Turbo V Supercharger?

[lotusguy]

Hi all,

A lively debate to be sure, but I see a lot of anecdotal information being bantered about here, so I have decided to inject some fundamental information which I hope some will find useful. To some, this information may seem redundant or condecending to your own knowledge (read knowledge - not impression or propaganda). I apologize in advance for the length, but it's necessary to convey a full understanding.

Lets start with a basic descrption. A Turbocharger consists of a compressor unit or ducted fan(something like a multi-bladed fan or tiny jet engine) coupled to a separate turbine unit (another ducted fan) sharing the same shaft and driven to speeds of up to 180,000 rpm by hot exhaust gases travelling through at speeds approaching 400 mph.

A significant advantage of this exhaust driven system is that the more boost you put into an engine, the more exhaust is generated. In turn, the more boost that can be generated and the more exhaust you get, and so on in a feedback loop of sorts. Centrifugal compressor airflow increases as a square of compressor rpm.

In order to keep a turbo engine from rapidly self-destructing due to overboost at wide open throttle (WOT), a wastegate- controlled these days, by a computer- is employed to bypass just enough exhaust around the turbo to maintian the desired level of boost in the engine's intake manifold.

At low compressor speeds, centrifugal compressor airflow falls off dramatically, but when coupled with the proper wastegate, the exponential nature of centrifugal compressor boost becomes a great asset for getting low-rpm boost up quickly.

The main fallout of exhaust driven centrifugal compressors is not a lack of boost, but a brief fraction of a second (sometimes infinitesimal) of 'turbo lag' that occurs upon application of full throttle at idle or light cruise while the compressor spools up to make boost. In properly setup turbo engines, turbo lag, while present in some quantity, is virtually non-existant. This is increasingly true of the newest generation of variable pitch centrifugal compressors .

Superchargers, on the other hand, are driven directly off the crankshaft by gears or belts. This can consume enormous amounts of horsepower, in fact a 'Roots' type blower on a Top Fuel Dragster at full speed is consuming 1,000 horses of the engine's power (which is still a good deal). By contrast, turbos are driven by exhaust gasses and heat-energy- which would otherwise remain largely wasted.

There are essentialy two types of Superchargers or blowers. The first is a 'Roots' type blower which consists of one or more 'rotors' or 'vanes'. These 'gulp' air, close to the 'gates' or intakes behind them and then compress and move the air in the general direction of the engine intake. These are often referred to as 'Fixed-Displacement' or 'Positive-Displacement' blowers. Positive-displacement blowers essentially pump a fixed amount of air per cycle, regardless of the speed at which they are rotating. I say essentially, because this does break down at very low and high speed due to changes in volumetric efficiency. Fixed-displacement blowers do deliver nearly instantaneous low-end boost.

The second type of Supercharger like the Vortech or Paxton, are essentially turbo compressors using ducted fans rather than the 'rotors' or 'vanes'. These turbo compressors are driven by gears or belts connected to the crankshaft. These do not suffer the volumetric inefficiencies of the 'roots' type blowers, but a fundamental problem still exists here namely when you match the centrifugal blower's gearing to achieve the correct multiple of engine speed for correct boost at maximum rpm, given the exponential nature of air flow changes with speed, your compressor speed will then typically be too slow to make any appreciable boost at low engine speed.

The bottom line is, both turbos and blowers can pack extra air into an engine; an important difference between the two is that blowers are a slave to the speed of the engine, while turbos are not. While a supercharger cannot lag behind engine speed as a turbo does, it also cannot surge ahead like a turbo to make more power while an engine lugs at low speed. Because the maximum boost on a blower is controlled by the size of the blower belt pulley or gear, boost cannot be dynamically increased to compensate for altitude as a turbo can. Nor can it be computer controlled to compensate for fuel quality or to provide short bursts of extremely high boost for passing. Without computer controls, in general, superchargers cannot operate in as high a state of tune as a computer controlled turbo. A supercharger is like an engine which has to drive a car with only one gear, whereas a turbo is like an engine coupled to an infinitely-variable transmission. From a technical point of view, the really interesting thing about turbos is that, in general, the more power they make, the more power they can make. Hope this helps...Jim '85TE

