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Go Back   TDIClub Forums > TDI Model Specific Discussions Areas > VW MKIV-A4 TDIs (VE and PD)

VW MKIV-A4 TDIs (VE and PD) This is a general discussion about A4/MkIV Jetta (99.5-~2005), Golf(99.5-2006), and New Beetle(98-2006). Both VE and PD engines are covered here.

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Old July 1st, 1999, 17:02   #1
cage
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Default How can diesels run cooler than gas?

Everyone says that diesels run so much cooler than a gas car... how? I thought diesel has more energy than gas and it blows up much harder and hotter hence the clatter and need for greater amount of egr. I understand that we get better fuel economy because diesel has more energy but doesn't it take the same amount of calories to move equal weight. We might get better milage but we must be burning the same btu's of energy as a gas car. Whats up?
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Old July 1st, 1999, 17:30   #2
mickey
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Default Re: How can diesels run cooler than gas?

Think of it this way: The idea behind an internal combustion engine is to convert chemical energy into mechanical energy. Heat energy is an unwanted byproduct. (Except in winter!) Any heat produced as a result of the combustion is "wasted" energy.

Diesel fuel does contain more available chemical energy than an equal volume of gasoline, but the difference is not nearly as much as I always thought it was. Someone pointed out to me a while back that the difference is only about 5%. But a diesel engine uses about 40% less fuel to produce the same power output as a gasoline engine with the same number of cylinders and total displacement. More mechanical energy from the same amount of fuel means less heat, since the chemical energy you've spent has to accounted for and the equation balanced. If it's not power, it must be heat! A major reason that a diesel engine is so much more efficient is the extremely high compression ratio. There is a lot more oxygen crammed into the cylinder at the top of the compression stroke, so the fuel is able to burn more thoroughly. A diesel engine is able to accomplish this because the fuel is not injected until the piston reaches the top of the compression stroke. It is only compressing air, not fuel. A gasoline engine compresses the fuel/air mix, so the compression ratio is limited by the fuel's ability to avoid self detonation, or "pre-ignition." (That's the "pinging" a gas motor makes when the octane is too low, or something else goes wrong.) By the way, many manufacturers are currently working on Direct Injection designs for gasoline engines which will greatly increase their efficiency and improve their emissions performance.

Both gasoline and diesel engines like to be "hot" in order to work efficiently. But with a gasoline engine the problem is "How do I get rid of all the EXCESS heat the engine produces?" With a diesel, the problem is just the opposite: "How do I conserve heat in the engine so that the fuel burns more completely?" On a cold day there's hardly any coolant circulating through your TDI, and the cooling fans will never even turn on! The problem of conserving heat also partially explains why the fuel mileage drops so much in the winter. ("Winterized" fuels containing less energy are also a problem.)

The "clattering" sound of a diesel has nothing to do with the amount of heat produced. A gasoline engine is burning fuel vapor mixed with air. A "flame front" moves outward from the spark plug. It is more of a rapid, smooth burn than an explosion. A diesel engine, on the other hand, burns very finely atomized droplets of liquid fuel. Ideally it would begin to burn instantaneously as it enters the combustion chamber from the nozzle of the injector, but in practice there is always a little bit of "delay" and a tendency for the fuel to explode all at once. Hence the "clatter" you hear. Many diesels, including TDIs, use Pilot Injection to help control the combustion noise. A tiny, metered quantity of fuel is injected several degrees before top dead center in order to start an "early" burn which helps ignite the main injection more rapidly. That also helps lower emissions. The new HEUI injector designs (High efficiency Electronically controlled Unit Injectors) use extremely high fuel pressures (about 10 times higher than most DI diesels)to more precisely meter the pilot injection, and to more thoroughly atomize the fuel spray for more efficient combustion. More complete burning means more power from less fuel, accompanied by lower emissions.

-mickey
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Old July 2nd, 1999, 09:56   #3
cage
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Default Re: How can diesels run cooler than gas?

but when I add more oxygen to my torch the flame gets hotter. Wouldn't it be the same in the diesel. I thought the reason a diesel gets such good milage is also the fact that it can burn leaner which means hotter.
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Old July 2nd, 1999, 11:33   #4
mickey
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Default Re: How can diesels run cooler than gas?

