free piston engine based off-road vehicles · cylinder (opoc) design • direct injection •...

Marquette University | Milwaukee School of Engineering | Purdue University | University of California, Merced | University of Illinois, Urbana-Champaign | University of Minnesota |

Vanderbilt University

Free Piston Engine Based Off-Road Vehicles

Chen Zhang, Keyan Liu, Feng Wang

Prof. Zongxuan Sun

University of Minnesota

Industry/University Engagement Summit

June 6 – 8, 2016

2

Outline

• Project Overview

• Control of FPE

• Trajectory based combustion control

• FPE based independent pressure and flow control

Basic concept

Demonstration through simulations

• Summary and future work

3

Project Overview Major

Objectives/Deliverables

Industry support

• What are your research goals?

Investigate the design, control and testing

of the FPE based off-road vehicles to

improve their fuel efficiency and reduce

emissions

• How does this project fit into the CCEFP’s

overall research strategy?

Increasing energy efficiency of fluid power

Improving and applying the energy storage

capabilities of fluid power

Reducing environmental impact of fluid

power

• What is the original contribution of this

project?

Controlling hydraulic FPE in real-time to

generate the required pressure and flow

rate Independently.

designing appropriate hydraulic actuation

system for both linear and rotary motion to

reduce or remove throttling losses.

How can industry help / contribute?

Providing operating duty cycle for off-

highway vehicles.

Providing industrial guidance on modeling,

experimental system design and

applicability of this technology

• What are the expected major objectives

and/or deliverables?

Control of the FPE to provide required

pressure and flow rate independently.

Design of efficient hydraulic actuation

systems for modular and digital fluid power

sources.

Evaluation of the FPE based off-road vehicles

and comparison with conventional vehicles.

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Control of FPE: the FPE at UMN

• Opposed Piston Opposed

Cylinder (OPOC) Design

• Direct Injection

• Uniflow Scavenging

Variable compression ratio

• Advanced combustion strategy

• Multi-fuel operation

Reduced frictional losses

Fast response time

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

5

• System Modeling– Combustion model

– Hydraulic model

– Gas dynamics

– Piston dynamics

• Hardware improvement– Sensor identification

– Sensor calibration

– Pre-charge system

– DAQ and control system

– Moog valve and Lee valves

– Ignition control

– High pressure DFI system

– Supercharger system

• 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

Control of FPE: Virtual Crankshaft

Experiment set-up in UMN test cell

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Control of FPE using virtual crankshaft:

Motoring Test

Gas pressure, hydraulic chamber pressure and piston

tracking performance (from top to bottom)

Virtual crankshaft is able to actively

regulate the piston motion of the

FPE to track any prescribed

trajectory reference. [1]

Feedforward controller is also

developed to further improve the

performance of the virtual

crankshaft mechanism. [2]

[1] Li, K., Sadighi, A. and Sun, Z. (2014). Active motion control of a

hydraulic free piston engine. IEEE/ASME Transactions on Mechatronics,

volume (19), pp. 1148-1159.

[2] Li, K, Zhang, C. and Sun, Z., "Precise piston trajectory control for a

free piston engine." Control Engineering Practice, 34 (2015): 30-38.

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(Top to bottom): Piston motion, combustion chamber

pressure, hydraulic chamber pressure and control signal

Continuous combustion

operation is achieved

Each fuel injection causes a

strong combustion occurrence

Virtual crankshaft is able to

maintain continuous engine

operation even with cycle-to-

cycle combustion variation

Frictional loss (FMEP):

50Kpa (conventional ICE

with the same size: 140Kpa)

Control of FPE using virtual crankshaft:

Continuous Combustion test

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Virtual

crankshaft

mechanism

Trajectory based

combustion control

• Improved thermal efficiency [3]

• Reduced emissions [4]

• Optimal trajectory based on load requirement and fuel property [5]

Independent pressure and

flow rate control

• Producing the required flow rate at different pressure in real time

• Fast response time to load variation

[3] Zhang, C., Li, K. and Sun, Z., “Modeling of Piston Trajectory-based HCCI Combustion Enabled by a Free Piston Engine”, Applied Energy, vol. 139, pp. 313-326, 2015.

[4] Zhang, C. and Sun, Z., “Using Variable Piston Trajectory to Reduce Engine-out Emissions”, Applied Energy, vol.170, pp. 403-414, 2016.

[5] Zhang, C and Sun, Z., “Optimization of Trajectory-based HCCI Combustion”, DSCC 2016.

9

Trajectory-based combustion control

Virtual crankshaft

Piston

Trajectory

Volume

Gas

Dynamics

Chemical

Kinetics

Pressure

Temperature

Species Concentration

Thermal Energy

Reaction Rate

Reaction Products

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Trajectory-based combustion controlAsymmetric piston trajectories:

1. Fixed CR and fixed frequency.

2. Compressions are the same.

3. The shape of each trajectory is

changed after the 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, the three asymmetric trajectories

can be easily achieved in the HFPE with the virtual crankshaft mechanism.

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Trajectory-based combustion control

By using asymmetric piston trajectories, both engine thermal efficiency and NOx

emission are improved simultaneously. (Compared to conventional ICE)

The performance gain achieved by asymmetric trajectory is more obvious at high

compression ratio.

