Design and Development of a Gasketless Cylinder Head / Block Interface for an Open Deck, Multi Cylinder, Highly Turbocharged Small Engine

This paper describes the design and development of a gasketless interface, which was used successfully to couple an aluminium cylinder head to an open deck design cylinder block. The cylinder block was manufactured from aluminium, featuring shrink fit dry cast iron liners. Extensive CAE modelling was employed to implement the gasketless interface and thus avoid using a conventional metal or fiber based cylinder head gasket. The engine was specifically designed and configured for the purpose, being a 430 cm, highly turbocharged (TC) twin cylinder in-line arrangement with double overhead camshafts and four valves per cylinder. Most of the engine components were specially cast or machined from billets. The new design removed the conventional head gasket and relied on the correct amount of face pressure generated by interference between the cylinder head and block to seal the interface. This had advantages in improving the structural integrity of the weak open deck design. Extensive FEM analysis determined the correct amount of interference needed for successful operation under all operating conditions. Extensive thermal analysis concluded that removing the conventional gasket had the advantage of improving the heat path between the cylinder head and block, as the gasket behaves as an insulator. The possibility of gasket failure due to abnormal combustion is also eliminated. The design proved successful in operation, withstanding knock amplitudes of 30 bar, in-cylinder pressures exceeding 100 bar and high combustion temperatures. The engine completed extensive static and transient testing with no interface problems after initial development, recording 25 bar brake mean effective pressure (BMEP) on pump gasoline. INTRODUCTION A clean sheet approach was taken in designing an engine specifically for SAE’s student Formula race-car competition. From the onset, an open deck design was an obvious choice for the cylinder block due to its well published advantages in cooling and end-gas knock avoidance [1,2,3]. The design brief featured high pressure ratio turbocharging, with maximum values predicted to reach 2.8 at mid range speeds [4,5,6]. Thus all design decisions were based on preventing knock and maintaining engine reliability. Sealing thin walled open deck designs has proven to be difficult in high BMEP normally aspirated (NA) engines, due to a lack of block structural integrity [7,8,9]. Excessive bore distortion is the underlying problem, which significantly increases piston blow-by. Therefore, preventing bore distortion was even more critical to the success of this program, as single compression ring pistons were relied upon for both gas sealing and oil retention. Difficulties also existed in the procurement of a sealing gasket, due to the engine’s prototype nature. Any form of gasket featured long lead times and excessive expense due to the tooling and development required, with no guarantee of sealing. Due to the type of turbocharging employed, manufacturers were very hesitant in supplying any components due to the high probability of gasket blowout. High knock intensity was expected to be encountered during the calibration phase, with accompanying piston land and gasket failures well documented for high BMEP engines [7,10,11]. The first author also expressed concerns to gasket manufacturers over unacceptable limits in conventional head gasket tolerances and bore alignment. This poor alignment could lead to bore and/or gasket edges being exposed and becoming hot spots within the cylinder, thus increasing the likelihood of end-gas knock and/or pre-ignition. Poor alignment could also cause increased crevice volume along the periphery of the chamber, which detracts from burning the available charge energy. This reduces engine performance and increases engine out hydrocarbon (HC) emissions [11]. Applying current day gasket technology to this particular engine was considered to be risky for the reasons previously outlined, and as such, a new design was sought and developed.