Who is happy the engine will be Naturally Aspirated?

[fatoni]

Originally Posted by madfast

that would be a roots, as its just an air pump. boost is directly proportional to rpm and the tq curve can be tuned to be super flat. it wont make the most power, but if you're ok with what it can make, then you can take advantage of what it does offer namely response and linearity. a rotrex usually isnt as linear, but if its close enough and makes a lot more up top, then we may see that being the more popular route to take.

its close but it isnt. there isnt a direct relation between one end of the system and the other. and for whatever reason its one of the most effecient forms of forced induction that i know of. although it really isnt my area of expertise

[serialk11r]

Originally Posted by old greg

That is incorrect. It is usually the case, but a well designed system will produce higher compressor outlet pressure than turbine inlet pressure over a certain operating range. The thing to keep in mind is that the work balance between the turbine and compressor isn't about pressure change, it's about enthalpy change.



Just ditch the wastegate, problem solved.

Sorry I didn't specify what I meant by pressure at the exhaust. I was talking about the "otto cycle" part of the engine, the operation of the pistons independent from the turbine and compressor. Aka the engine is always wasting some pressure when running at full volumetric efficiency, lower compression ratio/retarded timing means even more wasted pressure, boost wastes even more.

Also I think the nature of turbos (not needing all the energy in the exhaust to compress the air since there is significantly more pressure at the end of the expansion stroke than there is at the beginning of compression stroke) means the turbine is typically "undersized" for producing useful additional power, so the lower A/R, larger turbine wheel would do a bit better (especially since lower A/R would make it act more like an impulse turbine).

[Homemade WRX]

Originally Posted by serialk11r

Depends on how you look at it. If you look at it as a proportion of exhaust energy, then turbo reduces it. If you look at it in absolute terms (or in relation to total power output), turbo increases it because when you have close to equal compression and expansion, the gas ALWAYS has energy leftover. When you turbocharge you bring up the pressure in the exhaust as well. The pressure in the exhaust is always greater than the pressure at the intake; The energy in the exhaust is greater than the energy needed to supercharge the engine. The higher the boost, the bigger the gap becomes. A turbo behaves pretty similarly to a supercharger (okay, with its own electric drive) with a slightly lower energy input requirement (turbo still produces significant backpressure).

A few pages earlier (edit, in another thread rofl) I ran some very rough calculations for turbocompounding a typical gasoline engine (which has more pressure in the exhaust than diesel), and what you can pick up is rather small with a naturally aspirated engine. I believe turbocompounded diesels today have >2 bar boost, which as I said greatly increases the amount of pressure in the exhaust, allowing a turbine to be particularly effective. Turbocompounding a high boost gasoline engine would yield even better results, but at the end of the day the turbine is not going to be as good as a piston, and you ultimately dump more energy out the exhaust either way compared to a similarly powered naturally aspirated engine (of course high rpm friction makes things slightly complicated but...).

Actually something I'd like to try is connecting the shaft of a turbo to the crank via reduction gearbox. The turbine would have to be replaced of course, with a larger turbine with lower A/R to produce excess turbine power. One day when I have money...

I don't even know where to start with the pile if misinformation posted, really. I also am going to guess you didn't look up the turbine driven shaft technology that I mentioned

I'll simply say that I do this for a living (oem level, not some 'tuner' at a shop) and you way off base or misinformed.

Edit: you do realize that lower compression pistons actually have fewer pumping losses than high compression, right...LOL

[serialk11r]

Perhaps I expressed myself poorly as old greg misunderstood me, but I think I have a decent understanding of thermodynamics, thanks. Do turbo engines not have higher BSFC at full load? A naturally aspirated engine with decent compression has only 2 bar absolute pressure left at the end of the exhaust stroke, which amounts to less than 10% of power if you were to extend the expansion to let it reach atmospheric pressure. Whatever energy left in the cylinder of a turbocharged engine at the end of expansion stroke is far greater than what is needed to compress the intake air. I wouldn't call that reducing waste, I would call that recycling waste to produce more waste. A turbo motor eating 2 times as much air (and thus fuel) at 14psi typically can't produce even 70% more power right? The only efficiency "gain" is that it's better than a supercharger working on the same engine producing the same boost, still not as good as a bigger naturally aspirated engine.

Okay lower compression pistons have fewer pumping losses, but overall efficiency is better with higher compression. Whatever.

EDIT: okay I stopped reading the first sentence after "misinformation", I didn't have to look up the turbine driven shaft thing because I already did a few months ago. http://en.wikipedia.org/wiki/Turbo-compound_engine correct?

Anyhow I just read your other post more carefully again, and you said the only source of additional loss with a turbo is increased thermal loss, which I don't think is right? You blow more pressure out the exhaust because there's simply a lot more of it, you have to enrich fuel and pull timing or compression, anyways higher BSFC at full load is the result. Thermal losses through the block aren't even proportionally increasing with power, so that's not a source of "inefficiency".

