Merits and demerits of gas turbines

Other characteristics

The most discriminating feature of gas turbines against reciprocating internal combustion engines is the amount of gases to be processed in the same engine size. Gas turbines can process large amount of gases in a small engine, resulting in the very high power weight ratio. Can you imagine the 500kg weighing 2 cubic meter sized turbine engine provides the output power of 5000 hp ?  In diesel engines, the size will be the same of a large truck. Even a state of the art 300 KW electric motor used in the Shinkansen weighs about 300kg.

This picture shows the 25000KW gas turbine engine. You can guess the size of the engine by comparing the human standing next to it. The engine output is 25000KW, not 2500KW,  and it is not the maximum output, it is the continuous rated output. It is far more powerful than the Shinkansen series 500 train set, consists of  16 motive cars. To achieve this performance with other engine, it will be building-sized. The jet engine used in "Jumbo Jet" is as large as this. You may insist Jumbo's engine is far larger, but as a turbofan engine has a large propeller around the engine core, the engine core itself is very small. Next feature of a gas turbine is it can use many kind of flammable gases and liquids as a fuel.  For example, gasoline, light oil, kerosene, alcohol,  natural gas, hydrogen. Rregenerative fuels such as alcohol and methane have lately attracted considerable attention and gas turbine is well fit them.

The next feature is that gas turbines can produce the large torque at the low speed. As is mentioned above, this is an important feature to drive vehicles, which eliminates the sophisticated transmission and increases the acceleration. The next schema illustrates this advantage. This is a torque and output comparison of the turbocharged tank diesel engine  and the three shaft gas turbine (AGT1500) showing the gas turbine's high torque at low speed advantage.

Recently a high efficient and light weight electric transmission is available and this advantage has been fading away in the vehicle drive system, but it is convenient where simplicity and light weight are important factor.

The next is that gas turbine is vibration free and not so noisy. Gas turbine mainly produces high frequency noise and it is easily reduced by silencer.  On the other hand, diesel engine produces much low frequency noise with severe vibration and makes it difficult to reduce noise. You can experience this difference when you ride on a jetfoil. A turbine powered jetfoil is vibration free and less noisy, but a diesel boat is far noisy and full of vibration. The turbine powered jetfoil is well silenced, produces noise about 90-100dB at the near outside of the ship. Considering its output power level of 8000hp, it can be said that this is a silent vehicle. On the other hand, Japanese national railway's direct drive turbine powered test car Kiha391, produced early 70's was much more noisy, which produced noise over 120dB while starting at full power. 

Other feature is that gas turbines consume less lubricating oil than reciprocating engines and do not require a large cooling system.
Usually, reciprocating engines are hard to start up in cold weather but gas turbines are easy to start in such a condition and do not require long time idling to warm up. 
Gas turbines are environmentally more friendly than other internal combustion engines. When a diesel train starts at a station, the station will be filled with noxious blue-white or black smoke and you may be hard to see and breath.  Imagine a airport crowded with diesel powered large airplanes, if exists, there will be severe air pollutions around the airport.

What are defects?

The most serious problem is that a gas turbine consumes much fuel especially in a small one.　When working at the part load, the efficiency is seriously reduced. At full load some gas turbines may be more fuel efficient than some high speed diesel engines but this is not true at a part load condition.
The next graph shows the relationship between heat rate and output power.

When the output is reduced to 30% then the heat rate almost doubles.

The next graph shows the change of thermal efficiency of 5000 hp class diesel and gas turbine locomotives as a function of engine output. This class of gas turbine is classified as a middle size and it's part load efficiency is relatively good but still worse in comparison to the diesel locomotive especially at low output.

And what is even worse, gas turbines consume much fuel than other reciprocating engines at the idle condition. As is mentioned above, the compressor must spin at high speed continuously to produce the effective compressed air to maintain the engine idling, 　When a gas turbine is idling, a gas generator turbine must spins at 60% maximum speed or more. For example, 1000 hp class gas turbine should spin at over 10000 rpm, and if the engine is kept idling for one hour it will consume over 40kg of fuel, this amount will be four times or more than that of diesels.
This is not so serious in high speed rail applications where the long time high speed cruising is common and the high cruising power is required. But in ordinary rail applications the coasting time is much longer than the powering time resulting in the bad fuel economy. At some bad conditions, the fuel consumption may be twice as much as that of diesel trains. In American heavy duty freight trains, it is estimated that the 5000 hp class gas turbine operation will increase fuel consumption 25% against diesel operation according to GE Transportation..

Another aspect of the high fuel consumption is that the low speed operation of gas turbines worsen the fuel efficiency. It occurs even in two shaft gas turbines in spite of their high torque at the slow rotational speed. Turbine is designed to bring out it's best performance at specific speed. This rotational speed is called as "designed point". When turbine is spinning at this speed and the axial load increases,  its rotational speed will decrease and balance some rotational speed because the reduction of the turbine speed increases its torque. In this process, the amount of fuel consumed does not change. This is the distinguishing difference between turbines and reciprocating engines. In a  reciprocating engine, same amount of fuel is consumed at each explosion and then the fuel consumption is proportional to the rotational speed of the engine. But a  gas turbine is continuous combustion engine and the amount of fuel injection is not influenced by the engine speed. If the turbine torque doubles at the half speed, there is no problem but as the turbine efficiency worsens at rotational speed out of designed point, the turbine torque does not double but approximately 1.5 times. This means 25% loss occurs at this speed.

High cost of the engine is also significant barrier for rail applications. Mass produced micro gas turbines may have a competitive advantage over other reciprocating engines, but a 5000 hp class gas turbine costs three to four times higher than a comparable diesel engine. This cost is roughly the same as the entire cost of one diesel locomotive.

The turbine's high speed rotation requires sophisticated heavy reduction gear box. Middle class engines spin 10000 to 20000 rpm and small class over 100000 rpm. But recent advancement of the high speed alternator technology has enabled an alternator to couple directly to a turbine shaft, resulting in very light weight generator set.

Gas turbines absorb a lot of air and exhaust a lot. Consequently silencer and air filter occupy large space and this may influence the cabin space or the cargo space.

Gas turbines require clean air to keep good fuel efficiency because if compressor blades are polluted, the compressor efficiency decreases and the total efficiency decreases. There is no such problem in aircraft which flies the particle free high altitude but in land or sea applications engines must absorb particle rich air.

Gas turbines produce the large sound when it start up at its full power while the output shaft is stalled. This is the two shaft gas turbine specific phenomenon, occurs when a direct drive train starts at a station.

The cyclic load profile of the typical locomotive operation may be a challenge to gas turbines.  Gas turbines are usually used at constant power. But locomotive's operating power varies dynamically, from idle to maximum.  This increases the heat stress and affects the engine life.