# Why don't vehicles have electric water pumps and A/C units?

I believe it’s still the case that almost all production vehicles use a belt to run the water pump and A/C unit off the motor. I remember driving an under-powered 4-cylinder car once that told you the instantaneous gas mileage, and when I turned on the A/C the mileage dropped 3 mpg from the extra work the engine had to do to drive the compressor, but why do they still use this system?

The alternator is generating plenty of electricity to drive all kinds of devices. There must be a very good reason not to have an electric water pump or A/C compressor instead of one driven by a belt, which can sometimes fail catastrophically.

If you’re a car company striving to increase gas mileage why not make the switch?

Because if you have a motor -> DC -> motor system the losses increase. Good old laws of thermodynamics. You lose energy at each conversion.

When you’d switch on the AC the mpg will drop 4 mpg instead of by 3.

Also to power more stuff the alternator will have to be much bigger. Which means that you’re spinning more weight even when the AC is off. So the mpg goes down all the time.

It wouldn’t increase gas mileage at all, the inherent loss in energy transfer would probably make it less fuel efficient. Power off the alternator isn’t free, it adds resistance equal to it’s energy demand the same as a belt driven compressor or water pump. Want to charge your phone? It’s actually cheaper to charge it in your house than it is to charge it in your car.

There ain’t no such thing as a free lunch - or free energy.

Increased electrical demand will make the alternator harder to turn, again increasing the load on the engine.

I am not an auto-engineer by any stretch, but basic thermodynamics tells me that having an electric pump/compressor running off the alternator, which runs off the engine, is not likely to be more efficient (for the same output) than one running off the engine directly. To put it another way, the electricity generated by the alternator is not ‘free’ - any load on the alternator will increase the load on the engine. It’s just that for most electrical accessories (e.g. radio, lights) it’s not noticeable. If you drove around with your window demisting element on all the time (the biggest single item I can think of in terms of likely electrical load) you may well notice an impact on gas mileage.

ETA: a rare triple ninja!

Another issue would be the cost and weight of the electric motors needed to drive the water pump and air conditioner.

Driving these from the engine requires only a belt and a pulley (in the case of the AC compressor, the pulley incorporates a clutch).

Googling suggests a vehicle AC compressor requires about 5 hp. A 12v / 5hp electric motor would be notably large, heavy and expensive.

“notably large/heavy/expensive” is an understatement. Here’s a 5-horsepower motor that runs on 230 volts; it weighs 50+ pounds. If you’re gonna run a 5-horse motor on 12 volts, you’ll need to move about 300 amps of current through it. Giant windings and giant power connections, and a big fat solenoid contacter like the one on the starter motor. and an alternator that’s comparably oversized so as to feed this monster. Big bucks to make this happen. A pulley and V-belt (or poly-ribbed belt) is much cheaper, and probably also more efficient.

Hybrids and pure-EVs have less of an issue with this (and of course they don’t have a constant source of mechanical power). The propulsion battery on these vehicles has high power output capability and is typically very high voltage. Example, the Tesla Model S main battery is 375 volts; if you want to run a 5-hp AC compressor from this, you only need to draw about 10 amps, which you can supply through modest 14-gauge wires.

Interesting comment about a Tesla. I figured you could just increase the alternator size and there would be enough power to run a water pump and an A/C, but as has been said many times, “there’s no such thing as a free lunch”, and running directly off the engine is about as efficient as you can make things. I withdraw the question.

I understand you were just being polite, but no need to withdraw anything! I knew the general parameters of the answer beforehand, but it was still fun and educational to see the answers.

The Straight Dope. Rarely disappoints.

Depends greatly on how you design the motor how much it weights. https://chargedevs.com/newswire/elon-musk-cooling-not-power-to-weight-ratio-is-the-challenge-with-ac-induction-motors/

In reality electric A/C compressors exist and weigh not a great deal more than conventional compressors. The magic lives in modern motor design and power inverters. http://www.greencarcongress.com/2012/02/denso-20120226.html Don’t run the motors on 12v.