[serialk11r]

My bad my bad, slight correction: when boost pressure is higher than exhaust backpressure the turbo is putting a certain amount of energy back into the system, so that's how a turbo does better. Okay but that doesn't change the fact that a ton of pressure left behind in the cylinder that the engine cannot use. Anything wrong with what I'm saying sir? If your answer is yes, then I'll go back and read that physics textbook again.

[jonnyozero3]

Originally Posted by Ryephile

I'm thrilled the first iteration of the FT-86/FR-S will have a 2.0L naturally aspirated boxer. I for one do not want a turbo FT-86/FR-S. A proper momentum/light-weight sports car should be naturally aspirated for maximum connection and communication between driver and engine. As long as the engine sounds good and has enough power, that is what makes the experience satisfying.

Why N/A is better suited to this type of car:
*Driver/Engine "oneness"
*Predictable torque curve
*No throttle lag
*Pure & Harmonious sound
*Less complicated mechanicals
*Lighter powertrain


Who agrees?

No turbo-lovers allowed, so if that's you then tuck your small Johnson and GTFO.

Def not a turbo lover here, and I'm with you on most of this - but small johnsons or not, there is something downright intoxicating about a car that can kick you back into your seat coming out of a turn. I really hope Subaru can bring higher power versions of the car to market and not wreck the soul - oneness, torque curve linearity, throttle response, etc, as you say - of the car. I'm hoping the BRZ lineup is something like this:

BRZ (2.0L NA) ~$24k-$26k
BRZ STI (>2.0L NA) <$29k
BRZ GTS - street version of GT 300 (?.?L Supercharged) <$35k

Subaru, if you are reading this - I am a G37S driver and I will seriously consider putting my money where my mouth is and buy a ~300hp FI (Non turbo'd) BRZ. Give me the BRZ with 0-60 in under 5 seconds, highway mileage at 27mpg or better, better brakes/tires, etc as required for the power, and no ricer body kit at <$35k, I will be a very happy camper.

Subaru, don't do what Porsche did to the Cayman. Give the BRZ the power it wants and let it loose on the world....

[Homemade WRX]

Originally Posted by serialk11r

Do turbo engines not have higher BSFC at full load? A naturally aspirated engine with decent compression has only 2 bar absolute pressure left at the end of the exhaust stroke, which amounts to less than 10% of power if you were to extend the expansion to let it reach atmospheric pressure.

Yes, a turbo engine does have a higher BSFC but what is the point we're arguing for a performance engine? This isn't a prius forum
As for the higher pressure left at the end of the expansion stroke, you can't simply pick one point to look at. You'll also see that the peak pressure of combustion is MUCH higher on the entire combustion stroke. So take your delta P.

Originally Posted by serialk11r

Whatever energy left in the cylinder of a turbocharged engine at the end of expansion stroke is far greater than what is needed to compress the intake air.

well of course it is, it's taking in more than it's own swept volume. That energy is also part of what drives the turbo and this is where turbo sizing also comes into what portion of the range you want to target (just like any engine will have)...so are you turbocharging for fuel economy or power?

Originally Posted by serialk11r

A turbo motor eating 2 times as much air (and thus fuel) at 14psi typically can't produce even 70% more power right? The only efficiency "gain" is that it's better than a supercharger working on the same engine producing the same boost, still not as good as a bigger naturally aspirated engine.

Power gains is on a case by case situation, obviously. Now that you're looking at power from a larger NA engine that has far greater pumping and frictional losses at points other than full throttle, you're going away from all those points that you were hitting on with wasted energy. So now you're taking those trade offs, weight, package size, etc to try and make the same power as the smaller turbo engine.

Originally Posted by serialk11r

Anyhow I just read your other post more carefully again, and you said the only source of additional loss with a turbo is increased thermal loss, which I don't think is right? You blow more pressure out the exhaust because there's simply a lot more of it, you have to enrich fuel and pull timing or compression, anyways higher BSFC at full load is the result. Thermal losses through the block aren't even proportionally increasing with power, so that's not a source of "inefficiency".

Have you ever seen data on the where the total energy goes in an engine or more so various types of engines? I think you'd like to see how minimal a lot of them are and also how similar they are too.
The thermal losses through the block are always very small.
Pressure out of the exhaust isn't a comparable as it isn't a unit of energy and is also a physical characteristic of the exhaust system. I can put a 4" exhaust and lower the pressure, does that change the energy waste out of the tailpipe? OR I can cool it and reduce the pressure...does that mean that I didn't lose as much energy? no.

Originally Posted by serialk11r

My bad my bad, slight correction: when boost pressure is higher than exhaust backpressure the turbo is putting a certain amount of energy back into the system, so that's how a turbo does better. Okay but that doesn't change the fact that a ton of pressure left behind in the cylinder that the engine cannot use. Anything wrong with what I'm saying sir? If your answer is yes, then I'll go back and read that physics textbook again.

what pressure is left behind in the cylinder? are you stating that after the exhaust stroke that their is still lots of pressure? If so, I'm going to guess that you have never put pressure transducers in an engine. Cam overlap does a lot to control this and contrary to what most people think (they think of things as a static picture), scavenging effect still happens on a turbo engines, though they normally have much less overlap, because of the pressures at hand.
If you're now concerned with residual mass fraction, think of how small of a mass fraction that is in comparison to the incoming charge air.


