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Old 11-19-2007, 07:16 PM   #1
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Interesting observation

As we all know, diesel engines can run much leaner than a gasoline engine. But just how much leaner can they run in easy to see black and white terms?


Well, here is an example. Here are 2 completely different engines in this comparison;


2.8L gasoline 4 cylinder in a 2006 Chevy Colorado
175 hp
185 ft lbs
idle speed of 650 rpm
idle fuel consumption 0.3 gph

7.2L Cat C7 6 cylinder diesel engine in a 2007 model Blue Bird school bus
220 hp
520 ft lbs
idle speed of 700 rpm
idle fuel consumption 0.4 gph


Even though the diesel engine is 2.5 times the size of the gas engine in this comparison along with having 2 more cylinders, the gas engine only uses 25% less fuel at idle.



data was collected from a Scan Gauge 1 for the Chevy and the stock instrumentation for the school bus.
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Old 11-20-2007, 06:15 AM   #2
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That is one of the factors that contributes to diesels getting better FE. The other main thing is the fuel holding more energy than the same amount of gasoline. It was nice to see those figures, and eye-opening about just why larger trucks/buses favor diesel. I can only imagine what the idle would be on a similarly sized gasser.

Speaking of which, your Colorado is only a 4 cylinder? That is pretty big displacement for a four banger. I don't know that I have ever seen anything much bigger than 2.2 on a 4.

I have to question if some of the difference was due to the different testing instruments though.

Did you get both of the vehicles up to their running temps before taking the readings?
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Old 11-20-2007, 10:58 AM   #3
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1) I'm curious to know the idle fuel consumption of a VW TDI, which is similar in size to the Colorado....... Any Jetta owners out there with a scangage?

2) Large 4 cylinder engines have vibration at 2 times the crankshaft speed due to connnecting rod imbalance. The Colorado 2.8 and 2.9 liter engines use balance shafts to make them run smooth. Several other big fours have used balance shafts.

Many big fours like GM's 2.5 "Iron Duke" or the Vega 2.4 don't use them. They were noisy, especially the aluminum 2.4.

Many big diesel fours also do not have balance shafts, since they operate at much lower rpm than gas engines, and vibration is not as noticeable.
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Old 11-20-2007, 11:10 AM   #4
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Yes even in the the VW Jettas. The 1.9L diesel idles at about .25L/hr (.06gal/hr) and the gasser 1.8L idles at .6L/hr (.15gal/hr)
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Old 11-20-2007, 03:46 PM   #5
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Quote:
Originally Posted by Erdrick View Post
Speaking of which, your Colorado is only a 4 cylinder? That is pretty big displacement for a four banger. I don't know that I have ever seen anything much bigger than 2.2 on a 4.

I have to question if some of the difference was due to the different testing instruments though.

Did you get both of the vehicles up to their running temps before taking the readings?
Yep, my Colorado is only a 4 banger. Chevy basically took the 4.2L I6 from the Trailblazer and chopped it down to make the 3.5L 5 cylinder and the 2.8L 4 cylinder. I have been tempted to research how to disable the balance shafts to see if that would free up a bit of power/F.E.

Yes, both vehicles were completely up to tempurature when I took the readings. There could be a bit of variance because of the two differant computers.
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Old 11-27-2007, 05:16 PM   #6
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It isn't that the diesel runs lean...the combustion event in a diesel is completely different. Gasoline burns only as a vapor mixture. Diesel burns on the surface of the droplet. As long as there is enough air in the combustion chamber to completely burn the fuel diesels couldn't care less about mixture.

The diesel thermodynamic cycle is simply more efficient than the Otto cycle.

Here's insult to injury: 3000 HP railroad diesels idle burning only 3 gallons per hour.

BTW, I would not recommend disconnecting the balancing shaft in your 4-banger. The thing will shake every nut and bolt out of it within a few hours. Those engines need that balancing shaft.
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