T. C. Zannis

Operational and Environmental Evaluation of Diesel Engines Burning Oxygen-Enriched Intake Air or Oxygen-Enriched Fuels: A Review

A method to curtail emissions of smoke and other pollutants from diesel engines is to enhance the oxygen supply to their combustion chamber. This can be accomplished by enriching either the intake air stream or the fuel stream with oxygen. Experimental studies concerning the oxygen-enrichment of intake air, have revealed large decrease of ignition delay, drastic decrease of soot emissions as well as reduction of CO and HC emissions while, brake specific fuel consumption (BSFC) remained unaffected and increasing of power output is feasible. However, this technique was accompanied by considerable increase of NO x emissions. Experimental and theoretical studies with oxygenated fuels have demonstrated large decrease of soot emissions, which correlated well with the fuel oxygen content. Reduction of CO and HC emissions with oxygenated fuels was also obtained. However, penalties in both BSFC and NO x emissions have been observed with oxygenation of diesel fuels. In both cases one has to weigh the tradeoffs in fuel economy, in power output and in the emissions of various pollutants. Moreover, fuel cost, availability and supply infrastructure, as well as equipment and operational costs, are among concerns that apply to these techniques. This manuscript presents a comparative evaluation of the two techniques regarding engine performance characteristics, environmental repercussions and economy of operation. The primary objective is to contrast the benefits and the drawbacks of the two techniques in view of economic, operational and environmental parameters. Results have shown that the overall economy of operation of the two techniques may be comparable, if the price of oxygenated fuel blends is similar to that of diesel fuel. Their impact on pollutant emissions may also be comparable, if the oxygen enrichment of either technique is limited to a low level (<23% by mass in the cylinder mixture). However, there are possibilities of increasing the power density of engines with oxygen enrichment of the intake air.

Experimental Investigation to Specify the Effect of Oxygenated Additive Content and Type on DI Diesel Engine Performance and Emissions

The reduction of brake specific consumption and pollutant emissions are issued as future challenges to diesel engine designers due to the depletion of fossil fuel reserves and to the continuous suppression of emission regulations. These mandates have prompted the automotive industry to couple the development of combustion systems in modern diesel engines with an adequate reformulation of diesel fuels and have stirred interest in the development of clean diesel fuels. The use of oxygenated fuels seems to be a promising solution towards reducing particulate emissions in existing and future diesel motor vehicles. The prospective of minimizing particulate emissions with small fuel consumption penalties seems to be quite attractive in the case of biodiesel fuels, which are considered as an alternative power source. Studies conducted in diffusion flames and compression ignition engines have shown a reduction of soot with increasing oxygen percentage. However, the effects of the type of oxygenated additive and oxygen content on gaseous and particulate emissions obtained from modern DI diesel engines have not been fully investigated. An experimental investigation is conducted to determine the effect of oxygen content and oxygenate type on DI diesel engine performance and emissions. One conventional and three oxygenated fuels are examined having an oxygen content ranging from 0% to 9%. The fuels are prepared by blending a biodiesel compound (RME), Diglyme and Butyl-Diglyme with a low sulfur diesel fuel in various proportions. An experimental installation is prepared and engine tests are conducted on a naturally aspirated single-cylinder Ricardo Hydra research engine. The measurements are carried out at various operating conditions. The experimental findings reveal an increase of in-cylinder pressure due to the increase of cetane number. In addition, a slight increase of bsfc is observed due to the small decrease of fuel heating value with the increase of the oxygen content. A decrease of ignition delay is observed with increasing oxygen content following thus, the increase of cetane number. A considerable reduction of soot, carbon monoxide and unburned hydrocarbon emissions is witnessed while; nitric monoxide emissions are increased when the oxygen content is increased from 3% to 9%. Similar effects are observed when replacing the rapeseed methyl ester with a mixture of diglyme and butyl-diglyme and the oxygen percentage remains unaltered. As revealed, a reduction of tailpipe soot without overall considerable penalties in bsfc and NO x emissions can be achieved in modem DI diesel engines using oxygenated additives at elevated percentages (30% by mass).

