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Cruise Control Decel Function. - READ ONLY

66 messages,  Last post on Feb 14, 2007 at 9:43 AM

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What is this discussion about? Car Safety

Messages Page 5 of 7

#37 of 66 defogging by oldharry

Feb 08, 2007 (7:01 pm)

Willard West wrote:
 
"The surface temperature of the A/C cooling evaporator will never be driven below ~34F by design. To do so would result in freezing the condensate and eventually blocking all system airflow. So All modern day systems disable the A/C compressor as the OAT declines below ~34F."
 
This is incorrect, as most modern systems recirculate warm air with in the vehicle. Hence cooling the cabin air for dehumidifying before reheating it is not disabled in modern vehicles at ~34F.

#38 of 66 Wait a minute.... by andys120

Feb 08, 2007 (7:28 pm)

I NEVER allow my A/C compressor to operate at any time during the winter months and Lexus has provided two C-best options to facilitate that.
 
Most of the owners manuals I have seen recommend occasionally engaging the A/C compressor during the winter to keep the seals from drying out.

#39 of 66 Re: patent [rorr] by wwest

Feb 08, 2007 (4:22 pm)

Replying to: rorr (Feb 08, 2007 1:47 pm)
Now I really feel as if my leg is being pulled....
 
The efficiency of the A/C for helping to prevent windshield condensation and/or help to remove windshield condensation is strictly a function of local climatic conditions.
 
Throughout most of our world the colder the climate the lower will be the RH, relative humidity. Basically that means that the colder it is where you are trying to use the A/C to help prevent or remove windshield condensation the less likely it is that the will be of any help at all.
 
The surface temperature of the A/C cooling evaporator will never be driven below ~34F by design. To do so would result in freezing the condensate and eventually blocking all system airflow. So All modern day systems disable the A/C compressor as the OAT declines below ~34F.
 
What that means to you is that the dewpoint of the incoming fresh airflow must be above 34F in order for any dehumidification of the airflow to happen at all. And that 34F number is under ideal conditions. For instance an extremely low blower speed which will allow the airflow transit time through the cooling evaporator to be long enough for it to be cooled to its dewpoint.
 
In reality if the OAT is below ~47F, most of the time you may as well have the A/C turned off.
 
The BEST way to keep your windshield defogged, or remove condensation once it has formed is to HEAT the interior surface of the windshield as much as possible and as quickly as possible.
 
Turn up the temperature setpoint to maximum, the blower speed should follow, and then switch to defrost/defog/demist mode.
 
Once the windshield is cleared this method will no doubt begin to discomfort you with all of the heated airflow reflected off the windshield to your face.
 
Once that happens then turn the blower speed down manually while leaving the heat up and in 3D mode.
 
I NEVER allow my A/C compressor to operate at any time during the winter months and Lexus has provided two C-best options to facilitate that.
 
And by the by most newer cars have a variable displacement A/C compression so the engine load can be varied in a linear way as more or less cooling is required.
 
Additionally most modern day engine/transaxle control systems, ECUs, will "bump" up the air/fuel mixture feed to the engine just prior to engaging the A/C compressor clutch so that no "drag" on the engine is felt at the drive wheels.

#40 of 66 Re: defogging [oldharry] by wwest

Feb 08, 2007 (7:53 pm)

Replying to: oldharry (Feb 08, 2007 7:01 pm)
Most modern vehicles will not even allow you to operate the system in recirculate mode for more than a few minutes unless the A/C is in operation and I stand by my position that the A/C will automatically terminate operation with the outside climate near or below freezing.
 
And by the way it is NEVER a good idea to use the recirculate mode during the winter months due to the possibility of the cabin RH rising too high from human metabolism. Additionally when the windshield does eventually fog over due to improper recirculate use the system will automatically switch to fresh inlet airflow when you use the defrost/defog/demist mode. NOW the A/C will be disabled for certain sure!
 
If you can find documented evidence to refute my position I will be more than glad to listen and issue an apology.
 
