Durability Aspects of Turbocharged Vs Naturally Aspirated Racing Engines

Transcription

1 Durability Aspects of Turbocharged Vs Naturally Aspirated Racing Engines Doug Milliken Garrett Engine Boosting Systems, Inc. Copyright 22 Society of Automotive Engineers, Inc. ABSTRACT One of the attractive characteristics of the spectacle of the 24 Hours of LeMans is the technical variety of engine type and size solutions competing for the overall win. The question whether the turbocharging solution offers sufficient advantages in any racing application depends entirely on the technical restrictions governing that application. In the absence of technical restrictions on turbocharged engine output, obviously turbocharging offers significant advantages in outright power capability, as well as power/weight and power/size ratios. But what about the situation where the technical restrictions placed on turbocharged engines are carefully calculated to marginalize any peak power advantage and make all powerplant peak outputs essentially equal? Such is the case concerning the technical regulations currently in force (22) at the 24 Hours of LeMans, sanctioned by the Automobile Club de l Ouest (the ACO). Using a carefully calculated combination of restrictions on maximum airflow by use of air inlet restrictors on all engines plus boost limits on turbocharged engines, the ACO has created a technical formula that effectively gives all engines in the LMP9 and GTP categories similar maximum air massflow capability and therefore similar maximum power output potential, regardless of displacement, engine layout or induction method. (Ref 1). In this situation, what advantages are left for turbocharging to offer? There are several well understood ones. Smaller displacement gives reduced internal friction losses which contribute to minimizing fuel consumption. A smaller, lighter engine package allows the possibility for 1) vehicle handling improvements due to lower center of gravity and lower gross vehicle weight; and 2) body aerodynamic improvements due to smaller engine covers that can reduce drag and improve rear wing performance by reducing turbulence in front of the rear wing. This paper will not address these well understood advantages but instead address perhaps the most misunderstood, yet perhaps the most significant advantage for this type of racing, durability. Turbocharged racing engines offer the possibility to achieve equal performance with significantly lower internal inertia loading because of smaller and therefore lighter reciprocating components operating at lower speeds. This can be achieved without the need for high cylinder counts or exotic and costly low-density supermaterials to reduce the reciprocating mass per cylinder in order to reduce the maximum connecting rod tensile load. This paper will compare the effects of reciprocating mass per cylinder and engine speed on connecting rod load reversal cycles for typical engines of varying displacement, cylinder count and induction type to explain why turbocharged engines have demonstrated vastly superior durability results in the LMP9 and GTP classes at LeMans in recent years. INTRODUCTION While racing engines must obviously be developed to achieve competitive levels of performance, at the same time the most successful engines in endurance racing must also be optimized for safety margin against component failure due to fatigue loading. The fatigue life of the reciprocating components is linked to the maximum tensile stress, the number of load cycles in the component lifespan and the strength of the component materials. The ACO mandated air inlet restrictor diameters make all the engines in LMP9 and GTP classes at LeMans limited to maximum airflow of between.38 to.414 Kg/Sec (5.2 to 54.6 Lb/Min) which limits maximum power output to around 45 kw (6 HP). However, the engine configurations on the grid offer a variety of strategies for how that maximum airflow is achieved.

