I totally agree. I think the whole concept is one of **much** added complexity and headaches over the life of the vehicle for **extremely** marginal gains. Even in periods of low demand when it's running on, say, 4 cylinders, it can't be nearly as efficient as a full-time
4-cylinder engine because of the extra weight and friction of 4 dead cylinders. I can't believe the owner will come anywhere close to recovering the added initial expense by way of (mostly imagined) savings (even if there are no added repairs due to breakdowns from the added crap). Basically, a gimmick, of which we have way too many on our cars already IMO.
Bill Putney (to reply by e-mail, replace the last letter of the alphabet in my address with "x")
Chevy did something like this on the ZR-1 they had a switch that turned off a second set of injectors to get a low-power, more economical mode. I remember the early reviews on the ZR-1 some of which were under the impression the "valet" switch turned off cylinders.
I agree with you Bill, I don't think it's worth it. I suspect this is more sillyness to get past emissions.
If by "a while" you mean 24 years then yes... Cadillac tried this back in '81(ish). It was actually a pretty good system but the electronics just weren't "there" yet and the tecnical training wasn't there yet either. Most of them ended up being disconnected and running on all 8.
As far as I know this is Chryslers first try at this.
Why would you think it sucks when you say you know nothing about it? It's actually a pretty good idea as you don't loose all that much energy to the dead cylinders and still have 8 cylinder power and torque available when you want it. Economical for the work week and a tire smoker for the weekend. Doesn't get any better than that!
I'm trying to visualize an eight cylinder engine running on 8 cylinders and then running on only 4 cylinders with the other 4 free wheeling. For the life of me, I can't see why there would be less fuel consumed with a given amount of fuel concentrated over only 4 cylinders than the exact same total amount of fuel distributed over all 8 cylinders. I mean, (for that to be true) for a given load (obviously reduced if it's trimmed back to 4 cylinders), for firing on 4 vs. 8, why would the 4 firing be more efficient than all 8 firing with less fuel in each cylinder (same amount of power has to reach the driving wheels in either case, therefore same amount of fuel has to be burned **UNLESS** efficiency is somewhow magically improved just from the fact of having 4 cylinders firing to produce the same amount of reduced power).
IOW - what it boils down to is: Why is higher concentration of fuel in fewer cylinders more efficient than lower concentration of fuel across more cylinders? Honest question - deserves an honest answer.
Bill Putney (to reply by e-mail, replace the last letter of the alphabet in my address with "x")
I can't imagine the benefit being very great either and have always wondered about this scheme. Almost every engine is more efficient at higher throttle settings when the combustion event is more vigorous. This may be the advantage here. You get 4 larger bangs rather than 8 smaller ones, and the combustion is slightly more efficient when you rae burning more fuel per combustion cycle. However, I really can't imagine this being a huge gain. Now if you could disconnect the other 4 cylinders from the crank so that you didn't have their frictional loss, then I can see it beginning to have some real advantage.
I can see what you're saying about some gains with the 4 cylinders working harder maybe having some incremental efficiency gain over the eight working less hard. I still would question the net gain, everything else (complexity, weight penalty, initial cost, overall lifetime added cost with just one repair necessitated by the added immature technology) considered. The cliché "point of diminishing returns" comes to mind. Would be interesting to see what this vehicle will get in mpg compared to the old school engine in real world useage.
Here's a nother thing to consider: On that vehicle, the great majority of fuel usage is not going to be in the low power demand situation when it can drop down to 4 cylinders. I would think that if it can beat the regular engine in mpg, it will definitiely not be in stop and go driving (due to the parasitic drain of the extra weight and friction) - only for
95+% hiway driving - and then it will only be marginal, and the added costs will never be recovered. Throw in just a little stop-and-go, and any gains will have quickly been swamped out.
Regarding putting a clutch on a split crank, IIRC, that was done a few years ago - can't remember by whom (Ford I think, possibly Caddy), and I don't think it made its way off of the test tracks (even more weight than just shutting the valve train down). Man - imagine the repair cost if that clutch goes out (complete engine dismantle).
Bill Putney (to reply by e-mail, replace the last letter of the alphabet in my address with "x")
And above all, it can be implemented hydraulically with engine oil pressure the same way VTEC is done on Honduhs, rather than with electric solenoids as was done on the Cadillac V-8-6-4(-0) :-p all those years ago. The latter method has proven very simple and reliable.
Because combustion is more efficient the higher the cylinder pressure is. 4 Cylinders working at 100% of their output are more thermally efficient than 8 working at 50% of their output, even allowing for the losses of dragging the 4 unused pistons along (since both valves remain closed, the only loss is frictional since the energy taken away on the compression stroke is (mostly) returned on the downstroke).
The efficiency gain was quite measureable back with Caddy's V-8-6-4. The only problem was horrible reliability, but Caddy attempted to use electric solenoids to disengage the rocker arm fulcrums on the deactivated cylinders. There are much better, simpler, and proven ways to do this, such as the method Honda's VTEC uses to shift cam profiles. A small electric solenoid diverts oil pressure which then hydraulically locks or unlocks a pair of rocker arms (one following one cam profile, the other following a different cam profile. ) In the case of deactivation the default rocker could just follow a round "lobe" on the cam or simply rest on a pedestal in the overhead, and the second rocker would lift it off the pedestal when they're locked together. The advantage of the hydraulic type system is that the electrical solenoid is just a controller, the hydraulics do all the work, and the actual lock/unlock happens ONLY when the rockers naturally align during the cycle so its a "no load" shift.
Why do you say that? Even at 70 MPH most cars require less than 1/3 of their available horsepower. 4 of 8 cylinders should be adequate up to well above 70 MPH for most vehicles.
Thanks for the good explanation. I already understood the energy of the compression stroke being returned in the downstroke, but the bit about the greater efficiency at higher cylinder pressure took my understanding to another level - so, again, thanks - that's what I didn't understand.
My only reservation now is the added complexity. If a given component is divided into, say, 3 new ones with the same total volume and weight, the cost of the 3 new parts will be somewhere between one and three times the original one piece. I have to ask how many new parts have been added to the basic engine to accomplish this, and how much does the sticker price go up?
Bill Putney (to reply by e-mail, replace the last letter of the alphabet in my address with "x")
Several miles per gallon. Probably 10%, but I don't remember the numbers exactly- its been close to 30 years :-p. I do remember tha the V-8-6-4 with a 6-liter (368 CID) v8 got better mileage than the HT4100 (4.1 Liters, about 250 CID) in the same vehicles, AND had a lot more power available when needed. Come to think of it, the HT4100 didn't hold up a whole lot better than the 8-6-4 either, which should tell you something about the state of affairs at Cadillac in the late 70s.... and give confidence that Chrysler could make a reliable 8-6-4 engine.
I was thinking that you could have great mileage for half a tank, and then do a couple of hours of in-city stop-and-go in which all 8 were constantly on for rapid acceleration from a dead stop, and the average for the tank (total miles traveled ÷ gallons used) wouldn't be so good any more. May not be a fair argument.
Bill Putney (to reply by e-mail, replace the last letter of the alphabet in my address with "x")
It doesn't pay to get into thermodynamics on Usenet. Trust me, there's a reason. It's more efficient running less displacement at higher pressures to generate the same power. That's what they're trying to do.
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