Closed-Loop Combustion Control Using Ion-Current Signals in a 6-Cylinder Port-Injected Natural-gas Engine

High EGR rates combined with turbocharging has been identified as a promising way to increase the maximum load and efficiency of heavy-duty spark ignition engines. With stoichiometric conditions a three-way catalyst can be used which means that regulated emissions can be kept at very low levels. Obtaining reliable spark ignition is difficult however with high pressure and dilution. There will be a limit to the amount of EGR that can be tolerated for each operating point. Open-loop operation based on steady state maps is difficult since there is substantial dynamics both from the turbocharger and from the wall heat interaction. The proposed approach applies standard closed-loop lambda control for controlling the overall air/fuel ratio. Furthermore, ion-current-based dilution limit control is applied on the EGR in order to maximize EGR rate as long as combustion stability is preserved. The proposed control strategy has been successfully tested on a heavy-duty, 6-cylinder, port-injected natural gas engine and our findings show that 1.5-2.5% units (depending on the operating points) improvement in Brake Efficiency can be achieved

Closed-Loop Combustion Control for a 6-Cylinder Port-Injected Natural-gas Engine

High EGR rates combined with turbocharging has been identified as a promising way to increase the maximum load and efficiency of heavy-duty spark ignition engines. With stoichiometric conditions a three-way catalyst can be used which means that regulated emissions can be kept at very low levels. Obtaining reliable spark ignition is difficult however with high pressure and dilution. There will be a limit to the amount of EGR that can be tolerated for each operating point. Open-loop operation based on steady state maps is difficult since there is substantial dynamics both from the turbocharger and from the wall heat interaction. The proposed approach applies standard closed-loop lambda control for controlling the overall air/fuel ratio for a heavy-duty, 6-cylinder, port-injected natural gas engine. A closed-loop load control is also applied for keeping the load at a constant level when using EGR. Furthermore, cylinder pressure-based dilution limit control is applied on the EGR in order to keep the coefficient of variation at the desired level of 5%. This way confirms that the EGR ratio is kept at its maximum stable level all times. Pumping losses decrease due to the further opening of the throttle, thereby the gas exchange efficiency improves and since the regulator keeps track of the changes the engine all the time operates in a stable region. Our findings show that excellent steady-state performance can be achieved using closed-loop combustion control for keeping the EGR level at the highest level while the stability level is still good enough

Reducing Throttle Losses Using Variable Geometry Turbine (VGT) in a Heavy-Duty Spark-Ignited Natural Gas Engine

Stoichiometric operation of Spark Ignited (SI) Heavy Duty Natural Gas (HDNG) engines with a three way catalyst results in very low emissions however they suffer from bad gas-exchange efficiency due to use of throttle which results in high throttling losses. Variable Geometry Turbine (VGT) is a good practice to reduce throttling losses in a certain operating region of the engine. VTG technology is extensively used in diesel engines; it is very much ignored in gasoline engines however it is possible and advantageous to be used on HDNG engine due to their relatively low exhaust gas temperature. Exhaust gas temperatures in HDNG engines are low enough (lower than 760 degree Celsius) and tolerable for VGT material. Traditionally HDNG are equipped with a turbocharger with waste-gate but it is easy and simple to replace the by-pass turbocharger with a well-matched VGT. By altering the geometry of the turbine housing, the area for exhaust gases can be adjusted and results in the desired torque. Because of this the turbo lag is very low and it has a low boost threshold. Low boost threshold means that VGT can cover a big operation range of the engine from low engine speeds to high. In this operation range the throttle can be fully open and VGT is used instead of the throttle to control the desired torque which results in eliminating the throttling losses. This paper presents experimental results which show the feasibility of reducing throttling losses by means of VGT. The operating region which is appropriate for controlling the desired torque by VGT instead of throttle is specified. The gains in terms of gas exchange efficiency are quantified. Furthermore the dynamics of using VGT is quantified and compared with throttle. The experiments were performed successfully and the results showed at least 2 unit percent improvement in gas-exchange efficiency. A comparable dynamic to throttle is observed

Diluted Operation of a Heavy-Duty Natural Gas Engine - Aiming at Improved Effciency, Emission and Maximum Load

