Quote:
Originally Posted by Boss330
Garrett Turbo's view on eBoosting:
http://www.acarplace. com/ cars/ turbochargers.html
You oversimplify when you state the reason to go turbo is " to use the efficiency of the low or unboosted state where the engine uses significantly less fuel. To add a device that artificially maintains boost, is surely defeating the object? May as well stay with a larger NA."
Garrett's final report on Electric Boosting Systems:
http://www.osti.gov/bridge/servlets/...ive/841240.pdf
This report provides very interesting reading!
http://www.bmwblog.com/2011/07/09/ed...trical-assist/
What you are saying is that modern turbo engines don't boost before medium revs???
The fuel economy benefit of a smaller turbo engine vs a larger NA engine is more complicated than boost/no-boost:
Less cylinders means less frictional losses (significantly less). Fewer combustion cycles per rpm etc. A modern turbo engine is on boost from very low revs anyway. In a "M-mode" I wouldn't think fuel consumption was the main priority either... |
Agreed on the smaller engine having lower frictional losses.
I'll point out a little scenario that helps understand why turbo engines are so fuel efficient.
Take a 335i cruising on the highway at 70. In this steady state it only needs about 50hp to maintain the speed. 50hp of airflow is not much at all, so the throttle valves will only just be cracked open. In this state, regardless of engine revs, very little energy is flowing through the turbo, therefore it runs slowly, and produces little to no boost. Here we have a turbo charged engine running in almost NA engine conditions. Good fuel economy.
Flipside time....
........we all know that a turbo engine on full booost runs stoichiometric-ally richer than an equivalent NA engine. Therefore on boost a turbo is LESS fuel efficient, power for power. Proof if needed that it is only at low-No boost conditions where a turbo engine is actually able to run more efficiently than a similarly powered NA. Frictional losses/weight of powertrain aside.
Quote:
Originally Posted by Boss330
It's not only inertia that is the reason for going to a smaller turbo. A large turbo will need larger amounts of exhaust gas to spool up but will be able to handle higher rpms and the resulting larger volumes of exhaust gases. A small turbo spools quicker as it's a smaller amount of exhaust gas needed to rotatet the turbine.
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Well you do get two turbo's rated at similar flow, one having a smaller diameter than the other. In this case it is highly likely the smaller diameter one will have less inertia.