Powertrain & Calibration 101

1. Powertrain & Calibration 101 John Bucknell DaimlerChrysler Powertrain Systems Engineering December 4, 2006

2. Powertrain & Calibration Topics • Background • Powertrain terms • Thermodynamics • Mechanical Design • Combustion • Architecture • Cylinder Filling & Emptying • Aerodynamics • Calibration • Spark & Fuel • Transients & Drivability

3. What is a Powertrain? • Engine that converts thermal energy to mechanical work • Particularly, the architecture comprising all the subsystems required to convert this energy to work • Sometimes extends to drivetrain, which connects powertrain to end-user of power

4. Characteristics of Internal Combustion Heat Engines • High energy density of fuel leads to high power to weight ratio, especially when combusting with atmospheric oxygen • External combustion has losses due to multiple inefficiencies (primarily heat loss from condensing of working fluid), internal combustion has less inefficiencies • Heat engines use working fluids which is the simplest of all energy conversion methods

5. Reciprocating Internal Combustion Heat Engines • Characteristics • Slider-crank mechanism has high mechanical efficiency (piston skirt rubbing is source of 50-60% of all firing friction) • Piston-cylinder mechanism has high single-stage compression ratio capability – leads to high thermal efficiency capability • Fair to poor air pump, limiting power potential without additional mechanisms

6. Reciprocating Engine Terms Vc = Clearance Volume Vd = Displacement or Swept Volume Vt = Total Volume TC or TDC = Top or Top Dead Center Position BC or BDC = Bottom or Bottom Dead Center Position Compression Ratio (CR)

7. Further explanation of aspects of Compression Ratio

8. Reciprocating Engines • Most layouts created during second World War as aircraft manufacturers struggled to make the least-compromised installation

9. Thermodynamics • Otto Cycle • Diesel Cycle • Throttled Cycle • Supercharged Cycle Source: Internal Comb. Engine Fund.

10. Thermodynamic Terms MEP– Mean Effective Pressure • Average cylinder pressure over measuring period • Torque Normalized to Engine Displacement (VD) BMEP – Brake Mean Effective Pressure IMEP – Indicated Mean Effective Pressure MEP of Compression and Expansion Strokes PMEP – Pumping Mean Effective Pressure MEP of Exhaust and Intake Strokes FFMEP – Firing Friction Mean Effective Pressure BMEP = IMEP – PMEP – FFMEP

11. Thermodynamic Terms continued Work = Power = Work/Unit Time Specific Power– Power per unit, typically displacement or weight Pressure/Volume Diagram– Engineering tool to graph cylinder pressure

12. Indicated Work TDC BDC Source: Design and Sim of Four Strokes

13. Pumping Work TDC BDC Source: Design and Sim of Four Strokes

14. History of Internal Combustion • 1878 Niklaus Otto built first successful four stroke engine • 1885 Gottlieb Daimler built first high-speed four stroke engine • 1878 saw Sir Dougald Clerk complete first two-stroke engine (simplified by Joseph Day in 1891) 1891 Panhard-Levassor vehicle with front engine built under Daimler license

15. Energy Distribution in Passenger Car Engines Source: SAE 2000-01-2902 (Ricardo)

17. Using Exhaust Energy • Highest expansion ratio recovers most thermal energy • Turbines can recover heat energy left over from gas exchange • Energy can be used to drive turbo-compressor or fed back into crank train Source: Advanced Engine Technology

18. Supercharging • Increases specific output by increasing charge density into reciprocator • Many methods of implementation, cost usually only limiting factor Source: Internal Comb. Engine Fund.