Most heavy-duty engines are diesel operated. Severe emission regulations, high fuel prices, high technology costs (e.g. catalysts, fuel injection systems) and unsustainably in supplying fuel are enough reasons to convenience engine developers to explore alternative technologies or fuels. Using natural gas/biogas can be a very good alternative due to the attractive fuel properties regarding emission reduction and engine operation. Heavy-duty diesel engines can be easily converted for natural gas operation which is a very cost effective process for producing gas engines. However, due to the high throttle losses and low expansion ratio the overall engine efficiency is lower than the corresponding diesel engines. Moreover the lower density of natural gas results in lower maximum power level. In this thesis key features and strategies which may result in improved efficiency, increased maximum power and improved transient capability of a heavy-duty natural gas engines have been identified, validated and suggested. High EGR rates combined with turbocharging has been identified as a promising way to increase the maximum load and efficiency of heavy-duty gas engines. With stoichiometric conditions a three way catalyst can be used and thus regulated emissions can be kept at very low levels. Obtaining reliable spark ignition is difficult however with high dilution and there will be a limit to the amount of EGR that can be tolerated for each operating point. Extending the dilution limit of the engine and developing closed-loop control to operate the engine at its dilution limit has been the main method to reduce throttle losses. A new method for calculating cyclic variation was developed that significantly improved the transient capability of the engine control system. The method consequently applied on a closed-loop dilution limit control. Only applying closed-loop control to operate the engine at its dilution limit resulted in at least 4.5% improvement in specific fuel consumption at 1200 RPM. The dilution limit can also be extended by replacing the combustion chambers with high turbulence pistons which enhances the combustion. By extending the dilution limit the gain in efficiency will be even higher. In summary the key features to improve the performance of a stoichiometrically operated natural gas engine are identified as: right amount of EGR at different operating regions, right compression ratio, Variable Geometry Turbocharger (VGT), high turbulence pistons, long route EGR system and model-based control

Diluted Operation of a Heavy-Duty Natural-gas Engine

Fuel economy and emissions are the two central parameters in heavy duty engines; most of the existing heavy duty engines are run with diesel. The overall efficiency of diesel engines is good however they suffer from high levels of emissions mainly NOX and soot. Using alternative fuels like Natural-Gas has shown to be a good way to reduce emissions level. Diesel engines can be easily converted to Natural-Gas engines which is a very cost effective way for producing Natural-Gas engines. High EGR rates combined with turbocharging has been identified as a promising way to increase the maximum load and efficiency of heavy duty spark ignition Natural-Gas engines. With stoichiometric conditions a three way catalyst can be used which means that regulated emissions can be kept at very low levels. However efficiencies of Natural-Gas engines are lower than the corresponding diesel versions due to throttling losses. These losses are higher at low/part loads. Using EGR at lower loads can increase the efficiency; however obtaining reliable spark ignition is difficult with high pressure and dilution. There is a limit to the amount of EGR that can be tolerated for each operating point. A combustion stability parameter should indicate the EGR tolerance of the engine. Different combustion stability parameters derived from pressure and ion-current signals are applied in order to control the dilution limit with EGR. Furthermore closed-loop lambda control is applied to control air/fuel ratio. With help of these controllers and also a load controller, a tool is developed for finding the best positions of the throttle and EGR valve where the engine has its highest dilution while the engine stability is preserved. Two papers are written based on the results of this study i.e. in the first one the combustion stability is based on the pressure signals and in the second one the combustion stability is derived from ion-current signals. The proposed control strategy has been successfully tested on a heavy duty 6-cylinder port-injected natural-gas engine and the results show 1.5-2.5 % units improvement in Brake Efficiency. In another experiment, behaviour of the engine was investigated by running the engine with Hythane (Natural gas + 10% Hydrogen ) when the engine operates stoichiometric. Data from running a lean burn natural-gas engine with Hythane was available and it was desired to see the behavior of the engine with stoichiometric operation. The results do not show significant changes in knock margins, efficiency and emissions with stoichiometric operation. However, Lean limit and dilution limit can be extended somewhat by Hythane

Artificial Neural Networks Modelling for Monitoring and Performance Analysis of a Heat and Power Plant

Öresundskraft AB has a number of plants that produces mainly electricity and district heat to the city of Helsingborg and its surroundings. Västhamnsverket is the main plant of the company, with an installed capacity of 126 MW electricity and 186 MW heat. Västhamnsverket is a hybrid combined heat and power plant. In order to demonstrate an efficient control-monitoring model, several ANN models have been developed for different parts of Västhamnsverket. This master thesis is a part of a continuing ANN study applied at Västhamnsverket. The overall aim of this study is to develop an ANN simulator for the whole steam process at Västhamnsverket. In order to develop a better ANN model for the entire steam process, this process is divided into two sub-models, called case 1 and case 2. Consequently two cases have been studied separately and two different ANN models have been developed. These are then linked to each other

