Experimental comparison of efficiency and emission levels of four-cylinder lean-burn passenger car-sized CNG engines with different ignition concepts

Abstract

Today’s passenger car CNG engines are based on petrol engines which typically have restrictions preventing the exploitation of the full potential of methane based fuels, especially if they have to be operated also on petrol as a second fuel. Additionally, the use of three-way-catalysis limits the engine operation to λ = 1. Here, we present the efficiency potential and the raw emission characteristics for a dedicated four cylinder passenger car CNG engine without sticking to the usual combustion peak pressure and λ limitations. Lean com-bustion reduces the knocking tendency but, because of the higher pressure levels, increases the ignition energy demand. Therefore, dif-ferent ignition systems (spark plug, prechamber, Diesel pilot) have been used.

Control-oriented analysis of a lean-burn light-duty natural gas research engine with scavenged pre-chamber ignition

“…As derived in [10], the engine has its maximum efficiency close to an air-to-fuel ratio of λ Exh = 1.7. At this ratio, an operation point variation is conducted in order to investigate the effects of engine speed and load on prechamber scavenging.…”

“…They are based on a four cylinder passenger car Diesel engine, which was adapted to run with compressed natural gas (CNG). A characterization of these engines can be found in [10]. Figure 1 shows a schematic of the research gas engine 2.…”

“…An inverted pre-chamber injector map, then uses this reference injection duration and the amount of injected CNG to calculate the reference pre-chamber rail pressure. A further aspect to consider is that the pressure difference between rail and pre-chamber has to be at least 2 bar, in order to open a back pressure valve in front of the prechamber (see [10]). Therefore, the intake pressure plus 2 bar acts as a lower bound on the pre-chamber rail pressure reference value.…”

“…Within a research project with the objective to investigate the efficiency potential and emission characteristics at lean operation, a natural gas engine is set up on a test bench. In order to ignite lean mixtures, the engine is equipped with scavenged pre-chambers, as described in [10].…”

“…The implemented engine control strategy, which is described in section 2, is then used to investigate lean CNG combustion with scavenged pre-chambers. Results of this analysis are presented in [10]. Further, a control-oriented analysis of the effect of pre-chamber scavenging on engine efficiency and emissions is conducted and used to propose a pre-chamber scavenging control.…”