Investigation of effect of novel technologies' implementation to future internal combustion engines

Consumption of energy is becoming a crucial issue for the world as the human population rises. In addition to increasing population, the non-stop enhancement of the technology and technology becoming more and more widespread among population globally lead to a higher demand on energy. Although alternatives have emerged and those alternatives are anticipated to be future's main energy sources, contemporary main source of energy in the world is fossil fuels. While internal combustion engines are the dominant propulsion choice all over the world. Since their invention in 1876 by August Otto, reciprocating ICE are continuously evolving and improving in terms of efficiency, power density and emissions. Global politics, economics and environmental concerns have been main the driving factors of the improvement of ICE. Especially since early 1970's, when environmental concerns have started being spoken out louder and governments started legislating emissions, harmful emissions of ICE have been improving in a good way and as regulations constantly evolve and get stricter every year, ICE developers have been keeping forced to decrease harmful ICE emissions even further. Recently, while world is searching for better solutions for vehicle propulsion, electric motors became more and more prominent with their convenience for vehicle propulsion owing to their higher inner efficiencies and perfectly matching torque-speed outputs for land vehicle propulsion. However, since energy densities of modern batteries are way lower than that of fossil fuels, battery technology seems to be the most important bottleneck of electric propulsion. Moreover, unless 100% of the world electricity is generated from renewable sources i.e. wind energy or solar energy, electric vehicles cannot be considered real zero-emission. When a broader look at the use of energy sources are developed, well-to-wheel efficiency and total life cycle effect on environment gains importance. Thus, when propulsion alternatives are compared with a globally pragmatist approach, overall efficiency shall be considered rather than on vehicle efficiency. Although electric propulsion is promising for the future, there's no doubt that internal combustion engines shall be kept improving regardless of the future considering they still have a great potential for enhancement. As a machine that converts chemical energy in fuels into mechanical energy, main goal of an ICE is to do that as efficiently as possible. To oversee the obstacles between state of the art ICE and an ideal ICE, working principles of ICE and mechanisms that decrease their efficiency and lead to harmful emissions shall be understood well. First of all, it is obvious that most of the potential of a conventional ICE is inactive during daily uses, due to the structure of modern ICE that limit the flexible operation. Because of the limited flexibility of modern engines, there is a compromise between performance and fuel economy in ICE. Modern combustion technologies also have several disadvantages. For instance, knock phenomenon limits the efficiency of SI engines. As another example, CI engines suffer high NOx and soot emissions due to their non-premixed nature. Considering these facts, for a more versatile, more efficient and more environment friendly ICE, future ICE shall adapt to new technologies to become a long lasting rival to its alternatives. Future ICE shall be more power dense, more efficient and more environment friendly, and most importantly more flexible at varying conditions. Moreover, novel technologies that haven't made their way to serial production and are being researched, show the great enhancement potential of ICE. In this study, novel technologies that will enhance ICE, and potential effects of adoption of these technologies are investigated. First, principles and boundaries of ICE are stated to understand the theoretical limits and targets. It has been a very straightforward finding that thermal efficiency of ICE is theoretically limited, and with loss mechanisms like heat transfer, pumping losses, combustion inefficiency and friction, efficiency of real ICE is even lower than that of idealized ICE via Otto, Diesel and mixed cycles. Moreover, harmful emissions are another important concern that are considered about ICE. For higher thermal efficiency, ICE shall convert more of fuel energy into useful energy, and while doing this, it also needs to follow a path that will lead to less harmful emissions like soot, NOx, CO and UHC. As an ideal combustion product anything that will contribute to efficiency of ICE will bring concurrent drop in CO2 emissions. To improve modern engines, each undesired mechanism that lead to efficiency drop or harmful emissions shall be understood well. After that, methods to prevent these undesired mechanisms shall be developed. Improvement of efficiency of ICE according to ideal engine cycles require a high compression ratio and fast energy release, however these parameters are limited in real engines due to mechanical strengths and thermal characteristics of engine materials. In SI engines, knock is the most important limitation of compression ratio, therefore limitation of peak efficiency. Nevertheless, for lower harmful emissions, a premixed charge shall be burned with high combustion efficiency at low temperatures. However, premixing leads to knock tendency, and better combustion efficiencies lead to higher temperatures. Another important disadvantage of reciprocating modern ICE is its mechanical construction that prevent flexible operation. For instance, SI engines are manufactured and assembled with lower geometric compression ratios than that of non-premixed CI engines due to knock tendency that particularly increase at high loads. This geometric constraint limit the potential efficiency gain that can be achieved with higher compression ratios with a knock-free combustion at lower loads. There are modern methods that are widely used in engines to enhance them like gasoline direct injection that makes charge stratification possible in SI engines and CI engines have very complex aftertreatment systems to lower tailpipe emissions. Turbocharging is a very widespread method that is used in both SI and CI engines to benefit from waste heat out the exhaust. Exhaust gas recirculation is also a standard application in contemporary ICE to satisfy stringent NOx emission standards. All conflicting parameters mentioned in previous paragraph have led to researches about combining pros of combustion regimes while eliminating their cons simultaneously, and it is also not a secret that for a more flexible operation some