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Old 12-06-2010, 10:40 AM   #151 (permalink)
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Originally Posted by dcb View Post
diesels are also throttle-free, and have high compression ratios, and direct injection these days, those help too.
Agreed dcb, those are incremental improvements but the energy content difference is the largest factor, in my opinion.

The kwh value of a gallon if diesel is considerably higher than gasoline using the same method of measurement.

I worked for Mercedes when they went from throttled to throttle less in 1982. They calculated it resulted in a 7% improvement in mileage. Those were prechamber engines, but were otherwise identical. They had to add a vacuum pump to the throttle less engines to run the climate control and door locks.

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Old 12-06-2010, 11:13 AM   #152 (permalink)
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...and power brakes like vacuum.

Hard to get a good reading on energy content of fuels. What btu/gallon figure do you use for diesel and gasoline?

Here is one, admittedly less than perfect, way to look at the gains from diesel, epa figures for similiar cars:
2001 vw golf 4 cyl, 1.9 L, Manual 5-spd, Diesel : 35 city 44 hiway
2001 vw golf 4 cyl, 1.8 L, Manual 5-spd, Premium: 22 city 28 hiway

however I have no problems making over 50mpg in my gas 1996 saturn in the city, since putting a stick shift in it and p&ging the snot out of it. Hard to have the patience to beat 45mpg on the hiway though.
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Old 12-06-2010, 12:52 PM   #153 (permalink)
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Quote:
Originally Posted by Old Mechanic View Post
t vago

what do you think of the rotary engine design in the linked utube video? I understand it is operating as an air driven pump in the video, but I would be interested in your opinion.

I thought the greater efficiency in a diesel was due to the higher energy content of the fuel. Measured by the energy content of the fuel alone the diesels efficiency would be more comparable to a spark ignition gasoline engine.

Also what is you opinion of the Transonic type of super critical fuel injection?

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Mech
The difference in energy content of gasoline and diesel is about 10%, and diesel engines are typically 30% more efficient than gassers. It's not just the fuel.

And as a matter of fact, biodiesel has about the same energy content of gasoline, and I get better than 30% more fuel mileage over an equivalent gasser. Gasser Beetles average about 26 mpg, and diesel Beetles average about 42 mpg. That's about 60% better.
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Old 12-06-2010, 01:56 PM   #154 (permalink)
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One site I checked was 124k BTU for gas and 139k BTU for diesel, so that's 12% higher BTU content per unit of volume for diesel.

I wonder what the dyno figures would be in difference between gas and diesel engines at the load that would run both at best efficiency.

I think under those close to identical situations the difference is about 34% for typical gas engine and 40% for typical diesel, give or take a coupe percentage points for either.

Adding 12% to 34% would get you to 38% which is why I thought the most significant difference would be due to energy content in the fuel, with the rest due to other factors.

Pumping losses are something else I would like to see others opinions as to sources, but it would probably be better to start a different thread on that subject.

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Old 12-06-2010, 06:45 PM   #155 (permalink)
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I don't think you get the work back from compressing the valve springs -- the springs cannot be 100% spring back, they are pushing on the cam shaft, which is geared down, and there is a lot of friction involved in both directions. Can you imagine trying to spin the crankshaft by turning the crankshaft?

A rotating disk or cone or ball has to be less work than a poppet valve.

The Revetec engine is a "real" engine and it has been built, run, and tested.

The mechanical leverage that the connecting rod has on the crankshaft at 90d past TDC is much greater than it is very close to TDC or BDC. So, pressure in the cylinder at midstroke is *much* more important than pressure in the cylinder at TDC and BTC. All else being equal, the design I proposed would have much greater torque that a standard design.

