Daniel B. Martin

Design News is a respected publication, a trade journal which is distributed to engineers. The March 12, 2001 issue has an article about the industry-wide conversion of automotive electrical systems to 42 volts, expected to occur in 2004 - 2006.

The article quotes Michael Matouka, staff development engineer for General Motors' Electrical Center. Matouka points out that "power is not free" and says every additional 350 watts used by the vehicle results in a loss of about one MPG of fuel economy. That's a bigger fuel cost than I would have imagined. Some 4Gen Maxima owners boast about the powerful audio systems they have installed. Some 4Gen Maxima owners complain about excessive fuel consumption. Think about it.

CKNY

iTrader: (1)

interesting...

kratz74

Explain this one to me... It can't be the weight of the system that they are considering because that woul dbe deminimus. Are they saying that the electrical system can't geerate enough current to the plugs to ignite all the fuel in the cylinder, resulting in poor mileage? If so-couldn't you enhance your electrical?

I just put in a new Blaupunkt head unit and Polk 6 1/2" speakers and it sounds great to me-no need for 350 watts.....

NickStam

iTrader: (3)

I wonder what the audio people have to say about this.

emax02

iTrader: (1)

Now this explains why I use to get 10MPG in my 91 Explorer. I had a 2000 watt system in it which added about 200+ pounds and then my Brush guards etc. I used to have to fill up like every 3 days and I did not even drive far haha.

ericdwong

oh yea just my area... I have 2 15" woofers with about 1000 watts going to both (soundstream rubicon 1002 pushing bridged mono and paralleled woofers), a RF 500a2 amp pushing the fronts (about 125 or so to the fronts) and a Hifonics mercury X (80x2 to the rear). I recently lightened my enclosure, it used to be 90 lbs, but now I got them down to 60 for the woofers. I have an Optima Yellow Top. The electrical drain is tremendous when the system is cranked, even with the relatively high amperage alternator and the yellow top. My stereo can suck so much power out that you can see the RPMs of the engine bogging down. I know my car is a tad slower because of the extra weight, and the MPG suffers (either from weight or the electrical drain) but to me personally, it is worth it (even though this is probably why my spoiler got stolen). Funny part is, first time at the drag strips, I ran a 15.1 with only an intake but WITH my 90 Lb sub box. Last time I went, I had the ShopVac vaccuum intake, EMPTY TRUNK, and Y pipe, yet I only ran best of 15.5 that night. I am an avid audiophile and have quite a house stereo (enough that I got into Stereo Review Magazine when I was 13) and used to have a mobile DJ system.

I chose a maxima when I got it mainly because of the large trunk, so I could fit woofers (try that in a MR2 spyder!). This is also why the next 2 door I get (if i ever get one) will be a C5 Corvette with its 22 cubic storage space.

Its all about what you feel is and is not necessary. I've been into stereos pretty much all my life and probaly will keep on going.

Micah95GLE

The issue, according to DBM, is not weight, but power. The use of 350 watts is what causes the loss in MPG, not the weight of whatever it's being used for. Weight is an entirely different matter. Car makers will be using the 42-volt standard to accomodate all of the multiplexing that will be inherent to future (and to some extent, current) cars, with all of the computer controls and interior electronics.

Sprint

iTrader: (10)

I am at 1200 watts so i guess thats about 4 mpg .. so i guess thats not so bad

Daniel B. Martin

Matouka is saying that every watt of power dissipated by any load in the car came from the alternator. The alternator is powered by the engine and the engine develops power by burning gasoline. If you increase the load on the engine by towing a trailer you might expect the engine to use more fuel. It may not be obvious but making the alternator do more work also increases the load on the engine and makes it burn more fuel.

96SE_dsm

Im glad somebody knew the laws of physics.
I was getting tired of reading through all of the replies, looking for someone who had the right answer.

No offense to those who were just tyring to guess.

Daniel B. Martin

You are right, and it goes even farther than that. There are serious proposals for running some parts of the vehicle by electricity rather than mechanical drives. The Design News article has a table of these. Here are a few, with the peak power draw (kW) in parentheses...
Air conditioner compressor (4.0)
Valve actuation (3.2)
Brake by wire (2.0)
Steer by wire (1.8)
Coolant pump (0.5)

The car of the future won't have a timing belt and it won't have a timing chain because it won't have any camshafts! Each valve will be operated by its own electromechanical actuator. Variable valve timing will be handled by the Engine Control Module just as fuel delivery and spark timing are today.

