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MEAM-446-2012-13 page 1 Copyright © 2012 by the authors

MEAM-446-2012-13 Senior Design Project - Final Report April 26, 2012

Department of Mechanical Engineering and Applied Mechanics School of Engineering and Applied Science

The University of Pennsylvania Philadelphia, Pennsylvania, USA

HYDRAULIC DRIVETRAIN WITH REGENERATIVE BRAKING

Andrew Brown Karan Desai

Andrew McGrath Alfred ‘Hurst’ Nuckols Grant Wilson

Dr. Andrew Jackson1

Robert L. Jeffcoat PhD1

faculty advisor instructor

1 Department of Mechanical Engineering and Applied Mechanics, University of Pennsylvania

ABSTRACT

Since their development in 2006, hydraulic

drivetrain systems have gained considerable attention as a

result of their high efficiencies. However, due to size and

weight constraints, their application has been exclusively

for large trucks and commercial vehicles. Our senior design

project investigates the use of hydraulic drivetrains on

small-scale vehicles. We have established two main project

goals: 1) to create a working model of a hydraulic system

on a go-kart, and 2) to assess the efficiency of our hydraulic

system in relation to other mechanical and hybrid

drivetrains. As with many hydraulic drivetrain systems, our

model incorporates regenerative braking in order to further

improve efficiency values. The main components of the

model include an electric motor, hydraulic pump, hydraulic

motor and two 1.6 gallon accumulators. The present

document describes the research, design, construction, and

analysis of our hydraulic drivetrain system.

1. INTRODUCTION AND BACKGROUND

Vehicles with hydraulic drivetrains and regenerative

braking were first introduced over five years ago, with the

first working model produced in 2006.[1] These vehicles

are exclusively large and commercial, such as road tractors

and more recently, UPS delivery trucks. Hydraulic hybrids

show potential for increased efficiency over traditional

vehicles despite an increase in weight. According to the

EPA and various other organizations, the new UPS

hydraulic hybrids are 60-70% more fuel efficient than a

traditional delivery truck, and the regenerative braking

systems capture 70% of the braking energy compared to

only about 20% in current electric hybrid vehicles.[1]

However, hydraulic hybrid passenger vehicles are

virtually nonexistent. For passenger vehicles, size and

weight come at a much higher premium than with

commercial vehicles. Hydraulic systems are typically large

and heavy, which causes additional design challenges for

small vehicle manufacturers. Additionally, hydraulic

systems are very noisy, an undesirable feature for most

drivers. With these considerations at hand, few automotive

corporations are looking into hydraulic hybrid systems for

their passenger vehicles.

Our senior design project investigates the application

of hydraulic drivetrains with regenerative braking on small-

scale vehicles. We believe the potential gains in efficiency

of hydraulic hybrid systems could far outweigh the design

challenges currently faced by small vehicle manufacturers.

We have established two main project goals: 1) to create a

working model of a hydraulic drivetrain system on a go-


kart, and 2) to assess the efficiency of our hydraulic

drivetrain system in relation to other mechanical and hybrid

drivetrains. The present paper outlines our research and

efforts towards the completion of these project goals.

2. REQUIREMENTS AND OBJECTIVES

We established the following project requirements:

The project must:

a) Exhibit a functional hydraulic drivetrain on

the go-kart model.

b) Demonstrate hydraulic regenerative braking

on the go-kart model.

c) Ensure the go-kart model operates safely

under normal driving conditions.

d) Show, through theoretical analysis, the

potential benefits of the hydraulic system.

The project should:

e) Optimize the system efficiency with regards to

the electric motor, hydraulic circuit and

regenerative braking.

f) Allow for the synchronous use of both the

hydraulic drivetrain and regenerative braking

system in the go-kart model.

g) Compare, through theoretical analysis, the

efficiency of the system as applied to full size

vehicles.

With an understanding of our qualitative goals, we

established quantitative measures in order to

objectively evaluate the success of our project. Table

2-1 illustrates these goals.

Table 2-1 Quantitative Project Goals

Feature Goal

Maximum Speed 10mph

Maximum Discharge Time 30 seconds

Regenerative Braking Efficiency >0%

Hydraulic Drivetrain Efficiency 60%

3. CANDIDATE CONCEPTS

The following concepts emerged as possible candidates

for our hybrid drivetrain system:

Full Closed Center Hydraulic – Incorporates a closed

center hydraulic pump to power hydraulic motor.

Allows for high efficiencies, although very

expensive.

Full Open Center Hydraulic – Incorporates an open

center hydraulic pump to power hydraulic motor.

Provides predictable control and is simple to build.

However, system efficiency is low.

Hydraulic-Over-Electric – Utilizes a conventional

electric hybrid drivetrain in combination with

hydraulic components. Allows for precise control

over braking and acceleration. Involves complex

circuitry and expensive components.

Full Pneumatic- Utilizes atmospheric air as the

working fluid. Simplifies the storage process of the

working fluid (no reservoir required). Components

are costly, bulky and relatively imprecise.

3.1 Comparison and Downselection

During the downselection process, these four candidate

concepts were evaluated on the following three measures:

cost – does the price of the concept components exceed

our budget?

complexity – is the concept feasible considering our

backgrounds in engineering?

performance – to what degree does the concept satisfy

our project requirements and objectives?

Table 3-1 compares the four candidate concepts using

these three measures. Furthermore, we have provided a

concise evaluation of each design concept:

The full closed center hydraulic system is appealing due

to its high efficiency and feasibility. The closed

center pump offers many desirable features for the

system, such as variable displacement operation.

However, the high cost of the closed center pump

poses a considerable challenge with our budget

constraints. With sufficient sponsorship and

donations, this design is optimal.

The greatest assets of the full open center hydraulic

system are its simplicity and low cost. Both of these

measures allow for a high degree of optimization

within the system (few variables and large remaining

budget). However, performance is still the greatest

concern. The lack of critical pump features may

make several of the project objectives unfeasible.

On the surface, the hydraulic-over-electric system is

attractive due to its precise control mechanism. The

added electronics allow for a high level of system

performance. Further consideration shows that the

system’s complexity makes it nearly unfeasible. A

strong background in both electronics and hydraulics

is required. Additionally, the high cost of the added

electronics poses a considerable project challenge.

