ENERGY EFFICIENCY VIA ENGINE IMPROVEMENTS: A REVIEW OF DUAL FUEL ENGINE DEVELOPMENT E. Anye1 1
Department of Naval Architecture and Marine Engineering, University of Strathclyde, 100 Montrose Street, Glasgow G4 0LZ, UK,
[email protected]
ABSTRACT The continues growth in world population along with improvements in standards of living and in life expectancy are among factors contributing to a faster rate of consumption of available energy resources (especially fossil fuels). In the last couple of years attention has been placed on reducing emissions from engines. This is not surprising considering the spate of public attention about the negative health impacts of atmospheric pollution and the enforcement of stricter requirements to limit emissions from engines. Consequently, engine developers, end users, scientists, and policy makers are revisiting the idea of dual fuel engines. This paper attempts an exploratory discussion of the development that dual fuel engines have undergone over the years. The review notes that the potential identified in natural gas as an alternative fuel is very promising and further technological advances will only go a long way to make dual fuel engines more energy efficient, cost effective and reliable. This will be achieved while simultaneously reducing the emissions of nitric oxides and carbon dioxide from dual fuel engines, thus contributing towards meeting the global emission reduction targets. Keywords:
Dual
fuel
engine,
energy
efficiency,
diesel,
alternative
fuels
1. INTRODUCTION About a century ago, the concept of dual fuel operation for the internal combustion engine was being discussed but faded away because of the unrivaled thermal efficiency of the diesel-only engine. Nevertheless, this idea has now resurfaced drawing greater focus and attention from academia, industry and policy makers. These interests have developed especially because there have been encouraging results following breakthrough research on the use of Natural Gas as an internal combustion engine fuel (Mbarawa and Milton, 2005). Global research on engine performance and alternative fuels has been driven by stricter emission regulations in many countries, but market dynamics such as oil prices and the availability of cleaner fuels (especially Natural Gas) have also been contributing factors. Replacing some of the diesel with natural gas while retaining efficiency identical to that of a diesel-only engine currently appears to be the ideal option (Sahoo et al., 2009; Carlucci et al., 2008; Christen and Brand, 2013; Karim, 2003). It is therefore essential to examine relevant terminology so as to properly contextualize the discussion that will follow. ‘Dual-fuel Engine’ is the focus of this discussion. It refers to a compression ignition (CI) engine that burns simultaneously two entirely different fuels in varying proportions. One of the fuels is usually a gaseous fuel while the second is a liquid fuel. The gaseous fuel plays the role of providing a large amount of the energy that is released through combustion. The liquid fuel on its part is deployed basically to provide the energy that is needed for ignition. It also supplies the remaining fraction of energy released by the CI engine (Karim, 2015). With this in mind, alongside the environmental-friendly nature and increasing availability of natural gas, it becomes the gaseous fuel of interest to this discussion while the liquid fuel is diesel. Given the increasing popularity of dual-fuel engine research, development and application, several other terminologies have appeared in the published literature. A few of them are discussed below to further permit the reader completely understand the scope of the subject discussed in this paper. ‘Bi-fuel engine’ is more suited in the description of engines that employ two different fuels alternately and yet rely on an external source of energy for the provision of ignition (e.g. electric spark produced in engines that utilise spark plugs). ‘Gas-fuelled engine’ according to Karim (2015), is employed to refer to dual-fuel engines in which there is a direct injection of the gaseous fuel into the cylinder. The timing of this direct injection could be during early stages of compression or after the injection of liquid fuel and getting towards the end of the compression stage.