19. Mechanical Design

21. Two Valve Valvetrain • Pushrod OHV (Type 5) • HEMI 2-Valve (Type 5) • SOHC 2-Valve (Type 2)

22. Four Valve Valvetrain • DOHC 4-Valve (Type 2) • SOHC 4-Valve (Type 3) • DOHC 4-Valve (Type 1) • Desmodromic

23. Valvetrain • Specific Power = f(Air Flow, Thermal Efficiency) • Air flow is an easier variable to change than thermal efficiency • 90% of restriction of induction system occurs in cylinder head • Cylinder head layouts that allow the greatest airflow will have highest specific power potential • Peak flow from poppet valve engines primarily a function of total valve area • More/larger valves equals greater valve area

24. Combustion Terms • Brake Power – Power measured by the absorber (brake) at the crankshaft • BSFC - Brake Specific Fuel Consumption Fuel Mass Flow Rate / Brake Power grams/kW-h or lbs/hp-h • LBT Fuelling - Lean Best Torque Leanest Fuel/Air to Achieve Best Torque LBT = 0.0780-0.0800 FA or 0.85-0.9 Lambda • Thermal Enrichment – Fuel added for cooling due to component temperature limit • Injector Pulse Width - Time Injector is Open

25. Combustion Terms continued • Spark Advance – Timing in crank degrees prior to TDC for start of combustion event (ignition) • MBT Spark – Maximum Brake Torque Spark Minimum Spark Advance to Achieve Best Torque • Burn Rate – Speed of Combustion Expressed as a fraction of total heat released versus crank degrees • MAP - Manifold Absolute Pressure Absolute not Gauge (does not reference barometer)

26. Combustion Terms continued • Knock– Autoignition of end-gasses in combustion chamber, causing extreme rates of pressure rise. • Knock Limit Spark - Maximum Spark Allowed due to Knock – can be higher or lower than MBT • Pre-Ignition – Autoignition of mixture prior to spark timing, typically due to high temperatures of components • Combustion Stability – Cycle to cycle variation in burn rate, trapped mass, location of peak pressure, etc. The lower the variation the better the stability.

27. Engine Architecture Influence on Performance • Intake & Exhaust Manifold Tuning • Cylinder Filling & Emptying • Momentum • Pressure Wave • Aerodynamics • Flow Separation • Wall Friction • Junctions & Bends • Induction Restriction • Exhaust Restriction (Backpressure) • Compression Ratio • Valve Events

28. Intake Tuning for WOT Performance • Intake manifolds have ducts (“runners”) that tune at frequencies corresponding to engine speed, like an organ pipe • Longer runners tune at lower frequencies • Shorter runners tune at higher frequencies • Tuning increases local pressure at intake valve thereby increasing flow rate • Duct diameter is a trade-off between velocity and wall friction of passing charge

29. Exhaust Tuning for WOT Performance • Exhaust manifolds tune just as intake manifolds do, but since no fresh charge is being introduced as a result – not as much impact on volumetric efficiency (~8% maximum for headers) • Catalyst performance usually limits production exhaust systems that flow acceptably with little to no tuning

30. Tuned Headers Tuned Headers generally do not appear on production engines due to the impairment to catalyst light-off performance (usually a minimum of 150% additional distance for cold-start exhaust heat to be lost). Performance can be enhanced by 3-8% across 60% of the operating range.

31. Momentum Effects • Pressure loss influences dictate that duct diameter be as large as possible for minimum friction • Increasing charge momentum enhances cylinder filling by extending induction process past unsteady direct energy transfer of induction stroke (ie piston motion) • Decreasing duct diameter increases available kinetic energy for a given mass flux • Therefore duct diameter is a trade-off between velocity and wall friction of passing charge

32. Pressure Wave Effects • Induction process and exhaust blowdown both cause pressure pulsations • Abrupt changes of increased cross-section in the path of a pressure wave will reflect a wave of opposite magnitude back down the path of the wave • Closed-ended ducts reflect pressure waves directly, therefore a wave will echo with same amplitude

33. Pressure Wave Effects con’t • Friction decreases energy of pressure waves, therefore the 1st order reflection is the strongest – but up to 5th order have been utilized to good effect in high speed engines (thus active runners in F1 in Y2K) • Plenums also resonate and through superposition increase the amplitude of pressure waves in runners – small impact relative to runner geometry