2 The different configurations bias one or two of the three independent variables that determine maximum airflow: Displacement Volume Engine Speed (RPM) Air inlet Density (boost). This paper will show that choosing an engine configuration with a bias for achieving maximum airflow through boosted charge density minimizes the need for either large displacement (i.e. higher reciprocating mass) or high engine speed, which has an exponential effect on reciprocating load. Put simply, components of smaller reciprocating mass operating at lower piston speed for fewer load reversal cycles is the secret to the dramatic durability performance demonstrated by turbocharged engines at the 24 Hours of LeMans in recent years. MAIN SECTION ENGINE SPEED REQUIREMENTS Figure 1 shows a generalization of the calculated engine air massflow capabilities (pumping capacity) versus engine RPM for typical engine configurations in the LMP9 and GTP categories. An inlet restrictor depression of 5% is assumed for all engines. A turbocharger compressor efficiency of 7% and a charge air intercooler efficiency of 8% are assumed for all turbocharged engines. The chokeflow represented by the horizontal portions of the curves is a function of the mandated air inlet restrictor diameter. The slope of each curve is a straight forward function of the air pumping capacity of the engine based on displacement, engine speed and air inlet density (boost) according to the following formula: Equation A: Wa = ηv x D x RPM x ρ where, Wa = engine air massflow (Kg/Min) ηv = engine volumetric efficiency (dimensionless, assumed 1. for all engines for this analysis) D = engine displacement (Liter/Revolution) RPM = engine speed (Revolutions/Min) ρ = air inlet density (Kg/Liter) (atmospheric for N.A. engines and dependent on the A.C.O. regulated boost limit for turbocharged engines). Dropping the volumetric efficiency, a dimensionless constant for each engine, gives the following proportionality relation: Equation B: Wa α D x RPM x ρ The units are: (Mass/Time) α (Vol/Rev) x (Rev/Time) x (Mass/Vol) The actual RPM at which each engine achieves choke flow will vary from the generalizations made in Fig. 1 because of individual engine variations in friction, volume per cylinder, bore/stroke ratio, engine volumetric efficiency, the efficiencies of the turbocharger, intercooler and engine and many other variables. The air massflow values in Fig. 1 and Table 1 are not meant to indicate actual values for any particular engine or to imply any specific maximum power capability of one engine size/type compared to another one, but rather to show a simplified pumping capacity comparison to establish approximate RPM requirements. Table 1 shows what the three independent variables in Eq. B are for typical engine configurations in use at LeMans. Since the maximum engine (choke) massflow is fixed for all engines by the size of it s legislated air inlet restrictor, and all engines have similar maximum (choke) massflow, the engine designer has a choice as to how to bias the three independent variables in Eq. B to achieve that fixed massflow. As will be shown below, engine displacement D (per cylinder) and especially engine speed RPM have detrimental effects on engine reliability because of their affects on reciprocating inertia. Looking just at the turbocharged engines, Fig. 1 shows that, for this simplified analysis, all turbocharged engines from 3.2 liters to 4. liters displacement have essentially identical pumping capacity versus engine speed characteristics. All can achieve maximum (choke) massflow at approximately 67 RPM because all have the same air inlet restrictor size (two 32.4mm diameter restrictors) and each is legislated a maximum boost level (charge density) that allows it to achieve that choke massflow at approximately the same RPM of 67. (In actuality, there are small differences in RPM at choke between these engines due to differences in friction, volume per cylinder, etc.). The 2. liter turbocharged engine is allowed insufficient boost (3. bar) to achieve the airflow of it s larger brothers at low engine speed and needs an additional 5 RPM to achieve choke massflow.

3 .5 Fig. 1 Engine Airflow per ACO Rules L, 3.6L & 4.L Turbo 5L NA 4L NA 3L NA.35 6L NA Engine Airflow (kg/s) L Turbo Engine RPM TABLE 1. D RPM (ρ) Wa ACO Engine Engine ACO Air Inlet Engine Displacement Displacement Engine Speed Boost Restrictor Air Massflow Total no. of per choke Limit choke cylinders (liters) (liters) (RPM) (bar) (mm) (Kg/Sec) TURBOCHARGED x x x x NATURALLY ASPIRATED X X X X , X , X , X , X

