1. Technical Field
The present disclosure relates to multiple-cylinder internal combustion engines having intake and/or exhaust valves operated by a camshaft positioned in an engine block with an associated valvetrain.
2. Background Art
Conventional internal combustion engines use a camshaft-driven valvetrain to operate intake and exhaust valves that control the exchange of gases in the combustion chambers formed between the engine block and cylinder head. Engines are often categorized by the location of the camshaft relative to the valves, with overhead cam valvetrains driven by a camshaft in the cylinder head over the valves, and pushrod valvetrains or “cam-in-block” valvetrains having the camshaft located in the engine block with the valves operated using pushrods and rocker arms.
Current four-valve-per-cylinder pushrod engines include two intake valves and two exhaust valves for each cylinder. Each pair of valves is operated in tandem by a bridged valvetrain that includes a camshaft-driven cam follower (also referred to as a tappet or lifter) connected by a single pushrod to a rocker arm that drives a bridge coupled to the pair of valves (intake or exhaust). This bridged valvetrain is a cost-efficient design that achieves acceptable performance for many applications, although operation of the two bridged valves is not precisely synchronized because the force exerted on the bridge can not be perfectly balanced between the valves, the valves may have slightly different spring forces, and the valve components may experience slightly different wear. This may result in one valve opening late and/or one valve may seat first while closing causing the other valve to seat late with a higher than intended velocity. In addition, valve stem tips are edge loaded by the bridge with higher stresses resulting in higher rates of wear and potential noise, vibration, and harshness (NVH) concerns. While single overhead cam (SOHC) and dual overhead cam (DOHC) systems have independently controlled valves to address some of these issues, the SOHC and DOHC systems are significantly more expensive and have large package width relative to a cam-in-block design.
To provide various advantages over conventional pushrod, SOHC, and DOHC engines, an engine and valvetrain having dual pushrod lifters and independent lash adjustment has been developed as described in commonly owned and copending U.S. patent application Ser. No. 11/164,620 filed Nov. 30, 2005. While suitable for many applications, the number of pushrods utilized may impose packaging constraints on port placement in the cylinder head.