‘Multi-fuel engines’ refers to diesel engines of the compression ignition type utilising a liquid fuel. The term, when employed, suggests that such an engine has been accurately adapted to be able to effectively operate on other different liquid fuels including those that are perhaps not of the traditional diesel type. Therefore, of the four terminologies described above, dual-fuel engine is the focus in discussing energy efficiency via engine developments. The concept of dual fuel engines is not new in 2015. There is sufficient evidence that people began thinking about the possibility of engines operating on dual fuel as far back as almost a century (Boyer, 1949). It is however documented that several of these early attempts did not turn out to be successful. These unsuccessful early attempts have been attributed to the difficulties of running such engines at light load as well as very close to full load (Karim, 1983). In very recent years though, scientists as well as engine manufacturers are still striving to solve this dilemma that early attempts faced. Their efforts have been aided tremendously by advancements in technology and extensive knowledge built over the years. A lot of literature pertaining to this issue agrees on the fact that, the solution to the afore-mentioned problem rest in making dual fuel engines quite flexible, i.e. one that can revert to diesel-only mode (operation) during light load conditions. Despite the setbacks faced by early attempts, these did little to deter scientists as well as engine manufacturers from further exploring the dual fuel concept. The concerns associated with the fluctuating nature of fuel costs also contributed their part in furthering development of dual engines. Thus to combat rising fuel prices, people have continued to seek for solutions in alternative fuel sources. Just after the second, world war, it is believed that the interest in dual fuel engine utilization fluctuated based on the changing nature of prices of fuel as well as the nature of competition from alternative fuel sources. Thus with the documented increasing nature of fuels over the years (prior to 2015), the interest in dual fuel engines is reported to have increased significantly despite relatively insufficient mandatory research being done (Karim, 1983). Currently, there is tremendous pressure on scientist, manufacturers and end users of engines to reduce exhaust emissions. This pressure stems from the stringent legislation being put in place by several authorities, compelling all stake holders to reduce emissions. These legislations are themselves driven by the adverse effects that exhaust emissions have had on the world (Global warming). As a way of meeting the target reduction in emissions nowadays, many industries (marine, automobile, trucks, railway, power generation etc.) have re-visited the application of dual fuel engines. Consequently there are several companies and organizations that convert diesel-only engines to dual fuel engines. This is predominant because the diesel-only engine still has a relatively high efficiency, as well its structure can easily allow for the conversion to dual fuel, with minimal alterations involved to the original dieselonly engine. Across a multitude of applications, several fuel types have been used alongside diesel. Some of these include; Methane and Natural Gas (either compressed or liquefied), Biogases and Hydrogen. With this in mind, it is imperative to emphasize that most dual fuel engines have emanated from modifications to existing diesel engines (UNECE, 2012).
Figure 1: Overview of dual fuel engine working principle (Cummins, 2014) As shown in Figure 1 above, by modifying (adding dual-fuel specific hardware to) the traditional diesel engine, the dual fuel engine is achieved. This engine can operate in dual fuel mode as well as in diesel-only mode. However, it cannot operate in gas-only mode. In the dual fuel mode, there is the introduction of Natural gas in to the intake system. It is agreed that this Natural Gas has methane as its highest (leading) constituent. The chemical properties of methane suggest that it is superb when it comes to resisting Knock in engine. This therefore makes it an exceptional and unique fuel for engine applications with high compression ratio. Since
Natural Gas has been introduced into the system, the air/gas mixture is then drawn into the cylinder via the intake valve. Nonetheless, this mixture would not auto ignite on its own. There must therefore be a deliberate source of ignition. To this effect, when the piston is approaching the end of the compression stroke, a small amount of diesel is injected into the cylinder. This diesel fuel ignites and its combustion causes burning of the gas/air mixture. From the explanation in the previous paragraph, it is appropriate to state that, dual fuel engines borrow and combine the combustion concepts of both the SI and CI i.e. the air/gas mixture is carburetted first and then it is compressed like in a conventional diesel-only engine. Comparatively, the dual fuel engine is advantageous in the sense that, it employs the difference of flammability of two fuels. Furthermore, in most cases, the engine can be made to switch from dual fuel mode to diesel-only mode especially when there is lack/shortage of gaseous fuel. It is this particular advantage that has been successfully utilized to combat the dilemma of operating the dual fuel engine at light load.
2. DUAL FUEL ENGINE CHARACTERISTICS It is well documented that for burning (combustion) to occur in a conventional dual fuel engine, there is going to be some amount of liquid fuel pilot being injected into some mixture (pre-mixed) air and gas (Richards, 1999). With the above in mind, as well as with the fact that the dual fuel engine basically emanates from the diesel-only engine, it is prudent to have a look into what the combustion process of both these engine types involve. The slight difference between their combustion processes is that; in the case of the dual fuel engine, the process takes place in five stages. Whereas for the diesel-only engine, the process takes place in four stages (Sahoo et al., 2009).
