Using gas dynamic models to improve exhaust system design for large-bore, two-stroke engines

Abstract

It is vital to have accurate predictions of the gas exchange behavior of an engine in order to reliably study engine performance and emissions using engine simulation models. There are a multitude of factors, both upstream and downstream of the engine cylinder, which influence its gas exchange characteristics. Quite often these influences are interconnected in a non-linear manner that results in complicated feedback loops, which can introduce significant errors in the computed thermodynamic state of the post-breathing cylinder mixture. The effects of such bi-directional movement of pressure pulses are particularly pronounced in two-stroke engines. This study investigates the importance of exhaust system design on the scavenging characteristics of a piston-scavenged, cross-flow, two-stroke engine. A validated one-dimensional predictive model is used to study the effects of changing the exhaust port timing, exhaust system length, and exhaust port efficiency on the breathing performance of the engine, along with the consequent effects on the thermal efficiency and NOx emissions. Exhaust pressure waves and mass flows across ports are used to understand and explain the observed changes. The results show that while making design changes, thermodynamic efficiency considerations can act as a barrier to improving the scavenging efficiency of the engine; in addition, a trade-off between the two has to be considered in the design process to meet engine performance targets. The effects of such a trade-off on the NOx production are analyzed and two exhaust system modifications are discussed.

Trapped equivalence ratio determination in two-stroke engines

“…The height of the plugging pulse can be used as a trapping efficiency indicator. A more detailed discussion of exhaust wave phasing effects on the scavenging characteristics of the engine can be found in Bajwa et al 46…”

“…The height of the plugging pulse can be used as a trapping efficiency indicator. A more detailed discussion of exhaust wave phasing effects on the scavenging characteristics of the engine can be found in Bajwa et al 46 Figure 7 maps the calculated scavenging and trapping efficiencies on delivery ratio plots. Delivery ratio is defined as the mass of air delivered per cycle normalized by the total trapped mass.…”

Experimental investigation of scavenging in two-stroke engines using continuous CO2 sampling

“…Note that Figure 6 shows pressure waves for spark timings advanced and retarded by 10° (not 5°) to better convey the effects of spark timing changes. A more detailed discussion of exhaust wave phasing effects on the scavenging characteristics of the engine can be found in Bajwa et al 43…”

“…Note that Figure 6 shows pressure waves for spark timings advanced and retarded by 10°(not 5°) to better convey the effects of spark timing changes. A more detailed discussion of exhaust wave phasing effects on the scavenging characteristics of the engine can be found in Bajwa et al 43 To assess the variability in TRF values among individual cycles, Figure 7 shows individual cycle traces for a stable and an unstable operating point from the 12 operating points under investigation. The scatter in the individual traces is noticeably greater at the unstable operating point.…”

Performance improvement of turbocharged SI engine by post-oxidation enhancement in exhaust gas in-homogeneity

“…Pressure pulses interference were also studies and reduced. 22,23 According to another study, by controlling the pressure pulses in the exhaust manifold, the power production of thermo-electric generator had also improved up to 10% to 30%. This shows the benefit that comes when tackling pressure pulses effects in exhaust manifold.…”