Now how are we off on a geeky engine/thermo design debate?

If you want to see the trends of best BSFC and fuel consumption, looks at the truck diesel industry (as it's usually easier to find data from them than the train or boat world). Their is a reason they run turbos and not 32L NA diesel engines.

[serialk11r]

hahaha okay I think we are on the same page. I was being pretty vague still sorry about that. When I was talking about pressure I was talking about pressure at the end of the expansion stroke, but anyways...

[Homemade WRX]

Originally Posted by serialk11r

hahaha okay I think we are on the same page. I was being pretty vague still sorry about that. When I was talking about pressure I was talking about pressure at the end of the expansion stroke, but anyways...

Gotcha, meaning just before the exhaust stroke. A result and expected trade off when you have a peak cylinder pressure that is so much higher for so much longer than the same cylinder under NA conditions.
This is when you have to look at the pressure ratio of the two conditions and decide if you're making a fuel economy or performance engine. This is where you'll also view it from an efficiency point of view as you now have a ratio that you can compare between the two.

I'll take the trade off for power any day

I've come to find so many people say 'efficicency' is better in this or that but never specificy what efficiency. For instance, power desity or efficiecny of power per unit of displacement is better on the FI 2.0L making 400bhp than a 4.0L V8 making 400 hp. However the BSFC of the 4.0L NA is better....it all depends on what you're looking at

I'm going to see if I have anything at work that I can share about the break down of energy in an engine...ie things that won't get me fired for posting lol

[jonnyozero3]

Originally Posted by Homemade WRX

I'm going to see if I have anything at work that I can share about the break down of energy in an engine...ie things that won't get me fired for posting lol

Mmmmm...things blurring that line are always best to share with ye here peanut gallery

[arghx7]

Originally Posted by Homemade WRX

Yes, a turbo engine does have a higher BSFC but what is the point we're arguing for a performance engine?

Under conditions of enrichment it probably will, but if you look at most charts showing BSFC vs BMEP vs RPM the BSFC tends to be less at higher BMEP. But those charts contain isobars so the key is to keep the engine within a particular operating range. When you have to enrich to control exhaust temps and preignition that kind of throws everything out the window. Variable valve timing and direct injection reduces enrichment requirements.

Power gains is on a case by case situation, obviously. Now that you're looking at power from a larger NA engine that has far greater pumping and frictional losses at points other than full throttle, you're going away from all those points that you were hitting on with wasted energy. So now you're taking those trade offs, weight, package size, etc to try and make the same power as the smaller turbo engine.

Frictional losses related to displacement/engine size are one of the main reasons for the downsizing trend.

what pressure is left behind in the cylinder? are you stating that after the exhaust stroke that their is still lots of pressure?

depends on internal EGR rates

Now how are we off on a geeky engine/thermo design debate?

because it is relevant

If you want to see the trends of best BSFC and fuel consumption, looks at the truck diesel industry (as it's usually easier to find data from them than the train or boat world). Their is a reason they run turbos and not 32L NA diesel engines.

There is a lot of truth to that but diesels have their own set of emissions and driveability issues that lead to increased cost versus gas engines in the modern regulator environment.

[Homemade WRX]

I wasn't concerned with cost, they simply are a more no holds budget (though it is still a strong player) in comparison to gas engines for achieving what's being debated....with the diesel talk

As for the BSFC, I had assumed he was speaking purely at peak power/heavy load. Yes, you can actually run turbo applications lean, especially with direct injection.

If looking at an actual performance map (RPM or mean piston speed on the x-axis, BMEP on the y-axis and then having BSFC efficiency islands), things actually used for design, hands down the smaller turbo engine will outperform an NA engine of the same power...assuming we're still comparing 4-stroke piston engines.

[Capt Canuck]

Having owned a number of quick Renaults and Hondas I am a big fan of fun, sporty NA cars. Will have to take the FR-S BRZ for a test drive and see if they are quick and fun enough.

Of course once Toyota/Scion/Subaru offer an FI version - like the Exige to the Exige S or the Cayman to the Cayman S - the original lower powered version will go the way of the dodo.

[FT-86Mike]

Well I am not going to lie my 320WHP 2600lbs s13 is fast and i love it...but I still miss my old NA k24 with I/H/E... Loved the aspect of just pushing that engine like crazy at autoX The new sr20det i can barely go WOT on the AutoX track... and it is more temperamental and expensive and fragile. The NA just took abuse and was very cheap. SO i am gratefull this car is NA.

For boost and speed I would be looking at the new 270hp Hyundai coupe, not the BRZ!