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Old 07-06-2008, 07:15 AM   #169
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Diesel Power

Our "Diesel Weasel" '06 Jeep Liberty 2.8l CRD has been getting 21-22 mpg towing the boat lately.
Not quite as heavy as the Airstream but with A/C on the mileage has been taking quite a hit.
We picked this up as an alternative fuel vehicle, alternative as in if the gas dries up for a while.
I would love to find a '97 Dodge Cummings pick up but this Jeep was the best I could find.
Have spent quite a bit to bring it up to snuff as in lousy Origional Manufacturers Equipment (OME) that needed to be upgraded but the family likes it.
I would not spend the $40+K for a new diesel Jeep.
Honda, Toyota, etc. will probably come out with something soon too.
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Old 07-06-2008, 10:14 AM   #170
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Hi, I have to respectfully disagree with you on this subject. Smaller engine being pushed hard versus a larger engine that is not having to work as hard will get better mileage. I owned a 1988 Ford Ranger 2.3 L, 4 cyl. 5 speed stick and at the same time my wife owned a 1990 Ford Thunderbird SC 3.8 L, V-6, with automatic trans. [both purchased new] The Thunderbird got quite a bit better gas mileage, both in town, and on the road. Next, I had two company trucks, very close to the same size and weight; And both trucks were always loaded to the max. The Chevy 350 cu. in. got 4.8 miles per gallon and the Ford 460 cu. in. got 7.5 miles per gallon. So sometimes, but not always, a larger engine will get better gas mileage.
Two different engines, each in a different vehicle with massively different aerodynamics, one with a supercharger. Do you really believe that it's a fair comparison to evaluate the claims that I am making?

There are other factors at work like compression ratio, gearing, and in the example above an enormous difference in aerodynamics.

I'm not saying that a bigger engine is less efficient. I am saying this: A gasoline engine reaches its peak efficiency at full throttle. It's a simple fact of thermodynamics.

I will restate my example of multidisplacement engines. The new small block Chevys, and the Chrysler Hemi, as well as other engines now have the ability to stop the intake and exhaust valves from opening on certain cylinders. But shutting down cylinders you "make the other ones work harder." The computer opens the throttle at the same time so you never notice a change in power. There's no throttle cable, the throttle is controlled by the computer. The computer decides the combination of throttle position, transmission gear, and number of cylinders that will deliver the best fuel economy.
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Old 07-06-2008, 11:19 AM   #171
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My experience is with Dodge. As to whether the smaller engine working harder uses more fuel, this is what I have observed. All else being equal and unloaded the last generation of 360 and 318 cubic inch gas engines got the same fuel mileage. When loaded and driven at the same speeds the 360 did better. The smaller engine could very well have been operating at its' "maximum efficiency" but still did not do as well as the larger engine. I saw this enough that I eventually quit ordering the 318 engine when the 360 was available. I now see this same phenomenon with the current 4.7 and 5.7 engines - both pre and MDS versions.

Often someone will request the smaller engine hoping for better fuel mileage coupled with a lower gear ratio to help with power. This combination only makes things worse and these tend to be the customers that complain the most about their mileage. This too is a combination that I try to avoid unless cornered by Dodge - For instance the current 1500 Quad Cab ST is only available right now with the 4.7. If you don't want to pay the price for the SLT to get the Hemi then it might make sense to order the lower available ratio if you intend to tow.

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Old 07-06-2008, 12:14 PM   #172
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[quote=canadianguy;586220]To the originator of this thread: I won't flame you for trading diesel for gas, just for making a rather dodgy purchase.



Not true. A gasoline engine has maximum efficiency when working as hard as possible. ]


Not true. Easy test, drive you gas vehicle at full throttle all day and then do it at half throttle. At which position was it most fuel efficient? Now there is a point at which a gasoline engine performs more efficiently at a higher throttle setting than at a lower one but definitely not at full throttle as you say.
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Old 07-06-2008, 12:47 PM   #173
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Drive on the highway all day in top gear. The next day drive all day in your second from top gear. In which case was more fuel used? In which case was the manifold vacuum higher? In which case is the engine MEP (mean effective pressure) higher (ie. "working harder")?

