Engine technology thread.

[82mm 4g63]

Originally Posted by serialk11r

Perhaps in the real world it works better in some way, but if you just examine the thermodynamic cycle, and consider a turbine that works purely off a pressure drop, the piston has to supply most of the energy powering the turbine. The turbine has some losses, so at the end of the day we don't have much of an efficiency improvement.

Much of an efficiency improvement over what? Superchargers?

Originally Posted by serialk11r

If you draw out a P-V diagram for yourself you'll see what I mean. It's very important that the turbo collects the kinetic energy of the exhaust pulse wave to end up more efficient than a plain old supercharger (acting as an impulse turbine). If the exhaust manifold and turbine housing are not optimized, you have a definite loss in efficiency. The point of twin scroll appears to be faster spooling, but dividing the exhaust pulses improves its function as an impulse turbine (and also acts to scavenge exhaust gas from cylinders better), which is probably why BMW put a twin scroll on a 1.6L mini that only makes barely over 200hp.

There's no need to draw any diagrams, everyone knows there is no lossless form of forced induction. You must give power to receive power, but my point is: turbocharging wastes less and returns more than supercharging. Your basic Vortech S/C can take up over 50 HP just to turn. Your average turbocharger is estimated to waste just a couple HP, so right off the bat you're gaining over 45 HP with a turbocharger. Parasitic losses of a supercharger don't compare to the minimal wasted energy of a little bit of back pressure in the exhaust.

Originally Posted by serialk11r

What's more is as I said the turbine is always sitting in the exhaust stream, and the only method to relieve backpressure is to vent with the blowoff valve. However the blowoff valve itself has a large pressure drop. A supercharged engine with bypass valve or clutch simply acts like a naturally aspirated engine at low loads while the same engine with a turbocharger will always experience additional backpressure as described. I don't doubt for a moment that under heavy load a turbo is more efficient, it's just a turbo has some disadvantages a supercharger does not have, and that the advantage is not as big as people might think.

Yes, the turbine is always sitting in the exhaust stream. The supercharger pulley is always drawing power straight from the crankshaft. I'll take a little added back pressure over direct power loss.

They both have advantages and disadvantages, but when it comes to lost power and efficiency turbochargers [losing a few HP (3-5 HP)] versus superchargers [losing from 50 HP on a simple setup to a few hundred HP in high power setups], turbochargers win. Fact is superchargers will have to run 2-4 more pounds of boost and tax the engine that much more, as well as supply the extra fuel, to be on par with the turbocharger. That is just to recoup the parasitic loss from the pulley. Now you have to crank up the boost an additional 2-4 pounds, which in turn increases the charge temperature 20-60 degrees . 1/2 HP is lost per 1 degree F increase, so you're looking at 10-30 HP lost (assuming that added 2-4 PSI doesn't push that S/C out of it's efficiency range, then you could be losing more than you're gaining) combined with the 50+ HP lost from the pulley. Do I want to lose 3-5HP with a turbocharger OR 60-80+ HP with a supercharger? Hmm

[madfast]

Originally Posted by 82mm 4g63

IMO the only plus to supercharging is it's lag-less power band, but when you take a modern, ball-bearing turbocharger and a driver who knows how to drive a turbocharged car you won't have any lag complaints.

but that's a BIG plus on a street car... not to mention flatter torque curve and throttle response.

[82mm 4g63]

Originally Posted by madfast

but that's a BIG plus on a street car... not to mention flatter torque curve and throttle response.

It's a BIG plus if we were talking about 104mm non ball-bearing turbochargers with ridiculous power surge, not daily drivers with small-medium sized turbochargers which aren't very laggy to begin with.

If we're going to talk about big pluses for street cars, turbocharging a street car not only offers better fuel economy than supercharging, but that little delay/lag in power delivery actually provides a small cushion and doesn't beat on drive train components nearly as much as supercharging. Not to mention being able to dial the boost down for your less aggressive trips to the grocery story and only breaking the tires loose when you intend to do so.

Too much credit is given to supercharger throttle response. Sure it's great for burning the tires at a stop light, but not very good for a hard pull on the freeway. Big bottom end and moderate mid/high range isn't as exciting as moderate bottom end and big mid/high range on a street, track or drag car. I don't mind waiting a couple milliseconds for my turbo to spool.

[serialk11r]

I would be surprised if any turbo only used 5hp at maximum power...that would mean the exhaust manifold pressure can only be at most 0.25 atm (gauge) or so, if we're talking a 2L engine. You can compute an upper bound that is not a bad estimate pretty easily off displacement, rpm and exhaust manifold pressure. Can you supply some numbers? I've heard of turbos causing 1.5 atm backpressure in the exhaust manifold, this would be more like costing the engine 30hp, not 5hp.

