You’re describing a receiver*. But either way, once the system is shut down all that’s going to happen is that the pressure on the two sides will equalize** after a little while, but without anything to push it through the orifice (or TXV if it has one), you’re not going to get a significant enough pressure drop to for the liquid to turn to vapor. It would be like taking a bottle of windex and squeezing the trigger really slowly. If it doesn’t come out as a mist, it’s not going to work.
*An accumulator comes after the evaporator and separates out any remaining liquid so only vapor goes to the compressor.
**side note, while the pressure will equalize, refrigerant does have a tendency to migrate to the coldest location of the system, but that’s not really going to come into play here.
Ah. Well, scratch that, guess you’d just have to go to an electric compressor. Was reading up on the way the Prius does it as I was thinking of buying one. 3 phase 200 volt variable frequency AC, lol.
So this gives me another idea. Wouldn’t the most efficient thing to do be to close the expansion valve? High end A/Cs have a servo motor there and a multi-setting valve. If you close it, the liquid stays compressed, and you save a little energy and time on restart. Then re-open when you start the compressor again - reopen first, check a sensor to make sure the restart happened, then start the compressor motor…
It’s not as big a gas saving feature, but it is a pretty good one for the EPA testing as there is a long period of idling in the test (something like 1/3 of the test IIRC), so it’s a good way to get your MPG up that means not much in real world conditions.
As for the battery, it should not have problems as it should happen near the fully charged state which is fine for car batteries. However it may produce excess heat which can be damaging. A slightly larger battery should easy this significantly. As as mentioned vibration and deep discharging is what kills a battery, but also heat.
I saw that 5 to 10 percent figure, too, and I just don’t believe it. It also doesn’t seem to jibe with your fuel usage figures at idle. If a car only uses 0.16 to 0.39 gallons per hour at idle, how much idling would you have to do to increase your MPG by 5-10 percent? Someone better at math can check my calculations, but raising your city MPG from 25 to 27 (8 percent increase) would mean the car is idling for three to six hours during a 15-gallon tank of gas, over 375 miles. I live in the country and don’t do much true city driving, but that sounds like a tremendous amount of time idling at traffic lights for each tank of gas.
I’m guessing you are right. It’s a way to show a higher city figure for CAFE regs that doesn’t translate into real-world savings. The EPA city cycle is at idle 18 percent of the time. I don’t see any way that translates to 5 to 10 percent increase in MPG.
I just looked at a test paper prepared in 2012. It indicates that US and European government testing shows 0.5 to 14.4 percent fuel efficiency increases using three different vehicles in three different test simulations, but actual on-road tests show -0.7 to 2.0 percent fuel usage change (it actually went down for one of the vehicles). Complex, ineffective and costly hardware to satisfy government regulations that doesn’t translate into real-world benefit, looks like to me.
I’ve never seen a servo controlled TXV, I’ll have to read up on that. But the issue I see with what you’re proposing (I swear, I’m not looking to shoot down ideas) is that assuming a regular servo controlled TXV works like a regular TXV in that it takes it reads the temperature of the suction line, you’d now have to send that reading, as well as the control for the TXV through the PCM.
But ignoring that, I’m not sure I follow what improvement you’ll get if you let out a little poof of pressure into the evap coil, then start up the compressor. Keeping in mind, you had to pay to build that pressure up first.
You’d do better to get under your hood ever few months and make sure the condenser (and radiator) is good and clean and free of leaves and bugs so air can flow through it.
If you want to see something interesting with the Prius. When you’re not driving it, it pumps the coolant into an insulated flask to keep it hot. That way the next time you get use it, it can get the engine up to operating temperature much faster by filling the engine with still warm coolant.
In my Escape the air and heat keep running. And the radio and lights. If the load is too heavy it doesn’t shut off at all, or it comes back on before you start moving.
Really I hardly notice it anymore. I only notice it when I’m at a drive-thru window and have the car window open and the radio off. Then I can hear the restart.