Yes, diesels do run much "leaner", but that's still not the issue. The amount of energy extracted from a given volume of diesel fuel may be nearly the same as an equal volume of gasoline, but the diesel turns more of it into power instead of heat. I wish I understood more about thermodynamics than I do, because I have many of the same questions. All I can tell you is that a diesel's efficiency is the reason for the low amount of heat generated. The chemical energy released by combustion will either become Torque or Heat. Diesels make more Torque at the expense of Heat, but I'm not entirely clear on WHY they are so much more efficient. My big, long post up there represents my best effort to explain the situation as I understand it.

-mickey
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Old July 2nd, 1999, 13:44   #5
Tazzman
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Default Re: How can diesels run cooler than gas?

Mickey
I couldnt of said it any better,
but then I guess we're not ALL
mechanically incline.
need more laymans term ( presis effect )

JS SMKN

Taz
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Old July 2nd, 1999, 14:08   #6
christi
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Default Re: How can diesels run cooler than gas?

Diesels are more thermally efficient than petrol engines, because of the compression ratio. They certainly run cooler at tickover and light loads, but they also seem to dissipate more heat at high speeds. Many diesels (like all the Peugeots I've owned) have had mauch larger radiators than the equivalent petrol models. With Peugeots the non-turbo diesel model will often have a rad' which is nearly twice as big as even the GTi model. Here's my personal theory on this:-
At low speeds and tickover the diesel runs cooler because it is using hardly any fuel. This is due to the higher compression ratio, the fuel has plenty of time to burn completely, and the engine doesn't have to overcome the pumping losses of sucking air through a nearly closed throttle.
At high speeds the engine is running much less efficiently, with the fuel not able to burn before the piston has dissapeared off down the bore. Because of that compression ratio it runs hot, compressing air does make it hot.
I've always though that the biggest fuel savings of a diesel over a petrol are in city / slow driving. It's difficult for even the fast driver to drive flat out all the time, so even he makes a saving.
I once drove my 305 diesel with my accelerator flat to the floor on the Autobahn all day (Holland to Denmark). The economy was terrible (33mpg imperial). I recon that a petrol car would have been similar.

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Old July 2nd, 1999, 14:18   #7
goon
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Default Re: How can diesels run cooler than gas?

I beg to differ with Peter:
Total amount of heat transferred is unrelated to burn time. Combustion is complete at the exhaust stroke--whatever heat was going to be released has been released and must be dissipated. How quickly the combustion takes place during the expansion stroke does not really make a difference.

Let's not forget that, regardless of how much energy there is per volume, diesel gases expand slower than those in a gasoline engine--but with more force. The beauty of the TDI engine is that you can burn small amounts of fuel very efficiently to get gases expanding slowly, but with great force. This is great in the steady state (cruising on the highway) but the downside is that slow expansion, i.e. the engine is not "rev happy."

Engines "like to run hot" because a.) heat is required for lubricants to do their job per design b.) activation energy is required for the combustion process.

Actually, burning diesel fuel requires more activation energy than gasoline, that's why, at ambient temperatures, you may not be able to light diesel fuel with a match. The engine makes up for this with much higher pressure. Diesel fuel can burn efficiently at relatively low temperatures because the high compression ratio provides the required activation energy.

I think that diesels run cooler for the following reasons (in order):
1. The engine provides high torque at low rpm, which means you can run the engine slower with a taller gear, which means that less fuel is consumed, i.e. less energy expended per time.
2. Slower engine means less total mechanical friction
3. More efficient combustion process is more efficient (lower percentage of total energy consumed is lost to friction)

All of the above items are related, but some aspects have a bigger overall impact than others.
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Old July 3rd, 1999, 00:01   #8
Peter Cheuk
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Default Re: How can diesels run cooler than gas?

I'll take an uneducated stab at this one.

Diesel fuel burns extremely quickly in the combustion chamber, not like gas does (actually, it almost explodes). Since the flame is much quicker, there's less heat transfer, the power is discharged, the gasses just need to expand to push the piston down the rest of its way to BDC. In a gas car, the gas burns relatively slowly so it's expending its energy throughout the piston stroke to the point that burning gasses are exiting the combustion chamber. The long burn time transfers more heat.

Does that make sense?
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Old July 3rd, 1999, 05:38   #9
christi
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Default Re: How can diesels run cooler than gas?

So why do diesels have such huge radiators then?

