Optimized gas ICE for vehicle decarbonization

The Optimized gas ICE for vehicle decarbonization is functional prototype of a piston internal combustion engine, primarily focused on an advanced system equipped with an actively scavenged pre-chamber, that represents a future universal combustion system for spark ignition engines regarding new fuels, higher efficiency (significantly over 40%) and lower emissions (mainly NOx). The principle is already used, e.g. for racing purposes (F1, WEC), it is expected to be used in a future (new generation of ICE optimized for the use of alternative fuels is estimated more like around 2030 and later). Our current solution is roughly at the TRL 3-5 level (so far only Maserati has it in passive mode, probably only because of engine performance), standard spark plug thread was centrally mounted in the pre-chamber that is actively cooled by the engine coolant for reduced thermal stress and a safe operation even at high load, a new 2022 design of the fuel delivery (check valve and solenoid injector) was experimentally validated. This solution enables more precise fuel flow metering and control of charge mixing in the prechamber. There are still quite a few practical obstacles to be solved. Advanced combustion system for future gasoline engines brings higher efficiency of the combustion engine (and therefore lower CO₂) while simultaneously reducing some pollutants (mainly NOx) and enabling high combustion rates for various fuels (the flexibility suitable for the transition period from classic fossil fuels to an alternative fuels), which also positively affects the tendency to knock. 

From a global perspective, this solution contributes to decarbonization, the transition to renewable sources and sustainable mobility, including strengthening the competitiveness of the Czech manufacturer (Skoda Auto).

This functional sample was created in cooperation with Škoda Auto, within the NCC 1 project JOBNAC.

Efficient combustion of alternative fuels in piston combustion engines 

Piston combustion engines remain hardly replaceable vehicle powertrains. However, they are subject to ever increasing requirements regarding stricter legislation, be it from the perspective of emissions or recently also fuel consumption in the form of restricted total production of CO₂. The limit fleet average of CO₂ emissions for passenger cars set for the year 2020 is 95 g/km. In the case of diesel combustion, the limit implies fuel consumption kept down to 3.5 l/100 km or 4.1 l/100 km in the case of petrol. The planned further reduction of the CO₂ limit to 63 g/km in 2030 means that it will no longer be attainable in practice for bigger vehicles using conventional hydrocarbon fuels. Today, electric vehicles are wrongly considered “emission-free” vehicles, also from the perspective of legislation. However, if a fair approach were to be used, total actual impact on the production of CO₂ should be considered, particularly depending on the source of electric power, production and later disposal of batteries, etc. 

Despite very rapid evolution, mass adoption of electric vehicles in the near future is not realistic, particularly due to insufficient infrastructure. Thus, combustion engines keep their essential importance. However, continuous development is needed if they are to maintain their competitive edge. Searching for new solutions to increase the engine’s total efficiency or adoption of alternative low-carbon fuels is inevitable. Another path to pursue is the application of hybrid concepts where a combustion engine is combined with an electromotor in various arrangements to allow for its operation in modes with higher efficiency. 

Significant reduction of fuel consumption and emissions formation of combustion engines is one of the major research tasks of the Vehicle Center of Sustainable Mobility of the Faculty of Mechanical Engineering of CTU, Prague (CVUM). This mission is also associated with an investigation of options for efficient combustion of alternative fuels. A thesis from our research member ing. Zbyněk Syrovátka named „Efficient combustion of alternative fuels in piston combustion engines“ aims at fulfilling the given targets by means of a piston combustion engine using an advanced combustion system to reach maximum efficiency and minimum emissions.

Current progress in research of fuels and ignition systems

Natural gas seems to be a very suitable fuel from the perspective of its characteristics and availability of resources. The use of natural gas, mostly consisting of methane (CH₄), can reduce CO₂ production by approximately 25%, compared against conventional fuels if preserving the same engine power. Another benefit is its high resistance to detonation combustion which allows for preserving or increasing the engine compression ratio to improve its efficiency, eventually resulting in further reduction of fuel consumption or CO₂ production. 

Hydrogen (H₂) is recognized as the environmentally cleanest fuel, as in ideal cases its combustion only creates water. In the real world, where air is used as oxidizer, nitrogen oxides form as well; however, today’s technology can deal with such issue. Hydrogen is also used in fuel cells, directly transforming chemical energy to electric energy with a much higher efficiency. However, this process requires high-purity hydrogen, requiring high amounts of energy to produce it; hence, such production should also be included in the total impacts. Hydrogen combustion in a combustion engine puts no special requirements for the hydrogen in terms of purity. Thus, it is possible to benefit from the use of hydrogen created as a by-product in industrial production, such as in the production of chlorine. The main advantage of hydrogen is rapid combustion and an extensive flammable range, positively reflected also in its mixtures with other fuels. 

