Evaluation of the maximum horsepower of vehicles converted for use with natural gas fuel

Abstract

Tests to measure the maximum horsepower of commercial vehicles converted for use with natural gas fuel indicate a reduction of approximately 20% in horsepower compared with gasoline-driven vehicles. This reduction in horsepower resulting from the use of gas is due basically to the lower thermal efficiency of the cycle of natural gas compared with that of gasoline and to its lower volumetric efficiency, since natural gas is injected into the combustion chamber in gaseous form, unlike gasoline, in which part of the fuel entering the cylinder is in the liquid phase. Natural gas used as a fuel generates lower quantities of air pollutants, particularly unburned hydrocarbons (HCs) and carbon monoxide (CO). Despite the downside of reduced horsepower, the use of natural gas as a fuel for automotive vehicles equipped with Otto-cycle engines is economically viable owing to the wide availability of natural gas.

Introduction

Increasing energy consumption has led to a concomitant decline in air quality. This consumption is based on fossil-based nonrenewable energy whose level of deposits tends to decrease [1]. Solutions must therefore be found that can, on the one hand, contribute to reduce the emission of pollutants and, on the other hand, offer new answers in the field of energy. Natural gas ranks high in the field of alternative fuels.

Natural gas is traditionally seen as an abundant and clean fossil fuel for generating thermal and electric energy [2], [3], [4], [5], [6]. After treatment to remove undesirable components, this gas is used principally as a fuel. The chemical industry uses only 7% of this gas, e.g., in the production of ammonia and methanol [7], [8]. The use of gas as an electricity-generating fuel requires an infrastructure of local distribution to the end user, be it through gas pipeline systems or liquid natural gas (LNG) transportation and reevaporation systems. The composition of raw natural gas depends on a series of natural factors that determine its formation process and the conditions under which it accumulated in its original deposit. Like petroleum, natural gas is found in underground deposits in many parts of the planet, both on land and at sea, and a considerable number of deposits contain natural gas associated with petroleum. In these cases, the gas is known as associated natural gas. When a deposit contains little or no oil, the natural gas is called nonassociated [2], [9].

Moving natural gas over long distances through high pressure gas pipelines or in the form of LNG is a considerably costly operation and the margin of profit of natural gas from remote fields is low, basically due to high transportation costs [3]. The process of conversion of natural gas into chemical and fuel products easily transported in tanks, modifying the problem of transportation in gas form to liquid form, increases the product’s added value. This removes the restrictions on its use due to high transportation costs and access to distant markets. The use of natural gas depends on local market circumstances and can be based on small natural gas deposits [2], [3], [10], [11].

Year by year, environmental restrictions on automotive vehicle emissions increase, particularly in large urban centers overpopulated with automobiles, buses and trucks. The maximum permissible indices of contaminating gas emissions have declined steadily, a fact that has driven research into alternative fuels, which, in turn, has led to the increasing use of clean fuels. Natural gas is an abundant source of energy which is less harmful to today’s environment [12], [13], [14], [15], [16], [17], [18], [19].

Today there is a growing demand for fuels such as gasoline and diesel oil with increasingly restricted concentrations of sulfur [20]. The costs of technological transformation of vehicle fleets to LNG, hydrogen, methanol, ethanol and electricity must take into account the growing need for chemical raw materials to support worldwide industrial development and environmental restrictions. The conversion of gaseous fuels into liquid fuels predominates in the traditional production processes. The use of gas is economically attractive and generates smaller quantities of environmentally harmful residues.

Gasoline is a complex mixture of hydrocarbons that includes aromatic, naphthenic, olefinic and paraffinic compounds containing five to twelve carbon atoms with ebullition points ranging from 30 to 200 °C. This mixture results in incomplete combustion and the formation of partially oxidized compounds [21], [22], [23], [24], [25].

Natural gas can be used as a fuel in Otto-cycle engines by modifying only their injection system. The principal constituent of natural gas is methane, whose content may vary from 70% to 95% depending on its origin. In the US and Canada, natural gas usually contains 85% to 95% methane, the remainder consisting of carbon dioxide (CO), nitrogen and minor amounts of ethane, propane and butane. The natural gas that Russia supplies to Finland is very rich in methane, i.e., about 98% [26]. The volumetric composition of the natural gas used in southern Brazil today consists of about 89% methane, 6.5% ethane, 1.8% nitrogen and 1.4% propane [27].

Methane is the main constituent of natural gas, which contains 75% of carbon (in mass) compared with the 86% to 88% of carbon in the traditional liquid fuels. Under stoichiometric conditions, the level of carbon dioxide (CO₂) produced during combustion is 11.7% compared, for instance, to that of isooctane. Moreover, in a rich mixture, the level of CO is lower in methane and in hydrocarbons. The values of CO calculated for an air/fuel ratio of 1.10 are 2.2% for methane and 3.3% for toluene. Methane is also characterized by its relatively low flame temperature, which helps limit the formation of nitrogen oxides [2].

Gaseous fuels form homogeneous mixtures with air more easily, without the need for turbulence or heating. Well distributed to the engine’s cylinders, this homogeneous mixture makes starting easier, cold working relatively devoid of problems and burning more complete. On the other hand, the amount of gas required for combustion occupies a larger volume than if the fuel were liquid. Therefore, the amount of incoming air is smaller, which can lead to a loss of horsepower. However, the use of LNG provides the horsepower needed for the engine’s regular performance, not only in low gear at low rotation and without loads but also in situations requiring high horsepower, high rotation with loads, or even torque, low rotation and high loads [28].

The antiknock capacity of gasoline is measured by the octane number (RON or MON) [29], [30], which can be estimated based on the methane index for natural gas by extrapolation to 130 and 115, respectively [31]. To better represent the antiknock capacity of gaseous fuels, a new scale was developed called methane number standardized by the ISO 15403 standard [32], whose reference is pure methane (NM = 100) and hydrogen (NM = 0).

The latest forecasts of annual worldwide energy consumption indicate that 11.7 billion tons of petroleum equivalent will be used in 2010 and 14.2 billion in 2020, compared with the 8.9 billion consumed in 1997. Nevertheless, many countries already support the consumption of natural gas in detriment to oil, coal and nuclear energy. Thus, the consumption of natural gas is estimated to be in the order of 3.5 trillion m³ in 2010 and 4.7 trillion in 2020, against the 2.3 trillion consumed in 1997. This represents a growth of 52%, or 4.2% per year over the next decade. Natural gas is an abundant resource whose deposits total 146 trillion m³, which is equivalent to 60 yr of consumption. Probable additional deposits are estimated at 260 trillion m³, or an additional 83 yr at the consumption rate estimated for 2010. It is obvious that the general worldwide trend for environmental protection should strongly restrict the use of untreated petroleum by-products, leading to a forecasted drop in oil demand before the world’s oil reserves are depleted. Until that time, there will have been 50 yr of technological development associated with an ever decreasing availability of oil and natural gas [26].

In Brazil, 2005 saw a record conversion of light vehicles to VNG, with 216 thousand vehicles adapted to run on this fuel by the end of the year. This figure is far superior to that of 2003, when 194 thousand conversions to VNG were made. Brazil’s fleet in 2005 reached the mark of 17 million VNG-powered light vehicles [33].

In view of this reality, this study involved an analysis of the performance of vehicles adapted to run on natural gas, through the transformation of liquid fuel-driven engines (gasoline/alcohol) into bi-fuel engines (gasoline/alcohol–natural gas). The horsepower curves of natural gas are compared against the use of liquid fuel. An analysis was also made of the exhaust gases.