Another physicist weighs in on using the AC to slow the car (and I disagree with Tom and Ray)

The Show
1

I just heard Tom and Ray say that using the AC to slow the car probably won’t work due to the way car engine idle speeds are regulated today. I use it in my 1993 Civic and my experience says otherwise.

There are a number of reasons to use this practice YEAR ROUND. Read on for more info…

I recall my father using this trick on our 1971 Ford wagon as we descended Pike’s Peak in CO and, of course, it worked well at that time. I started doing it in my first car with working AC (that had not lost it’s charge by the time I bought it!) which is my present car, a 1993 Honda Civic VX hatchback that I purchased in 2001.

I can report that it works well to slow the car - slightly but noticeably deceleration occurs .

The other benefits of this practice: 1) less wear and tear on brakes; 2) when done in summer, you get free AC (I call this “poor man’s regenerative braking”); 3) when done in winter, this exercises the AC system, which is VERY IMPORTANT if you want the AC coolant NOT to leak out of the system over time (see below - I will explain how this works), and; 4) this practice can keep the car from getting away from you and lessen the likelihood of brakes overheating on steep hills.

I have heard one reason why this may not be a good idea - all that on/off cycling of the AC clutch might wear it out sooner. This does not seem to be an issue: the AC clutch will cycle every 20 to 30 seconds anyway so I am not giving it that much more of a workout than it would have had under normal usage. In my case, the car has 266K on it and the AC has never had a repair - not even a recharge after 18 years!

Which gets me back to the point above about exercising the AC system regularly to keep it working properly: your owner’s manual will tell you that you should run the AC for at least 10 minutes a week to keep it working- specifically to keep the coolant from leaking out. What happens is that the shaft seal (where the mechanical drive is attached to the flywheel pulley) will dry out if you do not circulate the coolant (once a week or so for 10 minutes), which is mixed with lubricating oil to keep the shaft seal flexible. It’s really nice to have to your AC working in the spring and not have an expensive repair bill.

(An aside, you will never have this problem with an old window AC, refrigerator, freezer, etc because their refrigerant systems are completely sealed with no moving parts - so you can store them for 20 years and then plug them in and they still work fine).

Since I am also a geeky physicist who hates to waste energy (my occupation), I will simply run my AC when descending hills (there are several on my regular commutes) during BOTH summer and winter: it is free all the time, I get AC in summer and I preserve the life of the system in winter.

One more thing: the engine in my Civic is small - so it really is slow to accelerate when the AC is on. Therefore, in summer, after slowing the car with the AC and often sitting at the light or stop sign with it on, I will turn it off to accelerate again and then, when I reach speed, if it is hot enough, I will turn the AC back on again as needed. This practice not only makes the car’s acceleration much better but it also increases gas mileage (slightly) while hardly affecting AC effectiveness.

So try this in your car and see if it slows your car down at all.

And even if it does not slow the car, you are still extending your AC life.

2

In addition to the above, the ECM will cut fuel if the RPM is above a threshold and the throttle position sensor indicates the closed position. The Idle Air Control valve may or may not be incremented open with the A/C ‘ON’ while the engine is in decceleration mode but that should have little effect.

3

“due to the way car engine idle speeds are regulated today”

“my 1993 Civic”

4

“due to the way car engine idle speeds are regulated today”

“my 1993 Civic”

Yup!
That is…shall we say…just a bit of an inconsistency.
Does the OP really consider his 18 year old economy car to be the latest in technology?

5

On my Toyota Yaris, the idle speed does kick up slightly when the AC is on. I’m pretty sure that in deceleration mode, the throttle or idle air bypass closes completely so that the catylitic converter doesn’t get cooled off by the air being pumped through the engine when the fuel is cut off.
With the AC off, I can feel the car come out of DFCO at about 18 mph in fifth gear. With the AC on, it’s about 24 mph.
When the car goes off of fuel cut off, I can feel a sudden decrease in engine braking, probably due to the throttle opening to the idle position from completely shut off as the engine reverts to a regular idle. When I feel that surge, I wait for it to slow down a couple of more mph and then I can float the transmission out of fifth gear without using the clutch as I coast the rest of the way to the intersection.

