Turbocharged spark-ignition (SI) engines, owing to frequent engine knocking events, utilize retarded spark timing that causes combustion inefficiency, and high turbine inlet temperature (Trb-In T) levels. Fuel enrichment is implemented at high power levels to prevent excessive Trb-In T levels, resulting in an additional fueling penalty and higher CO emissions. In current times, fuel-enrichment reductions are of high strategic importance for engine manufacturers to meet the imminent emissions regulations. To that end, the authors investigated the divided exhaust period (DEP) concept in a 2.2 L turbocharged SI engine with a geometric compression ratio of 14 by decoupling blowdown (BD) and scavenge (SC) events during the exhaust process. Using a validated 1D engine model, the authors first analyzed the DEP concept in terms of pumping mean effective pressure (PMEP) and engine knocking (KI) reduction. Subsequently, the authors examined the effectiveness of the DEP concept using a “low-restriction exhaust flowpath” and varying late intake valve closing (LIVC) duration. First, using DEP, significant PMEP and KI reductions benefits were observed at high power engine conditions along with a large increase in Trb-In T from the early blowdown event. Subsequently, use of a low restriction exhaust flowpath and a shortened LIVC duration further elevated the DEP benefits, including Trb-In T reduction that facilitated enrichment reduction. At 4,000 RPM/20 bar BMEP, ~70% lower PMEP and a 2.2 point increase in ITEg were noted relative to the base engine. However, the 2,000 RPM peak torque engine condition was compromised using DEP, due to knock limitation and deteriorated stock turbocharger performance. Finally, DEP design integrated with an off-the-shelf (new) turbocharger system remedied the low-end torque challenges and demonstrated a notable enrichment reduction and thermal efficiency benefits at the full load engine curve including the 200 kW rated condition.

Improving thermal efficiency of an internal combustion engine is one of the most cost-effective ways to reduce life cycle-based CO2 emissions for transportation. Lean burn technology has the potential to reach high thermal efficiency if simultaneous low NOx, HC, and CO emissions can be achieved. Low NOx can be realized by ultra-lean (λ ≥ 2) spark-ignited combustion; however, the HC and CO emissions can increase due to slow flame propagation and high combustion variability. In this work, we introduce a new combustion concept called turbulent jet-controlled compression ignition, which utilizes multiple turbulent jets to ignite the mixture and subsequently triggers end gas autoignition. As a result, the ultra-lean combustion is further improved with reduced late-cycle combustion duration and enhanced HC and CO oxidation. A low-cost passive prechamber is innovatively fueled using a DI injector in the main combustion chamber through spray-guided stratification. This concept has been experimentally demonstrated as detailed in this article to achieve 47.7% peak indicated efficiency and below 1 g/kWh engine-out NOx emission with initial single-cylinder engine hardware. Further systematic combustion system optimization is underway to demonstrate state-of-the-art efficiency and emissions at a wider operating range.

Gasoline Compression Ignition (GCI) is an engine-fuel technology which incorporates a combustion process using gasoline fuels in a compression ignition (CI) mode. GCI has the potential to offer high fuel efficiency while achieving ultra-low emissions. The objectives of the GCI engine development are to improve the fuel economy, meet the peak torque capabilities and comply with light duty emission regulations. The other key objective is to provide cost effective GCI engine design solution to automakers by minimizing the hardware complexity and maximizing compatibility with existing engine components. While GCI is under development for several years, there are several key technology risks (KTR’s) that need to be addressed. During cold start, the criteria pollutant emission is a challenge when the catalyst is not effectively warmed up. At low loads, misfires and partial burns lead to excessive HC and CO emissions. The increased pressure rise rate and particulate emissions are challenges at high load. The control challenges include transient Exhaust Gas Recirculation (EGR) control and combustion robustness control due to cycle to cycle stability needs. Furthermore, the fuel injection system durability and boosting system capabilities are required to be demonstrated. This book chapter presents strategies to address those KTR’s at different engine operating conditions. The engine is based on a gasoline engine architecture with a spark plug and high pressure gasoline fuel injection system, termed as spark assisted gasoline compression ignition (SAGCI) engine. The results reported in this chapter are specific this particular hardware configuration of GCI engine fueled by RON 91 gasoline. During cold start, a split fuel injection strategy with extremely retarded spark timing is proposed to rapidly warm up the catalyst and minimize the criteria emissions. At low loads, typical spark ignited (SI) combustion and spark assisted GCI strategies are preferred. During GCI operation at low loads, spark is enabled as a supplemental means for robust combustion control. GCI based on partially premixed compression ignition (PPCI) strategy is preferred at medium loads to meet the emission targets. Cold EGR, re-breathing and different fuel injection strategies are used as control variables to ensure better combustion control. The high load GCI is based on diffusion combustion with optimum fuel injection strategy that targets to reduce the pressure rise rate and soot emissions.