The combustion process itself deserves a more detailed examination. The main objective of the thesis of ing. Syrovátka is a detailed analysis of the ignition system with a scavenged pre-chamber for the combustion of a lean mixture in a piston combustion engine. The experimental engine was operated using alternative fuels, such as natural gas, propane-butane, hydrogen and a combination of these. Based on the conducted experiments and created simulation models, a detailed description of the ignition system was created. The solution employs 

3D simulation models of CFD (Computational Fluid Dynamics), focusing on the mechanism of mixture creation, quality of scavenging and development of combustion inside the pre-chamber as well as in the main combustion area. Sensitivity studies were used to examine the effect of particular parameters, to eventually optimize the geometrics of the pre-chamber. The selected pre-chamber variants were experimentally tested in a naturally aspirated gas engine to verify their functionality and life. Data collected from the conducted experiments and results from simulation models were used in further development of the ignition system and to calibrate the simulation models. The insights gained led to formulation of principles applicable to the optimal design and control strategy for the ignition system.

Lean mixture combustion 

Stoichiometric combustion of a homogenous mixture is generally known and widely used in spark ignition engines with a functional three-way catalyst. Another possibility is the use of the lean combustion concept. Combustion of a lean mixture decreases the maximum temperature and thus also decreases heat losses in the high-pressure part of the working cycle. Excess air in the lean mixture also increases the ratio of specific thermal capacities and leads to improvement of the engine’s thermal efficiency. Moreover, the lean combustion concept allows for reducing the necessity to choke the mixture flow at low power and thus reducing pump losses. An extremely lean mixture notably reduces NOx emissions due to lower maximum temperature during combustion, which is the main factor influencing their formation. 

Detailed research was conducted by Toyota and many excellent scientists (for example Yamaguchi, Ricardo, Gussak), yet many tasks remain to be solved to have the new engine type comply with the increasingly strict emission standards. For the time being, lean mixture combustion also comes with certain drawbacks in the form of reduced combustion speed, which prolongs combustion time. Also, demands for required igniting energy to reach stable initiation of the ignition and overall burning of the mixture are increased. Otherwise, a significant increase of unburned hydrocarbons occurs, eventually reducing the total efficiency due to unused fuel passing to exhaust. Further, the fact that the lean combustion concept is not entirely compatible with a three-way catalyst is fairly fundamental. Systems now commonly used in the segment of large stationary engines allow for significantly extending the ignition limit of the mixture. For example, these include bi-fuel gas engines with micropilot diesel injection enabling high-energy and multipoint ignition of the mixture. However, the use of liquid fuel also results in the formation of solid particles. 

Promising pre-chamber potential 

Other relatively known ignition systems are pre-chamber applications. Generally speaking, it consists of two separated combustion areas interconnected through one or more openings. Also, the pre-chamber may be equipped with a dedicated fuel intake, allowing for local enrichment of the mixture and scavenging of residual gas from the previous working cycle. Local enrichment is one of the key factors enabling a significantly expanded ignition limit for a lean mixture in the main combustion area. During compression stroke, the mixture inside the pre-chamber is thinned by the mixture flowing from the main combustion area. This mixture is then ignited using a spark and increasing pressure during combustion in turn displaces the contents of the pre-chamber through the interconnecting openings. Owing to increased ignition energy and multipoint ignition of the mixture, a scavenged pre-chamber is capable of extending the lean mixture ignition limit to reach values exceeding excess air coefficient 2 and significantly faster combustion. 

Formerly, pre-chamber ignition systems were mostly used in large stationary engines due to the large space required for their installation. Also, production use in the segment of passenger cars was prevented by insufficiently mapped behaviour in changing modes (rapid changes of revolutions and load), incompatibility with three-way catalysts and other undesirable effects. Nonetheless, today we see intensive work on development of such systems and their use in car engine applications to reduce fuel consumption and emissions, particularly NOx. The pre-chamber ignition system can be used for combustion of gas fuels as well as easily evaporable liquid fuels (such as petrol, etc.). Thus, the promising potential of the ignition system should drive further optimization efforts.

Ignition system with a future 

In his thesis, ing. Syrovátka designed and experimentally tested several prototypes of the pre-chamber ignition system. This created a rather extensive set of measurement data, evidencing the function of this ignition system and serving for calibration of numerical models. Comparison of the parameters of a pre-chamber engine and a traditional spark ignition engine shows that the use of a pre-chamber can result in a notably extended operating range of the engine and faster combustion. Upon partial load on the engine and extremely lean mixture, nitrogen oxides emissions in raw fumes can be reduced below values required by current emission legislation while increasing engine efficiency. Based on experimental and simulation results, a detailed description of the functioning of the pre-chamber system was created along with a number of recommendations regarding its optimal design and control strategy to reach as efficient operation of the engine as possible. The collected data and results related to the operation of a naturally aspired single-cylinder gas engine. The pre-chamber ignition system seems as a very efficient element for the combustion of gas fuels and it certainly is worthwhile to carry on in its further research and development. Also, if the ignition system were to be deployed in combination with the combustion of fuels with added hydrogen, further reductions of total carbon dioxide production would be attained. 

Contact:

Ing. Vojtěch Klír, Ph.D.

Department of Automotive, Combustion Engine and Railway Engineering FME CTU in Prague

E-mail: Vojtech.Klir@fs.cvut.cz

Phone: +420 246 003 709