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Electric Compressor Selection & Integration
- 1. ELECTRIC COMPRESSOR Selection and Integration Erik Huyghe, Director of Advanced Engineeringv
- 2. AGENDA i. What Heavy Duty and Off-road Applications Might Use Electric Compressor (EC)? ii. Compressor Sizing iii. Electrical System Constraints iv. Refrigerant System Integration v. Service vi. Control and Diagnostics p.2
- 3. HD and Off-road EC Applications
- 4. Electrification is Coming Fast! Publicly announced electrified vehicles: HD/MD TRUCKS • Freightliner eCascada (HD) and eM2 106 (MD) local delivery • Volvo FE and FL (HD) trucks • Scania, Navistar, MAN, etc. OFF-ROAD • John Deere electric powertrains • CAT wheel loaders, dozer, mini excavator • Komatsu, JCB, etc. FLEET • Electric buses (transit or school) from Cummins, Proterra, Volvo, BYD, etc. • Amazon invested $1B in Rivian, ordered 100k electric delivery trucks p.4
- 5. When is EC beneficial/needed? • Electric powertrain – No belt to drive the compressor – De-couple A/C performance from powertrain speed – Battery cooling, especially if DC fast charging • Operator or cargo cooling when powertrain is off – Parking cooler / anti-idle – Longer engine off during stops • Regulations / Customer Demand – Electric/hybrid mandates – Fuel economy regulations / anti-idle laws – Customer demand • Improved efficiency / Lower total cost of operation – Scroll compressors have higher COP vs usual piston comp – Keeping engine off saves fuel p.5
- 6. New Paradigm in Compressor Sizing
- 7. Compressor Performance Pyramid p.7 Operating Conditions [Temperatures, Refrigerant, Pressures] Electrical Design [Power, Voltage, Current, Energy] Mechanical Design [Displacement, Efficiency, Speed]
- 8. Compressor Sizing • Cannot compare EC and belt-driven compressor displacement • Cooling Performance = Speed × Vol. Eff. × Displacement × Refrig. Suction Density × Enthalpy Change p.8 Belt-driven Fixed Piston Electric-driven Scroll Operating Speed Idle ≈ 1000 rpm Cruising ≈ 2500 rpm Ratio of engine speed Idle: 1000 ~ 5000+ rpm Cruising: 1000 ~ 5000+ rpm Chosen by vehicle controls Volumetric Efficiency (effective displacement %) 1000 rpm ≈ 75% *NOTE 2500 rpm ≈ 65% 5000 rpm ≈ 45% 1000 rpm ≈ 86% *NOTE 2500 rpm ≈ 87% 5000 rpm ≈ 89% Performance at Idle per cc of Comp Displacement 100% (1000 rpm) 115 ~ 590% (1000 ~ 5000 rpm) AT IDLE a 24cc scroll (at 5000rpm) can perform similar to a 150cc piston (at 1000rpm) but only if the electrical system can keep up. At higher engine (and compressor) speeds, the 150cc piston would outperform the 24cc scroll *NOTE: These numbers are just trends for discussion. Operating conditions have a large impact to actual performance
- 9. Cool Down Behavior Comparison p.9 Stopligh t Accel -erate Cruise Engine Idle Accel -erate Cruise EvaporatorTemp EngineSpd/CompSpd Time ↑ Evap Freeze Temp EvaporatorTemp EngineSpd/CompSpd Time ↑ Evap Freeze Temp Belt- Driven Comp Electric Comp Since the electric comp speed isn’t tied to engine speed, it can provide max performance at any engine speed, unlike the belt-driven compressor
- 11. p.11 Operating Conditions [Temperatures, Refrigerant, Pressures] Electrical Design [Power, Voltage, Current, Energy] Mechanical Design [Displacement, Efficiency, Speed] Compressor Performance Pyramid
- 12. Electrical Limiters p.12 • Electrical power and energy available has to be enough to do the job • Current is a major driver to cost (increases copper mass of entire electrical circuit) – Double the current requires quadruple the copper area needed to keep temperature within safe limits • Current = Power / Voltage – Half the voltage doubles the current and quadruples copper cost • Higher voltage is better for performance
- 13. Electrical Integration • Common nominal voltage architectures: – 12~48V: 48V is practical limit to avoid high voltage electrical safety requirements (peak voltages <60V) – 300V: common with automotive, economy of scale – 600V: higher power & faster charging good for HD/MD, low scale so far, but increasing adoption • Electrical isolation compliance requires careful selection of the compressor oil since motor terminals submerged in oil and refrigerant. Use the right oil for manufacturing and service • Vehicle integration issues: – EMC/EMI – Voltage/current ripple (AC waveforms on DC bus) – Protecting HV parts during crash – HV bus discharge (make safe for service or crash) p.13
