I've always wondered why passenger aircraft don't use four smaller engines for take-offs only, then cut two of them for cruising and landing. Maybe even use retractable covers over the intakes of the two take-off engines to improve aerodynamics in flight. This would use less fuel and while it wouldn't improve the emissions during take off, it may be offset by fewer emissions during the flight itself.

@K5ING
I've heard of cargo planes (particularly C-130) being fitted with a system like that, except with rockets - and in one case, used to land and take off in the space of a football field. The drawback is that the rockets need to be replaced after each use. With what you're talking about, the plane would only need more of the same fuel it uses anyway.

@K5ING the reason is that engine not working in cruise is dead weight. Dead weight means less cargo and less money. So far the runways are sufficiently long. Also, as Angry Penguin pointed out military transports used RATO - rocket assisted take-off. It works well, because the RATO engines are discarded after the take off, so they aren't a dead weight in cruise. On the other hand, I don't think that people living in the neighborhoods of airfield would like empty rockets with some fuel still remaining to fall on their houses.

@K5ING not only would two extra engines add weight (as per flame_can), they also add drag, and between the two, you'd have one totally uneconomical aircraft. That's before you factor in the cost of buying and maintaining two additional engines.

Why not make it a flying wing? You could carry more passengers that way. Why not put the engines above the wings? That way the noise would be reflected upwards instead of downwards towards peoples houses.

Good idea to reduce fuel consumption. How about using a Maglev bogie with 2 arms above which push behind both wings near the fuselage. The Maglev track bed would be invisible beneath the runway. Not sure if they could be used for landing..

Even though the miltary planes used to use rockets, the system is known as JATO, not RATO, for Fet Assisted Take-Off.
This plane looks suspiciously like the Sonic Cruiser Boeing unveiled in 2000.
As for a flying wing, that has been studied, but flying wings aren't currently able to use existinfg airport gates, jetways and other infrastructure, which would need to be heavily modified to accomodate them.

Why not try to use laser propulsion from the ground to the plane rather than a buggy?

@ Luke Beauchamp The flying wing is a decent idea the issue with it is airports don't have a good way of boarding them as they don't exactly work well with existing jet ways. Gizmag had an article on a sort of drive through airport not too long ago that might make something like that more feasible but not without issues. Other issues presented is a lack of window seats. Carbon fiber brings the possibility of windows on the roof, but that probably still isn't going to cut it for most people as the view is substantially less, unless flying at night. I think a more practical solution to that is to have LCD screens that let people use them as windows or whatever. These would naturally be more geared towards long haul flights so you could create a different class structure where you have the sleeper seats in the middle along with other amenities and try and keep the passengers to the outsides as best as possible.
Something else that I think would be interesting is the treadmill "myth" I still have some trouble wrapping my mind around it but they tested it on a prop plane for mythbusters, I'd be interested in seeing if i works for jets as well.
In regards to things brought up in the article, computer automated ATC's have been demoed to be far more efficient than humans with less errors. Big surprise. I think in the next 10-15 years we'll start seeing them be implemented. Not that there aren't obvious security concerns. V formation flying, it's about time. I guess they decided it was too much work for pilots to do this on commercial flights so they waited for computer control. The distances cited seems a little large to me but still cool. I know another thing that's being looked at is ground effect flying over the ocean. Again, not without safety concerns but has the potential for more fuel efficient flight. The eco climb idea is a great idea however I don't think it'll get them out of the expense/weight of having more powerful engines as you'd still need them for aborted landings where you suddenly need massive amounts of power to pull away. I'm a pilot myself and I've had to do this more times than I'd like (deer, another plane, ducks) This of course being the safety issue with glide in landings. If they can figure out a way to start the engines instantly while gliding in, then you may be in business. Or just carry on RATO for aborted landings. I don't know if the fuel savings would = the cost of of fuel used to carry the RATO. But assuming RATO works, you could save more money in fewer, lighter, smaller, more efficient, etc engines

@K5ING
Despite some "smart" comments your suggestion makes sense. In some 10 or 12 cylinder cars fuel is not injected in some of the cylinders unless you really need the power and this has reduced fuel use and emissions.
However, you want to have all four on when landing -- if you need to abort the landing you'll really need all the power you can get.