physics of electrical v mechanical accessories

[dann092lx]

Hi

I see a lot of posts that talk about the advantages of electrical v. mechanical accessories. It seems to me that in both cases the engine is providing the power to run the accessories, its just that in the case of electrical the power is delivered indirectly, thru the alternator. All other things being equal indirect is less efficient than direct, so things cant be equal. What am I missing here? Thanks much

 

[2000xp8]

Dann, if i'm reading your post right, seems like you are answering your own question. (i think).

You start out by saying that you read about the advantages of electrical, and then end by sayind things can't be equal. Which is why the electrical has the advantage according to your opening statement.

Going electric does save a few horses, although i'm personally not a big advocate of electrical fans and water pumps on a car that just wasn't meant to have them. The hp gains are minimal, and if you read through all the posts since summer has hit, there has been a decent amount of people with E fan problems (although it's usually the fan controller or t stat that fails), and i don't see anyone with a fan clutch complaining.
Unless a car is maxed out of engine power, with the cost of E fans and water pumps, there is better places to spend the money.

 

[dann092lx]

For posterity

Thanks 2000xp8

I didnt answer my own question (or at least I dont think I did). That is, I still dont understand why electrical is more efficient - why arent things equal? Can you please elaborate? Thanks

 

[85_SS_302_Coupe]

Simple...anything mechanical, like a waterpump, clutch fan and power steering, is going to be driven by the engine's crankshaft. What you get is called parasitic drag, since the engine is using a portion of its power to turn/power those accessories.

Electric accessories are powered exclusively by the electrical system, which then frees up whatever portion of power those accessories were taking. This may be a small amount, but if you have a high amp alternator and a deep cycle battery, you can underdrive the alternator pulley, then get an electric fan, electric water pump and manual steering and you eliminate the parasitic drag that the water pump and power steering pump created. That can add up to a nice gain (or rather lack of loss) in power.

 

[stang&2Birds]

Do you want real physics, or BS?
I real physics, there are always loses in translating energy. So, when you go from mechanical (crank), to electrical (many loses in there), back to mechanical, it can not be as efficient as a "similar" mechanical-to-mechanical setup.

There are many many issues. For max cooling ability per HP, mechanical-to-mechanical will win. But, again, let's talk real-life and your real-life choices.

You do not want max cooling all of the time. Otherwise, a fixed fan is the way to go. After that, a very high quality low-loss thermal clutch fan is best. That's what came with the 5.0's from the factory. But, they do have loss because of slippage. You can get the FORD police/HD clutch and have less slippage and less loss. But, that also means more drag even if you don't need it at highway speeds.

You see, in reality, with real physics, there are real-life trade-offs. The huge problem is that only the manufactures really know the efficiency of things like the thermal clutches, alternators, and electric fans. EVERYONE else is either talking out of their rear, repeating BS, or making up stuff to make their product/decision look better.

Basically, per HP, you won't find anything that cools as well as the stock thermal clutch fan (per HP). BUT, the POS stock setup really isn't up to the job to cool a 5.0 at idle under all conditions. But, a MKVIII fan can provide full cooling at idle. It will draw more HP than the stock set would, but it would also cool more. So, we do not have a good apples-to-apples comparison. And, that's part of the problem.

Both setups are dependent on many things ehen it comes to airflow and HP draw.

IMHO, a 3G with a MKVIII fan will take zero power on the highway (the stock setup will have some draw). If you go with a DCController, in city driving and at idle, there's little loss, and much more potential gain in cooling.

FWIW: I do a lot of electrical control of mechanical stuff for a living, and power is always an issue.

 

[jrichker]

Some numbers to play with...
Electric fan steady state draw of 30 amps at 14 volts = 420 watts, or .56 HP. If 746 watts = 1 HP, then an electric fan draws less than 1 HP even when you consider the losses in generating the power to run it.

I have heard figures of 5-7 HP are required to drive a fan at cruse RPM and higher. If those figures are accurate, an electric fan could reduce the parasitic HP loss compared to a mechanical fan.

Edit: alternator efficiency figures from Alternator - Wikipedia, the free encyclopedia
Wiki says that the typical automotive alternator efficiency is in the range of 50%-62%. Using a worse case figure of 50%, 420 watts becomes 840 watts or 1.126 HP. That is still an improvement over an mechanical fan if the 5-7 HP parasitic draw figures I have are correct.

 

[Hawkeye_306]

Aside from the efficiency issues related to mechanical/electrical energy transfer, the theoretical advantage of electrical accessories is that you can store energy when your engine is under less demand, and use energy when it is under high demand. For example if you had an electric water pump, you could get your battery good and charged during your trip to the drag strip, then pull off your belt and run off the stored battery power during the ¼ mile without the accessory drag. If you had mechanical components, you could not do that (this example is only to illustrate a point – yes, some people with mechanical accessories disconnect them during the ¼ mile anyway). The point is with electrical accessories you have more control.

Ever notice that Hybrid cars have great city fuel economy, but only average highway fuel economy? It is because in the city the cars kick in stored battery power when the engine is less efficient – like when starting from a stop, and store energy when the engine is running more efficiently – like when the car is up to speed. (Yes, regeneteraive braking also plays a part). When running under more steady state conditions – like on the highway – you can`t take advantage of the efficiency tradeoffs, because they are much less significant.