- 14. Electrical System Constraints p.14 Electrification Type Nominal Voltage Sustained Current Power Runtime Applications Limiters Standard ICE w/ Aux Power Unit 24V Low ~ Medium Low Low ~ Medium Park Cooler, Aux Cooler (RV, Rear Box) Low voltage limits peak power, maintain temp, not pull down from hot soak Start-Stop (Mild) Hybrid 48V Low ~ Medium Medium Low Park Cooler *Cabin A/C *Capacity limited, very large wires Strong Hybrid (plug-in or not) 200 ~ 300V High High Medium Cabin A/C, Battery Chiller, Refrig. Trailer Some peak power limiting as battery voltage decreases Full Electric 300 ~ 600V High High High Cabin A/C, Battery Chiller, Refrig. Trailer, DC Fast Charge, Heat Pump 300V can get current limited for very high loads, 600V is very capable Vehicle electrical system has to provide enough power
- 15. Compressor Electrical Constraints • Current and power are limitations within the compressor as well – Motor sizing → maximum power and torque – Inverter (capacitors & transistors) → maximum current • Electronics have to be protected from overheat – Hotter inverter → current limit decreases – Hotter suction refrigerant → current limit decreases • Compressor design should make smart tradeoffs to match capability to the application p.15
- 17. p.17 Operating Conditions [Temperatures, Refrigerant, Pressures] Electrical Design [Power, Voltage, Current, Energy] Mechanical Design [Displacement, Efficiency, Speed] Compressor Performance Pyramid
- 18. A/C System Designs for ICE p.18 Standard Single Loop System • Single electric compressor • Single evaporator • Cool cabin exhaust air may be used for battery cooling on some mild hybrids Parallel Compressor System • Belt driven compressor for cabin cooling during driving • Electric compressor for park cooling • Compressors do not run at same time • Compressor oil shared by compressors, compatibility must be assured • Lower cost than two separate systems Electric Comp Belt- Driven Comp
- 19. p.19 Dual Loop Cabin and Battery • Single electric compressor • Evaporator for cabin • Chiller uses refrigerant to cool the battery coolant via refrig. evaporation • Evap and/or chiller operation modes • Powerful battery cooling for strong hybrids and EVs (Simple) Heat Pump System • Electric compressor used for heating of cabin and cooling of cabin & battery • Add valves and/or more heat exchangers • De-humidification makes more complex • Cost and controls difficulty increase • Supplemental heating likely needed • Efficiency benefits increase EV range Cabin Condenser (in HVAC module) Outside Heat Exchanger (in outside airflow) Heat Pump Mode Chiller A/C System Designs for EV
- 20. A/C Suction Pressure Impact Performance = Speed × Vol. Eff. × Displacement × Refrig. Suction Density × Enthalpy Change • Evap temp → evap press → suction density → performance • Battery wants 15~25°C chiller temp • Cabin wants 3~8°C evaporator temp • Compressor suction pressure will be the lowest pressure (only comp can increase pressure) p.20 Evap Temp C (F) Evap Press kPag (PSIg) Density kg/m3 Density Ratio Performance Ratio 25 (77) 564 (81.8) 30.53 201% 225% 15 (59) 387 (56.1) 22.51 148% 158% 8 (46.4) 287 (41.6) 17.99 118% 122% 3 (37.4) 225 (32.6) 15.22 100% 100% R134a, SC = 5K, SH = 10K, Condense Temp 75C (167F) Chiller
- 21. Refrigerant System Integration • Refrigerant choice – restrictions, CO2 credits • Battery size and heat transfer rate – Think of battery as thermal storage • System / compressor sizing points – Max vehicle speed – Towing up grade – DC Fast Charge – Noise and vibration can limit comp max speed • Small changes to critical areas have large impacts p.21
- 22. Why Heat Pump? • EV driving range highly impacted by outside temperature – battery, air density, thermal loads • Heat pump COP better than electric heaters • Reduce battery needed or increase range p.22 www.fleetcarma.com/nissan-leaf-chevrolet-volt-cold-weather-range-loss-electric-vehicle/
