Originally Posted by sonoronos
First of all, I'm not sure where this limit of 200psi BMEP for an engine came from. That's not a correct number.
One thing you have to understand is that BMEP is not related at all to cylinder pressure.
When one talks about BMEP limits, you're talking about the force being placed upon the pistons, piston bearings, rod, rod bearings, crankshaft, crank bearing, the main caps, and eventually the whole block. The cylinder wall takes some of this force as the piston moves downward as the force being exerted is not totally parallel to the cylinder. Tolerances, piston skirt, cylinder wall shape, piston material, and squareness of the engine contribute.
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Regardless of what the BMEP is, it will be the same for both the Supercharged and Turbocharged engine if the boost is the same. That is why most FI engines use turbos.
I'm not sure I understand that statement.
My understanding is that turbochargers can deliver significantly larger volumes of air at higher efficiencies with less parasitic drag. Superchargers are used mostly because they are simple to engineer into an existing engine and are space-efficient.
The BMEP limit of 200 psi is a "so called" theoretical limit for N/A engines. It is one of the design parameters. Another reference places the practical range at 125-150 psi but it really depends on what you plan to do with the engine and how reliable must it be. It is not the only measure of stress, but is a good general measure. High BMEP means more torque and more power at a given rpm. And more stress.
"One thing you have to understand is that BMEP is not related at all to cylinder pressure." Huh?? It is one of the measures of pressure (BMEP). It is a mathematical calculation of Mean Effective Pressure. You are right in that it is a way of expressing the mechanical stress on the pistions, rods, bearings, etc, etc. But it is only one measure. There are others such as rpm and piston speed that all come into play as well on the stress an engine experiences.
"Regardless of what the BMEP is, it will be the same for both the Supercharged and Turbocharged engine if the boost is the same. That is why most FI engines use turbos. "
My statement above may be too general. What I was attempting to explain is that the engine (pistons, rods, crank, etc), cannot tell from where the boost is being generated when you go to calculate the BMEP or the peak pressures or the IMEP, etc. All it knows is that the inlet tract is operating a some known boost pressure above atmospheric. The fact that this is from a turbocharger, or a supercharger is largely irrelevant when discussing the BMEP. Now it is true that the turbo, being driven from the hot exhaust gases has lower parasitic loads than the supercharger. Both can be made to pump more air, but at differing costs to the engine.
I'm not sure where you conclude that superchargers are used because that are easier to design and space efficient that turbos??? Turbos and superchargers are just two different ways to achieve a goal of more output. Turbos are more efficient but have other design issues to deal with. Heat, lag, flow rates, etc. It is not that one is better or worse, but just different. For a given boost pressure however and thus for a given BMEP limit, the turbo with produce the higher ouput torque and HP, simpley because it does not use any of the power to drive the compressor.
Go back and re-read the articles referenced by others for a more thorough explaination. I hope, this helps.
Irish