One convenient feature of the full pneumatic system is

the fact that it utilizes atmospheric air as its working


fluid. The overall complexity of the candidate is low.

However, the components necessary for operation

are oversized for the intended small scale system.

Additionally, the cost of pneumatic devices is very

high.

Table 3-1 System Comparison

Closed Center

Open Center

Hydraulic-Over-

Electric

Full Pneumatic

Cost High Low Very High High

Complexity Medium Low Very High Low

Performance High Low High Medium

We chose to pursue the full closed center hydraulic

system as it offers the greatest balance between cost,

complexity and performance. There are several distinct

advantages to having a closed center pump over the other

three designs. The variable displacement property allows

for greater system efficiencies as well as smooth operation.

Both of these gains are fundamental to the success of our

project. Although the system is relatively complex, we felt

capable of the research and study necessary to design the

system. Furthermore, we planned to moderate the high cost

of the design by pursuing sponsorship and discounts from

various suppliers.

4. DESIGN DESCRIPTION

4.1 System Components

The final design utilizes a complete hydraulic

drivetrain with regenerative braking. The main components

are summarized thus:

Hydraulic Charge Pump: Charges the system to

1,000 psi using an external power source. Pump is pressure

compensated.

Hydraulic Accumulators: Precharged with nitrogen

gas. Incompressible hydraulic fluid enters one side;

nitrogen is compressed on other side; gas and fluid

separated by piston. Hydraulic potential energy stored as

pressurized hydraulic fluid volume.

Hydraulic Pump/Motor: Drives wheel/axle using

pressurized hydraulic flow. Can be backdriven by

wheel/axle to produce pressurized hydraulic flow during

regenerative braking. Utilizes a reversible variable

swashplate.

Check Valve: Located at the outlet of the charge pump.

Prevents backflow of pressurized hydraulic fluid into pump

when off or when pressure exceeds 1000 psi during


One-Way Solenoid Valve: Separates hydraulic

connection between accumulators and pump/motor. Allows

the easy repositioning of swashplate from neutral. Prevents

undesirable rotation of wheel during regenerative braking.

Pressure Relief Valve: Prevents hydraulic system

pressure from exceeding 1500psi. Necessary to prevent

damage to system components and unsafe operating

conditions.

Reservoir: Stores hydraulic fluid at ambient

conditions. Provides hydraulic fluid for the charge pump

and pump/motor.

Filter: Placed on suction side of charge pump.

Prevents particulate contamination from damaging

components.

Engine: Mechanically connected to hydraulic charge

pump. Supplies power to hydraulic charge pump.

Larger components in their designed configuration can

be viewed in Figure 4-1.

Figure 4-1 Labeled System Design Diagram

4.2 Component Sizing

The design process included properly sizing the charge

pump, primary mover, pump/motor, hydraulic line, and the

hydraulic accumulators.

Hydraulic Charge Pump: This sizing was based on

the horsepower (hp) rating of the vehicle’s original power

source. The original vehicle had a gas engine with a peak

horsepower rating of 3 hp, but since our system could

provide momentary power surpassing the output of the

charge pump, a pump able to produce approximately 2 hp

of hydraulic power was chosen. The choice of the lower

pressure rating of 1,000 psi allowed the use of less

expensive components elsewhere in the system.

Primary Mover: This component was sized based on

matching the power required to operate the hydraulic

charge pump. An electric motor was chosen so that the

prototype could be operated safely indoors.

Hydraulic Pump/Motor: Sizing was largely dictated

by the available options from our supplier. The 13 CC/Rev

displacement pump was the largest pump/motor available

and would allow the pump/motor to operate in its optimal

RPM range for our expected operating speeds of 10 mph.


Hydraulic Line: The sizing of the hydraulic line was

determined by calculating inner diameter (ID) in order to

satisfy a target average fluid velocity based on Equation

4-1. The system required a line ID greater than 0.25” for the

pressure lines.

𝑇𝑢𝑏𝑒 𝐼. 𝐷. = 0.64√𝐹𝑙𝑜𝑤 𝑅𝑎𝑡𝑒

𝑉𝑒𝑙𝑜𝑐𝑖𝑡𝑦 Eq. 4-1

Hydraulic Accumulators: The configuration and

sizing of the accumulators were based on the physical space

of the system as well as simulations regarding discharge

behavior. Due to size constraints, two accumulators

connected in parallel were chosen rather than one large

accumulator. Simulations were performed predicting the

behavior of accumulator discharge at different precharge

pressures. Based on the results of the simulation and

physical constraints, two 1.6 gallon accumulators

precharged with nitrogen gas at 400 psi were chosen.

4.3 System Operation

Charging the System: For the system to operate, the

accumulators must be pressurized by the hydraulic charge

pump. The electric motor drives the hydraulic charge pump

which fills the accumulators to a pressure of 1,000 psi. The

pump is pressure compensated, so the flow rate will

decrease to zero as the system pressure approaches 1,000

psi. This process fills the accumulators with approximately

1.5 gallons of pressurized hydraulic fluid at 1,000 psi.

Figure 4-2 illustrates the flow diagram of the system while

it is charging.

Engine

Low Pressure Reservoir

Hydraulic Charge Pump

Solenoid Value

Wheel

Axle

Variable Displacement Hydraulic Pump/Motor

Relief Valve

Accumulators

Charging accumulators

Figure 4-2 System Charging Flow Diagram

Vehicle Propulsion: Following charging, the vehicle

can accelerate using the pressurized hydraulic fluid stored

in the accumulators. By changing the swashplate angle of

the pump/motor and energizing the solenoid valve, the

torque output of the pump/motor can be varied to

accommodate driving conditions. After the fluid flows

through the pump/motor, it is returned to the reservoir.

Figure 4-3 illustrates the flow diagram of the system during

propulsion.

Engine



Solenoid Value

Wheel

Axle


Relief Valve

Accumulators

Propulsion From Accumulators

Figure 4-3 Propulsion Flow Diagram

Vehicle Regenerative Braking: Regenerative braking

allows the hydraulic drivetrain to recapture much of the

kinetic energy that would be lost in conventional braking.

