Georgia Institute of Technology | Milwaukee School of Engineering | North Carolina A&T State University | Purdue University

University of Illinois, Urbana-Champaign | University of Minnesota | Vanderbilt University

2B.3 - Free Piston Engine Hydraulic Pump

Chen Zhang

Prof. Zongxuan Sun

University of Minnesota

2

Outline

• Introduction

• Previous Achievements

• Progresses in this year

Continuous combustion tests with supercharge

Trajectory effects on emissions performance

• Next steps

FPIRC 2015

10/15/2015

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Project Summary

2B.3 Hydraulic Free Piston Engine Pump

• Supply fluid power (10kW-500kW) in an efficient and compact manner for

mobile applications including both on-highway and off-highway vehicles with

hydraulic free piston engine

• It will address two transformational barriers as outlined in the ERC strategic

plan: compact power supply and compact energy storage.

• It will directly support the test bed: hydraulic hybrid passenger vehicle and it

is also applicable to the excavator test bed.

FPIRC 2015

10/15/2015

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Background and Motivation

Crankshaft

Based ICE

Rotational

Hydraulic

Pump

• In fluid power systems, the current practice for generating high pressure fluid onboard is to use crankshaft based gasoline or diesel engine with a rotational hydraulic pump.

• Is it possible to significantly improve the efficiency of both the ICE and the pump?

• Hydraulic free-piston engine with advanced combustion: leverage the high efficiency of advanced combustion and the power density of

hydraulic system.

Current fluid power generating unit for mobile applications

FPIRC 2015

10/15/2015

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Advantages of FPE

• Opposed Piston Opposed

Cylinder (OPOC) Design

• Direct Injection

• Uniflow scavenging

Variable compression ratio

• Advanced combustion strategy

• Multi-fuel operation

Reduced frictional losses

Higher power density

Internally balanced

Modularity

Exhaust Ports

Intake Ports

IntakePorts

ExhaustPorts

Check Valves

Servo Valve

On-off Valve

On-off Valve

LP

HP

Outer Piston Pair

Inner Piston Pair

Hydraulic Chambers

FPIRC 2015

10/15/2015

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Previously on 2B.3• System Modeling

– Combustion model

– Hydraulic model

– Gas dynamics

– Piston dynamics

• Hardware improvement– Sensor identification

– Sensor calibration

– Pre-charge system

– Lubrication system

– DAQ and control system

– Moog valve and Lee valves

– Ignition control

– Fuel Injection

• Implementation of Advanced Control– Virtual Crankshaft design

– Engine motoring tests

– Engine combustion tests

The developed robust repetitive controller acts as a

virtual crankshaft that would force the piston to follow

the reference signal through the hydraulic actuator.

• Engine start

• Misfire recover

• Real time frequency and compression ratio control

FPIRC 2015

10/15/2015

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Experiment Set-up and Subsystems

FPIRC 2015

10/15/2015

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Achievements in the last year• Upgrade of the FPE subsystems

– High pressure fuel injection system

• Boost the injection pressure to 1500 psi

• Reduce the fuel injection duration significantly

• Improve the air fuel mixing to benefit the combustion afterwards.

– Supercharge system

• Assist the mechanical scavenging pump to further boost the intake pressure

• Ensure sufficient fresh air blowing into the combustion chamber.

• Improvements on virtual crankshaft

– Feedforward control

• Further improve the tracking performance of the virtual crankshaft

– Transient control

• Eliminate the transient performance within an engine cycle

• Maintain appropriate TDC location in each cycle to realize continuous

combustion performance.

• Continuous combustion test

FPIRC 2015

10/15/2015

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Supercharge system for the FPE

Air tank Pressure regulator Intake manifold Gas filter

FPIRC 2015

10/15/2015

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4.2 4.25 4.3 4.35 4.4 4.45 4.5 4.55

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2x 10

6

Pa

4.2 4.25 4.3 4.35 4.4 4.45 4.5 4.550

10

20

30

40

50

60

Time[s]

pos

intake press

inj

With supercharge system, intake charge pressure is around 35 psi (2.5 bar)

1.9 1.95 2 2.05 2.1 2.15 2.2 2.25 2.3

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2x 10

6

Pa

1.9 1.95 2 2.05 2.1 2.15 2.2 2.25 2.30

10

20

30

40

50

60

Time[s]

pos

intake press

inj

Without supercharge system, intake charge pressure is around 21 psi (1.5 bar)

Intake charge pressure comparison

FPIRC 2015

10/15/2015

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Transition when switch from motoring to firing Piston motion after applying the transient control

Improvements on the virtual crankshaft:

Transient control

FPIRC 2015

10/15/2015

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(Top to bottom): Piston motion, combustion chamber

pressure, heat release rate and control signal

Continuous combustion test without supercharge

system

FPIRC 2015

10/15/2015

Multiple combustions are

achieved.

