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  #85  
Old 02-12-2014, 10:04 AM
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Re: Miles per Gallon

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Originally Posted by ExcursionDiesel View Post
BTW, diesels have less engine braking than gas motors so you may have noticed that going down steep hills produces higher rpms. This is due to the lack of a throttle plate to create a vacuum and slow the engine down when engine breaking.
This is incorrect unfortunately. Diesels have greater engine braking than gassers because of the higher compression generating more resistance while using the engine to brake. Engine braking works on the basis of compressing air in the cylinder against closed valves without the explosion. Higher compression, more resistance, more engine braking.
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Old 02-12-2014, 12:05 PM
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Re: Miles per Gallon

Zeros do horrible things to averages.

I was ironically able to average 17 mpg today by avoiding the highways and main roads. The tortuous route down the back roads does encounter frequent stop signs, but unlike the stoplights, stop signs aren't synchronized to require the maximum amount of 0 mph time.

So, I'm calling it a win.
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  #87  
Old 02-12-2014, 04:14 PM
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Re: Miles per Gallon

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Originally Posted by sc32valve View Post
This is incorrect unfortunately. Diesels have greater engine braking than gassers because of the higher compression generating more resistance while using the engine to brake. Engine braking works on the basis of compressing air in the cylinder against closed valves without the explosion. Higher compression, more resistance, more engine braking.
Unfortunately you are incorrect. Diesels do not have engine breaking. Although you are correct that Diesel engines work on high compression, a classic Diesel will act as a spring. During the compression phase the engine will convert rotational energy into heat (which is what's igniting the fuel) and into potential energy stored into the compressed air molecules (air is a compressible medium). During the power phase, if there is no fuel in the chamber, the compressed air will push down on the piston and cause all this stored compression energy (minus the energy converted into heat) to become back rotational energy which will move the vehicle forward (this is a classic case of physics "conservation of energy" law). The only way to get a Diesel to engine brake is to release the compressed air out of the chamber just before it imparts its energy back to the piston. This is done by opening the exhaust valves at the piston top dead center of the power phase. This is how Jacobs Engine Braking systems work (jake-brake) and when it is engaged you can hear all this air expansion energy with the typical machine-gun noise of the jake-brake.
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  #88  
Old 02-12-2014, 04:36 PM
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Re: Miles per Gallon

I agree with your points on a classic diesel. But you are assuming perfect conservation of energy and are leaving out other inherent restrictions AFTER the cylinder such as the DPF. A gassers restriction comes from the pressure gradient across the throttle body, while modern diesels have restriction through the exhaust. It may have been bold to say that diesels have better engine braking, but they ARE able to brake in a different manner compared to a gasser. You explanation of the J-brake was excellent. My apologies for starting the post saying that he was incorrect.
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  #89  
Old 02-12-2014, 04:50 PM
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Diesels have some engine breaking, no doubt. I can say unequivocally on my diesels, that they rev more freely down a hill than my gassers do. The VM3.0 really revs easy because of its small displacement vs the weight of the GC. My 7.3 Powerstroke in my 9k lb 4x4 Excursion engine brakes fairly well. My older 4R100 transmission is modified and programmed like the GC's 8 speed and downshifts with the torque converter locked to engine brake on decel. Its a real brake saver. Its just not as noticeable as a gas motor.
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Old 02-12-2014, 05:47 PM
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Re: Miles per Gallon

