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[quote]QUOTE: <i>Originally posted by Leon Silverman</i> <br /><br />To answer up829 question about variable valve timing; variable valve timing will not supplant the benefits of a larger turbo, which presumably would also provide a higher boost. Cylinder air has mass. This means it also has inertia and momentum. Increasing valve timing (duration and overlap) allows an engine to develop more power because it can breathe easier at high rpms. The increased valve timing gives the cylinder air more time to exit the cylinder by givng the air mass more time to get moving out of the cylinder ),overcoming its' inertia. The increased overlap (simultaneous opening of exhaust and intake valves) sucks more fresh air charge into the cylinder due to the siphoning effect of the fast moving exhaust gases. The momentum of the incoming air continues to add mass to the cylinder even after the piston has reached the bottom of its stroke. To take advantage of this, the intake valves are not closed until after the piston starts to move up again. This effect is usually not noticed until an engine reaches or exceeds about 3,000 rpm. <br /> The downside of this effect is that when use a cam timing that develops high horsepower at high rpms, you have a relatively unresponsive engine at low speeds) off the line. Conversely, an engine set up to hit its' torque peak at low rpms will be very responsive in stop and go traffic but is weak passing cars at highway speeds. <br /> Variable Valve timing gives you the best of both worlds . You wind up with an engine that is responsive at both high and low rpms. <br /> Diesel engines utilize a heavy construction in order to withstand the high compression ratios. This heavy construction limits the maximum rpms that the engine can safely operate at. Diesel engines develop a lot of torque at low rpm because they are generally large displacement engines. Changing valve timing so that a diesel can operate at higher rpms is a waste of time unless you can also reduce the weight of the reciprocating masses (pistons, crankshafts, and connecting rods) to permit the higher rpms. Increasing turbo boost can increase torque output and horsepower without requiring the engine to operate at a higher rpm. <br /> As I stated before, the horsepower boost from free breathing usually starts at around 3,000 rpm. Since locomotive diesels operate at a maximum of 1000 to1200 rpm, increased horsepower requires high boost pressures. Valve timing will not do it. <br />[/quote] <br /> <br />I was thinking more along the lines that the increased boost in Run 8 might decrease the scavenge time beyond the reduction provided by the modest increase in RPM. This would depend on the difference in boost between Run 1 and Run 8 and whether these engines are already optimized for Run 8 or compromised in order to allow cold startup. If I understand these engines correctly, the incoming charge is not compressed in the crankcase like a 2 stroke gasoline engine, instead a blower or turbo-supercharger is required to scavenge the cylinders.
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