4 Not surprisingly we see no 2.L turbocharged engines on the grid in LMP9 and GTP classes at LeMans. Looking just at the naturally aspirated engines, a 3. liter N.A. engine would have to operate at approximately 14,4 RPM in order to achieve the maximum (choke) massflow. Not surprisingly we see no 3.L N.A. engines on the grid in LMP9 and GTP classes at LeMans. A 4. liter N.A. engine must operate at approximately 1,5 RPM in order to pump the maximum (choke) massflow and a 5. liter N.A. engine must operate at approximately 8, RPM in order to pump the maximum (choke) massflow. Only the 6. liter N.A. engine can pump the maximum (choke) massflow at under 7 RPM like the turbocharged engines. Table 1 shows that the 6. liters needed for an N.A. engine to achieve choke massflow at an RPM similar to the turbocharged engines results in a 67% increase in displacement per cylinder for V8 engines. Bringing the associated reciprocating mass down to the level of the 3.6 liter V8 turbocharged engine requires the expense of higher cylinder counts and/or exotic low-density materials for reciprocating components. Even increasing the 6. liter N.A. cylinder count to 12 cylinders still gives a larger displacement per cylinder than the 3.6 liter V8 turbocharged engine. These effects of reciprocating mass per cylinder and engine RPM on internal inertia loading will be addressed below. CONNECTING ROD FATIGUE LIFE For classic cyclically loaded (tension/compression) test bars, mean compressive stress is beneficial and mean tensile stress is detrimental to fatigue life (Ref 2). Because of cylinder gas loading, connecting rods have a compressive mean stress, which is good for fatigue life. Whether at full throttle, partial throttle or no-load (downshifting), connecting rods of 4-stroke cycle engines are in compression for the entire combustion cycle except for a period near TDC at the top of the exhaust stroke, when the compressive gas load is nil because the exhaust valves are open. It is the value of the maximum tensile load applied to the connecting rod at this instant that determines the number of load cycles the connecting rod can endure before fatigue failure. This tensile load is simply the result of the inertia load caused by the mass of the reciprocating components (piston, rings, wrist pin and the top end of the connecting rod) changing directions. Gas pressure load and friction load do not enter the calculation because both are zero at the instant the inertia tensile component is maximum. (Gas pressure is essentially zero because the exhaust valves are open at TDC of the exhaust stroke and friction is zero at this instant because the piston velocity is zero at TDC). This maximum tensile connecting rod load can be calculated based on the reciprocating mass and the acceleration rate of the reciprocating components, which is a function of stroke, connecting rod length and RPM. Now let s take a closer look at the components of the load on the connecting rod. CONNECTING ROD RESULTANT LOAD The connecting rod resultant load consists of three components: Gas Load is always a compressive load on the connecting rod, with the maximum occurring just after TDC on the power stroke. (See Fig. 2). It has a positive effect on connecting rod fatigue life because it results in a compressive mean stress for the connecting rod. Friction Load is a smaller alternating load on the connecting rod due to the reaction to the friction forces between the piston rings and the cylinder walls. Fig. 3 shows that the friction load is zero when the piston speed is zero (TDC and BDC) and increases at low piston speed but decreases again suddenly as piston speed increases above a threshold. Friction load is not significant to connecting rod fatigue life because it s maximum tensile contribution is small and happens during the power stroke when it is completely canceled by the gas load under combustion. Inertia Load is simply the product of the reciprocating mass times the acceleration rate of the reciprocating components according to Newton s 2 nd Law, F=MA. Fig. 4. shows this load is a sine curve with maximum tensile load (detrimental) at TDC and maximum compressive load (beneficial) at BDC. The acceleration is maximum when the piston speed is zero as the piston changes directions. Fig. 5 shows the connecting rod resultant load versus crank angle over one combustion cycle for the Audi 3.6 liter twin turbo V8 operating at 67 RPM wide open throttle (WOT), the maximum race RPM in top gear. The dashed line shows just the inertia component of this resultant load. This graph clearly illustrates that the maximum tensile (detrimental) connecting rod resultant load occurs only at TDC of the exhaust stroke and is equal to it s inertia component, as gas and friction load components are zero at this instant when tensile inertia load is at it s maximum. The significance of this observation is that different engines can be compared in terms of maximum resultant connecting rod tensile load simply by calculating the value of the tensile inertia component at the top of the exhaust stroke, a value we will call It-max. The equal maximum tensile inertia component at the top of the compression stroke is completely canceled by cylinder gas load so has no net detrimental effect on connecting rod life.

5 Fig. 2 Connecting Rod Load, Gas Component Audi RPM W.O.T Load (Newtons) TDC 3 6 (Intake) BDC (Compr) TDC (Power) BDC (Exhaust) TDC 72 Crankshaft Angle (Degrees) Fig. 3 Connecting Rod Load, Friction Component Audi RPM W.O.T Load (Newtons) TDC 3 6 (Intake) BDC (Compr) TDC (Power) BDC (Exhaust) TDC 72 Crankshaft Angle (Degrees) Fig. 4 Connecting Rod Load, Inertia Component Audi RPM 15 1 Load (Newtons) TDC 3 6 (Intake) BDC (Compr) TDC (Power) BDC (Exhaust) TDC 72 Crankshaft Angle (Degrees)

