4 cyl Silverado gets worse mileage than the V8

Started by Payman, January 22, 2019, 11:39:43 AM


Xer0

C&D just had an article on this as well.  This engine is EPA gaming and it still cost them millions of dollars to develop while being objectively worse pretty much across the board from what they are already have.  So sad.

giant_mtb

Not surprised.

I didn't know they were starting to put 4-cylinders in Silverados.  Woooooof no thank you.  I'll take a GM V8 any day over that.

2o6

Most of these new 4cyl engines only get the economy achieved when out of boost.

I can't tell whether this means the 2.7T is crap, or the 5.3 is a very good engine.

Galaxy


MX793

The explanation they gave is a wive's tale.  The reason C&D's testing showed this is pretty straightforward.  At a steady cruise, the truck only needs maybe 50 horsepower to maintain speed, so both engines will be loading along at partial throttle.  The vehicle weight difference matters when accelerating, but is negligible for steady cruise.  Their respective aerodynic properties are the same.  The V8 has cylinder deactivation, so it will be running on only 4 cylinders (now a 2.65L 4-banger).  So now you have two engines of similar displacement generating similar power, so it becomes a game of efficiency.  The turbo 4 is likely off-boost in steady cruise like this, so it's efficiency will be worse than a naturally aspirated engine by virtue of high pressure ratios on the NA motor.

The C&D test would be like taking an NA 2.0 Focus and a Focus ST and running them at steady cruise.  Of course the NA motor is going to return better mileage.
Needs more Jiggawatts

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AutobahnSHO

so the question is, do people drive every day more like magazine writers, or the EPA test machine???....
Will

MX793

Quote from: AutobahnSHO on January 22, 2019, 12:29:45 PM
so the question is, do people drive every day more like magazine writers, or the EPA test machine???....

The EPA highway driving cycle assumes a mix of interstate and rural highway driving with changing speeds and at least one acceleration to highway speed from a stop.  Hence why most cars, when doing long-distance, steady speed interstate driving tend to have little difficulty beating EPA highway.
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FoMoJo

2.7 is pretty big for a 4 cylinder engine, but it really seems that GM hasn't got the hang of putting turbo on a smaller displacement engine for the sake of efficiency.  The comment about having to run it on racing fuel to achieve the numbers pretty much says that.

As for the V8, the cylinder deactivation no doubt adds something to the efficiency, but the heavier engine as well as dragging non-active pistons up and down in the cylinder sleeves really doesn't sound like the best approach to overall efficiency.  Done right, a small displacement turbo engine with comparable output stats can always achieve better mileage; unless you're carrying/dragging the maximum weight with the gas pedal to the floor.
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MX793

Quote from: FoMoJo on January 22, 2019, 12:55:01 PM
2.7 is pretty big for a 4 cylinder engine, but it really seems that GM hasn't got the hang of putting turbo on a smaller displacement engine for the sake of efficiency.  The comment about having to run it on racing fuel to achieve the numbers pretty much says that.

As for the V8, the cylinder deactivation no doubt adds something to the efficiency, but the heavier engine as well as dragging non-active pistons up and down in the cylinder sleeves really doesn't sound like the best approach to overall efficiency.  Done right, a small displacement turbo engine with comparable output stats can always achieve better mileage; unless you're carrying/dragging the maximum weight with the gas pedal to the floor.

When cylinders deactivate, the valves are closed so you basically have air springs on the pistons.  The net drag on the engine will be very low.  Certainly less of an efficiency hit than running a turbo off boost.
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giant_mtb

I've driven a lot of Silverados with cylinder deactivation (picking them up for detailing).  If I have a stretch of highway to tackle, I like to toggle to the info screen that displays whether you're in 4- or 8-cylinder mode.  And also the MPG screen, if course.  It doesn't deactivate as often as you might think or hope, but it does seem to make a difference...pennies add up.

