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

Doctoral thesis English OPEN
Kaiadi, Mehrzad (2011)
  • Publisher: Tryckeriet i E-huset, Lunds universitet
  • Journal: (issn: 0282-1990)
  • Subject: VGT | Dilution limit | Other Mechanical Engineering | Heavy-duty | Annan maskinteknik | Closed-loop control | Turbulence | Natural gas engine | Cyclic variation

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. <br /> 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.<br /> 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. <br /> 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.<br /> 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. <br /> 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. News about global warming, discussions about alternative fuels such as natural-gas and biogas, and commercials about gas engines sound familiar to the most people living in modern societies.<br /> Why global warming is important? What is the role of natural-gas engines in this subject? Are natural-gas engines efficient? Do they perform as well as diesel engines? If not, why? How can they be optimized? In the following two pages it is tried to answer very briefly to these questions. In this paper focus is on heavy-duty (i.e. medium size) natural-gas engines and not light-duty.<br /> World energy consumption is expected to increase by 50 percent over the next 20 years. The transportation sector is the biggest sector in liquid fuel consumption. Road transportation is the main part in the transportation sector. The fuels mainly used in internal combustion engines are petroleum products i.e. gasoline and diesel. This information is collected from Energy Information Administration report from 2005.<br /> Fossil fuels are of great importance since burning them produces significant amounts of energy, but they have a great impact on the environment due to the Carbon-Dioxide (i.e. CO2) production. CO2 is a strong greenhouse gas that contributes to the climate change which is an issue of growing international concern. Besides that, the local impact of other pollutants i.e. NOX and PM for diesel engines is a big issue.<br /> To reduce these impacts emission regulation were introduced in the mid-nineteenth century. The regulation has been becoming very sever which has been resulted in very pricey catalysts.<br /> Demand for lower pollutant levels together with increasing concern for climate changes force engine developers to find and investigate more efficient alternative engine management. Using alternative fuels such as natural-gas is one of the attractive solutions for engine application due to several facts. Natural-gas produces less CO2 since it has less carbon atoms. The CO2 reduction is much more if biogas is used. The availability and its high knock resistance are some other properties which make it attractive as engine fuel. However, heavy-duty natural-gas engines produce lower maximum torque than the corresponding diesel engines due to the lower density.<br /> Normally natural-gas engines use same combustion technology as gasoline engines due to the similar fuel properties. Natural-gas engines can be operated in different modes. Lund University has been researching about heavy-duty natural-gas engine since 1990.<br /> This project focuses on stoichiometric1<br /> 1 Stoichiometric is the ideal combustion process where fuel is burned completely. operation. By operating engine in this mode good engine performance can be obtained<br /> and a cost effective 3-way catalyst can be used to reduce all emissions simultaneously.<br /> The main objective of this project is to explore the reasons for the lower performance of natural-gas engine and develop relevant strategies to recover them. The developed strategies will be consequently validated by performing experiments in a test cell. A multi-cylinder 9.4 liter engine from Volvo was used for experiments (see Figure 1).<br /> Figure 1 Engine setup structure. A Linux based PC used to control ignition and injection<br /> The available heavy-duty natural-gas engines are generally diesel engines which converted for natural-gas operation. Due to that the engines are limited to some limitations.<br /> Heavy-duty natural-gas engines suffer from lower efficiency in compare with a corresponding diesel engine. There are mainly two reasons for that; first the lower compression ratio which is limited due to knock phenomena and second, the use of throttle for controlling the desired torque which introduces lots of pumping losses into the engine.<br /> Exhaust Gas Recirculation (EGR) can be used to minimize the throttling losses. The addition of inert exhaust gas into the intake system means that for a given power output, the throttle plate must be opened further, resulting in increased inlet manifold pressure and reduced throttling losses (see Figure 2).<br /> Figure 2 Adding EGR means that for a given power output, the throttle plate must be opened further which results in lower throttle losses<br /> By keeping the EGR effect in mind it is desired to operate the engine with the highest possible EGR rate (i.e. dilution limit). The dilution limit is imposed by increased cyclic variation of the combustion intensity that reduces the drivability. It can be improved by closed loop control of EGR based on combustion stability.<br /> The experimental results show at least 5% reduction in fuel consumption can be achieved which is significant. Moreover; implementing small modifications on the engine resulted in more efficiency improvement and maximum load extension. These modifications are applied on engine pistons, turbocharging system and EGR system.<br /> Finally it can be concluded that, there is a lot of space for improvement and optimization of the stoichiometric natural-gas engines. Our results showed by implementing simple modifications and designing simple regulators efficiency can be increased drastically.
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