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Fluid Power Controls Laboratory (Copyright – Perry Li, 2004-2010)
M..E., University of Minnesota (updated 12.2010)
181
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
2
Fluid Power Controls Laboratory (Copyright – Perry Li, 2004-2010)
M..E., University of Minnesota (updated 12.2010)
182
Pump - Introduction
3
Fluid Power Controls Laboratory (Copyright – Perry Li, 2004-2010)
M..E., University of Minnesota (updated 12.2010)
183
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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Fluid Power Controls Laboratory (Copyright – Perry Li, 2004-2010)
M..E., University of Minnesota (updated 12.2010)
184
Non positive displacement pumps
Impeller PumpCentrifugal Pump
5
Fluid Power Controls Laboratory (Copyright – Perry Li, 2004-2010)
M..E., University of Minnesota (updated 12.2010)
185
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%
6
Fluid Power Controls Laboratory (Copyright – Perry Li, 2004-2010)
M..E., University of Minnesota (updated 12.2010)
186
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)
7
Fluid Power Controls Laboratory (Copyright – Perry Li, 2004-2010)
M..E., University of Minnesota (updated 12.2010)
187
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
8
Fluid Power Controls Laboratory (Copyright – Perry Li, 2004-2010)
M..E., University of Minnesota (updated 12.2010)
188
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.
9
Fluid Power Controls Laboratory (Copyright – Perry Li, 2004-2010)
M..E., University of Minnesota (updated 12.2010)
189
Gerotor pump
• 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
Inlet portOutlet port
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Fluid Power Controls Laboratory (Copyright – Perry Li, 2004-2010)
M..E., University of Minnesota (updated 12.2010)
190
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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Fluid Power Controls Laboratory (Copyright – Perry Li, 2004-2010)
M..E., University of Minnesota (updated 12.2010)
191
Pressure Compensated Vane Pump
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Fluid Power Controls Laboratory (Copyright – Perry Li, 2004-2010)
M..E., University of Minnesota (updated 12.2010)
192
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
13
Fluid Power Controls Laboratory (Copyright – Perry Li, 2004-2010)
M..E., University of Minnesota (updated 12.2010)
193
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
14
Fluid Power Controls Laboratory (Copyright – Perry Li, 2004-2010)
M..E., University of Minnesota (updated 12.2010)
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)
194
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Fluid Power Controls Laboratory (Copyright – Perry Li, 2004-2010)
M..E., University of Minnesota (updated 12.2010)
195
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
16
Fluid Power Controls Laboratory (Copyright – Perry Li, 2004-2010)
M..E., University of Minnesota (updated 12.2010)
197
Piston Pump - flow ripples
• Each cylinder has a pumping cycle
• Total flow = flow of each cylinder
• More cylinders, less ripple
• Frequency:
Even # cylinders n*rpm
Odd # cylinders (2n)*rpm
• Can be problematic for manual operator (ergonomic issue)
• Noise
1 piston
Filling
Pumping
2 piston Total flow
• Displacement = # Cylinders x Stroke x Bore Area
17
Fluid Power Controls Laboratory (Copyright – Perry Li, 2004-2010)
M..E., University of Minnesota (updated 12.2010)
# of Pistons Effect on Flow Ripples
198
0 1 2 3 4 5 60
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Angle - rad
Flo
w -
n=2
n=3
n=4
n=5
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Fluid Power Controls Laboratory (Copyright – Perry Li, 2004-2010)
M..E., University of Minnesota (updated 12.2010)
199
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
19
Fluid Power Controls Laboratory (Copyright – Perry Li, 2004-2010)
M..E., University of Minnesota (updated 12.2010)
200
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
20
Fluid Power Controls Laboratory (Copyright – Perry Li, 2004-2010)
M..E., University of Minnesota (updated 12.2010)
201
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)
21
Fluid Power Controls Laboratory (Copyright – Perry Li, 2004-2010)
M..E., University of Minnesota (updated 12.2010)
202
Aeration and Cavitation
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Fluid Power Controls Laboratory (Copyright – Perry Li, 2004-2010)
M..E., University of Minnesota (updated 12.2010)
203
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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Fluid Power Controls Laboratory (Copyright – Perry Li, 2004-2010)
M..E., University of Minnesota (updated 12.2010)
•
Models for Pumps and Motors
204
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Fluid Power Controls Laboratory (Copyright – Perry Li, 2004-2010)
M..E., University of Minnesota (updated 12.2010)
205
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
Qi deal
Qact ual
Qloss
Ideal pump
leakage
Friction
Functions of speed, pressure and displacements
(Reverse if motor case !! )
Pump volumetric eff:
Pump mechanical eff:
Total efficiency:
Tin/out
Tideal vol
25
Fluid Power Controls Laboratory (Copyright – Perry Li, 2004-2010)
M..E., University of Minnesota (updated 12.2010)
206
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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Fluid Power Controls Laboratory (Copyright – Perry Li, 2004-2010)
M..E., University of Minnesota (updated 12.2010)
Hydrostatic Transmission
207
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Fluid Power Controls Laboratory (Copyright – Perry Li, 2004-2010)
M..E., University of Minnesota (updated 12.2010)
208
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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Fluid Power Controls Laboratory (Copyright – Perry Li, 2004-2010)
M..E., University of Minnesota (updated 12.2010)
209
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
29
Fluid Power Controls Laboratory (Copyright – Perry Li, 2004-2010)
M..E., University of Minnesota (updated 12.2010)
210
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 ratioVariable by varying
D1 or D2
Infinite and negativeratios possibleif pump can
go over-center
30
Fluid Power Controls Laboratory (Copyright – Perry Li, 2004-2010)
M..E., University of Minnesota (updated 12.2010)
211
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 Q
2
D1D2 Research opportunity!
