Durability Aspects of Turbocharged Vs Naturally Aspirated Racing Engines 2002-01-3362
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 (2002) 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 LMP900 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 super-materials 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 LMP900 and GTP classes at LeMans in recent years.