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