You also have to remember a turbine can only be 70% efficient at most, which is actually quite a large loss.

If you use a dynamic compressor (centrifugal supercharger) I know the frictional losses in driving the compressor should be relatively small. A supercharger is always connected to the engine, but if you open the bypass valve the supercharger has no air to push so it does not impart any energy into the air. With a positive displacement pump it's possible that mechanical friction during operation is very high that would certainly contribute to a loss in efficiency, but there is nothing stopping a positive displacement pump from having low mechanical friction.

Regarding what you just posted, fine, how about a dynamic compressor sized to provide more boost at higher rpm? You'd have the same "cushion".

A member running a supercharger posted that his typical fuel economy was not affected at all by adding the supercharger. This is a pretty good indication that when the supercharger is not in use, the power it consumes is very very small.

You say there's no need to draw diagrams, but maybe if you actually analyze what's going on you'll be surprised. If exhaust turbines worked purely off exhaust gas velocity, allowing exhaust gas to expand fully into the manifold, they wouldn't make much power, which is why turbos tend to introduce extra pressure to get more power. Of course with more boost you get more energy in the exhaust, which is why this works, and yes it does have the advantage of being independent of the crankshaft which frees up load on the engine, but I just want to point out that the actual efficiency gain is not what you might think it is.

[serialk11r]

By the way I'll say that if you have output controlled by valve timing rather than a throttle plate, a turbo has the potential to have a greater efficiency benefit because spooling it at part load would constitute an efficiency increase, not decrease. However most cars don't have this so we can't say this in general. In the future as valvetrains get better the turbo will actually be useful as a efficiency improvement add-on, as they are on the Mini (you could argue the BMW N55 engine counts too).

Turbo has quite a bit of potential for efficiency improvement, it just isn't really realized in today's cars. I described this a lot of pages back.

[madfast]

Originally Posted by 82mm 4g63

It's a BIG plus if we were talking about 104mm non ball-bearing turbochargers with ridiculous power surge, not daily drivers with small-medium sized turbochargers which aren't very laggy to begin with.

If we're going to talk about big pluses for street cars, turbocharging a street car not only offers better fuel economy than supercharging, but that little delay/lag in power delivery actually provides a small cushion and doesn't beat on drive train components nearly as much as supercharging. Not to mention being able to dial the boost down for your less aggressive trips to the grocery story and only breaking the tires loose when you intend to do so.

Too much credit is given to supercharger throttle response. Sure it's great for burning the tires at a stop light, but not very good for a hard pull on the freeway. Big bottom end and moderate mid/high range isn't as exciting as moderate bottom end and big mid/high range on a street, track or drag car. I don't mind waiting a couple milliseconds for my turbo to spool.

this is just me, but lag is lag. you can stand it, but you never want it. non linear power delivery, while it feels good when it kicks in, is never ideal.

i also disagree that the "cushion" is good. that "cushion" is load. if you're in high gear and you WOT, you're putting tremendous load on the engine. yes you can downshift but that's irrelevant to the point.

moderate mid-high? what are you talking about? are you thinking of the old school V8's that have everything down low? look at this dyno, gains across the board...



and after a flash tune:



so for me personally, unless you are not satisfied with this performance level, and must have more power at the expense of lag, then and only then should you go turbo. ymmv...

[82mm 4g63]

I guess I've lost track of the point you're trying to make. My point is superchargers, at full boost, can lose 1/3 of their power output passively. Are you trying to say that a turbocharger will lose equal power, because of the added back pressure?

The most efficient supercharger is a centrifugal s/c, which (as you know) is the compression half of a turbocharger strapped to a crank driven pulley. It is identically as adiabeticly efficient as a turbocharger at providing power, typically between 70-80% efficiency. Roots blowers are much lower, in the 40% ballpark.

If that was all there was to compare, everything would be equal, but it isn't. Yes, a 30% loss in efficiency from a turbine is substancial, but you're not taking 30% from the final power output of the engine like you are with a supercharger. You're taking that loss from the amount of energy used to spin the little turbine.

Superchargers waste 30% of the engines total horsepower output to spin the compressor wheel.
Turbochargers waste 30% of engines exhaust gases to spin the compressor wheel.

