Tractor Design & Testing 3(2+1)

Lesson 30. Alternative Fuels for IC Engines

1. Biomass based alternate fuels for IC engines:

Considerable research has been conducted and is still being carried in different disciplines of science and engineering to find alternate renewable fuels for Internal Combustion (IC) engines. However, agricultural scientists are more concerned about the biomass-based fuels, as the use of biomass appears to be a viable alternative, especially for the on-farm use of fuels in tractors, combines, and stationary diesel engines. Many biomass-based fuels have been tried in the IC engines, either as a partial or complete substitute to the gasoline/diesel fuel. These include alcohols derived from grains and starchy/sugar crops/plants/trees, biogas produced by the anaerobic fermentation of biomass (dung, refuge etc.), producer gas prepared by the partial burning (i.e. with less quantity of oxygen) of carbon rich biomass in a gasifier, and straight or modified plant oils derived from oil-rich seeds/nuts of farm crops/trees.

Engines or fuel modifications are required to exploit alcohol fuels in compression Ignition (CI) because of poor self-ignition characteristics of these fuels,. The extent and magnitude of modifications was dependent upon the proportion of diesel fuel substitution by alcohol. Thus, it has been found to be an expensive option. Biogas has been used satisfactorily in CI engines. However, in case of CI engines, it cannot replace the diesel completely. Because of its poor self-ignition characteristics, some minimum amount of diesel is required to start the ignition. Its use in IC engines for mobile operations could not be practically exploited due to very high liquefaction pressure of methane, the main combustible constituent of biogas. Like biogas, producer gas has also been used satisfactorily in CI engines. This gaseous fuel too has poor self-ignition characteristics and as such is not regarded as a complete substitute of diesel fuel in CI engines. It also has very high liquefaction pressure.

2. Plant oils as CI engine fuels:

Plant oils, straight or modified, are known to offer several advantages as engine fuel. These include better self-ignition characteristics, compatibility with fuel injection system of the CI engine, high-energy content, safe processing and handling. Above all, production of most plant oils can be realized within a short period after the need is felt. For most of the cultivated oilseed crops, the lead period is only few months. Due to relatively simple and low-cost technology for expelling and filtering, the plant oil can be processed on the farm itself, thus saving the transport cost, time, and energy. Based on simple calculations, researchers have indicated that one hectare of an oil-seed crop can fetch adequate oil to meet the energy needs of an 8 to 10 hectare farm (Bruwer et al., 1980).

In countries with shortage of edible oil, only non-edible oil need to be considered as an alternative fuel. However, during period of petroleum shortage, it might be unavoidable to perform certain urgent farm operations such as irrigation, plant protection, harvesting etc. Under such circumstances, a farmer might be compelled to use even edible oil as an engine fuel to prevent the loss/damage to the crop. As a strategy, the farmers should be encouraged to attain self-sufficiency in their fuel needs by exploiting all alternative energy sources. They must be encouraged to grow the most promising oil seed crops. Even if there is no short fall in the fuel supply from conventional sources, the additional production of oil can be utilized to meet the needs of edible oil in an area.

It has been reported that in diesel engines, crude plant oils can be used as fuel, straight as well as in blend with the diesel (Shyam, 1984). However, during extended operation of the engine, problems of injector coking, dilution of engine oil, deposits in various parts of engine, etc. have been reported. These problems are found to be rather serious in the direct injection CI engines. High viscosity of the plant oils was the major constraint although high acid value and presence of gums/wax etc. also adversely affected the engine performance. Different methods have been tried by different researchers to modify the plant oils. These include cracking of the plant oils, blending of the plant oils with appropriate additives like alcohol, heating of the plant oils before injecting into the combustion chamber, and chemically transforming the plant oils to convert into the esters (i.e. bio-diesels) by alcoholysis.

3. Bio-Diesel as CI engine fuel:

Based on the economic and other considerations, trans-esterification of plant oils by alcoholysis seems to be an effective way to reduce their viscosity. It converts them into methyl esters (known as bio-diesels) thereby making them more appropriate for use in the CI engines. This process also enhances the volatility of the fuel, which aids in better atomization of the fuel. Various problems faced, while using the plant oils straight as fuel, like injector coking, unusual carbon deposition on piston, cylinder head, exhaust manifold, etc. are thus eliminated to large extent by the use of bio-diesels. The glycerol, which is a by-product of this reaction, may be traded in the market.