If our cars ran on higher voltage systems (that could support electrical superchargers), you would see car companies replace mechanical superchargers with electrical superchargers. This is because the efficiency issues related to converting power from mechanical to electrical to mechanical are less significant when compared to the efficiency gained by having control over when you are storing power and when you are using stored power.

So are electrical accessories better in real life? I can`t answer that, I imagine it depends on the situation. But you asked for the physics based answer, not the practical answer.

 

[stang&2Birds]

I have heard figures of 5-7 HP are required to drive a fan at cruse RPM and higher. If those figures are accurate, an electric fan could reduce the parasitic HP loss compared to a mechanical fan.

5-7HP is required when the engine is over 4500 rpms. In real-life, that's a BS range to look at.

IMHO, there's no question at all that electric fans are better for highway or for quarter mile runs.

However, I deal with real-life. In real-life, city mileage often dominates. Otherwise, no one would give a rat's *ss about the hybrids. On pure highway driving, you can do better than a hybrid.

So, again, the question that *I* like to consider is for "normal/typical" city driving, which is better. From my point of view, and from all of my experience, I say that as long as the stock setup can adequately cool the Stang in question, then go with a thermal clutch setup.

Alternators are much less efficient at low speeds.

Also, as for storing energy, that's fine for the quarter mile with an alternator disconncted. But, for every street car out there, the alternator will charge as much as it can and try to keep charging the battery. Now again, we get into real life where alternators that are charging at full-tilt get hotter. As they get hotter, they get less efficient.

Doing very quick crunching of numbers is a good very rough first pass estimate. Basically, you're looking at factors of 10X or more when you start adding in a lot of factors. So, by looking at the very rough numbers, you can see that "it can be a toss up" under different conditions.

I've worked with electric motor braking (acting as an alternator). Many factors must be factored in to figure out a lot of real-life stats. Some is done by analysis, some is done by testing a number of units and adding in a safety percentage.

FWIW: I think the Stang stock cooling sucks so bad, that if you have AC and have to deal with big city bumper-to-bumper traffic in HHH weather, go with a MKVIII and a DCController. But, I also know that not everyone has to sit the the POS traffic that I have to sometimes deal with.

 

[Hawkeye_306]

I should clarify one thing – the alternator will pull harder when it is trying to charge the battery. So the earlier example of removing the belt at the ¼ mile is an analogy for the ability to control the engine-alternator power draw (depending on the situation).

BTW I have more or less agreed with what stang&2Birds has said so far. I just like theoretical arguments.

 

[85_SS_302_Coupe]

If clutch fans are superior then why does pretty much every new car come with an electric?

 

[Vipersix]

I'm no engineer so I can't compare the actual parasitic losses of each. However, it doesn't take a genius to realize that electric controlled fans are more reliable than mechanical clutch controlled fans (plus they're cheaper). Explains the move to electric fans in new cars anyway.

 

[85_SS_302_Coupe]

These guys are over analyzing this.


Like i said, it's simple. If it runs off the crank, it's taking power from the engine. Eliminate anything running from the crank and you'll put more power to the wheels.

 

[Maryland Stang]

The move to electric fans is simply explained. Most new cars are front wheel drive with the engine mounted "sideways". The difficulties involved in using a mechanical fan in these cars is difficult, complicated, expensive and involves the use of a mule drive.

In vehicles that must have a stone reliable cooling system mechanical fans are still the choice. Pop the hood of a F-350 and see what cools those radiators.

I chose to keep my factory setup simply because I believe it is reliable. I don't want to have to play around with my cooling system because the fan took a dump or the controller is acting up on a 100 degree day. I did the calculations on HP required with an electric fan a while back and there is no doubt, the electic fan uses less HP.

 

[Vipersix]

My 2007 F-250 has an electric fan.

 

[dann092lx]

when a mechanical fan clutch engages, does it do so completely (and this disconnect the fan from its power source completely) or does it do so partially?

I ask because I bet the heart of this issue, or at least a ventricle, revolves around how fine-grained the fan can be controlled. I speculate that mechanical fans are more crudely controlled than electrical, which means the gap between 'power actually consumed for cooling' and 'power actually needed for cooling' is higher for a mechanical fan. Any takers?

 

[2000xp8]

It's really very simple, take the whole belt off and you get about 25rwhp, that's no water pump, no alternator, no fan and no Power steering. It really doesn't take much power to turn the alternator, regardless of electrical load.
The PS and Water pump use up most of that 25rwhp.

I do not question that an E fan makes a car more efficient, what i question is how well it works on a car that does not have a factory E fan setup, that the computer controls.
So as for i do think it's more efficient, i also feel it's far less reliable on a car that wasn't meant to have an E fan. Same goes for electric water pumps.
Read through the last 20 pages of stangnet, there is plenty of posts about issue with E fans and controller problems.

When you calculate the cost of a good fan, controller setup or E water pump, they aren't exactly the most cost effective way for making more power for someone that doesn't have high end h/c/i parts.
There really is just better ways to make 5-10rwhp.

And even if a mechanical fan fails, which they rarely do, it will probably freewheel or lockup, either way you aren't stranded, won't work 100% right, but it still works.

Dann, they never completely lock up or release, if it completely locked up everytime you went full bore from a standstill it would likely be alot of stress on the belt, waterpump or fan shaft.