Your pet peeve, technical or otherwise.

[james_s]

I find the lag when starting up and the lurch to be obnoxiously irritating

Ah! Do you drive an automatic? Reason I ask is because both stop/start motors I drove were manual, and the act of depressing the clutch prior to selecting first (or second for the Dacia) got the motor running. So no lag and no lurch.

No, I loathe automatics, but the only cars with that stupid start/stop system I've ever driven were automatics. Rental cars are universally automatic in the USA, unless you go to one of those special exotic places and rent a sports car. I hate the way things are, but manual equipped cars are almost always a special order thing over here. Even 10 years ago when my uncle bought his BMW he had to custom order it with a manual, the dealer did not have a single one on the lot. >95% of the cars on US roads have slushboxes.

[james_s]

The articles I found (which is many, I got lots of results from Germany, too, which has similar laws) didn’t specify the test parameters. But what they do say is that cars with auto-stop use heavier-duty starters and batteries to make up for the more frequent starts, and that they won’t stop the engine until it’s warm, and that a warm restart doesn’t cause nearly as much wear and tear as a cold start.

As for fuel savings, it’s obviously highest (up to 15% according to tests) in city driving. For Switzerland as a whole (which is a lot of rural, non-freeway driving), they estimate a savings of about one tank of gas per year on average.

A tank of gas per year is negligible, not even remotely worth the annoyance, nevermind the higher costs of a heavy duty starter and battery, and shorter battery life. Having the starter motor replaced by a mechanic would easily consume a decade of fuel savings.

[coppice]

I'm told that Teslas, with their substantially larger batteries, have a "pet mode" where the AC runs off the battery while parked to keep pets at a comfortable temperature. AC is pretty power consumptive so I don't know how long that would last, but it's a great idea. I sure hope nobody uses it to rationalize leaving their kids in the car though.

If its a hot day the AC might take a couple of kW. Lets say you have happy to sacrifice 10% of your range for this. In a typical 60kWh car your pet can be left happy for 3 hours. If you leave it alone in the car longer than that you might want to rethink how you treat your pet.

[james_s]

I ran an experiment in our new Toyota Sienna Hybrid: I "turned on" the car (which is a totally silent act!) and then turned on the air conditioning. I wondered if it would instantly fire up the engine to provide rotating mechanical power, or if the drivetrain could turn the AC compressor from battery power alone. The latter turned out to be the case. The AC started up and got cool. My wife took longer than she'd promised so it ended up running for a while until the (smallish) battery ran down enough to cause the engine to start. This experiment confirmed my faith in Toyota's handling of the hybrid system.

I'm told that Teslas, with their substantially larger batteries, have a "pet mode" where the AC runs off the battery while parked to keep pets at a comfortable temperature. AC is pretty power consumptive so I don't know how long that would last, but it's a great idea. I sure hope nobody uses it to rationalize leaving their kids in the car though.

A lot of modern cars don't have an engine driven AC compressor, they have an electric compressor instead which makes this possible. Tesla does indeed have a "dog mode" and there's also a camping mode that will keep the HVAC going. You can also use the mobile app to turn on the AC (or heat) remotely so the car is comfortable by the time you get in it to go. They have ~80kWh of battery capacity so the AC could run for days if you wanted it to.

[IDEngineer]

A lot of modern cars don't have an engine driven AC compressor, they have an electric compressor instead which makes this possible.

Not sure which Toyota uses in the hybrid Sienna. But their clever transmission permits the drivetrain to be powered by the engine, the electric motor, or both simultaneously. I watched a detailed video teardown of that transmission and it's a marvel of packaging and creativity. They didn't go into the control algorithm but I bet there's a lot going on... to avoid "short circuiting" one prime mover into the other you'd need to detect lead/lag on both the engine and motor and modulate accordingly. My point is that they end up generating rotating mechanical power at the output of the transmission no matter which prime mover(s) is/are running. So they could be using either a mechanical or electrical compressor. I'll try to find out which.

Just ~1000 watts to power the AC? With the solar gain from all that glass surface area, outdoors? Impressive. A 12K BTU single-room window unit draws ~1200 watts and I'd think it would require more than that to overcome the solar gain. Seems like I heard a "automobile HP consumed" value in the mid single digits a few years ago, but maybe they were wrong. Definitely better than I expected, for sure.

