robrecht

I have heard some turbo experts disagree with this. No one would claim that there is much increased efficiency but it isn't all that uncommon to hear of turbo'd cars getting slightly better overall fuel economy. I know that's not the point of turbocharging (hardly!).

SolarYellow510

If we had rep here like the Honda dorks, I'd be adding to this guy's rep.

rotarenvy

well you obviously have experienced a very poor design of centrifugal SC. a good design will turn over easier than the crank of a rotary which isn't much effort. I don't believe there is any appreciable difference draw from a centrifugal SC than there is from a positive displacement SC.

both superchargers you compare, centrifugal and rotary both create boost and both create more power to overcome the small additional load they place on the motor.

you forget adiabatic efficiency of the two compressors you are comparing. a centrifugal sc with about 76% efficiency and a rotary compressor achieving what? I would guess it is worse than a roots style SC which isn't good.

I'm not knocking the Moller design it is just your argument that is flawed because you experienced a bad design of centrifugal compressor.

globi

Actually this compound wankel has been discussed before:
https://www.rx8club.com/series-i-tech-garage-22/efficiency-rotary-engine-37709/page2/

Here's another picture of the compound Wankel engine from the 1960's:
http://www.der-wankelmotor.de/Motore...lls-royce.html


Actually the compression efficiency of a rotary compressor is better than a roots style SC and even a centrifugal SC, because it seals well and can achieve very high compression ratios without provoking high losses (keep in mind a rotary engine has a compression ratio of 10 and a small centrifugal sc might reach 2).
In addition, since it is a positive displacement supercharger it generates the same boost over the entire rpm range and as opposed to a turbo there's practically no lag.

However, the main disadvantage of using a Wankel engine as a compressor as opposed to a centrifugal SC (mechanical or turbo) is that it takes way more room and especially more weight, which is rather a disadvantage if one wants to power an aircraft with it. Also at low loads, the efficiency compared to a turbocharged engine will be reduced as the compression/expansion rotor will keep on spinning and have an increased friction as opposed to a turbocharged engine.
Btw, in an aircraft-engine turbo-lag would not really be a problem either.

globi

If turbo-chargers are utilized to downsize an engine, this will lead to an increased fuel economy, because the smaller engine with turbo is always operated at higher loads which reduces pumping losses and frictional losses (relative to the engine output).
In addition, a smaller engine with turbo has usually less weight than a larger NA engine. (Less weight always increases fuel economy).

rotarenvy

I never considered it from that perspective. it just doesn't seem like that much compression and efficiency when you turn on over by hand.

neit_jnf

http://cgi.ebay.com/ebaymotors/ws/eB...MEWA:IT&ih=007


how bout that!

globi

At open throttle compressing a gas from 0 to 10 in a rotary or piston engine requires a lot of work and this work is significantly higher than any frictional losses.
Spinning a centrifugal SC by hand is no problem, because at these low speeds it doesn't really compress any gas and therefore there's no work involved.

rotarenvy

I thought my motor was easy to turn over by hand! must be down on compression lol

globi

A rotary engine has usually less friction losses than a piston engine, because it doesn't have to operate a valve-train.

The large surface area and shape of the combustion chamber is mainly responsible for the lower efficiency of the rotary engine.

Floyd

I love that we have so many smart people on this forum....sigh...makes me so proud....wait...maybe i'd better get my 8 first then be proud

Member since 03 and still no 8 <cry>

neit_jnf

same!

SolarYellow510

1. You can't really say this without a compressor efficiency map to back it up.

2. A compressor doesn't operate at a "compression ratio", but rather operates at a "pressure ratio", which can be anything from 1:1 (no boost) to (1+max_boost):1.

The mechanical "compression ratio" of a positive displacement supercharger must be infinite or close to it.

Have you ever gone to the Eaton web site and looked at compressor maps? They have a ramp-up to effectiveness at low speeds. Since they are effectively limited to about 12 psi (they start becoming rather inefficient at about 8 psi), you really only need to compare them to a turbo that can deliver 12 psi across the rpm range. Modern turbos that will do that are hardly laggy, especially in engine speed ranges that matter.

globi

The proof is obviously in the pudding:
A modern gas turbine uses extremely efficient axial compressors and yet they still don't reach the efficiencies of properly sealed positive displacement engines.
If the compression of a properly sealed piston or rotary engine would suck, it could never have a higher over all efficiency than a gas turbine.
http://people.bath.ac.uk/ccsshb/12cyl/


No kidding.


As far as maximum pressure ratios are concerned, a centrifugal charger can definitely not compete with a sealed piston or rotary engine.

Let's take a properly sealed positive displacement piston or rotary engine with a compression ratio of 10:
If air is adiabatically compressed in this engine, it will reach a pressure ratio of 25.11 or 364.11 psi (if you start with 14.5 psi). (p*V^1.4 = const.)

How many commercially available single stage centrifugal chargers can generate 360 psi of boost with little losses?


If you choose a turbo with a pressure ratio even remotely comparable to a properly sealed rotary or piston engine you'll end up with a lag like this:
http://www.youtube.com/watch?v=ag4nLpu1nJ0

robrecht

We all believe that. The more difficult issue is whether there is ever any increased net efficiency after turbocharging a specific engine through the use of otherwise wasted exhaust heat energy.

globi

Usually, an engine will have a lower specific fuel consumption g/(kW*h) (higher efficiency) after turbocharging. But this will be reached at a higher engine output, which again doesn't necessarily improve fuel economy.

