henry wrote: ↑
Fri Jan 11, 2019 9:49 am
PlatinumZealot wrote: ↑
Thu Jan 10, 2019 11:05 pm
Nope! I remember that thread! I won't re-enter!
There was no clear answer.. I have a Pressure-Volume graph with blow-down featured on it, which I might post if anyone is interested.. and some other very old references. But I would like a clear definition for everyone to be able to join the discussion. It's mostly the old-heads you hear with this blow-down term.
It seems unlikely that you would see much on a Pressure-Volume graph since blowdown is concerned with the kinetic energy of the exhaust stream outside the cylinder.
It’s in old references because it was pursued in an effort to make high performance aircraft piston engines more efficient, extracting work from the exhaust stream while not reducing the amount of work on the pistons. This was called compounding. The advent of turbines stopped pursuit of these routes to aero engine efficiency.
And now we have F1 power units in which compounding is mandated and, not surprisingly, the technique appears relevant again as part of mixed mode operation of the turbines.
This is only theory since the PU manufacturers don’t say much about how their units work at all. Honda are an exception but haven’t said anything on this.
You may, of course, not find this convincing. That’s your prerogative. But please don’t shout if you respond.
Hmm.. didn't say I don't know what it is. I was in that discussion too after all. But I feel the term just needs more clearing up so everyone is on one page without having to be a hardcore engine cycle-researcher.
Here is one such P-V graph.
Got the definition...simple and clean...
Blowdown is the amount of time/distance/degrees between
exhaust port opening and transfer port opening.
The absolute most accurate description of blowdown would
be... the amount of TIME between the exhaust and transfer
port openings. This "measurement" would of course only be
accurate at one rpm, so blowdown is commonly referred to in
degrees of crank rotation.
Also on page 9 here...blow-down loss.
http://www.iitg.ac.in/scifac/qip/public ... cycles.pdf
@ Tommy cookers..
Now that I have reconciled the term with what I know already.. the blowdown loss is the pulse of exhaust gasses at the "tail" of the power stroke. Basically only pulse tuned manifolds can utilize it. I also read that by tuning the shock-wave from other cylinders can push back any fresh intake air from escaping during overlap. It still is a loss to utilizing all the energy from expansion!. but I guess if the pulses can be utilized otherwise...
Since I have to deal with giant slow speed 2 stroke diesels at work these days.. I have seen the term in old references.. The engines used to use pulse manifolds for blow-down effect. It was most effective to join 3 cylinders to a collector.. 4 cylinders or more per collector and the pulses would crash into each other (hence the advent of two scroll turbines if this is to be preserved) The blowdown tuned exhaust manifolds had to be short and tubular to preserve the pulses too. Nowadays the multiple cylinders engines minimize the number of turbochargers to save space and cost. So more than three cylinders are normally connected to one big constant pressure receiver (or log manifold as people call it). The cheaper log manifold also has less thermal variations too.. and lighter turbine wheels could be made to take advantage of the more constant conditions.
In the giant two stroke diesel world.. they say the log manifold is about 5% more efficient than the pulse tuned manifold because the more constant conditions allow a reduced exhaust overlap (blowdown loss) thus increase utilization of expansion, and the turbocharger parts can be lightened/optimized for the lower peak loads...
I am not sure how the log manifolds were used by mercedes and honda in Formula 1 but it seems that pulse-tuning is definitely prefered these days. I get what you are saying, when you say the pulse is "free-energy" from the momentum of the gas.. but it in this efficiency formula supposed this blowdown energy is better utilized in combustion chamber than through the turbocharger? (I think this is what Honda's Tanabe alludes to when he mentions ICE power versus MGUH power)...
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