Do not understand how Alternator low voltage load increase MPG.

[brucepick]

kozaz,

I wouldn't go installing a higher output alternator in an effort to save fuel. Unless you need it to run a bunch of upgraded electrical items.

My understanding is, the alt's amp rating is it's maximum output. Measured at some particular rpm of course, but I don't know precisely. From my experience, alt output seems to reach it's max at something like 1.5 x the idle speed. Designed that way because if it's say, an 80A alt, and you NEED 80 amps, they can't build it so you have to get up to 6K rpm to get those 80 amps. You need them at 2K rpm just like you would at 6K. At idle the output can presumably be allowed to drop a bit (??) because the car won't spend much time at idle.

Anyway, the actual load the alt places on the engine at any given time
[which is what determines how much fuel need it's creating]
is mostly proportional to the actual electrical load the alt is "serving". That is, computer + injectors + ignition + fans + lights etc.

I'm sure that an alt with greater capacity, such as a 140A vs. 80A, will have a larger base mechanical load that's always present (which would hurt you if you install it). Because it will be heaver, and so will probably need more mechanical energy just to keep it spinning even if there would be zero electrical load on it. (That's where the mechanical load on engine isn't exactly proportional to the electrical load on the alt)

Of course if you've put in upgraded electrical everything plus a "bouncer" hydraulic suspension just for show and need to power all that stuff - then that's another story. But you probably wouldn't do those mods you' if you're trying to get better fuel economy.

[GasSavers_HAHA]

I kind of like the idea of using a Sterling engine to drive the alternator.
The Sterling would re-use the waste heat from the engine and cause little loss of efficiency. The numbers seem to work too for a normal car.

For example: 15 hp is used for driving the car forwards which means about 45 hp is wasted as heat. If we could suck up 25% of this for the Sterling engine (11,25 hp) and we managed 20% efficiency (very realistic), we still get 2.25 hp to drive the alternator, which seems about right.

I saw a reference to a system like this somewhere but it was for big trucks.

With some optimizations, the total efficiency would be much better and we would get an electrical surplus. With a hybrid system (like Hondas' IMA) we could use the power to offload the combustion engine even more.
The biggest practical problem is the availability of high output, low weight, Sterling engines. There really aren't any smaller ones above 1000W on the market It will also be much easier to experiment with this kind of system in a boat than a car due to space constraints and legal matters...

[Sludgy]

Originally Posted by HAHA

The biggest practical problem is the availability of high output, low weight, Sterling engines. There really aren't any smaller ones above 1000W on the market It will also be much easier to experiment with this kind of system in a boat than a car due to space constraints and legal matters...

No kidding. Using the copious amounts of automobile waste energy has been the goal of 1000's of tinkerers and dreamers. Significant wasted energy includes brake energy, coolant heat, and exhaust heat. So far the only waste energy that automakers have been able to recover on a commercial (i.e. economically viable) vehicle is brake energy, i.e. in hybrids.

Aside from cost, another trouble with stirling engines is weight. A simple (non-pressurized) stirling engine capable of ~2 kW weighs hundreds of pounds.

[GasSavers_HAHA]

Originally Posted by Sludgy

So far the only waste energy that automakers have been able to recover on a commercial (i.e. economically viable) vehicle is brake energy, i.e. in hybrids.

Turbos would perhaps count as being powered by waste energy.
Putting a turbine in the exhaust stream and connecting the output to the crankshaft also works. This principle was used on at least one aircraft engine (WWII?). I also saw some reference to something that looked like this on a current truck engine. They used both a turbo and another turbine for crank augmentation. For the aircraft engine, I saw a figure of 8% improvement in power if I remember correctly.
Sorry for running off-topic...

[brucepick]

Turbo to crankshaft - slick!
You'd have to gear it down like crazy I think - the rest of it is just a matter of finding places to route the belts etc.

Maybe this needs a new thread?

[theclencher]

"turbocompounding" has been around a loooong time; i don't know why it isn't in more widespread use.

[brucepick]

Originally Posted by theclencher

"turbocompounding" has been around a loooong time; i don't know why it isn't in more widespread use.

Same d**n reason that all our cars are set up for "performance" instead of safety, fuel economy and reliability.

[GasSavers_HAHA]

http://www.greencarcongress.com/2005...veloping_.html

BMW puts a steam engine in the exhaust system and reclaims 80% of the exhaust heat. This yields 14 hp on a 1.8 liter 4 cyl engine.