Pneumatic Hybrid – An alternative to electric hybrid (?)

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

Outline • Pneumatic Hybrid • Experimental Setup • Results: Evaluation of the Free Valve Technology system • Results: Pneumatic hybrid • Conclusions

Pneumatic Hybrid- Background - Electric hybrids have proven to have significant potential to improve fuel economy and reduce exhaust emissions  high customer attractiveness Cumulative reported US sales of hybrid vehicles during the period 1999-2007

Pneumatic Hybrid- Background - • 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

Pneumatic Hybrid- Background - • 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

Pneumatic Hybrid- Operating principal - • 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- Compressor Mode - • 1-2: Induction of fresh air • 2-3: Compression stroke • 3-4: Charging of pressure tank • 4-1: Expansion stroke

Pneumatic Hybrid- Air-motor Mode - • 1-2: Charging of the cylinder • 2-3: Expansion stroke • 3-4: Intake stroke • 4-1: Compression stroke

Experimental setup The Scania D12 Diesel Engine

Free Valve Technology System The Pneumatic Valve Actuators mounted on a Scania cylinder head Illustration of Cargine’s Pneumatic Valve Actuator

Free Valve Technology System • 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 of S1 • 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 Pneumatic Valve Actuation valve lift profile

Results: Evaluating the EPVA system- Evaluation objectives - • 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 - Testing valve stability - The valve lift duration remains constant when the valve lift height is varied Variation of valve lift at constant valve lift duration of 200 CAD and an engine speed of 1000 rpm

Results: Evaluating the EPVA system - Testing valve stability - The valve lift height remains constant when the valve lift duration is varied Variation of valve lift duration at constant valve lift height of 7 mm and an engine speed of 1000 rpm

Results: Evaluating the EPVA system - Testing valve stability - Cycle-to-cycle variations of valve lift and duration

Results: Evaluating the EPVA system - EPVA energy consumption - EPVA 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: Pneumatic Hybrid- Engine modifications- • 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- Initial testing of Compressor Mode - Tank Pressure = 6.5 bar Tank Pressure = 11 bar Tank Pressure = 6.5 bar Tank Pressure = 11 bar

Results: Pneumatic Hybrid- Initial testing of Compressor Mode - The overshoot in pressure increases with increasing engine speed

Results: Pneumatic Hybrid- Initial testing of Compressor Mode - 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

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

Results: Pneumatic Hybrid- Optimizing the Compressor Mode - • 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

Results: Pneumatic Hybrid- Optimizing the Compressor Mode - Large tank valve  = 28 mm Small tank valve  = 16 mm The flow area has been increase more than three times

Results: Pneumatic Hybrid- Optimizing the Compressor Mode - • 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

Pressure compensated tank valve 1 • Pneumatic spring cylinder • Spring retainer • Tank valve • Cylinder head • Pressurized air passages • 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 2 3 4 5 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 valve Solution: The valve actuator is fed with compressed air from the pressure tank 6

Results: Pneumatic Hybrid- Optimizing the Compressor Mode - Pressure losses over the 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 Small tank valve Large tank valve

Results: Pneumatic Hybrid- Optimizing the Compressor Mode - Optimization of the compressor mode Large tank valve Small tank valve

Results: Pneumatic Hybrid- Optimizing the Compressor Mode - Continuously open‐loop controlled CM operation based on optimized valve timings done at three different engine speeds

Results: Pneumatic Hybrid- Initial testing of Air-motor Mode - Negative loop contributing with negative IMEP. Occurs due to bad inlet valve operation

Results: Pneumatic Hybrid- Optimizing the Air-motor Mode - Optimized tank valve closing during AM for the large tank valve setup

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

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

Results: Pneumatic Hybrid- Regenerative efficiency- • 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-

Conclusions- Electro Pneumatic Valve Actuation - • 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- Pneumatic Hybrid- • 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- • 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.

Conclusions- Pneumatic Hybrid- • 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)

Thanks for your attention ????? Any questions?

Pneumatic Hybrid – An alternative to electric hybrid (?)