In the Toyota/Lexus line the A/C indicator will turn off automatically ay ~33F and you can manually turn it back on but in actuality the compressor itself will not be enabled even though the indicator is now on.

#41 of 66 ?? by oldharry

Feb 08, 2007 (9:28 pm)

""...never stops retarding the wheels until motion ceases..."
  
And this is different from..??
  
Put your FWD automatic transaxle in the 1st gear position, accelerate to, say, 15 MPH. Now take your foot off the gas and tell when you ICE stops retarding the wheels."
------------------------------------
Do that with your RWD stick or automatic, and then comment on the control you have. (Hint do not try it in traffic)
 
Harry

#42 of 66 Ya know.... by fezo

Feb 09, 2007 (7:52 am)

This topic would be a lot better with English subtitles.....

#43 of 66 Re: Ya know.... [fezo] by wale_bate1

Feb 09, 2007 (9:00 am)

Replying to: fezo (Feb 09, 2007 7:52 am)
Just follow the body english - you'll catch up...

#44 of 66 Re: Ya know.... [fezo] by rorr

Feb 09, 2007 (9:11 am)

Replying to: fezo (Feb 09, 2007 7:52 am)
"This topic would be a lot better with English subtitles....."
 
Right-o chap. I was jus' thinkin' that same thing me'own self.
 
Now I really feel as if my leg is being pulled....
 
"Crikey, what a load of bilge....."
 
Sorry mate, can't help with the rest. I've no idea whut'tha bloody-hell 'es sayin' either.....

#45 of 66 Re: ?? [oldharry] by wwest

Feb 09, 2007 (9:40 am)

Replying to: oldharry (Feb 08, 2007 9:28 pm)
Engine braking a RWD with STICK shift on a slippery surface can be fairly beniegn providing the driver has enough "stick" experience to know to use the clutch to modulate the level of engine braking on the rear wheels.
 
With a RWD automatic on an extremely slippery downhill run I would always advise a quick shift into neutral and then maybe judicious use of the e-brake in the same manner as the clutch in the above case.
 
It's the automatic transmission that's the "wild card" in these instances.

#46 of 66 Re: ?? [wwest] by rorr

Feb 09, 2007 (10:19 am)

Replying to: wwest (Feb 09, 2007 9:40 am)
"With a RWD automatic on an extremely slippery downhill run I would always advise a quick shift into neutral and then maybe judicious use of the e-brake in the same manner as the clutch in the above case."
 
Okay - back to seriousness.
 
Braking forces (whether they be due to engine compression or mechanical systems) cause weight transfer to the nose of the car. In fact, as you've pointed out before, the front brakes do vastly more work under normal braking than the rear brakes PRECISELY for this reason. Weight transfer.
 
So, physics tells us that under braking (or actually, ANY vehicle deceleration) you get weight transfer. Meaing, the force imparted DOWN through the wheels to the road is DECREASED at the rear of the vehicle and INCREASED at the front of the vehicle.
 
Now, the friction force available to brake is the product of that normal force ('weight') x the coefficient of friction. I'm going to assume that the coefficient of friction is identical for both the front and rear wheels (since the coefficient of friction is a function of the tire design and roadbed surface). Under vehicle deceleration, you get weight transfer FROM the rear TO the front, therefore the available friction necessary to slow the vehicle INCREASES at the front and DECREASES at the rear.
 
Prior to the advent of ABS systems, what usually happened with RWD vehicles (whether using the brakes or through engine compression) is that braking on icy surfaces caused the rear of the vehicle to unload and the REAR wheels to lockup. Result? Vehicle skid (ass-end coming around).
 
Using the e-brake on ice is, to put it bluntly, stupid. Not only do you BYPASS the ABS system, put you are braking using ONLY the rear brakes, the very end of the vehicle MOST LIKELY to lock up on icy surfaces.
 
Now consider FWD and engine compression on ice. The braking forces are acting ONLY on the end of the vehicle which GAINS traction under vehicle deceleration.
 
Honestly wwest, I think the disconnect that we are having is with the concept of weight transfer and how it affects available traction.

Messages Page 5 of 7

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