What about a typical ICE design is preventing it from being no better than ~25% efficient? How can we get 2-3X more efficiency out of an ICE?
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Old 12-06-2010, 07:10 PM   #156 (permalink)
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more torque != more power

Do you have a source you could link to that states the %50 efficiency gain?
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Old 12-06-2010, 07:20 PM   #157 (permalink)
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Quote:
Originally Posted by NeilBlanchard View Post
I don't think you get the work back from compressing the valve springs -- the springs cannot be 100% spring back, they are pushing on the cam shaft, which is geared down, and there is a lot of friction involved in both directions. Can you imagine trying to spin the crankshaft by turning the crankshaft?
Actually, you can and do, with only the heat lost in cam/rocker/lifter friction and spring internal damping. Mechanical advantages do not lose work energy. No imagination necessary.
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Old 12-06-2010, 07:22 PM   #158 (permalink)
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Quote:
Originally Posted by dcb View Post
more torque != more power
Exactly. The work extracted from the engine is mostly from the expansion from TDC to the exhaust valve opening. Diesels have higher compression that leads directly to greater expansion and work extracted.
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Old 12-06-2010, 07:36 PM   #159 (permalink)
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Quote:
Originally Posted by Old Mechanic View Post
One site I checked was 124k BTU for gas and 139k BTU for diesel, so that's 12% higher BTU content per unit of volume for diesel.
Yes.

Quote:
Originally Posted by Old Mechanic View Post
I wonder what the dyno figures would be in difference between gas and diesel engines at the load that would run both at best efficiency.

I think under those close to identical situations the difference is about 34% for typical gas engine and 40% for typical diesel, give or take a coupe percentage points for either.
I am not following what you are trying to identify. To compare the engines, you load them identically and measure the fuel consumption. Gassers are generally let out on the street packing 50% to 100% more power than diesel engines, so perhaps you are thinking at a cruising load the diesel would be operating in a more efficient load range.

You may be right. That would help explain how gassers can close the efficiency gap with hyper-miling techniques.

Quote:
Originally Posted by Old Mechanic View Post
Adding 12% to 34% would get you to 38% which is why I thought the most significant difference would be due to energy content in the fuel, with the rest due to other factors.
It's a difference for sure, but I don't think you have your baseline efficiencies correct, so I don't think the energy content is as significant as you claim.

Quote:
Originally Posted by Old Mechanic View Post
Pumping losses are something else I would like to see others opinions as to sources, but it would probably be better to start a different thread on that subject.

regards
Mech
I think pumping losses are completely on target for this thread. Obviously gassers have much higher losses when operated at a percentage of full load because of the throttle. I don't know how to quantify that loss though. I'm sure there is good data on factory engines out there though where you can make apples-apples comparisons, say for example equalizing displacement or rated power. And you can equalize the fuel energy content as well, just use B100!
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Old 12-06-2010, 08:56 PM   #160 (permalink)
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UFO

Lets look at it another way, bsfc for most small passenger car diesels are right at 200 or just slightly lower in their sweet spot (generally speaking). For gas engines it's right at 250 (also generally speaking). Take 200 and divide that by .88 and you get 227.2, round that off to 227 and you have more than 50% of the difference due to the energy content of the fuel.

Most of the rest is due to the ability of the diesel to run at mixtures as high as 50 to1 while the gas engine can not do that at this time. Throttle losses also reduce the efficiency in throttled engines by reducing the actual compression ratio in the combustion chamber which is the same as having a lower compression and no restriction.

Not sure if we agree that those two principle factors, fuel energy content and actual in cylinder compression are the majority of the difference in efficiencies between gas and diesel. If you use the best mapped bsfc for each you are placing them in their ideal state of operation.

Looking at the maps themselves reveals what you referred to when you stated that hypermiling will generally result in a greater benefit in a gasoline engine.

DCB
I am not disputing you belief that people should be better trained to drive economically. I just think that vehicles could be much better designed (like the INNAS configuration which was a diesel) to allow much of the improvement to be incorporated into the vehicle itself.

Neil
From personal experience, when we checked for cam free motion in the L24-28 series Nissan engines we rotated the cam by itself, and checked it at various stages as the valve train assembly was completed. The "spring back" was easy to feel and the actual work to roate the cam and operate the valves was much less than you might imagine, even without pressurized lubrication.

The cheapskates would only machine the bottom of the head, and if they did not machine the top of the head and shim the cam towers to get the timing chain sproket to crankshaft sprocket dimension back in specs, two things could happen.

The camshaft would break in to at the center cam bearing, and the timing chain tensioner would be hyperextended and the chain would rattle or in severe cases the tensioner block could actually come completely out of it's housing with catatrophic results.

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Mech

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