ducted fan use for air intakes on carbed engines

[slick1]

Has anyone experimented with a ducted fan , operated by an
electric motor(DC current), placed forward of the carb.
intake, then activated by throttle position . The fan is
variable speed of course also, the filtration system would
have to be augmented .

 

[patprimmer]

If you do a search on this site, you will find previous threads with lengthly discussions on electric morors for cheap or compound supercharging or turbocharging or overcomeing turbo lag

Regards
pat

 

[overmod]

Yah. But for a very specific purpose: short-term boost to allow better NA engine speed change under load.

As a supercharger, this idea suffers from the same problem as electrically-driven superchargers in general -- very high currents are required for the desired hp. of boost. The efficiency of a ducted fan generally suffers when the airflow through the duct is not smooth, and I might add also suffers when back pressure is high enough to spoil the flow. I would certainly think that an enclosed centrifugal design would give you better effectiveness than a df if the engine requires boost at widely different rpm ranges.

A generally preferred idea is the use of the 'electricity' to run a motor in the center section of a turbocharger, to spool the compressor up more quickly on engine speed change. This is a popular design idea on large diesel engines, and people have tried to promote it as a 'better' turbocharger for performance applications. Here the motor only serves to provide sufficient boost to spin the engine up to the point that the exhaust-gas flow induces adequate turbo boost registered at the intake valves (e.g., net of all pressurization and lags in the intake tract, intercooler, etc.) You are NOT trying to run the turbo as an electric supercharger. (I have also researched variants of this approach that use pressurized gas on either the turbine or the compressor vanes -- the latter either being an oxidizer or a combustible gas that supplements the principal engine fuel.)

On a carburetor, you have the additional problem that increased boost pressure blows the fuel out of the jets. That usually requires some form of external enclosure -- you've seen the polished aluminum 'pot' on the E-bird motors, right? In addition, I would expect the relatively primitive mixture adjusting capability of most carburetors to provide lousy emissions numbers over the desirable range of boosts. My advice to potential supercharger developers is to go directly to port injection or modulated DI, with the fuel admission programming using some form of pressure sensing to determine the boost and flow. This used to be expensive ;-}
but is now pretty cheap and do-able.


I would expect that a ducted fan would only be useful for sustained power output, in situations where the relatively steady-state flow in the intake tract matched the flow characteristics through the fan and duct. Variable vaning on the fan would provide a wider potential range, but probably isn't worth the cost for the linkage, actuators, etc. Likewise, 'feathers' or other variable geometry on the tail end of the duct might provide some measure of optimization. You begin to run into other problems, though: the physical size and packaging considerations -- presuming you want to run this thing in a car. Not to mention the incredible fun you are going to have with air filtration...

I do truly wish that electric supercharging worked -- I'd start in working out how to put some of the things on my M73 BMW motor as quick as I could figure out how to reverse-engineer the engine control chip... Unfortunately, even with the wonders of NIB magnets, the energy density and resistance-heating problems limit the developed HP to well below what any halfway-competent shaft-driven SC is expected to need.
I encourage you to look at some other lists to read discussions on the technologies involved...

RME

 

[Retracnic]

Anytime I hear of a new scheme to provide manifold boost by means of an electric compressor or fan, the phrase "There's no such thing as a free lunch" leaps to mind.

The problem is a simple matter of power. It takes power to compress air. Far more power than most people realize. So no matter which means you choose to compress air with, you have to supply it with ample power.

Using a standard 12V battery/alternator configuration, you can only power a 1-2 horsepower electrical motor for any significant duration of time. This is simply nowhere near enough power to drive a compressor capable of supplying an engine with an ample quantity of air.

It can take anywhere from 15-100+ horsepower to drive a compressor to produce appreciable levels of boost. Needless to say, this figure varies greatly with engine/compressor design. In any case, we can plainly see that a standard 12V system simply cannot supply that sort of power. And before you say "I could just use a more powerful alternator..." think about this: How much horsepower does it take to turn an alternator? Think about it. There's no such thing as a free lunch.

Regards,

Bryan Carter

 

[GregLocock]

To get 1 psi of boost for a 2 litre at 6000 rpm would need 700W at 100% efficiency. This might increase the crank SHP by 6%, or 6 kW say.

