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Advanced thermodynamic cycles are of significance in achieving greater engine performance and thermal efficiency. The first and second thermodynamic laws provide the potential and limits for engine thermal efficiency. Starting with the universal expression of thermal efficiency, Otto, diesel, and hybrid cycles are discussed to provide insights into maximizing thermal efficiency. Specific heat is also an important parameter responsible for thermal efficiency. Today, most IC engines in vehicles are operating at a four-stroke cycle with spark ignition or compression ignition.
The Otto cycle is often applied in SI engines where fuel and air are mixed in the intake manifold or engine cylinder, and then the premixed mixture is ignited actively by a spark plug . When the stoichiometric mixture is compressed, a fuel resistant to the auto-ignition, such as gasoline, must be used to avoid engine knock. Some drawbacks limiting the thermal efficiency of SI engine are as follows:
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Lower compression ratio (CR).
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Longer combustion evolution.
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Gas exchange losses by throttle valves.
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Lower specific heat ratio.
In general, two effective methods are used to improve thermal efficiency. The first one is increasing the CR through enlarging compression stroke or retarding exhaust valve opening timing. The second way is using lean burning to modify specific heat ratios. Dilution combustion is an efficient technique for overcoming engine knock and reducing heat loss. These obvious advantages have motivated the extensive applications of dilution combustion in IC engines in recent years. Nonetheless, engine knock and combustion instability are not solved and stay at the stage of fundamental studies.
Diesel cycle: Engine combustion with diesel cycle involves complicated, turbulent, and multi-phase processes that take place in high-temperature and -pressure environments. In the intake stroke, only air is introduced into the cylinder, and the reactive fuel is directly injected into the cylinder near the end of the compression stroke to achieve auto-ignition. Thanks to the high compression ratio and the lack of throttle intake loss, diesel engines have a higher thermal efficiency than SI engines . With the fuels tending to spontaneous ignition, diesel engines do not use a spark plug and combustion processes are characterized as diffusion combustion . Despite the advantages in thermal efficiency and reliability, diesel engines are generally associated with the trade-offs between thermal efficiency and pollution emissions, particularly NOx and PM. After-treatment systems, such as Selective Catalytic Reduction (SCR) and Diesel Particulate Filtration (DPF), are often employed in modern diesel engines .
Traditional turbocharged diesel engines are usually integrated with a turbocharger where the brake thermal efficiency, thermodynamic efficiency, and combustion efficiency are close to 44%, 50%, and 99.9%, respectively]. Low-temperature combustion strategies such as homogeneous charge compression ignition (HCCI), reactivity controlled compression ignition (RCCI), and partially premixed combustion (PPC) can deliver higher efficiency than traditional turbocharged engines. However, the LTC concepts are limited by combustion control and load extension, which in turn affects mechanical and gas exchange efficiency. Despite this, low-temperature combustion plays an important role in improving thermal efficiency.
A high-efficiency hybrid thermodynamic cycle can be defined as the configuration integrating various thermodynamic cycles. In this cycle, the air is squeezed into an isolated combustion chamber, enabling true isochoric combustion and lengthy combustion time. Specifically, the compact hybrid thermodynamic cycle adopts rotary pistons and independent revolving combustion chambers. Two pistons spin and reciprocate simultaneously and two roller bearings hold them in place. Intake and compression strokes are carried out by one piston, while exhaust and expansion strokes are done by the other. In contrast to traditional engines, one can expect a decrease in energy losses, moving components, and weight and height. The principal idea is to increase the thermal efficiency of IC engines. The benefits of the hybrid thermal cycle are the possibility of achieving constant combustion volume at higher compression ratios and over-expanding the working fluid. Consequently, more heat energy can be converted into useful work. The main characteristics of the hybrid thermal cycle include .
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Air is compressed to high CRs like those in the diesel cycle.
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Constant-volume combustion and isochoric combustion.
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Expansion volume is greater than compression volume.
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Water is added optionally during combustion and/or expansion.
It is known that around two-thirds of the thermal energy is released into the atmosphere as waste heat, i.e., by coolant, lubricating oil, and exhaust gas, and almost 40% of the heat energy is lost through exhaust gas. Several techniques have been adopted for further benefits of thermal and energy management in terms of thermal efficiencies, such as exhaust heat recovery (e.g., organic Rankine cycle and thermoelectric generator) and adiabatic IC engines.
Exhaust Heat Recovery:
Recovery of exhaust heat (EHR) essentially transforms the exhaust waste heat into some usable energy. The recovered energy can either be used to generate electrical energy for storage in batteries or can be reintroduced into the engine as mechanical energy. Here we will discuss some applications of the EHR which are represented by exhaust gas turbocharging. Organic Rankine cycle, and thermoelectric generation. The BTE of the current road diesel engine at full load is close to 43%, of which 28% of the fuel energy is wasted in the exhaust gas (comprising 4% for pumping losses), and 28% of the fuel energy is dissipated to the cooling medium as heat rejections to the environment (containing 4% for parasitic accessory power and mechanical friction), and 2% for miscellaneous heat loss.
There are strong interactions between exhaust gas energy and heat rejections, largely depending on pumping loss and EGR rate. When the EGR rate increases, exhaust energy becomes less, and the heat rejections become higher. Pumping loss usually is related to EGR rate and air-fuel ratio (or turbine area, turbocharger efficiency, and EGR flow circuit restriction). Such changes in energy distribution affect waste heat recovery strategies. Through the allocation improvement target for each part of energy distribution or each engine subsystem, a roadmap to achieve 50–55% BTE can be planned. The realization of various theoretical and experimental waste heat recovery (WHR) technologies showed that each technology affects the improvement of BSFC and operability of diesel engines under both steady-state and transient operation. In many diesel engine applications such as marine propulsion, transport vehicles, and electricity development, the WHR can effectively be applied .
Organic Rankine Cycle
The exhaust gasses and the cooling liquid waste two-thirds of the fuel energy consumed by IC engines (ICEs). The recovery from waste heat can greatly increase thermal fuel efficiency, reduce engine emissions, and decrease fuel consumption. The most suitable technology for mass processing uses an Organic Rankine cycle (ORC) to extract low-grade waste heat from these systems. It can be considered a promising solution for improving the efficiency of large vehicles such as trucks and buses. The ORC system does not generate steam from water but vaporized an organic fluid characterized by a higher molecular mass than water, resulting in slower turbine rotation, lessened pressure, and no corrosion of metal parts and blades. Moreover, the characteristics of the ORC are simple structure, high reliability, low cost, and easy maintenance, but its efficiency is reduced at higher temperatures (above 723 K) . The efficiency of an ORC varies typically from 10% to 20%; meanwhile, it is an attractive choice for heat recovery between 423~473 K . Leading engine manufacturers recently announced that they have developed an HD diesel engine for trucks with a BTE of 50%, and they plan to increase it to 55% using various Rankine cycle configurations. Therefore, when constructing an ORC, the selection of working fluid and the design of a suitable expander have to be given particular attention.
Few other methods to improve IC Engine performance are :
Impliment a cold air intake.
Ensure good compression
Install larger throttle body injectors.
Upgrade electronic control modules
Use forced induction
Replace exhaust manifold with real headers