Quote:
Originally Posted by GasperG
hcl, fuel flow restriction determines your max power (and torque @ rpm) that was my main point. Take a look at some BSFC maps:
https://www.google.si/search?q=bsfc+...w=1309&bih=895
They look all similar, no mater if diesel or petrol and probably every competitor would like to optimize their engine so that it achieves their max power at max fuel flow in that sweet spot of efficiency. The end product is obviously a compromise of size/weight and efficiency.
I try to get all the boost preasure, wastegate, exhaust preassure, and the only logic that I can get is if you design engine in a way that there is practically no exhaust pressure left (eg. atkinson cycle) then you are getting good efficiency. The fact that downsized turbo petrol must use waste gate that actually means that there is energy thrown out the exhaust (captured by MGU-H). I think there is a path in the middle, meaning that you can design a bigger engine with smaller turbo that wouldn't need much (or any) wastegate and I think Audi is on that path. If not they would definitely be running the MGU-H unit.
Direct injection can give you lean burn under lighter engine load, but this is still to light load to get peak efficiency, DI is also good to maximize the engine power but this means running on rich fuel ratio and your efficiency goes down the drain. Toyota has D4-S system in some of their road cars (GT86, IS 300h), the system combines direct and port injection, why do you think they kept the port injection? |
I really don't know what you mean about BSFC looks all similar... what engines do those charts belong ?
And don't worry, not even with atkinson cycle (eh! not even with diesel cycle) you run out of exhaust pressure. NEVER there is practically no exhaust pressure left with thermal efficiencies that are below the 50%... and if ever would be possible this 50%, with crap reciprocating architectures that are the only ones allowed, it will take yet quite some time and a bag full of tricks.
So there will be always some wastegate of sorts for a while, the problem of MGU-H is that it can generate quite additional back- pressures specially at high engine loads, that is, exactly when upon replacing the function of a wastegate is needed the most (a no-no), you need to have a very good designed exhaust system.
Also Audi doesn't have a MGU-H because their VGT ( variable geometry turbine) is quite advanced... and with higher displacement, the turbo lag issue is well mitigated. I can bet Audi engine is the same block of the 3.7L, with a higher stroker crankshaft
http://en.wikipedia.org/wiki/Stroker_Kit ...
the same for Toyota...
augments displacement, benefits power by the torque side, and can even reduce the weight of all rotating masses. (
higher displacement, more power, less weight)
Direct injection gives you much better control than a throttled system.period
A trottled system like Toyotas can give you more "mixturing time", that is , if fuel and air are mixed before the intake valve opens, and better pre-mixed fuel loads can have more efficiency. OTOH a throttled system has intake pumping loses because it is running against the quite higher vacuum it creates on the admission manifold ( a good part of the NA engines sound).