Figure 2: Stages of combustion process in dieselonly engine (Nwafor, 2003)
Figure 3: Pressure versus crank angle analysis of dual-fuel pilot injection (Nwafor, 2003)
Figure 2 graphically represents the changes in pressure with respect to the Top Dead Centre crank angle for a diesel-only combustion process. The vertical axis measures pressure in bar while the horizontal axis tracks the crank angle in degrees. The negative crank angles are used to depict angles of the crank prior to the piston reaching Top Dead Centre. It is evident from the above graph that for a diesel-only engine, four stages are involved in its combustion process. These stages are; A – B (Period of delay in ignition); B – C (Premixed combustion); C – D (Controlled combustion); D – E (Late combustion). For the dual fuel operation, there are 5 stages involved in the combustion process. The five stages revealed in the graph below are: A – B (Ignition delay of pilot liquid fuel); B – C (Premixed combustion of pilot liquid fuel); C – D (Ignition delay of primary fuel); D – E (Rapid combustion of primary fuel); E – F (Diffusion combustion). Figure 3 above graphically represents the relationship between pressure and crank angle for the combustion process in a dual fuel engine. Comparing and contrasting the combustion process of both the diesel-only and dual fuel engine operations, will further enhance the argument being made earlier in this work, that the dual fuel engine takes its inspiration from the diesel-only engine. Much more than just the fact that there are five stages involved in the dual fuel operation, some other major differences can be spotted (even by visual inspection of the graphs in Figures 2 and 3 above). Firstly, Figure 3 reveals that there is a much longer period (when compared to that for diesel-only operation) for which the injected pilot fuel delays before igniting. Several reasons could be advanced for this. One of such reasons would be the fact that, in a dual fuel engine, there is the addition of natural gas to air. The implication of this natural gas substitution for air is a consequent reduction in the concentration of oxygen. This reduced
concentration of oxygen then accounts for the increased ignition delay period (Sahoo et al., 2009). A second difference between the combustion processes is that there is relatively low pressure rise in the dual fuel operation when compared to that of the diesel-only operation. This low pressure rise could be attributed to only a small quantity of pilot fuel being ignited. Thirdly, owing to the auto-ignition temperature of the dual fuel (gas/air mixture), there is a longer ignition delay. This results in a time lag emanating between the development of the first and second pressure rise. Though there is an ignition delay, it is short in comparison with the opening delay period by virtue of the pilot fuel being injected. 2.1 FUEL INJECTION AND IGNITION This is a very vital characteristic not just for diesel-only engines but also for dual fuel engines. Fuel is usually injected into the combustion chamber prior to the piston approaching top-dead-centre. This happens at very high pressures. Presently the consensus is that the dual fuel engine still uses the normal diesel-only fuel injection system (Sahoo et al., 2009; Richards, 1999). Despite this existing consensus, it is believed that there is an avenue for future research to focus on optimising the injection characteristics of dual fuel engines so as to enhance its energy efficiency. Although the dual fuel engine relies on the traditional diesel-only fuel injection system to provide a pilot amount of liquid diesel fuel, the dual fuel engine induces a compressed premixed (gas/air) fuel into the combustion chamber. This compressed mixture is subsequently made to ignite due to the energy that emanates from the burning (combustion) of the diesel spray produced from the conventional dieselonly injection system. This diesel spray is also referred to as the pilot liquid fuel. Published literature reveals that there are some discrepancies in the amount of pilot fuel required to ignite the compressed mixture of gas and air. Sahoo et al. (2009) document that between 10 to 20% pilot fuel is required whereas Richards (1999) argues that between 5 to 8% is required. Another school of thought reveals that, a minimum liquid fuel quantity of 5 to 10 % of the maximum full load fuel is essential in order to achieve good spray penetration and atomization. The same school of thought goes on to emphasize that for a dual fuel engine that does not need to revert to a 100% diesel-only operation, the minimum pilot fuel quantity can be reduced to less than 1% while employing a much smaller fuel-injection system (Weaver and Turner, 1994). From exploring the research carried out over the years, there is significant evidence to argue that, there is no specific amount that can be expected to satisfy the purpose of all the existing engines in the world. Thus each engine would have a quantity range that best suits its operations. The required amount of pilot fuel would differ for engines for reasons such as design parameters as well as the point of operation of the engines. 2.2 EFFICIENCY, COMPRESSION RATIO AND KNOCK When it concerns internal combustion engines, the efficiency (theoretical) is said to be a function of the engine’s compression ratio. There is a direct proportional relationship between the compression ratio and the theoretical efficiency. Therefore, this implies that the higher the compression ratio, the higher the theoretical efficiency of the engine (Weaver and Turner, 1994). After establishing the theoretical efficiency possible for an internal combustion engine, it is necessary to suggest that another parameter (knock) affects the compression ratio of an engine. In other words, knock would also affect the efficiency of an engine through the intermediary of compression ratio. In this relation (between knock and compression ratio), it is understood that, the tendency of knock occurrence increases as compression ratio increases. Several fuels rely on their octane number as evident of their ability to resist auto-ignition. This (octane number) would also depict the ability of a fuel to resist knock. It is prudent to state (after the above description) that the fuel-efficiency of an engine would depend on the maximum compression ratio that can be utilized; this in turn depends on the knock resistance of the fuel. Given that Natural gas has a high research octane number (120 to 130), it means it is much more resistant to knock – thus making it a very suitable fuel for dual fuel engines (Weaver and Turner, 1994). Owing to extensive research, dual fuel diesel engines are reported to resist knock much more than spark ignition engines because, although they employ mixtures of gas and air, the fuel-air ratios are usually quite lean. It is still worth emphasizing, that despite this relative advantage that the dual fuel engine has over the spark ignition engine, there still remains a threat to the power output of the dual engine. This threat is imposed by the fact the knock still presents a problem to the dual fuel engine. Having enumerated all the above, there is substantial evidence that dual fuel engines produce power economically and at high thermal efficiencies that can sometimes exceed those of the modern diesel engines, while offering much lower exhaust emissions (Karim, 2015).
