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Old 01-19-2009, 04:07 PM   #1
Anatol
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Why diesel consumes less fuel?

Someone know why disel eat less fuel with combustion chamber size is same as petrol?
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Old 01-19-2009, 05:56 PM   #2
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Re: Why diesel consumes less fuel?

I think you mean 'displacement' (aka engine size).
A diesel engine with the same displacement has a smaller combustion chamber that a gasoline engine, due to its higher compression ratio.

Given that, a diesel has a more efficient thermodynamic cycle because the mixture of atomized diesel fuel and air can be compressed significantly more (before combustion begins) than a gasoline and air mixture.

Diesel engines take advantage of this by having higher compression ratios, ~18 to 22:1. Gasoline engines typically have compression ratios of ~8 to 10:1

Also, diesel fuel has slightly more chemical energy per volume than gasoline.
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Old 01-19-2009, 05:59 PM   #3
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Re: Why diesel consumes less fuel?

Diesels have MUCH smaller combustion chambers, but that has very little to do with it.

Diesel fuel has about 130,000 BTUs of energy per gallon. Gasoline only has about 115,000. So, let's say it takes 40 hp in your car to sustain highway speed. A diesel will require less fuel burned to make the same power.

Diesels are also higher compression, which means that of the BTUs stored in the fuel, more are released as power than in a comparable gas engine.
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Old 01-20-2009, 05:29 PM   #4
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Re: Why diesel consumes less fuel?

They also run a much leaner mix much of the time.
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Old 01-23-2009, 03:51 AM   #5
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Re: Why diesel consumes less fuel?

Quote:
Originally Posted by MagicRat
I think you mean 'displacement' (aka engine size).
A diesel engine with the same displacement has a smaller combustion chamber that a gasoline engine, due to its higher compression ratio.

Given that, a diesel has a more efficient thermodynamic cycle because the mixture of atomized diesel fuel and air can be compressed significantly more (before combustion begins) than a gasoline and air mixture.

Diesel engines take advantage of this by having higher compression ratios, ~18 to 22:1. Gasoline engines typically have compression ratios of ~8 to 10:1

Also, diesel fuel has slightly more chemical energy per volume than gasoline.
Why then with all the benefits diesel cars run slower and not many prefer them over gasoline englines?
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Old 01-23-2009, 06:14 PM   #6
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Re: Why diesel consumes less fuel?

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Why then with all the benefits diesel cars run slower and not many prefer them over gasoline englines?
Try driving one; then you will know.

Diesel engines are generally heavier; the weight may affect handling somewhat.
Typically they accelerate slower that gasoline engines due to a combination of less power and slower capacity to build revs.

Also, many diesel engines available elsewhere in the world do not meet US or Canadian emissions standards.

That being said, recent technologies have allowed some diesel car engines to produce similar performance and low emissions as gas engines, But these diesels are often considerably more expensive to buy, compared with gas engines.

Diesel fuel is less widely distributed than gasoline in North America. Also, most diesel pumps are 'high flow' models, designed to fill heavy trucks, and do not work in cars and light trucks.

Finally, there are many places in the world, (much of Europe and South America) where diesel cars have about 50%++ of the market. In those areas, diesel fuel has a lower tax rate and is cheaper to buy than gasoline.
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Old 01-25-2009, 01:29 PM   #7
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Re: Why diesel consumes less fuel?

MagicRat and I love to disagree on this one

I prefer driving a diesel, but much of the reason why we don't do diesel more often here in the states is perception. People think diesel and they think smokey mercedes from 1984, or an old smoky stinky 18-wheeler. But what they don't often realize is that a large percentage of newer mercedes and VWs are diesel, but they're so quiet and smoke-free that you'd never notice it. The incredible engineering that goes into them is unnoticed. My wife has a 99 mercedes diesel, and I have to tell people its a diesel. It doesn't sound like one, smell like one, or drive like one. The engine weighs about 30 lbs heavier than the gas option, but it makes much more torque and gets 35 mpg. The benefits are very clear to me. Right now with how much more expensive diesel is, its pretty much a wash on yearly fuel costs.

Right now diesel is going through some changes much like gas did in 1970. The beginning of the smog era meant a lot more technology on a gas engine which hurt reliability and power to an extent. Right now diesels are kinda in the same boat, but I look for the technology to expand and become reliable, just like smog equipment on a gas engine has become commonplace.

But having said that, I suggest you pick up a Diesel Power magazine or other fluffy diesel publication and read up on 5.9L diesels that make 1300 hp and 2100 lb-ft of torque at the WHEELS, get 25-30 mpg on the highway, all with just changes to the turbos and fueling. The beauty of diesel is how easy it is to make insane power and torque without ruining its streetability. The Duramax currently comes from the factory with 360 hp, 650 lb-ft of torque, 500k-mile reliability experience, and is only a chip, exhaust, and intake away from 800 hp. All in a package that weighs about the same as an old-school big block.

Here is a link to a video of a diesel grand national that makes 500 hp, gets 35 mpg, and turns in consistent 10 second timeslips in the 1/4 mile. http://video.google.com/videoplay?do...42729641&hl=en

Here is an 800 hp diesel mustang that gets 35 mpg and turns in 9-second timeslips, and he drives it on the highway frequently. http://www.nitrousexpress.com/Projec...duramaxgt.html

One of the diesel magazines outlined a VW TDI build that made over 300 hp from a 1.9L 4-cylinder. The owner reported 48-52 mpg on the highway. Can't find a link right now, but I think it was www.dieselpowermag.com

But, to answer your question specifically, most diesels in the US are engineered for trucks, or engineered for passenger cars with a strong nod toward MPG and reliability. No body really makes a high performance diesel because that is not what the market wants. Its very easy to do it as these guys have shown, but market perception is what is really holding it back.
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Old 01-25-2009, 01:58 PM   #8
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Re: Why diesel consumes less fuel?

Quote:
Originally Posted by curtis73
MagicRat and I love to disagree on this one

Just to reiterate what I have said before, I LIKE driving diesels, but I hate driving BEHIND diesels <cough,cough>....

That being said, I am all for a move to high tech, ultra-low emissions diesels because the availability of realistic alternatives to gasoline engines makes things better for everyone.
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Old 01-26-2011, 04:31 PM   #9
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Re: Why diesel consumes less fuel?

to cut this a simple as i can, lets say you have a 300hp gas motor, well a diesel can use less fuel to make that same hp witch gets u further and diesel also burns slower than normal gas, but thats why when u put a tune in a diesel you notice a big big change in power, i can go all day about how diesels, if you need to know what tune to put in a GM or Ford or Dodge just let me know ill give u the right one
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