The effects of LPG use over the performance of a Diesel engine

This study is based on the effects of using an auxiliary fuel such as gas on the performance of diesel engines. More attempts to use gas as a secondary fuel in a diesel engine were made over the years, resulting in significant increases of the engine performance. Other effects of dual fuel use, are lower fuel consumption, and an improved combustion, that has a direct effect on pollutant emission. For this investigation a 4-cylinder diesel engine with mechanical injection, equipped with a speed regulator disable system. An auxiliary fuel (gas) is introduced through the intake manifold of the engine using a volume base measurement system. According to the literature, by increasing the main injection advance, we can see further improvement in the engine performance, this being a result of better timing of the burn process. The regular burn process uses only a part of the oxygen in the cylinder, by introducing the gas in the intake, it uses the rest of the oxygen in the cylinder in the burn process. This in theory will improve the efficiency of the engine and thus reduces the emissions.


Introduction
The use of gas as an auxiliary fuel for diesel internal combustion engines has become an increasingly widespread topic nowadays, from the point of view of environmental protection.There are various methods to introduce the gas as a dual fuel into the engine.Many measurements were made using the gas injection on the intake manifold and they are described in the literature.In order to make an installation as simple as possible from a constructive point of view, we opted for the gas introduction on the intake, without using injectors.This solution offers a lower precision in adjusting a gas dose, but sufficient to obtain some relevant experimental results.
Diesel engines, often known for their reliability and the high powers they can develop, are also some of the most polluting sources for the environment due to the amount of polluting emissions from the composition of the exhaust gas.Specialized literature tells us that by introducing a quantity of gas into the cylinder of diesel engines, a proportional quantity of used fuel should be replaced [1].This phenomen must be studied in detail, because we also know from specialized literature that gas has a delay in ignition, so an analysis of this phenomenon could show us if we obtain a certain efficiency by using gas as a dual fuel in a diesel engine in what concerns power, fuel consumption or reducing polluting emissions.It is also considered from the literature that the use of gas for diesel engines would have a beneficial effect in changing polluting emissions by reducing the amounts of CO2 that would be considered the main factor of global warming, but also an increase in the level of hydrocarbons [2].
Diesel engines are much more efficient than spark ignition engines, but a major problem with them is related to high CO2 emissions.These emissions can be reduced by using a fuel with a low carbon content such as NG, LPG or biogas.For compression ignition engines, the main fuel cannot be completely replaced, which means that there is a minimum dose of diesel fuel required for its good operation [3].According to the measurements performed on our test bench, following the analysis performed on the measured data, is observed an increase in amount of HC and a decrease in the amount of CO2 and NOX which is inversely proportional to the amount of gas introduced in the combustion process.
Research studies [4] and [5] suggest that by increasing the dose of gas introduced into the intake manifold, the dose of air is reduced.This has an influence on the HC pollutant emissions from the exhaust gas which decreases in direct proportion with Lambda value.
According to the literature, by introducing the gas on the intake manifold, it replaces part of the air which entering in combustion chamber.Inside the cylinder, the gas is mixed with the diesel fuel, resulting the pre-mix phase, which leads to an ignition of the fuel before the top dead center, implicitly to an increase in engine power [6].Following the measurements on the test bench, we observe that by keeping the same engine speed and changing the dose of gas introduced into the combustion chamber, the specific consumption of diesel fuel decreases and result a significant increase in engine power.Into the references [7] and [8] are presented the effects of dual fuel supply on the duration of the internal combustion process.With increase in dose of gas introduced into the engine, ignition delay also increases, resulting an almost complete combustion of the fuel mixture

Experimental test bench
The experimental test bench is composed of a D110 diesel engine connected to a dynamometer which measures the force and torque to the engine crankshaf.The D110 engine is a compression ignition engine, equipped with direct injection, which develops a maximum power of 48 kW (65 Cp) and a maximum torque of 289.5 Nm at 1800 rpm.This experimental test bench is equipped with Arduinocontrolled electronic equipment for measuring engine parameters and a dual fuel supply system on the intake manifold.
In the following figure are represented the component elements of the dual fuel supply installation.From the fuel tank (1) the LPG or bio-gas is released by a pressure regulator (2), then it is transported through gas pipes in a gas meter (3) from which through a uni-directional valve (4) it is introduced in the intake manifold (5).In the intake manifold is made the mixture between LPG or biogas and air, which is then introduced into the engine (6).The hydraulic dynamometer is a brake that takes the mechanical energy produced during engine operation.During operation, quantities of water are introduced inside the hydraulic dynamometer depending on the required load.Transformation of mechanical energy into heat takes place at the friction contact between the water and the brake rotor.The principle of force measurement by hydraulic dynamometer is based on the measurement of the opposition force to the rotation of the dynamometer housing.This being done on the test bench by using a scale built with load cells connected to a HX711 conversion module.The measured weight is processed by an Arduino Nano microcontroller.
The electronic equipment used to measure the motor parameters is Arduino based.The monitored parameters are as follow: engine speed, coolant temperatures, intake and exhaust manifold temperatures, the load measured by the hydraulic dynamometer, but also fuel consumption reported per hour of operation.All these functional parameters are transferred to an interface made in LabView.Using calculation formulas we obtain data such as torque, power, and specific consumption of liquid fuel.

