Hybrid Book - Chapter 4

Efficiency is a measure of how well a vehicle uses its fuel and is calculated based on the ratio of output power to input power. It indicates how effectively the energy from the fuel is being converted into useful work. A higher efficiency means that the vehicle is able to generate more power from the same amount of fuel, resulting in better fuel economy. It is an important factor to consider when evaluating the performance and cost-effectiveness of a vehicle.

The Atkinson cycle is a type of internal combustion engine cycle that allows the compression stroke and expansion stroke to be set independently of each other. This results in a higher expansion ratio, which improves fuel efficiency. Unlike the traditional Otto cycle, the Atkinson cycle keeps the intake valve open longer during the compression stroke, causing some of the air-fuel mixture to be pushed back into the intake manifold. This reduces the effective compression ratio, allowing for a longer expansion stroke and more complete combustion. As a result, the Atkinson cycle is commonly used in hybrid vehicles to maximize fuel economy.

Approximately 15% to 20% of the power generated by a supercharged engine is usually required to do the work of driving the supercharger. This means that a significant portion of the engine's power is used to operate the supercharger, reducing the overall power available for other tasks. This highlights the efficiency trade-off of using a supercharger, as it provides increased power output but also requires a portion of that power to operate.

Electric traction systems for automotive use are designed as constant torque machines. This means that the torque produced by the motor remains constant regardless of the speed at which the motor is operating. This is important for automotive applications as it allows for consistent power delivery and efficient operation across a wide range of speeds. Additionally, constant torque machines are well-suited for applications where high starting torque is required, such as accelerating a vehicle from a standstill.

A high expansion ratio refers to the ratio between the volume of the combustion chamber at the beginning and end of the power stroke in an engine. When the expansion ratio is high, it means that the volume of the combustion chamber increases significantly during the power stroke. This leads to a longer power stroke, allowing more energy to be extracted from the combustion process. As a result, less heat is wasted in the exhaust, leading to improved efficiency of the engine.

A supercharger is added to an engine in order to increase the compression ratio. The Miller cycle is a modified Atkinson cycle that aims to improve fuel efficiency by keeping the intake valve open longer. However, this can result in a lower compression ratio. To compensate for this, a supercharger is added to increase the amount of air entering the engine, which in turn increases the compression ratio.

The LIVC and cam phasing techniques used with an idling Atkinson-cycle engine can yield fuel consumption savings of about 5% to 9%. These techniques optimize the engine's valve timing and camshaft position to improve efficiency during idling, resulting in reduced fuel consumption. By adjusting the valve timing and camshaft position, the engine can achieve better combustion and reduce energy losses, leading to fuel savings of 5% to 9%.

An Atkinson-cycle engine is designed to have a higher engine efficiency at low loads compared to a comparable Otto-cycle engine. This is due to the Atkinson-cycle engine's longer expansion stroke and shorter compression stroke, which allows for more efficient energy conversion. The longer expansion stroke allows the engine to extract more work from the combustion process, resulting in higher efficiency. In contrast, the Otto-cycle engine has a shorter expansion stroke, which leads to lower efficiency at low loads. Therefore, the engine efficiency of an Atkinson-cycle engine is higher than with a comparable Otto-cycle engine at low loads.