low compression diesel and fuel efficiency

[svizomang]

Hi, here is an explanation about mazda Skyactive d - ultra low compression diesel.
We know that higher the compression ratio beter the thermal efficiency. But as Mazda claims main diference of their low compression diesel engine is that they start main injection right after piston reaches TDC in contrast to others where main injection is switched 10-15 degres after TDC. Also since compression temp is lower the fuel has more time to better atomise inside the cylinder and so there are no hot spots that othervise contribute to soot and PM.

I am currios if all that is true since cx5 diesel compare to tiguan diesel is not that fuel efficient.

 

[btrueblood]

"cx5 diesel compare to tiguan diesel is not that fuel efficient. "

You mean the VW TDI diesel engine that runs dirty to beat mileage tests?

 

[enginesrus]

I'm out of the loop for the modern diesel tech, but sorry in the old days injection was just like spark advance in the old gasoline auto engines it was started before TDC. So nothing new there.
Low compression just satisfies to reduce the engines relation to a volcano (since that is big news now, Hawaii!) or what ever, for that particular crowd, it does not do a thing for efficiency.

 

[dgallup]

The only time I've seen decent efficiency coupled with "lower" mechanical CR is when using very high turbocharger boost pressures. Delayed injection certainly hurts efficiency. All these things are trade offs to try and clean up diesel exhaust with the least expensive after treatment possible.

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[RodRico]

svizomang,

There is a significant gain in efficiency from HCCI alone due to near instantaneous (constant volume) combustion, and the lower compression ratio reduces NOx emission relative to a diesel due to the lower combustion temperatures. In contrast, true diesels use constant pressure combustion and require NOx treatment systems due to the high compression ratio being used.

Rod

 

[Lou Scannon]

It's not the high compression ratio per se, but rather the local peak temperature of the flame due to it's being locally stoichiometric. The prevailing pressure during the burn obviously affects temperature (the driver for NOx) due to PV=nRT, so compression ratio plays a role, but combustion phasing wrt crank angle plays a strong role also.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[malbeare]

modern common rail diesels have multiple injection events per combustion event usually starting with a small pre injection at early timing and building with 4 more and avoid the usual diesel knock sound. Link I dont know how this works from a timing point of view . how do you get advanced timing if you are relying on compression pressure to create the injection pressure ? and also maybe how can you get multiple injection events . if injection holes are very small how do you get sufficient penetration into the outer areas of the combustion chamber ?

https://www.researchgate.net/profil...sented-by-DEC-1997-Courtesy-Dr-John-E-Dec.png

Link

A tidy mind not intelligent as it ignors the random opportunities of total chaos. Thats my excuse anyway
Malbeare

http://www.sixstroke.com

 

[svizomang]

Well I was talking about advertisement since Mazda claims their 14:1 compression ratio diesel is more fuel efficient than other on the market.
Well in my family we have 2015 Mazda CX5 with 2.2 skyactive D 150hp, 2017 Tiguan 2.0TDI with 150hp whic uses SRC aftertreatment and I have 2012 Audi A4 2.0 TDI 143hp - yes one with the engine included in fraud.
Mazda has compression ratio of 14:1 when both TDIs have someting round 16:1.
Fuel consumption on the 150km trip (70km of highway, with speeds od 120kmh and 80km of countriside roads with speeds up to 100kmh):
Mazda CX5 = 6.3 on the trip computer, real consumption is 6.6
Audi A4 = 5.8 on the trip computer, real consumption 6.3
Audi A4 (before beeing modified for NOx emmisions) = 5.3 on the trip computer, real consumption 5.9
Audi A4 (after beeing chip tunned and EGR deleted) = 4.7 on the trip computer, real consumption 5.2
Tiguan = 4.9 on the trip computer, real 5.3

*Those numbers are fuel consumption of liters per 100 kmh

so my conclusion is that EA288 engine in Tiguan that has comperable body and weight with CX5 is way way more fuel efficient. And EA288 was strictly tested and meets Euro 6 or Bin 5 emmisions standars.

So Mazda claiming that their engine is more fuel efficient is crap. VW consumes less since it has higher compression ratio and I don't believe they use retard injection as Mazda claims. They inject fuel at TDC and they indeed produce high amount of NOx, but those are aftertreated in the exhaust system using SRC technology.
You can't make fuel efficient and low NOx engine - it just don't add up. This is surely seen on my Audi A4 with EA189 engine. After dealer has modifiend ECU to meet the Euro 5 regulation the engine is completly diferent - less power and more fuel thursty. If they will add SRC (to costly I gues for VW) car will have same performance and same MPG.

And Mazda having no adwanced injection with 14:1 compression or VW having retard injection with 16:1 ratio is basicaly the same. You sacrify aditional fuel in order to met NOx emission without the need of SRC technology. Realy fuel efficient cars are those with SRC.

 

[enginesrus]

16:1 is the low side for a diesel.

 

[TugboatEng]

16:1 is very average. The higher numbers are all engines with precombustion chambers and their compression ratio is artificially high because the pre-chamber volume isn't included in the ratio calculation for whatever reason.

 

[RodRico]

svizomang,

Regarding your comment "You can't make fuel efficient and low NOx engine - it just don't add up," the consortium of national labs, universities, and automakers researching the topic under the Department of Energy's Advanced Combustion initiative disagreed. The combustion video and associated parameters posted at Sandia Labs indicate "High efficiency (~50%), limited to lower loads unless EGR and/or high boost." Enter the string between braces into Google for more: [HCCI LTC "efficiency" site:

http://www1.eere.energy.gov/vehiclesandfuels/pdfs/

filetypedf]


Your statement "Really fuel efficient cars are those with SRC[sic]," may be correct due to the higher compression ratios available, but study after study has shown SCR to be ineffective in consumer vehicles because so many owners only fill the SCR urea tank when it's time for an emissions test. Some of the newer vehicles monitor the tank, but a quick search reveals folks strive to bypass those sensors. Even those that don't are confronted with the need to buy another consumable, and SCR adds some complex subsystems, so it's less than ideal IMHO. Note, by the way, there's a nice brief on diesel NOx reduction on the Department of Energy site.


Rod