8 cylinder to 4 cylinder in new DC V8's

Hi guys,
Because in another thread about this subject people started to bash on
other things I'd thought a new thread would be better (so my view on
the subject get fresh new attention).

The cons:
- It could break down more easily. Jup, could be, dunno, time will tell.
If it is Chrysler only technology changes are the first 3 years it will
break down bad. After that, DC will have cured the problem and the engine
will be rocksteady for the next 20 years.

The pros:
- Mileage. If an engine runs as efficiently as possible, the amount of
fuel burned per second depends on 2 things.
First: cubic inches.
Second: revs.

If you have a large engine and want to have a decent fuel usage per second
you'll have to get the rpm's down. Otherwise it won't happen.
If you go from 3000 rpm to 1500 rpm your engine will use half the amount
of fuel a second. Now, because a typical torque curve goes way down below
a 1000 rpm it is impossible to gain something there. The engine will simply
stall (which of course gives you zero fuel usage a second but you won't
get anywhere either).

The only thing that remains is to decrease the engine size. By shutting
down have the cylinders the engine size is halfed too.
Now you have the same engine useage at 2000 rpm as you'd have at 1000 rpm
with all 8. And because of the way torque goes down you'll have more
power as well. Now you might worry if the smaller engine (still almost 3
litres) is powerful enough to drive at cruising speeds.

Well, my 2.5V6 Stratus has about 170 horsies at 6500 rpm.
Driving 120km/h (75mph) my car does about 2500 rpm. So the car uses
170 / ( 6500/2500) = 65 horses at that speed. Now, the new 300 is heavier
so lets assume it needs 75 horses to drive at the same speed.

If the cars has a maxrpm of 5500, it would go 75 x 5500 / 330 = 1250 rpm
at 75 mph. At 60 mph the engine would have to go to higher gear to avoid
stalling. Of course you haven't noticed any large engine car stall at
60mph. That's because the engine runs at higher revs, and gives you more
horsepower even if when you don't need it. But that extra power will cost
you. Now, with 4 cylinders shut, the car will do 2500 rpm at a steady
75 mph, and 2000 at 60 mph.

If the new V8 will be able to activate the 4 sleeping cylinders fast enough
to release its full power when needed I think Chrysler has a good hand
of carts. As fuel prices will go continue to go up, a normal large V8 has
no future.

I realize I've neglected gearing in the above, but there's only so much
to win there. Besides that, the 4 cylinder shutdown is a 50% fuel cut.
There's no way gearing can solve that without using at least a 8 speed.
And the engine would still stall below 750 rpm.

To be honest, noone needs a 300 HP car, but I won't start that war now.
Besides that, I want a 300 HP car as well.
Martin

 

If you compare fuel economy of the same car with different engine sizes
(caravan V6 vs. Caravan 2.4L 4cyl), you will find a dramatic difference in
city economy, but not very much highway economy. It will be around town
idling at stoplights that this sort of technology will shine (kind of like
hybrids). At cruising speed wind resistance and gearing (plus auto vs.
manual) play a big part.

Also when the engine won't stall at 1000 or 1250 rpm, it will just lug if
you floor it and it doesn't go to a *lower* gear.

Also, unless it stops the pistons of the unused cylinders, they will still
be turning, and there will be extra resistance.

 

I believe that the most fuel-efficient RPM for an engine to operate at
is where it peaks on it's torque curve (and I *believe* that's also
the same point where intake manifold vacuum is highest. That's why
vacuum gauges were common on cabin cruisers years ago - and why those
guages were labelled not only in inches of vacuum but also with labels
such as "economy cruise".

So take that RPM, pass it though your drive-train, and make it come
out the wheels such that you're going 65 mph.

BTW, what's the relation between displacement (ie bore and/or stroke)
and engine friction? Is a 5.7 liter V-6 more fuel efficient than a
5.7 liter V-8? Would they have the same peak-torque @ the same RPM?

 

I would call that the engines "most efficient maximum sustained output
point." Cruise typically requires FAR less torque than the engines
maximum torque potential, and so automobile engines typically run far,
far, FAR below the peak torque RPM in cruising mode. Look at the tach on
almost any modern car and you'll see it turning between 1500 and 1800
RPM in the 60-70 mph range. Doing that causes the engine to run with
comapratively high cylinder pressures and minimal ignition advance,
which results in better economy than running at the peak-torque rpm with
the throttle mostly closed to keep the torque down to cruise levels.
..