Your vehicle is geared to go faster with the throttle open. Every 10% increase in speed takes 21% more energy to cover the same distance. So while the efficiency of the engine improved in the wide open throttle case of your thought experiment, the vehicle's efficiency in using that energy to move down the road went down more than the engine's efficiency.
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Old 07-06-2008, 01:03 PM   #174
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Of course your fuel efficiency will be worse driven in a lower gear. I would want to see the equation that proves driving at full throttle is more efficient than driving at half throttle. Being geared to go faster with more throttle doesn't equate to better fuel efficiency. You said a gasoline powered car is more efficient working as hard as possible, every bit of my real world experience says otherwise, could you please explain this as I am curious as to how this is possible? Having asked I do realize that there is a point at which gas engines are operating at opitmum effiency and that that point is at a much higher rpm vs a diesel but how does efficieny continue to increase beyond that optimum level that in my experience has always been below wide open throttle?
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Old 07-06-2008, 01:26 PM   #175
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I don't believe I said driving as hard as possible is best for efficiency. I said that a gasoline engine is most efficient with the throttle wide open, this doesn't imply high RPM.

What I'm saying is this:
1.) There's this thing called the intake pumping loss.
2.) The higher the manifold vacuum, and the higher the RPM, and the bigger the engine, the higher the intake pumping loss.
3.) Intake pumping loss wastes more energy than the exhaust pumping loss.
4.) Opening the throttle, lowering the engine RPM, and a smaller engine reduces the intake pumping loss.
5.) There is new technology from the auto sector to reduce the intake pumping loss.
6.) Intake pumping loss is thermodynamic fact known for over 100 years.


Consider shifting down in a naturally aspirated gasoline engine in order to brake the vehicle on a long grade. Where is the breaking action coming from?

The throttle is closed so the engine basically has idle airflow, so there's idle exhaust flow, so hardly any exhaust backpressure. We conclude that the brake action doesn't come from exhaust back pressure.

Now what happens when the intake valve closes and you compress the gas in the cylinder? You expend energy, but they you let the gas expand, so you get your energy back. Just like a compressed air tool. But wait, the gas is a flammable mixture that ignites at the top of the compression stroke. So you actually gain energy from the combination of compression and power stroke.

So which stroke of the standard four stoke Otto cycle does this engine braking action come from?
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Old 07-06-2008, 09:43 PM   #176
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A gasoline engine IS at it's most efficient at WOT. For power. But not for economy. The least amount of throttle opening to do the work is fuel economy. For some engines, in some vehicles, WOT is what it will take to do the work. For another, 1/3-throttle.

Matching the vehicle to the task is the key. Too many Americans with too much vehicle. An Excursion MIGHT be okay for a family with [8] adult-sized members and their gear plus a trailer. For a family of three, it is a poor choice for "economy" when other, often much smaller tow vehicles, can do the same job.

Overkill is not economy.
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Old 07-06-2008, 09:59 PM   #177
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I am talking about fuel economy. The engine that needs WOT to make the power will be more fuel efficient than the one that needs 1/3 WOT. Again, consider driving on a level freeway. Are you more efficient in a higher gear with the throttle more open, or the lower gear with the throttle more closed?

There are probably exceptions with some older carburated performance engines that had the secondaries tuned to a more rich mixture to prevent engine knock at higher power output. I would expect that even with some severe enrichment that the fuel efficiency of the engine is still better at wide open throttle.

I would also like to add a further thought to my line of reasoning on the significance of this intake pumping loss. I would like to predict that a diesel engine with no "Jake Brake" or exhaust block valve, with it's lack of input pumping loss and output pumping loss, would have really poor engine braking performance in relation to a gas engine. Could somebody with first hand experience please confirm or deny my prediction?
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Old 07-06-2008, 11:06 PM   #178
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Sorry, Clear as mud!

Hi, in relation to gas engines only, the best fuel economy would be at the highest vacuum reading and in the highest gear in relation to speed. That being said, top gear at freeway speeds [55-65 mph] would be best, but if you are only going 20-25 mph maybe your highest vacuum reading would be in second gear. Now, while towing a trailer, you might find it better to drop down one gear to acheive a higher vacuum reading, while at the same time, less throttle and more RPM to maintain freeway speeds and more fuel efficiency.

