b) Economy
In the table, the above-mentioned economy criteria have been applied to different engine designs and assessed.
Among the competition, the assessment of specific operating costs is poor in the electric motor because of the generally low power output. The diesel engine with exhaust-gas turbocharging is assessed more favorably.
For the purpose of better comparability, the production costs are also referred to the power output. The underlying costs are average values for the propulsion units produced on large-scale. In the case of gas turbines and electric drives, however, estimate values are used for larger volumes.
Average values for the power-to-weight ratio for reciprocating engines were also determined. The turbocharged Otto engine has the smallest power-to-weight ratio, followed by the Wankel engine and the Otto engine. In the diesel power plant, the high operating pressures lead to a bulky design. A power-to-volume ratio comparison would show nearly corresponding relations with the power-to-weight ratio.
When considering the capability of propulsion units to use multiple-fuels, piston power plants can only be operated with alternative fuels following partly large and constructive modifications. In the gas turbine, however, for a large number of liquid fuels this is possible without additional measures. The electric motor can, at least indirectly, fall back on nearly all energy types, since wind, solar, and hydro-energy are used to produce electricity.
Maintenance costs are relatively small for all propulsion types. In case of piston power plants, the Wankel engine has a slight advantage due to the absence of a valvetrain. However, the increasing employment of valves with hydraulic compensation of clearance for the most part levels out this supposed disadvantage of other piston engines. Gas turbines and electric motors similarly require small maintenance costs as in the case of the Wankel engine, though today's lead accumulators still require regular maintenance.
When comparing the life-span of the various power plants, the diesel engine gets the highest values due to its robust design and the low speed level. The gas turbine is assessed lower because of its lower stage of development. It is true that the electric motor has the longer life, but as a total system it is assessed lowest because of its short battery life.
c) Environmental influence
In order to assess the environmental influence, the assessments for average exhaust emissions, noise, and vibrations of the different propulsion systems were compared and the results indicated in the table above.
The distribution of points for exhaust emissions was averaged by individually assessing different pollutants. In this case, the employment of exhaust catalysts which leads to a significant shift in favor of the Otto, stratified-charge, and Wankel engine, is not considered. The electric drive does not produce exhaust emission on a local level, but it should not be ignored that pollutants emerge when electricity is produced in conventional power plants.
When it comes to noise development, the electric drive is lower on average by 10 dB(A) when compared to conventional power plants. Also the gas turbine functions more quietly than piston power plants due to continuous combustion. The high frequencies of compressor and turbine however have a negative effect. Among piston engines, the Wankel engine has slight advantages compared to the Otto and stratified-charge engine due to the absence of a valvetrain. The diesel engine is the loudest. Finally, one has to distinguish between supercharged and naturally aspirated version: supercharged engines work insignificantly more quiet since the turbocharger causes a damping of the intake noise.
Considering the vibratory behavior, Wankel engine, gas turbine, and electric drive are better than the reciprocating power plants. High translational accelerations occur in reciprocating engines as a result of the piston kinematics. These accelerations depend on the number of cylinders and their arrangement and can only be partly compensated.
It has to be taken into consideration that the presented distribution of points can only demonstrate tendencies that do not have to be valid for every isolated case. However, they serve as an overview and also for orientation.