Towards High Efficiency Powertrains

Doktorsavhandling, 2022

In recent years there has been a great shift whereby conventional vehicles powered by an internal combustion (IC) engine are being partially or completely replaced by electrified alternatives; almost all major automotive manufacturers have made statements indicating a shift towards electrification. This shift has been driven in large part by concerns about climate change, which have prompted lawmakers to introduce increasingly strict regulations limiting vehicular emissions, particularly of carbon dioxide (CO2). Hybrid electric vehicles (HEVs) that combine an electric motor with an efficient downsized spark-ignited engine offer a viable solution to these challenges.

This thesis presents studies on two different strategies with the potential to improve the efficiency of spark-ignited engines and, by extension, that of hybrid systems. The first strategy is water injection, which was studied as part of a project seeking to optimize an SI engine for use in a high efficiency hybrid powertrain. The second strategy is cleaner engine starts, which was studied as part of a project seeking to improve the efficiency and reduce emissions during engine starts.

Downsizing SI engines makes it possible to reduce fuel consumption and improve efficiency without loss of power output. However, downsizing while maintaining high thermal efficiency leads to high cylinder pressures and temperatures, which increases the propensity for knocking combustion. Water injection (WI) has been used to mitigate knock and was therefore investigated during the first phase of the project. Experiments were conducted on a 3-cylinder 1.5L turbocharged engine with a port water injection (PWI) system to assess the effects of water injection on knock and efficiency. To account for the variation in the research octane number (RON) of commercially available gasoline blends, experiments were performed using gasoline blends with RONs of 91, 95, and 98. The first test campaign showed that WI enables stoichiometric operation and advancement of ignition timing while suppressing knock. A follow-up experimental campaign focused on investigating the effect of the relative humidity (i.e., the water content of the ambient air) on the efficiency benefits of WI. The engine was operated at three different humidity levels, which were established and maintained using a humidity control system developed in-house. This campaign revealed that the knock suppressing effect of WI in the studied engine was mainly due to charge dilution; the charge cooling effect due to the injected water’s heat of vaporization was insignificant. Finally, a simulation study was performed in GT-Suite to assess the feasibility of using WI in a hybrid vehicle. The simulations showed that the improvement in BSFC due to WI was maximized in highly downsized engines.

Engine starts were investigated during the second phase of the project. Since, any driving event in a hybrid vehicle will inevitably involve multiple engine starts and/or restarts, the objective during this phase was to develop methods to study engine starts and to use these methods to find ways of improving the engine’s starting efficiency. The first investigations in this area were conducted on a hybrid system; later experimental work focused on an isolated engine setup. The hybrid system featured a 1.5L turbo-charged SI engine with Port Fuel Injection (PFI) in a P2.5 Hybrid architecture. Tests were performed under various drive cycles including WLTC and RTS95. The start events were categorized into three different categories (cold, mild, and warm starts) based on the initial three-way catalyst (TWC) temperature, and it was found that warm starts were most common. The second campaign therefore investigated electric motor (EM)-assisted warm engine starts in a Gasoline Direct Injection (GDI) engine. EM-assisted starts were modeled by performing dynamometer-assisted starts on the engine test bed. During this work, methods were developed for categorizing, understanding, and optimizing engine starts for different powertrain architectures. On the basis of a simple case study of a hybrid system, it was estimated that engine start optimization could reduce CO2 emissions by approximately 1.75 g per kilometer if comparing the most efficient conditions to the standard engine starting condition.