I frequently turn the AC on for extra engine braking and ‘free’ air conditioning when I coast down to an intersection or when going down a grade that demands some braking to keep a safe speed. I can’t prove it has any benefits but I don’t see how it can hurt.

6

With my 1998 Saab 900, the drag of the A/C compressor is very noticeable. The difference it makes may be equivalent to descending a hill in 5th gear or 4th gear at the same speed with the same throttle position. In fact, according to the car’s owner manual, at high load conditions (more than 85% throttle opening) the A/C is temporarily disabled to transfer all the power to the wheels. So, it is clear that the A/C provides enough drag to help with braking. Also, most advice on increasing gas mileage or reducing CO2 emissions include minimizing A/C usage. But the question still remains whether the caller’s husband’s practice of manually controlling the A/C saves fuel for the same cabin temperature and avarage speed over the same terrain. I think it does.

The long-term-average cabin temperature depends on the duty cycle of the A/C; so, that is not changing. If the A/C is controlled in such a way that the descents and decelerations coincide more with the “ON” portion of the duty cycle, and ascents and accelerations coincide more with the “OFF” portion, then a given average cabin temperature is achieved with less heat being dissipated at the brake pads. That additional heat would ultimately have had to be generated by burning fuel.

There is a corollary question: In my car the temperature control system does not include a thermostat for cabin temperature. Instead, during A/C operation, heated air is mixed with cooled air. The driver adjusts the heater knob to whichever setting achieves the desired cabin temperature with the internally-set duty cycle of the A/C (I think that this type of control is found in modern cars, too). In this case, even if no braking is involved, can fuel be saved by shortening the A/C’s duty cycle by setting the temperature knob to the coldest position and switching the A/C on and off manually as needed? I think so, but the down side is that the temperature variations are generally greater.

7

On my 1996 Honda Civic four door, whenever I turn on the a/c the engine cranks down a gear and has to work harder. Turn it off, and vrrroom it goes. I’m not sure about this as a method for braking, but there’s no question there’s a difference in engine power with the a/c on and off.

8

I drive a 2008 ford focus with a manual transmission, and when going down hill yesterday there was a cop behind me so i didn’t have my foot on the gas but continued to speed up anyways, I turned on the ac and started slowing down. not sure about another cars, but it definitely worked for me, though i might be biased, I am also a physicist.

9

Mine is a small car too, 1993 escort. All this time I’ve been regenerative slowing down, I thought I was the only nut in the world. Just shows, all the nuts must gravitate towards the Car Talk community!
Seriously, like you others mention, it really does make sense, if you need to slow down, might as well use the [10nm X 3000 rpm] POWER to slow down vehicle instead of turning it into brake system heat.
What the guys seemed to gloss over is the idle speed re-set is irrelevant at higher RPM of engine braking. The A/C re-set idle speed may be 1100 RPM, but for engine braking I get 2000 - 3000 RPM because you should be using the shifter to force the transmission into LOW.
By sequentially pulling the shifter all the way to LOW, I keep off brakes, spin up the A/C, and slow down to around 25 MPH!
Ah, you will want maximum airflow setting so the A/C clutch will have less tendency to cycle off on low suction pressure. Meaning, if the A/C isn’t cooling enough air, it will shut off to avoid freezing the coil. I can see the difference because I have wired the A/C indicator light to go off when the A/C clutch does.
In conclusion, whenever you slow down, just put A/C fan on high and then start downshifting. The rest of time drive around with the A/C fan set to anything else. DON’T TURN IT OFF AND PISS OFF THE WIFE AND KIDS YOU’LL BE A NUT THEN!

10

I even have a name for this. I call it “Breezing to a stop”. The caller’s question wasn’t about idling, it was in regards to slowing down. I thought all cars since the late 80’s have a fuel bypass solenoid that opens on deceleration. I drive with a ScanGauge II and see when my fuel consumption drops to zero when I take my foot off the gas. If I want to get a nice “free” blast of cold air and also slow down faster, I’ll crank up the AC as I coast down but then turn it back off before the fuel pressure kicks back in.
Of course I usually only indulge my inner cheapskate when my wife’s not in the car. And um hey my Dad’s a retired physicist.