There are also many environmental benefits of bio-diesel. There is a clear contribution to the reduction of greenhouse gases at least 3.3 kg CO₂-equivalent per kilogram of bio-diesel (Korbitz, 1998); those results have been improved since then by lower inputs in raw material production and by more efficient process technology. It is well established, that there are significant locally impacting emissions e.g. 99% reduction of SOx-emissions, and reductions of 20% for CO, 32% for HC, 50% for soot and 39% for particulate matter. But there is a slight increase of NOx-emission. However, with delayed injection timing a decrease of 23% can be obtained (Korbitz, 1998). Bio-diesel also appears to be an ideal synergistic partner for the oxidation catalytic converter (oxicat). Not surprisingly, as a plant derivative bio-diesel has a very low toxicity as a compound. This is also the reason for its high biodegradability (more than 90% within 3 weeks) and a substantial reduction of toxicity risks to fresh water organisms such as trout, daphnia, watercress and algae, which is of advantage in case of accidental spillage.

4. Preparation of Bio-Diesels:

There are four distinct stages in the preparation of plant oil bio-diesels. These are (a) heating of oil, (b) adding alkaline alcohol to oil, stirring and heating the mixture, (c) settling and separation of glycerol, and (d) washing of bio-diesel with water and removal of water. For preparing bio-diesel at farm scale, any procedure involving long time duration or requiring controlled temperature conditions might be unacceptable to the farmer.

Excessive reduction in the time required for bio-diesel preparation may result in decreased purity of bio-diesel i.e. lesser oil to bio-diesel conversion. It has been suggested that esterification yields be at least 90% if the esters were to be used in CI engines (Hawkins et al., 1982). It was, therefore, clear that for practical purposes, a small variation in oil to bio-diesel conversion would not matter much as long as this conversion is >90%. As such, it would be beneficial to save time and labour through a simplified process at the cost of small reduction in oil to bio-diesel conversion.

Most of the studies relating to plant oil bio-diesels have been confined to preparation of methyl esters either at laboratory scale or at commercial/large scale under controlled conditions. This requires complicated procedure as well as equipment. Considering the limitations and capabilities of a common Indian farmer and the requirements of a well prepared bio-diesel, a simple, inexpensive and less time consuming system has been developed at PAU for farm level bulk production of bio-diesels (Gupta, 1994). It requires not only less time but also less heating and the whole process is completed in about five hours.

In this method, oil is heated to a little below 60°C. On the other hand, alkaline methanol is prepared by dissolving 10 gms of sodium hydroxide pellets in 200 ml of methanol (for every one kg of oil). This alkaline methanol is added to the heated oil and the mixture is stirred manually for 5 minutes. The mixture is then kept for glycerol settlement. After four hours of settlement, glycerol is left at the bottom and bio-diesel is decanted from the top. Then the bio-diesel is washed two or three times with water to remove impurities like sodium etc. It also removes excess methanol. Use of warm water for washing gives better results and reduces the number of washings required. After each washing, water is allowed to settle for about 5 minutes and then it is removed from the bottom. At the end of last washing, water is allowed to settle for 15 minutes and is then removed leaving behind the bio-diesel. The washed bio-diesel is heated so that it becomes almost completely free of moisture.

The whole process for single batch production takes about five hours. If more number of containers are available (for heating oil and mixing it with alkaline methanol) different stages can be carried out simultaneously. Then the average time per batch will be quite less. However, the quantity of bio-diesel prepared in a batch will depend upon the size of the reaction vessel used. The process does not require any specialized equipment. Heating of oil could be done in any metallic container available with the farmer. Dissolving of sodium hydroxide in methanol can be done in a glass or a stainless steel container. The mixture can be stirred with the help of a wooden stick or plastic/stainless steel/glass rod. Glycerol settlement should be carried in an open stainless steel container, particularly at low ambient temperature (below 15 °C), because the glycerol solidifies and does not flow at low temperatures. At higher temperatures, final traces of bio-diesel in the glycerol can be separated using a separating funnel.

In case of farm level bio-diesel production, the mixing and settlement can be carried in one and the same container. But for commercial scale production, separate mixing will require mechanical stirrers and, therefore, separate container would be required for settlement to take care of the problems arising out of solidification of glycerol at low temperatures. Washing of bio-diesels with water and subsequent removal of this water can be carried in plastic bucket, again easily available with every farmer. For removing leftover traces of water in the washed bio-diesel, the separating flask, mentioned above, can be used.