[Kyle_from_somewhere]

My pet peeve is when I ask a question and I don't get told the answer, just told I am stupid.

"It's obvious"

If it was obvious then I wouldn't be asking.

[tom66]

All Toyota hybrids use electric AC compressors.  It's been this way since the first gen Prius.

AC power for these systems is entirely dependent on the temperature delta.  If it's only 10C or so (18C cabin, 28C exterior) then 1kW is probably enough - and 200W or so will go to running the interior fan and (if stopped) the exterior fan.   For bigger differences, of course power needs to go up.  The system in my car is rated somewhere around 24000 BTU, which is about what you'd need to cool an entire floor of a small house in the UK.

[IDEngineer]

All Toyota hybrids use electric AC compressors.  It's been this way since the first gen Prius.

Thanks for confirming!

AC power for these systems is entirely dependent on the temperature delta.  If it's only 10C or so (18C cabin, 28C exterior) then 1kW is probably enough - and 200W or so will go to running the interior fan and (if stopped) the exterior fan. For bigger differences, of course power needs to go up.

Interesting. I did not expect the compressor to be a variable load. I presumed it simply "ran" like a pump and temperature was controlled by mixing chilled vs. ambient air in the cabin. Most pressure pumps (which is what a compressor really is) don't run across a wide RPM range. Maybe there's a different pump design, because obviously running its electric motor over a wide range of RPM's would be trivial.

The system in my car is rated somewhere around 24000 BTU, which is about what you'd need to cool an entire floor of a small house in the UK.

So about twice a typical whole-room window AC unit. That sounds about right, given the roof area and huge solar gain from all that glass.

Thanks for the feedback!

[JohanH]

Interesting. I did not expect the compressor to be a variable load. I presumed it simply "ran" like a pump and temperature was controlled by mixing chilled vs. ambient air in the cabin. Most pressure pumps (which is what a compressor really is) don't run across a wide RPM range. Maybe there's a different pump design, because obviously running its electric motor over a wide range of RPM's would be trivial.

Many ground source heat pumps are nowadays variable. In our house, the heat pump (Nibe) has a Mitsubishi compressor with a BLDC motor that is varied between 20 rps and 120 rps depending on load.

There are multiple benefits. It is whisper quiet at light loads (most of the time), startup is easy for all involved components (no sudden current rush), temperature regulation is optimized and so is heat output (efficiency).

[tom66]

Interesting. I did not expect the compressor to be a variable load. I presumed it simply "ran" like a pump and temperature was controlled by mixing chilled vs. ambient air in the cabin. Most pressure pumps (which is what a compressor really is) don't run across a wide RPM range. Maybe there's a different pump design, because obviously running its electric motor over a wide range of RPM's would be trivial.

Yes, it's variable speed.  One consequence of this is your valves need to be electronically metered as well... not exactly sure how that works though.  A simple 'bang bang' controller as used on a regular ICE doesn't need this.  As to why they do it?  My guess is the motivation is the same as for fridges with inverters, it's slightly more efficient.  The motors are usually 3 phase AC induction, so speed control is trivial, it's just a VFD which you needed anyway. 

For EVs, the system is often reversible as a true heat pump to provide mild heat in winter (usually reinforced with a PTC air heater to get the cabin up to temperature a little quicker in very cold conditions.)

On my vehicle, the AC system runs year round to provide demisting and humidity control; you can often see it pulling just 200-300W at very light load to provide this function (you need a fraction of the power to dehumidify as you do to cool.)  I don't know if variable speed control makes this more practical, but maybe it makes it less noisy and that's one benefit especially for an otherwise virtually silent vehicle (barring the odd pump whirr I hear sometimes.)

[SeanB]

Variable apeture valves operate using a  solenoid to pull the needle away from the seat. They need feedback, mostly from having pressure transducers in the compressor high side by the accumulator after the refrigerant is compressed and cooled, pressure by the suction of the compressor, and temperature of the condenser coil, evaporator and also interior and exterior temperature. This then is used to control the flow so the maximum possible drop is possible with the flow from the compressor, but also keeping the evaporator coil just above freezing, so it does not ice up. If it detects the coil is freezing up, it will throttle back the compressor, and open the valve to a higher flow rate, so the hot refrigerant can defrost the coil.

