Question, does the alternator vary its load on the engine or is it constant?

Discussion in 'Fuel Economy' started by MateriaPanama, Jan 18, 2009.

  1. MateriaPanama

    MateriaPanama Well-Known Member

    i ask because supposedly running electronic devices in you car will adversely affect fuel economy, but unless there is some other phenomenon, wouldn´t this only be true if the alternator varied its load on the engine

    and doeskin running the ac worsen it because it has components directly attached to the engine

    im not challenging anyone´s statement, i am simply curious about this

    other question

    for slow city driving, with short intervals of medium speed (say 35mph) keeping in mind that i live in a hot place, and have electric windows

    should i roll up my tinted windows and run the fan

    roll down my windows

    run the ac in bursts (if so at what setting)

    of alternate between fan and windows
  2. basjoos

    basjoos Well-Known Member

    The alternator puts a load on the engine proportional to the electrical load that is on the alternator. If you have a scangauge or supermid on your car and turn on various electrical accessories while the car is idling, you can watch the fuel consumption rate increase as the electrical load increases.

    Running the AC causes a big increase in the fuel consumption rate, especially on cars with smaller engines. Running the AC uses much more fuel than running the ventilation fan. Once way to save gas in stop and go urban driving is to only turn on the AC (on its max setting) when you need to slow down, so you can use the AC load as a form of braking. Leave the AC off when accelerating and cruising.
  3. MateriaPanama

    MateriaPanama Well-Known Member

    you have really thought of this mpg stuff, havent you

    it never would have occurred to me to use the alternator as a brake do that, in a way it has the effect of regen braking

    thats really cool ill try that

    and thanks for the info
  4. NiHaoMike

    NiHaoMike Well-Known Member

    Yes, and it's known as Lenz's Law. The current in the alternator coils resists the movement of the rotating magnets, the magnitude of which is proportional to the magnitude of the current.

    Interestingly enough, that is why (in hot weather) shaving your legs can indirectly improve fuel economy. It helps with staying cool, so less A/C usage and therefore less current from the alternator. Yes, I shave my legs, and no, I did not start doing that for hypermiling. (I did it to stay cool in my KINE classes.) For what it's worth, Texas A&M's best hypermiler (Jennie Chen, who also helped set a world record) shaves her legs. I guess I'll just "follow the leader" and continue to shave as well!

    I have an idea for a "mild plug-in hybrid" upgrade that uses its own batteries to substitute for the alternator as long as it has charge. A pair of deep cycle batteries and a step down (buck) converter should be all that's needed.
  5. xcel

    xcel PZEV, there's nothing like it :) Staff Member

    Hi NIHaoMike:

    ___You will find some Honda alternators are smart will cycle between on and off using the larger 12V as the buffer. It is probably cycles on the 12V's output and you can see it when electrical load is at a minimum on any number of them. The system output will cycle form 13.x to 14.x and drop out to 12.x for a while and then back comes the alternator.

    ___Good Luck

  6. Right Lane Cruiser

    Right Lane Cruiser Penguin of Notagascar

    I see this in the Insight as well -- though there it is a DC-DC converter and the swing isn't as much. Usually between about 13.7 and 14.1V
  7. NiHaoMike

    NiHaoMike Well-Known Member

    I wonder if there is some control line that can be tapped into to disable the alternator as long as the "mild hybrid" upgrade still has charge. This way, the operation can be seamless to the driver - the alternator simply resumes operation when the upgrade is discharged. (I wonder if the logic will freak out when it finds over 13v when the alternator is supposed to be off, as if there is a control problem or something.) And maybe force the alternator on if the brakes are applied above a certain speed, regening to charge the upgrade.
    That sounds like a pulse charger. I have seen something very similar with a Belkin UPS. The battery voltage would sit at 13.6v most of the time, but every 15 minutes or so, it spikes up to as high as 15v for a few seconds. (The spikes were so high a current that they interfered with nearby analog audio equipment!) I think the purpose is to prolong the life of the battery. And it worked - the battery lasted over 5 years instead of 3 like the ones in other UPSes. I analyzed the circuit and it seems to be a line-interactive UPS with the transformer connected to the output and some relays to connect the input. An winding on the transformer is rectified to provide voltage for the charging circuits. One of those is a LM317 regulator circuit and the other is a MOSFET that simply connects the rectified voltage to the battery.
  8. Right Lane Cruiser

    Right Lane Cruiser Penguin of Notagascar

    It seems to be engine load dependent. I can see it drop when I hit a steep hill, for example.
  9. ATaylorRacing

    ATaylorRacing ATaylorRacing

    Here's an example for you guys. I have been drag racing since 83 (bracket racing) and it took me a few weeks to fugure out that my low HP car ran much slower in the night compared to the day (.10 or more) due to having my headlights on. I started from then on to make all passes with my lights on if the races were going to be continued into the night. At idle you can see the revs drop on ECU cars due to the extra drag when you turn on the lights, heater, wipers...etc.
  10. fuzzy

    fuzzy Mild hypermiler

    As another example of load dependence, my Honda's idle RPM sometimes varies up and down in time with the turn signal blink rate.
  11. chilimac02

    chilimac02 Bible Professor & Minister

    My 2001 Accord's alternator turns on and off intelligently. For this reason, it was obvious when I needed a new battery. My MPG went down a few. My cruising mpg wasn't as good as normal.

    However, my 2000 Explorer doesn't have a smart alternator and is a 4litre. The A/C only puts a mild strain on the engine, and interior electronics seem to make no difference. This is probably because the alternator pulls on the engine pretty uniformly on this vehicle, whereas the Accord's alternator only kicks in when the battery drops below about 12 volts (according to my SgII).
  12. 99LeCouch

    99LeCouch Well-Known Member

    My 99 Buick LeSabre is the same as your Explorer. Fuel consumption is the same with lights on as with lights off. The A/C puts a little strain on, then turning on the recirculate function makes it cycle a lot less. There's little MPG difference between recirculating the A/C and running with the vent fan on and windows cracked.

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