As the esterification process developed at PAU is quite simple, less time consuming and inexpensive, it can be easily used by the farmer for preparing bio-diesels in bulk at a very short notice and without needing much additional equipment. Some precautions are, however, necessary to avoid any problems in bio-diesel preparation. Firstly, the oil should not be heated beyond 60°C otherwise there would be a chance of vaporization of methanol that has low boiling point (about 65° C). Secondly, sodium hydroxide pellets should be kept in airtight bottles. These should be properly mixed preferably in glass containers in the whole quantity of methanol to be used i.e. it should form almost homogeneous mixture, otherwise saponification may take place. Thirdly, the oil should have low acid value (<1.0) otherwise saponification may occur.

5. Characteristics of plant oils and Bio-Diesels:

Bio-diesels of several plant oils have been prepared, characterized, and used in diesel engines and tractors at PAU, Ludhiana. Some of the characterization results are given in appendix A2 and A3.

Kinematic viscosity of bio-diesels was reduced to about one sixth of that of the parent oil. The viscosity of the studied bio-diesels was about 1.28-2.16 times that of diesel. All the bio-diesels had quite high flash point compared to that of diesel, which is a beneficial safety feature. All the bio-diesels had a little lower gross heating value compared that of diesel. Esterification of linseed oil increased the heating value. Density was a little higher in case of bio-diesels than for diesel. Bio-diesels, in general, had lower pour point than of diesel. Distillation temperatures were higher for bio-diesels compared to diesel.

The sulphur-free plant oil is the reason for the very low SOx-emissions of bio-diesel. Generally the cetane number is higher for bio-diesel, resulting in a smoother running of the engine with less noise. Bio-diesel of rapeseed oil is by nature an oxygenated fuel with an oxygen content of about 10%. Oxygen is responsible for bio-diesel’s favorable emission results, but it is also the reason for calorific value a little lower than conventional diesel (Korbitz, 1998).

6. Performance of CI engine fuelled with Bio-Diesels:

Soni and Verma (1993) used blends containing 10-25% HSD in rapeseed oil methyl ester as alternate fuel in 3.67 kW direct injection CI engine and compared their performance with that of HSD. Tests were conducted at fuel injection pressure of 210 Kg/cm² and 240 Kg/cm². All the fuel performed better at fuel injection pressure of 210 Kg/cm². At this pressure, the highest brake thermal efficiency values were recorded for blends containing 10% HSD in RME, 25% HSD in RME, unblended RME and HSD in that order. There was no significant difference in the exhaust gas temperature recorded for various fuels.

Gupta (1994) reported comparable performance of an unmodified, direct injection 3.67 kW diesel engine on bio-diesels of rice bran oil, cotton seed oil, linseed oil and raya oil. The effect of different injection timings and injection pressures on brake thermal efficiency was also studied. It was reported that the fuel injection timings had more pronounced effect on the brake thermal efficiency as compared to fuel injection pressure. Optimum values of fuel injection timing and pressure were found to be 25° BTDC and 25 Mpa respectively for all bio-diesels studied except rice-bran oil bio-diesel and unwashed linseed oil bio-diesel for which the optimum values of fuel injection timings were below the range studied. Higher compression ratio resulted in increased optimum value of fuel injection. Pandey (1996) reported that sunflower bio-diesel gave brake thermal efficiency comparable to diesel.

Gupta (1994) and Pandey (1996) reported that by using bio-diesel as fuel in 5 hp (3.72 kW) direct injection engine, the smoke number increased with load with higher rate of increase at higher loads. However, carbon monoxide (CO) concentration in exhaust gases increased abruptly with increase in load above 50 per cent of rated torque and hydrocarbon (HC) emission increased abruptly with increase in load beyond 75 percent of rated torque.

7. Related research at PAU, Ludhiana:

In an adhoc ICAR project (Verma et al., 1999), investigations on methyl bio-diesels of four plant oils, namely linseed oil, sunflower oil, rice-bran oil and Jatropha curcas oil were carried out. In addition to characterisation studies reported above, studies were also conducted on the following aspects:

 

Method for Estimation of Oil to Bio-diesel Conversion:

A spectrometric method was developed and standardized for estimating quantitatively the oil to bio-diesel conversion during the trans-esterification process used for preparing the bio-diesels. Oil bio-diesel conversion as well as recovery of bio-diesel was observed to be the highest in case of linseed oil (94% and 91% respectively) while the lowest values were obtained for rice-bran oil.

