Natural Gas Fuelled City Bus - is it environmental or no 1 by po2378


									Natural Gas Fuelled City Bus - is it environmental or no?
Michal Takats, CTU Prague, Josef Božek Research Center of Internal Combustion Engines &Automotive Engineering Technická 4, 16607 Praha, 02 20395127, Fax: 02 24352500

1. Strategic Consideration
It is generally well known that world reserves of conventional hydrocarbon fuel will be expired relatively soon. This statement is generally valid for both crude oil and natural gas. As concern the quantitative evaluation of these phenomena, there exist differences. Particular data of final expiration of fossil fuel reserves differ depending from estimation of future trends of source exploitation and fuel consumption by various authors. Nevertheless all expectations agree that duration of natural gas sources will exceed the duration of reserves of crude oil. Thus the replacement of liquid hydrocarbon fuel consumption by consumption of natural gas can lead to prolongation of time which is at disposal for introduction of new types of energy converter which will not use fossil fuels as energy carrier at all. As concern road transport vehicles featuring following properties seems to be good candidate for shift from consumption of liquid hydrocarbons to use of natural gas as a fuel. a) fleet vehicles whose are able to be supplied by fuel from one refueling station b) vehicles performing limited amount of kilometers per day not demanding storage of high amount of fuel on vehicle board c) vehicles which do not fully utilize their loading capacity during typical operation. Weight penalty due to high mass of fuel storage tank does not influence payload capacity of vehicle in this case d) vehicles, whose makes possible by proper design and arrangement of their power unit to exploit the environmental advantages of natural gas use. In this way the still more strict environmental regulation demands can be fulfilled or even lower emission of pollutants can be obtained City transport vehicles meet the demands described in point a) of previous listing. In certain cases (i.e. all delivery vehicles whose transport capacity is limited by volume - this means not by weight - of transported commodity or which cannot be fully exploited concerning the possibility of management of their exploitation) fulfill the demands of point b) perfectly. Taking into account the delays during loading and unloading, the demands according to the point c) is fulfilled by delivery van also. Anyway city buses do not met the demands of point c). Any user of this public service (at least in Czech Republic) sometimes experiences the trip in highly overloaded bus. Usually the daily performance of city bus (except supporting vehicles used for peak shaving) is more than 500 km. The requirements according to point b) are highly overstepped. Nevertheless the city bus is many times favorite subject for use of natural gas as a fuel. This popularity is caused probably by good compatibility with demands according to point a) including the possibility of exploiting the advantage of so-called slow-fill system. Thus it seems be meaningful to investigate how are fulfilled the demands of point d) of above mentioned list.

2. Default advantages of use of natural gas as a vehicle fuel
First advantage is given by fuel composition. Natural gas (from the chemical point of view mainly methane CH4) contains 75 % of carbon and 25 % of hydrogen by mass. Diesel oil which is composed from heavy hydrocarbons (normalized substitute chemical formula is CH1, 82) contains approx. 87 % of carbon and 12 % of hydrogen by mass. 2.75 kg of carbon dioxide is formed by complete oxidization of 1 kg of methane. Complete oxidization of 1 kg diesel oil produces approx. 3.15 kg of CO2. At the same time heating value of methane is higher than those of diesel oil in ratio approx. 50/43 (MJ/kg). Even if slightly lower efficiency of gas engine (compared to diesel one) is taken into account the production of CO2 from natural gas powered vehicle is significantly lower than those from vehicle equipped with diesel engine. Natural gas fuelled engine practically does not produce any particles. Premixed combustion which takes place in combustion chamber of gas engine avoid the reactions which caused in diesel engine combustion chamber to fuel thermal decomposition and formation of soot. If diesel engine is replaced

by gas fuelled one the emission of particles (soot) is eliminated. There exist a hypothesis declaring soot emission to be most significant environmental drawback of diesel engine powered vehicles. It is assumed that emitted particles serve as carrier of strongly unhealthful PAH supporting their negative influence on living organism by deposits of pollutants inside lung alveolus. Natural gas features high antiknock performance not demanding the addition of antiknock ingredients to the fuel. In the past this advantage was declared in comparison with use of leaded gasoline for spark ignited engines. Contemporary vehicle gasoline engines use unleaded fuel only, nevertheless good antiknock quality of natural gas is usable by design of learn burn natural gas fuelled engine as will be described later.

