MAF question for techs

[theheff]

I am fabricating a low-pressure, electrically-driven supercharger for my Q and was wondering-- does the computer simply read off the MAF sensor and then add fuel accordingly or is there some upper limit of airflow above which it won't add more? Basically I want to know if I just can cram more air in and the ECU will take care of making more power with the added air...

Thx in advance,

Heff

[maxnix]

Out of curiosity, is it a 1996+ with OBDII? Might check out http://www.keuylianmotorcars.com/ as they have done it for 2002+ Q45s (about $4K before install?). Whether it may be adaptible to earlier 2nd generation models or the 1996 is a guess, but it could have some pointers for your project.

[Q45tech]

Good luck! As the electrical requirements are very high......since motors are not very efficient it usually take 1200 watts of electricity to make a single horsepower and a super charger that would be meaningful [would create 5 psi] woud need from 12-15 HP to work against the air volume required for a 4.5 liter engine.......that would be close to 1,000-1,500 amperes at 12 volts...............you would ned 4-6-10 batteries in parallel ......the other problem is high HP electrical moter are BIG..........you might find some older style electric torpedo motors.The other problem is that most super chargers [axial type run at 35-40,000 rpm so even the 6,000 rpm crankshaft must be stepped up 7 times [then there are heavy gear losses].

[Q45tech]

The electric superchargers available usually produce between 0.1 and at best 0.5 psi [so they produce at best 6.6 x 0.5 = 3.3% more power] but they are in the air path so the motor inefficiencies are translated to air temperature rises as the incoming air cools the motor. ......it gets hotter.

Many times ther is a breakeven or a power loss due to the reduced density of the air!

[Q45tech]

Back to MAF the only way you know how much range [reserve] is built in is to measure the MAF voltage at 6,000-6,500 rpm.

A Q MAF might read 4.0-4.44 volts out of a possible 5.000000 volts.........on OBD2 ecu the Consult can read grams per second with 255 grams per second equalling around 375 HP........200 gm/s = around 300 HP .

Generally the 90-95 ecu had the ability to deal with up to 10% more air flow at 70F to deal with a WOT acceleration at 20F [5%] more than normal SAE rated airflow.......with a little reserve.

JWT ecu reprogramming for Nox works up to 350 -360 HP again with a little reserve [375 HP]..........above that the MAF should be changed for a larger unit.

20% reserve is a common safety parameter used by designers.

Side note: when GM upgrade the current Corvette they realized the 255 gram limit [375 HP worth of air ] would be exceeded so a whole new group of MAF needed designing same with Supercharged Cobras. 255 gms worked well for the past 15 years when few [no] US cars had more than 375 HP.

Lots of designers added a pressure monitor so that the ecu would go to <255 gm/s then increment 6.6% per psi [corrected for plenum air temperature] to deal with turbo engines that might exceed 375 HP..........what ever the injector open time was at 300 HP [say 11 milliseconds] [from MAF then multiply by 1.066666 per PSI boost..........this was usually a piggy back circuit boad.

[theheff]

Q45Tech-- your concerns for my motor/battery situation are appreciated but I work for an engineering firm developing an advanced, battery-powered motor for a DARPA project. Heh heh... this is an 8 HP axial flow fan that runs on just 7 lbs. of an advanced polymer battery pack-- and I get one of the prototypes! The whole system will weigh just over 10 lbs. Just wanted to make sure that I won't lean out my motor too much. I plan to set it up to only engage under WOT...

[DAEDALUS]

Impressive! My first guess is that it is for a silent Predator type vehicle. No response needed. If the motor only engages under WOT, won't it cause a lot of drag when it isn't spinning?

Measure the MAF voltage while driving before and after installation to see the impact.

[theheff]

I don't see much adverse effects happening when not spinning, since the motor is a ring around the blades (which are in the flow). When not spinning, I will basically have the equivalent of "The Tornado Supercharger" from those stupid infomercials-- should be pretty harmless...

Correct, it is for a UAV...

[DAEDALUS]

Very cool. Keep us informed. Maybe the answer is to vary the "non-engaged" current with a speed controller using the MAF voltage as an input, such that the fan speed matches the stock airspeed. Or is the battery pack absolutely necessary? I like the ring idea...kind of like hubless wheels.

[Q45tech]

Speaking of the tornado, to prove how well it DOESN't work measure the MAF voltage at WOT 6,000-6,500 rpm with and without one! It is a major restriction at high flow rates! 350-375 CFM.

