The abrupt rise of fuel economy in the US was a direct result of a shift of fuel economy policy in 1975, in response to the oil price shock of the early 1970s. This caused a transition towards smaller cars with less powerful, smaller engines.
Manufacturers took note of this and started exploring technologies that would bring power and robustness back to their vehicles while still maintaining good fuel economy.
An engine extracts energy from the burning of gasoline. It does this by first taking in a mixture of fuel and air into a cylinder. The total working volume of all of the cylinders in an engine is known as its displacement.
It then compresses the mixture and ignites it with a spark plug. As the mixture burns, it expands, pushing down a piston, which rotates a crankshaft. The spent gases are then pushed out through the exhaust. Power is sent from the rotating crankshaft, through the drivetrain, then to the wheels.
The first step is to reduce the size of the vehicle. We can now reduce the size of the powertrain. A lower displacement engine with fewer cylinders loses less energy getting power to the wheels. This is called a parasitic loss.
The amount of fuel-air mixture an engine can aspirate to create power is directly related to its displacement and number of cylinders. By reducing engine displacement size, you lower the amount of power an engine can make but also the amount of fuel it consumes.
The first steps were to control the fuel usage of the engine more accurately. In order to do this, we need to understand when fuel is used most and why.
Engines in cars have 5 modes of operation. Starting, idling, accelerating, cruising, and decelerating. Acceleration and cruising. These two modes are where most fuel consumption occurs.
Throttling open an engine to make more power is where its highest fuel consumption occurs. Cruising, on the other hand, occurs when the throttle is held slightly open, keeping the engine speed and power output steady. This is where we can hone in the fuel efficiency of an engine.
Most of the fuel we use driving is caused by a combination of short bursts of acceleration and longer periods of cruising. The key to balancing power and fuel economy is having strong acceleration characteristics but efficient cruising characteristics
The ideal ratio of air to gasoline is 14.7 to 1. This is known as a stoichiometric mixture. But in practice, this ratio becomes difficult to achieve.
To compensate for this more fuel is added, enriching it. This allows more fuel to be burned without ideal mixing. Enriching is used primarily under acceleration to ensure maximum power generation. Unburnt fuel is wasted.
With cruising, since our power requirements are constant and relatively low, mixtures closer to 14.7 to 1 or even slightly higher are used. This is known as running lean since were not utilizing all of the air in combustion. Running lean uses less fuel but can be damaging. Uncontrolled self-ignition of the mixture is called detonation and it can cause overheating and damage to an engine.
Incoming fuel is used to cool the combustion chamber and control the rate of burning, reducing the chances of detonation. This limit how lean we can run an engine.
Up until the 1980s, most cars relied on carburetors to meter out fuel. Because of its mechanical nature, carburetors lack precise control over air-fuel mixture and required maintenance to keep them functioning correctly. Electronic fuel injection was embraced by manufacturers.
Fuel injection works by precisely spraying pressurized fuel through computer controlled injectors. The computer that meters out fuel is known as an engine control unit or ECU. Some of the key parameters measured are engine rpm, air temperature, air flow into the engine, throttle position and engine temperature. Manufacturers could now tune fuel systems much closer to the ideals for both power and fuel economy.
Because the injected fuel is sprayed at higher pressures, better air-fuel mixing occurs. It requires less enrichment overall and improves both fuel economy and power.
On most engines, the fuel injection system and the ignition system are merged. This allows the ECU to adjust the ignition point timing relative to the combustion cycle. Creating a spark earlier in the cycle, or advancing the timing can produce more power by starting combustion sooner.
Another advantage of fuel injection is that it allows for the use of feedback in the fuel delivery system. During cruising, the leanness of combustion is monitored by an oxygen sensor is in the exhaust stream, providing feedback to the ECU. The ECU can use this data to trim the air-fuel mixture closer to ideal, boosting fuel economy.
Sensors to detect detonation are also present on some fuel injection systems. Early sensors work by listening for the acoustical signature of detonation on the engine block.