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Old 12-06-2010, 11:08 PM   #171 (permalink)
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Quote:
Originally Posted by autoteach View Post
Once again, effects of CR on power
Engine Compression Ratio - Tech - Popular Hot Rodding Magazine

Now, you dont have to believe it at all. In fact, you dont have to read it. But ignoring facts doesnt make your imagination reality. That is only unfortunate for you. Diesel and gasoline engines benefit greatly in their efficiency from higher CR's.
Okay, then. Let's take an example, using the numbers presented in that article you so thoughtfully provided. Let's go from 9:1 to, say 16:1. 16:1 is a rather low compression ratio for a diesel, but it should work for illustration purposes. Increasing the compression ratio in this manner, according to the article, results in about an 14.5% theoretical increase in the amount of useful power extracted from burning the fuel. Conversely, that could also be thought of as saying that the engine should be able to use 14.5% less fuel to achieve the same output as before. Now, if you want to be nit-picky about it, we could go all the way up to 22:1, and have a whopping 21% or so increase from 9:1. Alternately, you could go with 19:1, and use 18.4% as your percentage.

Somehow, though, diesel engines tend to get more fuel economy than what can be accounted for by merely bumping up the compression ratio. While I admit that comparing a 1.8L gasoline wastegated turbo engine and a 1.9L diesel variable nozzle turbo engine may not be totally kosher, the fact remains that the 1.9T, at 19:1, gets about 36% better fuel economy than the 1.8T, at 9.5:1. Clearly, 36% is better than 14.5% or 18.4% or even 21%. And, no, variable nozzle technology is not that superior to wastegates, so as to cause the remainder of the fuel efficiency increase.

Therefore, not all of the fuel economy gain of a diesel over a gasoline engine is due simply to compression ratio. Other factors play a significant part, particularly throttle losses.

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Old 12-06-2010, 11:39 PM   #172 (permalink)
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Quote:
Originally Posted by t vago View Post
The higher compression ratio of a diesel engine does not provide as much fuel economy savings as is generally thought.
This is your quote from before, and there are other conditions existing that you are neglecting so you can feel okay about making flawed statements. A 15-20% gain on CR alone is pretty worthless, I will admit. That is why I am lowering the compression on everything I own. How about the 13.5% increase in BTU or energy content of the fuel? Almost there... No throttle, higher running temp, and direct injection... Yeah, that should do it. Now that you have dispelled the myth that CR doesnt...well, read your quote. I am pretty sure that if I were to offer everyone a 15% increase in FE they would come running for that miserably small gain.
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Old 12-07-2010, 06:48 AM   #173 (permalink)
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Quote:
Originally Posted by dcb View Post
sorry dude, but bsfc is by weight (grams), not volume, and gas and diesel are about the same in the weight department (gas is actually a little higher)
http://upload.wikimedia.org/wikipedi...gy_density.svg

I think UFO explained the effects of increased compression ratio pretty well, higher expansion ratio=more energy extracted.
I was using a chart that listed the BTU content per gallon dcb.

While bsfc is in grams, the energy content should be in the quantity you normally purchase, which in this case was gallons.

If bsfc was measured based on the available energy content per unit of measurement, then diesel would have more energy per unit of weight, but my percentages were not based on weight but volume, since you don't generally by fuel by weight.

With gas heavier than diesel (based on your statement) then the difference by weight would be greater than 12%.

dude, LOL

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Old 12-07-2010, 08:18 AM   #174 (permalink)
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I don't know about that whole tap dance there, glad I could clear up how bsfc charts are used for you.

But weight is a much more consistent unit of measure when dispensing energy, as you can see from that scatter plot, hydrogen at various pressure (or temperature) levels contains the same energy content for a given amount of weight. Hopefully that is intuitive.

And it is also interesting to note that your 200-250 bsfc numbers (which was NOT power/gallons, in case there is still confusion there no thanks to mech) clearly put the improved efficiency of diesels in a new light. Despite gasoline having slightly more energy by weight, gas engines are frequently found with %25 worse fuel consumption at peak bsfc, per unit of power output.

