Performance and Emission Study on a Dual Fuel Engine with Modified Gas Inlet

Fossil fuels are the dominant global source of air pollution and their combustion is showing greater threat to clean environment. The economic cost of the effect of this pollution on human health and environment has been estimated at 0.4 % of gross domestic product [1, 2]. However, it is possible that in some areas the costs may be substantially higher. The limits for reduction in the emission levels of emission have been agreed [3] by various nations.

Gaseous fuels are capable of performing a prominent role to reduce emission. Various gaseous fuels such as biogas, producer gas, hydrogen, LPG and CNG (Compressed Natural Gas) are suitable for Internal Combustion Engines [4]. But LPG and CNG are considered better alternatives because of their simpler structure with low carbon content, resulting in reduction of exhaust emissions drastically [5]. When CNG was used as fuel in automobiles, it results in lower carbon monoxide and particulate matter emission as compared to diesel operation.

In India, LPG is easily available compared to CNG [6, 7]. Hence, for the present work LPG was taken as primary fuel. In dual fuel engine, the primary fuel releases large amount of energy and secondary fuel is required to start the combustion of the primary fuel [8]. The dual fuel engines are capable of switching from one fuel to the other without interruption in power generation [9].

The performance and emission of LPG-Diesel dual fuel engine is comparable with diesel operation at higher load [10]. The cycle variation on the performance of the LPG-Diesel dual fuel engine was found to be lower between the LPG flow rates of 0.2 to 0.3 kg/hr, fluctuations in various parameters like peak pressure, rate of pressure rise and indicated mean effective pressure was minimum [11, 12].  

Phase equilibrium is a basis of calculation of enthalpy and entropy, and it is a clearly reveal of liquefied degree of raw natural gas [13, 14]. Main variables affecting the LPG-diesel dual engine are the LPG flow rate and diesel consumption. It is shown that there is an optimum LPG flow rate for maximum efficiency and also at lower loads LPG will not burn completely due to excessive lean mixtures. Higher secondary fuel quantity is to be used at low outputs to ensure proper combustion of the gaseous fuels [15, 16].  

The secondary fuel LPG is premixed with the intake air and supplied to the engine. LPG duel fuel engines have a good thermal efficiency at high output but the performance is less during part load conditions due to the poor utilization of charges. This problem can be solved by varying injection timing, composition of the gaseous fuel and intake charge conditions, for improving the performance, combustion and emissions of dual fuel engines [17]. The objective of the present work is to develop a dual fuel engine test rig operating on LPG-diesel fuel with some modifications in the intake manifold for better mixing of the secondary fuel and to compare the engine performance characteristics without modifications and along with sole fuel.

A single cylinder, four stroke, direct injection, water cooled diesel engine was used for conducting the experiments as shown in figure 3. The engine was modified to work in the dual fuel mode by attaching an LPG line to the intake manifold directly and through venturi of different size for better turbulence of the secondary fuel. The flow rate of LPG can be varied manually, and measured by a wet type gas flow meter.

The engine is connected to an eddy current dynamometer. It is provided with temperature sensors for measurement of jacket water inlet and outlet temperatures, pressure sensor for combustion pressure and encoder for crank angle measurements. Provision is also made for air and liquid fuel flow measurement. The LPG cylinder is connected to the inlet manifold through a rubber hose provided with a valve. The gas flow rate is measured by INSREF 06 wet type gas flow meter.

The set up enables study of indicated power, brake power, thermal efficiency, fuel consumption, air fuel ratio, heat balance, emission of CO,CO₂,HC,O₂ and Oxides of Nitrogen.

The experiments were carried out in four different phases. These were neat diesel, then by directly admitting LPG in to the intake manifold at the flow rate of 0.3, 0.5 and 0.8 kg/hr along with diesel. The experiments were repeated by providing a venturi attachment of three different diameters (20, 15 and 12mm).

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Figure 1. Modified intake manifold

Table 3. Engine details

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Figure 2. Venturi attachments of three different diameters

The dual fuel engine was started by hand cranking with diesel as fuel and slowly LPG was introduced into the cylinder through the air intake manifold. The LPG flow rate was adjusted to 0.3 kg/hr and the engine speed was maintained at 1500 rpm. At no load and at steady state condition, important observations such as diesel flow rate, air flow rate, exhaust gas temperature and exhaust emissions were recorded. Then the load was increased up to full load for the increment of 25%. Similar procedure was followed for the LPG flow of 0.5 and 0.8 kg/hr.

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Figure 3. Schematic diagram of dual fuel setup

From this analysis, it is observed that diesel consumption is reduced considerably by using the modified venturi configuration. The design of venturi configuration is optimized by comparing all the configurations tested. Carbon monoxide values are reduced considerably using 0.3kg/hr LPG flow with V3 venturi. But in the case of hydro carbon the emission are little high at low loads and is reduced with increase in load. NOx are reasonably reduced in the modified design. V3 design with LPG flow rate of 0.3 Kg/hr was found to be optimum in all conditions.