[madfast]

Originally Posted by 82mm 4g63

The most efficient supercharger is a centrifugal s/c, which (as you know) is the compression half of a turbocharger strapped to a crank driven pulley. It is identically as adiabeticly efficient as a turbocharger at providing power, typically between 70-80% efficiency. Roots blowers are much lower, in the 40% ballpark.

do you keep abreast of the latest superchargers? Eaton 6th Gen TVS compressor map: 75% adiabatic efficiency. this aint your dad's roots blower...

[serialk11r]

Originally Posted by 82mm 4g63

I guess I've lost track of the point you're trying to make. My point is superchargers, at full boost, can lose 1/3 of their power output passively. Are you trying to say that a turbocharger will lose equal power, because of the added back pressure?

The most efficient supercharger is a centrifugal s/c, which (as you know) is the compression half of a turbocharger strapped to a crank driven pulley. It is identically as adiabeticly efficient as a turbocharger at providing power, typically between 70-80% efficiency. Roots blowers are much lower, in the 40% ballpark.

If that was all there was to compare, everything would be equal, but it isn't. Yes, a 30% loss in efficiency from a turbine is substancial, but you're not taking 30% from the final power output of the engine like you are with a supercharger. You're taking that loss from the amount of energy used to spin the little turbine.

Superchargers waste 30% of the engines total horsepower output to spin the compressor wheel.
Turbochargers waste 30% of engines exhaust gases to spin the compressor wheel.

You're oversimplifying things. The amount of power a supercharger uses depends on a lot of things. A supercharger does not waste 30% of the engine's power necessarily. Take my example from the previous page, where a compressor providing 0.5atm boost intercooled to a 2L engine at 8000rpm consumes about 20hp after losses are factored in (assuming 70% adiabatic efficiency). Let's bump it up to 30 to account for the very rough nature of the calculation. 30hp is less than 15% of the engine's output when naturally aspirated, and after accounting for the increase in power from supercharging this proportion is even smaller. The loss in overall efficiency of a supercharger is the increased loss from the compressor itself being not as efficient as the pistons (times the proportion of energy going into breathing) along with part of the heat rejection from intercooling (lower temperature means the compression stroke does less work, but the expansion stroke does less work too, except the residual pressure is also lower) and primarily, the increased pressure that is leftover after combustion. In a naturally aspirated engine the "blowdown loss" represents about 10% or less of the power output of the motor that could be recovered. Supercharging increases this last category at a rate greater than proportional to boost. A turbo is able to recover a portion of this energy so a turbo can maintain or increase overall efficiency over a small range of operating conditions, but efficiency drops with higher boost levels because the amount of energy that the engine supplements to the turbine increases.

A turbo wastes at least 30% of the exhaust gas energy, but you have to remember the engine is actively putting energy into the exhaust during the exhaust stroke. So the turbo directly wastes engine power as well, just not as much because a portion of this energy is not supplied directly by the engine but by the "free" energy left in the exhaust. However the part of it that is not free can only be recovered partially, while a supercharger wastes very little of the non-free energy. The compressor side of a turbo is usually better than say a Roots type supercharger, but it also has associated losses.

Again I encourage you to draw a PV diagram and think about the thermodynamics for yourself. btw sorry for all the edits rofl.

[82mm 4g63]

Honestly, no I don't follow much of the latest supercharger tech as I really couldn't care less about superchargers. Turbocharging is the only form of forced induction I'll ever use as they have no downside. There are many ways to eliminate lag (the only "downside") if it was really that big of a nuisance.

Congratulations to roots supercharges for catching up to centrifugal/turbocharger efficiency.

Are you saying a supercharger doesn't offer more bottom end power? A turbocharger doesn't offer more top end power? What is that dyno supposed to show, that a supercharger offers more power than the N/A motor it was put on? I sure hope so.

This was a comparison of a supercharger and turbocharger on an older LS Integra from last year. Exact same car/motor.




The supercharger, as expected, was superior up until 3800 RPMs when the turbocharger walked away from it.

If they completely eliminate parasitic loss from a supercharger the outcome will remain the same. Superchargers will always have better bottom end power because they are directly connected to the crankshaft. And turbochargers, being able to free spin at high RPM will always have better higher end power.

[serialk11r]

That's not how it works, it has to do with compressor and turbine sizing, aspect ratio, etc. You could put a smaller turbo on a car and it would create a lot of torque at low rpm because the turbine operates optimally, and then at higher power there is too much mass flow and the turbine makes comparably less power and so you get less torque. Case in point: BMW N55 engine. Poops out after 5000rpm, has maximum torque basically right at idle. If you use a higher aspect ratio turbine housing you optimize the turbine for higher flow, but decrease the exhaust gas velocity at lower speed and lose torque there.