>> Edited by lotusguy on Monday 6th January 17:10

>> Edited by lotusguy on Monday 6th January 17:15

>> Edited by lotusguy on Monday 6th January 17:22

>> Edited by lotusguy on Monday 6th January 21:02

>> Edited by lotusguy on Tuesday 7th January 06:49

[GreenV8S]

Thanks for the interesting and informative post. Just to qualify one small point you made, that supercharger boost cannot be controlled (by computer) as turbo boost can: a supercharger dump valve can be used to achieve the same thing as a turbo waste gate, i.e. to limit the actual boost. It's slightly less efficient but does the job. In particular, this allows the peak boost to be limited to suit the conditions, so that you can run a higher pulley ratio to run high boost pressure in the winter, and then drop it down on hot days.

I don't know whether anyone has a computer-controlled supercharger dump valve, but a manually-adjusted one seems relatively straight forward.

[boosted ls1]

If you look at after market efi systems then there's a lot of features to help 'control' the turbo. Modern systems are versatile in this respect. I reckon Saab have this sort of stuff sorted. I drive a 900 turbo 16 as well as a 225 bhp 9000 2.3 litre. I have no complaints of lag with either car and the 9000 is a great tool for daily driving. Both ecu's allow overboost for overtaking and have temperature correction, knock sensor inputs etc.

[lotusguy]

GreenV8S said: Thanks for the interesting and informative post. Just to qualify one small point you made, that supercharger boost cannot be controlled (by computer) as turbo boost can: a supercharger dump valve can be used to achieve the same thing as a turbo waste gate, i.e. to limit the actual boost. It's slightly less efficient but does the job. In particular, this allows the peak boost to be limited to suit the conditions, so that you can run a higher pulley ratio to run high boost pressure in the winter, and then drop it down on hot days.

I don't know whether anyone has a computer-controlled supercharger dump valve, but a manually-adjusted one seems relatively straight forward.

Hi,

I'm glad to see that my post was actually read. Nothing goes unchallenged by this astute group!

While you are correct that a supercharger can be fitted with a dump valve, the fact that the rotor(s) are not freewheeling, as a turbo compressor is, means the dump valve serves mainly to disperse (or dump) excess or unwanted boost rather than control (read: Increase/Decrease) compressor speed. It can also prevent shockwaves from damaging the rotor on rapid deceleration such as when downshifting.

But, on a turbo, the wastegate can close allowing more exhaust gas to spin the turbo at higher speed creating more boost, not just limit it, as I said in my previous post, boost can be dynamically increased, while a blower's cannot. A BOV or Dump valve can also be fitted to a turbo to protect the compressor from shockwaves and to aid in spooling it back up. As the turbo is freewheeling, it's speed is independent of the crankshaft speed rather than being a linear ratio to it (therefore limited) as in a blower. Hope this clarifies...Jim '85TE

>> Edited by lotusguy on Monday 6th January 19:37

[funkihamsta]

Cheers Lotusguy, think l've just lost all enthusiasm for superchargers.
OK so what about twin turbos. I presume one small to lessen lag and one preposterous one to bruise your kidneys.
What sort of peak pressures do these set ups where found normally produce, (i.e. not including lunatic one offs) for in my humble inexpert opinion this seems the ideal solution.

[lotusguy]

funkihamsta said: Cheers Lotusguy, think l've just lost all enthusiasm for superchargers.
OK so what about twin turbos. I presume one small to lessen lag and one preposterous one to bruise your kidneys.
What sort of peak pressures do these set ups where found normally produce, (i.e. not including lunatic one offs) for in my humble inexpert opinion this seems the ideal solution.

Hi,
Looks like you are seeing the light! There are numerous other reasons that a turbocharger is superior to a supercharger, but unless it's requested, I'll save the bandwidth.