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Old July 3rd, 1999, 10:03   #10
mickey
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Default Re: How can diesels run cooler than gas?

Mine has a little, tiny radiator because it never has to do anything but move around a 3000 lb Beetle. Heavy duty diesel trucks have huge radiators for those times when they do a huge amount of work, like pulling a heavy trailer up a steep grade. When they're just cruising down the highway without a heavy load the radiator is just along for the ride. The thermostat won't even open. I think with a diesel engine you have the choice between burning a lot less fuel and producing a lot less heat while doing the same work, or burning about the same amount of fuel and producing a lot MORE heat while doing a lot MORE work! I think that also explains the sudden drop in fuel mileage that a TDI experiences above a certain speed. The "curve" seems to drop off a lot more steeply than it does with a petrol engine. Diesels are capable of producing a lot more torque than petrol engines for the same amount of fuel, but they are the most efficient at low speeds and light loads. Or something.

-mickey
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Old July 4th, 1999, 02:47   #11
christi
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Default Re: How can diesels run cooler than gas?

Maybe that's why VW tell you to modify the cooling system for towing (at least on by B4) and to watch the temp guage, where as Peugeot just fit a gigantic radiator as standard, and don't give any warnings. I suppose that only 2% of owner's actually do pull a heavy trailer and four people up the side of the Alps. Funny thing is, I'm sure that the petrol Peugeots, with their much smaller radiators, don't overheat in the same circumstance either though.

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Old July 4th, 1999, 17:05   #12
Kiwi_ME
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Default Re: How can diesels run cooler than gas?

The question was posted "why do diesels run cooler than a gas car?"
We'll the thermostat has about the same setpoint as a gas engine so therefore it runs at about the same temp as far as coolant goes. Does it produce more waste heat? Waste heat is the fuel energy lost in the "heat engine cycle" and must be less than a gas engine given the same conditions or we wouldn't be getting better fuel mileage, would we.
Why is the diesel more efficient? Fundamentally, (but as others have mentioned, probably not exclusively,) it's the higher (real) compression ratio, one that is present at all power levels, and allows a greater expansion ratio for the burning fuel/air mass. It's not good to compare the diesel directly with the gas engine as the latter has more vices, not the least of which is throttling the intake to vary the output power. With any throttling of the intake, the effective compression ratio is reduced thus the lower efficency. If 1/3 atmosphere (intake manifold) absolute pressure is compressed 10 times, you only get 3.3 atmospheres. If 1 atmos is compressed 10 times you get 10 atmos, and so on. The gas engine is always at a disadvantage at part load due to this fundamental compromise along with the fact that gas can't take any geometric CR greater than ~10.5 w/o pre-igniting when wide-open.

Do diesel cars have bigger radiators? A radiator is sized for the job the car is designed for and really should differ little, gas or diesel.
More importantly, the diesel (radiator) does not need a lot of air flow at idle as the efficiency is still very high, and therefore electric fans may be smaller or not present.
Hope this is understandable, wrote in a hurry, must get out to see fireworks.

-paul
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Old July 5th, 1999, 00:22   #13
mickey
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Default Re: How can diesels run cooler than gas?

I agree. I'm starting to realize that saying "diesels run cooler" is only telling part of the story. They produce less heat when they're not working hard, but more heat when you really push them. Hmmmm. They're just odd motors in general. Well, I don't pull a trailer with mine, so mine runs cooler than a gas motor.

-mickey
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Old July 8th, 1999, 18:58   #14
cage
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Default Re: How can diesels run cooler than gas?

Paul your answer sounds the most convincing to me. I never understood someone saying that diesel turns the combustion into energy better than gas since combustion is combustion. ie.. if you burn rocket fuel it will burn hotter and quicker than charcoal yet over time with enough charcoal you can equal the energy of rocket fuel. My question was if gas moves the same vehicle at the same speed for the same amount of time why is diesel cooler. I never thought of changing compression ratios.
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Old July 8th, 1999, 21:36   #15
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I was thinking "compression ratio" too, but I was not thinking in the right direction. Good answer, Paul! Equal heat energy "expanded" into bigger relative volume means lower temperature! The energy that would have become excess heat instead becomes mechanical energy as the piston is pushed that "extra" distance, and the equation is balanced. That's the answer I was looking for! Simple thermodynamics.

-mickey
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