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Pumps
•
Source of hydraulic power
•
Converts mechanical energy to hydraulic energy
• prime movers - engines, electrical motors, manual power
•
Two main types:
• positive displacement pumps
• non-positive displacement pumps
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Pump - Introduction
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Positive displacement pumps
•
Displacement is the volume of fluid displaced cycle of pump motion
• unit = cc or in3
•
Positive displacement pumps displace (nearly) a fixed amount of fluid
per cycle of pump motion, (more of less) independent of pressure
• leak can decrease the actual volume displaced as pressure increases
•
Therefore, flow rate Q gpm = D (gallons) * frequency (rpm)
•
E.g. pump displacement = 0.1 litre
• Q = 10 lpm if pump speed is 100 rpm
• Q = 20 lpm if pump speed is 200 rpm
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Non positive displacement pumps
Centrifugal Pump
Impeller Pump
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Non-positive displacement pump
•
Flow does not depend on kinematics only - pressure important
• Also called hydro-dynamic pump (pressure dependent)
•
Smooth flow
•
Examples: centrifugal (impeller) pump, axial (propeller) pump
•
Does not have positive internal seal against leakage
•
If outlet blocks, Q = 0 while shaft can still turn
• Volumetric efficiency = actual flow / flow estimated from shaft speed
= 0%
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Positive vs. non-positive displacement pumps
•
Positive displacement pumps
•
•
•
•
•
•
most hydraulic pumps are positive displacement
high pressure (10,000psi+)
high volumetric efficiency (leakage is small)
large ranges of pressure and speed available
can be stalled !
Non-positive displacement pumps
•
•
•
•
many pneumatic pumps are non-positive displacement
used for transporting fluid rather than transmitting power
low pressure (<300psi), high volume flow
blood pump (less mechanical damage to cells)
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Types of positive displacement pumps
•
Gear pump (fixed displacement)
• internal gear (gerotor)
• external gear
•
Vane pump
• fixed or variable displacement
• pressure compensated
•
Piston pump
• axial design
• radial design
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External gear pump
•
•
•
•
Driving gear and driven gear
Inlet fluid flow is trapped
between the rotating gear teeth
and the housing
The fluid is carried around the
outside of the gears to the outlet
side of the pump
As the fluid can not seep back
along the path it came nor
between the engaged gear teeth
(they create a seal,) it must exit
the outlet port.
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Gerotor pump
Inlet port
Outlet port
• Inner gerotor is slightly offset from external gear
• Gerotor has 1 fewer teeth than outer gear
• Gerotor rotates slightly faster than outer gear
• Displacement = (roughly) volume of missing tooth
• Pockets increase and decrease in volume corresponding
to filling and pumping
• Lower pressure application: < 2000psi
• Displacements (determined by length): 0.1 in3 to 11.5 in3
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Vane Pump
•
•
•
•
•
•
Vanes are in slots
As rotor rotates, vanes are pushed
out, touching cam ring
Vane pushes fluid from one end to
another
Eccentricity of rotor from center of
cam ring determines displacement
Quiet
Less than 4000psi
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Pressure Compensated Vane Pump
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PC Vane Pump (Cont’d)
•
Eccentricity (hence displacement) is varied by shifting the cam ring
•
Cam ring is spring loaded against pump outlet pressure
•
As pressure increases, eccentricity decreases, reducing flow rate
•
Spring constants determines how the P-Q curve drops:
• small stiffness (sharp decrease in Q as P increases)
• large stiffness (gentle decreases in Q as P increases)
•
Preload on spring determines
• pressure at which flow starts cutting off
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Axial Piston Pump
•
•
•
Each piston has a pumping cycle
Interlacing pumping cycles produce nearly uniform
flow (with some ripples)
Displacement is determined by the swash plate angle
•
Generally can be altered manually or via (electro-)
hydraulic actuator.
Displacement can be varied by varying swashplate angle
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Bent-Axis Piston Pump
•
•
•
Thrust-plate rotates with shaft
Piston-rods connected to swash plate
Piston barrel rotates and is connected
to thrust plate via a U-joint
• More efficient than axial piston pump
(less friction)
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Radial Piston Pump
•
•
•
•
•
Similar to axial piston pump, pistons
move in and out as pump rotates.