To answer your question, I must first ask you some. Why are you going the forced induction route? I mean what are you intending to do? I am an older guy with a factory turbocharged car (Lotus Esprit Turbo) so I am stuck with it. Also, I do appreciate that there is somewhat of a frenzy, especially among younger car guys, to pump up one's bragging rights with lots of sexy add on hardware. I'm not making a value judgement here, just pointing it out.

Realize that forced induction, whether it be turbo or super charging, came about in an effort to make small motors produce the power of larger ones. But there's real truth to the old adage "There ain't no substitute for cubic inches".

Just look at today's market and you'll see that manufacturers are moving toward swapping small turbocharged 4 cylinders for modern N/A 6 cylinder motors. This is because with variable cam timing, modern ECU's, dry port injection, free flowing exhausts, they are making as much horsepower or more while preserving the economy of the smaller turbo motors. Just chipping the ECU, swapping injectors, adding a free-flow air intake and filter and/or a free flow exhaust and doing some porting can give you vast improvements which are more reliable than any bolt-on kit and probably for less expense. After all, the real bragging rights go to the guy who goes the fastest regardless of the hardware that got him there.

That having been said, going the twin turbo route is a way of reducing the turbo lag, albeit with very complex plumbing and controls. Adding a new variable pitch single compressor might be cheaper, easier to install and maintain. But, don't be fooled by the myth that "turbo lag = slower speed".

This myth has created great appeal amongst Hot-Rodders for superchargers. Mainly because of the 'Halo' effect of seeing them used on Top Fuel Dragsters. Some think that if turbo engines really had great acceleration - for many the real litmus test of an engine- Fuelers would use them. But, remember, Top Fuel clutches are designed to slip away most of an engine's power until very near the end of a 5 second quarter mile run because the drivetrain cannot handle 6,000 or so horses. The fact that turbos often win, over blowers, in classes where they are legal is not widely known.

Take the Miata for example (I use this example as there are Supercharge and Turbocharge kits available and published test data is known). Stock Miatas do 0-60mph in 8.0 seconds. Company published performance figures for a Miata with an Eaton 'roots' blower kit claim 0-60 in 6.5 sec. With a BEGI Autorotor blower (centrifugal compressor), it goes 0-60 in 5.9 sec. Time drops even further to 5.6 sec. w/ a BEGI Aerodyne VATN and 5.4 with a BEGI classic turbo. Using this example, and the fact that turbos have a power advantage over superchargers of at least ten percent (due to much more efficient compressors and using 'free' exhaust energy rather than crank energy) it becomes clear that turbo lag, while perhaps unsettling, is really of little or no consequence at all. It may seem that a car with turbo-lag isn't as fast, but the facts really don't bear that out, especially when compared to a supercharger. Hope this helps...Jim '85TE

[joust]

Just a thought about the gearing problems with superchargers - couldn't you fit a small CVT box to it to give infinate variable speeds????

Complex I know - but might solve some of the low speed issues...

J

[greenv8s]

joust said: Just a thought about the gearing problems with superchargers - couldn't you fit a small CVT box to it to give infinate variable speeds????

Complex I know - but might solve some of the low speed issues...

J

In theory, with CVT drive to the supercharger you could get away without a throttle (torque is directly controlled by the gear ratio, from zero to ). In practice I think the inertia would give such poor throttle response that it would be undriveable, but it's a nice idea.

[lotusguy]

joust said: Just a thought about the gearing problems with superchargers - couldn't you fit a small CVT box to it to give infinate variable speeds????

Complex I know - but might solve some of the low speed issues...

J

Hi,

You raise an interesting point. But, rather than infinitely variable speeds, you would need very precise definite speeds.

To meet this requirement, you would need a variety of sensors such as MAP, MAT, TPS, Lambda (to monitor fuel variations by extrapolating this from CO data) to monitor engine function.