Measurement procedure
To perform the measurements, we opted for the adjustment of a constant engine speed regime, of approximately 1402 rpm, with a deviation of 1.03%.Also, to keep the same constant engine speed, we used a locking device for the injection pump regulator.The operating principle is based on locking the injection pump rack to obtain a constant engine speed and a variable load.The fuel used for dual fueling is LPG and consists of a mixture of mostly propane and butane.In this sense, according to the literature, gas mixtures with propane content give the best contribution to the performance of dual fueled diesel engines.[9] During the measurements, 3 different operating cases were studied.In first case we analyzed the operation of the engine only with diesel fuel, and in the following two cases we studied the operation of the engine by reducing the dose of diesel fuel and increasing the dose of gas, but also keeping the dose of diesel at the value of approximately 1.05 kg/h with a deviation of 8.4% and the increase in the gas dose.
The pollutant emissions measurements were made with Capelec CAP 3201.For a better understanding of the values on the following graphs, it is necessary to know the emission values for diesel operation only.On the side of polluting emissions, after the tests, we can see an improvement due to the use of gas as an auxiliary fuel.The gas introduced into the intake manifold replaces part of the air but also of the diesel fuel introduced into the cylinder during compression.This fact has an observable contribution, modifying the combustion process.Following the analysis of the measured results, we observed a significant decrease in NOX and CO2 from the exhaust gas, with the increase the dose of gas.In addition to the positive values obtained on the emissions side, we also observe an increase in HC which is not beneficial.Although it is assumed that the gas mixed with diesel should provide a better combustion, the increase in HC emissions, are the result of the incomplete combustion of the gas [10].Decreasing the diesel dose requires an increase of the gas dose in order to keep the power and the speed constant.On the engine efficiency side, no significant improvement can be observed.The engine efficiency is around 26% during operation with only diesel, and by adding gas it has a deviation of 10% from its initial value.The addition of gas does not have a positive contribution to the engine's performance, because with large doses of gas introduced, a slight decrease of the engine efficiency is observed due to irregular operation.
Regarding to the price of the fuel required for the operation of the engine under the conditions listed above, the mixture of gas and diesel brings a significant benefit because with small doses of gas introduced, a significant amount of diesel is substituted, and the total price of the two fuels is lower than the one required to operate only with diesel.

Conclusion
We consider that the use of gas as a dual fuel for diesel engines has a quite noticeable impact in the case of polluting emissions from the exhaust gas.The decrease in emissions specific to diesel engines can be seen through the decrease in NOX, but on the other hand, an increase in HC due to unburned gas is also obtained.Also, the reduction of CO2 is important because it is considered the main factor of global warming.
On the other hand, we do not see an improvement in terms of engine efficiency.At low rates of diesel substitution, the engine performance remains almost identical to the case of operating only with diesel, but when substituting a larger amount of liquid fuel, a slight decrease in efficiency is observed.For this reason, we believe that there should be a minimum dose of diesel for perfect operation in order to correlate engine performance with the improvement of polluting emissions.
The analysis that was made on the financial side shows us that it is profitable to use gas as a dual fuel for diesel engines, but the calculations were made only on the basis of current LPG prices.If another gas with the same thermal properties were used, such as biogas, which has a lower price, it would result in a significant price decrease compared to the price of diesel.

Figure 2 .
Figure 2. General overview of the test bench and the monitored parameters.

Figure 3 .
Figure 3. Variation of NOx and HC as a function of Gas consumption.

Figure 4 . 5 Figure 5 .
Figure 4. Variation of NOx and HC in function of gas consumption when we keep a constant Diesel dose and increase gas dose.

Figure 6 .
Figure 6.Variation of CO2 and CO in function of Gas consumption when we keep a constant Diesel dose and increase Gas dose.

6 Figure 7 .
Figure 7. Differences between Diesel and LPG+Diesel price for 1h engine operation.

Table 1 .
Emission Values for Diesel Operation Only.