How can one justify higher brake thermal efficiency of diesel engine fuelled with biodiesel having lower calorific value and higher cetane number compared to diesel fuel?
In general, many articles in the literature report lower BTE and torque with BD when compared to neat diesel. However, even I could find few articles that report higher BTE with BD.
In my knowledge, the first parameter one should look into is the product of fuel flow rate and calorific value, to relate the heat input to the BTE.
Higher the value of this product is, lower would be the BTE, provided the torque produced in the constant speed engine under test is same with both the fuels, which is not the practical case.
Irrespective of its poor energy content, BD can at times yield SLIGHTLY higher torque. This can be due to its oxygen bound nature which can improve its combustion, thus yielding slightly higher torque and BTE.
BTE being an indicative of percent of heat input that gets converted into useful work, its trend can also be related to the percent of remaining heat input (radiation, exhaust heat, cooling water heat) of the two fuels.
I hope these inputs would help you in analyzing your results.
Regards
Kiran
@ Master Kiran Raj Bukkarapu and Prof. Yılmaz ,
Thank you for your wonderful contributions.
The calorific value of a fuel definitely have impacts on its performance as it pertains to the engine BTE, however, other properties of the fuel also does shows to impact the BTE ( check " compression ignition engine performance as a function of its fuel properties "), also the engine combustion chamber geometry could favour better in-cylinder charge motion with the use of BD which will result in better combustion and increased engine BTE (check - "impact of conical piston crown equipped compression ignition engine on performance and experimental investigation of the performance and emission characteristics of a ci engine equipped with a modified truncated cone piston crown operated on diesel and shea butter biodiesel).
Other literature on this can be sought.
Several ways to improve the B.T.E. of a diesel engine:
If the effect of load on brake thermal efficiency is analyzed, it can be observed that the B.T.E. goes on increasing with the increase in load up to a great extent.
For a bio-diesel as a fuel, the calorific value is less. So after complete combustion bio-diesel will produce a low torque or in turn will produce less mechanical power.
But still a bio-diesel fuel will produce slightly higher torque & B.T.E. can be upto 32% approx (experimental) depending upon the Cetane no. of the fuel used.
The following are few explanations:
Dear Olusegun D. Samuel,
Biodiesel from vegetable oil sources have been recorded as having a cetane numberrange of 46 to 52, and animal-fat based biodiesels cetane numbers range from 56 to 60. Dimethyl ether is a potential diesel fuel as it has a high cetane rating (55-60) and can be produced as a biofuel. Cetane number is an important parameter in evaluating the quality of biodiesel fuel.
Biodiesel is an alternative fuel that is cleaner than petrodiesel. Biodiesel can be used directly as fuel for a diesel engine without having to modify the engine system. It has the major advantages of having high biodegradability, excellent lubricity and no sulfur content. It is safe and efficient alternative fuel and has a low impact on the environment. The calorific value of biodiesel is about 37.27 MJ/kg. This is 9% lower than regular Number 2 petrodiesel. Variations in biodiesel energy density are more dependent on the feedstock used than the production process.
The biodiesel increased volumetric fuel consumption due to its chemically bound oxygen content. In contrast the petroleum derived fuels showed about the same consumption results. Lower percent of blends (B5, B20) give a good improvement in the engine power.
Emission
Furthermore they were found to improve exhaust emissions. It can be used safely in the diesel engine, at least in small blending ratios with normal diesel fuel. The fuel consumption increases as the biodiesel content in the fuel rises due to its lower heating power. It should be noted that the biodiesel maintains approximately the same engine efficiency at that obtained with diesel fuel. Increasing the biodiesel content reduces the particulates in the engine exhaust prior to the after treatment. The engine after-treatment reduces particulate emissions drastically, hiding the potential benefits of biodiesel.
Regarding NOx emissions the results obtained in this study1 show that the higher the biodiesel content, the higher the NOx emissions. It should be underlined that the effect of the fuel composition is less important than the effect of the EGR reduction due to the lower heating power of biodiesel. The increase in engine speed caused an increase in fuel consumption rate, brake thermal efficiency, equivalence ratio, and exhaust gas temperature, while at the same time decreasing the emission indices of CO2, CO and the NOx for the fuels.
Hope this is helpful.
Ashish
Dear Dr. Olusegun D.Samuel,
The result you have mentioned is quite interesting and in my opinion it might be theoretically possible, however, in real time the chances of getting higher BTE is very less.
In case of your result, the biodiesel has reduced calorific value, but in view of its high cetane number, my assumption is that the calorific value will also be slightly higher but less than diesel (because only saturated Fatty acid esters will give high CN). Assuming this, your biodiesel will have high CN, slightly reduced CV than diesel , hence, high fuel bound oxygen content and also viscosity. Here, high CN means reduced ignition delay, so duration of combustion will be slightly longer.
Answering your question, shortly after injection, your biodiesel will start igniting due to reduced ignition delay, which progress into combustion. Due to high viscosity and less volatility, combustion becomes slow, where the fuel bound oxygen is consumed and undergoes complete combustion. Because of this, maximum amount of energy will be liberated thereby deriving maximum work output. Thus, high BTE.
PS: Even traces of alcohol in biodiesel tends to produce high cetane number but still CV will remain less.This might also tend to produce increased BTE.
Hope this might answer your question Sir. All the very best and good luck Sir
All the very best and Good luck Sir
Regards,
Goku
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