In Poland, bus transport is present in all cities with municipal transport services. Buses have become a common and primary means of urban transport nationwide. Their share in public transport passenger traffic reaches 66%. An important feature of bus transport, which influences its attractiveness for urban transit, is the ability to launch new routes without major capital expenditure. Emerging transport needs related to the development of suburban areas and the creation of new residential districts can be met practically “on the spot.” Bus transport in large urban agglomerations is also an example of the negative impact of modern motorization on the natural environment. Diesel-powered internal combustion bus engines emit significant amounts of substances harmful to humans into the atmosphere. Diesel exhaust fumes are multi-component mixtures of chemical compounds formed as a result of the combustion process of diesel and engine oil. Combustion products are emitted into the atmosphere in the form of gases and solid particles that have not undergone combustion.
The main components of diesel engine exhaust are:
• nitrogen oxides
• carbon oxides
• sulfur oxides
• aromatic and aliphatic hydrocarbons
• solid particles containing carbon (soot)
• paraffinic hydrocarbons, phenol derivatives, esters.
The direct impact of these substances on the human body can cause many dangerous diseases. An example of such a negative impact is the effect of solid particles contained in diesel engine exhaust on the human body. The International Agency for Research on Cancer (IARC) has concluded that there is sufficient evidence of their carcinogenic effect. In addition to environmental pollution by liquid fuel combustion products, public transport vehicles emit significant amounts of noise. Noise is a factor that has a very negative impact on humans and significantly affects their health. The characteristics of operating municipal buses cause the negative impact of this type of transport on the natural environment to be significantly intensified. Higher emission of pollutants results from the fact that:
• a municipal bus covers hundreds of kilometers daily in a densely populated and built-up area
• due to numerous stops and relatively short distances between them, the engine’s operating characteristics in such conditions cause higher fuel consumption compared to long-distance travel
• with uneven engine operating characteristics (frequent stopping and starting from stops), significantly higher amounts of noise are emitted than in the case of a vehicle moving at a constant speed.
Urban areas are also characterized by a large number of motor vehicles using roads within cities. A significant number of vehicles owned by private individuals, institutions, business entities, etc., move within every urban agglomeration. The number of cars in every major city is growing year by year. Problems related to the negative impact of rapidly developing motorization on the natural human environment are intensifying. Therefore, the problem of limiting the amount of pollutants and noise emitted by motor vehicles is becoming so important. The use of alternative fuels in cars moving within city areas can contribute to solving this problem or at least significantly mitigating it. One of the ways to reduce exhaust emissions from bus engines is to use compressed natural gas (Compressed Natural Gas – CNG) as fuel. Natural gas has many features of an ideal fuel. It is cheap, ecological, safe, and convenient to use. Natural gas available on the Polish market contains no less than 92% methane by volume. Other components occurring in small amounts are: higher saturated hydrocarbons, nitrogen, carbon dioxide, hydrogen sulfide, water, and other components in trace amounts. The composition of natural gas, which determines its physicochemical properties, makes it an excellent engine fuel without the need for major modifications and processing. Engines powered by compressed natural gas emit much smaller amounts of harmful substances into the atmosphere than engines powered by traditional liquid fuels. The emission of harmful substances is so low that CNG buses meet current and future environmental protection standards in force in Europe (Table 1).