Realistically fans are more like 75% efficient, motors are typically 60% efficient, and alternators are typically 40% efficient at high speeds.

So of your 6 kW you need 0.7kW/.75/.6/.4, or 3.9 kW, leaving you a net gain at the flywheel of 2%, but using 6% more fuel. Not an especially attractive option.

Admittedly it would be possible to find motors and alternators that are far more efficient, but they are not easy to come by.



Cheers

Greg Locock

 

[Andybee]

As an amateur on a shoe string budget running a 998cc Austin Mini I find all this fascinating...not to mention useful. I did briefly consider trying to run a garden vac (on blow) into the intake not to raise boost, but to assist induction. I realise that a piece of equipment like that would not significantly pressurise the intake air, but would I be right in thinking that it would improve effieciency? A standard IC engine is only 25% efficient - isn't this because it has to expend energy in drawing in air, thereby wasting a percentage of the power gained from the fuel it has just burned?

Anyway, the electric supercharger idea was quickly dismissed, mainly for the reasons stated above - complexity of feedback for varying engine speeds, lack of power from the battery, never mind the strain on the alternator...

But am I thinking along the right lines - any assistance in the way of pressurised air given to an engine will increase its efficiency, thereby improving it's output? Any gains from a 998cc engine are welcome

 

[patprimmer]

I don't know how much air a garden vac pumps, but if it is less than the engine uses, it will be a restriction, not a supercharger.

If it blows more air than the engine uses, it will be a supercharger and will improve the volumetric efficiency, but probably severly decrease the mechanical efficiency, as you need to include the vac moror as part of the overal car motor package.

I thought most modern motors were over 30% efficient, with about 1/3 going to the transmission, 1/3 going out the exhaust and about 1/3 going out the cooling system.

I expect that a turbo might improve on that figure at full load due to the energy recovery from the exhaust, but a mechanical supercharger will be worse as the parasitic losses driveing the supercharger are substantial

Regards
pat

 

[Andybee]

Thanks - I appreciate that airflow tests will have be done to compare the output of the impeller to the air drawn into the engine. I hadn't thought that an ineffiecient impeller might actually reduce the airflow - worth considering.
the 25% figure was a guesstimate, but the 'A' Series is by no means a modern engine - design dates back to pre WWII, and when it is squeezed into an Austin Mini, adding a turbo requires major structural surgery. So a Supercharger (or airflow assistor) seemed like the way to go.

I have already had some success with a large diameter air scoop behind the front grille 'ramming' the air into the carb, so I was looking to a supercharger being the next logical step.

On a Carb'd engine, how does the system compensate the fuel/air mixture when a larger volume of atmosphere is squeezed into the chamber? Larger volume = weaker mixture?

 

[coughdrop]

Andybee,
There were some "Frogeye" sprites fitted with superchargers. I helped to convert a standard sprite as a "works team replica" some 20 years ago. The carb jets were bigger and we used WD40 in the dashpot to shorten carb "piston lift" response.
I suggest you contact the "Sprite Owners Club" for some archival data which would help you in your quest !

Regards,
John.

 

[Andybee]

WD40 is thinner than the 20W-50 I have always been advised to use - might try that tomorrow! As it's thinner, it'll need topping up a lot more though won't it? Also, isn't WD40 inflammable? Fire risk?

 

[PEW]

You can experiment with different oils in the dashpot of an SU. Try ATF, 3 in 1 oil, jack oil or a lower viscosity engine oil.

I have twin SUs on my Reliant 850 engine and I am forever trying different ideas. Interestingly, one of the dashpots uses oil quicker than the other.

Have you tried a "blue" piston spring? This is a lighter spring (2.5 oz)which allows the piston to open more for the same engine condition. It might (or might not) improve your engine.

PW

 

[Andybee]

I have stocks of the various oils you mentioned - I'll try them in turn - thanks for the tip.

As for the piston spring I have fitted - I'll have to check waht sort it is - it's not blue, more a kind of murky grey! (lol)

A Reliant 850 TC? Should be blinding off the mark!

 

[Pancholin]

Hi Slick 1
I seem to recall there was a Latham axial flow supercharger that was belt driven and was basically in competition with the Paxton centrifugal.
Pancholin.