- 23. HP Suction Pressure Impact p.23 Evap Temp *C (*F) Evap Press kPag (PSIg) Density kg/m3 Density Ratio Performance Ratio 3 (37.4) 225 (32.6) 15.22 100% 100% -10 (14) 99 (14.4) 9.57 63% 58% -20 (-4) 31 (4.5) 6.47 43% 37% -30 (-22) -17 (-2.5) 4.23 28% 22% R134a, SC = 5K, SH = 10K, Condense Temp 75C (167F) Performance = Speed × Vol. Eff. × Displacement × Refrig. Suction Density × Enthalpy Change • Outside Heat Exchanger temp → suction press → density → performance • Outside heat exchanger will be same as ambient (-10*C to -40*C) • At -30*C ambient, lose 78% of performance compared to cooling mode • Heat pump needs a larger compressor and/or supplemental electric heating Cabin Condenser (in HVAC module) Outside Heat Exchanger (in outside airflow)
- 24. Heat Pump Integration • Low density creates oil circulation issues • Needs extra durable compressor – Bearings – Scroll – Inverter (capacitors) • Requires extra HX, valves, seals, plumbing, sensors to change refrigerant flow, de-humidify • Control of system is more critical • Trade refrigerant system cost for more range (or decreased battery size/cost) p.24
- 25. Service
- 26. Diagnosing System • Communication and diagnostic data should be first step if compressor is not running. Could be electrical issue instead of refrigerant system issue • If no speed command to compressor, it’s not the compressor’s fault it isn’t running! It wasn’t asked to run! • Why is the vehicle not requesting operation? What does the vehicle check for? Voltage, temperatures, pressures, electrical faults • If you can, try to command low speed operation for a few seconds with a scan tool • Many faults are self-resetting after key cycle. If it wasn’t working before, but does now, what has changed? • Is the refrigerant system in proper working condition? p.26
- 27. Servicing Electrical System • Orange wires or connectors indicate high voltage (>60V) • Current (at any voltage) can kill. Affects muscles and heart • Get the training and follow proper procedures! • Don’t modify power circuit, current is too high for repairing wires or connectors. Replace them! • Ensure power connectors are fully seated and no visible internal damage. Intermittent connections cause failures • Check the fuses (power and communication) p.27
- 28. Servicing Refrigerant System • Compressor motor is sitting in the refrigerant loop! • Motor must be cooled by and is susceptible to damage from the A/C system • Debris in A/C loop will scratch the insulation off the motor windings and short circuit it. – If find debris in system, flush lines, replace heat exchangers or you will be replacing the compressor again • Moisture in system can form acids and cause motor short circuit. – Need to replace desiccant if oil or desiccant is open to air for significant time • Oils are specifically chosen for high electrical resistance. A lot of variation based on specific product, even from same brand! – Use only the recommended oil! – Don’t cross contaminate oils p.28
- 30. Compressor Protection • Compressor self-monitors voltage, current, power, temperature • Near electrical limits, it will slow down or stop • Slowing or stopping are the only safe actions for compressor to take on its own • System still has to protect compressor from refrigerant system issues • Refrigerant system should implement discharge and suction pressure sensors p.30
- 31. Control and Diagnostics • PWM, LIN and CAN communication available • Commands to compressor are speed request and immediate shutdown flag • Compressor reports back status, actual speed, voltage, current, temperature • Controls should avoid requesting unrealistic operation from compressor • Accurate diagnosis is critical, no trouble found returns are extra costly for electric comp p.31
- 32. THANK YOU For all your electric and belt-driven compressor needs, contact: sales@TCCImfg.com www.tccimfg.com
Editor's Notes
- There’s a clear trend in the industry towards greater electrification. Governments, customers and investors are demanding companies offer electrified vehicles. So it’s no wonder that you see announcements from companies across the industry about their EV products or plans. Much more work behind the scenes as well. Yes it is a small percentage of sales today, but that will change quickly and nobody wants to be left out of that growth
- If you want a lot of performance out of an engine, you obviously need to pick an appropriate displacement and design. The same is true for compressors. So how do we size a compressor? We need the same performance from the electric as the conventional belt-driven compressor, so it’s tempting to say that that means we need the same displacement. But that’s not true. Well let’s take a look at an example.
