Half a croissant, on a plate, with a sign in front of it saying '50c'
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Vacuum-powered alternator

Power a turbine off engine vacuum and use it to drive the alternator or other load
  [vote for,

In a gasoline engine, the intake stroke produces a partial vacuum in the air intake. The function of the throttle is to restrict air going into the motor, thus producing a deeper vacuum. In a normal engine, the throttle represents a pure waste of energy. I would propose having a turbine on the air intake which was wired up electrically so as to provide a mechanical load that would vary with the accellerator pedal while getting as much useful electricity out as possible (if the battery was fully charged, energy beyond that required to operate electrical systems would have to be dissipated as heat).

As an added bonus, depending upon the exact turbine configuration, the addition of suitable electronics might allow for using this turbine as an electric supercharger when rapid accelleration was required.

Unlike some other proposals here for driving a turbine off the exhaust (which would increase back-pressure and reduce efficiency), this proposal would make use of the vacuum which is already required to exist, and which is currently produced via an entirely-wasteful method.

supercat, Mar 14 2004


       I'm sure this won't work.   

       There's not enough pressure differential to power an alternator here. For the moment, I'm ignoring the added complexity of maintaining a proper fuel / air ratio with such a device.   


       //Unlike some other proposals here for driving a turbine off the exhaust (which would increase back-pressure and reduce efficiency)//   

       Why do you think that this won't reduce efficiency? You've got the same fundamental problems here.   

       Quite frankly, "powering" anything with the air going into or coming out of the engine is problematic. The disruption in engine efficiency is staggering without a lot of additional complexity.   

       I'd suggest a quick read on fuel-air ratios.   

       Finally, all of the above aside, just compare direct drive efficiency with turbine efficiency. BIG difference. You don't accomplish anything with this unless you SAVE something somewhere.   

       This dog won't hunt... although he might find some fishbones.
zigness, Mar 14 2004

       The efficiency of a motor will be at maximum when the intake pressure and output pressure are as close to each other as possible. Since, for practical purposes, the output pressure is going to be above atmospheric unless one spends energy pushing it below, and the intake pressure is going to below atmospheric unless one spends energy raising it, maximum efficiency occurs when both are as close to atmospheric as possible.   

       Unfortunately, an engine which is running with its intake near atmospheric will generate more power than is needed in most circumstances. Consequently, engines are equipped with a throttle which reduces the amount of air available to the engine and thus draws a partial vacuum.   

       Energy spent drawing this vacuum is wasted in typical engine designs. Allowing the energy to be put to some other use would thus provide a source of "free" energy.   

       To be sure, the amount of energy wasted in the throttle isn't huge, but then again nor is the amount of energy required by the electrical system. I would suspect, however, that they should be roughly comparable.
supercat, Mar 14 2004

       zigness: Correct me if I'm wrong on this, but a 3.0 liter engine running at 3000RPM will draw 75 liters/second. Each PSI of vacuum drawn will represent about 0.8hp or 620 watts. While I'm by no means an expert on cars' vacuum or electrical requirements, it would seem that cruising at moderate throttle should provide something on the order of 2-3psi, which if harnessed with 50% efficiency should be a bit more than enough to power the alternator.
supercat, Mar 14 2004

       Won't work. Alternator draws several horsepower (you can often hear engine idle momentarily drop as you turn fans, lights on). As for using manifold vacuum to power anything, fifties Ford UK products had vac powered wipers, which would grind to a halt under full throttle operation.
unclepete, Mar 15 2004

       Well, I really don't know what a typical cruising vacuum is, but I was being pretty conservative with a 2-3psi guess (idle is about 10psi, while WOT is less than 0.5psi). So there may be some power there, especially on larger engines.   

       I'm aware that vacuum-powered wipers were a problem, since wipers need continuous power. I would expect, though, that something like an alternator would not need continuous power; if the alternator drops out momentarily when a driver floors the accelerator, the battery should be able to supply ample current for a few seconds while the car gets up to speed.
supercat, Mar 15 2004

       Pretty interesting idea. I had been thinking of something rather more of the other way round where there is this vacuum switch, connected to the intake manifold, which cuts out the alternator load at full throttle, where the manifold vacuum will be zero, and thus relieves the engine of the alternator load at full throttle, allowing more power to reach the wheels. Had also been thinking of a similar way that relieves the engine of the air-conditioner compressor load under the same situation.
rthlng, Aug 10 2004

       Certainly in theory, this works. Worth pointing out that the throttle is the major factor in a petrol engine's part-load inefficiency - Diesels, with no throttle, are much more efficient at low power settings. Harnessing this would be sensible if it didn't disrupt the engine's operation.
david_scothern, Aug 10 2004

       Sure it could be *made* to work. Yes, the vacuum is wasing energy. But loading the draw will not be a free use of this energy. The engine is simply driving the alternator load via a different transmission method; the 50% efficiency sounds conservative. But how about a near-100% method -- like a belt and pulleys?
bpilot, Aug 10 2004

       Zigness said: //Quite frankly, "powering" anything with the air going into or coming out of the engine is problematic.// You realize that engine vacuum has been used for many decades to power such things as brake-assist, windshield wipers, radio antenna (50's cadillac). That's not on the same scale as an alternator provides, but it's certainly "power".
musicator, Aug 10 2004

       rthlng, I think they already have a cutoff for the AC compressor. In my 1995 Geo Metro, hot air blows out of the vent when you mash the gas for very long.
roland3c, Aug 11 2004

       bpilot: Powering the alternator with a belt means the alternator is using power that's being "paid for". Every watt of power used to drive the alternator is one more watt of power the engine has to produce. By contrast, vacuum power is "free"; every watt of power harnessed from the vacuum is one less watt of power that has to be wasted.
supercat, Aug 12 2004

       The kinetic energy of gases exiting through the exhaust is always going to be greater than the kinetic energy of gases entering through the intake manifold.   

       So true that energy is free but it is so small that it is probably not feasible to harness (given simple losses in converting kinetic energy).
madness, Oct 04 2005


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