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Old 12-15-2008, 10:45 AM   #1
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The downside of DFCO

I was thinking about it today... There is a situation where DFCO isn't a plus.

Watching the snow outside I was thinking about this time last year when I drove up to the crest shortly afte the first snow fall. It was a nice. 1AM, nobody else around, 45mph winds @ sub 20 degrees. Nice for about 10 seconds anyways.

Well, after getting myself out of waist deep snow and back into my car I started my decent where I lost heat and engine temperature plummeted. Up until now I had always wondered why that actually happened but knowing a little more about the car and everything I've realized it was DFCO.

Anyone that's driven down a mountain pass knows that engine braking is pretty much the only way to get to the bottom without worrying about your brakes fading and possibly losing the use of them. That's what I did and as a matter of fact, the car stayed in 2nd the whole way up and the whole way down.

DFCO is nice in that it means your engine is using no fuel(duh) but no fuel means no heat generation. The engine turned ~3k rpm for about 30 minutes while I descended. The water pump, mechanical fan and cabin heat all sucking from a heat source that no longer exists.

Now, I should point out that the temperature only fell to a point. At around 90 degrees coolant temp the DFCO is canceled until temps get above 120 again. There is a HUGE difference in the amount of heat you get between 120 degrees and 195-200 degrees.
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Old 12-15-2008, 10:58 AM   #2
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Sounds to me like an argument for a grille block
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Old 12-15-2008, 10:59 AM   #3
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I guess that would be another benefit to having an electric fan?

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Old 12-15-2008, 11:22 AM   #4
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E-fan won't help when the car isn't burning any fuel.
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Old 12-15-2008, 12:32 PM   #5
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Grill block and e-fan could help if water is being passed through the radiator. Either way, you're taking heat from a source that isn't making its own anymore.

Think of it like a water heater. Get it up to temp and turn it off. You can stick a low flow shower head on there but it'll eventually run out either way. I think I lost heat within the first 10 minutes.
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Old 12-15-2008, 07:36 PM   #6
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My logic is that the mechanical fan is still spinning, even if the clutch is not engauged. An electric fan will stop if its cool. My vote is for grille block and E fan. It won't eliminate the problem, but it will reduce the problem. Also, you can get a 12V electric creamic heater. I have one for my truck to help heat it up faster in the mornings. I think I got it for like $15 or $20. not the greatest, but its better than nothing, and the electricity is "free" in DFCO. Also, as soon as you enter DFCO put the blower fan on the lowest speed to make it last as long as possible.

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Old 12-15-2008, 07:45 PM   #7
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Certainly freezing is not worth fuel,especially at todays prices.

Potential beneficial thoughts.

Dress as if you would be stranded if your car dies.

Install a hand throttle to adjust idle just above fuel shutoff point.

Of course use recirculate to get cabin temp as high as possible.

Some kind of seat heater?

Higher temp thermostat to block coolant flow to radiator, as well as blocking off most of the radiator.

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Old 12-15-2008, 11:51 PM   #8
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interesting. Can't say I've experience that, but at idle rpm, any temp under 10 above the Grand Prix doesn't produce enough heat to feel warm in the cabin. It's warm enough, but still cold feeling.

Growing up in Alaska taught me to always wear as if the heater doesn't work in your car; you can always take layers off (if the drive is long enough to merit it). I do recall one trip it was -30 in the whole state (colder in places) and driving back from Anchorage to Fairbanks my brother and I wore snowpants and down jackets to stay warm, because the heater put out just enough to keep 80% of the windshield from frosting over. That was a crappy 6 hour drive.
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Old 12-16-2008, 03:27 AM   #9
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Quote:
Originally Posted by R.I.D.E. View Post
Certainly freezing is not worth fuel,especially at todays prices.
[...]
Install a hand throttle to adjust idle just above fuel shutoff point.
You missed the important part: It's a LONG descent (DFCO 30 minutes @ 3000rpm @ 2nd gear), and lots of engine braking is required for safety (not for saving fuel). Adjusting the idle to get rid of DFCO will reduce the effectiveness of engine braking.

What about a manually controlled radiator bypass that prevents the coolant from ever entering the radiator?

Gary, how would your hydraulic hybrid system handle braking during excessive input? Would it bleed off some stored pressure to enable additional braking when it's full?
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Old 12-16-2008, 04:51 AM   #10
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The hydraulic drive would require an emergency brake system just like any other car. Basically this would be accomplished by intentionally placing a restricting valve in the flow circuit with a bypass when the accumulator is charged to its maximum pressure. Bypass and restriction would occur in each individual wheel drive.

Traction control would shift the "gear ratio" to a higher "gear" to limit wheel slip. This is accomplished by dumping the pressure that forces the individual wheel motor to a "lower gear". With the pressure dumped the wheel motor would shift to neutral almost instantaneously.

The pumps themselves would inherently seek neutral, and require imput to maintain "gear ratios".

Initial acceleration starts in the highest "gear" ratio and rapidly moves to a lower "ratio" until the desired rate of acceleration is achieved. Then the "gear ratio" would decrease (lower ratio-higher gear) as wheel RPM increases.

This means you have redundancy at each wheel without the failure of even 3 of 4 wheels leaving you with no braking ability. The same applies to drive power. I am not saying it would be fun driving the car on a single wheel, but it would be possible to limp home or at least to a repair facility if you suffered a 3 of 4 failure. The odds of that happening would be in the 1 in many millions.

DFCO in DK's example has very negative effects, with cold engine enrichment costing him mileage every time he encounters an extended DFCO as is the situation in his example. I did read and understand the example HC and it's definitely an extreme situation (30 minutes at 3k downhill). Remember people had to endure the same situation in cars that did not have DFCO.

It may be that he needs to combine some brake activity with engine braking without DFCO, while balancing both activities to maintain higher coolant temperatures. In most cars the thermostat will choke off the majority of flow through the radiator, but not the flow through the heater core. In fact in such an extreme situation the heater core itself would probably provide all the cooling he might need.

I also read DK"s post about the accident. I would consider an emergency repair to his vehicle (if practical) that basically concentrated on rough alignment of the frame and sheetmetal, that corrected the basic frame and sheetmetal misalignment. The body shop I deal with could probably accomplish that repair in less than 8 hours, at a cost of $350. The Mercedes" side collapsed like a catchers mit and spread the impact out, instead of concentrating it at a single point.

Not saying this would make sense to him, but if he knows an honest shop it might be an option. The body shop would also consider the fact that he will not be concerned with cosmetics, so there is no reason for a comeback as long as the basic chassis and body alignment is close to specs.

regards
gary
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