
Porsche 919 – Racecar Engineering - dmmalam
http://www.racecar-engineering.com/cars/porsche-919/
======
vegabook
It's interesting how at the high end the piston engine increasingly looks like
a jet turbine. In the 919 we essentially have more than half the energy
created by a rotating set of turbos, capturing a part of the huge energies
usually expelled down the exhaust, in way reminiscent of the rear (hot)
turbines in a jet (in this case the "compression stage" is provided by the
piston engine). The big advantage, of course, shared with wankel engines, is
no waste of energy accelerating and decelerating pistons hundreds of times per
second, and lower frictional losses too. Porsche captures this fact by making
a small V4, not some big v6 or V8. In essence, we're getting closer to pure
turbine style for extracting the available energy in the fuel, because the
electric part of the engine creates a way of "buffering" the energy and
releasing it on demand, which of course is the major problem with jet turbines
(constant output difficult to modulate). Take it to its logical conclusion
and, if turbines could be made small enough, we might no longer have a
physical link between the fuel-burning part of the engine and the road. Not a
ICE engineer myself so happy to be corrected but this seems to be where fuel-
burning vehicle engine technology is headed (I note the proliferation of
turbos in road cars).

~~~
rebootthesystem
Interesting comment but I am having trouble wrapping my head around the idea.

A turbocharger doesn't create energy. It's function is to increase the
volumetric efficiency of the IC engine. How? By allowing more fuel to be
pumped into the same size engine. It really is that simple.

Ideal combustion of gasoline occurs at the stoichiometric ratio, which is 14.7
to 1 (roughly, lots of variables). Let's call it 15 to 1.

That means you need 15 grams of air [1] for every gram of gasoline you pump
into the engine.

How do you generate more power?

You burn more fuel per unit time.

How do you burn more fuel per unit time?

You build a bigger engine or you figure out how to pump more fuel into the
same engine.

What is the limiting factor in being able to pump more fuel into the same
engine?

You can't get enough air into a naturally aspirated engine in order to
maintain the roughly 15 to 1 ratio. If you can't get more air into the engine
you can't pump more fuel into it. Period.

How do we get more air into the engine?

You build a compressor. The compressor can be powered directly from the
engine's drive shaft, in which case it is called a "supercharger", or it can
be powered by exhaust gasses, which is known as a "turbocharger".

A turbocharger doesn't turn a piston engine into a turbine. It's function is
to forcefully shove more air into the combustion chamber in order to allow a
proportionally greater amount of fuel to be burned per unit time, thereby
producing more power per engine unit volume, often termed "volumetric
efficiency".

A greater volumetric efficiency does NOT mean an engine is more efficient.
This is often a mistake made by consumers when buying cars. A turbocharger
allows you to make more power with the same engine. It does not make it more
efficient other than recovering some of the energy lost through exhaust.

Having owned several turbocharged and twin turbocharged fire-breathing
monsters over the years I can tell you fuel efficiency was never better than
non-turbo cars and, in my case at least, never a criteria for buying them. The
latter said with a big fucking grin. And I have the speeding ticket collection
to prove it.

    
    
        officer: Do you know how fast you were going? I could not
                 catch up to you until I turned on the lights and
                 you slowed down.
        driver (not to be identified):
                 Not sure.  I stopped looking at the speedometer
                 at 120.
        officer: If you want to get out of this ticket you have
                 to let me take this thing for a spin.
        driver (now absolutely perplexed):
                 Here are the keys, sir
        
        That was the longest uncomfortable 45 minute wait standing 
        by a CHP patrol car in the middle of the California desert.
        The grin in the officer's face told the driver (not to be 
        identified) all would be well.  Warning issued.  Set cruise
        control to 100 for the rest of the desert crossing.
    

[1] Actually, I think it's oxygen, not air. I don't remember, it's been
decades since I studied this. I'll just call it "air" as a generalized yet
possibly inaccurate term.

~~~
vegabook
well burning more fuel certainly increases output, but isn't it the case that
the forcing the compression takes energy? That must be found from somewhere?
Supercharged engines are generally less efficient than turbocharged engines,
as far as I understand, because the turbocharged engines are doing the
compression work using energy in the exhaust.