Displacement is determined by cam
profile (i.e. eccentricity)
Displacement variation can be achieved
by moving the cam (possible, but not
common though)
High pressure capable, and efficient
Pancake profile
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Piston Pump - flow ripples
1 piston
•
Pumping
•
Filling
•
•
2 piston
Total flow
Even # cylinders n*rpm
Odd # cylinders (2n)*rpm
•
•
•
Each cylinder has a
pumping cycle
Total flow = flow of each
cylinder
More cylinders, less ripple
Frequency:
Can be problematic for
manual operator
(ergonomic issue)
Noise
Displacement = # Cylinders x Stroke x Bore Area
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# of Pistons Effect on Flow Ripples
1
n=2
n=3
n=4
n=5
0.9
0.8
0.7
Flow -
0.6
0.5
0.4
0.3
0.2
0.1
0
0
1
2
3
Angle - rad
4
5
6
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Pumping theory
•
Create a partial vacuum (i.e. reduced pressure)
•
Atmospheric / tank pressure forces fluid into pump
• usually tank check valve opens
• outlet check valve closes
•
Power stroke expels fluid to outlet
• outlet check valve opens
• tank check valve closes
•
Power demand for prime mover (ideal calculation)
• (piston pump) Power = Force*velocity = Pressure*area*piston speed
= Pressure * Flow rate
•
If power required > power available => Pumps stall or decrease speed
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Aeration and Cavitation
•
Disastrous events - cause rapid erosion
•
Aeration
•
•
•
•
•
air bubbles enter pump at low pressure side
bubbles expand in partial vacuum
when fluid+air travel to high pressure side, bubbles collapse
micro-jets are formed which cause rapid erosion
Cavitation
• dissolved air in fluid evaporates in partial vacuum to form bubbles
• bubbles expands then collapse
• as bubbles collapse, micro-jets formed, causing rapid erosion
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Causes of cavitation and aeration
•
For positive displacement pumps, the filling rate is determined by
pump speed; (Q-demand) = D * freq)
•
Filling pressure = tank pressure - inlet pressure
• Q-actual = f(filling pressure, viscosity, orifice size, dirt)
•
If Q-actual < Q-demand, inlet pressure decreases significantly
• This causes air to enter (via leakage) or to evaporation (cavitates)
•
To prevent cavitation/aeration
• increase tank pressure
• low viscosity, large orifice
• lower speed (hence lower Q-demand)
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Aeration and Cavitation
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Hydraulic Motor / Actuator
•
Hydraulic motors / actuators are basically pumps run in reverse
•
Input = hydraulic power
•
Output = mechanical power
•
For motor:
• Frequency (rpm) = Q (gallons per min) / D (gallons) * efficiency
• Torque (lb-in) = Pressure (psi) * D (inch^3) * efficiency
• efficiency about 90%
• Note: units
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Models for Pumps and Motors
•
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Non-ideal Pump/Motor Efficiencies
•
•
•
•
•
Ideal torque = torque required/generated for the ideal pump/motor
Ideal flow = flow generated/required for the ideal pump/motor
Torque loss (friction)
Flow loss (leakage)
Signs different for pumping and motoring mode
Qact u al
Pump volumetric eff:
Friction
Qi deal
Pump mechanical eff:
Ql oss
Tin/out
Tideal
(Reverse if motor case !! )
leakage
Total efficiency:
vol
Ideal pump
Functions of speed, pressure
and displacements
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Hydro-static Transmission
•
A combination of a pump and a motor
• Either pump or motor can have variable displacement
•
Replaces mechanical transmission
• By varying displacements of pump/motor, transmission ratio is changed
•
Various topologies:
•
•
•
•
•
single pump / multi-motors
multi (pump-motor)
Open / closed circuit
Open / closed loop control
Integrated package / split implementation
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Hydrostatic Transmission
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General Consideration - Hydrostats
•
Advantages:
•
•
•
•
•
•
•
Wide range of operating speeds/torque
Infinite gear ratios - continuous variable transmission (CVT)
High power, low inertia (relative to mechanical transmission)
Dynamic braking via relief valve
Engine does not stall
No interruption to power when shifting gear
Disadvantage:
• Lower energy efficiency (85% versus 92%+ for mechanical transmission)
• Leaks !
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Closed Circuit Hydrostat Circuit
Notes:
• Charge pump circuit (pump + shuttle valve)
• Bi-directional relief
• Circuit above closed circuit because fluid re-circulates.