- So the cooling performance equation is given at the top here. Let’s focus on the variables related to compressor design. We’ll look the refrigerant effects later. Compressor speed, volumetric efficiency and displacement all affect it’s performance. Comp speed is decided by engine speed and pulley ratio for belt-driven, typically 1000rpm at idle. For EC it’s whatever you want within the capability of the compressor (1000-5000 rpm) Volumetric efficiency is essentially the percentage of the theoretical displacement that you actually can use due to internal clearances If we multiply speed times vol. eff. then we will see that for every cc of comp displacement we can get up to 5.9 times the performance with EC This means that at idle a 24cc EC can perform like a 150cc belt-driven
- This is a theoretical example of a cool down form a hot soak. The belt-driven piston comp has less than desired performance at idle due to low speed. As the vehicle starts driving perf improves but when idling at a stoplight we are hurting for performance again. At higher engine speeds we have too much capacity and have to cycle the clutch. The electric compressor speed is anywhere from max to min speed, based on vehicle controls request and available power. Evap temp can be kept steady regardless of engine speed
- Even a powerful engine won’t perform well if you starve it of fuel and air. You can’t get enough airflow through a drinking straw for anything but the smallest engines. Well electricity is the fuel for EC so we have to make sure we can feed it to get the performance we need.
- There’s no free lunch If you want a lot of performance it takes a lot of electrical power Current is the major limiter. Double the current costs 4 times as much in copper material to deal with thermals Increasing voltage is the antidote to high current. As voltage goes up current goes down one to one. Voltage selection is driven by powertrain, not thermal, so typically we have to deal with it, but for APU applications it’s a very important consideration.
- The vehicle powertrain solution will typically determine what voltage, current and energy are available for the compressor to use
- The final piece to performance is the set of operating conditions. Engines perform differently depending on the air pressure and temperature. Compressors are the same except they care about refrigerant pressure and temperature, which determine the density.
- So the first factor in compressor operating conditions is the refrigerant system design. Here we see two types are likely to be found on vehicles with engines.
- Here are two system designs that are more common for EVs
- Evaporating temperature determines suction pressure and density, which has a very large performance impact Battery can be happy with 15~25°C chiller temp, increasing system performance substantially When cabin and battery need cooling at same time, suction pressure must be lower (evap temp 3~8°C) to maintain comfort, which places high demand on A/C system Compressor size may be driven by highest combined battery and cabin thermal loads (Max speed? Towing up grade? DC Fast Charge?) Condensing performance can also be an impact, especially at real high pressures where dome curves more
- Why do a heat pump system? Increased vehicle driving range at cold temps. Heat pump has a higher COP than electric heater, so increases range, meaning you can use a smaller battery for the same range or increase range for the same battery.
- Outside heat exchanger temperature determines suction pressure and density, which has a very large performance impact Heat pump operation may require a much larger compressor than what is needed for cooling only (supplemental electric heating can help)
- Heat pump operation can cause oil circulation issues since very low density refrigerant doesn’t carry oil with it back to the compressor Heat pump needs high durability compressor (upgraded bearings, scroll, inverter) vs non-heat pump compressor Trade refrigerant system cost for more range (or decreased battery cost) Control of system during mode changes is critical
- Compressor is trying to protect itself. Better to have a temporary degradation than a permanent failure. Compressor only knows about electrical status, nothing about refrigerant system. The vehicle controls need to help protect the compressor from damaging refrigerant system conditions Slowing or stopping are the only safe actions because compressor does not know state of rest of vehicle (system pressure, vehicle crash, controller software bug) System still has to protect compressor from adverse suction and discharge pressures, low charge, poor oil flow, evaporator freeze, etc. Refrigerant system should implement suction and discharge pressure sensors with electric compressors the business case is usually justified with EC
- LIN is good balance of cost and capability for diagnostics. CAN costs more, PWM is very basic. Signals can of course be customized, but these are typical. Master tells the compressor what to do and the compressor tells the master what it is doing and sees. Controls should avoid requesting unrealistic operation from compressor, so self-protection does not activate Diagnosis needs to be done carefully and service instructions need to include checks of connectors and harnesses. No trouble found replacements are expensive