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Plug-in Blackfly: going alternator optional nets +10% mpg
Posted Monday, October 23/06 in Mods & Tests
How much energy does a car's electrical system use? Would shifting the electrical load from the gasoline-driven alternator to just the battery return a measurable fuel economy improvement? Will a modern computer-controlled car even run properly off its battery alone? Inquiring minds want to know!
My curiosity about this topic was piqued when I read about a guy who installed solar panels on the roof of his VW bus, added a couple of deep cycle batteries, and ditched his alternator. Apparently, the panels and the extra batteries were enough to drive round-trip from Texas to Seattle.
So, I tried it (minus the solar panels on the car). And it turns out that not only does running without the alternator result in a real MPG gain, it turned out to be the largest single fuel economy improvement of all the mods I've tried to date. But there's a big caveat: the immediate fuel savings may not make financial sense in the long run when you consider the effect on battery life.
Unbelted alternator overview ...
Smart alternators: old idea ...
Before undertaking this little experiment, I had little doubt that alternators exact some fuel consumption penalty - the question was just how much. It's not hard to find examples where alternator use is minimized or abandoned entirely, either to maximize engine power or reduce fuel consumption:
- Drag racers are known to run alternator-less (or switch them off) when racing, for example.
- The high efficiency Honda Civic VX and CRX HF models reduce alternator charging under certain cirucumstances. The VX computer instructs the alternator to drop from 14.5 volts to 12.5 (ie. not charging the battery) when all of the following are true:
- Load below 10A
In other words, the alternator is essentially "idling" when the load on it is low, the car is warmed up and being driven gently. Note that the alternator reverts to normal charging when braking or decelerating (fuel cut-off mode), which is like a simple form of regenerative braking found in modern hybrids and electric vehicles. (Thanks to Randy for the VX info.)
Estimating electrical load ...
So it's clear there's some fuel efficiency benefit to be gained by fiddling with the alternator. But how much?
To figure that out, we need to start by estimating how much electrical energy is required to run the engine and accessories. Fortunately for me, Randy has already done the work. He measured the electrical power required to run his '94 Civic EX:
With that info in hand, a back of the envelope calculation begins: around 163 watts for running the engine, ECU & fuel pump; marker lights @ 95 watts; cruise control at 30 watts (just a guess). For a total of about 288 watts.
Estimating fuel needed to run the alternator ...
The alternator isn't 100% efficient. In other words, the engine has to contribute more than 288 watts to generate 288 watts. According to Wikipedia, a modern alternator is between 50-62% efficient.
At 60% efficiency, it would take 480 watts of work from the engine to generate 288 watts of electricity.
But wait, there's more!
Let's not forget losses to belt friction and aerodynamic drag at the alternator pulley fan (which, being smaller than the crank pulley, spins much faster than engine RPM). I'm not sure if pulley drag is figured in the Wikipedia efficiency calculation or not, but since this is a back-of-the-envelope exercise, we'll say it isn't. Let's say belt & aero losses consume an additional 50 watts.
So the back of the envelope tells us that my car requires around 530 watts, or nearly two thirds of a horsepower (745.7 watts = 1 hp) to run its alternator (for the load specified).
I've calculated elsewhere that the Firefly has to generate 5.4 hp to maintain 70 km/h in ideal circumstances. So reducing that power requirement by 0.64 hp should reduce fuel consumption by a similar amount: which is 11.9% (at that speed).
Worth trying? Sure!
Unbelting the Blackfly: the water pump conundrum ...
To do this test, removing the belt that turns the alternator was easy. Unfortunately, that belt also turns the water pump.
Of course, I could have just disconnected the alternator electrically and left the belt in place, but I wanted to do the full Monty and shed belt & pulley fan losses too. This required a bit of MacGyvering to get the water pump spinning again:
Panty hose? No...
The first approach I tried (and the one used for this test) was to make 2-ply loop from a 3/4 inch wide strip of rubber cut from an old bicycle inner tube. I cut & stapled one together, stretched it taut over the crank & water pump pulleys, and surprisingly, it worked (long enough to test with, anyway).
Experiment: testing an unbelted alternator ...
June 7, 2006 - 8 PM
- The experiment
This was an A-B-A comparison on my usual nearly level test route, where A = alternator connected as usual, and B = alternator belt removed (with the temporary rubber belt running the water pump from the crank pulley).
Speed was 70 km/h (43.5 mph) using cruise control set once, cancelled with the brake between runs, and resumed for subsequent runs.
@ 70 km/h / 43.5 mph - MPG (US)
A = alternator connected
A/B ... West ... East ... Average MPG (US) / L/100 km / MPG (imperial)
A ..... 71.2 ... 68.1 ... 71.65 / 3.28 / 86.05
B ..... 78.1 ... 76.3 ... 77.20 / 3.05 / 92.72
A ..... 75.0 ... 66.6 ... 70.80 / 3.32 / 85.03
A average ... 70.78 mpg (US) / 3.23 / 85.0
7.3 mpg (US) - difference of between A avg. and B avg. runs
VW verification: corroboration by another tinkerer ...
Skeptical of my claims? Well, you don't have to take my word for it!
Fellow efficiency enthusiast Brock did a similar test with his 2003 VW Jetta TDI, and reported similar results, though he had to be a bit more creative in his approach, since the TDI draws a lot more current when cold (glow & coolant plugs), and the engine computer was unhappy when the volts sagged too low:
Thanks for the corroboration, Brock!
The BIG caveat: batteries are consumables ...
At first blush, the 10% gain makes this seem like a no-brainer mod. But it may not actually make sense from a strictly financial point of view. You could end up spending more than you'll save, particularly because a battery is a consumable that wears out relatively quickly when discharged deeply and repeatedly.
A conventional "starting" battery is not at all suited to this kind of punishment. I have read that you may only expect to get 50 deep discharges from a starting battery before it's damaged beyond usability. To use this mod, a more durable flooded or deep cycle battery is definitely required.
Some really rough calcs, using my own situation:
If I had saved 10% by running without the alternator over the past year, then my fuel bill would have been reduced by roughly $34 over 8,700 km, or 0.39 cents/km. (And that's at higher Canadian fuel prices.)
Assuming I get 200 cycles from a deep cycle battery before it's toast, and can drive about 75 km before it needs recharging, that's 15,000 km per battery. The battery cost (assuming $90 per) is therefore 0.6 cents per km.
So already I'm "paying" for my increased efficiency (0.6 cents out, 0.39 cents saved), and I haven't even factored in the cost of electricity yet (which is admittedly small where I live). Unless you happen to have a source of cheap or free batteries, you need to do the math before jumping on this mod as a way to save money.
On the other hand, if I look at it from an emissions perspective, it's a different picture: 2/3 of Ontario electricity is GHG-free (1/3 nuke, 1/3 hydro). That's arguably better than using gasoline to generate my car's electricity.
Alternative alternator ideas ...
Recognizing the fuel consumption penalty of conventional charging systems, a number of companies are working on alternatives:
darin AT metrompg D-O-T com, or here