34. Effects of Intake Runner Geometry

35. Tuning in Production I4 Engine

36. Aerodynamics • Losses due to poor aerodynamics can be equal in magnitude to the gains from pressure wave tuning • Often the dominant factory in poorly performing OE components • If properly designed, flow of a single-entry intake manifold can approach 98% of an ideal entrance on a cylinder head port (steady state on a flow bench)

37. Aerodynamics con’t • Flow Separation • Literally same phenomenon as stall in wing elements – pressure in free stream insufficient to ‘push’ flow along wall of short side radius • Recirculation pushes flow away from wall, thereby reducing effective cross-section: so-called “vena contracta” • Simple guidelines can prevent flow separation in ducts – studies performed by NACA in the 1930s empirically established the best duct configurations

38. Aerodynamics con’t • Wall Friction • Surface finish of ducts need to be as smooth as possible to prevent ‘tripping’ of flow on a macro level • Junctions & Bends • Everything from your fluid dynamics textbook applies • Radiused inlets and free-standing pipe outlets • Minimize number of bends • Avoid ‘S’ bends if at all possible

39. Induction Restriction • Air cleaner and intake manifolds provide some resistance to incoming charge • Power loss related to restriction almost directly a function of ratio between manifold pressure (plenum pressure upstream of runners) and atmospheric

40. Exhaust Restriction

41. Compression Ratio • The highest possible compression ratio is always the design point, as higher will always be more thermally efficient with better idle quality • Knock limits compression ratio because of combustion stability issues at low engine speed due to necessary spark retard • Most engines are designed with higher compression than is best for low speed combustion stability because of the associated part-load BSFC benefits and high speed power

42. Valve Events • Valve events define how an engine breathes all the time, and so are an important aspect of low load as well as high load performance • Valve events also effectively define compression & expansion ratio, as “compression” will not begin until the piston-cylinder mechanism is sealed – same with expansion

43. Valve Event Timing Diagram • Spider Plot - Describes timing points for valve events with respect to Crank Position • Cam Centerline - Peak Valve Lift with respect to TDC in Crank Degrees

44. Valve Events for Power • Maximize Trapping Efficiency • Intake closing that is best compromise between compression stroke back flow and induction momentum (retard with increasing engine speed) • Early intake closing usefulness limited at low engine speed due to knock limit • Early intake opening will impart some exhaust blowdown or pressure wave tuning momentum to intake charge • Maximize Thermal Efficiency • Earliest intake closing to maximize compression ratio for best burn rate (optimum is instantaneous after TDC) • Latest exhaust opening to maximize expansion ratio for best use of heat energy and lowest EGT (least thermal protection enrichment beyond LBT)

45. Valve Events for Power • Minimize Flow Loss • Achieve maximum valve lift (max flow usually at L/D > 0.25-0.3) as long as possible (square lift curves are optimum for poppet valves) • Minimize Exhaust Pumping Work • Earliest exhaust opening that blows down cylinder pressure to backpressure levels before exhaust stroke (advance with increasing engine speed) • Earliest exhaust closing that avoids recompression spike (retard with increasing engine speed)

47. Engine Power and BSFC vs Engine Speed

48. Summary • Component’s Relative Impact on Performance • Cylinder Head Ports & Valve Area • Valve Events • Intake Manifold Runner Geometry • Compression Ratio • Exhaust Header Geometry • Exhaust Restriction • Air Cleaner Restriction

49. Powertrain Closing Remarks • Powertrain is compromise • Four-stroke engines are volumetric flow rate devices – the only route to more power is increased engine speed, more valve area or increased charge density • More speed, charge density or valve area are expensive or difficult to develop – therefore minimizing losses is the most efficient path within existing engine architectures • Highest average power during a vehicle acceleration is fastest – peak power values don’t win races

50. Break