Engine urbo/super Charging Super and urbo-charging Why super/ turbo-charging? Fuel burned per cycle in an IC engine is air limited (F/A) stoich = /4.6 orq m Q f, v fuel conversion and volumetric efficiencies f f HV m f fuel mass per cycle Q HV fuel heating value n n R for -stroke, for 4-stroke engine R N revolution per second V D engine displacement a,0 air density Power orq N m F f V a,0 V A D Super/turbo-charging: increase air density
Super- and urbo- Charging Purpose: o increase the charge density Supercharge: compressor powered by engine output No turbo-lag Does not impact exhaust treatment Less efficient than turbo-charging urbo-charge: compressor powered by exhaust turbine More directly utilize exhaust energy urbo- lag problem Affects exhaust treatment Intercooler Increase charge density (hence output power) by cooling the charge Lowers NO x emissions Suppresses knock Additional benefit of turbo-charging Can downsize engine while retaining same max power Less throttle loss under part load in SI engine Higher BMEP reduces relative friction and heat transfer losses
Engine Losses Spark retard/enrichment for SI; 5 th gear, smoke limit for diesel flat road 0 9 4 th gear, flat road Relative 8 efficiency = Heat transfer 7 Combustion speed, pumping loss BMEP (bar) hrottle + ht transf + friction 6 5 5 g/kw-hr =0.88 =0.78 =0.70 4 88 =0.64 3 3 rd gear, 34 =0.58 flat road 360 =0.54 =0.50 000 000 3000 4000 5000 5 Engine speed (rpm) Data from SAE 90676; Saturn I4 engine Society of Automotive Engineers. All rights reserved. his content is excluded from our SI engine efficiency opportunity urbo DISI as enabling technology Fuel in-flight evaporation cools charge Regain load 0 More knock head room by turbo-charging resistant 8 Issues BMEP (bar) 6 Shift op. points up by downsizing Knock 4 Peak pressure Boosting capacity Cold start emissions aurus FP 0 sec-by-sec HC - PM 0 000 000 3000 Speed (rpm) 3
Charge-air pressure regulation with wastegate on exhaust gas end..engine,. Exhaust-gas turbochager, 3. Wastegate Exhaust-gas turbocharger for trucks.compressor housing,. Compressor impeller, 3. urbine housing, 4. Rotor, 5. Bearing housing, 6. inflowing exhaust gas, 7. Out-flowing exhaust gas, 8. Atmospheric fresh air, 9. Pre-compressed fresh air, 0. Oil inlet,. Oil return From Bosch Automotive Handbook Robert Bosch GmbH. All rights reserved. his content is excluded from our Creative Commons license. For more information, see https://ocw.mit.edu/help/faq-fair-use. urbo-charger Waste gate Source: BorgWarner urbo Systems BorgWarner urbo Systems. All rights reserved. his content is excluded from our 4
Variable geometry turbo-charger Variable Guide Vane Variable sliding ring Source: BorgWarner urbo Systems BorgWarner urbo Systems. All rights reserved. his content is excluded from our Compressor: basic thermodynamics Compressor efficiency c W W ideal m c Wactual W m c ideal p Ideal process P P Actual process P P P W actual m c p c P s W actual m c p 5
urbine: basic thermodynamics urbine efficiency 4 t W W actual t W ideal 3 m W m c 4 ideal p 3 3 Ideal process 3 4 4 P 3 P 4 Actual process 4 P 4 P 3 3 P W actual t m c p 3 4 P 3 s 4 3 W actual m c p Properties of urbochargers Power transfer between fluid and shaft RPM 3 ypically operate at ~ 60K to 0K RPM RPM limited by centrifugal stress: usually tip velocity is approximately sonic RPM also limited by shock waves Flow devices, sensitive to boundary layer (BL) behavior Compressor: BL under unfavorable gradient urbine: BL under favorable gradient 6
orque characteristics of flow machinery Angular momentum theorem orq rv V da x rv V x da V x V both V x and V are fixed by the blade angle so that both are RPM, therefore: V x orq RPM Power RPM 3 V V V x Rotor stress Force balance over mass element from r to dr A r A rdr m Adr or d A m A r dr Crosssection r area A o illustrate effect, say A is independent of r, then : Max at m root (r) R r m R t t ensile stress Material density Angular velocity = N ip radius r R root R t r r dr 7
ypical super/turbo-charged engine parameters Peak compressor pressure ratio.5 BMEP up to 4 bar Limits: compressor aerodynamics cylinder peak pressure NOx emissions Compressor/urbine Characteristics Delivered pressure P P = f( m,r,p,n,d,,, geometric ratios) Dimensional analysis: 7 dimensional variables (7-3) = 4 dimensionless parameters (plus and geometric ratios) P N m f(,,re,, geometric ratios) P R / D P R D R Velocity Velocity Density High Re number flow weak Re dependence For fixed geometry machinery and gas properties P N m f, P P 8
