to enable consistent motoring

N2 supply system enables motoring with increased pressure and flow rate

Vacuum pump used to evacuate BC vent manifold at start Allows for more efficient He venting

New oil distribution system installed; allows for controlled, continuous lubrication

New head pressure regulator extends to 1500 psi

New additions to the system were required for combustion experiments

Hydrogen fuel injector and flow meter calibration

Failure Modes Effects and Criticality Analysis (FMECA) completed and reviewed with site ES&H personnel Control electronics and software completed for fuel injector control and safeguards

Zero and span gases for analyzers

O2, NOx, CO, CO2, and HC analyzers calibrated

Regulator

Flow meter

Fuel injectors

Hydrogen fuel supply system

Solenoid valve

Pressure gage Buffer volume

Continuous motoring time increased exponentially once system improvements were completed

2012 2013

New vent manifolds and stop system

N2 system installed

New BC head manifold

New valve plates and lubrication system

Motoring tests have demonstrated passive piston synchronization through alternator force

Synchronization error: Relative offset in piston position with respect to engine centerline Many motoring tests have shown synchronization error less than 1-2 mm

Piston travel is ~220 mm Stop system triggers at 12.7 mm error

Piston synchronization error has significant effect on electrical output Synchronization error of 11 mm reduces output by ~25%

25% reduction when error is 11mm

5 longest motoring tests

Errors generally less than 4 mm High error at end of tests due to stop system

Motoring energy balance shows work-to-electrical efficiency ~50%; high frictional losses (25-35%)

• Power input calculated from bounce chamber P-V integral • Electrical output calculated from measured currents and load resistance • Thermal losses calculated from combustion chamber P-V integral • Resistive losses calculated based on coil and lead resistances • Alternator efficiency assumed to be 85% or 95%; eddy current losses • Friction losses taken as the remainder of the energy balance

Motoring Tests

Several preliminary combustion experiments were carried out prior to replacing pistons

• Single shot combustion tests carried out in Dec. 2012 – H2/air mixtures from 1% to 4% corresponding to equivalence ratios of

0.024 to 0.1 were tested – P-V calculations indicate that all mixtures compression ignited and

burned • Motoring/combustion with 1% H2 mixture (phi = 0.024) in Feb. 2013

– Injectors were triggered after 10 motoring cycles – Oil and H2 combustion over first 5-6 injection cycles – H2 HCCI inconsistent over remaining cycles – Piston motion died out after a little over one second

• Motoring/combustion with 1.5% H2 mixture (phi = 0.036) in Feb. 2013 – Injectors were triggered after 15 motoring cycles – Oil and H2 combustion over first 5-6 injection cycles – H2 HCCI fairly consistent over remaining cycles – Test lasted for ~10 seconds before motion died out

Integrals of combustion chamber P-V curves show H2 combustion energy (preliminary)

• Combustion data is from 1.5% H2 case • Motoring data typically shows a negative

CC energy on the order of 100 J/cycle – Assumed to be thermal and gas losses

• Combustion data shows positive CC energy with 127 J/cycle average

• Gross CC energy added per cycle is ~227 J • H2 energy added per pulse is ~360 J • Gross ind eff = 63%; Otto cycle eff = 76%

Example logP vs logV curves Motoring Energy Summary

Combustion Energy Summary

Cycle Number

Cycle Number

Preliminary combustion results show higher power but lower efficiency

• Net energy input is ~15% higher with combustion – 1300 J/cycle vs 1115 J/cycle

• Energy output is only 2.5% higher – 572 J/cycle vs 558 J/cycle

• Efficiency drops from 50% to 44%

• Power output increases from 18 kW to 19.5 kW due to increased speed

Motoring Energy Summary

Combustion Energy Summary

Cycle Number

Cycle Number

COLLABORATION AND COORDINATION

We have been collaborating with GM and U of M since May 2009

General Motors / University of Michigan • A new phase of collaboration with the GM/UM CRL began in May

2009. • GM/UM CRL will assess Sandia free-piston engine (FPE) potential

with a model built in a MATLAB/Simulink framework featuring 0-D thermodynamics, a linear alternator model, a bounce chamber air control model, and dynamic force balance.

• The model will be validated with respect to Sandia FPE experimental data and used to explore conditions outside the experimental matrix.