OTOH, there is no way I would have an electrically driven water pump. Failure of drive there kills the engine. Don’t mess with trivially simple proven mechanisms when you don’t need to.

There’s not much reason for an electric water pump – you generally want it running 100% of the time when the engine is running anyway, so mechanically tying it to engine rotation makes sense. It’s more efficient to drive it directly than going mechanical -> electrical -> mechanical. It’s one thing you really, really don’t want to fail. And packaging isn’t an issue. The only exceptions I’m aware of are auxiliary pumps for turbo cooling (so they can run after the engine shuts off), and racing applications where packaging is an issue, or where any source of parasitic loss is on the chopping block.

As for electric A/C compressors, those exist on electric cars obviously, as well as plug in hybrids. E.g., my Ford C-Max Energi has an electric A/C compressor, and it’s amazing. Chevy Volt, BMW i3, etc. Since more cars are shutting down the engine at stoplights, it’d be nice if this tech would trickle down to non-hybrids. I’ve had rental cars where you stop at a stoplight and start sweating 30 seconds later because the vents start blowing hot air at you.

ISTR that years ago the Toyota Prius used (and may still use) an electric coolant pump. This conferred two benefits:

• the electric motor was magnetically coupled to the impeller. There was no mechanical shaft connecting these parts, and so there was no seal that could wear out.

-Coolant flow rate could be tailored to actual cooling need, minimizing pump power requirement. Contrast this with a belt-driven pump, which has to deliver enough coolant flow at low RPM/high-load conditions, and consequently delivers a wretched excess of coolant at high RPM/low-load conditions, wasting mechanical power.

At some point in the 70s-80s cars went from having a cooling fan driven by the engine to draw air across the radiator to electric 12V fans turned on and off by a bimetal temperature switch to modern ones that are toggled by the computer and can continue to move air over the rad even after the engine is shut down, to take care of heat soaking problems.

There were downsides to the old engine driven fans in stop and go traffic. The fan would never keep up with the cooling needs on a hot day because the fan’s speed and resulting airflow was related to the RPM of the engine and idle RPMs didn’t move enough heat away.

So in that case a switch from mechanical to electric was a big win.

We tend not to appreciate how powerful our car air conditioners are. Cars aren’t very well insulated, and receive a lot of heat from sunlight, and car owners today expect them to cool down from 120F+ to 75F in a few minutes. So even a small car has a >10,000 BTU air conditioner. You may have seen pictures of “redneck engineering” with window air conditioners mounted on cars, but you would actually need 2 or 3 window AC units to match the AC that was originally in that car.

The Prius has 2 water pumps (and coolant systems), the pump for the power inverter is electric but the water pump for the ICE is still mechanical.

Scratch above, the 3rd gen Prius moved to an electric engine pump. Sorry for the wrong info.

I had a motor home with a belt driven fan. On a hot day crossing highway 17 from Santa Cruz to San Jose the traffic was stop and go. When the speed was high enough for the engine to shift to a higher gear the engine would begin to heat up. I would manually down shift to low range and the temp would drop. With an electric fan that would not be necessary.

They do. pretty much every hybrid/plug-in hybrid has an electric A/C compressor. EVs do as well for obvious reasons. they’re pretty much limited to hybrids and EVs at this point because they’re powered by the high-voltage propulsion battery system; I forget how much horsepower it takes to run them but I’m all but certain they require too much to feasibly power one from the 12 volt system.

electric water pumps are rarer, but they do exist. BMW has used them, and a couple of upcoming GM engines will have them. Their advantage is that the speed can be independent of engine RPM, so you don’t have to waste power pumping water at a high rate when cooling needs are lower.

Yeah the Prius has done this since 2010. It’s an efficient practical vehicle optimized for most people’s actual transportation needs. But many people believe that having more horsepower or driving something else makes them more manly or whatever.