Highly Turbocharging a Restricted, Odd Fire, Two Cylinder Small Engine - Design, Lubrication, Tuning and Control

This paper describes the mechanical component design, lubrication, tuning and control aspects of a restricted, odd fire, highly turbocharged (TC) engine for Formula SAE competition. The engine was specifically designed and configured for the purpose, being a twin cylinder inline arrangement with double overhead camshafts and four valves per cylinder. Most of the engine components were specially cast or machined from billets. A detailed theoretical analysis was completed to determine engine specifications and operating conditions. Results from the analysis indicated a new engine design was necessary to sustain highly TC operation. Dry sump lubrication was implemented after initial oil surge problems were found with the wet sump system during vehicle testing. The design and development of the system is outlined, together with brake performance effects for the varying systems. Tuning an odd fire engine with an intake restriction and upstream throttle location was explored together with varying injector locations and manifold geometry. To improve engine efficiency, turbocharging and specific engine downsizing were employed in conjunction with a lean burn strategy at low brake mean effective pressure (BMEP). This engine package and tuning strategy resulted in the Melbourne University Formula SAE vehicle being very successful in competition, finishing first in the fuel economy event at the 2004 Australasian competition. Peak BMEP values of 25 bar, believed to be the highest recorded for small engines on pump gasoline were also achieved. INTRODUCTION AND BACKGROUND The design objectives for the engine featured specific downsizing when compared to the typical four cylinder 600 cm maximum capacity engines used in the Formula. Downsizing had obvious packaging advantages including large reductions in mass, physical size and centre of gravity (CoG) height, which all contributed to improving the dynamic performance of the Formula vehicle [1,2,3,4]. A specifically designed downsized engine also allowed the positioning of many major components, including manifolds and the turbocharger to suit Formula applications. Brake power could also be increased, as the maximum mass flow was limited by the mandatory 20 mm diameter intake restriction. If the restriction could be choked, delivered power would increase due to the reduction in frictional losses associated with the smaller capacity [1,2,3,5]. To compensate for the reduced capacity, the engine would feature high pressure ratio turbocharging with maximum boost values dictated by turbocharger limits. Turbocharging offered additional benefits, including maintaining maximum mass flow through the restrictor over a wide speed range via boost regulation together with efficiency gains [3,6,7,8,9]. This had advantages in reducing fuel consumption and minimising gear shifting for much of the Formula competition [1,2,3]. The engine was installed into successive MUR Motorsport vehicles in 2003 and 2004 and became the first prototype engine to successfully compete in the competition’s 25 year history. The final version is displayed in Figure 1. Figure 1: The UniMelb ‘WATTARD’ engine together with the fuel economy trophy from Formula SAE-Australasia 2004. THEORETICAL ANALYSIS ENGINE CAPACITY AND CONFIGURATION The engine capacity was selected with the aid of Figure 2, which shows the predicted volumetric efficiency (ηVOL) needed to maintain sonic flow in the restrictor for varying engine capacities and operating conditions. A validation point for the model simulation is shown with an experimental result from the team’s previous Suzuki GSX-R600 engine. On the basis that a boost pressure ratio of 3.2 (250% ηVOL assuming losses) is the expected turbocharger limit, an engine size of 400 to 450 cm with an operating speed range of 6000 10000 rev/min was selected. The engine configuration was heavily influenced by the turbocharger. A larger number of cylinders reduced the flow velocity fluctuations experienced by the exhaust turbine but increased frictional losses due to the greater piston rubbing area. The final design featured a two cylinder in-line configuration as a compromise between the competing affects. Figure 2: Predicted ηVOL needed to maintain sonic flow in the restrictor for varying engine capacities and operating conditions.