2.3 AIR - FUEL RATIO AND POWER OUTPUT These characteristics of dual fuel engines are worth discussing because they are of utmost importance to the success (or otherwise) of the dual fuel engine. Generally, the air-fuel ratio of a dual fuel engine impacts seriously on the power output of that engine as well as its efficiency and emissions of environmental pollutants. Given the impact air-fuel ratio can have on the above mentioned, it is interesting to note how power output in a dual fuel engine is being controlled. Unlike most engines, the dual fuel engine (diesel engine as well) is not equipped with throttles. Therefore, controlling the power output in such an engine requires a different strategy. In a dual fuel engine, the strategy employed is to vary the concentration of natural gas that is being admitted into the cylinder while maintaining the quantity of pilot diesel required. When air-fuel ratio is said to be high, this implies that the fuel concentration is low and conversely when air-fuel ratio is low, the implication is that fuel concentration is high. In the latter case, it is reported that the lowest air-fuel ratio occurs at maximum engine torque. This is however restricted by the engine’s knock limits (Weaver and Turner, 1994). The problem associated with controlling power output in the above described manner is that, as there is a reduction in engine load, this causes the ratio of air to fuel to become leaner until it eventually cannot burn efficiently (Karim and Jones, 1992). The problem discussed in this paragraph has been studied and researched in detail (Karim, 1991). Some exploration of the research reveals that there is an increase of the quantum of unburned fuel in the exhaust emissions as the air-fuel ratio approaches a critical limit (Karim and Jones, 1992).
3. DUAL FUEL ENGINE PERFORMANCE AND EMISSIONS The gas/fuel ratio of the dual fuel engine is very vital when it comes to examining the dual fuel’s engine performance and emission characteristics. It is however quite essential to also establish the fact that, the gas-tofuel ratio is not the only factor that is used in depicting the performance and emissions of dual fuel engines. Some other factors (engine design and operation parameters) include: pilot fuel injection timing, the conditions surrounding the intake manifold, the type of gaseous fuel utilized, load, the speed and compression ratio. Using several different types of test engines which all employ a wide array of various gaseous primary fuels and pilot fuel quantity, many studies have been done by a plethora of researchers. The studies referred to in this instance, are those focused on examining the effects of the already listed parameters on performance, emissions and combustion characteristics of dual fuel engines. These are summarized in Table 1. 3.1 EFFECT OF PILOT FUEL MASS/QUANTITY ON PERFORMANCE AND EMISSIONS Given the focus of this paper, it is quite necessary to explore the impact that the mass of pilot fuel injected has on the performance and emission characteristics of the dual fuel engine. It is argued that, this factor is one of the most important factors (Sahoo et al., 2009). This argument supports particularly the extensive work done pertaining to the operation of the dual fuel engine at light load (Karim, 1983). Despite the agreement of the importance of the pilot mass, there is evidence to suggest that in the past, several diesel engines have witnessed poor combustion and atomization within their injection systems. This is reported to be caused by the quantity of pilot fuel injected per cycle being well under 5 – 10 % of the optimum design level. Previous research (Badr et al., 1999) reveals that the quantum of pilot fuel would no longer affect carbon monoxide emissions as well as the unburned methane gas when the engine operates way beyond a certain limiting equivalence ratio (see figures below)
Figure 4: Total equivalence ratio versus methane concentration and carbon monoxide concentration for different pilot fuel quantities at 1000rpm (Badr et al., 1999)
The Figure above consists of two comparisons. The first one (a) analyses the relationship between the total equivalence ratio and the concentration of Methane. It is evident from the curves that, as the total equivalence ratio increases, there is a corresponding rise in Methane Concentration. However, for each of the different pilot quantities used in the study, there is a certain equivalence ratio, beyond which the Methane concentration begins to drop. The same can be said for the second graph (Figure b). The implication of both these graphs could point to the fact that there is a certain equivalence ratio limit for excellent propagation of flame from the pilot ignition centres. As well as investigating the variations of both CH 4 and CO concentrations in exhaust gas with total equivalence ratio for different pilot fuel amounts, the variation of flame spread limits (FSL) with changes in the quantity of pilot fuel were also investigated (Badr et al., 1999).
