US truck fuel economy and Obama's new rules 5

[JayMaechtlen]

In thread71-223596 it seems that fuel economy has taken a real hit in the last few years due to emissions requirements.
Now, we see "Obama Signs Order to Boost Trucks' Fuel Economy"
Obama said the government believes that "we can increase fuel economy by as much as 25% in tractor trailers using technologies that already exist today."

http://www.ttnews.com/articles/basetemplate.aspx?storyid=24442

(There was a discussion in Pat's Pub, but this is a better place to discuss this.)




Jay Maechtlen

http://www.laserpubs.com/techcomm

 

[JayMaechtlen]

Cost of blipping a carburated gasoline engine is probably far greater than with a diesel engine.
In fact: if a diesel is under load in upper rpm and dragged to lower rpm by power shift, what is change in airflow versus fuel flow? Does that cause a brief overfuel condition?
Does dropping the load and picking it up again (during the non-power shift) result in worse or better net fuel combustion?

Jay Maechtlen

http://www.laserpubs.com/techcomm

 

[SomptingGuy]

Purely hand-waving...

If a diesel were subjected to a sudden decrease in engine speed at full load, I'd expect it to lean out until the turbo slowed down. Fuel injection quantity (unless mapped to engine speed) shouldn't change. Boost pressure would temporarily be higher than the steady-state condition.

- Steve

 

[patprimmer]

Certainly diesels with mechanical drive injection pumps had WOT fuel delivery tied to rpm as the pump was tied to rpm.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

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[SomptingGuy]

... I meant quantity per shot, not fuel flow rate.

Maybe quantity per shot is also mapped these days with common rail. It never was with mechanical FIE. Maybe modern truck engines have clever control systems to run the engine during rapid transients? Some light-duty diesels have such systems.

- Steve

 

[rmw]

Dicer,

Every HD vehicle automatic transmission (torque converter type) I have ever encountered operated where the engine to change RPM as it shifted through the various gears.

The only difference between them and the manuals was that the transmissions shifted when they wanted to, not when the driver chose to make the shift and did it without clutch action - the torque converter took care of that.

Maybe someone can adapt the Cat powershift transmission to HD vehicles and Obama can get his fuel mileage increase out of it.

rmw

 

[tbuelna]

Maybe this is stating the obvious, but an immediate and cost-free approach to reducing the amount of diesel fuel used by trucks on US roads would be for Obama and Congress to quit over-regulating and over-taxing US businesses. If more goods were produced within the US, they would not have to be trucked in from places like Mexico.

Encouraging production of goods domestically and closer to their point of sale would also have the added benefit of reducing prices.

 

[catserveng]

You may find the CAT powershift a bit heavy, in reality it is VERY heavy, but the transmission weight really isn't an issue in a bulldozer is it?

CAT actually had an on road automatic transmission under test for a number of years, along with a turbine based on road truck driveline. The tranny was a beast, nearly indestructable, but nearly twice the weight of a competitor at the time. Can't remember the model number, but it was a commercial failure and got shelved.

In the early to mid 90's CAT spent a ton of money on owner and driver education, and getting the best possible fuel economy. They made significant improvements and worked with truck builders to come up with some helpful changes. In earlier electronic engines they introduced "advance on steady state" timing strategy, which helped fuel economy with very little negative impact on emissions, but ended up costing them (and some of the other manufacturers who tried the same thing)millions of dollars in fines to the EPA.

I think in a nutshell in this country we don't have a well integrated energy and environmental policy, it seems the two get in conflict not only in trucking but in other areas, like marine and electric power as well. I did some cogeneration projects in Denmark and Holland in the late 90's and was impressed by the emissions regulatory people I met, as they seemed to be more technically proficient in engine technology and their related emissions, and would actually try to help improve a system instead of just "whining and fining" like so many of the regulators I dealt with here at home.

 

[ptours99]

today,the big deal is gear it to run fast and drive slow.this works good on flat land and no restrictive winds while under cruise control,which all truckers select.climbing hills,windy conditions,rain effect mpg as we all know.this doesn't cause it to go lean,it's removing the air from the equation and lowering volumetric efficientcy.you can't run a diesel in a high load demand under full t/position and expect it to get near same results as less load stress.under these conditions using c/control should be eliminated.the closest you can get to 100%of V/E is @peak torque.peak tq is above 1600rpm.egt and boost guages should be used extensively for mpg,higher manifold press and egt means less mpg.learning how to use the operation of the motor needs to be relearned . more mpg can be obtained just by letting air catch up to fueling under load in a 2 second delay manner while accel and will gain a 10%boost in mpg.

 

[BrianPetersen]

All modern diesel engines have fuel injection quantity per shot mapped to power demand, intake pressure and temperature, etc and modern control systems are capable of recalculating the required injection quantity and changing it for each individual shot. If engine rpm changes rapidly then the injection quantity will be recalculated before the next injection shot happens. Transient conditions with regards to the fuel delivery aren't an issue any more. Also, if the turbocharger comes off boost during a long manual gearchange, the new injection quantity will be automatically recalculated to compensate for this. No more puffs of black smoke on taking up the load like in the old days, and no more wasted fuel making that puff of smoke.

Having said that, I don't think manual vs automatic vs powershift or whatever you call it is the main culprit behind higher truck fuel consumption nowadays. Maybe in mountainous areas or in city driving there might be some effect, but most long haul trucking is done at more or less steady speed on main highways, and then the only thing that matters is how mechanically efficient the transmission is (and whether it's geared appropriately to the engine).

It's the emission controls. Injection timing has to be moved away from best-BSFC settings in order to cut engine-out emissions and/or regenerate the various catalytic converters.

I don't know if the heavy trucks are using active regeneration of their DPF catalysts but if they do (like the new VW diesels and pickup truck diesels do) that's costing some fuel consumption. The new VW diesels have higher fuel consumption during real world city driving than the previous pre-DPF models. On the highway, they're not too different, perhaps even a little better (but they've got the benefit of gear number 6 in the manual transmission).

Engines that are using in-cylinder NOx control are probably compromising fuel consumption relative to those that are using SCR and AdBlue. Even on those using AdBlue, if the compromise has been shifted towards cutting engine-out NOx in the interest of cutting AdBlue consumption, that could cost something.

The envirofreaks fail to realize that you cannot achieve perfection and you cannot have everything your way.