Porsche 919 – Racecar Engineering | Hacker News Reader

Porsche 919 – Racecar Engineering

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60 pointsdmmalam40 comments

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).

Jet engines don't dominate airline usage because of good fuel consumption (their BSFC [specific fuel consumption, or "how much fuel is needed to create a set amount of power?] is not competitive with piston engines).

Jet engines dominate because of high power to mass, highly reliable power production, and a good match between power available and power required in different phases of flight.

Non-flat-rated jet turbines lose power with altitude and temperature right off the deck. Some are flat-rated (higher thermodynamic power capability than physical power capability), but even those lose power at some altitude. This is a good match for high power required at takeoff and less power required at cruise altitudes.

why do most power stations use turbines and not pistons? There is no need for power to mass ratios there?

That's a great question. I do not know for certain, but I suspect the answer lies in reliability and much longer intervals between servicing and overhauls.

Piston aircraft engines have a recommended time between overhaul of 1200-2000 hours (with outliers above and below).

Jet aircraft engines have a recommended time between overhaul of several times that, with required inspections typically at slightly longer intervals than piston engine overhauls. (In a fixed install turbine plant, you might be able to skip the inspections as well, and just use trend monitoring data to determine when to take the turbine out of service for maintenance/overhaul. In an aircraft carrying people, the risk profile is a little different.)

There's a difference between turbine and jet engine. Natural gas peaker plants are very expensive to operate due to fuel cost. Most big power plants do use turbines to spin the generators but the turbine is fed steam, not hot combustion gas as in a jet engine.

It depends, there are also many power plants that use diesel engines.

Knowing about ship propulsion, compared to turbines, diesels have higher efficiency and longer times between overhaul. Turbines are lighter. Both mean that if you use turbines, you pay more for fuel and your ship must be docked more often which are a big nono in commercial shipping.

Generally only the military uses turbines in ships.

Sometimes they are installed in for example cruise ships if they need a lot of power and there's no space, / there are balance issues with diesels.

Probably on the land side if you only run it rarely, or your natural gas is extremely cheap, then a less efficient turbine is fine. Generally power plant frame turbines are more old fashioned and less efficient than aircraft turbines.

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.

The turbine reference isn't to a traditional turbocharger. The 919 has two turbines. The first is a traditional turbo, but the second is used as an energy recovery system (stored in batteries, released on-demand).

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?)

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.

I actually didn't realise that the energy extracted by a turbocharger, more than any residual gas expansion from still-burning fuel (if any), is also dropping the temperature of the exhaust gases. So we are extracting some of the heat energy using a turbo. neat. Yes I take your point on diminishing returns. Still it seems that there is quite a big chunk of energy otherwise being thrown away, and turb[os/ines] are getting better and better at harvesting it.

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

Right. Yes. I believe I said that here:

    powered by exhaust gasses, which is known as a "turbocharger"

Yes, a lot of energy goes out the exhaust and turbochargers use some of it to compress air for the intake. Even with that there's still energy wasted out the exhaust. If it is hot and makes noise there's energy going out the exhaust. Principle of Conservation of Energy.

At the most basic level thermodynamic machines are about max delta T.

You actually don't waste any energy accelerating pistons as the system is balanced. If it wasn't you shake the engine apart due to huge amounts of vibration. The crankshaft has counter weights so that it is balanced and the individual pistons all balance each other out.

It's like a see-saw, yes the mass isn't evenly distributed but it is balanced. And it takes only a small amount of force to make it oscillate relative to the force required to support it.

I stand corrected, and I think I've figured it out. Even without burning fuel, in a frictionless system, the decelerating piston would be putting its energy back into the crankshaft? So it draws energy then puts it back every 180 degrees? Takes energy on the acceleration, puts it back on the deceleration?

I suppose the friction is the main loss source then?

Conversion of linear motion to rotational motion is the loss you're looking for.

To your point about effectively gas turbine/electric drive trains from your OP, the problem is efficency in burning the fuel. This comes from the lack of achievable compression ratio in a turbine system. 6:1 is good for a jet, 13:1 is good for a N/A piston engine (not sure what race engines run exactly). Higher compression ratio = better fuel efficency.

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

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...

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.

> 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.

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.

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.

When they started talking about the changes that were coming to F1 in 2014 the regulations initially called for inline four engines[1], but that had nothing to do with the engines that Ferrari manufactured, so they compromised with V6 turbo hybrids.

[1]: https://en.wikipedia.org/wiki/Formula_One_engines#2014.E2.80...

Is interesting to see how hybrids now dominate Le Mans.

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.

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.

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.

Since Le Mans is an endurance race, the vehicles that can run longer with less pit stops have a strong advantage. This is what the Audi diesels dominated for a good part of the 2000s.

I hope nobody has forged an emissions test.

"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.

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.

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 ...

What vw has done sucks yet, what you are siggesting is completly without relevance to racing. There is no connection whatsoever.

>But if one has to explain ...

Comments this far off topic and irrelevant need explanation.

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.

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.

That explains downvoting, it doesn't explain the replies that clearly did not recognize any attempt at humor. I really missed that mark.

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.

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...

Race officials are always looking for possible cheating.

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