There's not a real simple relationship between friction losses and
engine displacement. Short-stroke/big bore engines typically have less
friction loss and less air-pumping loss than long-stroke/skinny bore
engines, BUT engines with the bore and stroke more comparable to each
other (or the bore slightly smaller than the stroke) have lower inherent
CO and HC emissions since the cylinder surface area is lower compared to
the cylinder volume. That's why recent engine designs have backed away
from the big-bore trend, which peaked in the 70s and then hung around
through the 90s. In general, (VERY general) keeping the displacement
constant and reducing the number of cylinders will lower the torque peak
RPM, and possibly increase HC emissions. OTOH, having a whole bunch of
very small cylinders is inefficient also. Thats why things have settled
out such that 4-bangers are mostly in the 1.5-2.5 liter range,
6-cylinders range from 2-4 liters, v8s range from 4 to 6 liters, and for
bigger displacements (over 7 liters) v10s have come into vogue. And also
why you don't see many 3-liter v8s or 4-liter V-12s.

 

You bring up something that I had thought of but decided not to just to
keep things simple. But since you did...

I learned somewhere a while ago (very likely from you, Steve, on this
very ng) that emissions are lower and/or easier to control with more and
smaller cylinders. Considering that, for a given load, smaller total
displacement results in higher cylinder pressure (==> higher efficiency)
**PLUS** more but smaller cylinders for the same displacement results in
lower emissions, rather than a 8-6-4 configuration, maybe the overall
better answer would be a 10-7-5, or maybe 12-9-6. I guess the downside
would be the number of individual parts increasing with number of
cylinders which would tend to inherently push the initial cost higher
for a given total displacement. As usual, everything's a tradeoff.

Bill Putney
(to reply by e-mail, replace the last letter of the alphabet in my
address with "x")

 

when the air fuel ratio gets as close to 25-1 then the gas milage is at
maxium effiency. dam i can't think how to spell that dam word it starts
with a s________ ratio

 

I think that is only true at full throttle. Since you have to use only
a very small throttle opening to run at 65 MPH at the RPM corresponding
to the torque peak, I don't believe this relationship holds. That is
why you see cars with deep OD transmissions that run the engine at a
cruise RPM MUCH below the peak torque RPM.

I believe the peak torque RPM is the most efficient way to generate the
amount of horsepower that is available at that RPM, but if you don't
need that much horsepower and must run at a reduced throttle setting,
then you no longer are most efficient at that RPM.


Matt

 

Why are emissions easier to control or lower for smaller cylinders?


Matt

 

You mean the stoichiometric ratio. I believe it is closer to 15:1 than
25:1. This has little to do with gas mileage.


Matt

 

According to Steve (as quoted above), "BUT engines with the bore and
stroke more comparable to each other (or the bore slightly smaller than
the stroke) have lower inherent CO and HC emissions since the cylinder
surface area is lower compared to the cylinder volume. I think it has
to do with cold/hot areas and edge effects of cylinder walls interfering
with even fuel/air distribution, flame front propagation, etc. Steve
will have to elaborate.

Bill Putney
(to reply by e-mail, replace the last letter of the alphabet in my
address with "x")

 

I'm no expert, but here's what I've read and heard:

Part of it has to do ONLY with bore size- the volume above the top
cylinder ring but below the piston crown is a place where combustion is
impossible (because of the quenching effect of the cold piston and
cylinder walls) but where air/fuel mix can "hide" and then be exhausted
unburned. That makes small bores work better (and very short piston
crowns, but then you may break the top ring land because its so weak).
The other part has to do with getting a good even combustion process in
a very large volume of a big cylinder. Its not as much of a problem with
a diesel engine where only air is ingested into the cylinder and then
the fuel injector creats an anchored flame within the cylinder (in fact
it probably is easier with huge cylinders), but a gasoline engine
pre-loads the WHOLE cylinder with a combustible mix and then you need to
get uniform flame-front propagation (no anchored flame at all) across
the entire volume. Not easy with a big volume, because rich and lean
pockets can develop even with very good port fuel injection.

 

Best thermal efficiency tends to occur on the lean side of
stoichiometric, and best power production tends to occur on the rich
side. That's one reason Chrysler's old "Lean Burn" system actually
worked so well for its day- it would lean the mixture out far past
stoich under light load conditions and got amazing efficiency. Its
downfalls were 1) the electronics of the day were inadequate (mainly the
electromechanical controls- a variable mixture carb is very
trouble-prone and inaccurate compared to EFI) and 2) very lean
combustion turned out to be hard as hell on engine parts.

 

I'm beginning to suspect that there's an sort of optimum cylinder size,
and that its in the 3.small/medium" bore and 3.small/medium" stroke
range. If you look at all the engines produced in large numbers for sale
in the US, those sorts of numbers just turn up in droves. So the "more
smaller cylinders for the same displacement" argument probably only
holds true if it does NOT drive you down to 2" sized cylinders.

 

I'm beginning to suspect that there's an sort of optimum cylinder size,

I think this might be the case. I've been told (although I'm FAR from a
production engineer) that these sizes are also influenced heavily by tooling
concerns, which is why they've picked an optimal boreXstroke geometry
(roughly) and have stuck to it.