Compressor Map Pressure ratio Corrected Flow rate m /P = inlet temperature (K); P = inlet pressure (bar); N = rev. per min.; m = mass flow rate (kg/s) (From Principles and Performance in Diesel Engineering, Ed. by Haddad and Watson) Compressor stall and surge Stall Happens when incident flow angle is too large (large V /V x ) Stall causes flow blockage Surge Flow inertia/resistance, and compression system internal volume comprise a LRC resonance system Oscillatory flow behave when flow blockage occurs because of compressor stall reverse flow and violent flow rate surges 9
urbine Map Efficiency Mass flow Source: BorgWarner urbo Systems BorgWarner urbo Systems. All rights reserved. his content is excluded from our Compressor urbine Matching Exercise For simplicity, take away intercooler and wastegate Given engine brake power output ( W E ) and RPM, 4 compressor map, turbine map, and engine map Find operating point, i.e. air C flow ( ), fuel flow rate ( ) m a m f turbo-shaft revolution per second (N), compressor and turbine pressure ratios ( c and t ) etc. m f 3 Diesel Engine W E Q L 0
Pressure ratio Compressor/ turbine/engine matching solution Compressor Flow rate m /P Procedure:. Guess ; can get engine inlet conditions: c P c P c c. hen engine volumetric efficiency calibration will give the air flow m a that can be ' swallowed' 3. From m and, the compressor speed N can be a E f f E c obtained from the compressor map 4. he fuel flow rate m may be obtained from the engine map: W m LHV (RPM,W,A/F) f 5. Eng ine exhaust temperature may be obtained from energy balance (with known engine mech. eff. M ) W E (m a m f )c p 3 m a c p m f LHV Q L 6. Guess, then get turbine speed N from turbine map t and mass flow 7. Determine turbine power from turbine efficiency on map W t t t 8. Iterate on the values of and until W W and N N 3 M t c t t c t c BorgWarner urbo Systems. All rights reserved. his content is excluded from our Compressor/ Engine/ urbine Matching Mass flows through compressor, engine, turbine and wastegate have to be consistent urbine inlet temperature consistent with fuel flow and engine power output urbine supplies compressor work urbine and compressor at same speed C Inter- Cooler Wastegate Compressor characteristics, with airflow requirements of a four-stroke truck engine superimposed. (From Principles and Performance in Diesel Engineering, Ed. by Haddad and Watson) Engine Ellis Horwood Ltd. All rights reserved. his content is excluded from our Creative Commons license. For more information, see https://ocw.mit.edu/help/faq-fair-use.
Advanced turbocharger development Electric assisted turbo-charging Concept Put motor/ generator on turbo-charger reduce wastegate function Benefit increase air flow at low engine speed auxiliary electrical output at part load Inter- Cooler C Engine Wastegate Motor/ Generator Battery Advanced turbocharger development Electrical turbo-charger Concept turbine drives generator; compressor driven by motor Benefit decoupling of turbine and compressor map, hence much more freedom in performance optimization Auxiliary power output do not need wastegate; no turbo-lag Inter- Cooler C Motor Engine Battery Generator
Advanced turbocharger development Challenges Interaction of turbo-charging system with exhaust treatment and emissions Especially severe in light-duty diesel market because of low exhaust temperature Low pressure and high pressure EGR circuits ransient response Cost EGR/ turbo Configurations From SAE 007-0-978 Society of Automotive Engineers. All rights reserved. his content is excluded from our 3
Hybrid EGR From SAE 009-0-45 Society of Automotive Engineers. All rights reserved. his content is excluded from our wo stage turbo with HP EGR loop SAE 008-0-06 Society of Automotive Engineers. All rights reserved. his content is excluded from our 4
MI OpenCourseWare https://ocw.mit.edu.6 Internal Combustion Engines Spring 07 For information about citing these materials or our erms of Use, visit: https://ocw.mit.edu/terms.