Braking occurs when the swashplate is reversed in the

pump/motor. This mechanically reverses the flow of the

fluid through the pump/motor, thus when the pump/motor

remains rotating the same direction, it will produce

pressurized flow back into the accumulators. The solenoid

valve was chosen to allow one-way flow back into the

accumulators without being energized, preventing the

vehicle from changing direction after coming to a stop.

Figure 4-4 illustrates the flow diagram of the system during


Engine



Solenoid Value

Wheel

Axle


Relief Valve

Accumulators

Figure 4-4 Regenerative Braking Flow Diagram

The pressure relief valve bleeds off pressure after the

system has reached the set maximum pressure, preventing


catastrophic failure and allowing non-regenerative

hydraulic braking. The flow diagram in Figure 4-5 further

illustrates the behavior of this component.

Engine



Solenoid Value

Wheel

Axle


Relief Valve

Accumulators

Regenerative Braking at or past

Figure 4-5 Non-Regenerative Braking Flow Diagram

4.4 Design Improvements

Several characteristics of the final prototype were

chosen due to outside constraints, including monetary

resources, readily available products, and ease/simplicity of

operation.

Hydraulic Charge Pump: The prototype utilizes a

variable vane hydraulic pump as the charge pump. This

pump lacks in operating efficiency compared to a similar

sized piston pump. Due to the small scale of our project, a

pressure compensated piston pump was not readily

available from suppliers that would satisfy our hydraulic

output requirements.

Additionally, the 1,000 psi system pressure was largely

a product of the maximum pressure of the vane pump and

other readily available pressure compensated hydraulic

pumps. By incorporating a piston pump operating at higher

pressures, higher efficiencies are possible as well as the use

of smaller accumulators.

Hydraulic Accumulators: The accumulators on the

final prototype were generously provided by Parker

Hannifin. While meeting the volume determined necessary

to satisfy the system performance goals, these accumulators

were under-utilized. Rated at 4,000 psi, the accumulators

were capable of much higher energy density than the

system could produce. Weight savings could have been

better utilized by these accumulators, decreasing the

required volume, and therefore component weight.

Hydraulic Pump/Motor: The prototype incorporates a

White Drive Products CP series variable displacement

hydraulic pump as the pump/motor. While White Drive

donated the CP pump to the project, their engineers were

unsure of its ability to perform as a variable displacement

hydraulic motor. The major concern was lack of efficiency,

especially at smaller swashplate angles (lower

displacements). Improvements in efficiency from further

pump/motor design would allow for higher overall system

efficiency (both in braking and propulsion).

5. PROTOTYPE REALIZATION

Mounting: The system bears all of the components

included in the original design. The placements of the

components on the go-kart depended on their ease of

integration with the go-kart chassis. Mounts were

specifically built for several of the components with

asymmetric geometry, such as the reservoir, pump and

motor. The two accumulators, being the largest

components, were placed horizontally on either side of the

driver in order to fit safely. The most concerning mounted

component is the reservoir, as it is very heavy and sits high

atop the go-kart’s spoiler. Several bolts were used to ensure

its secure mounting. On the whole, our team was very

successful in integrating the design components onto the

go-kart in a professional-looking manner.

Figure 5-1 Final Prototype Picture

Operation/Controls: The two main components that

the driver handles when operating the go-kart are the

variable piston pump/motor and the solenoid valve. These

components must be used synchronously by the driver. The

driver can control the pump/motor by adjusting the position

of a lever located to his left either forwards or backwards.

This lever is connected by a steel rod to the swashplate

lever on the pump/motor, which subsequently controls the

output torque of the pump/motor.

The driver can control the solenoid valve by pressing a

push-switch mounted atop the lever. The push-switch is

powered by a 12V battery that is mounted towards the rear

of the go-kart. When not pressed, the solenoid valve is

closed and allows no hydraulic flow into the pump/motor.

When pressed, the solenoid valve opens and allows flow to

the pump/motor. The solenoid also functions as a one-way

check valve, in that fluid cannot flow to the pump/motor

unless the button is pressed, but allows the charging of the

accumulators through regenerative braking.

To accelerate forward, the driver must move the lever


forward and press the push-switch, which allows fluid from

the accumulators to flow to the pump/motor. To brake, the

user must pull the lever back which reverses the swashplate

angle; the solenoid is open for flow in this direction

whether or not the button is pressed. In order to drive in

reverse, the driver must pull the lever backwards and press

the push-switch to allow flow from the accumulators to the

pump/motor. To brake during reverse, the driver must move

the lever forward.

Functionality: Having installed the control

mechanisms, the go-kart operates successfully in both

driving and regenerative braking. The go-kart can be driven

in two ways; either by using long extension cords to power

the electric motor, or by plugging in the electric motor

periodically to charge the accumulators. Additionally, due

to the continuous nature of the swashplate angle, the

drivability is similar to that of a passenger vehicle: the

driver does not experience violent accelerations.

6. EVALUATION AND TEST

Tests and evaluations were conducted to adjudicate the

overall success of the project as well as the accuracy of the

models. Tests were divided into three areas: propulsion

with accumulator, the efficiencies of the overall system, and

the regenerative braking.

6.1 Propulsion with Accumulator

To test the validity of the adiabatic assumption

pertaining to the accumulators, trials were run that isolated

the effects of the accumulators. Figure 6-1 shows the

difference between the experimental and theoretical

pressure versus time graphs of the accumulators.

Figure 6-1 Pressure Behavior During Discharge

Subsequent tests measured speed and energy during the

system’s propulsion phase. Figure 6-2 shows the actual

speed of the system plotted against a theoretical velocity

curve. The theoretical curve was based on energy methods

and the maximum speed was based on the physical

constraints of the system components.

Figure 6-2 Velocity During Discharge

Based on the total distance travelled and the energy

dissipated in rolling resistance, a total useable potential

energy stored in the accumulator was derived. This

quantity, along with the kinetic energy and work due to

non-conservative forces, is plotted on Figure 6-3.