Figure 5: Experimentally established flame spread limits (FSL) versus pilot quantity for methane operation (Badr et al., 1999) Figure 5 above provides insight into how the FSL varies with pilot quantity. This was generated using Methane at a compression ratio of 14.2:1 and 1000 rpm. It is prudent to suggest that the study carried out, established the flame spread limits by way of experiments. Having observed from the previous two Figures (6a and 6b) that the variations portrayed are indicative of the optimum equivalence ratio limit for flame propagation, this Figure (7) could not agree better. The graph puts forward the fact that, the FSL is lowered owing to an increase in pilot quantity. There are several suggestions advanced to explain this behaviour, some of which include: “greater energy release on ignition, correspondingly improved pilot fuel injection characteristics, larger pilot-mixture envelope size, larger ignition centres, higher rate of heat transfer to the unburned gaseous mixture and increased contribution of hot residual gases” (Sahoo et al., 2009).
Table 1: Some test engines and their respective fuel types used previously by researchers, adapted from (Sahoo et al., 2009). RESEARCHER(S)
TEST ENGINE USED
PILOT PRIMARY FUEL FUEL
Abd, Soliman, Badr, Abd (2000,2002)
Single cylinder, 4-Stroke, water cooled engine (Ricardo E6)
Diesel
Methane, Propane
Badr, Karim, Liu (1999)
Two single cylinder, 4-Stroke, Water cooled, DI, normally aspirated laboratory dual fuel engines
Diesel
Methane
Bari (1996)
Two cylinder, 4-stroke cycle diesel engine (16.8 KW at 1500 rpm, Model-2105 Nang Chang Company, China), water cooled, naturally aspirated with double swirl combustion chamber
Diesel
Biogas
Henham, Makkar (1998)
Two-cylinder, 4-stroke, water-cooled, IDI Lister Petter LPWS2 diesel engine
Gasoil
Biogas
Krishnan et al. (2004)
Single-cylinder DI, CI engine
Diesel
Natural Gas
Kusaka , Okamato, Daisho, Kihara, Saito (2000)
Water-cooled, 4-Stroke-cycle, and 4- cylinder conventional DI diesel engine
Diesel
Natural Gas
Mansour, Bounif, Aris, Gaillard Naturally aspirated, V-8 Deutz FL8 412F 4-cycle diesel (2001) engine
Diesel
Natural Gas
Nwafor (2000, 2002, 2003)
Petter model AC1 single cylinder, air-cooled, high speed, IDI, 4-stroke diesel engine
Diesel
Natural Gas
Nwafor and Rice (1994)
Petter model AC1 single cylinder, air-cooled, high speed, IDI, 4-stroke diesel engine
Diesel
Natural Gas
Papagiannakis and Hountalas (2004)
Single cylinder, naturally aspirated, 4-stroke, air cooled, direct injection, high speed, Lister LV1 DI diesel engine with bowl in piston combustion chamber
Diesel
Natural Gas
Selim (2004)
Ricardo E6 single cylinder variable compression IDI diesel engine
Diesel
CH4, CNG, LPG
Selim (2001)
Ricardo E6 single cylinder variable compression IDI diesel engine
Diesel
CNG
Singh, Singh, Pathak (2007)
Naturally aspirated multi cylinder DG with matching alternator
FD
Producer Gas
Uma, Kandpal, Kishore
Direct injected 6-cylinder, vertical, 4-stroke engine with mechanical injector
Diesel
Producer Gas
In a separate study, an indirect injection diesel engine was employed in a bid to investigate the impact of three different pilot fuel amounts on the performance and emissions of the engine (Abd-Alla et al., 2000)
Figure 6: Variation of unburned hydrocarbon concentration with total equivalence ratio for different quantities of pilot fuel (Abd-Alla et al., 2000)
Figure 7: Variation of carbon mono oxide concentration with total equivalence ratio for different quantities of pilot fuel (Abd-Alla et al., 2000)
The Figure above portrays that a combination of minute (small) amount of pilot fuel and light load operating conditions results in excessively high levels of concentration of unburned hydrocarbons being noted. The reason advanced for such observation is that, when a mixture is too lean, the flame that emanates from the ignition of the pilot fuel cannot effectively spread throughout the entire combustion chamber in other words, there is just partial oxidation taking place (Sahoo et al., 2009). Consequently, both the emission levels of carbon monoxide and unburned hydrocarbons are comparatively higher. This can be seen by visually inspecting Figures 8 and 9. In contrast to the behaviour observed at light loads, it is revealed that, when the concentration of the gaseous fuel in the air charge surpasses the lean combustion limit at higher loads, the flame generated from the ignition of the pilot fuel is able to spread, unaided, covering a wider portion of the combustion chamber. According to the study carried out, it is not out of place then, to conclude that there is little effect derived from varying the quantity of pilot fuel at higher loads.
