Reducing the dimensions of the engines and increasing the power can help to lower plane weight and fuel weight and, as it is used less fuel for the same operation there is a consistent reduction in the pollution

Great. Aircraft increasing speeds is also an issue?

Using electric motors , and employing fuel cell will be the future of Aircraft engines.

I do not think so because hydrogen has a very low density and it reuquires to be compressed to have a sufficient storage ; everybody understand that a 200 atm compressed gas is dangerous and diffiucult to handle. A solution could be a reengineering of the airships as was the Zeppelin in the 30's

Fuel consumption by passenger and cargo aircraft is huge. A typical 747 holds nearly 50,000 gallons of fuel. Enginneering advancements are currently playing a large role in the increased efficiency of aircraft engines. As engines burn fuel more efficiently, less pollution is emmitted and more energy is harnessed to power the engine. Therefore, I can only see positive results from the "New" aircraft engines. Engines such as Pratt & Whitney's Geared Turbo Fan (GTF) are prime examples of innovation. Rolls-Royce's "Trent" series of engines has also proven to be highly efficient and is saving the aircraft industry countless thousands of gallons of fuel. Our collective engineering expertise will continue to design and manufacture more efficient engines and there are no perceived negative results.

Faster aircraft is so far only an option for 'niche' markets (business aircraft).

Main developments today in aircraft engines for commercial transport are made to increase performances (incl. fuel consumption), reduce costs and to soften the impact on environment (emission and noise).

Looking from energy density perspective (driving factor for aircraft), fuels remain by far the most relevant energy source for propulsion. Developments are made in the field of alternative (e.g. Gas to Liquid) or renewable (from biomass) fuels so that they will be seamlessly used in today engines, offer alternatives to crude oil and further reduce environmental impact (if CO2 absorbed by plants used for fuel production is counted).

Reducing the dimension of the engine decreases the bypass ratio and increases the fuel consumption. Research is conducted to increase as much as possible the size of the engines without increasing too much the friction drag due to an increased wet surface. This is the idea behind Contra Rotative Open Rotor. Turboprops engines are also more efficient than turbofans in terms of power/fuel ratio, but not in term of power/mass ratio. Increasing the velocity highly decreases the performance of the aircraft by increasing compressibility effects (Mach number), shock waves and also skin friction (V² term).

So for decreasing the fuel consumption we need : Fly slower and larger engines (fans)

The solution is to redesign the aircraft/engines to make it more traveler-friendly. The use of a new type of more efficient energy would be reduce the environmental impact as well.

As battery energy density doubles in the next few years, no one will want to put fuel in an airplane anymore.

Electric planes can go higher and therefore faster than air-breathing planes. Electric engines can be smaller with less drag, because again- they don't need to collect oxygen.

Note the CAFE / Google Green aviation challenge resulted in a winning plane from Pipstrel that gets over 200 MPG / passenger at 200MPH for 200 miles. That's fuel. But it points out that designing planes for fuel efficiency allows them to be much more efficient than compact cars.

One possibility is a coupling of turbofan engines and ramjets on the same plane (if I remember well there was something like this on the SR71A ) in this way turbofan is for low speed range and ramjet for high speed.

There are two major reasons why using electric energy is a major issue :

Weight is everything in aeronautics, batteries are super heavy for the same amount of energy as kerosene. More fuel / batteries = more loads = more structural mass to support the loads = more loads = more structural mass...

Using oxygen, for that reason, is an excellent thing ! Actually the aircraft grabs half its energy soruce (oxygen) in the sky without having to bring it from takeoff... As rockets do because they fly too high. This is a huge mass saving, by a factor of much more than 2. Then, with current technologies, electric aircraft is , I think a very bad compromise.

I do not believe in electric or hydrogen fueled aircraft because the fuel/motor weight is likely to be to large for a given payload, say for passenger*kilometer. High speed turboprop are best for short routes. Decreasing the aircraft drag and noise is probably the future for long routes medium sized planes like 787 or A350 and will have the most positive influence on mankind.

the major problem in aircraft engines is the "T3" temperature (the temperature in the firs stage of the turbine. Because of the materials being used in the engines, this temperature can not be too high, so the efficicy of the thermodynamic cicle of the engine is quite low compared with the reciprocating engines. Then, when the tecnology available allows increase the temperature in the turbine' first stage, the engines becames more efficient (low fuel consumptio whith high thrust). But it is not the only issue: fuel calorific values (increase of them), reduce of internal loses in the engine, increase of the compressor/turbine efficiency and many other issues that may be investigated and improve.

Also at low altitudes the drag force is greater than at high altitudes, then more energy to drive the airplane

It is a question of boon or ban only.It redused travelling time at the same time redused life time of human being.Unless we find solar or battery aircraft,it is going to be ban.

The lift force is also reduced in altitude (1/2.Rho.Cz.S.V2), the consumption is in fact driven by the ratio lift/drag given by the Breguet Leduc equation :

Instant Consumption = Engine Specific Consumption . Mass. G(9.81) / (Lift/drag)

While lift must always stay equal to weight.

So it is all a matter of balance...

I think that to give us a more energy efficient and eco friendly engine in aircrafts, the technology needs to be upgraded in many points. One of that points are the fuel used in engines: they are so rich in sulfur and have a low mass/energy factor. This is only one point. Upgrade all technology in the modern aircrafts will be take a very long time...

Inter continental transport (long range) with reasonable quick time is only possible with supersonic and hypersonic flights. With a major issue on sonic boom, high supersonic or hypersonic transport is possible in upper atmosphere (>20 km) with air breathing propulsion devices. It may  an option in future for long range flights. However It has many technical challenges compared to the air breathing missiles.