An ECU would be needed to interpret the sensor data and achieve rapid sampling rates. It would also need to compare this data with a pre-programmed set of maps and send the proper control signal to actuators controlling the CVT box and the injectors.

As a DIY, the only way to ascertain this map data would be experimenting on a dynomometer, and then having a custom chip programmed (this is actually easier than it sounds by the use of a laptop programmable chip). Then, as the engine aged, if any of the parameters of the maps changed with it, these would need to be corrected.

The CVT box would need to 'know' it's current state and send that data back to the ECU so an appropriate RPM could be selected by adding or subtracting the RPM necessary to match the map. That means a MAP, MAT or speed sensor on the CVT or blower as well.

All this would have to happen in virtual real-time for the data to mean anything. The slightest time lag and the instruction to the CVT would be useless. The sampling rate would need to be at least 0.1 sec.

And, despite all this complex, cumbersome and surely costly system, you would only gain the dynamism naturally inherent in a turbocharged system. But, you would still be saddled with the inefficiency of a blower, nearly twice the adiabatic and residual heat, 90% of the power a turbo makes, and a drain on crankshaft energy. And another factor we have yet to mention, namely the noise common to blower units. Not really a good trade off IMHO...Jim '85TE


>> Edited by lotusguy on Tuesday 7th January 06:45

[AJLintern]

I don't think there is much doubt that a turbo engine is going to be more efficient and so more powerful - its the lag that's the issue. If you have two engines, both with the same power output, one turbocharged and the other supercharged - which is going to be more rewarding to drive? One that has lag (even if small) or one that gives smooth even power across the rev range? I think that is the main issue - if you just want power and lots of it, then turbos are the way to go. If driveability and balance is prefered then its a blower.
IMHO, of course

[kevinday]

A probably silly question from an non-engineer:

Could you not turn a supercharger using hydraulics or electricity to avoid the low speed low boost problem?

[lotusguy]

AJLintern said: I don't think there is much doubt that a turbo engine is going to be more efficient and so more powerful - its the lag that's the issue. If you have two engines, both with the same power output, one turbocharged and the other supercharged - which is going to be more rewarding to drive? One that has lag (even if small) or one that gives smooth even power across the rev range? I think that is the main issue - if you just want power and lots of it, then turbos are the way to go. If driveability and balance is prefered then its a blower.
IMHO, of course

Alan,

I couldn't agree with you more. As I said, tubo-lag can be unsettling, but one does get used to it. In fact, I think it bothers many people simply because it is one of the only shortcomings of a turbocharged car that others can throw back in their face - ala "yea, but the turbo-lag is terrible...". Turbocharged cars are very driveable.

It's somewhat analogous to saying that an F-16 is less flyable than a Piper Cub. To be sure, the F-16 is much more complex and demanding, but certainly just as flyable. And, if it's smoothness you want, it's hard to beat Auntie's Buick or Granny's Cadillac. I own a Lincoln Continental (very powerful and responsive V-8), which is very rewarding to drive from a comfort, reliability and confidence standpoint, but not nearly so much fun as my Lotus Esprit.

However, one small point, a supercharger's power delivery is anything but smooth and even across the rev range. This is due to their crank speed dependent rotation. At low rpm and high rpm, the speed of the crank exceeds the optimum airflow characteristics of a blower.

That having been said, the best driving experience is always achieved with a properly cam'd naturally aspirated engine. The modern N/A engines, w/ variable valve timing display incredibly smooth and linear power curves. In fact, there are really only a few forced induction examples on the list of Most Desireable Cars. Happy Motoring...Jim '85TE



>> Edited by lotusguy on Tuesday 7th January 08:26

>> Edited by lotusguy on Tuesday 7th January 08:27

[lotusguy]

kevinday said: A probably silly question from an non-engineer:

Could you not turn a supercharger using hydraulics or electricity to avoid the low speed low boost problem?

Kevin,

There's no such thing as a silly question. And never apologize for not having an engineering degree. I'm not an engineer, but I'm acquainted with plenty and there is an abundance of 'C' students amongst them.

