Pneumatic Hybrid – An alternative to electric hybrid (?) Bengt Johansson Sasa Trajkovic, Div. of...

transcript

Pneumatic Hybrid –An alternative to electric hybrid (?)

Bengt Johansson Sasa Trajkovic,

Div. of Combustion EnginesLund University

2008-11-19

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Outline

• Pneumatic Hybrid• Experimental Setup• Results: Evaluation of the Free Valve Technology

system• Results: Pneumatic hybrid• Conclusions

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Outline

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Electric hybrids have proven to have significant potential to improve fuel economy and reduce

exhaust emissions high customer attractiveness

Pneumatic Hybrid- Background -

Cumulative reported US

sales of hybrid vehicles during

the period 1999-2007

Cumulative reported US

sales of hybrid vehicles during

the period 1999-2007

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• The electric hybrids suffer from a high end-product price due to the additional propulsion source and batteries

• Also, the limited life-cycle of the batteries contributes to a higher life-cycle cost

• One way of reducing the extra cost due to vehicle hybridization is the introduction of the pneumatic hybrid

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• The pneumatic hybrid is a quite simple solution utilizing only an internal combustion engine as propulsion source

• Instead of batteries, the pneumatic hybrid uses a relatively cheap pressure tank to store energy

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• Compressor mode, CM– During deceleration, the engine is used as a compressor that

converts the kinetic energy of the vehicle into potential energy in the form of compressed air which is stored in a pressure tank

• Air-motor mode, AM– During acceleration, the engine is used as a air-motor that

utilizes the pressurized air from the tank

• Air-power assist mode, APAM– The stored pressurized air is used for supercharging the engine

when there is a demand for higher torque

• Stop-start functionality– During idling the combustion engine can be completely shut off

with no fuel consumption during this period as a result

Pneumatic Hybrid- Operating principal -

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Pneumatic Hybrid- Compressor Mode -

• 1-2: Induction of fresh air• 2-3: Compression stroke• 3-4: Charging of pressure tank• 4-1: Expansion stroke

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Pneumatic Hybrid- Air-motor Mode -

• 1-2: Charging of the cylinder• 2-3: Expansion stroke• 3-4: Intake stroke• 4-1: Compression stroke

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Outline

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Experimental setup

Displaced Volume 1966 cm3Bore 127.5 mmStroke 154 mmConnecting Rod Length 255 mmNumber of Valves 4Compression Ratio 18:1Piston type FlatInlet valve diameter 45 mmExhaust valve diameter 41 mmValve Timing VariablePiston clearance 7.3 mmFuel injection PFIFuel Isooctane

The Scania D12 Diesel Engine

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Outline

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Free Valve Technology System

Illustration of Cargine’s Pneumatic Valve Actuator

The Pneumatic Valve Actuators mounted on a Scania cylinder head

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Free Valve Technology System

Pneumatic Valve Actuation valve lift profile• The valve lift event consists of three sections

– Open period– Dwell period– Closing period

• Solenoid 1 (S1)– Starts the flow of pressurized air into the

actuator → starts the opening of the valve– A hydraulic latch prevents the valve from

returning as long as S1 is active – The valve duration is set by the deactivation

• Solenoid 2 (S2)– Stops the air charging of the actuator →

determines the valve lift– May not be deactivated before S1 since it

would lead to an additional valve event

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• The objective of the evaluation of the EPVA system can be divided into two parts– Testing EPVA system performance

• Valve timing and lift• Energy consumption

– Testing of three different valve strategies enabled by EPVA• HCCI with Negative Valve Overlap• HCCI with Reberathe Strategy• HCCI with Atkinson/ Miller Strategy

Results: Evaluating the EPVA system- Evaluation objectives -

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Results: Evaluating the EPVA system - Testing valve stability -

Variation of valve lift at constant valve lift duration of 200 CAD and an engine speed of 1000 rpm

The valve lift duration remains constant when the valve lift height is varied

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Variation of valve lift duration at constant valve lift height of 7 mm and an engine speed of 1000 rpm

The valve lift height remains constant when the valve lift duration is varied

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COV (%)

Valve Lift Height 0.1805

Valve Lift Duration 0.3107

Cycle-to-cycle variations of valve lift and durationCycle-to-cycle variations of valve lift and duration

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EPVA energy consumptionEPVA energy consumption

The air consumption per engine cycle increase with increasing valve lift due to longer actuator piston stroke

The air consumption per engine cycle is not engine speed dependant

Results: Evaluating the EPVA system - EPVA energy consumption -

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Outline

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• The Scania engine was converted to work as a pneumatic hybrid engine

• A 50 liter pressure tank was connected to one of the inlet ports

• The corresponding inlet valve was converted to a tank valve with a valve head diameter of 16 mm (originally 45 mm)

Results: Pneumatic Hybrid- Engine modifications-

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Tank Pressure = 6.5 bar

Tank Pressure = 11 bar

Results: Pneumatic Hybrid- Initial testing of Compressor Mode -

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The overshoot in pressure increases with increasing engine speed

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Continuously open loop controlled CM operation done ‐at three different engine speeds