Figure 6-3 Energy Behavior During Discharge

Discussion

For Figure 6-1 and Figure 6-2, the overall shape of the

empirical data roughly follows that of the theoretical

curves. The discrepancies are a result of drivetrain

inefficiencies, which are further studied in section 6.2. As

seen in Figure 6-1, the accumulators were able to discharge

for 36.3 seconds, considerably longer than our goal of 30

seconds. Figure 6-2 also shows that we were able to reach a

maximum speed of 11.3mph, also meeting our goal of

10mph. Figure 6-3 is useful because quantities such as

energy, pressure, and velocity can all be related assuming

the same initial pressure and displacement.

6.2 Overall System

Two tests and calculations were run to evaluate the

efficiencies of different areas of the physical system.

First, the efficiency of the motor to accumulator was

measured by using Equation 6-1, the data in Table 6-1, and

the initial potential energy in Figure 6-3.

0 5 10 15 20 25 30 35 40200

300

400

500

600

700

800

900

1000

Time [s]

Pre

ssu

re [p

si]

Comparison of Theoretical and Empirical Pressure Discharge

Theoretical Pressure Discharge

Testing

0 2 4 6 8 10 12 140

2

4

6

8

10

12

14

16

18

Time [s]

Ve

locity [m

ph

]

Comparison of Theoretical and Empirical Speed Tests

Theoretical based on Discharge

Maximum System Velocity

Testing

0 2 4 6 8 10 12 140

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

Time [s]

En

erg

y [J]

Energy Behavior

Potential Energy

Kinetic Energy

Work


= 𝐼 =

√

𝑉

√ Eq. 6-1

Table 6-1 Charging Data

Iavg [A] Vavg [V] P [W] tcharge [s] ηEA [-]

3 110 165 37.7 .80

Based on Figure 6-2, the efficiency from accumulator to

wheel can be calculated to be 0.67 based on the energy

required to achieve the theoretical maximum speed and the

empirical kinetic energy. Using Equation 6-2, the overall

efficiency is 0.54.

𝑜𝑡 = ∏ Eq. 6-2

Discussion

The overall efficiency of 0.54 is rather low,

considering that the electric motor should have a very high

efficiency rating. The 0.80 efficiency of the engine to

accumulator is most likely a result of the Nachi hydraulic

pump’s inefficiencies. The 0.67 efficiency of the

accumulator to wheel is actually relatively high, as the

components were chosen to give an efficiency of 0.60 using

conservative estimates. In a system using a pump more

appropriately suited for this application, the pump and

electric motor combination would most likely have a much

higher efficiency, probably around 0.90 using Equation 6-2

and assuming both components have efficiencies of

approximately 0.95. The overall efficiency of the system

would then be raised to 0.605. Overall, the efficiencies are

high enough to judge the project a success in this area as

they prove potential for small and light-weight applications

of hydraulic hybrid vehicles.

6.3 Regenerative Braking

A series of tests travelling with the system moving at

maximum speed were run and analyzed to retrieve the

amount of energy recovered during braking. From the data

shown in Figures 6-1, 6-2 and 6-3, it is possible to relate a

pressure to a velocity and energy. Therefore, to calculate

the amount of energy recovered, only the pressure at the

inception of braking, the change in pressure during braking

to rest, and the initial speed need be known. Trials were run

in which the accumulators were fully pressurized and then

discharged at full displacement for a certain amount of time

before braking to rest. Table 6-2 shows the test data which

gives an average of 24.3% of kinetic energy recovered.

Table 6-2 Regenerative Braking Trials

Initial

Pressure

[psi]

Change

in

Pressure

[psi]

Kinetic Energy

Recovered [%]

410 110 24.3

425 105 24.0

500 90 24.6

Discussion

The testing proves beyond a doubt that the system does

regenerate energy usually lost during braking, which

accomplishes the set goal of greater than 0%.

There are potential problems with the method of testing

used. The tests occurred at nearly the same initial pressure,

and are subject to error in the pressure gauge itself as well

as the precision of reading the meter. The data, although a

small sample size, suggests that as initial pressure rises, the

recovered change in pressure decreases. This follows our

expectations that a higher initial pressure would provide

more resistance to the fluid being pumped by the hydraulic

pump/motor. At higher pressures, a smaller increase in

pressure corresponds to a higher recovered energy than at a

lower pressure. The assumption is made, and somewhat

verified by the small dataset, that the recovered energy is

simply proportional to the initial kinetic energy.

Another consequence of the differences of braking

behavior of different charge pressures in the accumulator is

the responsiveness of the system. Because a higher pressure

means more difficulty in pumping fluid to the accumulators

by the hydraulic pump/motor, braking at high pressures

slows the system down faster compared to braking at low

pressures.

7. DISCUSSION

Although the project may be viewed as a success, the

overall design of the project has strong and weak aspects.

Moreover, we have advocated many different areas of the

project for future work that would result in a higher

performing technology.

7.1 Strengths of Design

The areas of strength in the design and prototyping

phases include the simulation of individual components, the

intuitive nature of the control system, and the ease of use of

the regenerative braking system.

MATLAB Simulation: Our MATLAB simulation of

the system components proved to be very helpful in the

sizing and design processes. The simulations allowed us to

subject the components to various operating conditions and

witness the resulting effects. One example of this involves

the accumulators. The MATLAB simulation of the

accumulators correctly predicted their behavior during

discharge at several different initial charge pressures. From

this simulation, we were able to choose the most well-suited


configuration for our application. This same process was

performed for many of the other system components.

Simple Control System: A simple and easily operated

control system was accomplished by making the lever

positions intuitive for the driver; that is, adjusting the lever

forward moves the go-kart forward, and adjusting the lever

backwards moves the go-kart backwards.

Simple Regenerative Braking Routine: Braking is

remarkably easy for the driver, requiring only one action.

As previously discussed, the solenoid was implemented

such that it can allow flow to reverse without requiring any

battery power. Consequently, the operator must only

position the control lever backwards in order to brake and

regenerate energy.

7.2 Weaknesses of Design

The project weaknesses include the lack of utilization

of the Simulink simulation, and the inconsistencies in

braking behavior.

Utilization of Simulink: A Simulink model was

devised in which two system designs were evaluated: our

system, and a traditional drivetrain system of similar size.

The model outputted only relative values and was not

wholly verified. For this reason, the Simulink model was

not applied in choosing the best configuration of

components as initially intended.

