The first gasoline power plants worked with carburetors, and this solution is still alive and well in classic cars.
However, carburetors were not the most efficient way to mix air and gas, so engineers started improving the spark-ignited engine so they can get more power out of it. Back then, the horsepower war was not going on at the same time with an emission challenge, so improving efficiency was all that mattered.
Since our story is focused on reducing emissions from gasoline engines, we would count the four-stroke gasoline engine as the first norm in reducing emissions. This one did not have to be fueled by a mix of oil and gas to provide internal lubrication of the cylinders, so it did not generate the hateful blue smoke. We consider this the first step in improving gasoline engines.
The next massive step in that direction was the Mechanical Fuel Injection (MFI) system. This was developed in WWII for fighter planes, but eventually saw its way into production vehicles. While it was reserved for expensive cars when it first came out, it eventually got into the hands of the average consumer.
Unlike carburetors, the MFI reliably provided a precise quantity of fuel and air for every combustion cycle. These systems were perfected over the years and ended up being manufactured well into the late 1980s. Meanwhile, some economy cars were still using carburetors, so mechanical fuel injection was still an expensive technology back then. Unfortunately, like carburetors, MFI had its drawbacks and required adjusting once in a while, but it could theoretically operate more efficiently than a carbureted engine.
Exhaust Gas Recirculation Valve (EGR)
The Exhaust Gas Recirculation valve was one of the first systems that reduced emissions from gasoline engines while also improving their efficiency. This method involved injecting exhaust gasses back into the combustion chamber to reduce the temperature. Using EGR, engines would reach operating temperature faster, and the choke of the carburetors would be turned off. In turn, the reduced temperature in the combustion chamber prevented pre-ignitions and detonations, making the engine run more smoothly.
However, the first EGR systems were crude and impacted the operation of the engine. Eventually, carmakers started controlling EGR systems to provide improved startup and idling, as well as enhanced performance on high loads by disabling the system. Most modern cars still feature an EGR system, while some engines have managed to go without it through various solutions.Electronic Ignition Systems
One of the most important steps in the world of gasoline engines was the implementation of electronic ignition systems. These used a variety of simple sensors to replace the vacuum-operated and centrifugal timing advance mechanisms of traditional distributors to provide the spark required to ignite the air-fuel mixture at precisely the right time.
These were initially fitted to carbureted engines as a way of improving their efficiency, and they eventually became the norm for gasoline engines. Along with adjusting a carburetor, setting up the ignition timing on a distributor was a nightmare without adequate tools and experience. The lack of one or another element mentioned in the previous phrase led to millions of liters of fuel wasted by consumers over the years.Lean Burn
The idea of a lean burn engine was to steer away from the stoichiometric mixture of air and gas considered optimum for the Otto cycle so that efficiency would be improved. The solution was called lean burn, and it involved using a different air/fuel ratio, several times higher than the stoichiometric 14.64:1.
This solution became prevalent in the late 1970s, and was utilized for some Chrysler, Honda, Nissan, Mitsubishi, and Toyota models, among others. The idea of this type of engine was that it reduced throttling losses (those that occur because of the design of the throttle body) and enhanced fuel economy.
They did not become the norm, as they required complicated catalytic converters that were incompatible with the current three-way catalytic converter systems. Therefore, this concept hasn't been used as the only way of operation for gasoline engines since the 1990s. Electronic Fuel Injection (EFI)
The next real advancement in reducing emissions of gasoline engines (and, later, diesel engines) was the introduction of electronic fuel injection. This was more precise than the mechanical solutions that preceded it, and provided an even better control of the amount of fuel that went into the combustion chamber.
Initial applications were single-point injection systems, but the EFI technology evolved quickly to multi-point, and multi-port solutions. The latter is still rarely used, as it was replaced by Direct Injection out of efficiency reasons.
Unlike mechanical fuel injection systems, EFI systems had even more sensors, but also achieved improved precision. They also required dedicated computers for operation, which worked with the electronic ignition system. Together, these systems could provide the engine with a stoichiometric mixture of air and fuel at any speed and moment of operation.
The new fuel injected engines started requiring the so-called “Lambda Sensors.” These are oxygen sensors, and are placed on the exhaust gallery to evaluate the efficiency of the combustion cycle and on pollution-reducing devices like catalytic converters.
Modern cars have two or more oxygen sensors, and these are placed in a stream on the exhaust gallery (usually before and after the catalytic converter). Ignition timing and injection are continuously adjusted to parameters set from the factory to ensure that the gasoline engine fully complies with emission regulations. Moreover, it is all done thanks to these Lambda Sensors, along with EFI.
As you have noticed, we mentioned the catalytic converter in the paragraphs above. This is a device that works with the electronic fuel injection to provide lower emissions for the engine it equips. It works by creating an oxidation reaction inside of its shell with the aid of the rare metals it contains.
These systems have worked together for over two decades to provide the world with engines that will not choke everybody on the street when the cars that feature them are stuck in traffic. However, a slight malfunction in either system will cause the other components to fail if it's not fixed in time, so keep an eye on that “Check Engine” light and service your car at the intervals specified in the manufacturer's service manual to ensure that your vehicle is running optimally. Direct injection
While not a recent invention, direct injected gasoline engines have been on the rise in the last few years. In contrast to multi-point and multi-port systems, these deliver the gas at a higher pressure and directly into the combustion chamber. Instead, classical fuel injection provided the fuel at a lower pressure (similar to the one you use to inflate your tires on a passenger car) in the intake gallery.
Direct injection gasoline engines took inspiration from diesel units and had the injectors deliver the fuel into the combustion chamber. Thanks to ingenious engineering, these injectors are capable of spraying an even smaller amount of gas in the combustion chamber, and can even do this to form a particular pattern that will ignite optimally.Other Combustion Cycles
The Otto combustion cycle is not the only way gasoline engine can operate. Back in 1882, James Atkinson developed the combustion cycle that bears his last name. It was not as popular as the Otto cycle because of the reduced power density it provided when compared to the Otto internal combustion cycle. However, modern hybrids have turned to the Atkinson cycle thanks to its improved overall efficiency.
Some engines have been developed to compensate the reduced power density of the Atkinson cycle with the aid of a mechanical supercharger, and these are called Miller-cycle engines. Unlike Otto-engines using the Atkinson cycle, these were not widespread.
Some automakers have developed spark-ignited engines that work in Atkinson cycle at low loads, and then turn to the Otto cycle to provide a high output. Mazda does this with its 2.0-liter Skyactiv-G units, and Toyota uses a similar concept on the Lexus NX200t’s engine, among others.What the future holds for spark ignited engines
We expect future gasoline engines to feature improved spark plugs, like the Laser-operated units currently in development by Mazda for its rotary engines. Furthermore, the throttle body might be eliminated because of the pumping losses it generates (BMW already does this on its Valvetronic engines), while the combustion cycle could be further improved through fully adjustable timing.
Currently, variable valve timing is the norm in the auto industry along with direct injection, but Koenigsegg has already developed a camless engine with fully variable timing to improve power and efficiency. It does not use it in any of its cars yet, though, as it is just a prototype for now.