Servo controlled TVXes are the key component of variable stage A/Cs, are they not? As I understand it, with a mini split or other common variable setting A/C, the servo valve has discrete notches, about 3-5 of them. The motor driving it counts steps or uses a reference.
So there’s a target delta T on the evaporator coil. There’s air temperature sensors on the intake and exit. You adjust the servo valve up and down to maintain that dT. Then, at a given rate of gas flow, you need a given amount of compression cycles. I’m not sure what sensors you use, but this is where you need inverter drive, so the compressor can run at the speed needed to compress enough gas to keep up with the setting on the expansion valve. What makes this tricky is that hotter gas needs more compression cycles, and I don’t know how you detect if you are falling behind and the gas pressure is rising. Maybe there’s a gas pressure sensor in these high end A/Cs.
Then given a certain air temperature, there’s an optimum dT across the condensor coils. Too low a dT, and you are running the outside fan too hard and wasting energy. Too high a dT, and you are making the compressor do more labor to make the condensor coils hotter.
So it all comes together, with 3 inverters and 2 large circuit boards full of parts, and you can get energy efficiencies double what the conventional systems give. For double the equipment cost, and if anything serious fails in a mini split, you replace it…
Automatic transmission? That should have down-shifted when you slowed & stopped, and will shift back up through the gears as you start & speed up. Really no change.
And the auto shutoff/restart feature is computer controlled, and is temperature sensitive. A friend with a Prius several years ago said that it didn’t shut off the IC engine during Minnesota winters – too hard to restart. I think on the newer Priuses, this is controlled by engine temp rather than ambient temp – once the engine is warmed up, it will restart easily, so it can do the shutoff/restart cycles. Possibly something similar applies down South in their hot months.
I think it’s been mentioned more than once that the stop/start is deactivated in a host of situations. For BMW’s,
Reasons why the stop/start may not shut down the engine include:
The car is coasting (manual transmission)
Brake vacuum reading is too low (manual transmission)
Insufficient brake pedal pressure is being applied to hold the vehicle stationary (automatic transmission)
Stopping on a hill/downhill with a 12% or greater gradient
If the steering angle is more then 6 degrees
If the steering wheel is being moved after approximately one second of coming to a stop
The system has not yet be activated by going over a speed of 5mph for automatics, 3mph for manuals since the last engine switch off
If the gas pedal is being pressed (engine must be at idle speed only)
You’re coming to a stop from reverse
The driver gets out of the car
If Hill Decent Control (HDC) is activated
If the engine is not up to temperature
If the carbon canister is being purged
if the grade of the fuel is too low for the engine
Transmission adaptation is active (automatic transmission)
The hydraulic pressure accumulator is not yet up to pressure (automatic transmission)
Stop-and-go traffic - system is only activated by driving over 5mph for automatics, 3mph for manuals
Batter charge is low
Outside temperature is colder then 37.4 F / 3 C
Outside temperature above 95 F / 35 C and the air conditioning system is on
Fogging of the windshield is detected by the IHKA system
Climate control system is on but passenger compartment has not yet reached desired temperature
ABS system was used when coming to a stop
Alternatively the car may automatically restart outside the standard restart procedure if:
The driver is moving the steering wheel
The driver presses the gas pedal
The transmission was shifted from D to P and subsequently back to D
The transmission is shifted to N or R
The battery charge runs low
Outside temperature rises above 95 F / 35 C and the air conditioning is on
Fogging of the windshield is detected by the IKHA system
Low evaporator temperature is detected
From a drivers’ forum, and as it says not necessarily comprehensive. Mine starts back up (to run the AC) after awhile at stop lights apparently based on cabin temperature rising more than a certain amount above the set point, not just on it being more than 95F outside, but only with the car in ‘Comfort’ mode. Etc.
Not just for start/stop but all kinds of things cars do now you have to rely on complex control software working, but it almost always does IME. I used to drive a VW Bug way back. I understood and could fix every system in the car with simple tools and written specs. No more. But I like current cars. I don’t shout at them to ‘get off my lawn!’