Figure 8: Variation of Unburned hydrocarbon concentration with total equivalence ratio for different quantities of pilot fuel (Abd-Alla et al., 2000)
Figure 9: Variation of carbon mono oxide concentration with total equivalence ratio for different quantities of pilot fuel (Abd-Alla et al., 2000)
Whereas the variations in Figures 10 were derived using methane as primary gas, those in Figures 9 and 11 are generated from the same sort of study, using propane as primary. It is revealed from the latter pair of figures that there is a slight reduction in the levels of unburned hydrocarbon and carbon mono oxide emissions. This is attributed to a switch in oxidation reactions (from unsuccessful to successful flame spread). Another pertinent aspect being examined is the effect that variations of pilot fuel quantity have on oxides of Nitrogen (NO x) constituents in emissions. The review has shown that there is an increase in charge temperature when pilot fuel quantity is increased for the same total equivalence ratio. The consequence of this increased charge temperature is an increase in the production of NO x. The figure below provides evidence to this assertion.
Figure 10: Variation of NOx with total equivalence ratio for different quantities of pilot fuel with methane as primary fuel (Abd-Alla et al., 2000)
Figure 11: Variation of NOx with total equivalence ratio for different quantities of pilot fuel with propane as primary fuel (Abd-Alla et al., 2000)
To a great extent, it is prudent to state that both the quantity of pilot fuel and the overall equivalence ratio have tremendous influence on the quantum of NO x production. From the graph (Figure 11), it is shown that the use of
larger amounts of pilot fuel quantities alongside high charge equivalence ratios tend to lead to considerable increase in NOx production. Moving on from the effect of pilot fuel quantity on emissions, it is argued that when large quantities of pilot fuel are employed, the result is an increase in power output of the engine. An explanation advanced to support this observation is that there is a much more successful spreading (propagation) of the ignition flame owing to an increase in pilot fuel quantity, hence, increased power output.
Figure 12: Variation of brake power with total equivalence ratio for different quantities of pilot fuel with methane as primary fuel (Abd-Alla et al., 2000)
Figure 13: Variation of brake power with total equivalence ratio for different quantities of pilot fuel with propane as primary fuel (Abd-Alla et al., 2000)
Figure 14: Effect of pilot fuel flow rate on knocking torque (Abd-Alla et al., 2000)
Figure 15: Pressure versus crank-angle for pilot injection of a dual fuel engine (Nwafor, 2002)
As far as knock is concerned, Figure 14 highlights the idea that, at low loads, when larger amounts of pilot fuel are used in an attempt to enhance the combustion process, there is an increase in the tendency of knock being experienced by the engine at higher loads. A much more detailed study that explored the issue of knock has been carried out and documented (Nwafor, 2002, Nwafor and Rice, 1994). The main theme addressed in the study is the impact/effect of the pilot fuel/gas ratio on the knock characteristics of a dual fuel engine. A typical comparison between the knock characteristics of pure diesel and those of dual operations are provided. From Figure 15 above, ripples can be seen between points B-C-D-E and even slightly after E. The appearance of theses ripples is indicative of the occurrence of combustion knock. It is argued that the extent of knock during longer ignition delay depends very much on the ratio of the primary fuel (natural gas) to the pilot fuel (diesel), and thus on the load and speed of operation for the dual fuel engine (see Table 2 below) Table 2: Effect of pilot fuel/gas ratio on knock characteristics of dual engine operating at 3000 rpm (Nwafor, 2002)
In dual fuel engines, increasing the pilot fuel and reducing the primary fuel is believed to lead to reduction of the knocking occurrence. It is worth stating, that, while this is beneficial to combatting knock, it is not the ideal solution as it defeats the environmental and cost effective purposes behind using a lot more natural gas as primary fuel. This further enhances the necessity for further research into the gas/fuel ratio a dual fuel engine.
Figure 16: Pressure versus crank angle diagram of diesel fuel operation (Nwafor, 2002) Keeping constant the engine speed, pilot fuel injection timing and compression ratio, the effect of pilot fuel quantity on a dual fuel engine investigated using a Ricardo E6, single cylinder, variable compression, IDI diesel engine (Selim, 2004). The investigations show that when the quantity of pilot diesel fuel is increased, there is a corresponding increase in torque output of the engine. Using three different primary fuels (methane, Compressed Natural Gas, and Liquefied Petroleum Gas) it is shown that when the pilot diesel fuel quantity is increased, the results achieved include: greater energy being released on ignition; improved pilot fuel injection characteristics: larger size of pilot mixture envelope with greater entrainment of the gaseous fuel; a larger number of ignition centres requiring shorter flame travels and a higher rate of heat transfer to unburned gaseous fuel-air mixture (Badr et al., 1999). All these factors (achieved results) enumerated above tend to increase both the thermal efficiency and power output of the dual fuel engine (Abd-Alla et al., 2000).