FoMoJo

Quote from: MX793 on January 22, 2019, 01:04:16 PM
When cylinders deactivate, the valves are closed so you basically have air springs on the pistons.  The net drag on the engine will be very low.  Certainly less of an efficiency hit than running a turbo off boost.
Still gotta compress that air.  Anyways, the least elegant way of achieving improved efficiency, imo. 
"The only reason for time is so that everything doesn't happen at once." ~ Albert Einstein
"As the saying goes, when you mix science and politics, you get politics."

CaminoRacer

Quote from: FoMoJo on January 22, 2019, 01:19:05 PM
Still gotta compress that air.  Anyways, the least elegant way of achieving improved efficiency, imo. 

The net difference of compressing the air and having it spring the cylinder back is (hopefully) negligible.

I think deactivation makes a lot of sense.
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FoMoJo

Quote from: CaminoRacer on January 22, 2019, 01:22:02 PM
The net difference of compressing the air and having it spring the cylinder back is (hopefully) negligible.

I think deactivation makes a lot of sense.
No doubt, with the advance of computer technology, the engine will behave a lot better than the previous version from a few decades back and actually save a bit of fuel when not under load.  The trick, I guess, is deciding when it's okay to deactivate cylinders without impacting the performance so that it's not too noticeable to the driver.  A friend mine, years ago, had one of the Cadillac 4-6-8 models and it was an absolute pig.

As for comparing the cylinder deactivation technology with a comparable performance version of a small displacement turbo engine, I think the small displacement turbo engine has a lot more flexibility in tuning; that is in respect to determining when power is needed vs. efficiency; if it's done right.
"The only reason for time is so that everything doesn't happen at once." ~ Albert Einstein
"As the saying goes, when you mix science and politics, you get politics."

MX793

Quote from: FoMoJo on January 22, 2019, 01:19:05 PM
Still gotta compress that air.  Anyways, the least elegant way of achieving improved efficiency, imo. 

But then that compressed energy is released on the downstroke, cancelling out the work put in to compressing it.
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GoCougs

Quote from: 2o6 on January 22, 2019, 12:11:13 PM
Most of these new 4cyl engines only get the economy achieved when out of boost.

I can't tell whether this means the 2.7T is crap, or the 5.3 is a very good engine.

All turbo motors under deliver and the GM 2.7T is no different - see the countless comparos that show the Silverado 6.2L getting as good or better MPG than the F-150 Ecoboost. 98% of MPG is weight and aero. A 2.7T DOHC is gonna weigh say 100-150 lbs less than a pooprod V8, but in a 5,000-6,000 lb vehicle, the % difference is negligible, plus then ad the issues of bias toward a richer A/F mixture and low compression ratio (= lower efficiency) off boost. As with turbo motors, automakers full well know that turbos under deliver, but they can game the EPA test method and prepare for European-esque displacement taxes. All the while WtP are left with a worse product that is worse for the environment.


GoCougs

Quote from: MX793 on January 22, 2019, 01:42:48 PM
But then that compressed energy is released on the downstroke, cancelling out the work put in to compressing it.

Cancelling some of the work of compression ;).

giant_mtb

GM did a good job programming their cylinder deactivation on Silvies. It's totally unnoticeable, and probably errs on the conservative side.

Lebowski

Quote from: Xer0 on January 22, 2019, 11:40:59 AM

This engine is EPA gaming and it still cost them millions of dollars to develop while being objectively worse pretty much across the board from what they are already have.  So sad.



In other words it's the predictable outcome of onerous regulation.

Galaxy

Since the topic of cylinder deactivation and compression of air has been brought up; would it not make sense to make them non interference engines and keep the valves open when deactivated?

FoMoJo

Quote from: MX793 on January 22, 2019, 01:42:48 PM
But then that compressed energy is released on the downstroke, cancelling out the work put in to compressing it.
Yes, that is true.  The non-functioning pistons still require energy to be moved.
"The only reason for time is so that everything doesn't happen at once." ~ Albert Einstein
"As the saying goes, when you mix science and politics, you get politics."