Of course with the tiny amount of R1234YF refrigerant in the system, it will set an error code, if only 2% of the refrigerant is lost through the compressor seals. That is why you need a computer controlled charging system to get the system charged, it has to communicate with the vehicle, to use all those sensors and verify them over a wide range.

[IDEngineer]

This then is used to control the flow so the maximum possible drop is possible with the flow from the compressor, but also keeping the evaporator coil just above freezing, so it does not ice up. If it detects the coil is freezing up, it will throttle back the compressor, and open the valve to a higher flow rate, so the hot refrigerant can defrost the coil.

Very interesting. I had thought about such variables with respect to an idea I had about chilling our waterfront home using water drawn from below the lake's thermoclime. I haven't studied modern refrigeration systems in a very long time, and back when I did they were hyper-simple constant speed zero sensor systems. Sounds like they've gone the way of internal combustion and turbine engines... a basic version can be very simple but to get that last percent of efficiency adds heaping gobs of instrumention.

For my lake-chilled idea I figured I'd need to modulate flow based on temperature drop across the coil. There's no chance of coil freeze-up with liquid water refrigerant but might as well minimize the GPM flow to ease wear and power consumption.

EDIT: We just added an AC compressor to our house last year, and the parts of its installation that I didn't do personally I watched very carefully. The coil in the house has no sensors that I saw, and the compressor runs at a constant speed using a standard AC synchronous motor. The control wiring going to the outside compressor consists of ground and one signal wire, which use 24VDC to close the contactor that powers up the unit (e.g. not variable speed). Given your comments above I'm surprised that this latest-generation system isn't more instrumented... it's a LOT bigger (60K BTU) and therefore represents a better energy efficiency target than a vehicle AC system.

[tooki]

The articles I found (which is many, I got lots of results from Germany, too, which has similar laws) didn’t specify the test parameters. But what they do say is that cars with auto-stop use heavier-duty starters and batteries to make up for the more frequent starts, and that they won’t stop the engine until it’s warm, and that a warm restart doesn’t cause nearly as much wear and tear as a cold start.

As for fuel savings, it’s obviously highest (up to 15% according to tests) in city driving. For Switzerland as a whole (which is a lot of rural, non-freeway driving), they estimate a savings of about one tank of gas per year on average.

A tank of gas per year is negligible, not even remotely worth the annoyance, nevermind the higher costs of a heavy duty starter and battery, and shorter battery life. Having the starter motor replaced by a mechanic would easily consume a decade of fuel savings.

Again, there’s no evidence that the starter motors or batteries are suffering from this, and these systems have been around for a while now.

As for the increased cost of the new car: I’d be surprised the added cost exceeded $100, and it would not surprise me in the least if it was more like $10. Cars are fundamentally built down to cost to an extreme degree, so adding a bit of reinforcement likely costs very, very little.

[Bassman59]

I ran an experiment in our new Toyota Sienna Hybrid: I "turned on" the car (which is a totally silent act!) and then turned on the air conditioning. I wondered if it would instantly fire up the engine to provide rotating mechanical power, or if the drivetrain could turn the AC compressor from battery power alone. The latter turned out to be the case. The AC started up and got cool. My wife took longer than she'd promised so it ended up running for a while until the (smallish) battery ran down enough to cause the engine to start. This experiment confirmed my faith in Toyota's handling of the hybrid system.

My Honda CR-V hybrid is the same as your Sienna. Even here in the Sonoran Desert, where AC is a must, the AC runs even when the car's motor is not.

One thing i noticed is that the engine will start when you run the cabin heater, and it will continue to run until the engine reaches some standard temperature. I presume that the car uses a standard heater core fed by the coolant. Maybe a resistive heater is too complicated/power-hungry?

[IDEngineer]

One thing i noticed is that the engine will start when you run the cabin heater, and it will continue to run until the engine reaches some standard temperature. I presume that the car uses a standard heater core fed by the coolant. Maybe a resistive heater is too complicated/power-hungry?

"If you want heat, burn something."  Resistive heat is fine for small things (like TCXO's) but if you're heating larger spaces it's a poor approach. Yes, I know entire buildings are heated with electricity but that doesn't make it a good idea.