Virtual crankshaft is able to

maintain engine operation

even with large cycle-to-cycle

combustion variation

What is the reason caused this

cycle-to-cycle variation and

how to deal with it?

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(Top to bottom): Piston motion, combustion chamber

pressure, heat release rate and control signal

Continuous combustion test with supercharge

system

Each fuel injection causes a

strong combustion occurrence

Supercharge system forces

sufficient fresh air flowing

into the combustion cylinder.

Virtual crankshaft mechanism

with the transient control is

able to realize the continuous

combustion performance in

the HFPE.

FPIRC 2015

10/15/2015

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Trajectory-based combustion control

Fuel Injection

Valve Timing

Spark Timing

Piston Trajecotry

Free Piston Engine

Fuel Injection

Valve Timing

Spark Timing

Conventional ICE

• Cycle-to-cycle discrete control

• Limited effects on engine cycle performance

• Only apply to a specific fuel

• Continuous in-cycle control

• Affect the processes prior, during and after

combustion

• Apply to any types of fuel (alternative fuels)

Fuel Economy

+

Emissions

FPIRC 2015

10/15/2015

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Trajectory-based combustion control

Virtual crankshaft

Piston

Trajectory

Volume

Gas

Dynamics

Chemical

Kinetics

Pressure

Temperature

Species Concentration

Thermal Energy

Reaction Rate

Reaction Products

FPIRC 2015

10/15/2015

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Higher efficiency is achieved in HFPE due to less heat loss.

HFPE has the capability of igniting extremely lean fuel.

Efficiency gain achieved by HFPETemperature profiles under extremely

fuel-lean condition (AFR = 30)

Trajectory effects on engine efficiency

FPIRC 2015

10/15/2015

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Trajectory effects on emissionsCharacteristics of the piston trajectories:

1. Fixed CR and fixed frequency.

2. Compressions are the same.

3. The shape of each trajectory is

changed after TDC point, which

means each trajectory has different

expansion process.

4. Compression trajectories are

determined to ensure the combustion

occurs at the TDC point and

expansion processes are designed to

reduce NOx emission.

Due to the ultimate freedom of trajectory movement, this asymmetric trajectory can be

easily achieved in the HFPE with the virtual crankshaft mechanism.

FPIRC 2015

10/15/2015

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Trajectory effects on emissions

Temperature profiles along three trajectories NOx emissions along three trajectories

Trajectory Indicated efficiency NOx emission [ppm]

Blue 52.89% 504

Green 53.47% 339

Red 53.84% 242

FPIRC 2015

10/15/2015

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The dominant reaction for NOx

production: (based on Zeldovich

Mechanism)

O + N2 => NOx + N

Trajectory effect on emissions NOx productions

][][][

2NOK

dt

NOxd ]/38000exp[106.7 13 TK

2 factors affect NOx production rate:

• Reaction rate constant K.

• Species concentrations [O], [N2].

FPIRC 2015

10/15/2015

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Trajectory effect on the emissions production

2 factors affect NOx production rate:

• Reaction rate constant K.

• Species concentrations [O], [N2].

][][][

2NOK

dt

NOxd

Piston trajectory causes important influences on the emissions production.

The dominant reaction for NOx

production: (based on Zeldovich

Mechanism)

O + N2 => NOx + N

Species amount / Volume

FPIRC 2015

10/15/2015

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Free Piston Engine Hydraulic Pump Major Objectives/Deliverables

Next StepsProgress

• Project Goal: Supply fluid power in an efficient

and compact manner for mobile applications.

• Two transformational barriers are addressed:

compact power supply and energy storage

• It directly support two test beds: hydraulic hybrid

passenger vehicle and the excavator test bed.

• It is the first time to achieve continuous

combustion performance in the hydraulic FPE

with the similar architecture.

• The tracking performance of the virtual

crankshaft is improved by adding feedforward

control and transient control method.

• The installed supercharge system improves the

combustion performance.

• The investigation of trajectory-based combustion

control demonstrates that both engine efficiency

and emissions are enhanced by applying the

optimal piston trajectory into the HFPE.

• Achieving the optimal piston trajectory based

on various loading conditions and chemical

kinetics of utilized fuels.

• Continuously improve engine combustion

performance and test virtual crankshaft at

different loading conditions.

Task 1: Trajectory-based combustion control

development • Investigation on the trajectory effects on the engine

performance

• Optimization of the HFPE piston trajectory

Task 2: Enhancement of HFPE system capability• Installation and testing of the supercharge system

• Installation of the necessary sensor to quantify the

engine efficiency

Task 3: Optimization of HFPE performance for

different mobile applications

FPIRC 2015

10/15/2015