Open-cycle engine simulations of a passive pre-chamber operating inside a direct-injected gasoline engine are performed and analyzed in this study. A comprehensive three-dimensional computational fluid dynamics (CFD) model has been formulated with state-of-the-art physical sub-models to account for the complex processes of engine gas exchange, fuel injection and mixing, ignition, and combustion. Numerical results are validated against experimental measurements under low load condition with high internal EGR. Realistic modeling considerations are discussed to ensure proper fidelity. In particular, the mixture conditions and flow motions could present very different features between the pre- and main chamber, which requires comprehensive simulation of the full engine cycle and imposes challenges for combustion modeling. Practically validated chemical kinetics models are essential for proper prediction of cylinder pressure history of pre-chamber jet combustion systems. Detailed analysis is then carried out to highlight key processes associated with pre-chamber operation, including residual scavenging, fuel/air mixture formation, flow pattern and turbulence development within the pre-chamber, and the ignition of main chamber mixture by issued turbulent jets. Numerical evaluation of pre-chamber design variants has been attempted, and less commonly investigated geometry parameters such as swirl nozzles and nozzle umbrella angle are found impactful for pre-chamber ignition performances.

Knock is a major challenge for high load operation of spark ignited gasoline engines with higher compression ratios, since the end-gas undergoes higher temperature and pressure trajectories during combustion. Pre-chamber combustion creates long-reach ignition jets that have the potential to mitigate knock due to their rapid consumption of end-gas. However, conventional pressure oscillation-based knock metrics may not accurately capture the end-gas autoignition severity in pre-chamber systems due to differences in ignition and combustion behavior. This work investigates the knock behavior of both traditional spark ignition and pre-chamber combustion (including different nozzle designs) in a high compression ratio engine fueled with regular octane certification gasoline. The data was analyzed using statistical methods to show the random nature of knock events. Detailed analysis was used to explain the pressure oscillations of both knocking and non-knocking cycles of pre-chamber jet combustion and show that conventional pressure oscillation-based knock metrics may not adequately quantify end-gas autoignition severity. A novel knock metric is introduced to avoid consideration of the non-knock related pressure oscillation and better quantify the end-gas autoignition severity. The new metric was used to explain the knock mitigation mechanism for pre-chamber jet combustion and demonstrate an additional pre-chamber jet ignition benefit of reduced combustion variability during engine operation with cooled exhaust gas circulation within its dilution limit.

The work presented here seeks to compare different means of providing scavenging systems for an automotive 2-stroke engine.It follows on from previous work solely investigating uniflow scavenging systems, and aims to provide context for the results discovered there as well as to assess the benefits of a new scavenging system: the reverse-uniflow sleeve-valve.For the opposed-piston engine, once the port timing obtained by the optimizer had been established, a supplementary study was conducted looking at the effect of relative phasing of the crankshafts on performance and economy.This was found to have a small effect on fuel consumption for a significant change in compression ratio, suggesting that, if available, variable crankshaft phasing could be a very important control actuator for gasoline compression ignition in such an engine.Importantly, it was found that existing experiential guidelines for port angle-area specification for loop-scavenged, piston-ported engines using crankcase compression could also be applied to all of the other scavenging types, this having been done here in order to provide a starting point for the work.This important result has not been demonstrated before for such a wide range of architectures.The optimizer employed then allowed further improvements to be made over the starting point.The paper therefore presents a fundamental comparison of scavenging systems using a new approach, providing insights and information which have not been shown before.

One of the attractive alternatives to traditional spark ignition engines is the gasoline compression ignition (GCI) engine technology. Fuels with octane numbers lower than those of market gasolines have been identified as a viable option for GCI engine applications. Their longer ignition delay time characteristics compared to diesel fuel and their similar volatility features compared to gasoline fuels make them interesting to be explored. In this study, we have numerically investigated the effect of different injection timings at part-load conditions using a research octane number (RON) 75 fuel in gasoline compression ignition single cylinder engine. Full cycle GCI computational fluid dynamics (CFD) engine simulations have been successfully performed while changing the start of injection (SOI) timing from −60° to −10° CAD aTDC at 5bar net indicated mean effective pressure (IMEPn). The effect of SOI on mixing, combustion phasing and engine-out emissions is investigated using detailed equivalence ratio-temperature maps. Also, the effects of different rates of exhaust gas recirculation (EGR) on the combustion and emissions characteristics are investigated. Rebreathing valves profiles along with double injection strategies are also examined in the current study. Fuel consumption, soot, nitric oxides (NOx), hydrocarbon (HC) emissions and combustion phasing (CA50) are the targeted parameters throughout this study. Optimum engine parameters to obtain the best combination of the targeted properties were identified.