Comparative engine performance and emission analysis of CNG and gasoline in a retrofitted car engine

Abstract

A comparative analysis is being performed of the engine performance and exhaust emission on a gasoline and compressed natural gas (CNG) fueled retrofitted spark ignition car engine. A new 1.6 L, 4-cylinder petrol engine was converted to the computer incorporated bi-fuel system which operated with either gasoline or CNG using an electronically controlled solenoid actuated valve mechanism. The engine brake power, brake specific fuel consumption, brake thermal efficiency, exhaust gas temperature and exhaust emissions (unburnt hydrocarbon, carbon mono-oxide, oxygen and carbon dioxides) were measured over a range of speed variations at 50% and 80% throttle positions through a computer based data acquisition and control system. Comparative analysis of the experimental results showed 19.25% and 10.86% reduction in brake power and 15.96% and 14.68% reduction in brake specific fuel consumption (BSFC) at 50% and 80% throttle positions respectively while the engine was fueled with CNG compared to that with the gasoline. Whereas, the retrofitted engine produced 1.6% higher brake thermal efficiency and 24.21% higher exhaust gas temperature at 80% throttle had produced an average of 40.84% higher NO_x emission over the speed range of 1500–5500 rpm at 80% throttle. Other emission contents (unburnt HC, CO, O₂ and CO₂) were significantly lower than those of the gasoline emissions.

Introduction

The scarcity of petroleum fuel resources and turmoil in the oil market along with the acutely growing demand of oil threatens the security of energy production. The necessity of fuel has gained the ground for adaptation of suitable energy policy for the transportation sector in order to balance the demand and supply of oil and to contain the overall release of the greenhouse gases with the eventual undesirable environmental impacts. The drive created by the energy security, climate change and the rapidly growing demand of transport fuel lead to a quest for clean burning fuel.

Energy policy and planning with the related orientation have become a very important public agendum of most developed and developing countries nowadays, as a result of which, the governments are encouraging the use of alternative fuels of petroleum oil in the automotive engines. When evaluating different alternative fuels one has to take into account many aspects [1]:

• Adequacy of fuel supply,

• Process efficiency,

• Ease of transport and safety of storage,

• Modifications needed in the distribution/refueling network in the vehicle,

• Fuel compatibility with vehicle engine (power, emissions, ease of use, and durability of engine).

Numerous researches are going on worldwide in alternative fuels/sources of energy, such as, biodiesel, bioethanol, hydrogen cell, solar energy and compressed natural gas have so far been most common approaches in this arena. Solar powered car are still not market adaptive as it requires more dedicated design features. Hydrogen fuel has low volumetric efficiencies and frequent pre-ignition combustion event because the power densities of premixed or port-fuel-injected hydrogen engines is significantly lower than gasoline [2]. Many academic researchers on the hydrogen economy have queried the rationale on why hydrogen might not be the best alternative transport fuel, including safety, cost and overall efficiency [3], [4]. On the contrary, biodiesel and bioethanol require no engine modification for smooth operation, but they create various problems in the long term operation and in the higher percentage usage, especially when biofuels are mostly derived from vegetable oils and crops-seeds. These alternatives are strongly criticized for its environmental impact and phenomenal threat to food security [5], [6], [7]. Apart from experimental investigations, several theoretical researches are proceeding in the quest for alternative fuels. Saidur et al. [8] evaluated the effect of partial substitution of diesel fuel by the natural gas on performance parameters of a four-cylinder diesel engine. Other types of alternative fuels, such as, methyl and ethyl alcohol, boron, liquefied petroleum gas, biomass, electricity solar energy, etc., are also potential alternative sources of energy in the internal combustion engine [9]. Artificial neural network has been applied to predict the gasoline engines emission and performance [10]. As the consequence of these studies, researches on CNG fueled engine are also progressing throughout the world due to its potential as an alternative fuel for the spark ignition (S. I.) engine. The difference between the operation of the conventional gasoline fueled and the CNG-engine system arises from the physical and chemical properties of these two fuels. It is a well known fact that petroleum fuels are liquid at room temperature and CNG remains in a gaseous state at a much lower temperature (−161 °C). CNG has a lower density but higher octane number then gasoline. It can easily operate in a high compression ratio and higher self/spontaneous ignition temperature makes it a safer fuel in case of leakage [11]. Table 1 represents the comparison between the physiochemical properties of CNG and that of the gasoline.

As a gas, CNG requires a different approach of fuel induction mechanism at all normal temperatures and pressures. This has resulted in an increased interest in the use of CNG as fuel for the internal combustion engines and hence CNG has now been used to power vehicles of various ranges, starting from light delivery trucks to full size urban buses and other varieties of applications [13], [14]. But most of the CNG-engine vehicles used today are retrofitted from the gasoline engine. This type of engines cannot advantageously perform on CNG as an engine fuel. However, the research has somewhat succeeded to minimize the drawbacks of the CNG in retrofitted cars and harvests the maximum obtainable from the CNG the result of which concludes that a dedicated CNG engine is a must. In this experimental study, a comparative evaluation of the performance of gasoline and CNG fueled retrofitted spark ignition car engine had been performed. The engine was converted to a computer incorporated bi-fuel system and operated with either gasoline or CNG using an electronically controlled solenoid actuated valve system. The engine was operated at constant throttle positions with a variable speed to evaluate the performance and exhaust emission for both the fuels.