Inconsistent Braking Behavior: We discovered that

the intensity of the go-kart’s braking varied with the

pressure in the accumulators. At high accumulator

pressures, the braking is more severe as the flow encounters

more difficulty in reversing direction. At low accumulator

pressures, the braking is slow as the flow encounters less

pressure resistance. This inconsistency makes braking less

predictable and the system more difficult to drive.

7.3 Future Work

Our project and subsequent analysis show a large

amount of potential for hydraulic hybrids. Given that

potential, many areas of improvement are advocated by the

team:

Carbon Fiber Accumulators – Weight is at a

premium on passenger vehicles, and the heaviest

component of a hydraulic hybrid is the accumulator. Steel

accumulators offer better performance currently due to their

higher maximum pressure, but an effort to make these

carbon fiber accumulators stronger would greatly improve

the efficiency of the system.

Lightweight Components – Hydraulic components

are often very heavy parts because companies have little

incentive to produce lighter components. With results of our

and other concurrent projects, it is now obvious that

engineering lightweight components could help

revolutionize the hydraulic hybrid industry.

Turbines – Turbines are capable of high efficiencies,

but only in a very narrow range of rotational speeds and

loads. Because the axle is decoupled from the motor in our

system, it is possible to take advantage of the increased

efficiency of a turbine.

A Set Minimum Pressure in the Accumulators – The

accumulators behave most ideally and effectively at higher

pressures. For this reason, it would be best to set a high

minimum pressure for the accumulators. This would

increase efficiencies due to fewer losses during discharge as

well as mitigating the inconsistencies caused by differing

accumulator pressures.

8. CONCLUSIONS AND RECOMMENDATIONS

Our project demonstrated both the feasibility and

performance of hydraulic hybrids on a small scale. With

more time for research and an extensive budget, we are

confident the project would have further shown the

advantages of a series hydraulic hybrid system. In moving

forward, we believe that hydraulic hybrid systems have a

greater potential for performance in passenger vehicles than

current hybrid designs. We strongly recommend that further

research be conducted by universities and automotive

companies to explore this potential.

9. ACKNOWLEDGEMENTS

Our final product was made possible by the help of our

faculty advisor, Dr. Andrew Jackson, as well as the senior

design course instructor, Dr. Robert Jeffcoat and the

course’s teaching assistant, John Martin. Additionally, we

received assistance from the UPenn Formula SAE team.

We also received generous donations from several

companies. Parker Hannifin graciously donates two

accumulators, while White Drive Products donated the

variable piston pump/motor. SKF and Colonial Seal also

assisted our team by supplying a seal when one of our

components temporarily faltered.

10. NOMENCLATURE AND DEFINITIONS

A Amperes

AC Alternating Current

CC/Rev Cubic Centimeter Per Revolution

Displacement Volume of Hydraulic Fluid Transferred

Per Motor Revolution

ηEA Efficiency Between Engine and

Accumulators

I Current

P Power, in Watts

p Pressure, in psi

psi Pounds Per Square Inch, Unit of Pressure

rms Root Mean Squared

V Voltage


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Web.

<http://onlinedigitalpublishing.com/publication/?i=97071>.

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consumer_tips/vehicle_energy_losses.html>.

Gray, Charles L. Hydraulic Hybrids. Houston: Houston

Advanced Research Center, 17 Oct. 2006. PDF.

Hansen, Alex, Tom Coletta, Ryan Weaver, Shawn Wilhelm,

Dan Mittelman, and Eric Weiland. Hydraulic Series

Hybrid Baja Car. Tech. WORCESTER

POLYTECHNIC INSTITUTE, 2010. Print.

Holmberg, Kenneth, Peter Andersson, and Ali Erdemir.

"Global Energy Consumption Due to Friction in

Passenger Cars." Tribology International 47 (2012):

221-34. SciVerse. Web. 4 Apr. 2012.

Industrial Hydraulic Service Inc. Tube Selection Chart.

Sherwood, Oregon: Industrial Hydraulic Service Inc.

PDF.

Nikkel, Cathy. "EPA Innovates Hydraulic Hybrid System."

Automedia.com. Automedia.com, 2006. Web. 26 Apr.

2012.

<http://www.automedia.com/EPA_Innovates_Hydrauli

c_Hybrid_System/dsm20060801eh/1>.

Van De Ven, James D., Michael W. Olson, and Perry Y. Li.

Development of a Hydro-Mechanical Hydraulic Hybrid

Drive Train with Independent Wheel Torque Control

for an Urban Passenger Vehicle. The Pennsylvania

State University. Web. 3 Apr. 2012.

<http://www.google.com/url?sa=t&rct=j&q=&esrc=s&

source=web&cd=2&ved=0CFwQFjAB&url=http%3A

%2F%2Fciteseerx.ist.psu.edu%2Fviewdoc%2Fdownlo

ad%3Fdoi%3D10.1.1.148.8237%26rep%3Drep1%26ty

pe%3Dpdf&ei=HJGYT6rmDaqX6AGLt8n1Bg&usg=

AFQjCNG2h7cJlOlJGE5RQkihZCgPuX-8IQ>.

Van Rensselar, Jean. "Power to Spare: Hydraulic Hybrids."

Tribology and Lubrication Technology (2012): 34-43.

Print.

INTELLECTUAL PROPERTY

This document, in whole or in part, in electronic or

other form, may be freely reproduced and modified, without

restriction, provided only that the source be properly

attributed.

CORRESPONDING AUTHOR

Inquiries should be addressed to Alfred ‘Hurst’

Nuckols at [email protected].


APPENDIX A MATERIALS AND COST SUMMARY

Our allotted budget for the project was $1,500. The materials cost of the entire project was approximately $1,208, leaving an

excess of $292. The details regarding the project purchases can be seen in Table A1 and Table A2.

