# Strategies for Reducing Automobile Fuel Consumption
## 3 Vehicle Design and Technology
### Advances in Internal Combustion Engines (ICEs)
Furthermore, the vaporization of LNG has been identified as a potential heat sink for the Rankine Cycle, as pointed out by [PERSON] [26]. Moreover, the Miller Cycle offers another way to enhance fuel efficiency. At an ICE technology level, innovative solutions such as increasing the _mean effective pressure_ (MEP), supported by multistage turbochargers, and integrating turbocompounding are being explored. Engine downsizing_ (ED), which consists of reducing engine displacement while sustaining (or even increasing) power and torque densities, has become a focal strategy among many ICE manufacturers [27]. The primary objective behind ED lies in the drive to operate engines within specific speed/mean-effective-pressure regimes, yielding diminished frictional and heat transfer losses, thereby optimizing efficiency [27]. In effect, ED can replace larger, multi-cylinder engines with smaller, fewer-cylinder engines, thereby minimizing friction. Such _downsized engines_ can complement hybrid electric vehicles, whether in parallel or series configurations. In the former, the engine benefits from supplementary battery power; in the latter, it predominantly serves as a generator, providing electricity either for direct propulsion or battery replenishment. In this case, an electric machine propels the vehicle. In the current market, a significant number of gasoline engines are categorized as downsized engines, which typically exhibit a reduction factor, also called _downsizing factor_ (\(DF\)), of approximately 35-40%. The \(DF\) is a metric that quantifies the decrease in engine displacement (when compared to traditional engines) and can be defined by the following expression:  
\[DF=\frac{V_{\textit{Susept}_{NA}}-V_{\textit{Susept}_{\textit{Downsized}}}}{V_{ \textit{Susept}_{\textit{NA}}}}, \tag{3}\]  
where \(DF\) is the downsizing factor, \(V_{\textit{Susept}_{\textit{NA}}}\) is the swept volume of a naturally aspirated traditional engine with a given output power, and \(V_{\textit{Susept}_{\textit{Downsized}}}\) is the swept volume of a downsized alternative engine with a similar power output. According to [PERSON] [27], a reduction factor of 40% can result in an improvement in fuel economy of approximately 12%. This improvement is attributed to the reduction in the engine's swept volume and the corresponding decrease in friction within the cylinders. The _optimization of modern engines_ across a wide range of operating regimes (i.e., speed-torque ranges) is now achievable through a comprehensive understanding of the factors influencing fuel consumption. Such factors include intake air flows, fuel atomization, combustion, and exhaust gas formation, as well as their interaction with other factors, such as chamber/piston geometry and fuel injection strategies. Currently, these interactions can be modeled with _robust computational tools_. Moreover, _advanced ignition systems_, such as High-Energy Inductive Systems (HEISs), Plasma, Corona, and Laser, have significantly reduced combustion variability. Furthermore, the development of Variable Valve Timing (VVT) and Variable Compression Ratio (VCR) strategies for gasoline engines, along with multiple-pulse injection (MPI) techniques, have enabled higher thermodynamic efficiency within a wide range of operating regimes. Engineers are also exploring the use of multiple pulses and multiple fuels in the same combustion cycle to further enhance engine performance. Another approach considered for improving fuel efficiency in ICEs is the implementation of the _start-stop system_. This system automatically shuts off the engine when the vehicle comes to a stop (e.g., at traffic lights) [28; 29]. During this idle period, all systems operate using battery power. In second-generation start-stop systems, the engine shuts off at a specific speed during deceleration, with the speed being a determining factor for the system's efficiency. The engine remains off throughout the periods of zero speed in each cycle, contributing to energy savings and enhanced overall efficiency [30].