Sprint

iTrader: (10)

So Dan is it better to race with the radio off?

Daniel B. Martin

If you have a weeny base-model radio it doesn't make any difference. If you have a monster Melt Your Eardrums aftermarket system, then turn it off... just the same as you would turn off your air conditioner.

CustomMax

I have 1680 watts cranking through my car and I have not noticed a significant MPG change from when I had the stock radio installed. Did this journal offer any charts are stat sheets illustrating their results?

chris j vurnis

In terms of horsepower vs. watts --

About 760Watts equals 1 horsepower --

1MPG for most city driving is about 5% of the total mileage --
5% of say 200HP is about 10 horsepower --

So by GMs account, a 350W system should suck up about 10 horsepower --- I don't think so !!!

O.K. GM makes a fairly decent Corvette -- their one and only car out of dozens that can be considered world class --
But aside from that, I don't look to them for any source of technical excellence ---

Daniel B. Martin

No, but that's because the relationship between electrical demands and fuel consumption was not the main point of the article. The title of the article is "Automotive engineers look to 42v architectures." Many Design News articles are available online, but this one is not. Perhaps it will be put On The Web in the future. I don't know whether to believe Michael Matouka's figures but I have to respect his industry credentials.

brubenstein

You are assuming 100% effeciency, which is not a valid assumption. You have to go from AC to DC, run power through resistive wires, power devices that have their own losses, etc.

Once upon a time, it was common on roadracing motorcycles to run total loss ignition systems (no charging system, only powered by the battery), because generators/alternators use quite a bit of engine power.

Keven97SE

Your argument is flawed because you're assuming that the electrical system is 100% efficient. It's not. If your stereo puts out 760W, that does NOT mean it's only consuming 760W, ie 1 HP. The efficiency is going to be far below 100%. I am NOT an electrical engineer, so I have no idea what the actual draw might be for a monster stereo system, but if it's say 25% efficient, it would draw 4 HP to make 1 HP (760W) worth of sound.

But then again, I really don't fully understand how increasing the load on the alternator could decrease MPG. The alternator certainly doesn't have any way of mechanically adding or decreasing resistance on the motor. It simply spins and the voltage regulator dials the V up or down as needed. I can see how a high draw would divert energy from the ignition system and result in worse firing, hence worse mpg (and a slight HP drop), though.

Can somebody please try to explain to me how increased current draw off the alternator can cause an increased mechanical load on the engine?

Daniel B. Martin

This is the central element in your misunderstanding. The alternator doesn't "simply spin". It is an energy conversion device which converts mechanical energy into electrical energy. If you want more electrical energy to come out you must put more mechanical energy in. There is no free lunch.

98BlaximaSE

Its just like driving with the A/C on.

BLacKMax69

every1 saying watts ...is it in rms or max power you know what i mean????

brubenstein

A car's electrical system is DC (rectifiers are built into the alternator and the battery acts as a filtering cap). RMS and "max power" only apply to AC.

ezemax99se

How does a higher output alternator affect MPG? Does it rob more HP compared to a stock alt.? And if the high output alt. is used instead of a stock one to just run the normal accessories, will that change the HP and MPG? Thanks.

chris j vurnis

O.K.

I can understand about the energy efficiency thing ---
So I can now perceive up to 5HP loss driving 750Watts - just in the efficiencies of the amps/head unit --

Thats still about 2% of total power of the engine at full throttle -- but since most folks are only at light throttle, that percentage becomes a lot higher when we are just talking about cruising ---

So I guess the guy quoted in Design News magazine must be fairly on target with his statements ---

I always run my 700W system kinda loud -- my city mileage is about 17MPG at best -- was something closer to 19MPG in times past -- It would be a good experiment -- I just filled up my tank -- I'll drive for 10 days with no tunes just to see if it really is true --


Later ---

Daniel B. Martin

An alternator "steals" power from the engine in proportion to the electrical demand placed on it at that moment. If your power-hungry accessories are turned off the engine power required to spin the alternator is very low. If you turn them on the alternator requires more engine power. The difference between a stock alternator and a high output alternator is that the H-O unit has a higher maximum output than the stocker.