8 cylinder to 4 cylinder in new DC V8's

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")

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

Actually, vacuum is higher with closed throttle and the engine being driven by the drive train as when slowing down from high speed.

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 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

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:

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.