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Old 04-20-2012, 10:47 PM   #21 (permalink)
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That chart reported -3 as ideal if i am reading the chart right.
I do not buy it, seems counter intuitive to real life data of hard starting and poor mileage diesel owners experience during winter.

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Old 04-21-2012, 12:31 AM   #22 (permalink)
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The premise is that the car will make less hp and use less gas.

My deduction could easily be flawed but here it is in a nut shell. Cold air has more explosive potential and hp than warm air when in a compressed cylinder because the air is denser.
The sensors on the car keep the air fuel mixture correct, the colder denser air would need more fuel to stay at the correct level. With warm air the car would lean out the mixture to correct the air fuel mix. As a bonus the warmer air fuel mix ignites a substantial amount easier.
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Old 04-21-2012, 01:07 AM   #23 (permalink)
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Diesels seem to want as much mass as possible stuffed in their cylinders to get the best fuel economy.
EGR delete, free flowing intake, ram air, water injection and turbo chargers all show FE gains on diesels.
One of the most important factors with FE is getting your diesel engine up to operating temperature.
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Old 04-21-2012, 05:36 AM   #24 (permalink)
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Quote:
Originally Posted by oil pan 4 View Post
Diesels seem to want as much mass as possible stuffed in their cylinders to get the best fuel economy.
EGR delete, free flowing intake, ram air, water injection and turbo chargers all show FE gains on diesels.
One of the most important factors with FE is getting your diesel engine up to operating temperature.
If you think about it, that all leads to conclusion that as piston compress stuff certain amount, if there is more stuff you gain higher CR and higher CR always improves things until it becomes too high.

That is at least image what I'm getting.

Real life results have always a lot of variables, cold starting, engine not staying at optimal temp, if short driving, drivetrain has more loss because oils and greases becoming thick and most of effect comes from cold, maybe using winter tires, winter diesel etc.

Also while air has more oxygen, it has less water, that might have something to do with something too, at least skin and throat becomes dry more easily
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Old 04-21-2012, 12:51 PM   #25 (permalink)
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Quote:
Originally Posted by ecomodded View Post
The sensors on the car keep the air fuel mixture correct,
Here is where you have a problem. Diesels operate with excess air, hence the reason they have no throttle.
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Old 04-21-2012, 09:32 PM   #26 (permalink)
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Quote:
Originally Posted by ecomodded View Post
The premise is that the car will make less hp and use less gas.

My deduction could easily be flawed but here it is in a nut shell. Cold air has more explosive potential and hp than warm air when in a compressed cylinder because the air is denser.
The sensors on the car keep the air fuel mixture correct, the colder denser air would need more fuel to stay at the correct level. With warm air the car would lean out the mixture to correct the air fuel mix. As a bonus the warmer air fuel mix ignites a substantial amount easier.
I think it's a bit more complicated than this, this is what I am seeing.

A diesel doesn't need to maintain any kind of air fuel mixture, it just injects however much fuel it needs and spits out however much power. Press less on the pedal, less fuel is injected.

The issue is that since colder air has a greater amount of oxygen available, the fraction of fuel injected for the same power level is lower. I'll be taking a sort of metatheoretical view just thinking about temperatures. Less fuel fraction means when the fuel burns, you have a smaller change in temperature and pressure, which is not good. However, a lower temperature to start with increases the maximum possible theoretical efficiency, so it's really not clear what will happen.

Coming back to reality, we see there's another thing about cold air that makes it more complicated. The heat ratio of gases is not truly a constant, and tends to decrease with temperature. In an ideal cycle with ideal gases higher heat ratio is better, but I imagine in the real world the advantage is dulled a little.

This is all not considering combustion quality, which of course can be affected. If you're having trouble starting a diesel engine, that probably means combustion stability is an issue and you'll be seeing a greater effect from that than the ~10-20% absolute temperature changes that happen between seasons.
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Old 04-22-2012, 01:58 AM   #27 (permalink)
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Piwoslaw or Diesel Dave or anyone else if you have a fuel economy gauge and a hair dryer or as Stan suggested, run a heat safe hose from over your exhaust to your intake and note any differences
for the good or bad by preheating the air.
If I had a gauge I would be doing the test myself.
I could try one day when the engine will be hot, though now it's getting warm outside, so I'd be comparing a W(arm)AI with a H(ot)AI, I'd have to wait 6 months to test a C(old)AI.

But I wonder if this test would show anything conclusive. Since I don't have a dyno, then I'd just be idling on the driveway. At idle I see ~23% load and ~760rpm, while 80% and 1800-2200rpm would be ideal from a hypermiler's point of view.

Also, my turbo has variable geometry, so it may be that the amount of air getting scooped up depends on more than just the amount of exhaust gases coming out. For example, the ECU will see that the intake air is warm, so it will change the turbo's geometry to force more air into the cylinder. Then the only difference between tests would be the temperature of the air in the cylinder, not the amount, i.e. the amount of air (mass-wise) is constant, but if it is warmer, then it gets crammed in at a slightly higher pressure.
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Old 04-22-2012, 11:26 AM   #28 (permalink)
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Quote:
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Also while air has more oxygen, it has less water, that might have something to do with something too, at least skin and throat becomes dry more easily
I run water injection.
When I have air in the system and the water pressure is randomly cycling up and down I can feel the difference as the water cuts on and off.
I can tell my diesel likes the water injection, it did not like propane injection.
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Old 04-23-2012, 01:01 AM   #29 (permalink)
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Thanks everyone for their input, I am still curious to test the ole hair dryer in the intake out. I am going to get Vag Com installed on my laptop , I had the free version before but lost it due to a reformatting the hd.

It may be best to have a light foot on the old gas pedal thereby using less fuel, ha ha bet we all new that already!!

What everyone has been saying makes sense and helps refresh my mechanical knowledge, or lack of.
This is my first diesel
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Last edited by ecomodded; 04-23-2012 at 01:06 AM..
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Old 04-23-2012, 02:36 AM   #30 (permalink)
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My idi 2.0L Mazda RF diesel engine has an air heater plate between the intake and the air filter. Get's pretty warm to the touch.
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