Also in this 919, not only are turbos(turbines?) being used to compress more
mixture into the car, but also directly to drive a generator. So there must be
unused energy in the exhaust (isn't this why highly tuned drag cars have fire
literally coming out of the exhaust?)

~~~
cellularmitosis
You are keying into some great points here.

The energy recovered by a turbo is not entirely a free lunch. A turbocharger
gets some of its energy "for free" and some of it not so free.

To the extent that the turbo is acting like a turbine (extracting energy by
dropping temperature and pressure across the turbine blades), you are
recovering that energy "for free".

However, to the extent that the turbo appears as additional back-pressure to
the engine as each cylinder's exhaust valve opens, that portion of the energy
is not had "for free".

Theory tells us that the amount of energy left to be extracted from exhaust
gas is proportional to how much hotter it is than the ambient air temperature
(even 1 degree means there is energy left in the exhaust).

However, every practical Carnot-cycle heat engine (steam, piston ICE, gas
turbine, etc) ever manufactured leaves some energy on the table.

Practically, the process of extracting additional energy from the exhaust
means running it through another "stage" of the engine (e.g. an additional
ring of turbine blades in a turbine or an additional cylinder in a compound
steam engine).

But here's the rub: Have you ever noticed that in every turbine of every jet
engine, each successive ring of blades is physically larger in diameter than
the one before it? And in a compound steam engine, each additional cylinder is
larger than the one before it?

This is because the process of extracting energy from the exhaust requires
allowing the gas to expand, requiring that the next stage be even larger in
order to extract additional energy.

So in any practical engine, you reach a point of compromise where you decide
that adding another stage is either too expensive, adds too much weight (or
perhaps adds just enough friction as as to be counter-productive!) that it
isn't worth it to chase after that last few percent of heat energy left in the
exhaust.

Edit: This is also why you see the advent of combined cycle power plants. At a
certain point, whatever heat is left in the exhaust is more practically used
to provide heated water, than to try and convert it to kinetic (and then
electrical) energy.

~~~
aunty_helen
To your point about the increasing blade size, this is to allow the gas to
decompress, however its less about extracting more energy and more about using
the energy in the gas to keep it moving out the rear. One of the biggest
problems with gas turbines is that theres nothing to stop the flow from
reversing if the (intake pressure - forward momentum of the gas stream) is at
a lower energy state than the exhaust. When this does happen you get what is
known as a compressor stall. This is dangerous and costly in aviation (11 mil
of overhaul for a GE90-115) and so a little less effiency is the compromise.
You could run 2 stages of the same size of turbine blade but that would be
dancing with the devil.

Also, just to add to your comment, another form of energy that you get for
free from a turbo application is harnessing the sound pulses from the exhaust.
You wouldn't think it's much but all you have to do is compare the F1 V10 vs
the V6T
[https://www.youtube.com/watch?v=jS4Dh_EAfJI](https://www.youtube.com/watch?v=jS4Dh_EAfJI)

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zackmorris
I'm a little surprised they didn't go with a 4 cylinder boxer engine since
vibration was such a problem:

[https://en.wikipedia.org/wiki/Flat_engine#Porsche](https://en.wikipedia.org/wiki/Flat_engine#Porsche)

~~~
jdietrich
It's a packaging issue. Boxers are excellent for weight distribution, but
their width would restrict airflow to the rear diffuser. V engines fit neatly
behind the cockpit, with plenty of space for ancillaries. Rear aerodynamics
are a key aspect of Nissan's decision to go front-engined; putting the engine
up front allows for an absolutely huge diffuser tunnel.

[https://s-media-cache-
ak0.pinimg.com/736x/1a/4a/73/1a4a73715...](https://s-media-cache-
ak0.pinimg.com/736x/1a/4a/73/1a4a737152125a0a74d1b2946c7ca513.jpg)

~~~
FrankBooth
Also because it's a stressed member, which is significantly easier to build in
a V layout than a flat layout.