I know, some people claim that they reached a higher fuel economy after a turbo conversion, but this has more to do with the fact that they generally drive in a higher gear given the more torque available.

Keep also in mind, 90% of the time a car is driven at partial load not needing any boost. If there's no boost generated, there is also no wasted exhaust energy recycled and at this point a turbo can't really do anything other than adding some extra weight.

rotarygod

Somehow I missed this until now. Show me a centrifugal supercharger. Any one. Pick one. Pick your favorite. I don't care. I'll still call it the piece of **** that it is! Centrifugals suck on small high revving engines. That's fact. That's physics. You can't change that with experience either.

I'm not arguing the differences or advantages/disadvantages of different types of course as there are many in each direction but for small engines, centrifugals always have and always will suck. Centrifugal superchargers are inherently bad by design due to the necesary aggressive internal gearing. I didn't make them that way. Therefore I can't have a flawed argument as there isn't a good centrifugal design out there. There are only those that suck less.

rotarenvy

I make no claims about advantages and disadvantages of centrifugal compressors I merely refute your comments about "HUMONGOUS parasitic draws on an engine. ...it's a product that is a dead end with little real world potential"

have a look at the rotrex superchargers. they have an efficient and very high gearing compared to the pro-chargers and vortech SC's. they have proven very capable on small displacement toyota and honda motors.

if these superchargers were such humongous power robbing devices they wouldn't make the HP they do. obviously like all superchargers they make a lot more power than they rob!

Nubo

I wouldn't get too excited by any Moller innovation. This dude has been leading the media around by the nose for 30 years or more with his "skycar" "innovations" and last I checked nobody has ever taken a trip in one.

rotarygod

I never said they don't make more power than they rob. That's obvious. Even turbos rob some power.

Just because some moron installs something on their car doesn't suddenly make it good. Quite frankly the general automotive public and even enthusiast are a bunch of idiots that believe whatever they are told. I could crap in a bag, take a picture of it sitting on top of an engine, attach a dyno chart, make a claim, market it, and someone would probably buy it and then they'd probably rave about it!

If the Rotrex is the best example of a centrifugal then that really means it is the least sucky of the centrifugals. Forward this to them and tell them I said so. I don't care. I also don't care how efficient something is, parasitic, adbiatic or otherwise. If it isn't spinning fast enough to make boost, it sucks! It's a power robbing device with no benefit. At least not until it's too high in the powerband to be terribly useful. This is what centrifugals do. If the Rotrex is geared so aggressively that it actually builds boost early, what compromises are they making on the top end to do it? It's a centrifugal. They HAVE to be making a big compromise. Especially if they are using a smaller wheel.

Have you ever tried to spin a Vortec or Paxton supercharger by hand? It's not easy. They just don't spin easy. I'm not saying a roots is easy to spin but a centrifugal is still a pretty big parasitic draw. I guarantee it's easier to free spin a rotor than it is a centrifugal supercharger! They just can't make power anywhere useful. At least not for our needs. By the time it gets good, it's time to shift. For small engines centrifugals suck. They always have and will. I don't care who's name is on it. The more aggressively it's geared, the worse it will get. Keep these things on large engines with low redlines. That's where they can be useful. For a small engine, the best overall power will come from the device that can give it power where it needs it most, not least. That means a centrifugal is the opposite of what is desirable on a small high revving engine on the street and a roots or twin screw would actually be desirable units. It's not all about peak power.

PhillipM

They've been a godsend to the FWD world as the torque gain builds gradually from low rpm, without any big kicks, so it's letting FWD (and RWD, come to it) put down power a lot more smoothly without issues.
There's a few 400bhp FWD cars over here using them that have no real traction issues even without a LSD.

For a rotary I would have thought they were ideal (well, if it weren't for the fact you can get a turbo kit...), as they tend to develop more torque in the midrange and higher rpm ranges than a rootes type charger.

They may have power sapping gearing, but a roots puts so much heat into the charge it has far more power loss.

Here's one from a lower boost level car:



versus a virtually standard car:



Far, far more midrange, a noticable torque increase right from 2000rpm to the redline, with a smooth, progressive graph.
I think they deserve a little more credit than you give them.
And because they don't generate the heat in the air charge of most other supercharger designs, that engine is unmodified bar the supercharger, no intercooler either.

edit: sorry about the picture size!

robrecht

Thanks for the explanation, but I don't quite understand what you mean by driving in a "higher" gear improving fuel economy. Did you mean "lower" gear?

Also, generally speaking, is there a greater increase in efficiency when turbocharging a Diesel as opposed to a gasoline engine?

Phil's 8

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rotarenvy

and just because some 'know all' makes biased clames without logic or proof dosn't make it bad.

no gearing is gearing. the rotrex still has the same problems as other centrifugal compressors but it is more efficient in spinning and compression.

have you tried a to spin a rotrex? I reckon you could do it with a couple of fingers!

yeah only if you drive a high reving motor like a granny. have you looked at the dyno comparison thread? the only centrifugal sc there is the DNA one and it is almost the same as the pettit SC at similar boost levels. then if you integrate under the curve between 6.5K and 8.5K it is very similar to both the pettit and the greddy (at 3 psi more!).
the DNA even out guns the mazsport turbo at under 4.5K rpm. i don't belive the dna graph it is too good to be true IMO. but still it's better proof than your lack of.

centrifugal compressors do have their idiosyncrasies but as PhillipM points out they can work very well in many instances.

HD-Paschke

A another Rolls Royce Wankel compound concept.