3. Possible way to optimization of environmental properties of natural gas fuelled engines
Circumstances mentioned in previous paragraph takes place automatically as soon as the liquid fuel is replaced by use of natural gas. Circumstances, which will be, described latter will works after carefully performed optimization procedure. Conventional busses, which are used in city transport, are powered by diesel engines. Their conversion for use of natural gas as a fuel usually means rearrangement of engine to spark ignited (SI) one equipped with external mixture formation. There exist two ways to minimize the tailpipe emission of pollutants from natural gas fuelled SI engines. Stoichiometric engine is principally designed similarly to gasoline engines usually used as a prime mover of state-of-the-art passenger car. Low level of tailpipe emission is reached by exhaust gas aftertreatment using so called three-way catalyst (TWC). It is necessary to obtain good efficiency of exhaust gas aftertreatment to maintain mixture composition, which contains amount of air just necessary to complete oxidization of whole amount of fuel. Mixture strength is usually expressed in terms of air excess λ. At stoichiometric engine the value of λ = 1 must be maintained. Raw exhaust gas originates from combustion of rich mixture (λ < 1) does not contain enough oxygen to additional oxidization of products of incomplete oxidization. If lean mixture (λ > 1) is burned then condition are not reach inside the catalytic reactor to additional decomposition of nitrogen oxides (NOX). Closed loop λ-control must by applied to maintain mixture strength at demanded value according to the output voltage from λ-sensor installed in exhaust manifold upstream of the catalyst. If gasoline us used as a fuel the application of λ = 1 / TWC system only possibility to obtain favorable tailpipe emission of pollutant. If natural gas is used a fuel there exist another possibility. In fig.1 courses of certain engine parameters is plotted depending from air excess value. Data were measured during operation of SI engine on natural gas maintaining the same engine speed and the same throttle position (fully opened). Ignition timing was maintained constant also, only fuel flow was changed. In the figure curves of brake mean effective pressure pe (it can be interpreted as specific torque per unit of engine displacement), molar fractions of nitrogen oxides and carbon monoxide and brake specific fuel consumption mpe. It is to seen that raw exhaust gasses contain very small amount of presented pollutants if (extremely) lean mixture is burned. To utilize this favorable feature lean burn engine is designed. Manufacturers of conventional gasoline engines keep learn burn concept in touch as a perspective solution of problem of Fig. 1 Engine properties depending from mixture strength engine environmental behavior. Its practical implementation is practically disabled due to circumstance, which can be also observed in fig.1. Due to the low energy density of extremely lean mixture the engine power decreases significantly (see the pe curve in fig.1). Engine with such poor

specific power is not applicable for vehicle use. Application of turbocharging to obtain acceptable engine power is complicated by (relatively) low knock resistance quality of conventional gasoline. Compression ratio must be decreased significantly to avoid danger of knocking combustion. In this way the engine efficiency is deteriorated. Exhaust gas temperature at turbine inlet is too high, demanding use of special (and expensive) turbine design. Extremely high knock resistance of natural gas fuel can be utilized and turbocharged lean burn natural gas fuelled engine can be designed with compression ratio even higher (i.e. ε = 13) as is usually chosen for conventional naturally aspirated gasoline engines. Conventional design of turbocharger can be used and gas engine efficiency is almost comparable with efficiency of diesel engine.

3.1 Stoichiometric natural gas fuelled engine
As it was already mentioned natural gas fuelled λ = 1 / TWC engine is designed and it operates similarly to gasoline one. The same type of λ-sensor and catalyst itself are often used. Great volume flow of gaseous fuel can be exploited to simplify the design of an actuator of closed loop λ-control system as variable cross sectional area of fuel metering orifice. Nevertheless medium pressure intake manifold gas injection fuel system (similar to multipoint gasoline injection) is available on the market. So-called part load characteristic (this means the course of chosen parameters depending from engine load at constant engine speed) of natural gas fuelled stoichiometric engine is introduced in fig.2 as an example of environmental properties of such kind of engine. Engine load is expressed in terms of fraction of maximum load for given engine speed. To make the graph user-friendlier values are recalculated as a ratio between actual specific emission of particular pollutants and valid limit of European regulation, which is valid for evaluation Fig. 2 Stoichiometric natural gas fuelled engine properties of environmental behavior of (among others), bus engines. Thick lines describe the composition of raw exhaust gas; thin lines plot the composition of tailpipe gas. Logarithmic scale is used for this plot. Comparison with limit is not quite correct since the result of prescribed test is constructed as a weighted average from total of 13 measured points. Nevertheless the presented values illustrate typical behavior of λ = 1 / TWC natural gas fuelled engine. Exhaust gas aftertreatment is sufficiently effective as concern the additional removal of CO and particularly NOX On the contrary, the afterburning of hydrocarbons is not effective enough and tailpipe emission of this pollutant lies near the limit. Main individual organic compound (approx. 95 % of whole amount of emitted hydrocarbons) emitted from natural gas fuelled engine is methane. Low reactivity of methane itself causes deterioration of catalyst efficiency during exhaust gas aftertreatment. On the other hand low reactivity of methane itself is evaluated as low level of its harmfulness. Certain environmental regulations (i.e. those valid in California) take into account only Non-Methane Hydrocarbons (NMHC). Tailpipe exhaust gas composition of natural gas fuelled λ = 1 / TWC engine is more favorable compared to diesel engine exhaust gas composition.