25.1>27.5mpg in BMW 3 series over a 77 mile 3 hour course.
BMW claims ‘an mpg or two’. I haven’t tested it on ours, very similar car to the one in the test. We only get slightly more than the car’s 22mpg city rating around town, lots of stoplights and traffic jams, though I wouldn’t have thought our avg speed including stops is much below 25mph. IOW the absolute mpg’s that guy got seem high. But we live in a different traffic landscape than So Cal so maybe. Just saying 25 or 27mpg with enough spotlights to make the avg speed only around 25mph sounds like quite skilled mpg’ing in that car, IME.
I see no reason to believe it’s negligible in that particular car though. And again I like zero engine noise when stopped. The 2L 4 cyl in that car is a little on the rough side idling which is another factor (though I like the engine overall, 300hp with the aftermarket tuning software, amazing kick for its size, but up to 40 mpg highway in warm weather under my foot).
Well, not exactly. In a manual it is releasing the clutch (after putting the car in neutral) that stops the engine. If I come to a stop and put the car in first gear (clutch engaged), the engine remains on. Personally, this is what I typically do, unless I come to a stop light where I know it is going to take a while.
Orwell, I agree that the conflict between the low amount of fuel used at idle and the seemingly good gains in mileage with stop/start is confusing. I too was surprised at the results of the tests, and I wonder if the tests just weren’t rigorous enough. One circuit with the feature off, and one with it on just doesn’t seem like enough. There could be more traffic, different temperatures, even drivers driving differently knowing the feature was on or off, all sorts of things to throw off the results. I’ll be curious to see more real world results.
Yes, jz78817 is correct, I had never heard of the Bolt until this thread (only the Volt) and had to look it up, it would appear that when I got back to this thread I typed Volt instead of Bolt.
I don’t think the result in link I gave is that shocking, except as I mentioned the absolute mpg’s are pretty high for that car in both cases, IME with almost the same car. The course was roughly 77 miles in 3 hours, 26mph including time stopped. Say you average 35mph between stops. Then it’s 0.8 hrs stopped. Edmunds got 25.1 mpg with system turned off and 27.5mpg with it turned on, 77/25.1-77/27.5=.26 gallon savings. That implies a consumption of .26/.8=.33 gal/hr idling at a stop, within the range earlier quoted. And those numbers are actually highly variable for a given car, can vary by a factor of 2 by A/C load.
I would grant the savings would vary a lot depending on what ‘real world’ is to a given driver. Cruising from where my daughter lived in a rural house to the two traffic light (OK a stop sign too ) town and back? OK 9% is unbelievable. Where we do a lot of our local driving, being stopped 25% of time at lights or in jams is unfortunately not that unusual, and an answer of an mpg or two doesn’t seem that strange.
The other thing to consider among such systems is that the one in BMW’s recovers braking energy to charge the battery. I suppose other current ones do too. But it’s possible earlier systems from results years ago simply turned the engine off then restarted it a lot more often using battery energy the engine had to replenish by driving the alternator. In the newer systems some or most of the extra electrical energy used for the extra starts would be thrown away as heat in the brakes otherwise.
I drove a friend’s 3-series with the feature and it seemed ok. He had a manual transmission, though, which may have ameliorated any defects (no creep mode, for instance). I didn’t even notice the feature right away, though you could feel a small clunk as the engine restarted.
Here are the EPA test cycles. the most relevant one is probably the second one on that page, the urban driving cycle. There are indeed substantial idle periods in that test. Improving your window-sticker MPG is nice for marketing purposes, but I’ll wager that emissions reduction is at least as important a goal for manufacturers. If stop-start technology lets your car pass EPA certification with a smaller cat, less EGR, less rigorous engineering, etc., then you reduce the per-unit manufacturing cost, which is at least as big a win as being able to advertise a slightly higher MPG. Manufacturers sell hundreds of thousands of each model, so they scrutinize vehicle manufacturing costs down to the penny.