Brake Specific Fuel Consumption (BSFC) Maps - EcoModder

What the bsfc charts are telling you is that, for essentially the same ENERGY input, diesels are still more efficient. You need to rethink your diesel assertions.
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Old 12-07-2010, 01:36 PM   #175 (permalink)
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"Gasoline having slightly more energy by weight."
Talk about adding confusion, LOL.

Are you talking about liquid hydrogen, or hydrogen gas?

You could do a lot more for the education of those who might show an interest in this other than arguing the obvious.

I used the quantity of fuel that you can purchase. Anyone see a fuel pump dispensing by weight?

The bsfc comparison was using the same unit of measurement, which makes your argument irrelevant. As long as the basis of calculation is the same the comparison is valid.

The point about weight between gas a diesel was valid, the rest was ptui.

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Old 12-07-2010, 02:40 PM   #176 (permalink)
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A reasonable considerate post.

Uh Mech you goofed, the bsfc should be factored from 200 to 172 for a diesel because it is calculated based on weight not volume.

So the comparison should be the percentage difference between 171 and 250 which is a more accurate representation of the difference in efficiency between gas and diesel engines.

A diesel engine produces 250/171 of the useful work of an equivalent gasoline engine. This is about a 46% improvement based on the weight of the fuel.

By volume the difference would be lower because gasoline is lighter than diesel by about 14%.

Got it.

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Old 12-07-2010, 02:51 PM   #177 (permalink)
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I was talking about all states of hydrogen, for a given weight you get the same energy. The same cannot be said for volume. But this is tiresome trying to keep you honest.
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Old 12-07-2010, 02:57 PM   #178 (permalink)
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Quote:
Commonly BSFC is expressed in units of grams per kilowatt-hour (g/(kWh)). The conversion factor is as follows:

BSFC [g/(kWh)] = BSFC [g/J](3.6106)

To calculate the actual efficiency of an engine requires the energy density of the fuel being used.

Different fuels have different energy densities defined by the fuels heating value. The lower heating value LHV is used for internal combustion engines efficiency calculations because the heat at temperatures below 150 C (300 F) cannot be put to use.

Some examples of lower heating values for vehicle fuels are:

Certification gasoline = 18640 BTU/lb = 0.01204 kWh/g
Regular gasoline = 18917 BTU/lb = 0.0122225 kWh/g
Diesel fuel = 18500 BTU/lb = 0.0119531 kWh/g

Thus a diesel engine's efficiency = 1/(BSFC*0.0119531)

and a gasoline engine's efficiency = 1/(BSFC*0.0122225)
Brake specific fuel consumption - Wikipedia, the free encyclopedia

Using 200 for diesel and 250 for a typical gasser, that gives a diesel efficiency of 42% and a gasser efficiency of 33% independent of the fuel energy density.

From the charts dcb linked, it looks like the hybrid engines are coming close to rivaling the TDI if operated in a specific manner. The Insight uses lean burn techniques, and the Prius uses a high static compression of 13.5:1 ratio to get the expansion volume coupled with a late intake closing to get a more manageable 9.5:1 dynamically.

The TDI has a 19:1 static compression ratio.
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Old 12-07-2010, 03:17 PM   #179 (permalink)
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Quote:
Originally Posted by dcb View Post
I was talking about all states of hydrogen, for a given weight you get the same energy. The same cannot be said for volume. But this is tiresome trying to keep you honest.
I have your honest dangling.
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Old 12-07-2010, 08:09 PM   #180 (permalink)
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Hi,

Typical gasoline has 116,090 BTU/gallon and diesel has 129,488 BTU/gallon (according the X-Prize MPGe spreadsheet).

If you have more torque, you just need to use taller gear ratios to get the work done?

Quote:
Originally Posted by Old Mechanic View Post
Looks similar to your original configuration Neil.



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Yes, it does; complete with superchargers.

So, if I am off base -- how do we get a much more efficient ICE design?

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Last edited by NeilBlanchard; 12-07-2010 at 08:19 PM..
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