Possibilities to Achieve Future Emission Limits for HD DI Diesel Engines Using Internal Measures

The diesel engine is currently the most efficient powertrain for vehicle propulsion. Unfortunately it suffers from rather high particulate and NO x emissions that are directly related to its combustion mechanism. Future emission legislation requires drastic reduction of NO x and particulate matter compared to present values. Engine manufacturers in their effort to meet these limits propose two solutions: reduction of pollutants inside the combustion chamber using internal measures and reduction at the tailpipe using aftertreatment technology. Currently there are various opinions considering the final solution. Taking into account information related to aftertreatment technology, an effort should be made to reduce pollutants inside the combustion chamber as much as possible. The last is obvious if we account for the even more strict emission limits to be applied after 2010 that will require a combination of aftertreatment and internal measures. For this purpose it is examined in the present using a simulation tool the possibility for achieving EURO-V emission limits using conventional Dl diesel engine combustion technology and internal measures only. Towards this aim, the advancement of fuel injection timing and the increase of injection pressure in conjunction with the increase of EGR rate and boost air pressure are considered. These technologies are evaluated comparing performance and emission data against experimental ones referring to the baseline operation of a single cylinder HD Dl diesel test engine. The results are used to examine the potential for meeting EURO-V emission limits using the previous combination of internal measures. The idea is to control soot using internal measures only and use A/T devices to control NO x emissions where necessary. From the results obtained, indication is also provided for the required efficiency of after-treatment systems to fulfill future emission limits. Thus one can decide to use internal measures to control emissions up to a certain point and/or less demanding aftertreatment systems to further fulfill his requirements. Another outcome of the present analysis is the information provided concerning the required increase of boost pressure, which will be demanded by future boost air systems.

Use of a Multi-Zone Combustion Model to Interpret the Effect of Injector Nozzle Hole Geometry on HD DI Diesel Engine Performance and Pollutant Emissions

A major challenge in the development of future heavy-duty diesel engines is the reduction of NO x and particulate emissions with minimum penalties in fuel consumption. The further decrease of emission limits (i.e., EPA 2007-2010, Euro 5 and Japan 05) requires new, advanced approaches. The injection system of Dl diesel engines has an important role regarding the fulfillment of demands for low pollutant emissions and high engine efficiency. One of the injection system parameters affecting fuel spray characteristics, fuel-air mixing and consequently, combustion and pollutant formation is the geometry of the nozzle hole. A detailed experimental investigation was conducted at UPV-CMT using three different nozzle hole types: a standard, a convergent and a divergent one to discern the effect of nozzle hole conical shape on engine performance and emissions. According to the experimental findings, an increase of soot and decrease of NO x was observed for the divergent nozzle hole compared to the other two. Conventional heat release rate analysis did not show any significant effect of nozzle hole geometry on the combustion mechanism. However, the use of a modified procedure to account for the differences of fuel injection rate between the three nozzles, revealed a slower combustion rate for the divergent nozzle. The results of the experimental analysis motivated the present group to conduct a computational investigation using a multi-zone combustion model to interpret the mechanism behind the different behaviour of divergent nozzle compared to others. The model was used as a diagnostic tool to capture the effect of the three nozzle hole geometries on engine performance and emissions. For this reason an automatic calibration procedure has been developed and applied to estimate model constants to predict engine performance. The only parameter that had to be modified between the three nozzle geometries examined was the air entrainment rate inside the fuel jet. Thus, it was concluded that for the divergent nozzle a lower fuel-air mixing rate occurs compared to the other two nozzle configurations. Using the estimated model constants, an endeavour was made to assess computationally its ability to capture the effect of nozzle hole geometry on emissions. Hence, predictions for soot and NO tailpipe values were made for the three types of nozzles at various engine-operating conditions using the multi-zone model. The analysis revealed that predictions for pollutant emissions are in agreement with corresponding experimental data for all cases examined confirming the fact that nozzle hole geometry affects the mixing rate of injected fuel with surrounding air. Furthermore, the usefulness of the phenomenological model to identify the underlying mechanisms of the different combustion phenomena was acknowledged providing a reasonable explanation for the observed effect of nozzle hole geometry.

Comparative Evaluation of the Effect of Intake Charge Temperature, Pilot Fuel Quantity and Injection Advance on Dual Fuel Compression Ignition Engine Performance Characteristics and Emitted Pollutants