The one thing I do agree with you about is if I were to every use forced induction I would use a turbo. I just mean to say the "advantages" of a turbo aren't exactly what they seem to be, and can even be disadvantages in some situations, although as I said with variable intake duration many of them will be eliminated. Variable intake duration definitely makes supercharging better too. We better all pray that it becomes mainstream sooner rather than later...it would nearly eliminate the efficiency decreases at part load that forced induction setups typically see.

[82mm 4g63]

Originally Posted by serialk11r

That's not how it works, it has to do with compressor and turbine sizing, aspect ratio, etc. You could put a smaller turbo on a car and it would create a lot of torque at low rpm because the turbine operates optimally, and then at higher power there is too much mass flow and the turbine makes comparably less power and so you get less torque. Case in point: BMW N55 engine. Poops out after 5000rpm, has maximum torque basically right at idle. If you use a higher aspect ratio turbine housing you optimize the turbine for higher flow, but decrease the exhaust gas velocity at lower speed and lose torque there.

The one thing I do agree with you about is if I were to every use forced induction I would use a turbo. I just mean to say the "advantages" of a turbo aren't exactly what they seem to be, and can even be disadvantages in some situations, although as I said with variable intake duration many of them will be eliminated. Variable intake duration definitely makes supercharging better too. We better all pray that it becomes mainstream sooner rather than later...it would nearly eliminate the efficiency decreases at part load that forced induction setups typically see.

That's not how what works? I've been working with turbocharged DSMs and RX7's for the last decade. I know from experience how turbochargers and their components effect the power output. My Talon has gone through 6 different turbos and the RX is awaiting it's 5th. Each upgrade fulfilled a different goal in power delivery. They really aren't all that difficult to understand. Turbocharging 101: Small turbine/low A/R = quick spool = tops out sooner. Large turbine/high A/R = slow spool = tops out later. Anything you can do to reduce friction in the rotating assembly or reduce the weight of the rotating assembly speeds up spool. When have I suggested differently?

The last thing I really have to say regarding supercharging is this. Put whatever the hell you want on your car, it's your money. For me though, there isn't one characteristic about a supercharger that I like that a turbocharger doesn't match or improve upon. In both of my turbocharged cars, regardless of turbo size, I have never had any lag issues that the turbo couldn't more than compensate for in other areas. When I had an FP Red on my Talon I sprayed it down low, in doing so we eliminated the lag while keeping the ridiculous high end power that turbo offered. Turbo technology, and as it seems supercharging technology, is evolving. Lag is really a non issue with modern turbochargers, and it seems superchargers aren't as inefficient as they once were. But the fact still remains, turbochargers size for size will always out flow a supercharger simply because it isn't attached to a pulley. Until that handicap is removed from superchargers, that will always be the case, but then it would just be called a turbocharger.

[madfast]

Originally Posted by 82mm 4g63

Honestly, no I don't follow much of the latest supercharger tech as I really couldn't care less about superchargers. Turbocharging is the only form of forced induction I'll ever use as they have no downside. There are many ways to eliminate lag (the only "downside") if it was really that big of a nuisance.

Congratulations to roots supercharges for catching up to centrifugal/turbocharger efficiency.

Are you saying a supercharger doesn't offer more bottom end power? A turbocharger doesn't offer more top end power? What is that dyno supposed to show, that a supercharger offers more power than the N/A motor it was put on? I sure hope so.

This was a comparison of a supercharger and turbocharger on an older LS Integra from last year. Exact same car/motor.




The supercharger, as expected, was superior up until 3800 RPMs when the turbocharger walked away from it.

If they completely eliminate parasitic loss from a supercharger the outcome will remain the same. Superchargers will always have better bottom end power because they are directly connected to the crankshaft. And turbochargers, being able to free spin at high RPM will always have better higher end power.

the purpose of the dynos were to illustrate the proportional gains across the entire rev range and the shape of the curve. if all you care about are numbers and not HOW the actual power is delivered then turbo all the way. but if the shape of the curve means anything to you then SC is preferred. that's the way i see it, ymmv.

[madfast]

Originally Posted by 82mm 4g63

Lag is really a non issue with modern turbochargers,

that's really something personal that is different for eveyone. the 335i makes peak tq very low and for all intents and purposes lag is imperceptible, however there are still people who say they cant take the slight lag. that the feel is still not like the older NA engines that were less powerful, but had more linear power delivery. the point is that lag is still a problem for some. and just because you can take it doesnt mean everyone can.

SC on the other hand has no such issue. the main problem is that people want more power and the SC cant deliver on that front. but like i said if your goals are modest, SC is probably the better option for most people. if your goals are modest...