You could use hydraulics or electricity to turn a supercharger, but the inefficiencies increase because these too will draw their power from the rotating engine crank, but they will tack on their own inefficiencies to the equation. A hydraulic power steering pump can consume 15 horsepower and an alternator, just running standard accessories will rob 4-10 horses. Say it takes 10 HP to power the alternator sufficiently that it makes enough electricity to power the blower, because of inefficiencies, that 10 HP will yield say 12 HP of added power to the engine. Whereas the blower taking power directly from the crank, may rob 10 HP but return 15.5 HP. These numbers are merely examples and are not verifyable computationally.

In physics, most everything is a tradeoff. Energy is never consumed, it is merely transformed into work or different forms of energy, often heat. An engine's total energy is never converted to usable work. This is due to poor designs, friction, heat etc. In fact, the most modern and efficient engines can only convert about 65% of all the energy contained in a given volume of gasoline into usable work , the other 35% is lost as heat or remains uncombusted. Another way to look at that is that $0.35 of every $1.00 you spend on gas yields nothing to you. OUCH!! And that is if your engine is in perfect tune!

The energy used to power a turbo has already been made by the engine but was heretofore wasted by releasing it through the exhaust system. Recapturing it makes it essentially 'free'. On a supercharged engine, you still waste this exhaust energy, but you must now draw additional energy from the engine to turn the blower. Hope this is a little clearer...Jim '85TE

[JonGwynne]

lotusguy said:

kevinday said: A probably silly question from an non-engineer:

Could you not turn a supercharger using hydraulics or electricity to avoid the low speed low boost problem?

Kevin,

There's no such thing as a silly question. And never apologize for not having an engineering degree. I'm not an engineer, but I'm acquainted with plenty and there is an abundance of 'C' students amongst them.

You could use hydraulics or electricity to turn a supercharger, but the inefficiencies increase because these too will draw their power from the rotating engine crank, but they will tack on their own inefficiencies to the equation. A hydraulic power steering pump can consume 15 horsepower and an alternator, just running standard accessories will rob 4-10 horses. Say it takes 10 HP to power the alternator sufficiently that it makes enough electricity to power the blower, because of inefficiencies, that 10 HP will yield say 12 HP of added power to the engine. Whereas the blower taking power directly from the crank, may rob 10 HP but return 15.5 HP. These numbers are merely examples and are not verifyable computationally.

In physics, most everything is a tradeoff. Energy is never consumed, it is merely transformed into work or different forms of energy, often heat. An engine's total energy is never converted to usable work. This is due to poor designs, friction, heat etc. In fact, the most modern and efficient engines can only convert about 65% of all the energy contained in a given volume of gasoline into usable work , the other 35% is lost as heat or remains uncombusted. Another way to look at that is that $0.35 of every $1.00 you spend on gas yields nothing to you. OUCH!! And that is if your engine is in perfect tune!

The energy used to power a turbo has already been made by the engine but was heretofore wasted by releasing it through the exhaust system. Recapturing it makes it essentially 'free'. On a supercharged engine, you still waste this exhaust energy, but you must now draw additional energy from the engine to turn the blower. Hope this is a little clearer...Jim '85TE

But, to be fair, doesn't the turbo-charger indirectly steal power from the engine by creating back-pressure?

I ask because having owned both types of cars, I prefer supercharging because it provides smoother power across a wider band and provides far more (and better) low-end boost.

[kevinday]

Thanks for the detailed reply, you learn something everyday.

[funkihamsta]

Lotusguy,
Am already down the induction route having bought a turbo car. I like the lightweight engine sizes and corresponding body-styles. I also enjoy turbo driving and lag gives me a chance to say "engage".

[lotusguy]

JonGwynne said: [quote

But, to be fair, doesn't the turbo-charger indirectly steal power from the engine by creating back-pressure?