[james_s]

One thing i noticed is that the engine will start when you run the cabin heater, and it will continue to run until the engine reaches some standard temperature. I presume that the car uses a standard heater core fed by the coolant. Maybe a resistive heater is too complicated/power-hungry?

"If you want heat, burn something."  Resistive heat is fine for small things (like TCXO's) but if you're heating larger spaces it's a poor approach. Yes, I know entire buildings are heated with electricity but that doesn't make it a good idea.

Most modern EVs use a heat pump for both heating and A/C. When you already have an internal combustion engine though there is a huge supply of waste heat so it makes sense to use that to heat the cabin. Even with the engine shut off you will have heat for quite some time if you keep the coolant circulating.

[james_s]

Again, there’s no evidence that the starter motors or batteries are suffering from this, and these systems have been around for a while now.

As for the increased cost of the new car: I’d be surprised the added cost exceeded $100, and it would not surprise me in the least if it was more like $10. Cars are fundamentally built down to cost to an extreme degree, so adding a bit of reinforcement likely costs very, very little.

I only have a sample size of one which is not very significant, but I had a coworker with an SUV that had one of these systems and he said he was replacing the battery every couple of years. It's hard to say if it was due to that system, but it stands to reason that the more you use something, the faster it will wear out.

Either way the main issue for me is that they are annoying, the starting and stopping engine is jarring to me, and the ones that use a conventional starter are noisy. The fuel savings are negligible and the whole thing is political more than anything and a way to game the way emissions are measured.

[james_s]

We just added an AC compressor to our house last year, and the parts of its installation that I didn't do personally I watched very carefully. The coil in the house has no sensors that I saw, and the compressor runs at a constant speed using a standard AC synchronous motor. The control wiring going to the outside compressor consists of ground and one signal wire, which use 24VDC to close the contactor that powers up the unit (e.g. not variable speed). Given your comments above I'm surprised that this latest-generation system isn't more instrumented... it's a LOT bigger (60K BTU) and therefore represents a better energy efficiency target than a vehicle AC system.

Most of them are that simple, I think current efficiency standards have essentially mandated a TXV rather than the fixed orifice that was pretty much the norm for a long time. They're starting to use ECM (BLDC) motors for the fans in more and more of them these days too rather than the old PSC induction motors. Quite a bit of energy savings there, especially at low speeds. Really high end systems use inverter drives so the compressor is variable speed and have sensors but you don't really see that until the top end of the efficiency range and you pay a substantial premium. If you live in a really hot climate it could make sense to go with something like that but where I am there are diminishing returns on increasing A/C efficiency, I only spend about $20/mo on additional electricity during the peak summer heat with the 18 year old single stage system I installed 18 years ago.

[james_s]

Not sure which Toyota uses in the hybrid Sienna. But their clever transmission permits the drivetrain to be powered by the engine, the electric motor, or both simultaneously. I watched a detailed video teardown of that transmission and it's a marvel of packaging and creativity. They didn't go into the control algorithm but I bet there's a lot going on... to avoid "short circuiting" one prime mover into the other you'd need to detect lead/lag on both the engine and motor and modulate accordingly. My point is that they end up generating rotating mechanical power at the output of the transmission no matter which prime mover(s) is/are running. So they could be using either a mechanical or electrical compressor. I'll try to find out which.

Just ~1000 watts to power the AC? With the solar gain from all that glass surface area, outdoors? Impressive. A 12K BTU single-room window unit draws ~1200 watts and I'd think it would require more than that to overcome the solar gain. Seems like I heard a "automobile HP consumed" value in the mid single digits a few years ago, but maybe they were wrong. Definitely better than I expected, for sure.

The drivetrain yes, but an engine driven A/C compressor is on the accessory end of the engine so it won't rotate unless the crankshaft is rotating. Technically I suppose it would be possible to mount the compressor on the transmission instead but I've never seen it.

It's hard to find specs on this but I recall being told once that a typical automotive A/C system is about 2 tons, or 24k BTU/hr. It takes about 1HP per ton of refrigeration capacity to drive the compressor, so an electrically powered system with a hermetic compressor draws about 1kW per ton. With an engine driven compressor you have belt losses which for a V-belt can be pretty significant and I'm sure the efficiency is not helped by the fact that engine speed (thus compressor speed) varies without any relation to needed cooling capacity.