31
Fluid Power Controls Laboratory (Copyright – Perry Li, 2004-2010)
M..E., University of Minnesota (updated 12.2010)
Hydraulic Hybrid Vehicles
32
Fluid Power Controls Laboratory (Copyright – Perry Li, 2004-2010)
M..E., University of Minnesota (updated 12.2010)
How Hybrid Vehicles Save Energy?
With a secondary power source/storage, it is possible to:
Required for maximum
performance
Normal drivingoperating
range10-15%
38%
Example vehicle on EPA-UDDS cycle: • Baseline (10% engine efficiency): 24 mpg• Engine management (38% efficiency): 95 mpg• Above with regeneration: 140 mpg
• Manage engine operation
• Store/reuse braking energy
• Turn off engine
• Downsize engine for continuous power
Engine
Energy storage
Drive-train wheel
33
Fluid Power Controls Laboratory (Copyright – Perry Li, 2004-2010)
M..E., University of Minnesota (updated 12.2010)
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
Toyota PriusMetrics
• Fuel economy
• Cost
• Performance
34
Fluid Power Controls Laboratory (Copyright – Perry Li, 2004-2010)
M..E., University of Minnesota (updated 12.2010)
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
• Accumulators are 10x more power dense than batteries
• Efficient power electronics are expensive
• 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
Accumulator
EngineHydraulic
pump/motor
Battery &ultracapacitor
EngineElectricmotor/
generator
limits acc/braking and hence regenerative
braking
Parallel Hybrid Electric Parallel Hybrid Hydraulic
35
Fluid Power Controls Laboratory (Copyright – Perry Li, 2004-2010)
M..E., University of Minnesota (updated 12.2010)
Hydraulic Hybrids Versus Electric HybridsElectric Fluid
Power
Performance
• acceleration -- ++
• regenerative braking -- ++
Component efficiency + -
Regenerative efficiency
-- ++
Weight -- ++
Cost -- ++
Reliability -- ++
Environmental impact - +
Energy storage ++ --
NVH ++ --
Realize opportunities for • Both performance & efficiency• Cost and reliability
Overcome threats in• Inefficient components • Low density energy storage • Noise, vibration, harshness
+
+
36
Fluid Power Controls Laboratory (Copyright – Perry Li, 2004-2010)
M..E., University of Minnesota (updated 12.2010)
Parallel ArchitectureExample: HLA system for F150 & garbage trucks
• Regenerates braking energy
• Utilizes efficient mechanical transmission
• Does not allow full engine management
Achievable engine op. points
37
Fluid Power Controls Laboratory (Copyright – Perry Li, 2004-2010)
M..E., University of Minnesota (updated 12.2010)
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
Normal driving
operating range
10-15%
38% Required for maximum
performance
Normal drivingoperating
range10-15%
38%
38
Fluid Power Controls Laboratory (Copyright – Perry Li, 2004-2010)
M..E., University of Minnesota (updated 12.2010)
Power-Split: Hydromechanical Transmission (HMT)
• Power split between mechanical and hydraulic paths
• Hybridized HMT – i.e. w/ regeneration
Efficient Mechanical
Transmission
RegenerativeBraking
Full enginemanagement
39
Fluid Power Controls Laboratory (Copyright – Perry Li, 2004-2010)
M..E., University of Minnesota (updated 12.2010)
Hypothesized hydraulic & overall efficiencies
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Mean hydraulic efficiency
Overa
ll eff
icie
ncy
Urban
series
parallel
hmt
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
Mean hydraulic efficiency
Overa
ll eff
icie
ncy
Highway
series
parallel
hmt
• Series / HMT at peak engine efficiency (38.5%)• Parallel at lower engine efficiency (33%)