turbo alternator

[kunundrum0]

hello im new been a watcher for a while now and now i have a aquired a 94 metro 4 door still have not done anything to it at all and getting about 40mpg with a/c off i know evil but small evil to me.

anywho i was wondering if any one has done any experimenting with a turbo alternator. small aircraft have them it basically is a turbo that scoops air and uses it to spin and alternator, in a car I've seen it be a modified turbo but just leeches off of the exhaust and what would normally be the intake boost side is just an alternator.

so has anyone seen one or made one id kinda like to make one or would the backpressure be bad for the engine in any way?

i moved this from a diff area

im thinking of researching and possibly making on for my geo

[markweatherill]

As the current draw on the alternator varies, the load it presents varies. With all lights blazing and after an engine start, the alternator would be a bigger restriction in the exhaust than with no lights on on a warm day. And then there's heat soak...
But I've thought about this too, and maybe it could work. Any such system needs to be able to cope with the car's maximum demand for electricity in a 'worst case' situation.

[chuckm]

TANSTAAFL. Remember that your engine would then have to work harder to push exhaust gases out. That would translate into poorer mpgs, if not engine performance problems (exhaust not properly evacuated from the cylinders).

[nemesis]

Originally Posted by chuckm

TANSTAAFL. Remember that your engine would then have to work harder to push exhaust gases out. That would translate into poorer mpgs, if not engine performance problems (exhaust not properly evacuated from the cylinders).

Not necessary, I've lost couple of mpg with free flowing exhaust, which leads me to believe that the exhaust with more back pressure might get better gas mileage, I don't have any proof since I haven't done it personally, but it might work

[chuckm]

Sure, some back pressure is necessary. But it is an engineered system; it's not a "If some is good, more is better" deal. Rather, the exhaust system is designed to a local optimum.
Let's think about the power requirements of the alternator for a moment. Let's assume it generates 50A at 14V. That's 700 watts of power. Assuming an total efficiency of 70% (that's unrealistically high, especially for turbo!), you'd need to extract 1000 watts of power from the exhaust, or about 1.3hP. Again, "There Ain't No Such Thing As A Free Lunch." The exhaust gas pressure simply isn't high enough to extract that kind of power. An alternative way to think about it: how would your car behave if you stuck a 1/2" diameter exhaust pipe on it, all while leaving the catalytic converter and muffler in? Better yet, here's a real example of a what happens when you restrict the exhaust.

[DonR]

http://ecomodder.com/forum/showthrea...tive-8796.html

You could control backpressure by removing the stock muffler, adding the turbo & then useing a Supertrapp to control noise.

Don

[markweatherill]

I wonder if this concept would be better with a diesel engine. They seem to have the gas flow to get a turbo spinning at lower RPMs.
But, an alternator is a larger mass to get spinning in the first place than a turbo impeller. Especially after an engine start, it's going to be hard to get it spinning.

[evolutionmovement]

Another one of those persistent myths! Back pressure is NOT helpful. Ever. The changes you're seeing are from changing the scavenging effects in your exhaust. Different diameter pipes as well as volume changes throughout the pipe's length change the velocity of the exhaust pulses as well as move the rpm range of the scavenging effect of their reflected waves. A larger pipe will slow velocity, but move more volume. Changes in diameter (from a bend, for example) or volume (muffler, converter, etc.) reflect pressure waves back to their source (exhaust valve). These reflected waves, when the timing of them coincides with the exhaust pulse of a new cycle, can help pull the new pulse out by creating a low pressure zone for it to move into. At a different rpm range than the optimal one for scavenging, the reflected pulses will hinder it by creating a high-pressure zone at the open port.

The longer the distance before a change in volume initiates a reflection, the longer it takes the pulses to return (lower rpm scavenging). Larger pipe diameter will have the same effect by slowing the pulse and can cause back pressure if there's too little energy or volume in the exhaust (which is why 4" pipe doesn't help a 1.6 liter engine, but might work well on a big block or a diesel). This is where exhaust tuning comes into play (these same principles apply to the intake side. The reason for variable length intake manifolds is to increase the rpm range(s) where reflections in the intake help to push air into the cylinder) and depends on the intended use of the car (high or low rpm optimization). Back pressure helps at no speed, it just reduces pumping efficiency at all rpm.

[markweatherill]

...but production engines are designed with back pressure in mind, with a stock restrictive exhaust as part of the design. In that respect, back pressure is 'helpful' in that the engine can have a gentle exhaust cam profile. That's my theory anyway. Of course for sheer power you need no intake or exhaust restrictions and therefore a hotter cam.

[evolutionmovement]

Stock cars aren't usually optimized for pulse tuning alone as there are so many considerations that compromise design. For emissions purposes, some exhaust is reburned with the fresh intake charge, which exhaust overlap would allow and back pressure would assist, if that's designed by intention on the pipe side.