Figure 17: Effects of pilot fuel mass of engine performance and noise (Selim, 2004) The Figure 17 (a-d) above reveals the impact of pilot fuel mass on the performance and Noise of a dual fuel engine operating at 1300 rom, with an injection timing of 35 BTDC (Before Top-Dead-Centre) and Compression Ratio of 22. There is a much higher maximum combustion pressure being attained by virtue of increasing the pilot diesel fuel mass (see Figure 17c). In Figure 20 d, the rate of maximum pressure rise is generally reduced
when there is an increase in pilot fuel quantity. The combustion noise is measured through the intermediary of the change in pressure with respect to temperature (dP/d) and this is seen to decrease when the pilot diesel fuel quantity is first increased. The reason to support this observation is the increase in flame volume that emanates from an increase in pilot fuel quantity which burns the gaseous primary fuel smoothly and at a low combustion rate (Sahoo et al., 2009). However, increasing the quantity of pilot diesel fuel beyond a certain threshold, the period of ignition delay of the pilot diesel is being increased and thus the pressure rise rate (dP/d) for the gas-air mixture increases (Selim, 2001). 3.2 EFFECTS OF OTHER PARAMETERS ON DUAL-FUEL DIESEL ENGINE PERFORMANCE In terms of the effect of the engine load, there is one school of thought that advocates that, as the engine load is being increased, there is a corresponding increase in the combustion noise. Emphasis by this same school of thought is made to clarify that, the combustion noise in the case of a dual-fuel engine is always higher than that for a pure diesel engine (Selim, 2001). In another study, it is shown that, at part load conditions (for both dualfuel mode and diesel-only mode), there is a decrease in the performance of the engine. While acknowledging that NOx and SO2 emissions are reduced (without increasing particulate emissions) in the dual fuel mode, it is argued that CO emissions are substantially higher at all operated conditions for a dual-fuel engine when compared against its diesel engine counterpart. The same study also suggests that HC emissions are slightly higher in a dual-fuel engine (Uma et al., 2004). Another study observed a 1-2% reduction in the engine output – they called this a minor reduction in comparison to the diesel case. In the case of this study that used producer gas and rice bran oil on a CI engine, it is demonstrated that while HC and exhaust gas temperatures rise, the CO, CO2, NO and NO2 emissions are seen to drop (Singh et al., 2007). In yet another study, lower levels of NOx as well as drastic decreases in soot emission, alongside higher CO and HC emissions are reported. The study goes on to highlight the fact that there is longer combustion duration for a dual-fuel engine than for a diesel only engine at their respective low load operations. However, at higher loads, the combustion duration is shorter in the former when compared to the later (Hountalas and Papagiannakis, 2000). There is agreement that both CO and HC emissions are higher for dual fuel than for diesel (Hountalas and Papagiannakis, 2000, Uma et al., 2004). While Signh et al., (2007) agree with the above two about an increase in HC for dual fuel operation, they contrast the above as far as emissions of CO are concerned. Another parameter which studies have focused on in the past is the effect of engine speed on performance and emissions of the dual-fuel engine. As concerns this parameter, Maximum combustion pressure is marginally higher for all engine speeds in the case of dual-fuel. At a given speed condition, the dual-fuel possesses a slightly higher equivalence ratio (Mansour et al., 2001b). According to Selim (2001, 2004), the rate of pressure rise is shown to decrease with increase in the speed of engine. When compared to the diesel case, it is observed that the rate of pressure rise is higher for the dual-fuel case. Concerning the effect of the pilot fuel injection timing, there is an increase in the efficiency of fuel conversion between an injection timing of 15-45 BTDC. After this injection timing range, it thought that the fuel conversion efficiency decreases. At a timing of 45 BTDC, the NOx emissions are higher than those observed at 15 BTDC (retarded timing) or 60 BTDC – advanced timing(Krishnan et al., 2004). Still on the same subject matter, Selim (2001) observes that as injection advances increases from 25-40 BTDC), the pressure rise rate of a dual fuel engine is higher than for a 100% diesel engine. In a separate study, the same author (Selim, 2004) suggests that, there is a reduction in torque output as well as in engine thermal efficiency as a consequence of advancing the timing of the pilot fuel injection. In yet another study, it was evident that, the consequence of advancing the injection timing was improvement in thermal efficiency. However, when this advancement of injection timing was done at medium and high load operating conditions, knocking occurred early. Also, with advance in injection timing, there was increase in NOx, reduction in CO and unburned hydrocarbon emissions (Abd-Alla et al., 2002). Another investigation on the effect of pilot fuel injection timing reveals that with advanced injection timing, there are higher Hydrocarbon emissions as well the exhaust temperatures being high. Furthermore, the study highlights the fact that standard dual timing shows longer period of delay at high loads than the advanced injection timing operation (Nwafor, 2000). Concerning the effect of engine compression ratio on dual fuel engine performance and emissions, Selim (2004) explains that by using a high compression ratio in a dual fuel engine knock commences earlier – t his more pronounced when the fuel utilized is liquefied petroleum gas (LPG). Generally, combustion noise is thought to increase when compression ratio is increased. The engine intake manifold conditions have also been studied and the following observation pertaining to its impacts and effects on the performance and emissions of dual fuel engine made. A