Thermal efficiency comparison NOx emission comparison

12


Virtual

crankshaft

mechanism

Trajectory based

combustion control

• Improved thermal efficiency [3]

• Reduced emissions [4]

• Optimal trajectory based on load requirement and fuel property [5]

Independent pressure and

flow rate control

• Producing the required flow rate at different pressure in real time

• Fast response time to load variation

[3] Zhang, C., Li, K. and Sun, Z., “Modeling of Piston Trajectory-based HCCI Combustion Enabled by a Free Piston Engine”, Applied Energy, vol. 139, pp. 313-326, 2015.

[4] Zhang, C. and Sun, Z., “Using Variable Piston Trajectory to Reduce Engine-out Emissions”, Applied Energy, vol.170, pp. 403-414, 2016.

[5] Zhang, C and Sun, Z., “Optimization of Trajectory-based HCCI Combustion”, DSCC 2016.

13

Independent Pressure and Flow control

Virtual

Crankshaft

Hydraulic FPE

IPFC

Output flow rate

at load pressure

Piston Position

Servo

valve

signal

Reference

Measured

load pressure

Required flow rate

Fuel injection Amount

• The key component is the Independent Pressure and Flowrate Controller (IPFC),

which is able to synthesis a unique trajectory reference for the hydraulic FPE,

and derive the corresponding fuel injection amount, according to required flow

rate and measured load pressure.

• The synthesized trajectory reference is then sent to the virtual crankshaft, which

ensures accurate piston motion tracking by adjusting the opening of the servo

valve through different servo valve signal.

• The variable opening of the servo valve can also affect the output flow rate at

different load pressure produced by the FPE

Basic Concept

+Error

-

14

Positive

Chamber 1&3

Load

To Load

Net flow

Chamber 2

From Tank

To Tank

From Load

Negative


1

3

2

Basic Concept

15


Switching Point+

-

+

-

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Trajectory Generation


Net Hydraulic Force

Left Gas Force Right Gas Force

• The piston pair is subject to the

hydraulic force and the gas

forces.

•The hydraulic force is the net

force in all hydraulic chambers,

while the gas force is subject to

ideal gas law.

•By switching the servo valve

between the top position and

bottom position, the hydraulic

force direction is changed.

•The piston motion is subject to

the Newton second law 𝑥 = −𝐹𝑔_𝑙 − 𝐹𝑔_𝑟 ± 𝐹ℎ𝑦

𝑚

𝐹𝑔_𝑙 𝐹𝑔_𝑟

𝐹ℎ𝑦

𝐹𝑔_𝑙: Ideal gas law,

Instantaneous combustion model

𝐹𝑔_𝑟: Ideal gas law

𝐹ℎ𝑦 = (𝑃𝑙𝑜𝑎𝑑 − 𝑃𝑡𝑎𝑛𝑘) × 𝐴𝑝𝑖𝑠𝑡𝑜𝑛, Constant

17

Different piston trajectory leads to various flow rate at a specific load pressure.


18

Desired

actuator speed

Actuator

pressure

Valve opening cmd

Hydraulic

actuator

+

+ PIDFree Piston

Engine Actuator

speed

Flow

Fraction of

displacementValve

Hydraulic plant

Hydraulic pressure source

Fluid

capacitor

_

Delta

pressure +

+

Source

pressure

PID

_

Combining FPE with the actuator


Control scheme of hydraulic controls for the FPE and actuator

Virtual

Crankshaft

Hydraulic FPE

IPFCPiston Position

Servo

valve

signal

Reference

Fuel injection Amount

+Error

-

Output flow rate

at load pressureMeasured

load pressure

Required flow rate

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Case study: wheel loader working hydraulic circuit

S

C

R

PM2

PM1

ICE

Final

drive

Lift

cylinder

Tilt

cylinder

Working hydraulic system

Hydraulic power split drivetrain

LA LB TA TB

LS

compensator

Pressure

limiter

Loading-sensing pump

PM1–Pump/motor1

PM2–Pump/motor2

Planetary

gear set

LA – Lift chamber A

LB – Lift chamber B

TA – Tilt chamber A

TB – Tilt chamber B

Working hydraulic circuit

Drivetrain

20


21

Working hydraulic circuit simulation results

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Summary and future work

• With proper reference trajectory and fuel

injection strategy, the FPE can work as a fluid

power source that independently control the

output flow and pressure.

• Power source side simulation shows that the

corresponding trajectory can be acquired and

the working principle has been verified with

simulations.

• Actuator side simulation shows that such flow

source can be utilized by the off-road vehicle

hydraulic circuit.

• Next, we will combine the models of the

power source and the actuator to further verify

the proposed idea.

23

Hydraulic FPE vs. Digital Pump

𝑄𝑜𝑢𝑡 = 𝑄𝑙𝑜𝑎𝑑 𝑄𝑜𝑢𝑡 = 𝑄𝑙𝑜𝑎𝑑 − 𝑄𝑑𝑟𝑎𝑖𝑛𝑄𝑜𝑢𝑡 = 𝑄𝑙𝑜𝑎𝑑

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Trajectory Generation


Load

Fraction of

displacement

Start

Set the left chamber at the TDC

Set valve timing according to Dx

Adjust fuel amount so that the piston returns

to the same TDC

Numerically Calculate the trajectory

Calculate corresponding

flowrate

End

Record the fuel amount and trajectory

Off-line

Sweeping

free piston engine based off-road vehicles · cylinder (opoc) design • direct injection •...

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