Table A1 - Embedded, expended, and consumed items and charges

Supplier QTY Description Price Total

Amazon 2 3/4 Bearing UCP204-12 + Pillow Block Cast Housing Mounted Bearings $ 5.99 $ 11.98

Amazon 1 Spypoint 12-Volt 7.0 AH Rechargeable Battery and AC Charger $ 34.99 $ 34.99

Discount Hyd. Hose 1 #6 ORB x 3/8 NPSM swivel $ 1.53 $ 1.53

Discount Hyd. Hose 3 3/8 NPTF Female x 3/8 JIC $ 2.12 $ 6.36

Discount Hyd. Hose 1 run tee: 3/8 JIC x 3/8 JIC x 3/8 JIC Female $ 2.68 $ 2.68

Discount Hyd. Hose 1 3/8 JIC male x #6 ORB $ 1.28 $ 1.28

Discount Hyd. Hose 1 flareless 1/4 x #4 ORB $ 2.08 $ 2.08

Discount Hyd. Hose 1 1/4 NPTF male x 3/8 JIC female swivel $ 1.67 $ 1.67

Discount Hyd. Hose 1 1/4 flareless x 1/4 female pipe $ 5.61 $ 5.61

Discount Hyd. Hose 1 Hex Head Plug #6 SAE/ORB $ 0.79 $ 0.79

Discount Hyd. Hose 1 #8 SAE/ORB x 3/8" NPSM Swivel (steel) $ 1.73 $ 1.73

Discount Hyd. Hose 1 1/2" JIC x #6 SAE/ORB (steel) $ 1.33 $ 1.33

Discount Hyd. Hose 2 #8 SAE/ORB x 3/8" NPTF Female (steel) $ 1.32 $ 2.64

Discount Hyd. Hose 2 1/2" Push-Lock barb x 3/8" Male Pipe (Brass) $ 1.69 $ 3.38

Discount Hyd. Hose 6 1/2" Black Push-Lock Hose (per foot) $ 1.76 $ 10.56

Discount Hyd. Hose 6 Wormgear Hose clamp 0.56"-1.25" $ 0.56 $ 3.36

Discount Hyd. Hose 1 #6 JIC Cap $ 2.55 $ 2.55

Discount Hyd. Hose 1 #4 ORB to 3/8 female pipe $ 9.75 $ 9.75

Discount Hyd. Hose 1 1/2" hose barb Tee $ 14.13 $ 14.13

Discount Hyd. Hose 3 Wormgear Hose clamp 0.56"-1.00" $ 1.65 $ 4.95

Discount Hyd. Hose 1 3/8" NPTF Female x 3/8" JIC Female Swivel (Steel) $ 2.12 $ 2.12

Discount Hyd. Hose 4 90° Street Elbow, 3/8" NPTF Male x 3/8" NPTF Female (Steel) $ 2.55 $ 10.20

Discount Hyd. Hose 2 90° Street Elbow, 1/2" NPTF Male x 1/2" NPTF Female (Steel) $ 3.35 $ 6.70

Discount Hyd. Hose 4 Hex Bushing, 3/4" NPTF Male x 1/2" NPTF Female (Steel) $ 1.36 $ 5.44

Harbor Freight 1 Tube Bender $ 5.99 $ 5.99

Harbor Freight 1 Funnel Set (4 piece) $ 0.99 $ 0.99

Harbor Freight 1 Tube cutter-Mini $ 4.99 $ 4.99

Harbor Freight 1 Rip Hammer W/ Wood handle $ 2.99 $ 2.99


Harbor Freight 1 8" Pipe Wrench (steel) $ 2.99 $ 2.99

Harbor Freight 1 1/4" threaded brass ball valve $ 3.99 $ 3.99

Harbor Freight 1 4 piece adjustable wrench set $ 9.59 $ 9.59

Harbor Freight 1 2 HP 120V Electric Motor Compressor Duty $ 139.99 $ 139.99

Harbor Freight 1 0-5000 PSI 1/4NPT filled gauge $ 10.39 $ 10.39

HGR Industrial

Supplies (Ebay) 1 VDS-0B-1A3-E11 Nachi Hydraulic Pump, variable vane $ 35.09 $ 35.09

Home Depot 1 Mini hack saw $ 8.97 $ 8.97

Hurst 1 Test Stand $ - $ -

Hurst 1 Electric Motor $ - $ -

Hurst 1 12V Hydraulic Power Pack $ - $ -

Lowes 1 M10-1.25 nuts $ 1.64 $ 1.64

Lowes 1 Mixed wire nuts $ 7.25 $ 7.25

Lowes 1 flat washers m10 $ 0.68 $ 0.68

Lowes 1 M10-1.25x25 $ 1.76 $ 1.76

Lowes 1 M10-1.25x30 $ 1.87 $ 1.87

Lowes 3 Hex Bolts 3/8-16x1-1/2 GR5 (2pk) $ 1.09 $ 3.27

Lowes 2 3/8 x 2" black pipe nipple $ 1.17 $ 2.34

Lowes 1 3/8 x 1/4 black iron bushing $ 1.43 $ 1.43

Lowes 1 25 packet 3/8 flat washers $ 3.24 $ 3.24

Lowes 1 3/8 black iron tee $ 1.76 $ 1.76

Lowes 6 Hex Nuts 3/8 $ 0.12 $ 0.72

Lowes 6 Lock washer 3/8 $ 0.20 $ 1.20

Lowes 1 3/8 black iron coupling $ 1.65 $ 1.65

Lowes 1 3/8 black iron 90 degree elbow $ 1.64 $ 1.64

Lowes 1 3/8" x 1/4" black iron bushing $ 1.43 $ 1.43

Lowes 1 1/4" brass hex nipple $ 1.82 $ 1.82

Lowes 1 Cobalt 10PC SAE Long Arm Allen key set $ 3.88 $ 3.88

McMaster Carr 1 1/4" steel tubing x 36" length $ 5.37 $ 5.37

McMaster Carr 2 5/8 flex coupler half $ 4.17 $ 8.34

McMaster Carr 1 7/8 coupler half $ 4.17 $ 4.17

McMaster Carr 1 Coupler spider $ 5.69 $ 5.69

McMaster Carr 1 Steel Finished-Bore Roller Chain Sprocket for #35 Chain, 3/8" Pitch, 18 Teeth, 5/8" Bore

$ 11.98 $ 11.98

McMaster Carr 2 Ball Joint Linkage Shielded, Steel, 5/16"-24 Right-Hand Thread Size $ 3.86 $ 7.72