mechanical parts within ICE need to be replaced with electrified components. A wide literature search showed that there are a lot of novel technologies that haven't made their way to serial production, but has great potential to make future ICE get closer to its theoretical limits. For instance, low temperature combustion strategies like PCCI, HCCI and RCCI provides concurrent enhancement in thermal efficiency with NOx and soot emissions reduction. There are also new technologies that will facilitate flexible operation of ICE and lead to extinction of compromise between performance and fuel economy. Fully variable valve actuation, variable compression ratio, hybridization, e-boosting and e-turbocharging are some of the mechanisms that will lead to uncompromising enhancement all over the operating range. Moreover, new material technologies like composites and high entropy alloys will also contribute the improvement of future ICE. When findings of the literature research are blended with goals and limits that are set in the previous chapters, it is seen that combination of these methods will increase potential improvements even further. An ideal architecture is forecasted for the ultimate ICE of the future: it is most likely that in the future, ICE will be available to run with fuels with different reactivities i.e. gasoline, diesel or biofuels, available to operate at different combustion regimes like SI, CI, HCCI or RCCI depending on load and engine speed conditions, owing to having gasoline and Diesel fuel injectors and spark plugs in same engine head. FVVA will have a great synergy with LTC strategies in terms of load control and flexible operation. Moreover, with transition to electro-hydro-pneumatically driven valves from mechanical valve trains, flow control will be carried out only by cylinder valves rather than using a throttle, and flow to each cylinder can be controlled independently thanks to FVVA during all combustion regimes. FVVA will also facilitate actuation of CDA without requiring any other component. Besides, by varying CR with either multilink piston rod-crank structure or adjusting effective CR with altering IVC, higher efficiencies at low load conditions and knock free combustion at high load conditions will be ensured during SI regimes. In the next chapter an engine operating with the most potent LTC regime according to literature research and findings achieved by previous tests: RCCI is investigated in a CFD software AVL Fire. An RCCI engine, and the CI engine that this RCCI engine is based, are modeled and investigated in AVL Fire ESE Diesel module. RCCI results are compared with CI in terms of efficiency and emissions. Also potential increase in efficiency by using HEA that have higher strengths at elevated temperatures as engine material is modeled and investigated in simulations by increasing engine wall temperatures. Simulations demonstrated that, RCCI engine provides higher brake thermal efficiencies compared to CI engines and despite slight volumetric efficiency penalty, increased wall temperatures have also led to increase in efficiency in both CI and RCCI engines. Afterwards, these efficiency enhancements are reflected into a base CI engine bsfc map. Different engines are compared in WLTC driving cycles by matching their engine maps with reference vehicles to see effects of RCCI, HEA and CDA on fuel consumption and tank-to-wheel efficiency over this new cycle that reflects real world driving conditions. 1-D WLTC simulations demonstrated that combination of turbocharging, RCCI, HEA and CDA via FVVA provides a tank-to-wheel efficiency increase of 100% over the base naturally aspirated GDI SI engine. It shall be noted that, according to simulation results, while being slightly energy efficient than CI engines, RCCI engines have similar fuel consumption to CI engines. This result is due to usage of nearly 80% gasoline in RCCI combustion and gasoline having lower density leads to usage of higher volume of fuel. Thus, with their disadvantageous CO and UHC emissions, RCCI regime is needed to be investigated further, as it needs more improvements than current findings for user recognition, considering the fact that a vehicle needs to have at least to fuel tanks and two sets of different injectors to operate with RCCI. The dominant and most promising technology according to results are CDA via FVVA with an average tank-to-wheel efficiency gain of 25% on SI, CI and RCCI engines. It is also considered that contribution of FVVA will be higher, since effect of varying valve timings and lifts are not simulated in the scope of this research. There is no doubt that FVVA, VCR and e-boosting will lead to higher efficiency gains with its synergy with all combustion regimes. In the last chapter, comparison of thermal and electric propulsion alternatives is carried out, via a well-to-wheel approach. Energy flow pathways are simulated in a 1-D model, in which efficiency values are either obtained from literature review or results of previous chapter in which effects of different technologies' implementation on fuel consumption and tank-to-wheel efficiencies are modeled in WLTC cycle. According to results, with average well-to-wheel efficiency of 29%, contemporary battery electric vehicles have considerably better well-to-wheel efficiencies than that of contemporary ICE powered vehicles. Moreover, projections show that with improved efficiency parameters, electric vehicles will even be better in terms of efficient use of energy sources. Nevertheless, ICE engine model results have also demonstrated the big enhancement potential of ICE with implementation of novel technologies. To illustrate, RCCI engine with higher wall temperatures thanks HEA and CDA via FVVA can achieve 24% well-to-wheel efficiency. It also shall be noted that contribution of FVVA and other technologies like e-boosting is not entirely modeled in the scope of this study and their contribution will absolutely lead to higher well-to-wheel efficiencies than 24%. To conclude, it is obvious that electric vehicles are and will be better options for vehicle propulsion in terms of efficient use of sources and tailpipe emissions. If they can exceed technological barriers they currently face like battery technology, advantages of this technology will increase further. However, with their great enhancement potential, ICE shall be kept improving regardless of the long-term future of vehicle propulsion, considering the great potential that they were also proven with model results. It shall also be noted that ICE will still be the best option for heavy duty vehicles regardless of conditions. Therefore, any improvement on ICE will be beneficial for both environment and efficient use of energy sources.