Clearly, you're in good company with your comments about the trend to more
cylinders for larger displacements (i.e. >6L gets 10 cyl), this is widely
known and spoken about in the various things I've read about it. Although
it's interesting that the iron-block V10 has been dropped for the Ram truck
line; I suppose they've figured that they'll get their horsepower by
increasing volumetric efficiency via the hemispherical heads in the V8s,
etc. I'd be willing to bet that if the V10 wasn't so tightly wrapped in the
image of the Viper, it would disappear from that line, too, in favor of a
large (and probably better-sounding) hemi configuration. But that's just a
guess. Smaller displacement blocks are, well, smaller, and probably easier
to get to meet CAFE.

More OT, I spent a considerable amount of time last week in dealerships in
the last week, and the interest in the 300 series is very intense. A LOT of
people came in the doors looking to drive one.

--Geoff

 

The "hidden" fuel mixture theory sounds pretty bogus, for two reasons:

1. Modern pistons have very little clearance between the cylinder and
the piston so this volume is miniscule. Also, at dimensions that small,
it is very unlikely that this unburned fuel mix does much other than
stay there and ride up and down. I don't believe for a second that you
can exhaust that volume on each exhaust stroke and then reload it on the
next intake stroke.

2. The displacement also increases with the piston diameter, thus this
hidden volume will be roughly the same fraction of the overall charge
volume and thus the relative emissions will be the same.

The flame propogation theory seems a little more sensible on the
surfact, however, the flame front travels very fast indeed and you'd
need a very large engine before this became a significant issue. And
since larger engines turn lower RPMs, there is more burn time as
cylinder displacement increases.

Matt

 

the closer it is to 25.1 the more efficient the engine runs and the better
the milage gets at least thats the magic ratio they tell us at obd 3 classes

 

Man, did you stay home from school on the wrong day.

You made all that up. "Miniscule?"

Totally wrong. I bet you can remember that combustion chamber volume goes up
by the square of piston diameter. But the volume above the ring land
doesn't. Can you think of a formula for that volume?

You made this up too. "Significant?" "Very Large?". Come on, you can do
better than that.

 

Well, I don't know what 25-1 or 25.1 means, but if you are talking about
the stoichiometric ratio for combustion of gasoline, the ratio is 15:1,
actually 14.7:1 to be more precise. See:
http://www.chevron.com/prodserv/fuels/bulletin/motorgas/ch5.shtml

If your class is telling you to use 25:1, they don't know what they are
talking about and an engine likely won't even run at a ratio that lean.


Matt

 

Clearances haven't changed since the 60s, EXCEPT for hypereutectic
pistons (which have other problems...). Still, if you're talking about
the primary source of unburned HC, then it makes sense that reducing it
helps, even if its already "miniscule" Beyond that, I suspect that the
quench area formed in the "corner" where the cylinder crown meets the
cylinder wall extends fairly deep into the combustion volume and isn't
STRICTLY limited to the space trapped by the upper ring land.

Larger bore engines DON'T typically turn lower RPMs, they turn higher
RPMs. Compare a 340 (big bore) to a 225 (small bore) or 318 (medium
bore). The 340 comes to life at 5500-6000 RPM, and the 225 is done by
4000 and the 318 by 5500.

Also, its not an issue of flame front speed, necessarily. Its an issue
of maintaining a uniform mixture distribution over such a large area
(big bore engine) versus a smaller area in a small-bore engine of the
same displacement.

 

I guess insults are the best you can offer when you don't have any facts.

Yes. The forumla for a cylindrical shell's volume is: V = 2*pi*r*h*w,
where r is the radius of centroid of the revolved rectangle, h is the
height of the shell and w is the thickness of the shell. I checked a
few places and found typical piston/cylinder clearances of 0.0005 -
0.002". Let's use the large value which is the w in the above formula.
A 4" bore isn't all that uncommon so we'll use that which gives an r
of 2". I didn't find any listings for distance from the piston groove
to the top of the piston, but pistons I've seen in the past were
probably around 0.25" so I'll use that for h. This gives a volume of
6.28 x 10^-4 cubic inches which is miniscule in my book.

Makes little difference. Let's assume the above cylinder has a stroke
of 3.5". The volume swept by the piston is therefore: pi*r^2*h or 44
cubic inches if I punched the calculators buttons correctly. If the
compression ratio is about 10:1, that gives a cylinder volume of
approximately 5 cubic inches. This means that the volume of cylindrical
shell above the top ring amounts to ~0.001%. So even doubling or
halving the diameter of the engine bore will make diddly squat
difference and wouldn't contribute enough to exhaust emissions to be
even measureable.

Have you been embarrassed enough yet? Can you find any flaws in my
assumptions or calculations or, better yet, provide some calculations of
your own that support your theory about this contribution to engine
emissions?

And the above percentage is assuming that the entire volume of gases
trapped in this annulus is exhausted unburned on each cycle. Anyone
with a modicum of knowledge of fluid mechanics knows that you'll have a
very hard time moving the air/fuel mixture into and out of this
cylindrical shell that is only 0.002" thick or less. So the effective
percentage above is probably more like 0.00001%.


Matt