The open-loop controller is based on valve timings calculated with the polytropic compression law

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Results: Pneumatic Hybrid- Optimizing the Compressor Mode -

• Optimization of CM has been done with regards to tank valve opening, TankVO

• Tank valve closing, TankVC = 10 CAD ATDC• Inlet valve opening, IVO = 35 CAD ATDC• Inlet valve closing, IVC = 180 CAD ATDC

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• There is a difference in IMEP

• However, the difference is quite small

• The reason might be high pressure losses due to a small tank valve diameter

The tank valve diameter was changed

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Small tank valve = 16 mm

Large tank valve = 28 mm

The flow area has been increase more than three times

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• However, increasing the valve diameter does not come without a problem

• Due to the increased valve area, the force acting on the underside of the valve head is larger and thus the valve actuator has to open with a larger force.

• Due to limited supply pressure, achieving an adequate opening force is not possible

• The solution is to make the valve pressure compensated. For this purpose a in-house developed pneumatic spring has been used

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Pressure compensated tank valvePressure compensated tank valve

1. Pneumatic spring cylinder2. Spring retainer3. Tank valve4. Cylinder head5. Pressurized air passages6. Tank valve port• Blue arrows: Pressurized air entering the

pneumatic spring• Yellow arrows: The pressurized air acting on the

underside of the spring retainer and on the upside of the tank valve head

One Problem: When the tank valve is open the force acting on the upside of the tank valve head is canceled and the net force is acting to close the valveSolution: The valve actuator is fed with compressed air from the pressure tank

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Pressure losses over the tank valvePressure losses over the tank valve

Small tank valve Large tank valve

Two new problems arise with the pneumatic spring:

– The hump-like behavior occurs due to bad interactions between the check-valves when switching pressure source

– The increase in pressure drop with increasing number of engine cycles is due to a insufficient pressure in the pressurized air supply line feeding the tank valve actuator. To compensate for this, TankVO has to occur earlier than optimal

Two new problems arise with the pneumatic spring:

– The hump-like behavior occurs due to bad interactions between the check-valves when switching pressure source

– The increase in pressure drop with increasing number of engine cycles is due to a insufficient pressure in the pressurized air supply line feeding the tank valve actuator. To compensate for this, TankVO has to occur earlier than optimal

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Optimization of the compressor modeOptimization of the compressor mode

Large tank valve Small tank valve

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Continuously open loop controlled CM operation based ‐on optimized valve timings done at three different

engine speeds

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Results: Pneumatic Hybrid- Initial testing of Air-motor Mode -

Negative loop contributing with negative IMEP. Occurs due to bad

inlet valve operation

Negative loop contributing with negative IMEP. Occurs due to bad

inlet valve operation

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Results: Pneumatic Hybrid- Optimizing the Air-motor Mode -

Optimized tank valve closing during AM for the large tank valve setup

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Continuously open loop controlled AM operation based ‐on optimized valve timings with the large tank valve setup

• An remarkable increase in positive work can be seen (>30 %)

• This is due to a larger tank valve diameter in combination with proper valve timing

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• In order to estimate the potential of the pneumatic hybrid a so called regenerative efficiency has been defined

• The regenerative efficiency is the ratio between the energy recovered during AM and the energy consumed during CM

• It also can be defined as the ratio between positive and negative IMEP:

Results: Pneumatic Hybrid- Regenerative efficiency-

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ηregen

Engine speed 600 900 1200

Small tank valve setup, unoptmized 32 33 25Large Tank valve setup, constant IVC during AM 44 40 37

Large tank valve setup 48 44 40

Results: Pneumatic Hybrid- Regenerative efficiency-

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Outline

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• Various tests have clearly shown the potential with EPVA

• Results show the ability to operate in the desirable range associated with heavy duty engines

• Great flexibility as valve lift and timing can be chosen almost without constraints and independently of each other

• Successful test runs with various valve strategies have shown the great benefits with a fully flexible VVA system

Conclusions- Electro Pneumatic Valve Actuation -

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• Initial Pneuamtic Hybrid testing showed the potential of the concept with a ηregen of up to 33 %, increased to 48% with larger valve.

• The optimization of the compression mode shows that there are optimal valve timings for every tank pressure.

• In order to increase the efficiency the tank valve diameter had to be increased from 16 to 28 mm

• The new tank valve geometry was combined with a pneumatic spring in order do ensure proper valve timing at higher pressures

Conclusions- Pneumatic Hybrid-

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• A method for optimizing valve timings during both compressor mode and motor mode has been developed with good results

• The regenerative efficiency has been increased from 33%, achieved during initial testing of the concept, to 48%.

• Further improvements of CM can be done with an estimated increase in efficiency by up to 5 units.

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• Pneumatic hybrid can absorb more power than electric hybrid

• Energy storage is much simpler; an air tank is MUCH less complex than an electric battery.

• Low end torque with small turbocharged engine can be handled with air supplied from tank

• Turbocharger lag can be compensated giving instant load change (if desired)

Any questions??????

Thanks for your attention

Pneumatic Hybrid – An alternative to electric hybrid (?) Bengt Johansson Sasa Trajkovic, Div. of...

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