FUTURE WORK

There is a large operating space to explore with combustion experiments

Combustion Goals – Explore the limits of thermal efficiency – Operate with at a variety of compression ratios and equivalence

ratios to demonstrate the engine’s flexibility

Control Knobs: BC head pressure, vent pressure, and fuel load Variable Parameters: Equivalence ratio, compression ratio, and operating frequency Performance Metrics: gross indicated efficiency, fuel-to-electrical efficiency, scavenging efficiency Emissions: NOx and O2 for H2; CO, CO2 and HC’s indicate oil combustion

New pistons should greatly reduce friction; increase system reliability and “uptime”

Full rings Half Rings Velocity m/sec

Force N

Velocity m/sec

Force N

13.0 -710 13.0 -342 -12.1 812 -12.2 560 8.6 -640 9.2 -317

Tests with cut rings showed significant reduction in friction force

New Combustion End

New Bounce End

New Backiron

New pistons are ready to be assembled

Rider ring groove Compression ring grooves

We are pursuing a staged extension to demonstrate all project goals with new pistons

• Stage 1: Install new pistons and demonstrate improved performance

• Stage 2: Investigate a range of equivalence ratio, compression ratio, and operating frequency using simple control scheme

• Stage 3: Investigate a broader parameter space and carry out emissions analysis with enhanced control capability

• Documentation: Final project report, journal publication(s) by September 2013

Summary • Numerous improvements were made to the system to

enable continuous motoring • New hardware and software were added to the system to

allow for hydrogen combustion experiments • Passive synchronization of the pistons was successfully

demonstrated through continuous motoring tests • Combustion experiments began in Dec. 2012 and continuous

motoring/combustion was achieved in Feb. 2013 • New, low-friction pistons were designed, fabricated and

installed; experiments ongoing • Collaboration with GM/UM CRL on modeling of the Sandia

FPE prototype engine is ongoing

BACKUP SLIDES

An air-driven dynamometer will be used to initiate combustion tests

Air-Driven Dynamometer • First cycle initiated by high pressure

injection of helium through solenoid valves and check valves in each head

• Motoring of engine accomplished by air injection into bounce chambers through valves actuated by each piston

• Control of air injection pressure and vent pressure allows adjustment of compression ratio while motoring

• As fuel is introduced, vent pressure is increased with injection pressure held constant, reducing the energy input by dynamometer

Modeling used to predict operating regime for motoring with different pin lengths

1.100" pins 1.225" pins 1.350" pins air drive pressure

compression ratio

valve actuation

vent pressure

compression ratio

valve actuation

vent pressure

compression ratio

valve actuation

vent pressure

850 ----- ----- ----- ----- ----- ----- 26 0.16 122 900 ----- ----- ----- ----- ----- ----- 33 0.22 128 950 ----- ----- ----- 26.7 0.08 121 41 0.31 135 1000 ----- ----- ----- 33 0.17 126 50 0.4 142 1050 ----- ----- ----- 40 0.24 133 ----- ----- ----- 1100 30 0.08 122 ----- ----- ----- ----- ----- ----- 1150 36 0.15 127 ----- ----- ----- ----- ----- ----- 1200 43 0.21 132 ----- ----- ----- ----- ----- -----

Modeling predicts that transition to combustion with one fuel injector is feasible

1.350" pins, 1mg H2 addition air drive pressure

CR before fuel addition

CR after fuel addition

valve actuation before fuel addition

valve actuation after fuel addition

vent pressure

850 26.2 33.4 0.13 0.24 122 900 33.1 42 0.22 0.33 128 950 41 52.2 0.32 0.42 135

1000 50.1 63.9 0.4 0.5 142

• Two control channels for fuel injectors • 1 injector on channel A and 4 injectors on channel B • Start with channel A injector and vary pulsewidth to get from 1 mg/pulse

to 4 mg/pulse • Then introduce channel B injectors and vary pulsewidth to get from 5

mg/pulse to 20 mg/pulse (max load of 72 kW at 30 Hz)

Compensation by electrical load suggested by model; confirmed by experiment

• From model: Friction Difference = 11%

• Friction Difference = 11%, Coil #2-7 removed

• This effect was confirmed with motoring tests.

Single shot combustion tests with H2 have been performed at several equivalence ratios

• Air motoring was disabled for these tests

• Combustion chamber was pre-filled with desired H2/air mixture

• H2/air mixtures from 1% to 4% corresponding to equivalence ratios of 0.024 to 0.1 were tested

• P-V calculations indicate that all mixtures compression ignited and burned

Combustion Chamber Pressure

Piston Position

TDC

TDC

Time (sec)

Increasing piston speed and travel with fuel load

* Ringing in pressure measurement may be caused by transducer adapter which is being replaced

*