Diesels, not my game, so someone else can chime in on them.
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Old 07-07-2008, 07:01 AM   #179
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I think I have seen this in another thread where you have a point trying to be made only using one part of the solution, and not considering the other part that plays into it.
Fact 1; all engines have a power curve where it starts at operating low RPM (not idle) and then as the RPM increases so does power efficiency to peak then tapers off as the RPM continues to climb until the RPM peaks. An engine at max RPM has less power
Fact 2; An engine operating at max RPM will wear out its parts twice as fast as one operating at mid RPM, and 4 times as fast as one using low RPM.
Fact 3; a smaller displacment engine will have to use more power & gas longer to reach its operating torc point, where a larger displacment engine uses a little less power to reach torc but will reach it faster, both will use the fuel to reach that point.
Fact 4; that mentioned in #2 does not dictate fuel economy, what dictates the fuel economy is which one reachs that point the quickest thus easeing off the gas quicker.
Fact 5; it does not matter whether an engine is gas or diesel the fuel economy can not be found without adding the gearing into the solution. 3 engines exactly the same will have different results if one is attached to a low geared trans & rear end, the other to high geared trans & rear end, the other attached to mid range trans & rear end. Reason is because the gearing dictates whether they are operating below, above, in the max power curve. Another secondary solution to gearing is the set up between the trans & rear end:

Just thought I would throw this into the mix, now maybe a certified mechanic in here can give better explanations as to how these set ups have a influence on fuel economy as well as what the best set up for peak effiency, fuel, engine wear.

Sarge
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Old 07-07-2008, 08:54 AM   #180
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Well done Sarge. I like facts. I have always contended, and stated this in other threads, that best fuel efficiency is achieved when the engine runs at peak torque (the high point of the power curve). Peak torque is ,"The maximum torque a machine can exert, achieved at a certain rpm. After torque peaks, it will decrease with increasing rpm."

The basic reason a peak torque exists, is that every engine has an RPM that it runs the most efficient at. With the hundreds of moving parts in an engine, with varying coefficients of friction, and all the parts reaching different temperatures and expansions, etc. etc., the peak torque is the RPM where all these factors combine to allow the engine to run most efficiently.

It is this RPM (or very close once air resistance is mixed in) that I believe peak fuel efficiency is realized. Just my 2 cents.
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Old 07-07-2008, 02:27 PM   #181
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Originally Posted by SARGE/AF View Post
I think I have seen this in another thread where you have a point trying to be made only using one part of the solution, and not considering the other part that plays into it.
Fact 1; all engines have a power curve where it starts at operating low RPM (not idle) and then as the RPM increases so does power efficiency to peak then tapers off as the RPM continues to climb until the RPM peaks. An engine at max RPM has less power
Fact 2; An engine operating at max RPM will wear out its parts twice as fast as one operating at mid RPM, and 4 times as fast as one using low RPM.
Fact 3; a smaller displacment engine will have to use more power & gas longer to reach its operating torc point, where a larger displacment engine uses a little less power to reach torc but will reach it faster, both will use the fuel to reach that point.
Fact 4; that mentioned in #2 does not dictate fuel economy, what dictates the fuel economy is which one reachs that point the quickest thus easeing off the gas quicker.
Fact 5; it does not matter whether an engine is gas or diesel the fuel economy can not be found without adding the gearing into the solution. 3 engines exactly the same will have different results if one is attached to a low geared trans & rear end, the other to high geared trans & rear end, the other attached to mid range trans & rear end. Reason is because the gearing dictates whether they are operating below, above, in the max power curve. Another secondary solution to gearing is the set up between the trans & rear end:

Just thought I would throw this into the mix, now maybe a certified mechanic in here can give better explanations as to how these set ups have a influence on fuel economy as well as what the best set up for peak effiency, fuel, engine wear.

Sarge
Now this is what I have always heard, learned from experience and believed. I have had a few motorcycles dynotuned and all of them increase in power until a certain rpm and then level off or lose power. I had one bike in particular that I knew I needed to shift at or just before 5200 rpm's(this bikespeak in it's torque curve)otherwise I would lose a little power and acceleration.
Thought I was losing it for a while there.
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Old 07-07-2008, 02:30 PM   #182
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Well done Sarge. I like facts. I have always contended, and stated this in other threads, that best fuel efficiency is achieved when the engine runs at peak torque (the high point of the power curve). Peak torque is ,"The maximum torque a machine can exert, achieved at a certain rpm. After torque peaks, it will decrease with increasing rpm."

The basic reason a peak torque exists, is that every engine has an RPM that it runs the most efficient at. With the hundreds of moving parts in an engine, with varying coefficients of friction, and all the parts reaching different temperatures and expansions, etc. etc., the peak torque is the RPM where all these factors combine to allow the engine to run most efficiently.

It is this RPM (or very close once air resistance is mixed in) that I believe peak fuel efficiency is realized. Just my 2 cents.

This also jives with my experiences. I am however only an amatuer mechanic at best.
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