A: Not really Jon, when operating 'Off Boost', the wastegate is completely open and the turbocharger is freewheeling (now, the normal flow of intake air is usually sufficient to power this freewheeling, operation of the compressor), but once the wastegate closes and the turbo comes 'On Boost', it provides much more intake pressure than the back pressure it is using. Also, due to valve overlap, present on virtually all cars, some of this boost pressure goes directly into the exhaust system increasing rather than decreasing it's flow.

I ask because having owned both types of cars, I prefer supercharging because it provides smoother power across a wider band and provides far more (and better) low-end boost.

---

A: Not smoother power across a wider powerband - On a turbo system, once the turbo has spooled up, it is the wastegate's responsibility to see that boost is perfectly linear by changing turbo speed to account for fluctuations in engine volumetric efficiency regardless of the engine's crank speed. So, the torque curve of a turbo is always more linear than that of a blower in the mid-range and top-end revs. A blower, is more efficient at low speeds and so is more linear in that range, say the first 15-20% of available boost.

The key here is adaptability. The turbo's is infinite while the blower's is non-existent, in fact, by design. Superchargers were originally conceived and designed not for boosting modern, high compression, spark-ignition engines. They were designed for boosting the output of un-throttled two-stroke diesel powerplants, then available with various numbers of 71 cubic-inch cylinders- which is why 'Roots' blowers of inceasing capacity are referred to as 4-71s, 6-71s, 8-71s, and even 12-71s. The advent of the turbocharger came about precisely because of the supercharger's inability to cope with throttle controlled engines.

Superchargers do provide lots of low end boost because this is what they were designed to do - make more 'grunt' for the diesel. Turbochargers were designed to increase the power at the top-end of the engine's range. Happy Motoring...Jim '85TE

[funkihamsta]

Hang on,
A wastegate can only control the boost curve so subtley if it is controlled by an ECU solenoid or the like. If the pressure acting on the diaphragm is more direct say via a manual bleed it is more a safety device to cap overall boost levels. Wastegate creep could give you a more gentle on-curve, but normally this is regarded as a weakness of the diaphragm rahter than by design.

[lotusguy]

funkihamsta said: Lotusguy,
Am already down the induction route having bought a turbo car. I like the lightweight engine sizes and corresponding body-styles. I also enjoy turbo driving and lag gives me a chance to say "engage".

Funki,

I thought about your question overnight and found what I believe is a solution to your quest. Rather than a twin turbo set-up, one common Hot Rodding trick is to use a computer controlled shot of nitrous injection and fuel enrichment to 'spank' up a turbo nearly instantly to full boost with a nitrous-enhanced gust of heat and kinetic energy, then to smoothly terminate the nitrous as high boost approaches. As you can imagine, the entire torque curve shifts suddenly and dramatically upward.

I met a guy this past fall from L.A., while attending the annual Lotus Owner's convention, who set-up nitro injection on a Twin Turbo Lotus Esprit V-8. Talk about your 'E' ticket ride! That dam*@d thing made over 650 HP in a fraction of a second. The last time I felt a 'seat of the pants, neck snapping' kick like that, I was catapulting jets from the deck of an aircraft carrier 20 years ago. A really radical ride... Jim '85TE

>> Edited by lotusguy on Tuesday 7th January 17:15

[lotusguy]

funkihamsta said: Hang on,
A wastegate can only control the boost curve so subtley if it is controlled by an ECU solenoid or the like. If the pressure acting on the diaphragm is more direct say via a manual bleed it is more a safety device to cap overall boost levels. Wastegate creep could give you a more gentle on-curve, but normally this is regarded as a weakness of the diaphragm rahter than by design.

funki,

While an ECU controlled wastegate is 'better' (because it can interpret data other than just vacuum), a wastegate controlled by other methods, say using vacuum pressure, on the other side of the diaphram for instance, accomplishes the same thing really and can be very subtle and dynamic. Wastegate 'creep' is really the result of small variations in the exhaust flow caused by fluctuations in engine volumetric efficiencies, not the spring or diaphram in the wastegate...Jim '85TE