5.2L Supercharged V8 Confirmed

[mamula]

This is interesting thread. Don’t really know a lot about v6 turbo vs v8 SC.

 

[mezger]

In an engine designed and tuned from the ground up to be turbocharged, the backpressure cost of spinning the turbo is trivial. The 1.5L V6TT Mercedes uses in its F1 car has a staggering 50% thermal efficiency. Compare that to their old NA V8 that was 29% efficient.

Without making some back of the envelope calculations, the power required to compress air across 20 psig at the rate 450hp is going to consume it is non-trivial. The way I see it is the only way the backpressure cost of spinning said turbo is trivial is if it's getting 100% of its energy from the exhaust's heat... which it isn't. It is getting some of it from waste heat, but I expect it's not anywhere close to 100% efficient. And the compressor side also has thermodynamic efficiency loss.

I don't argue that a turbo has advantages over a supercharger because it can recover heat from the exhaust, but I don't think its power cost at full boost is non-trivial.

Now there are other improvements because it's a boosted application, but those same improvements apply to the supercharged engine.

I'm not super well versed on heat engines, so I'm interested to hear the counterpoints.

 

[jaz13]

Without making some back of the envelope calculations, the power required to compress air across 20 psig at the rate 450hp is going to consume it is non-trivial. The way I see it is the only way the backpressure cost of spinning said turbo is trivial is if it's getting 100% of its energy from the exhaust's heat... which it isn't. It is getting some of it from waste heat, but I expect it's not anywhere close to 100% efficient. And the compressor side also has thermodynamic efficiency loss.

I don't argue that a turbo has advantages over a supercharger because it can recover heat from the exhaust, but I don't think its power cost at full boost is non-trivial.

Now there are other improvements because it's a boosted application, but those same improvements apply to the supercharged engine.

I'm not super well versed on heat engines, so I'm interested to hear the counterpoints.

You are correct, exhaust turbines are nowhere near 100% efficient. But they don't need to be since it is using energy that would be lost otherwise. It is like paying taxes on lottery winnings. Sure you are losing a huge pile of money to the government (inefficiencies), but you still end up with a ton money (usable energy) that you didn't have before.

Typical pressure in the cylinder just before the exhaust valve opens is around 9atm, which is primarily heat energy released from burning the fuel (a little is mechanical energy from the cylinder moving higher. but of course it is moving higher because of heat energy turned into mechanical energy and is present in both NA and supercharged engines).

In a NA or supercharged engine, all of that pressure (heat) energy is lost when it is dumped out the tailpipe.

The only "cost" of a turbo is a tiny bit of extra exhaust gas and residual pressure left in the cylinder that wouldn't be present in a freer flowing NA application. But in a well-tuned engine, designers use cleverly controlled pressure shockwaves (momentum and reflected waves) to scavenge (suck) most of that excess exhaust out.

It takes a lot of energy to pressurize inlet air, but all of it comes from the high-pressure exhaust that is being dumped into the atmosphere in NA and supercharged engines. And because turbos pressurize the air upstream, they are actually recycling a lot of that energy because higher pressure in = higher pressure out = more energy back into the turbo.

 

[mezger]

Typical pressure in the cylinder just before the exhaust valve opens is around 9atm, which is primarily heat energy released from burning the fuel (a little is mechanical energy from the cylinder moving higher. but of course it is moving higher because of heat energy turned into mechanical energy and is present in both NA and supercharged engines).

Agreed, I considered this from the outset.

In a NA or supercharged engine, all of that pressure (heat) energy is lost when it is dumped out the tailpipe

Agreed, I also considered this from the outset.

The only "cost" of a turbo is a tiny bit of extra exhaust gas and residual pressure left in the cylinder that wouldn't be present in a freer flowing NA application. But in a well-tuned engine, designers use cleverly controlled pressure shockwaves (momentum and reflected waves) to scavenge (suck) most of that excess exhaust out.

Agreed, but there is a lot of tuning for scavenging in a well designed NA engine, does this tend to be better implemented or more beneficial for a turbo'd engine?

Now I claim it's not a tiny pressure difference compared with cylinder pressures if it's 9 atm when the valve opens; could be on the order of 7% if 10 psig delta on the turbine side is required to drive 20 psig max boost...(the rest coming from scavenged heat)... 20 psig is 1.36 atm, so the pressures involved aren't trivial compared to 9 atm.*

That said, I volunteer that there are a lot of counterintuitive dynamics (pressure waves, etc.) ignored by my simplistic assumptions of average pressures. Also that back pressure from the turbo means it's hotter ahead of the turbine and makes for a bigger delta temperature after the turbine, which hurts my argument, though I guess this is just another statement of heat scavenging and efficiency.

It takes a lot of energy to pressurize inlet air, but all of it comes from the high-pressure exhaust that is being dumped into the atmosphere in NA and supercharged engines. And because turbos pressurize the air upstream, they are actually recycling a lot of that energy because higher pressure in = higher pressure out = more energy back into the turbo.

Somewhat embarrassed I hadn't considered this effect, since it does move in the direction of offsetting the piston pumping through increased pressure effect I was basing my hypothesis on. But then you charge cool, so this reduces this recovery, which I guess is a statement of compressor efficiency < 1.

Running the thought experiment in my head with your additional points, things are definitely moved in the direction of a turbo being very efficient. But summing of all of these effects, I don't see an overwhelmingly obvious bottoms up answer without some hard data; maybe rough numbers put the lost crank power very near zero no matter what, so more exact modeling won't change the result much, maybe they don't.

