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#3188344 - 11/15/13 08:22 AM Gas & diesel engines are converging
blackman777 Offline


Registered: 01/08/13
Posts: 981
Loc: Santa Ana, California
Gasoline/petrol and diesel engines appear to be converging in their technologies. Gas engines are adopting diesel techniques like direct injection, lean operation, higher MPGs (40+) while diesels are moving towards low compression, richer burns, and lowering MPGs. (Example: Jetta TDI dropped 45 downto 42.)

Example from Mazda engines:
- SkyActive G-motor is 14:1 (highest compression for a gas engine)
- SkyActive CDI engine is 14:1 (lowest compression for a diesel)
They are almost identical in design, except one is spark ignition & the other is not. Cool. :-D



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#3188352 - 11/15/13 08:33 AM Re: Gas & diesel engines are converging [Re: blackman777]
brandini Offline


Registered: 10/08/09
Posts: 1005
Loc: Richmond, VA
Uh identical being that the diesel has 2 turbos? Seriously?

And the jetta also grew in size and has to run ULSD now and has more strict emissions requirements... not to mention the change in MPG measurement in 2008.
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#3188356 - 11/15/13 08:40 AM Re: Gas & diesel engines are converging [Re: brandini]
blackman777 Offline


Registered: 01/08/13
Posts: 981
Loc: Santa Ana, California
I didn't realize the CDI had 2 turbos. I guess that helps achieve the ultra-low compression? Also the numbers quoted for the older Jetta ARE the new test results, because the original EPA rating was 49 highway (and then revised to 45).

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#3188363 - 11/15/13 08:50 AM Re: Gas & diesel engines are converging [Re: blackman777]
asand1 Offline


Registered: 12/16/12
Posts: 818
Loc: Oregon coast
Lol. This is not a case of technologies evolving naturally. This is a case of goverment mandates fprcing cars to be less efficient for the sake of being cleaner.
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#3188398 - 11/15/13 09:19 AM Re: Gas & diesel engines are converging [Re: blackman777]
NateDN10 Offline


Registered: 01/10/10
Posts: 1406
Loc: Rochester, NY
One of the main reasons diesels typically use such high compression is for cold starts - the higher the CR, the more the temperature in the combustion chamber increases when the piston moves from BDC to TDC on the compression stroke. Mazda had to do some interesting things with injectors and valve timing to get reliable cold starting with low CR.
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#3188455 - 11/15/13 10:07 AM Re: Gas & diesel engines are converging [Re: blackman777]
440Magnum Offline


Registered: 02/01/09
Posts: 6344
Loc: Texas
The degradation in diesels is almost entirely emissions-driven. Once the manufacturers figure out the best way to orchestrate particulate filtration, aftertreatment, and EGR I hope (knocking wood) that diesel specific fuel consumptions will drop back down and continue to exceed spark-ignition. Given what actually goes on in the little chemical refinery that is a modern diesel exhaust system, I'm surprised they work at all, let alone work as well as they do. It'll get better.

But in some ways you're right- DI on the gasoline side at least opens the door to a true "all-fuels" engine of the type that was being researched by Ford and Texaco (among others) back in the early 80s. At that time, a real stumbling block was fuel pumps and injectors that could direct-inject very low-viscosity low-lubricity fuels like gasoline and alcohols and hold up over the long term. But that's obviously sorted out now, and its emissions, sensors, and engine management required to operate on such a wide range of fuels. Imagine what would have to go on with the engine management system if a driver started with a tank of diesel, filled up with gasoline at 3/8 of a tank, and then bought E85 when the needle read 1/4 tank! It CAN be done, but right now the market isn't there to support the required cost.
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#3188473 - 11/15/13 10:27 AM Re: Gas & diesel engines are converging [Re: 440Magnum]
sdowney717 Offline


Registered: 10/01/05
Posts: 236
Loc: Newport News, VA
Even if such an engine existed, people would soon figure out what was the cheapest fuel that gave the highest mpg and use only that.

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#3188491 - 11/15/13 10:56 AM Re: Gas & diesel engines are converging [Re: blackman777]
asand1 Offline


Registered: 12/16/12
Posts: 818
Loc: Oregon coast
And then what would you put pn tje window sticker for epa estimated mpg?
_________________________
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1996 Maxima 300K
2008 Uplander 80K

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#3188611 - 11/15/13 01:12 PM Re: Gas & diesel engines are converging [Re: sdowney717]
440Magnum Offline


Registered: 02/01/09
Posts: 6344
Loc: Texas
Originally Posted By: sdowney717
Even if such an engine existed, people would soon figure out what was the cheapest fuel that gave the highest mpg and use only that.