11

Of course turning on the A/C slows the car and saves fuel. The A/C compressor puts a load on the engine which no one should doubt. When slowing down the throttle is generally closed so the ECU does NOT increase idle speed. However so little time is spend with the car in this condition so if this is the only time the A/C is engaged it would not keep the car very cool. It is a dumb way to save fuel. It won’t save fuel over not running the A/C at all so why bother to run the A/C so little in the first place? Unless you can prove the interior temperature is enough lower to “feel” different to the occupants doing this rather than running the vent alone is silly. If it keeps the car ~2°C cooler no one would “feel” any cooler so subjectively feeling cooler is more important than any small measurable temperature difference. Correct idea in theory and practice but also irrelevant in real world driving.

12

“my 1993 Civic”

My 1985 Accord SEi had DFCO.

@Jthorn65, don’t knock it until you’ve tried it!

13

I have a 1998 and 2003 year cars and four cylinder motors… Both benifit from cutting the ac off ( but not the fan ) when accelerating. And both slow quicker when the ac is on… Just a idea for the lady caller. Use the ac in recirc mode and she may notice as I have that the car will remain cooler longer without the ac compressor running verses the outside are being cooled.

I plan on getting a new car, four cylinder, three pedels and I hope a simple two button radio. If you hear of one please give a yell as I have trouble as I have big trouble with email and tech in general!!

Bob Fallin

14

Pressing the A/C whilst decellerating uses the kinetic energy of the car to run the A/C and provide some cooling for free (no fuel used). As to whether its worth it is another question. I found a blast of cold air followed by nothing can feel worse than keeping the A/C off in the first place.

15

IMHO, “Physicist/Mathematician” deserves an apology from Tom and Ray for pooh-poohing his attempt to save energy. It’s hard enough already to be a physicist/mathematician without having people make fun of good ideas. And if, as T &R suggested, it doesn’t work in some cars because the engine computer is programmed to give it more gas when the AC is turned on, even if the brakes are on, that’s incompetence on the part of the programmer, not a defect of "P/M"s idea.
The basic idea here is that it takes energy to cool air, so the cool air represents a potential energy-storage reservoir, just as the gas tank or a lithium-ion battery are. Except the air is free and the others are pricey. Using the kinetic energy of the car to charge this reservoir instead of dissipating it in the brakes is a great idea. It’s true that its practical utility is limited by the whiny-wife syndrome, but if discussing it would encourage some car manufacturers to program their cars to cycle on the AC when they’re braking, that would be a positive thing.

16

"And if, as T &R suggested, it doesn’t work in some cars because the engine computer is programmed to give it more gas when the AC is turned on, even if the brakes are on, that’s incompetence on the part of the programmer, not a defect of “P/M"s idea.”

So if the physicist doesn’t understand the 19th century technology he’s working with, it’s the programmer’s fault?

“The basic idea here is that it takes energy to cool air, so the cool air represents a potential energy-storage reservoir, just as the gas tank or a lithium-ion battery are.”

I look forward to your explanation of how cooling air stores energy and how you propose to recover it.

Seriously?

17

Technically I should have said “free energy”, but I didn’t want to assume the reader knows about thermodynamics. In thermodynamics one learns that it requires mechanical energy to pump heat from a cold reservoir to a warm one, and that this energy can be recovered (completely, if the processes are reversible) by a heat engine. But my point can be understood much more intuitively – having cold air is equivalent to having energy (in the form of gas, say), because it makes it unnecessary to burn that gas to run the AC to cool the air.

18

Ah, thermodynamics! Why didn’t you just say so?

19

"And if, as T &R suggested, it doesn’t work in some cars because the engine computer is programmed to give it more gas when the AC is turned on, even if the brakes are on, that’s incompetence on the part of the programmer, not a defect of “P/M"s idea.”

You have several flaws in your logic…

First off…it CAN’T be any incompetence on part of the programmer…The programmer will design a system to work with the system he’s given. If you turn your AC on while going down hill IF there isn’t any increased gas your engine will stall. So how is that the programmers fault??? Usually people with a Physics or Math background have some foundation in computers and how to program them.

“Which gets me back to the point above about exercising the AC system regularly to keep it working properly: your owner’s manual will tell you that you should run the AC for at least 10 minutes a week to keep it working- specifically to keep the coolant from leaking out.”

I suggest you re-read your owners manual…probably says once a month…like every vehicle I’ve owned in the past 30 years that had AC…this included a few Honda’s.