Unburned Hydro Carbon (HC) estimation using a self-tuned heat release method

An estimation model which uses the gross heat release data and the fuel energy to estimate the total amount of emissions and unburned Hydro Carbon (HC) is developed. Gross heat release data is calculated from a self-tuned heat release method which uses in-cylinder pressure data for computing the energy released during combustion. The method takes all heat and mass losses into account. The method estimates the polytropic exponent and pressure offset during compression and expansion using a nonlinear least square method. Linear interpolation of polytropic exponent and pressure offset is then performed during combustion to calculate the gross heat release during combustion. Moreover the relations between the emissions specifically HC and Carbon Monoxide (CO) are investigated. The model was validated with experimental data and promising results were achieved

How hythane with 25% Hydrogen can affect the Combustion in a 6-Cylinder Natural-Gas Engine

Using alternative fuels like Natural Gas (NG) has shown good potentials on heavy duty engines. Heavy duty NG engines can be operated either lean or stoichiometric diluted with EGR. Extending Dilution limit has been identified as a beneficial strategy for increasing efficiency and decreasing emissions. However dilution limit is limited in these types of engines because of the lower burnings rate of NG. One way to extend the dilution limit of a NG engine is to run the engine on Hythane (natural gas + some percentage hydrogen). Previously effects of Hythane with 10% hydrogen by volume in a stoichiometric heavy duty NG engine were studied and no significant changes in terms of efficiency and emissions were observed. This paper presents results from measurements made on a heavy duty 6-cylinder NG engine. The engine is operated with NG and Hythane with 25% hydrogen by volume and the effects of these fuels on the engine performance are studied. Different experiments were designed and performed to investigate the parameters like knocking margin, dilution limit, lean limit, different efficiencies, emissions and maximum load of the engine. The experiments were performed successfully and the results showed modest improvement in lean, and dilution limit. Slightly changes in emissions and knocking margins are also observed

Using Hythane as a Fuel in a 6-Cylinder Stoichiometric Natural-gas Engine

Combination of right EGR rates with turbocharging has been identified as a promising way to increase the maximum load and efficiency of heavy duty spark-ignited natural gas engines. With stoichiometric conditions a three way catalyst can be used which means that regulated emissions can be kept at very low levels. However dilution limit is limited in these types of engines because of the lower burnings rate of natural gas with higher EGR rates. One way to extend the dilution limit of a natural gas engine is to run the engine with Hythane (natural gas+ some percentage hydrogen). Previously benefits of hydrogen addition to a Lean Burn natural-gas fueled engine was investigated [1] however a complete study for stoichiometric operation was not performed.This paper presents measurements made on a heavy duty 6-cylinder natural gas engine. Three different experiments were designed and tested to investigate first of all if the engine encounters too severe knocking problems, second how and why, Hythane affect the running and finally how lean limit and dilution limit will be improved. The experiments were performed successfully and the results showed no significant differences between natural gas and Hythane in terms of efficiency and emissions when engine operates stoichiometric

Improving Efficiency, Extending the Maximum Load Limit and Characterizing the Control-related Problems Associated with Higher Loads in a 6-Cylinder Heavy-duty Natural gas Engine

High EGR rates combined with turbocharging has been identified as a promising way to increase the maximum load and efficiency of heavy duty spark ignition Natural Gas engines. With stoichiometric conditions a three way catalyst can be used which means that regulated emissions can be kept at very low levels. Most of the heavy duty NG engines are diesel engines which are converted for SI operation. These engine's components are in common with the diesel-engine which put limits on higher exhaust gas temperature. The engines have lower maximum load level than the corresponding diesel engines. This is mainly due to the lower density of NG, lower compression ratio and limits on knocking and also high exhaust gas temperature. They also have lower efficiency due to mainly the lower compression ratio and the throttling losses. However performing some modifications on the engines such as redesigning the engine's piston in a way to achieve higher compression ratio and more turbulence, modifying EGR system and optimizing the turbocharging system will result in improving the overall efficiency and the maximum load limit of the engine. This paper presents the detailed information about the engine modifications which result in improving the overall efficiency and extending the maximum load of the engine. Control-related problems associated with the higher loads are also identified and appropriate solutions are suggested