Quote:
Originally Posted by 82mm 4g63
I guess I've lost track of the point you're trying to make. My point is superchargers, at full boost, can lose 1/3 of their power output passively. Are you trying to say that a turbocharger will lose equal power, because of the added back pressure?
The most efficient supercharger is a centrifugal s/c, which (as you know) is the compression half of a turbocharger strapped to a crank driven pulley. It is identically as adiabeticly efficient as a turbocharger at providing power, typically between 70-80% efficiency. Roots blowers are much lower, in the 40% ballpark.
If that was all there was to compare, everything would be equal, but it isn't. Yes, a 30% loss in efficiency from a turbine is substancial, but you're not taking 30% from the final power output of the engine like you are with a supercharger. You're taking that loss from the amount of energy used to spin the little turbine.
Superchargers waste 30% of the engines total horsepower output to spin the compressor wheel.
Turbochargers waste 30% of engines exhaust gases to spin the compressor wheel.
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You're oversimplifying things. The amount of power a supercharger uses depends on a lot of things. A supercharger does not waste 30% of the engine's power necessarily. Take my example from the previous page, where a compressor providing 0.5atm boost intercooled to a 2L engine at 8000rpm consumes about 20hp after losses are factored in (assuming 70% adiabatic efficiency). Let's bump it up to 30 to account for the very rough nature of the calculation. 30hp is less than 15% of the engine's output when naturally aspirated, and after accounting for the increase in power from supercharging this proportion is even smaller. The loss in overall efficiency of a supercharger is the increased loss from the compressor itself being not as efficient as the pistons (times the proportion of energy going into breathing) along with part of the heat rejection from intercooling (lower temperature means the compression stroke does less work, but the expansion stroke does less work too, except the residual pressure is also lower) and primarily, the increased pressure that is leftover after combustion. In a naturally aspirated engine the "blowdown loss" represents about 10% or less of the power output of the motor that could be recovered. Supercharging increases this last category at a rate greater than proportional to boost. A turbo is able to recover a portion of this energy so a turbo can maintain or increase overall efficiency over a small range of operating conditions, but efficiency drops with higher boost levels because the amount of energy that the engine supplements to the turbine increases.
A turbo wastes at least 30% of the exhaust gas energy, but you have to remember the engine is actively putting energy into the exhaust during the exhaust stroke. So the turbo directly wastes engine power as well, just not as much because a portion of this energy is not supplied directly by the engine but by the "free" energy left in the exhaust. However the part of it that is not free can only be recovered partially, while a supercharger wastes very little of the non-free energy. The compressor side of a turbo is usually better than say a Roots type supercharger, but it also has associated losses.
Again I encourage you to draw a PV diagram and think about the thermodynamics for yourself. btw sorry for all the edits rofl.