GoCougs

Quote from: Galaxy on January 22, 2019, 01:49:16 PM
Since the topic of cylinder deactivation and compression of air has been brought up; would it not make sense to make them non interference engines and keep the valves open when deactivated?

That would create issues with vacuum and the combustion process of activated cylinders as the deactivated cylinders would be pushing air back up into the intake manifold.

FoMoJo

Quote from: Galaxy on January 22, 2019, 01:49:16 PM
Since the topic of cylinder deactivation and compression of air has been brought up; would it not make sense to make them non interference engines and keep the valves open when deactivated?
I think that might introduce a few other problems regarding air blown into the intake plenum as well as exhaust from the functioning cylinders.
"The only reason for time is so that everything doesn't happen at once." ~ Albert Einstein
"As the saying goes, when you mix science and politics, you get politics."

MX793

Quote from: GoCougs on January 22, 2019, 01:57:15 PM
That would create issues with vacuum and the combustion process of activated cylinders as the deactivated cylinders would be pushing air back up into the intake manifold.

Plus the pumping losses.  Closing the valves isn't totally loss-less, but it's a very low loss solution
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Eye of the Tiger

Quote from: GoCougs on January 22, 2019, 01:44:18 PM
All turbo motors under deliver and the GM 2.7T is no different - see the countless comparos that show the Silverado 6.2L getting as good or better MPG than the F-150 Ecoboost. 98% of MPG is weight and aero. A 2.7T DOHC is gonna weigh say 100-150 lbs less than a pooprod V8, but in a 5,000-6,000 lb vehicle, the % difference is negligible, plus then ad the issues of bias toward a richer A/F mixture and low compression ratio (= lower efficiency) off boost. As with turbo motors, automakers full well know that turbos under deliver, but they can game the EPA test method and prepare for European-esque displacement taxes. All the while WtP are left with a worse product that is worse for the environment.



Fiesta
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BimmerM3

Quote from: MX793 on January 22, 2019, 02:21:21 PM
Plus the pumping losses.  Closing the valves isn't totally loss-less, but it's a very low loss solution

Compressing the air within the cylinder is better than pumping it in/out of the manifold?

Not discounting the other downsides, just curious about that particular aspect.

Soup DeVille

Quote from: BimmerM3 on January 22, 2019, 03:46:21 PM
Compressing the air within the cylinder is better than pumping it in/out of the manifold?

Not discounting the other downsides, just curious about that particular aspect.

Yes. The trapped air acts like a spring. The energy used to compress it is mostly returned on the downstroke. Pumping air in and out uses significant energy both ways.
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Eye of the Tiger

Quote from: Soup DeVille on January 22, 2019, 03:55:24 PM
Yes. The trapped air acts like a spring. The energy used to compress it is mostly returned on the downstroke. Pumping air in and out uses significant energy both ways.

Yeah, but what if you diverted some exhaust pressure from the active cylinders into the compression strokes of the deactivated cylinders, and let them re-exhaust just enough of that exhaust to balance the net energy loss of the deactivated cylinders.
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Soup DeVille

Quote from: Eye of the Tiger on January 22, 2019, 04:06:06 PM
Yeah, but what if you diverted some exhaust pressure from the active cylinders into the compression strokes of the deactivated cylinders, and let them re-exhaust just enough of that exhaust to balance the net energy loss of the deactivated cylinders.

I'm gonna have to lie down for a while and think about that.
Maybe we need to start off small. I mean, they don't let you fuck the glumpers at Glumpees without a level 4 FuckPass, do they?

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FoMoJo

Quote from: Eye of the Tiger on January 22, 2019, 04:06:06 PM
Yeah, but what if you diverted some exhaust pressure from the active cylinders into the compression strokes of the deactivated cylinders, and let them re-exhaust just enough of that exhaust to balance the net energy loss of the deactivated cylinders.
You might need a free valve system to make that work.
"The only reason for time is so that everything doesn't happen at once." ~ Albert Einstein
"As the saying goes, when you mix science and politics, you get politics."