3.2 Lean burn natural gas fuelled engine
An example of properties of particular implementation of lean burn concept is introduced in fig.3. Part load characteristic of particular engine with particular arrangement and particular adjustment is plotted. Environmental properties are evaluated again as the ratio of achieved value to the valid limit. This time linear scale is used for plot. Lean burn concept is fully implemented at full load and high part load. In those regimes emission of nitrogen oxides understeps the limit level significantly. Emission of CO lies safe below the limit value, its further improvement is easy feasible by use of simple (and relatively cheap) oxidization catalyst. At low part load mixture enrichment takes place to ensure good starting properties of engine

and minimize its running roughness during idle operation. Specific emission of both CO and NOX increases in those regimes. Emission of hydrocarbons oversteps the limit level λ multiple. Special measure concerning exhaust gas aftertreatment must be applied to meet the limit value. Conventional oxidization catalyst is ineffective. Nevertheless as concern the harmfulness of emitted hydrocarbon, the same is valid as was already mentioned in previous paragraph. Similarly to stoichiometric engine, main advantage is decrease of emission of nitrogen oxides. This result is particularly valuable taking into account well known fact that all measures leading to minimize emission of NOX leads at the same time to deterioration certain other engine properties i.e. soot emission from diesel engines. On the whole the use of natural gas as fuel for road Fig. 3 Lean burn engine properties transport means can be evaluated as advantageous from the environmental point of view even if not all problems concerning emission of pollutants are solved.

4. Non-environmental aspects of use of natural gas in transportation
There exist certain environmental hazards connected with use of natural gas as a vehicle fuel. The insufficient care dedicated to optimization the environmental behavior of vehicle power unit plays leading role. Various reason for this omission are listed as follows: • ignorance of basic laws of thermodynamics followed by overestimation of default advantages of use of natural gas • insufficient personal and technological equipment of certain subjects whose are active in rearrangement the vehicles for natural gas use in combination with (sometimes wrong) assumption that this activity is highly remunerative • insufficient resources for development of gasified variant (only limited number of pieces will be realized usually) especially to investigation of reliability and durability corresponding devices • shift the cost / quality relationship to accent low cost of vehicle rearrangement for natural gas use followed by negative consequence particularly concerning durability, reliability ant repeatability of properties • non-critical generalization of partial environmental contributions Following statement can serve as an example of combination of certain from above listed circumstances: Natural gas fuelled vehicle is absolutely environment friendly since very low value of molar fraction of CO was measured at engine idle. Thus objectively was investigated: • one emission factor (molar fraction) • one pollutant (carbon monoxide) • one running regime (idle) whilst for correct judgement it is necessary to monitor: • all relevant emission factors (such as global mass flow of pollutant and/or specific mass flow of pollutants per unit of effective output) • all pollutants (or at least all pollutants defined in corresponding environmental regulations) • all running regimes (or at least all running regimes whose are significantly occurred during typical vehicle operation It was determined in previous paragraphs that carefully optimized natural gas fueled vehicle can compete concerning its environmental properties even if it is evaluated correctly. Non-critical tendentious presentation of only illusive advantages (followed by its disclosure) leads to disbelief to idea of use of gaseous fuels at all. Henceforth all concerned subject will be reticent. Both on-board and service diagnostics systems are special problem as far as rebuilding of existing vehicle is concerned. Linking to auto-diagnostic function of included control units is complicated by necessity of knowledge of corresponding codes. Those codes are keep secret (by vehicle manufacturers) intentionally since this knowledge makes possible the manipulations, whose otherwise

are declared as unauthorized. Reaching the standard of diagnostic procedures as it is usually implemented in design of state-of-the-art vehicle represent a big challenge for designer and manufacturer of gasified version of vehicles. Eventual absence of diagnostic techniques (at least) complicates periodical monitoring of vehicle properties after its rearrangement for use of gaseous fuel.

5. Conditions to be held to enforce advantages of natural gas fuelled vehicles
Main condition, which is necessary to be held to enforce the advantages of natural gas use as a vehicle fuel, was already mentioned repeatedly. Research and development work must be performed in high quality by designer of gas fuelled vehicles. System of checking and management of quality of production must be implemented into manufacturing of gas fuelled vehicles. Accomplishment of mentioned (axiomatic) demand leads to increase of purchase price of gasified vehicles. To maintain the natural gas fuelled vehicles to be attractive for their potential user it is necessary to keep the cost of natural gas at refueling station low compared with cost of conventional (liquid hydrocarbon) fuels. This task can solve state administration by management the development of excise duties for various vehicle fuels. At the same time it is necessary this intention (if it exist at all) to be clearly declared. The uncertainty concerning the development of cost relation in the future retards the development of gasification of road transportation significantly. Especially as concern the urban bus service it is to recommend to change the system of management of subsidize. It is necessary to introduce the system motivating to effective behavior of urban transportation enterprises instead of system covering economical waste as it is sometimes established. Acknowledgment Research was supported by Ministry of Education of Czech Republic project # LN 00B073

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