Post: Car manufacturers are constantly trying the following methods to make engines more fuel efficient

Increasing displacement: More displacement means more power because the engine can burn more gasoline per revolution. Displacement can be increased by making the cylinders larger or by adding more cylinders. 12 cylinders seems to be the practical limit.
Increasing compression ratio: The higher the compression ratio, the more power is produced, but the higher the compression ratio. However, the more the air/fuel mixture is compressed, the more likely it is to spontaneously ignite (before the spark plug ignites). High-octane gasoline prevents this premature combustion. This is why high-performance cars often require high-octane gasoline—their engines use higher compression ratios to make more power.
Cram more air into each cylinder: If you can cram more air (and fuel) into a cylinder of a given size, you can get more power out of the cylinder (in the same way as increasing the cylinder size) without increasing the fuel required to burn it. Turbochargers and superchargers pressurize the incoming air to effectively cram more air into the cylinder.
Cooling the incoming air: Compressing air raises its temperature. However, you want the air in the cylinders to be as cool as possible, because the hotter the air is, the less it expands when burned. For this reason, many turbocharged and supercharged cars have intercoolers. An intercooler is a special radiator that the compressed air passes through to cool before it enters the cylinders.
Make it easier for air to get in: As the piston moves down on the intake stroke, air resistance can sap the engine’s power. By installing two intake valves in each cylinder, you can greatly reduce air resistance. Some newer cars also use polished intake manifolds to eliminate air resistance there. A larger air filter can also improve air flow.
Make it easier for exhaust gases to get out: If air resistance makes it difficult for exhaust gases to get out of the cylinder, it can make the engine lose power. Air resistance can be reduced by adding a second exhaust valve to each cylinder. Cars with two intake valves and two exhaust valves have four valves per cylinder, which can improve performance. When you hear a car commercial tell you that the car has four cylinders and 16 valves, the commercial is talking about an engine with four valves per cylinder.
If the exhaust pipe is too small or the muffler has a lot of air resistance, it can cause back pressure, which has the same effect. High-performance exhaust systems use headers, large tailpipes, and free-flowing mufflers to eliminate back pressure in the exhaust system. When you hear that a vehicle has a “dual exhaust,” the purpose is to improve exhaust flow by using two exhaust pipes instead of one.
Make Everything Lighter: Lightweight components help improve engine performance. Every time a piston changes direction, it uses energy to stop the stroke in one direction and start the stroke in the other. The lighter the piston, the less energy it uses. This can improve fuel efficiency and performance.
Injecting Fuel: Fuel injection meters fuel to each cylinder very precisely. This can improve performance and fuel economy.