Towards High Efficiency Powertrains
Doctoral thesis, 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.

hybrid powertrain

water injection

Downsized spark-ignition engines


efficient engine starts

knock mitigation

warm engine starts

Opponent: Dr. Graham Conway, Southwest Research Institute, TX (USA)


Jayesh Khatri

Chalmers, Mechanics and Maritime Sciences (M2), Combustion and Propulsion Systems

Water Injection Benefits in a 3-Cylinder Downsized SI-Engine

SAE International Journal of Advances and Current Practices in Mobility,; Vol. 2019-January(2019)p. 236-248

Journal article

Effect of relative humidity on water injection technique in downsized spark ignition engines

International Journal of Engine Research,; Vol. 22(2021)p. 2119-2130

Journal article

Water Injection System Application in a Mild Hybrid Powertrain

SAE Technical Papers,; Vol. 2020-April(2020)

Journal article

Climate change is a serious threat to the planet with increasing global carbon dioxide emissions resulting in higher earth temperature. The transportation sector is one of the major contributors of greenhouse gas emissions and this has led to more stringent emission regulations and a shift towards electrified vehicles, partly (hybrid electric vehicles) or completely (battery electric vehicles). Even though the share of hybrids and pure electric vehicles will increase: the efficiency of internal combustion engine equipped vehicles will remain to be a key parameter to lower fuel consumption and lower CO2 emissions for decades to come.

The work presented in this dissertation investigates two technologies – water injection and strategies for cleaner engine starts, to achieve higher internal combustion engine (and subsequently powertrain) efficiency. Historically, water injection has been used in the military aircrafts (fighter planes) during early 1940s and has recently received fresh attention in IC engines for passenger vehicles. As part of this project, water injection was investigated to assess its benefits and underlying mechanisms when operated on gasoline fuels with different octane rating. Considering the daily variation in atmospheric conditions, experiments were also conducted to investigate the effect of varying humidity levels on water injection. The results from simulation of water injection benefits in a hybrid powertrain showed improvements in brake specific fuel consumption (BSFC) and overall gains when operated on three different drive cycles.

During the second part of the work in this thesis, engine starts were investigated, and a methodology was developed to identify, categorize and optimize engine starts. The developed methodology can be used to study engine starts in a variety of powertrain systems and based on the three-way catalyst (TWC) temperature it can be used to optimize individual engine starts using AVL CAMEO tool. Through a simple case study, CO2 reduction of 1.76 g per km was obtained using optimized engine start conditions when compared to engine starts with base calibration for the specific hardware.  In conclusion, a combination of different technologies can help improve powertrain efficiency and reduce greenhouse gas emissions.

High efficient hybrid powertrain

Swedish Energy Agency (43325-1), 2016-12-01 -- 2019-12-31.

Subject Categories

Mechanical Engineering

Driving Forces

Sustainable development

Areas of Advance





Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 5075





Opponent: Dr. Graham Conway, Southwest Research Institute, TX (USA)

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