Very interesting. Check this out:
General Motors has proved the value of modern overhead valve (OHV) engine design with its Generation III small-block V8 engines, and the forthcoming Generation IV engines will continue to advance the architecture once considered obsolete. But GM Powertrain recognized the inherent handicaps of a two-valve per cylinder head design that is typical in OHV engines. To further extend the life of the Generation IV V8 and its revised High Value V6 engines, the company has devised a three-valve head design to work with these pushrod engines.
Adding a second intake valve improves intake air flow, which provides some value, but perhaps more importantly the design relocates the exhaust valve and the spark plug within the combustion chamber. The exhaust valve moves from alongside the intake valve to the other side of the cylinder, so less exhaust heat transers to the intake port. The resulting cooler intake charge boosts power and efficiency.
Positioning intake and exhaust valves on opposite sides of the combustion chamber leaves space in the center for the spark plug, which improves combustion efficiency. Together, these improvements boost power output by 10-15% according to the company.
The three-valve engine has the intake valves in a straight line, operating them with a forked rocker arm off a conventional rocker shaft. Because the exhaust valve is on the other side of the spark plug and is not mounted in the same plane as the intakes, some ingenuity was needed to enable the engine to operate that valve using the same block-mounted camshaft arrangement used in two-valve engines.
The solution is a pair of rocker arms. One mounts on the rocker shaft where the exhaust rocker arm would be on a two-valve engine. Instead of acting on the valve, however, the first rocker pushes on a short, nearly horizontal pushrod that runs across the head to a freestanding stud-mounted rocker arm.
The second rocker arm pivots the movement into the right direction for the exhaust valve.
The three-valve heads are about 1 in (25mm) wider than the two-valve heads, but they maintain the low profile that is an advantage of OHV engines. The new heads will work with the displacement-on-demand (DOD) cylinder-deactivation system that will arrive on the two valve Generation IV engine and High Value V6 for better fuel economy.
"The three-valve design adds complexity but uses the type of components with which GM is very familiar, so reliability shouldn't be a problem," said Frederick Rozario, Development Engineer, Advanced Powertrain at GM. "And while the added mass in the valvetrain might seem to be an obstacle to high rpm operation,
the Corvette engine will rev to 7000 rpm with a 30% margin of safety. It can go to 8000 rpm safely," he added. A special jig will hold the parts together so the whole valve actuation assembly can be installed as a unit on the head.
"A weakness of single camshaft engines is the inability to separate intake-cam timing from exhaust-cam timing for maximum efficiency and minimal emissions. But a cam phaser that adjusts the advance or retard of even a single cam, depending on conditions, is still vey valuable," Rozario said. "The cam phaser on this engine provides 80% of the benefit of a system with separate intake and exhaust phasers.
The phaser mounts under the water pump in the traditional timing chain location, so it does not require any additional space underhood. The water pump is new too, because the higher-output engine increases the demand for cooling.
The 6.3-L version of the engine will produce 500 hp (373 kW), for example. So the new pump produces twice the 80 gal/min (300 L/min) of coolant provided by the two-valve engine's water pump.
The DOD system and cam phaser increase the demand for oil pressure, so both the V8 and V6 engines get improved oil pumps. The V8's is a two-phase oil pump, switching between high and low flow as needed to maintain the necessary oil pressure without suffering excessive parasitic losses when lower pressure is sufficient. The V6 is even more efficient, with a variable displacement oil pump that continuously adjusts its output for maximum efficiency.
The engines also feature optimized exhaust manifolds with equal flow runners for each cylinder.
Dan Carney
Source: SAE Tech Briefs - 9/2003
http://www.sae.org/automag/techbrief...1-111-9-26.pdf
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[Press] GM Smallblock Gets Three-Valve Head
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