You need 1.2 CFM per HP with air at 60F and 40% humidity.

[reggiegsd]

After thinking about this for a while, I have a few questions/observations.

Max boost would occur at the minimum air flow rate (Max back pressure), just like any other constant speed centrifugal pump.

Minimum boost would occur at maximum air flow rate (Minimum back pressure).

If the Q motor flows 275 scfm at WOT, then the fan must flow at least this much into 0 backpressure to prevent a loss at WOT.

Wouldn't this device be most effective at the lowest RPMs where the Q needs help and would provide a decreasing boost as RPMs increase where the Q needs little or no help?

All discussions I have seen about these fans compare them to superchargers or turbo chargers. Those devices vary their speed with engine speed, either mechanically of due to increasing exhaust flow. This means higher engine speed equals higher boost. The fan works just the opposite. Constant speed with decreasing boost.

Electrical load is based on flow rate x pressure increase. Generally speaking, load will be highest at low air flow (not zero) conditions (Max Boost) and lowest at high air low (Minimum Boost). What would the electrical load be at 2500 rpm (about ~100 cfm) and 1 pound of boost? (For the technoweenies, try the power to compress air formula)

I suspect about 300 Watts or 25 amps per pound of boost at these conditions.

Maybe this thing could work at low RPMs where it would do the most good.

[Q45tech]

Think you are undercalculating the air flow: 274/2 x [4,000 rpm]/1728 x 90% VE = 285 CFM or checked another way 307 lb/ft x 0.7616= 234 HP x 1.2= 280 CFM.............330 CFM at 6,000 rpm [276 HP] with a natural vacuum [resistance to filling] of maybe up to 25" water column. [The worst case vac after MAF is 12"WC] at 6500 rpm.

1CFM= 34.688 grams=191 grams per second at 276 HP.

http://www.allmeasures.com/

[reggiegsd]

I don't doubt I am making some assumptions that are off the mark such as air flow. My assumption of 100scfm was at an intermediate engine speed. If the max air flow is 300 vice 275 then 100 occurs at a lower rpm. I also assumed standard atmosperic pressure, temperature, and humidity. That does not take into account the vacuum felt in the intake line.

Redoing the calcutaltions for 150 scfm and 1 pound of boost requires about 40 amps. Thats 40 amps PER POUND OF BOOST!

The whole point is, this is on the verge of being possible. It would stress the crap out of the electrical system, but it could work, at low RPMs, where we need it.

[Q45tech]

Superchargers are 70% efficient based on air temperature changes , there must be some relationship in that the best may take 15 HP to make 6 psi on a 5 liter engine most take 20 HP [some take 30 HP] [this comes from the before after air flow in and the net crank shaft power what you would expect vs what is measured...........the active pump on a Q takes 5-7 HP to run under extreme conditions [also a guess from the sizes of drive belts used on supercharger confirms].

1.0 psi is at best 6.8% more density less the temperature rise....15.7/14.7=...1.0680272 and the rule of thumb for the most efficient is a 15F rise per psi at 70F so that decreases the density by 15/11=1.3636% so the net is 5.4364% per psi assumming the electrical power is free.

I am concerned that the electric motor will heat the air more and reduce the density more. You just cannot make an efficient design when the motor is in the air path.

What if the realationship was 3 mechanical HP per psi would that not be 746 x 3=2,238 watts plus motor inefficiency say 3,000 watts per psi vs your 40 amps x14= 560 watts or are you using a 48 volt motor = 1,920 watts. What about using a massive power convertor to step up to 480 volts or so.......10,000 watt units are not that hard to design.

Obviously a motor draws more current as the backpressure increases.

Great question for a PHD in Physics which I'm no where close to.

I went thru a similar exercise trying to design a hyro pump/motor to drive a supercharger by using a Q45 active pump needed 4 times the capacity to get 5 psi.

[reggiegsd]

My formulas come from calculating efficiencies for gas turbine compressors. Should be close.

[Q45tech]

Build it and let us know. All my motor knowledge is too old but the Q starter is very efficient and it draws 65 amps at 11.0 volts and makes around 0.52 HP.........that would be 1375 watts per horsepower for a 54.25% efficency at 3500 rpm no load [there is a gear reduction to match the flywheel].

I may be in error concerning the input horse power required to move the turbine because of belt slippage and the power lost in the gear step up drive to raise the engine rpm from 6,000 to 45,000 rpm for the turbine. We could always measure the oil lube temp rise and calculate the drive inefficiency?