The simultaneous reduction of nitrogen oxide emissions and particulate matter in a compression ignition environment is quite difficult due to the soot/NOx trade off and it is often accompanied by fuel consumption penalties. Thus, fuel reformulation is also essential for the curtailment of diesel pollutant emissions along with the optimization of combustion-related design factors and exhaust after-treatment equipment. Various solutions have been proposed for improving the combustion process of conventional diesel engines and reducing the exhaust emissions without making serious modifications on the engine, one of which is the use of natural gas as a supplement for the conventional diesel fuel (Dual Fuel Natural Gas/Diesel Engines). Natural gas is considered to be quite promising since its cost is relative lower compared to conventional fuels and it has high auto-ignition temperature compared to other gaseous fuels facilitating thus its use on future and existing fleet of small high speed direct injection diesel engines without serious modifications on their structure. Moreover, natural gas does not generate particulates when burned in engines. The most common natural gas/diesel operating mode is referred to as the Pilot Ignited Natural Gas Diesel Engine (P.I.N.G.D.E). Here, the primary fuel is natural gas that controls the engine power output, while the pilot diesel fuel injected near the end of the compression stroke autoignites and creates ignition sources for the surrounding gaseous fuel mixture to be burned. Previous research studies have shown that the main disadvantage of this dual fuel combustion is its negative impact on engine efficiency compared to the normal diesel operation, while carbon monoxide emissions are also increased. The specific engine operating mode, in comparison with conventional diesel fuel operation, suffers from low brake engine efficiency and high carbon monoxide (CO) emissions. The influence becomes more evident at part load conditions. Intake charge temperature, pilot fuel quantity and injection advance are some of the engine parameters which influence significantly the combustion mechanism inside the combustion chamber of a Pilot Ignited Natural Gas Diesel Engine. In order to be examined the effect of these parameters on performance and exhaust emissions of a natural gas/diesel engine a theoretical investigation has been conducted by using a numerical simulation. In order to be examined the effect of increased air inlet temperature combined with increased pilot fuel quantity and its injection timing on performance and exhaust emissions of a pilot ignited natural gas-diesel engine, a theoretical investigation has been conducted by using a comprehensive two-zone phenomenological model. The results concerning engine performance characteristics and NO, CO and Soot emissions for various engine operating conditions (i.e. load and engine speed), comes from the employment of a comprehensive two-zone phenomenological model which had been applied on a high-speed natural gas/diesel engine. The main objectives of this comparative assessment are to record and to comparatively evaluate the relative impact each one of the above mentioned parameters on engine performance characteristics and emitted pollutants. Furthermore, the present investigation deals with the determining of optimum combinations between the parameters referred before since at high engine load conditions, the simultaneous increase some of the specific parameters may lead in undesirable results about engine performance characteristics. The conclusions of the specific investigation will be extremely valuable for the application of this technology on existing DI diesel engines.Copyright

Development and Validation of a Multi-Zone Combustion Model for Predicting Performance Characteristics and NOx Emissions in Large Scale Two-Stroke Diesel Engines

In the present study, an existing multi-zone combustion model has been modified and applied to predict the performance characteristics and the NOx emissions of a large-scale two-stroke diesel engine (i.e. 12.5 MW rated power). Initially, an attempt was made to examine whether the multizone model can predict with sufficient accuracy the main performance parameters of the stationary diesel engine, using input data from the shop tests (i.e. engine speed, fuel consumption and scavenging pressure at different engine loads). Hence, it was verified that the model is capable of describing the main performance characteristics of the engine with satisfactory accuracy (i.e. reference state). Further on, an experimental investigation was conducted on the specific engine, consisting of a series of performance and NOx emissions measurements that were conducted at constant speed and at three different engine loads. Then, the proposed model was applied at the present engine condition to evaluate its ability to predict the combustion mechanism using the measured cylinder pressure trace as basis for comparison. Furthermore, the predicted engine out NOx emissions were compared to measured values to examine if the model can estimate at least qualitatively the effect of engine load on them. From this combined theoretical and experimental analysis is revealed that the developed model is capable of adequately predicting both engine performance and NOx emissions of large-scale two-stroke diesel engines.Copyright

Theoretical Investigation of the Factors Affecting the Performance of a High Speed DI Diesel Engine Fuelled With Natural Gas

