**7. Conclusion**

*Transportation Systems Analysis and Assessment*

fan engines are the current state of the art [64].

propeller engine on the roof of the fuselage.

**6.6 Continuous descent operations: CDO**

part of the air through the compressor and the combustion chamber, rather than around the engine. The first stage of the compressor, also known as a fan, works like a giant blower. The fan accelerates this circulating mantle air. The so-called turbo-

The shroud flow ideally requires a relatively low speed for the large fan and high speed in the high-pressure range. This created the two-shaft engines. The axles of these engines can rotate counter-wise. One of them is the slow-speed low-pressure shaft, which is driven by the rear turbine stages just before the exhaust outlet. At the same time, the first compressor stages are rotated. The other one is the very fast-rotating high-pressure shaft. The high-pressure shaft is operated by the turbine stages behind the combustion chamber and thus moves the high-pressure part of the compressor. Optimization of the engine concept has been in progress for many years. First and foremost, the approach is followed to change the amount of air that has passed through. The difference in speed should not be too big between thrust and airspeed. Ideally, a very large amount of air is pushed back very slowly from the engine. Another approach for increasing efficiency is the turbine including the combustion chamber. The hotter the combustion is, the more efficient the process becomes. Here, the materials are pushed to their limits. The first stage of the turbine is under most stress because it gets the full heat of the combustion chamber. Other developments are heading back in the direction of the classic propeller. Ideas in this area run under the slogan "open rotor concept." However, the mounting size, which makes mounting on the wing difficult, as well as the noise, proves to be problematic. Aircraft could look completely different in the future, for example, with a huge

Continuous descent arrival (CDA) is an aircraft operating technique designed to reduce aircraft noise, fuel consumption, and emissions. In this method, an incoming aircraft sinks with minimal engine performance and largely avoids horizontal flight phases. Ideally, this happens at idle. Without the use of CDA, an airplane goes down step-by-step. When performing CDA, the aircraft lingers high up in the air for extended periods of time, operating at a low engine thrust. This causes a reduction

**128**

**Figure 7.**

*Visual representation of CDA [65]. NM = nautical mile; FAP = final approach point; FT = foot.*

To sum up, energy efficiency management should always be used and aspired to because of the overwhelming economic and ecologic benefits in the transport sector. One of the most important advantages is, above all, the potential for saving fuel, since low fuel consumption leads to cost savings and consequently to reduced CO2 emissions. The application of energy-efficient technologies and methods therefore not only has economic aspects but is also good for the environment. This chapter shows that there are already several propulsion technologies and developments in the areas of rail freight and airfreight, but they are not yet completely revolutionized and provide room for further improvements.

Rail freight currently has mainly internal combustion engines and electric motors in use. In particular, the invention of electrically powered railroads has been a significant advance in improving energy efficiency. By using modern drive technologies and methods in rail freight transport, many potential savings can be achieved. In the field of electric locomotives, regenerative braking can be used, for example, with rechargeable energy storage systems (RESS). This allows the current generated during dynamic braking to be stored and reused, and it offers a potential of 10–20% of energy consumption to save. Other powertrain technologies such as the dual power hybrid locomotive as well as diesel multiple units (DMUs) and electric multiple units (EMUs) are particularly effective and efficient. The dual power hybrid locomotive convinces with the advantage that the diesel engine can be converted to a purely electric drive with just a single push of a button. Although EMUs prove to be a high financial burden, they are considered to be very environmentally friendly and energy-efficient. The most fuel-efficient and advanced engine in this regard is the highly clean diesel-electric locomotive with repower kits. Compared to other locomotives, this model has the advantage of not requiring urea additives to reduce NOx emissions. Since the purchase of a completely new diesel-electric locomotive is extremely expensive, repower kits are a cost-effective option that can also reduce fuel consumption and emissions. This can save up to 25% on fuel and about 50% on lubricating oil. To optimize energy efficiency, the use of energy management and control technologies should be promoted. It has been proven that the distributed power controller is 5% more efficient than the traditional push-pull configuration. In conjunction with electronically controlled pneumatic brakes, it is possible to achieve fuel savings of almost 30%. Rail freight transport should focus on natural gas locomotives in terms of renewable energy. With this technology hardly more diesel fuel is needed. Only 5% of the diesel fuel is needed to reach full power, with the remaining 95% being replaced by natural gas. Natural gas locomotives can save over 1 million liters of diesel per locomotive per year.

Airfreight is extremely attractive as a transport method. The aircraft as a means of transport brings many benefits. The reliability, safety, and speed ensure steady growth in this sector. It should be noted, however, that fuel consumption, as measured by effective numbers, is highest. Due to the specific CO2 emissions, airfreight drives global warming. All the more important are the technologies and opportunities that contribute to improving energy efficiency. The potential here is in different areas. Propulsion technologies, aerodynamics, composites, as well as flight behavior itself can have a huge impact on the energy efficiency of freighters. The solar energy systems announced by the EU for the production of "solar kerosene" act primarily as an optimal way to supply clean fuel to aircraft, but in this process, CO2 is also

produced by combustion. Electric motors in aviation are currently still in the development phase but could become very relevant in terms of energy efficiency in the future. The developments in the field of aerodynamics have proven to be effective and operational for many years. With the help of winglets and riblets, air resistance can be reduced by 20% and load capacity increased by almost 9%. This technology can be applied to any aircraft and offers around 1.5% fuel savings.

In aircraft construction, the processing of composite materials can result in optimized fuel consumption and a reduction in CO2 emissions. A disadvantage of carbon fiber-reinforced plastics is the high manufacturing and processing costs. In the long term, however, these investment costs are offset by the respective savings. Composites are particularly interesting because of their high specific strength. Changing the flight behavior can result in a reduction in fuel consumption, emissions, and aircraft noise. This aircraft operating technique is called continuous descent arrival. The state of the art is currently the turbofan engine. The developments have shown that ideas in the direction of the classic propeller cannot be ruled out.