combination of intake heating and exhaust gas recirculation (EGR) is shown to improve thermal efficiency. The practice of EGR controls the rate of pressure rise and when the EGR ratio exceeds 50%, it leads to deterioration of engine combustion characteristics. In addition, EGR alongside intake heating will lead to a reduction of NOx emissions as well as THC (Kusaka et al., 2000). Turning attention to the effect of the type of gaseous fuel on engine performance and emissions, the deterioration in engine performance when 40% CO2 in biogas was used, was observed to be much more in comparison to an engine in which 96% methane was used as primary fuel. However, 30% CO2 in biogas is depicted to improve engine performance as compared to the same, running on methane as primary fuel (Bari, 1996). It is also suggested that the overall efficiency drops with gas substitution and adding CO2 affects this more at elevated speeds. In addition, that, the exhaust temperature is affected more by Natural gas substitution than by CO2 addition with exception to this being observed when there is maximum natural gas substitution. Furthermore, CO is affected mainly by natural gas substitution and not very much by gas quality (Henham and Makkar, 1998).
4. CONCLUSIONS Engines that operate in the dual fuel mode, to a great extent are usually traditional diesel-only engines that have been converted to permit them burn a gaseous fuel while relying on the conventional liquid fuel injection system of the engine to facilitate ignition. It is reported that these engines require a minimum amount of modification from their corresponding diesel versions. They also possess the added advantage of being able to operate on a wide range of gaseous fuels while retaining the ability to operate as a conventional diesel engine whenever required. They have been seen to produce power much more economically and at superior thermal efficiencies compared to those of their equivalent diesel engines. Furthermore, their levels of exhaust emissions are favourable and the engines prove to be quite reliable. Improvements to the traditional diesel engine have led to the concept of dual-fuel engines being revisited. There are significant benefits in terms of energy efficiency that have been observed across a number of industries/sectors where these dual fuel engines have already been utilised. It is evident that this kind of engine is suitable for several applications, spanning across a few industries. Some of these applications where dual fuel engines have been employed include; marine vessels, buses, trucks, vehicles and locomotives. 4.1 MARINE AND OFFSHORE With the abundance of marine carriers in the world nowadays, there is the utmost need for them to be more cost efficient while simultaneously complying with stringent emissions legislation. In a sense, there is paramount interest for these carriers to be cost transparent and reliable. In view of this, utilizing natural gas rather than diesel or heavy fuel oil has demonstrated tremendous potential to achieve reductions in fuel cost, while simultaneously offering a decrease in emissions. Therefore, this permits the numerous carriers to comply with the current strict environmental legislation. It is therefore not uncommon to assert that the application of dual fuel engines in the maritime industry has been predominant on ships where there is a readily available source of gas e.g. Liquefied Natural Gas (LNG) carriers (Richards, 1999). Gas diesel and typical dual fuel low speed, twostroke engines for propulsion have been developed. Companies such as Wartsila, MAN B & W, Cummins and Caterpillar have all significantly contributed to establishing dual fuel practice within this class of engine (slow speed, 2-stroke engines). However, for high speed engines, there is still much to be done to enhance and make dominant the practice of dual fuel. Turning attention the offshore industry, it is believed that oil platforms are an ideal environment in which the application of dual fuel engines should thrive (Richards, 1999). It is argued that, this is because there is a ready availability of Gas at virtually minimal (if not Zero) cost. Despite, this suggestion of ready availability of gas, caution is required as there cannot be a total reliance on the supply of these proclaimed gas availability. Looking at the traditional fixed oil platform, and comparing with the floating, production, storage and offloading (FPSO) vessel, it is suggested that the dual fuel engine applications are more beneficial to the latter rather than the former (Richards, 1999). 4.2 OTHER INDUSTRIES There has been growing pressure on several industries to reduce the amount of emissions produced. The power generation sector has not been left out. Consequently, the prevalent institution of emission regulations to power generation units as well as stationary plants provides a reasonable avenue in which dual fuel engine
applications can thrive. In addition, in several parts of the world, there is the practice of gas pipeline. This offers a readily available (and in most instances cheaper) source of natural gas, thus enhancing the ease of application of dual engines. Other sectors such as the automobile, trucking, rail and buses have seen tremendous benefits in terms of energy efficiency via the utilisation of dual fuel engines.
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