THE MOST EFFICIENT INTERNAL COMBUSTION ENGINE

 

On August 8 2017 in Tokyo, Japan, the President of Japanese multinational automaker Mazda Motor Corp, Masamichi Kogai introduced a new engine called the SKYACTIVE-X. The engine was to target 20 to 30 percent greater fuel efficiency and thermal efficiency roughly to about 56 percent as a part of Mazda’s “Sustainable Zoom-Zoom 2030” vision.

In the latter half of 2019, the Skyactive engine became the first commercial automotive internal-combustion engine to turn the majority of its fuel’s energy into power, rather than waste in the form of friction or heat loss. Before this, the most thermally efficient automotive internal combustion engine belonged to F1 engines (Mercedes AMG F1), with an efficiency of slightly over 50 percent. 

The engine is found cleaner to run than all electric vehicles by tallying the pollution generated by both fossil fuel production and utility electricity generation to compare Skyactiv and EV emissions. This analysis reflects the reality that, currently, much electricity is generated through fossil fuels. In regions where electricity comes from wind, solar, or hydroelectric, the EV would clearly win the argument, but that's not the case for many customers today. Mazda made it possible using the age old type of combustion called HCCI.

How? – HCCI

HCCI stands for Homogeneous Charge Compression Ignition Engine. Unlike the regular petrol or diesel engines which use a spark plug or a fuel injector respectively, these engines doesn't use a spark or fuel injector for ignition. Instead, a homogeneous mixture of air and fuel is ignited simply by compression. The advantage of this is that in such combustion, there is no point of source from where the flame originates. This indicates that the flame front is nonexistent; decreasing the overall internal working temperature of the engine and therefore the creation of nitrogen oxide and soot is prevented, resulting in improved emissions and better efficiency. Also a lean mixture of fuel and air (lesser fuel) can be used due to homogeneity which improves the fuel efficiency.

The HCCI  potentially offers the advantages of both the diesel and the spark-ignition engine. If HCCI can be made to operate at very low equivalence ratios (i.e., very lean), then it will permit light load engine operation without throttling. Such operation should give fuel economy equivalent to that of the diesel engine. At the same time, because it is a homogeneous-charge engine, it can be operated in spark-ignition mode at full power, and give the same specific output as the gasoline engine. It should have the great advantage over the diesel engine of producing very low particulate emissions, because homogeneous charge combustion generates very low particulate emissions.

Why isn’t HCCI implemented yet?

The biggest challenge in this kind of an engine is to ensure the simultaneous and complete combustion of the homogeneous mixture. The best method to overcome this challenge is to maintain a particular temperature range in the cylinder during the compression stroke. If the cylinder is too cold, the ignition performance of the engine will be heavily affected. If the cylinder is too hot, knocking might occur and causes unfavorable combustion. Hence, in order to achieve a perfect combustion, it is necessary that these temperature restrictions are met. Also, in cold weather conditions it becomes impossible for the motor to get going.

Variations: 

In order to overcome the disadvantages, various companies have been working on various variations of the HCCI engines. Mazda for one have introduced spark control combustion ignition. Mazda’s breakthrough technology uses a traditional spark plug to begin the combustion process, then leverages the pressure rise from the resulting flame to trigger compression ignition in the remainder of the cylinder. They have used air supply technology to reach higher combustion ratios in the Skyactiv. This engine is currently being used in their Mazda 3. Nissan has introduced variable compression technology along with exhaust gas recirculation (EGR) where the mixture is preconditioned by the addition of EGR or heating or both. Volkswagen has introduced Innovative Variable Valve Timing (VVT) which is used to control the compression ratio to maintain the required temperature in the cylinder. 

Many other companies like GM, Honda, BMW, and Daimler have also begun working with HCCI. Different solutions are already in the marketplace, and more are on the way. Development is ongoing, and don’t be surprised to see in the near future solutions that combine HCCI with other advanced technologies.

Entry Ignition Combustion:

Entry ignition is a theoretical engine that uses four different chambers during its functioning. It starts by pulling air into the cylinder without fuel and compressing it. This brings the temperature inside the cylinder close to the auto-ignition point of gasoline, meaning the fuel is close to being able to ignite without a spark, but not quite there. That high-pressure, high-temperature air is then sent into a mixing chamber, where fuel is injected. Slider valves allow the air-fuel mixture to travel from the mixing chamber into the cylinder, where it ignites instantly, hence the name entry ignition.

The efficiency benefits of entry ignition include the ability to use higher compression ratio with a lean air-fuel ratio just like the HCCI engine. It also gets more work out of combustion by allowing for greater expansion, which pushes the piston down further. The advantage this theory has over HCCI is that the ignition can be controlled. In an SAE technical paper published in April 2020, researchers estimated that entry ignition could boost thermal efficiency—the percentage of energy converted into work within the engine—to 63%, compared to 49% for a standard Otto-cycle gasoline engine.

However, entry ignition is an unproven technology. Among other things, it uses different-size pistons and cylinders, which could create balance issues, Fenske noted. Furthermore, no production automotive engine has ever used the slider valve called for by entry ignition either, so reliability is also a question mark. It's also worth noting that no automaker has discussed using this technology in a production car yet.

Conclusion: 

While electric cars are being hampered by high battery costs, lack of charging infrastructure and low customer interest, HCCI and entry ignition engines could very well be the future of automotive engines. Of course everything will turn electric at some point, but in that interim, however short or long it may last, these engines will play a significant role in letting us move around without destruction to the environment – for now at least.

References:

https://www.nissan-global.com/EN/TECHNOLOGY/OVERVIEW/hcci.html

https://ca.finance.yahoo.com/news/mazda-touts-gasoline-engine-drive-longer-term-strategy-044456888--finance.html?guccounter=1&guce_referrer=aHR0cHM6Ly9lbi53aWtpcGVkaWEub3JnLw&guce_referrer_sig=AQAAADb4zgW77huXPpENCcOB3iRcX6-lH8c57yM4-eNMhMZEFHr_i9BPTrrumu0P9bTqnl9ygoLCjVh8ILxtAjbkPaScgY420zl8zzdksHe5k7IPZ-E-BVMJ5I1fbnPqoDAoi6Y9MG2GeT70eruhrvDMsFFpo3ocUHZ02Wut1pBvJMGX

https://www.ni.com/en-in/innovations/white-papers/11/subsystems-required-to-control-low-temperature-combustion-engine.html

Research Paper: Zhao, Fuquan; Asmus, Thomas W.; Assanis, Dennis N.; Dec, John E.; Eng, James A.; Najt, Paul M. (2003). Homogeneous Charge Compression Ignition (HCCI) Engines: Key Research and Development Issues. Warrendale, PA, USA: Society of Automotive Engineers.