[IDEngineer]

They're starting to use ECM (BLDC) motors for the fans in more and more of them these days too rather than the old PSC induction motors. Quite a bit of energy savings there, especially at low speeds.

Is that residential, or commercial? Really big systems for commercial installations could justify the additional expense but I wonder if it would be worth it for a single family residence.

[james_s]

All Toyota hybrids use electric AC compressors.  It's been this way since the first gen Prius.

AC power for these systems is entirely dependent on the temperature delta.  If it's only 10C or so (18C cabin, 28C exterior) then 1kW is probably enough - and 200W or so will go to running the interior fan and (if stopped) the exterior fan.   For bigger differences, of course power needs to go up.  The system in my car is rated somewhere around 24000 BTU, which is about what you'd need to cool an entire floor of a small house in the UK.

IIRC it's not the temperature delta but the condenser temperature. The pressure in the condenser coil is directly related to the temperature of the refrigerant within it and the mechanical load on the compressor is directly related to the pressure of the refrigerant it is compressing. The higher the condenser temperature, the more mechanical energy is required to drive the compressor. The temperature of the evaporator will be fairly constant, although the more heat it absorbs from the air, the more refrigerant needs to be admitted into it by the TXV. With my home AC the measured power consumption is certainly dependent on outdoor temperature, though the indoor temperature doesn't vary much.

[IDEngineer]

I'm sure the efficiency is not helped by the fact that engine speed (thus compressor speed) varies without any relation to needed cooling capacity.

I've often wondered how AC compressors handle the varying input RPM's, which can have a 10:1 dynamic range in a gasoline engine (diesels are about 5:1 or 6:1 max). This has been a problem since AC was first introduced in cars back in the, what, 70's? Back then it would have had to be a purely mechanical solution. I wonder if they just put a pressure regulator on the output and short-circuit the unnecessary flow back to tank like most fuel systems. To get adequate cooling while idling means the compressor would be massively oversized given that typical engine operation at cruising speeds is ~2000 RPM. Typical idles are about 1/3rd of that.

[IDEngineer]

With my home AC the measured power consumption is certainly dependent on outdoor temperature, though the indoor temperature doesn't vary much.

You'd expect that regardless of the design, since the system is being asked to remove excess heat and a higher outdoor temperature means a greater delta to your constant indoor temperature.

[james_s]

They're starting to use ECM (BLDC) motors for the fans in more and more of them these days too rather than the old PSC induction motors. Quite a bit of energy savings there, especially at low speeds.

Is that residential, or commercial? Really big systems for commercial installations could justify the additional expense but I wonder if it would be worth it for a single family residence.

Residential. I have put in a couple of forced air furnaces recently that had ECM blowers, virtually all of the mid to high efficiency units already have those and the efficiency requirements keep getting higher. I've put in a couple of those mini split heat pumps that are inverter drive and super high efficiency, something like 30 SEER. A friend of mine had a larger Mitsubushi split system installed in his house last year that is inverter drive. I looked at them when I did the install at my brother's house but the variable speed AC would have been more than double the cost and in the climate here it would probably never pay back, we only need A/C for a month or so out of the year.

[Bassman59]

Again, there’s no evidence that the starter motors or batteries are suffering from this, and these systems have been around for a while now.

As for the increased cost of the new car: I’d be surprised the added cost exceeded $100, and it would not surprise me in the least if it was more like $10. Cars are fundamentally built down to cost to an extreme degree, so adding a bit of reinforcement likely costs very, very little.

I only have a sample size of one which is not very significant, but I had a coworker with an SUV that had one of these systems and he said he was replacing the battery every couple of years. It's hard to say if it was due to that system, but it stands to reason that the more you use something, the faster it will wear out.

Either way the main issue for me is that they are annoying, the starting and stopping engine is jarring to me, and the ones that use a conventional starter are noisy. The fuel savings are negligible and the whole thing is political more than anything and a way to game the way emissions are measured.

I can barely hear (or feel) when the engine on my hybrid CR-V turns on. It's remarkable. Now I suppose that's not fair to compare the hybrid system with a standard gasoline-only car, as there might be a slight lag while the gas motor starts up in the latter. In the hybrid, the batteries can move the vehicle until the engine starts and the transition between the two is seamless.

As for battery life, well, I live in the Sonoran Desert, and replacing car batteries every two years is just standard.