That is somewhat correct. Per your suggestion, I tested this out today on the way home from work. I'll share that info below.
Originally Posted by Loren
Correct! It is designed to charge the battery and keep up with the electrical loads that the car demands. However, I added a deep cycle battery that will take care of the electrical loads for non-long trips, so I don't really need it most of the time.
You have a noble idea, but Mitsubishi beat you to it. The alternator is already as small as it can be, and operates as efficiently as it can to operate the electrical systems in the car and keep the battery charged.
I'm going to keep the research stage here short. Its already been done by others thankfully. This will actually be the 3rd car I've done this mod to as well as many overs on EcoModder. MetroMPG's testing is on his website here. He goes as far as removing the accessory belt and it nets him a 10% increase in fuel economy. According to the EcoModder wiki page on this mod, it will net a 5-10% gain in fuel economy. That is pretty huge as far as a single mod goes.
As with anything of this sort, you're best to start with LOTS of research, and some instrumented testing. You should be able to tap into the alternator control circuit (the wire that profides the "field current" voltage to the alternator to turn it on) and wire that to a light on your dash, or even to a logging system. With that, you'd be able to see exactly when and for how long the alternator is actually loading the engine.
If you put a toggle switch in that same circuit, you could turn the alternator off at-will.
I'm going to bet that MetroMPG knows a bit about this from years of playing with this kind of stuff and keeping up with the Ecomodder forum. It totally makes sense on an older "dumb" car, like a Metro. But, I'd be VERY surprised if there was anything to gain by playing with it on the Mirage because it's already ECU-controlled and optimized for fuel economy.
Alright, enough talk. Lets hear about the testing.
I have about a 25 minute commute home. I monitored battery voltage the entire ride home, and I took some pictures at the end of the trip to eliminate any 'start up recharging' situation. Voltage will tell us roughly how much the alternator is putting out. Higher voltages are required to charge a battery faster. Typical alternators keep battery voltages anywhere from 13.5 to 14.5 volts.
When I fired up the car at work, the battery voltage jumped up to 14.5V. This is the start up recharge phase. It slowly went down as I drove for a while, and ultimately settled right around 13.9V for most of the trip. However, I wanted to test it under different conditions to see how the ECU controls things. So, I snapped a few pictures.
In our first picture here, I am cruising through town (slightly downhill) at 35 mph. 13.8V
In this picture, I am in gear slowing down for a red light. 13.9V
Here I am sitting at said red light idling. 14.0V
Now, I am accelerating away from the light at WOT. 13.8V.
So, yeah it seems like the Mirage really likes 13.9V +/- .1V. I was hoping to see the voltage drop a bit more under acceleration as that would free up a bit of power as Loren suggested. However, I really didn't see that happen. I didn't just quickly goose it, I was WOT all the way to the speed limit which as we know takes the Mirage a bit longer than most cars.
In either case, this mod will completely eliminate ANY electrical load the car will be using. Doing a little bit of math will give us a very rough idea of what this will do for us.
1 horsepower = 745 watts
Lets assume that the Mirage uses 20A in normal use. 20A * 13.9V = 278W
That is nearly 1/3 hp just to run the electric stuff in the car like the spark plugs, the plethora of sensors, abs, traction control, etc.
But wait, it gets better. If you look up the efficiency of an alternator, they peak at about 60%, more likely we're looking at 50%. This means, the engine is actually being loaded down by 556W or 3/4 horsepower. That may not sound like a ton, but it is significant. According to the Ecomodder power & MPG calculator the Mirage needs 9.7 horsepower to maintain speed on level ground at 55 mph. Add another .75 hp to that, and you add almost 8% to your power requirement. Interestingly enough, that falls into our 5-10% fuel economy gain from eliminating the alternator.
View my fuel log 2015 Mirage DE 1.2 manual: 45.4 mpg (US) ... 19.3 km/L ... 5.2 L/100 km ... 54.6 mpg (Imp)