Which would (eventually) drive fuel to be priced per BTU instead of per gallon. It would take more gallons of gasoline to equal the same BTU content in diesel, but the cost to the consumer would be the same. Which would ultimately allow more effective use of crude fuels (including crude oil, unprocessed recycled oil, unprocessed bio-oils, etc.)
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'66 Dodge Polara & '69 Dodge Coronet R/T both 440/727
'08 Ram 1500 4.7/545RFE
'12 Challenger SRT8 392/6-speed
'99 Cherokee 4.0, '11 Grand Cherokee 3.6

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#3188769 - 11/15/13 04:08 PM Re: Gas & diesel engines are converging [Re: 440Magnum]
blackman777 Offline


Registered: 01/08/13
Posts: 981
Loc: Santa Ana, California
If you compare the BTU/mile of a modern Jetta diesel vs. Jetta gasoline, they are almost identical (~3150). Diesel has lost its efficiency advantage.

And in terms of cost, it's a tossup which is cheaper to operate..... I would guess the gasoline model, since diesel often costs the same as supreme.

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#3188803 - 11/15/13 04:34 PM Re: Gas & diesel engines are converging [Re: NateDN10]
Shannow Offline


Registered: 12/12/02
Posts: 26811
Loc: a prison island
Originally Posted By: NateDN10
One of the main reasons diesels typically use such high compression is for cold starts - the higher the CR, the more the temperature in the combustion chamber increases when the piston moves from BDC to TDC on the compression stroke. Mazda had to do some interesting things with injectors and valve timing to get reliable cold starting with low CR.


Nope, C.R. equals thermal efficiency...that's a diesel fundamental.

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#3189247 - 11/16/13 07:12 AM Re: Gas & diesel engines are converging [Re: Shannow]
blackman777 Offline


Registered: 01/08/13
Posts: 981
Loc: Santa Ana, California
Mazda says their low-compression diesel is More efficient (versus their standard H.C. diesel) wwith more thorough burning of the fuel & less soot produced. They were also able to increase the redline to almost 6000 rpm, like a gasser

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#3189406 - 11/16/13 10:46 AM Re: Gas & diesel engines are converging [Re: blackman777]
TiredTrucker Offline


Registered: 01/04/08
Posts: 1015
Loc: Kellogg, IA
I often wonder, if gassers were required to have the same emissions control devices on them that the current crop of diesels have, how the public would react. Imagine.... soot filters like DPF's on diesels, having to add Urea based products for SCR units, etc. And the associated costs when these systems take a dump.

I am convinced that the lower compression ratios being tried out in diesels has more to do with getting the engine to a level that would facilitate eliminating the EGR systems on them. This would reduce NOx levels to where the SCR could be the single solution, and it would reduce soot levels that have been a big part of EGR use on diesels so that the particulate filters last longer and have longer regen intervals. Once everything is moved off of the engine, and all emissions stuff is kept downstream, engines would have greatly reduced maintenance issues.
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#3189834 - 11/16/13 09:14 PM Re: Gas & diesel engines are converging [Re: blackman777]
artificialist Offline


Registered: 09/23/07
Posts: 6899
Loc: Florida
Originally Posted By: blackman777
Mazda says their low-compression diesel is More efficient (versus their standard H.C. diesel) wwith more thorough burning of the fuel & less soot produced. They were also able to increase the redline to almost 6000 rpm, like a gasser

Yeah, Mazda bragged about their new technology making a 5300 RPM rev limit possible, that can't exactly be true. They said their 14:1 compression ratio made it possible.

Back in the 80s, there were numerous diesels in cars that revved to about 5000 RPM, and they had about 21:1 compression, because engines using pre-chambers needed extra compression.
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#3189982 - 11/17/13 02:42 AM Re: Gas & diesel engines are converging [Re: blackman777]
Shannow Offline


Registered: 12/12/02
Posts: 26811
Loc: a prison island
Originally Posted By: blackman777
Mazda says their low-compression diesel is More efficient (versus their standard H.C. diesel) wwith more thorough burning of the fuel & less soot produced. They were also able to increase the redline to almost 6000 rpm, like a gasser


the basic thermodynamic cycle that IS the diesel cycle increases efficiency by increasing the compression ratio...yes, less soot is "more efficient" than more soot, but not to the degree that compression drives efficiency.

The reduction in soot is a design specific, and soot reduction can be applied to a diesel of high compression also...similarly, compression doesn't change redline...my 3.0L turbodiesel Nissan will pull 4,200 RPM (redline is 4,500 but governor won't let it go there)

TiredTrucker, IMO is spot on that they are trying to reduce NOx with lower compression.

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