But I don't have the rough numbers, so if you're involved at the level where you've seen and interpreted these data I'll defer. Any well known white papers out there on this? I'd be interested in reading them as these subjects interest me.

Either way, your points have been interesting.

*I pulled all these numbers out of my ass.

 

[RAPTERRIER]

Common man your gonna throw comments out like this at least back it up, i have a 11 Raptor 6.2 with the Rousch and a 17 GT350. I went out to the Larry H miller Sports track (every buyer gets a day out there to learn how to drive these cars) it was amazing the GT350 ran the whole track in 3rd gear (per the instructors) including the straight aways and never once did it hiccup 100 + on the straights. Asked the professional drivers that where teaching the classes if they had any engine problems and it was a fanatic no (they dont work for Ford either) they run the same cars day in and day out all summer spring and fall getting new cars once a year, the old ones sent back to Ford to study and evaluate. The most amazing thing was the ride with a professional driver around the course that you had been driving all day.....i sat there and laughed, i thought i was doing well driving until he showed us how it was done and talking to me like it was no sweat the whole time.....the GT350 is a frickin awesome car from the power plant to the brakes, which never once faded and they got abused!!!!!. The typical driver around the streets will never even touch the capability of these cars. I have yet to put this car in the Drag stip mode (with line lock) but it is on the bucket list.

Go over to M6G and get up to speed about that 5.2 FPC. FP stopped racing that engine years ago.

 

[rtmozingo]

Either way, your points have been interesting.

The main takeaway about turbos is that while they may use a bit of energy, it is all energy that would be lost anyway without the turbo. In a nutshell, you are taking exhaust gas (still usable energy) and putting it to work to boost your engine. The exhaust itself is a loss - turbos allow you to reclaim some of that energy. It doesn't "cost" the engine energy because it is already considered lost anyway.

Superchargers require more energy from the engine (just like your AC or alternator), and thus is parasitic. It does add more power, but it is not efficient. Kind of counter-intuitive, but so it is.

Superchargers got popular because they brought power faster than turbos of old, and linearly at that. However, twin turbo tech and decades of progress have pretty much made the supercharger obsolete, excluding size considerations.

 

[mezger]

The main takeaway about turbos is that while they may use a bit of energy, it is all energy that would be lost anyway without the turbo. In a nutshell, you are taking exhaust gas (still usable energy) and putting it to work to boost your engine. The exhaust itself is a loss - turbos allow you to reclaim some of that energy. It doesn't "cost" the engine energy because it is already considered lost anyway.

Understood, but my question|contention is how much of the turbo's power suck is from the exhaust heat (free) and how much is due to increased back pressure (at least partially not free). To level set, although this isn't my focus, I'm not a complete layman when it comes to thermo. I expect the amount of power required to move the air ingested at 450 hp across 20 psig is probably in the tens of hp, and that's before turbine and compressor efficiency < 1.

Unless its energy is 100% from exhaust heat, there is some parasitic loss.

That said, given that this isn't my specialty, I'm willing to accept that much (or maybe effectively, all) of it is from the exhaust's heat, but would like to see some formal measurements or technical papers on the subject if jaz13 or some others have some at their fingertips.

If not, I'll probably look up some on my own as I'd like to learn the particulars.

 

[charliebrown]

Very interesting thread.

What I’m reading here about turbos capturing the expanding (lost heat energy) exhaust gases to in turn compress inlet air seems the more efficient approach to building power when needed. Also, I get that crank driven super chargers require mechanical energy to compress air.

So I’m wondering; if a given supercharged engine is making say 700 hp at the crank, and it takes (for easy math) 100hp to drive the supercharger, does that mean the engine is making 800hp?

I ask because when building blown engines in the past (Vortech supercharger days) we would build them to withstand the “net crank hp” when perhaps we should have been building the internals to handle the gross power and that’s maybe why we’d blow some of them up.

 

[EricM]

So I’m wondering; if a given supercharged engine is making say 700 hp at the crank, and it takes (for easy math) 100hp to drive the supercharger, does that mean the engine is making 800hp?

Yes, that is correct. You need 800 HP worth of fuel and air to get 700 HP out the back of the crank if the blower is sucking up 100 HP.

 

[EricM]

A supercharger takes about 20% to run, so on a 700hp engine, you're looking at 140hp. Compressing 23,000L of air every minute at 6k RPM requires a TON of work.

But, again- how often are you ruining 100% throttle at max RPMs? Because that is the ONLY time your numbers are applicable. I think your estimate is way outdated as well, and is about double the needed power to move 700 HP worth of air with today's PD blower designs- but whatever. Any time you are not in boost, the blower takes basically nothing to turn, maybe 2 or 3 HP. Similar to cruising with or with A/C running.

If you get into boost- yeah sure the S/C engine drinks the fuel, but again, same thing with the TTV6. 8 to 10 MPG all day long in boost. Yes, turbos ARE more efficient, no doubt. If you want max off road mileage, yes a TT V6 will beat out a S/C V8. Physics are physics though, and in the end the total efficiency of the two engines vs amount of energy available in the fuel will be way more similar than different.

Remember your Gen 2 is lighter and has better gearing in the 10 speed to help achieve the better MPG. A S/C V8 would get those same advantages too.

And, you ask, why have that engine if you aren't running around at 100% WOT all the time? Really? I mean shit, why not stick a 4 cylinder diesel in it? We could get everywhere we need to with that and go twice as far as any boosted gas engine. I like to have the power available *if needed* but I don't have to drive at 100% WOT everywhere all the time just because it's available.