6 Fig. 5 Connecting Rod Resultant Load and it's Inertia Component Audi RPM W.O.T. Resultant Load Inertia Component Load (Newtons) TDC 3 6 (Intake) BDC (Compr) TDC (Power) BDC (Exhaust) TDC 72 Crankshaft Angle (Degrees) Fig. 6 Max. Connecting Rod Tensile Load (It-max) Audi 3.6 L It-max (Newtons) Max RPM Engine RPM

7 The average gas load is higher for turbocharged engines and this results in higher mean compressive (beneficial) connecting rod loads. Figure 6 shows the maximum connecting rod tensile load It-max versus engine RPM for the Audi 3.6 liter Turbo V8. This figure shows clearly the exponential effect that engine speed has on connecting rod peak tensile load. Fig. 6 also shows that for the Audi engine the maximum connecting rod tensile load is 12,7 Newtons at its maximum engine speed of 67 RPM. Because maximum connecting rod tensile load It-max is proportional to reciprocating mass (according to Newton s 2 nd Law, F = MA), it is easy to estimate the significance of the effect of displacement per cylinder on It-max. Since the 6. liter N.A. V8 has 67% higher displacement per cylinder, without exotic low-density materials for reciprocating components it must have on the order of 5% higher reciprocating mass and therefore 5% higher maximum connecting rod tensile load It-max than the 3.6 liter Turbo V8. It can also be seen from Fig. 6 that if the Audi engine were to be operated at the 1,5 RPM that a 4. liter N.A. engine must operate at to achieve choke massflow, the 57% increase in engine speed would result in a 15% increase in connecting rod peak tensile load from 12,7 to 32, Newtons! Additionally, because of the higher average engine speed requirement, a 4. liter N.A. engine s connecting rod will see more than 5% more load reversal cycles in the timed event. safety factor afforded by sacrificing 1. engine speed and 2. reciprocating mass for charge air density - boost by turbocharging. REFERENCES 1. A. C. O. Technical Regulations 22 for Le Mans Prototype. Available at 2. Metal Fatigue in Engineering ; H.O. Fuchs and R.I. Stephens. John Wiley & Sons. Section Mean Stress Effects. CONTACT Doug Milliken is the Motorsports Manager for Garrett Engine Boosting Systems, Inc. division of Honeywell International, Inc. He has a Bachelor of Science degree in Mechanical Engineering from California State University at Los Angeles. He can be contacted at: Garrett Engine Boosting Systems, Inc. 321 W. Lomita Blvd. Torrance, Ca website: doug.milliken@honeywell.com CONCLUSION At maximum engine speed of 67 RPM, Audi s 3.6 liter Turbo V8 has 12,7 Newtons (2,855 Lb.) of tension yanking on the connecting rod once every combustion cycle as the piston reverses direction at the top of the exhaust stroke. A 4. liter naturally aspirated V8, because it sacrifices charge density (boost) for engine speed (RPM) to achieve legislated choke massflow, has over twice that load yanking 5% more times in the 24 hour period of the race that is the 24 Hours of LeMans. While a 6. liter V8 sacrifices charge density (boost) for displacement instead of engine speed (RPM) to achieve legislated choke massflow, it still has a significantly higher maximum connecting rod tensile load It-max due to it s 67% higher displacement per cylinder and the higher reciprocating mass associated with it. Under 22 ACO rules, an engine designer must bias one or two parameters among displacement, engine speed, or charge density in deciding how he will achieve the choke massflow limit set by the ACO rules for the 24 hour endurance classic. Durability results at LeMans in recent years have demonstrated the effectiveness of the

8 DEFINITIONS, ACRONYMS, ABBREVIATIONS ACO Automobile Club de l Ouest, the sanctioning body of the 24 Hours of LeMans. LMP 9 GTP N.A. BDC TDC F = MA It-max RPM WOT LeMans Prototype, 9 Kg weight ACO open cockpit classification for the 24 Hours of LeMans race. Grand Touring Prototype also 9 Kg weight. ACO closed cockpit classification for the 24 Hours of LeMans race. Naturally Aspirated. An engine with atmospheric inlet density, not boosted. Bottom Dead Center. When the piston is at the bottom of it s stroke. Top Dead Center. When the piston is at the top of it s stroke. Force equals mass times acceleration. Newton s 2 nd Law of motion. Maximum tensile connecting rod inertia load. Revolutions per Minute. Wide Open Throttle, a term for full engine load.