Surplus Center 1 3/8 NPT CHECK VALVE $ 8.99 $ 8.99

Surplus Center 1 1/2 NPT 16 GPM 1000-2500 PSI RELIEF VALVE $ 41.95 $ 41.95

Surplus Center 1 12 VDC 5 GPM N.C. 2-WAY SOLENOID VALVE $ 49.99 $ 49.99

Tractor Supply 1 3/8 x 60" hose (2 wire) $ 16.99 $ 16.99

Tractor Supply 1 3/8 x 36" hose (2 wire) $ 12.00 $ 12.00

Tractor Supply 0.4 Gr 2 bulk hardware (3/8 bolts, nuts, and washers) $ 1.99 $ 0.80

Tractor Supply 1 1/2" ORB x 3/8 Female swivel pipe thread $ 5.99 $ 5.99


Tractor Supply 1 5 Gallon Reservoir $ 89.99 $ 89.99

White Drives 1 CP Variable Drive pump $ - $ -

Total Cost

$ 681.33

Table A2 - Recoverable items

Supplier QTY Description Price Total

Advanced Auto Parts 1 AW46 Hydraulic Oil $ 47.48 $ 47.48

Advanced Auto Parts 1 Funnel $ 3.80 $ 3.80

Amazon 1 Koch 7435100 Roller Chain, #35, 10 Feet $ 12.70 $ 12.70

Amazon 1 Koch 7535040 Roller Chain Connector Link, 4-Pack, #35 $ 10.39 $ 10.39

Discount Hyd. Hose 1 1/2" Push-Lock x 3/8" Male Pipe (Brass) $ 1.69 $ 1.69

Discount Hyd. Hose 4 3/8" Flareless x 1/2" Male Pipe (Steel) $ 3.73 $ 14.92

Discount Hyd. Hose 2 3/8" Flareless Compression Tee (Steel) $ 8.42 $ 16.84

Discount Hyd. Hose 6 3/8" Flareless x 3/8" Male Pipe (Steel) $ 2.46 $ 14.76

Discount Hyd. Hose 4 1/2" Push-Lock x 1/2" Male Pipe (Brass) $ 2.31 $ 9.24

Discount Hyd. Hose 2 3/8" Flareless x 1/4" Female Pipe Tee (Steel) $ 7.51 $ 15.02

Discount Hyd. Hose 1 Bottom-Mounted Pressure Gauge, 0 to 2,000 psi $ 17.50 $ 17.50

Discount Hyd. Hose 2 #12 SAE/ORB x 3/4" NPTF Female (Steel) $ 2.57 $ 5.14

Harbor Freight 1 Miscellaneous Shipping Costs $ 6.99 $ 6.99

Harbor Freight 1 Hand-held Digital Scale $ 14.99 $ 14.99

Harbor Freight 1 5-Function Clamp on Multimeter $ 9.99 $ 9.99

Harbor Freight 1 50ft x 14GA Outdoor Extension Cord $ 21.99 $ 21.99

HGR Industrial Supplies

(Ebay) 1 shipping costs $ 11.75 $ 11.75

Home Depot 1 2x3 Stud $ 1.86 $ 1.86

Home Depot 3 Wood Screws $ 1.18 $ 3.54

Home Depot 2 Metal Screws $ 1.18 $ 2.36

Home Depot 1 3/4" x 1/16" x 36" angle aluminum $ 3.97 $ 3.97

Home Depot 1 Flat plain steel 1x3/16x36 $ 6.25 $ 6.25

Home Depot 1 Threaded rod zinc 5/16 x 36 $ 2.63 $ 2.63

Home Depot 1 1/2" 2x4 ft. handy panel plywood sheet $ 10.17 $ 10.17

Home Depot 1 9' black power tool cord $ 12.97 $ 12.97

Home Depot 2 5/16 nuts, washer, and lockwasher kit $ 2.47 $ 4.94

Home Depot 1 1/4 nuts, washer, and lockwasher kit $ 2.36 $ 2.36

Home Depot 1 Sheet metal screws $ 1.18 $ 1.18

Home Depot 1 Corner Brace 1" 4pk $ 2.27 $ 2.27

Home Depot 1 1/4" washers $ 1.18 $ 1.18

Home Depot 2 Hex bolt 1/4 x 1 $ 0.12 $ 0.24

Home Depot 4 Hex bolt 5/16 x 3/4 $ 0.16 $ 0.64

Home Depot 4 Hex bolt 5/16 x 1 $ 0.17 $ 0.68


Home Depot 4 Hex Bolt 1/4 x 1-1/2 $ 0.17 $ 0.68

Home Depot 4 Hex bolt 1/4 x 3/4 $ 0.11 $ 0.44

Lowes 1 Angle Iron (enough to make 2 mounts for the pump) $ 16.57 $ 16.57

Lowes 2 Roll of Teflon tape $ 1.07 $ 2.14

Lowes 1 Flat steel 3/16" x 3"-3' for pump mounting $ 17.64 $ 17.64

Lowes 1 1/4 SAE flat steel washer $ 1.18 $ 1.18

Lowes 1 3/8" black pipe plug $ 1.12 $ 1.12

McMaster Carr 1 Plain Steel Threaded Rod 5/16"-24 Thread, 3' Length $ 2.56 $ 2.56

McMaster Carr 1 Plain Grade 2 Steel Hex Nut 5/16"-24 Thread Size, 1/2" Width, 17/64" Height, packs of 100

$ 2.76 $ 2.76

McMaster Carr 4 Clamping U-Bolt Steel, 3/8"-16 Thread, for 5" Outside Diameter $ 2.97 $ 11.88

McMaster Carr 1 Zinc-Plated Steel Large-Diameter Flat Washer 5/16" Screw Size, 2" OD, .04"-.06" Thick, packs of 25

$ 9.01 $ 9.01

McMaster Carr 1 Zinc-Plated Steel U-Bolt W/Plate, 1/4"-20 X1-1/4" L Third, for 1-1/4" OD, 435#Wll, packs of 10

$ 8.40 $ 8.40

McMaster Carr 1 Slide-on Flat Grip Smooth Surface Fits Flat Bar 3/16" X 1", Red, packs of 6 $ 8.87 $ 8.87

McMaster Carr 1 Steel Drive Shaft 3/4" OD, 36" Length $ 35.50 $ 35.50

McMaster Carr 1 Forged Clevis Rod End Steel, 5/16"-24 Female Thread Size, 2-1/4" Length $ 4.98 $ 4.98