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Originally Posted by sc32valve View Post
I agree with your points on a classic diesel. But you are assuming perfect conservation of energy and are leaving out other inherent restrictions AFTER the cylinder such as the DPF. A gassers restriction comes from the pressure gradient across the throttle body, while modern diesels have restriction through the exhaust. It may have been bold to say that diesels have better engine braking, but they ARE able to brake in a different manner compared to a gasser. You explanation of the J-brake was excellent. My apologies for starting the post saying that he was incorrect.
I agree. Another possible source of braking power is the turbo which should cause some vacuum in the intake manifold when there's barely any flow of exhaust gases. The combination of this and the backpressure in the DPF and CSR laden exhaust you mentioned should present some type of resistance. Unfortunately, as ExcursionDiesel mentioned, the small engine footprint on a relatively heavy vehicle is leaving a lot to be desired as far as engine braking is concerned. I'm not planning to drive down Hawaii state road 240 any time soon, but if I would, I'd be concerned of overheating my brakes with an SFU.
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  #91  
Old 02-12-2014, 06:57 PM
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According to my Scanguage II, intake pressure (aka Boost) doesn't get much under -3 psi when deceling. Idle is -2.3 psi. 60 mph cruise may make 1-2 psi. WOT makes around 25 psi with my Bluespark tuner box.

Edit: Just did a quick test. Mild deceleration cause a slight vacuum increase but paddle shifting down at speed to generate higher engine braking actually decreases vacuum. The pumping action of the motor at higher rpms actually spins the turbo enough to make boost. This sounds like a perpetual motion machine but I suspect the air heated due to compression and engine temperature results in expansion and greater exhaust flow. Who knows? Its not what I expected.
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Old 02-12-2014, 11:45 PM
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Re: Miles per Gallon

Another way to produce engine braking is to add backpressure to the exhaust system via the Variable Geometry Turbocharger, or via an aftermarket butterfly valve in the exhaust pipe. The Cummins 6.7 pickup engine used in the Ram trucks uses the VGT to produce a impressive amount of braking.
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Old 02-13-2014, 04:11 AM
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Re: Miles per Gallon

Yeah, reducing the blade angle works pretty well. The poor mans jake, (a butterfly in the downpipe as used on the Cummins and others) also works quite well. Had both. I'm not too thrilled with the engine braking on this car. I brought it home across Snowqualmie Pass and had to brake to hold and it is not a steep pass. I'm curious to see how it goes once bass season starts up and I begin towing.
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  #94  
Old 02-13-2014, 04:18 AM
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Re: Miles per Gallon

Ive got all of you diesel sniffers beat in my SRT.....30.1 (s)miles per gallon....BEAT THAT BITCHES!
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  #95  
Old 02-13-2014, 08:16 AM
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Re: Miles per Gallon

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Ive got all of you diesel sniffers beat in my SRT.....30.1 (s)miles per gallon....BEAT THAT BITCHES!
What was your fuel economy driving to the peak (top) of those massively-imposing, Himalayan-dwarfing Kentucky mountains, before starting your descent on that looong, deeep, valley road?

I didn't get just 30.1 mpg driving downhill from Mt. Evans Colorado in a 2011 3.6L V6 Grand Cherokee...from 14,000 feet to the interstate. So...you're not trying hard enough...
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Old 02-13-2014, 08:06 PM
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Re: Miles per Gallon

Well I have had my GC for a bit over 1 month now. I had hoped to get decent mpg for my commute (~36 miles one-way) which is mostly freeway. So far, I am getting about 25 mpg on average over 3 tank fulls.

The best however was to see how it would do pulling my snowmobile trailer. I previously pulled my rig and gear with a 2010 Toyota Tundra, 4.6 L V8. Round trip up to Maine from Massachusetts averaged 10 mpg in the Tundra. I did the trip 2 weeks ago, same rig and gear, with the GC and the 3.0 L diesel. Phenomenal in my opinion! I got 19.2 mpg. I thought I didn't calculate correctly, but the round trip cost me 2 tanks of fuel.

I was averaging about 70 mph for most of the trip on the main highways, and then slowed down when we got to the curvy up and down secondary roads. Overall, really impressed with the ride and fuel consumption.

Noticed that if I stay in the 70 - 72 mph range I get a few more mpg. With cruise control, flat highway, I am getting 30 -32 mpg which for the 4 WD was great.
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