Originally posted by Rara:
Paul,

just a hint, but have you looked into the power requirements to drive an electric fan to sufficiently supply enough air to run any sort appreciable boost pressure on a given engine? Have you compared those power requirements to the additional drive loads for an appropriately sized alternator?


Yes, I have. The power requirements are a reasonable match with larger starter motors, I think.

Alternator capacity is not an issue unless you use boost semi-constantly. One might even want to use an alternator cutoff.

Originally posted by Rara:
I believe you will be in for an unpleasant surprise. Even more so when you consider the extra weight associated with that large of an electric motor as well as any additional batteries.


There may have to be a second battery, or a single red-top might do fine. They'd have to be moved to the trunk, I'm sure. One possibility I'm considering is to run the motor on 24 volts, if I can think of a way to make a suitable charger for the second battery that doesn't create safety risks.

Originally posted by Rara:
I think electrically driven forced induction has a good potential for short-term transient conditions, but to say that it take less power to drive is short sighted at best. It will take the same mechanical energy (probably more, due to conversion inefficiences) from the engine to drive the alternator to make the required electrical energy as it will to drive a mechanical supercharger.


Actually, I suspect that most mechanical superchargers waste more energy than an electrical setup would... except maybe for Vortech type setups that only work at high revs. Positive displacement pumps are less efficient, right?

Originally posted by Rara:
Granted, the electrical system does allow you to spread out the time when the power is generated, but only so much.


"Only so much" in that you're limited in how long the blower can run before you have to fill the battery back up, but the key thing for performance is that during WOT, it allows you to defer all of the load until later.

Originally posted by Rara:
Further, a turbocharger is driven by mostly wasted energy (outgoing heat and flow velocity in the exhaust system) and really only costs power in that it eliminates any exhaust scavenging effects. If ever there was such a thing as free power, the turbo is about as close as you can get.


That's why I had the idea that hybrid-electric cars should use turbine-powered generators. It's been demonstrated that exhaust energy can significantly help hybrid cars... by a guy who augmented a diesel engine with an exhaust-heat-powered steam engine.

The turbo does impose some exhaust restriction when taking power out of there, so it will impose minor costs when operating, like maybe comparable to using a non-performance exhaust system.

Originally posted by Rara:
Also, as far as component cost, I think you are grossly underestimating the costs associated with electric motors and thier controllers at the required power levels. Keep in mind the power levels required to provide the required flow at the desired pressure. A good example is on a typical Ford 5.0L engine with a centrifugal supercharger and a power output in the 400hp range, it takes somewhere in the neighborhood of another 70hp to drive that supercharger (ie, engine would be putting out 470hp in the exact same setup if it didn't have to drive the blower). Even if you cut that in half for the 200hp range you are looking at, that is still ~35hp.


My previous estimate had been that it should require 10 horsepower or less. Have I missed something?

This is a key point... if 10 horsepower can do the job, then the project can be done with affordable components. If it requires 25 horsepower, then it's completely infeasible on a budget.

Anyway, there's some wiggle room there because that maximum power demand occurs only at top revs, and there are worse fates in life than having your boost pressure droop lower as you go above 5000 rpm.

Let's calculate the actual work that needs to be done on the airstream by an ideal compressor... if we want to make a Zetec have an volumetric efficiency of 133% above normal, that makes a maximum consumption of about 0.15 cubic meters per second at top revs, and the pressure would be at least 5 psi, or 35000 Pa, so the minimum work required to compress the air is, if we call the cross sectional area of the intake A, the force (35000 * A) times the distance (0.15/A per second), making the minimum ideal power need about 5200 watts, or 7 horsepower. (Damn, I think it came out smaller last time I calculated it!) The compressor inefficiency pushes that to about 9 horsepower, I think, and heat losses etc mean that you have to push more than 5 lbs, making the demand 11 or more... it looks to me like a 10 HP motor should be able to do a useful job of boosting through at least a large part of the powerband, if not perhaps at the tip top.

Originally posted by Rara:
Don't get me wrong, I would be overjoyed to see a functional electrically driven supercharger, but the current level of technology doesn't allow it to be anywhere near competitive with other methods of driving a compressor. The closest I have seen to a succesful system design was the "dynapac" I believe it was called. And even then, its benefits were quite limited and very expensive.



The only Dynapac I could find on the web is something to do with earthmoving equipment.