McMaster Carr 2 Carbon Steel seamless tubing 3/8" OD, .305" ID, .035 wall, 6' Length $ 10.08 $ 20.16

McMaster Carr 1 Steel Finished-Bore Roller Chain Sprocket for #35, 3/8" Pitch Chain, 35 Teeth, 5/8" bore

$ 27.95 $ 27.95

McMaster Carr 1 High DC Inrush Current Relay Standard, SPST-NO, 5 Terminals, 12V DC Control, W/Tab

$ 7.68 $ 7.68

McMaster Carr 1 Heat-Treatable Steel Standard Key Stock 3/16" X 3/16", 12" Length $ 2.74 $ 2.74

Surplus Center 1 3/4" NPT 9 GPM FA1100-25 SUCTION FILTER $ 19.95 $ 19.95

Surplus Center 1 Shipping costs $ 12.91 $ 12.91

Tractor Supply 1 2 gal tractor fluid $ 18.99 $ 18.99

Total Cost

$ 526.54


APPENDIX B ACCUMULATOR SIZING

Accumulator sizing and configuration was decided by

identifying which parameters could and could not be

modified, simulating the behavior of discharge, and

manually choosing the best attainable accumulator for the

application.

Parameter Identification

Parameters which cannot be modified are as follows:

1000 psi hydraulic charge pressure

25 psi difference between minimum working

pressure and initial charge pressure of accumulator

4:1 working pressure to initial charge pressure

ratio

0.8 cubic inch per revolution maximum variable

axial piston hydraulic motor displacement

700 rpm maximum variable axial piston motor

speed

150 kg net mass of go-kart, drivetrain system, and

operator

60% efficiency from accumulator to axle

Parameters to be set within the simulation are as follows:

Mode of heat transfer (isothermal or isentropic)

Size/Weight of accumulator

Initial charge pressure

Rolling resistance coefficient and aerodynamic

drag

The mode of heat transfer depends on the amount of

time the accumulator discharges. If the discharge time is

under one minute, heat transfer is approximated as fully

isentropic. If the discharge time is over two minutes, heat

transfer is approximated as fully isothermal. In between one

and two minutes, heat transfer is some hybridization of the

two modes.1 Next, the optimal size and weight

configuration of the accumulator is the largest viable

capacity with the lightest weight. The initial charge

pressure must be at least 250 psi because the working

pressure of the system is 1000 psi. Finally, the rolling

resistance coefficient and aerodynamic drag must be chosen

to best describe the system.

Research led to a coefficient of rolling resistance to be

approximated at a nominal value of 0.01, which is a

common value for car tires.2 Although a normal go-kart

1 Design Engineers Handbook. Cleveland: Parker Hannifin, 1979.

Print.

2 "Low Rolling Resistance Tires." Alternative Fuels and Advanced

likely has a rolling resistance coefficient lower than 0.01,

the additional weight of the drivetrain and accumulator

makes this comparison to car tires a valid approximation.

Based on the speed of the go-kart, calculations showed

aerodynamic drag to be negligible. It was later found that

the rolling resistance coefficient was actually closer to 0.02.

Simulation and Results

Two versions of code were produced, one for

isentropic discharge and the other for isothermal discharge

to use depending on the discharge duration. A Euler

integration scheme was developed to find a collection of

quantities including power provided and the conditions in

the accumulator at any given time. Given the power

provided by the accumulator, the speed of the go-kart can

be computed based on the weight, the coefficient of rolling

resistance and the efficiency of the drivetrain using another

integration scheme.

Figures B-1 through B-3 show the behavior of the

chosen accumulator at various initial charge pressures

between 200 and 700 psi.

Figure B-1 Pressure During Discharge

Vehicles Data Center:. Web.

<http://www.afdc.energy.gov/afdc/vehicles/fuel_economy_tir

es_light.html>.

0 10 20 30 40 50 60100

200

300

400

500

600

700

800

900

1000

Time (s)

Hyd

rau

lic P

ressu

re in

Accu

mu

lato

r (p

si)

Hydraulic Pressure at Full Displacement

Po = 200 psi

Po = 300 psi

Po = 400 psi

Po = 500 psi

Po = 600 psi

Po = 700 psi


Figure B-2 Volume During Discharge

Figure B-3 Power During Discharge

From figure B-3, it is possible to find the overall speed of

the go-kart as a function of time given the parameters set

previously. Figure B-4 shows the result.

Figure B-4 Velocity During Discharge

Figure B-4 shows calculations based on only energy

considerations; however, there are limits as to rotational

speed and flow rate in individual components.

Consequently, all shown initial pressures would be

expected to reach the max speed of the system and sustain

the speed because it is under all the maximum velocities

shown.

Final Choice

Based on physical restrictions, the largest accumulator that

our system could support was a 3 gallon accumulator.

Inspection of Figure B-4 shows that 400 psi gives us over

30 seconds discharge and the largest maximum velocity

besides 300 psi. However, Figure B-2 shows that the initial

volume using a precharge of 300 psi is about 1.8 gallons,

which is over half of the available fluid in the reservoir.

Thus, a 3 gallon accumulator with a 400 psi nitrogen

precharge was chosen.

0 10 20 30 40 50 600

0.5

1

1.5

2

2.5

Time (s)

Vo

lum

e o

f H

yd

rau

lic F

luid

in

Accu

mu

lato

r (g

allo

ns) Hydraulic Volume at Full Displacement

Po = 200 psi

Po = 300 psi

Po = 400 psi

Po = 500 psi

Po = 600 psi

Po = 700 psi

0 10 20 30 40 50 600

0.2

0.4

0.6

0.8

1

1.2

1.4

Time [s]

Po

we

r [h

p]

Motor Power at Full Displacement

Po = 200 psi

Po = 300 psi

Po = 400 psi

Po = 500 psi

Po = 600 psi

Po = 700 psi

0 10 20 30 40 50 600

5

10

15

20

25

30

Time (s)

Ve

locity (

mp

h)

Velocity vs Time

Po = 200 psi

Po = 300 psi

Po = 400 psi

Po = 500 psi

Po = 600 psi

Po = 700 psi

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