Bio-diesels could not be prepared from raw rice bran oil with very high FFA content. The oil tried had FFA > 25%. From parboiled rice-bran oil (FFA 3%), bio-diesels could be prepared but the percent oil to bio-diesel conversion and bio-diesel recovery were quite low (75% and 50% respectively). But for physically refined rice-bran oil, the values were 85% and 70% respectively. Presence of wax in rice-bran oil as well as its bio-diesels created problems in extreme winter during washing of bio-diesel. Use of warm water helped in effective washing.

Oil yield from Jatropha curcas seeds purchased from Rajasthan was quite low, possibly due to inferior quality of seeds and non-availability of proper extraction facilities for Jatropha curcas seeds in Punjab. However, the bio-diesel quality was quite good. Oil to bio-diesel conversion was 91% and recovery was 78%. The quality of Jatropha curcas oil purchased from Udaipur (Rajasthan) was not up to the mark resulting in only 85% oil to bio-diesel conversion and 70% bio-diesel recovery.

Storage Studies:

The FFA contents of the bio-diesels after a period of 1 to 1.5 years storage increased marginally. However, the oils had higher increase in FFA content as compared to their respective bio-diesels. During storage, the viscosity of the bio-diesels increased by 30-60% over a period of seven months. The highest increase was observed in case of linseed and sunflower oil bio-diesels. The effect of storage time on weight gain of oils and their bio-diesels was also studied. Samples were kept in half and fully filled containers. In case of linseed oil, sunflower oil and sunflower oil bio-diesel, the weight gain was more in half-filled containers. But in linseed oil bio-diesel, weight gain was similar in both types of containers. There was no weight increase in case of Jatropha curcas oil and its bio-diesel. However, in case of rice-bran oil and its bio-diesel, the weight gain was more in case of fully filled container. Except linseed oil bio-diesel, the weight gain was partly due to absorption of moisture and partly due to reaction with air. But in case of linseed oil, it was mainly due to reaction with air. In case of linseed oil, linoxygen deposited on the rim of half filled container even after 15 days of storage.

Effect of bio-diesels on Engine Components

During the studies on the effect of plant oil bio-diesels on engine components, it was found that no significant change was observed in the dimensions of metallic components, namely copper washer, aluminium washer and piston rings. However, the colour of copper washer dipped in linseed oil bio-diesel and sunflower oil bio-diesel changed. The peach colour of precipitate layer was found deposited on copper washer in case of sunflower oil bio-diesel. Among non-metallic engine components, the maximum changes were observed in case of rubber washer followed by insignificant changes in case of plastic pipe. However, no change was observed in case of gaskets.

Use of Plant Oil Bio-diesels in Stationary as well as Tractor Engines

Fuel consumption tests were carried out in accordance with modified BIS engine Test Codes. The tests included the fuel consumption test, performance test and endurance test at the prescribed loads. The tests indicated that among the four bio-diesels, the specific fuel consumption was the lowest in case of linseed oil bio-diesel followed by sunflower oil bio-diesel. Specific fuel consumption was comparatively higher in case of rice-bran oil bio-diesel and Jatropha curcas oil bio-diesel. That is why brake thermal efficiency was the highest in case of linseed Oil bio-diesel followed by sunflower oil bio-diesel.

Long duration .testing of diesel engines using the methyl bio-diesels was also performed. Engines were run for endurance tests on bio-diesels of linseed oil, rice-bran oil and sunflower oil for 282, 243 and 272 hours respectively. Due to late availability of Jatropha curcas oil, endurance tests on its bio-diesel could not be taken-up. Standard measurements of engine components before and after the endurance tests with each bio-diesel indicated that there was no significant wear in various components.

Exhaust emissions namely CO, NOx and total combustibles were also monitored. NOx emissions were a little higher in case of bio-diesels compared to diesel particularly in sunflower oil bio-diesel. On the other hand, CO emissions were significantly lower in case of all the four bio-diesels compared to diesel. Combustible emissions were also lower in case of the bio-diesels as compared to diesel.

An Eicher tractor of 35 hp (26.1 kW) was run on rice-bran oil bio-diesel for seedbed preparation with a field cultivator and disc harrow. No visual smoke in the exhaust was observed with the use of the bio-diesels. NOx emissions were marginally lower with the bio-diesel than diesel.

On the whole, it was observed that the performance of the existing unmodified diesel engine run on all the four plant oil bio-diesels was quite promising. In fact better thermal efficiency was achieved and the bio-diesels were more environment-friendly as compared to diesel in terms of exhaust emissions.