• Open circuit systems draw and return flow to a reservoir
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Hydrostatic Transmission
•
•
•
Let pump and motor displacements be D1 and D2, with one or both
being variable.
Let the torque (Nm) and speeds (rad/s) of the pump and motor be (T1,
S1) and (T2,S2)
Assuming ideal pumps and motors:
Transmission ratio
Variable by varying
D1 or D2
Infinite and negative
ratios possible
if pump can
go over-center
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Hydraulic Transformer
•
•
•
Used to change pressure in a power conservative way
Pressure boost or buck is accompanied by proportionate flow decrease
and increase
Note: Hydrostatic transmission can be thought of as a mechanical
transformer (torque boost/buck)
Q1
Q2
D1
D2
Research opportunity!
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Hydraulic Hybrid Vehicles
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How Hybrid Vehicles Save Energy?
With a secondary power source/storage,
it is possible to:
•
•
•
•
Manage engine operation
Store/reuse braking energy
Turn off engine
Downsize engine for continuous power
Energy storage
Engine
Drive-train
wheel
38%
Required for
maximum
performance
Example vehicle on EPA-UDDS cycle:
• Baseline (10% engine efficiency): 24 mpg
• Engine management (38% efficiency): 95 mpg
• Above with regeneration: 140 mpg
10-15%
Normal driving
operating
range
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Why Hydraulic Hybrids?
Why not stick with electric hybrids?
• Electric batteries / ultracaps (cost, reliability, recycling, power density)
• Electric motors & inverters (cost, power density)
• Affect overall cost, weight, and power
Metrics
• Fuel economy
• Cost
• Performance
Toyota Prius
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Hybrid Hydraulic versus Hybrid Electric Vehicle
•
•
•
•
Hydraulic pump/motor have significantly higher power density than electric
motor/generator (16:1 by weight , 8:1 by volume)
Hydraulic drives have much lower torque density than electric drives
limits acc/braking and
Accumulators are 10x more power dense than batteries
hence regenerative
Efficient power electronics are expensive
braking
•
•
Batteries have 2 order magnitude higher energy density than accumulators
Current hydraulic systems tend to be noisy and leaky
•
Overall tradeoff: Hydraulic hybrids can be significantly lighter and cheaper than
electric hybrids if energy density limitation can be solved.
Accumulator
Battery &
ultracapacitor
Hydraulic
pump/motor
Engine
Engine
Accumulator
Parallel Hybrid Hydraulic
Electric
motor/
generator
M..E., University of Minnesota (updated 12.2010)
Parallel Hybrid Electric
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Hydraulic Hybrids Versus Electric Hybrids
Electric
Fluid
Power
• acceleration
--
++
• regenerative braking
--
++
Component efficiency
+
-
Regenerative
efficiency
--
++
Weight
--
++
Cost
--
++
Reliability
--
++
Environmental impact
-
+
Energy storage
++
--
NVH
++
--
Performance
Realize opportunities for
• Both performance & efficiency
• Cost and reliability
Overcome threats in
• Inefficient components
• Low density energy storage
• Noise, vibration, harshness
+
+
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Parallel Architecture
Example: HLA system for F150 & garbage trucks
• Regenerates braking energy
• Utilizes efficient mechanical transmission
• Does not allow full engine management
Achievable engine
op. points
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Series Architecture
•
Example: Eaton/UPS (truck), Ford/EPA (Escape), Artemis (BMW-5), …
– Regenerates braking energy
– Allows for full engine management
– Independent wheel torque control possible
– All power must be transmitted through fluid power components
38%
38%
Required for
maximum
performance
10-15%
10-15%
Normal driving
Normal
operating
driving
range
operating
range
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Power-Split: Hydromechanical Transmission (HMT)
•
Power split between mechanical and
hydraulic paths
• Hybridized HMT – i.e. w/ regeneration
Efficient
Mechanical
Transmission
Regenerative
Braking
Full engine
management
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Hypothesized hydraulic & overall efficiencies
Highway
Urban
0.45
0.7
0.4
0.6
0.35
0.3
Overall efficiency
Overall efficiency
0.5
0.4
0.3
0.25
0.2
0.15
0.2
series
parallel
hmt
0.1
0
0.05
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Mean hydraulic efficiency
0.8
0.9
series
parallel
hmt
0.1
1
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Mean hydraulic efficiency
0.8
0.9
1
• Series / HMT at peak engine efficiency (38.5%)
• Parallel at lower engine efficiency (33%)
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