Abstract
Natural gas is well-suited as a fuel in the transport sector. Due to its excellent combustion characteristics, engines operating with compressed natural gas (CNG) reach high efficiency, especially if operated at lean conditions. However, CNG engine research mainly focusses on stoichiometric conditions in order to use a three-way catalytic converter for the exhaust gas after treatment system. With the objective to explore the potential of CNG engines operated at lean conditions, a turbo-charged CNG engine with high com-pression ratio is developed and optimized for lean operation. In order to increase the ignition energy, the CNG engine is equipped with scavenged pre-chambers. A specific control structure is developed, which allows to operate the engine at a pre-defined (lean) air-to-fuel ratio. Further functionalities such as the combustion placement control and algorithms to estimate the conditions inside of the pre-chamber are implemented. The first part of this paper describes this engine control structure, which is specifically developed for the lean-burn CNG engine. In the second part, the effects of pre-chamber scavenging on engine performance criteria such as the combustion stability, engine efficiency or engine emissions are analyzed. With the objective to use pre-chamber scavenging to improve engine performance, a scavenging feed-back control strategy is proposed. In order to control the ignition delay, this strategy adapts the amount of CNG injected into the pre-chamber with a linear controller or an extremum seeking algorithm depending on the air-to-fuel ratio of the main chamber.
Highlights
Natural gas as a fuel for passenger cars is a promising alternative to Diesel or gasoline [8]
We conclude that for a strategy where the main chamber air-to-fuel ratio is controlled to a constant value of λMC = = 1.8 and the combustion center is controlled to a constant value, there exists an optimal amount of compressed natural gas (CNG) to be injected into the pre-chamber, which leads to the shortest ignition delay and to the highest ignition energy for the main chamber
A detailed analysis of pre-chamber scavenging at lean engine operation revealed that early pre-chamber injection leads to an increased homogeneity of the CNG-air mixture inside of the pre-chamber, which leads to lower hydrocarbon emissions
Summary
Natural gas as a fuel for passenger cars is a promising alternative to Diesel or gasoline [8]. For the operation of this research engine, a custom engine control strategy is developed and implemented It offers extended functionalities such as the combustion placement feedback control and the precise control of lean air-to-fuel ratios. Further it allows to set a specific CNG mass to be injected into the pre-chambers in order to increase the ignition energy for lean mixtures. The implemented engine control strategy, which is described, is used to investigate lean CNG combustion with scavenged pre-chambers. Results of this analysis are presented in [10]. To the authors knowledge, there is no literature about pre-chamber scavenging feedback control
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