New technology has made changes in engines within things such as cylinder arrangement, piston bore diameter, stroke displacement etc. how have each improved the fuel economy greatly in recent years.
It might be wise here to give an overview of the internal combustion engine and the basics of how it works. The heart of a gasoline engine is the fuel cylinder. The cylinder is a metal cylinder that has the gas/air intake at the top along with the spark plug. The center of the cylinder is drilled out to make a bore tube opening at the bottom that is fitted with a metal rod called a piston. So the piston fits into the cylinder. The gas/air mixture enters the cylinder at the top and is ignited by the spark plug to produce a highly contained explosion. This explosion causes the piston to shoot downward and the resulting cooling of the air after the explosion causes it to return upward. In this way, the piston is constantly moving up and down within the cylinder. This up/down cycle of motion is called a stroke. The bottom of the piston is connected to a crankshaft, which translates the linear movement of the piston into the rotation of the transaxle, which in turn causes the axle and ultimately the wheels to rotate. This is a simplified view, but it suffices to show that the engineering of the cylinder, piston, and crankshaft all play a significant role in translating the energy obtained from the igniting of the gasoline into the motion of the wheels. Finding the optimal parameters of this system such as the arrangement of the cylinders about the crankshaft, the size of the cylinders and pistons, and motion of the piston in terms of stroke depth all help contribute to maximizing the flow of this energy chain, which in turn improves fuel economy. This is a system that converts chemical energy (fuel) into mechanical enegy (motion).