The lower CoG thing with flat engines is kind of a myth. You end up with the
exhaust headers below the engine, so the engine sits higher than an equivalent
V layout engine would.

------
Gonzih
> The cars power unit was at first thought to be a derivative of the never
> raced early 2014 spec Volkswagen F1 inline four engine

I had no idea VW had f1 engine, is it used? I know that they have some
presence in f3 in that sense, had no idea about f1 though.

~~~
mrcarrot
Perhaps F1 is just a model number here? VW certainly don't supply any Formula
1 teams currently, and a 2014 spec F1 engine would have to be a V6, not a four
cylinder.

~~~
Gonzih
I did some research on the topic and apparently audi was planning to jump on
to the f1 game (based on redbulls plans). But sadly with recent VW related
issues audis presence in f1 will not happen soon.

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elektromekatron
Is interesting to see how hybrids now dominate Le Mans.

~~~
nialo
An important point about racing cars is how completely their performance is
determined by the rules of the series. Hybrids dominating says more about the
details of the rules than about hybrids.

~~~
jdietrich
By design, the LMP1 rules permit a diverse range of solutions. Porsche run a
turbocharged V4 petrol engine, Audi use a turbocharged V6 diesel, Toyota a
naturally aspirated petrol V8. Audi's hybrid system has half the energy
capacity of Porsche's. Teams running a large hybrid system are penalised with
a lower fuel rate limit and vice-versa.

The technical regulations are significant in any racing formula, but the LMP1
rules are carefully balanced to prevent the dominance of any one technological
approach. Diesel engines have been significantly gimped in recent years to
curtail the dominance of Audi.

~~~
magila
None of what you said is wrong, but it also doesn't refute the central point:
current WEC regulations are very much designed to make the LMP1 hybrid formula
the premiere class. While there is a non-hybrid LMP1 formula, it is a
distinctly lower performance class which exists to accommodate privateer teams
who can't afford a complex hybrid system.

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ExpiredLink
I hope nobody has forged an emissions test.

------
sandworm101
"the internal combustion engines have no air restrictors or capacity limits
which has seen Audi opt for a diesel fuelled V6"

Well, I think they might want to take a closer look at that one.

~~~
jdietrich
They're correct. Engine capacity limits and intake restrictors were abolished
in 2014 and replaced with a fuel flow limit. This more closely aligns the LMP1
rules with the goal of developing commercially relevant fuel efficiency
technology.

Engine capacity limits do apply to other classes, and to privateer teams in
LMP1 who opt not to use a hybrid system.

~~~
sandworm101
I meant to imply that, as Audi is owned by VW, that officials might want to
look for possible cheating given the current VW controversy re diesel engines.
But if one has to explain ...

~~~
jotux
>But if one has to explain ...

Comments this far off topic and irrelevant need explanation.

~~~
sandworm101
Wow. I have seriously over-estimated this crowd. The saying goes that if one
has to explain a joke, then it wasn't funny in the first place.

I had assumed that people reading this thread would see the humor behind
linking Audi, VW, diesel and cheating. Oops. Then I assumed that readers would
understand the "if it needs explaining..." remark as a reference to jokes not
being funny if they need explaining. Oops again.

I'll set my lowest common denominator a bit lower next time.

~~~
bryanlarsen
Humour is regularly down-voted on Hacker News to avoid the Reddit phenomenon
where humour bubbles to the top and pushes meaty comments down. Hacker News
has collectively decided that humour belongs on the bottom of the thread. That
doesn't necessarily mean we don't like jokes, but it's hard to tell the
difference.

~~~
krapp
And yet the dismissive and sarcastic comments all too often wind up on top.
For some reason, gratuitous cynicism tends to be seen as more highbrow, and
appropriate for HN, than gratuitous humor. Yet the former tends to cause more
rancorous discussion then the latter.

~~~
bryanlarsen
I find that dismissive & cynical comments tend to attract quick votes, but
over the long term more substantial positive comments do win out. Of course,
most stories don't stay on the front page long enough to have a long term...

