Power Units for Transportation

Abstract

For more than 125 years, gasoline and Diesel engines have prevailed as the exclusive drive unit in road transportation. None of the other power units invented to date has been able to make use of the energy content of mineral oil with the piston engine’s same good efficiency.Combustion is the fundamental process by which the chemical energy of fuels is converted into thermal energy and further into mechanical work. If hydrocarbon-containing fuels were completely burnt, the resulting products would be carbon dioxide and water vapor only. Since it is impossible to obtain a 100% complete combustion the exhaust gases always include a great variety of combustion products, the most important are: carbon monoxide, unburnt hydrocarbons, nitrogen oxides and particulate matter.During its 125 years of existence, the Otto (gasoline) engine – as it was called after its inventor – has been developed into a mature combustion engine which is characterized by an excellent efficiency and low pollutant emissions. The properties of the gasoline engine strongly depend on the composition of the air-fuel mixtures and ignition parameters. The influence of the so-called engine design parameters on combustion and exhaust emission is no less important.The emission of many of the exhaust-gas constituents can be influenced and minimized at their place of origin, that is in the combustion chamber by correctly selecting and adapting the relevant engine design and operating parameters. If optimization of engine-internal parameters for further reducing of the exhaust gas emissions are not enough anymore, so-called engine-external measures must be additionally taken. It was found that so-called three-way catalyst reduces the three aforementioned pollutants by clearly more than 90%, provided that a precisely stoichiometric A/F-ratio is used.Thanks to the strict maintenance of a precise stoichiometric air/fuel mixture the three-way catalyst allows very low HC, CO and NOx pollutant emissions to be achieved. However, in this operating range, fuel consumption is 8 to 15% higher (with a resulting higher CO2 emission) than during lean-burn operation.One of the technically most useful solutions to reduce the fuel consumption and CO2 emission of gasoline engines is to make them tolerate lean air/fuel mixtures. The future of the lean-burn gasoline engines will almost exclusively depend on the successful development of NOx-exhaust-gas after-treatment technologies for lean air/fuel mixtures.Diesel engines are internal combustion units with the highest thermal efficiency. Mixture formation is achieved through high pressure fuel injection. The fuel leads to self-ignition in the highly compressed air of the engine cylinder. The power and torque characteristics of modern Diesel engines are comparable with those of spark ignition (Otto) power units of equal capacity, the fuel consumption however is approx. 20% lower.The Diesel power unit has achieved a high status in transport. The world wide share of Diesel engines in passenger vehicles is now approx. 20%, whereas in freight transport on the roads and by water the share is approaching 100%, diesel being the only cost effective alternative.Increasingly, new methods for injection combustion, exhaust gas recirculation and after treatment (NOx-Cat, Diesel particle filter) are being pursued to meet the ever stricter emission legislations, aimed at limiting the effects on the environment.Ever since its invention, the 4-stroke reciprocating piston engine has been considered as a rather complex thermal unit which should better be replaced by far less complicated designs.When summing up all the properties required to smoothly operate cars over wide speed and load ranges and a long lifetime, all alternative concepts have never succeeded in edging the Otto and Diesel engines out of their top positions. Further optimized versions of gasoline and Diesel engines will continue to prevail in the automotive domain in the coming 15 to 20 years. Due to their theoretically high efficiency and low pollutant emissions, fuel cells are among the most promising alternative energy sources of the future.