Reduction of exhaust emissions is a major research task in diesel engine development in view of increasing concern regarding environmental protection and stringent exhaust gas regulations. Simultaneous reduction of NOx emissions and particulate matter is quite difficult due to the soot/NOx trade-off and is often accompanied by fuel consumption penalties. Towards this aim, automotive engineers have proposed various solutions, one of which is the use of alternative gaseous fuels as a supplement for the commercial liquid diesel fuel. This type of engine, which operates fuelled simultaneously with conventional diesel oil and gaseous fuel, is called “dual fuel” diesel engine. Among alternative gaseous fuels, natural gas is considered to be quite promising due to its low cost and its higher auto-ignition temperature compared to other gaseous fuels facilitating thus its use on existing diesel engines. Previous research studies revealed that natural gas/diesel engine operation results in deterioration of brake engine efficiency, CO and HC emissions compared to conventional diesel fuel operation. In attempt to curtail these negative effects, various theoretical and experimental studies were carried out examining the influence of various parameters such as pilot fuel quantity, diesel fuel injection timing advance and intake charge conditions on “dual fuel” engine performance characteristics and pollutant emissions. However, there are more to know about the proper combination of these engine parameters to attain the optimum results regarding reduction of CO and HC emissions without further deteriorating, if not improving, brake engine efficiency. Hence, in the present study, a theoretical investigation is conducted using an engine simulation model to examine the effect of the aforementioned parameters on performance and exhaust emissions of a natural gas/diesel engine. Predictions are produced for a high-speed natural gas/diesel engine performance characteristics and NO, CO and Soot emissions at diverse engine speeds and loads using a comprehensive two-zone combustion model. The main objective of this comparative assessment is to elaborate the relative impact of each one of the above mentioned parameters on engine performance characteristics and exhaust emissions. Furthermore, an endeavor is made to determine the optimum combinations of these engine operational parameters. The conclusions of this study may be proven to be considerably valuable for the application of this technology on existing DI diesel engines.Copyright

Effects of Boost Pressure and Spark Timing on Performance and Exhaust Emissions in a Heavy-Duty Spark-Ignited Wood-Gas Engine

AbstractWood gas represents a viable energy source, particularly for stationary electric power generation, as it allows for a wide flexibility in fossil fuel sources and can be used as full supplement fuel in conventional heavy-duty (HD), turbocharged (T/C), spark-ignited (SI) engines. For such engines fuelled with wood gas, spark timing and boost pressure are critical parameters that affect both engine performance characteristics and NO and CO emissions. Thus, the main objective of this study is to investigate theoretically the effects of these parameters on the performance and exhaust emissions (NO and CO) of such an existing engine fueled with wood gas. The investigation is conducted by using a comprehensive two-zone phenomenological model. The predictive ability of model was tested against experimental measurements, which were obtained from the operation of such an engine fueled with wood-gas fuel under various operating conditions. The experimental results were found to be in good agreement with the ...

Intake-Air Oxygen-Enrichment of Diesel Engines as a Power Enhancement Method and Implications on Pollutant Emissions

Increasing the in-cylinder oxygen availability of diesel engines is an effective method to improve combustion efficiency and to reduce particulate emissions. Past work on oxygen-enrichment of the intake air, revealed a large decrease of ignition delay, a remarkable decrease of soot emissions as well as reduction of CO and unburned hydrocarbon (HC) emissions while, brake specific fuel consumption (bsfc) remained unaffected or even improved. Moreover, experiments conducted in the past by authors revealed that oxygen-enrichment of the intake air (from 21% to 25% oxygen mole fraction) under high fuelling rates resulted to an increase of brake power output by 10%. However, a considerable increase of NOx emissions was recorded. This manuscript, presents the results of a theoretical investigation that examines the effect of oxygen enrichment of intake air, up to 30%v/v, on the local combustion characteristics, soot and NO concentrations under the following two in-cylinder mixing conditions: (1) lean in-cylinder average fuel/oxygen equivalence ratio (constant fuelling rate) and (2) constant in-cylinder average fuel/oxygen equivalence ratio (increased fuelling rate). A phenomenological engine simulation model is used to shed light into the influence of the oxygen content of combustion air on the distribution of combustion parameters, soot and nitric oxide inside the fuel jet, in all cases considered. Simulations were made for a naturally aspirated single-cylinder DI diesel engine “Lister LV1” at 2500 rpm and at various engine loads. The outcome of this theoretical investigation was contrasted with published experimental findings.Copyright

Effect of injector flow rate on heavy-duty DI diesel engine performance and emissions for various injection pressures

A computational study to examine the effect of injector flow rate at various injection pressures on the combustion process and mainly, on pollutants emitted from a heavy duty DI diesel engine was conducted using a multi-zone combustion model. The relation of injector flow rate with injection duration and mean injection pressure was taken into account in the present analysis to assess the overall effect of injection parameters. As observed, injector flow rate affects the combustion and pollutants formation mechanism through its effect on the fuel-air mixing rate. Higher injection rates are more beneficial to the improvement of bsfc-NO and soot-NO trade-offs at low engine speed whereas the opposite occurs at high engine speed. Overall, it was revealed that considerable improvements in terms of engine performance and pollutant emissions can be attained using low injector flow rates at the entire engine operating range and significantly increased injection pressures to keep the injection duration constant.