The content of this article is based on the technical assessment report completed by Roush Industries on behalf of CAELP in 2021. It systematically sorts out the technical routes for efficient gasoline engines in 2025 and beyond, revealing the key technologies for improving thermal efficiency and reducing emissions. Many of these technical solutions have been implemented in the current market.
1. Mixed gas dilution technology
The specific heat ratio γ value is increased through EGR or air dilution, thereby increasing the amount of work transferred by the piston during the expansion stroke. The following figure shows the influence of the compression ratio (CR) and the 𝛾 value on the fuel conversion efficiency of the constant volume cycle. The 𝛾 value of the air mixture is approximately 1.4, while the values of the combustion products (carbon dioxide and water vapor) are lower, close to 1.3. The 𝛾 value of the in-cylinder mixture diluted with air is higher compared with the mixture diluted by cooling exhaust gas recirculation (cEGR). This makes the engine with lean combustion (air dilution) more efficient under the same equivalent dilution (i.e., the ratio of fuel to non-fuel gas mixture in the cylinder).
1. Mixed gas dilution technology
The specific heat ratio γ value is increased through EGR or air dilution, thereby increasing the amount of work transferred by the piston during the expansion stroke. The following figure shows the influence of the compression ratio (CR) and the 𝛾 value on the fuel conversion efficiency of the constant volume cycle. The 𝛾 value of the air mixture is approximately 1.4, while the values of the combustion products (carbon dioxide and water vapor) are lower, close to 1.3. The 𝛾 value of the in-cylinder mixture diluted with air is higher compared with the mixture diluted by cooling exhaust gas recirculation (cEGR). This makes the engine with lean combustion (air dilution) more efficient under the same equivalent dilution (i.e., the ratio of fuel to non-fuel gas mixture in the cylinder).
1.1 Non-catalytic Special in-cylinder reforming (D-EGR)
The dedicated EGR system developed by Southwest Research Institute (SWRI). This system generates reformed gas with high concentrations of H₂ and CO by converting one cylinder to an oil-rich combustion mode. These reforming gases are introduced into the intake ports of other cylinders and complete combustion in SI combustion [10]. The SWRI D-EGR demonstration test conducted on the 2.4-liter PFI naturally aspirated engine indicates that fuel economy has improved by more than 10% throughout the engine's operating range.
1.2 Catalytic waste gas recirculation loop reforming
One of the cylinders operates at a lean air-fuel ratio and adopts a secondary fuel injection technology after combustion. The exhaust gas from this cylinder is treated by the catalyst bed and generates hydrogen-rich reforming gas through endothermic reactions. In the test of the 2.0-liter GM Ecotec LNF DI engine, when the rotational speed reached 2000 RPM and the boost value was 4 bar, a hydrogen intake concentration of 5% was achieved. The volume ratio of waste gas recirculation has been raised from less than 25% to over 50%. The efficiency of this engine at the operating point has increased by 8% compared to the benchmark value.
2. Expansion ratio optimization
An engine with a high geometric compression ratio but an effective compression ratio lower than the effective expansion ratio is an effective way to improve engine efficiency. In mass-produced engines, over-expansion cycles are usually achieved by closing the intake valves in advance (EIVC) or delaying the closing of the intake valves. The shorter intake stroke leads to a reduction in the amount of compression drawn in by the engine per cycle, so a larger-displacement engine is needed to maintain the same torque/power level as non-Atkinson/Miller engines. For turbocharged engines, the loss of intake stroke volume can be compensated for by increasing the boost pressure. Engines with turbocharged Miller cycles have a higher expansion ratio and lower exhaust temperatures, thereby reducing the demand for lean combustion.
One challenge faced when adopting EIVC and LIVC strategies is that the turbulence at the end of the compression stroke will weaken. Figure 6 below shows the changes in the turbulent kinetic energy (TKE) within the cylinder when EIVC and LIVC strategies are adopted compared with the reference engine. This reduction in TKE will lead to a decrease in combustion efficiency and combustion instability. In some cases, compared with the benchmark engine, this may even lead to reduced efficiency and increased emissions.
The following figure shows the design optimization plan required for the EA888 third-generation B-type engine (2.0-liter four-cylinder) to maintain the in-cylinder turbulence level of the previous generation non-Miller cycle engine. This engine adopts EIVC technology to achieve the Miller cycle, and it is necessary to maintain the turbulence inside the cylinder and combustion efficiency through engine design optimization.
3. A smaller cylinder bore stroke ratio
The following figure shows the changes in the three main factors that determine the optimal bore stroke ratio of an engine: piston speed, surface area to volume ratio, and pressure drop on both sides of the intake valve. The optimal bore stroke ratio is determined by the following factors:
Average piston speed: The longer the stroke, the higher the average piston speed, while limiting the maximum engine speed. Whether it is turbocharged or naturally aspirated engines, most of them nowadays have not reached the upper limit of the average piston speed that current technology can achieve (about 25 m/s).
Intake valve pressure drop characteristics: Engines with a large bore ratio design can increase the valve size, forming a larger flow passage area (i.e., the flow zone), thereby reducing the pressure drop at both ends of the valve and improving volumetric efficiency. However, at high speeds, the volumetric efficiency of low bore ratio engines will decline, resulting in premature attenuation of torque and power within the engine's speed range.
Heat transfer efficiency: A lower compression ratio (BSR) will reduce the surface area to volume ratio of the combustion chamber (especially near the top dead center of combustion), thereby weakening the heat transfer effect of combustion. As the compression ratio increases, the surface area to volume ratio of the combustion chamber grows, leading to an increase in heat transfer losses and offsetting some of the advantages brought by the efficiency improvement. This effect is particularly significant in Atkinson-Miller cycle engines with extremely high geometric compression ratios.
Detonation resistance caused by flame propagation distance: A smaller cylinder diameter ratio will shorten the flame propagation distance, thereby reducing the heat release time (increasing the proportion of constant volume combustion). The shortening of the combustion time will also reduce detonation (the time for the terminal gas to reach the self-ignition condition is shorter). This makes a higher compression ratio possible.
Turbulence in the cylinder and combustion rate: As the piston speed increases, turbulence in the cylinder intensifies. When the bore and stroke are relatively low, under the same rotational speed and displacement conditions, the piston's rotational speed is actually higher. This enhanced turbulence can accelerate the combustion rate and reduce the tendency of detonation (as the time for the terminal gas to reach the self-ignition condition is shorter). This enables the engine to adopt a higher compression ratio.
4. Thermal management optimization
A typical SI engine generates a large amount of heat during combustion, of which about one-third is transferred to the cylinder wall and another one-third is lost to the coolant. The key technologies for reducing heat transfer loss include:
Increasing the retained lean mixture can lower the combustion temperature, thereby reducing heat transfer losses
Engines designed with a low specific surface area ratio (BSR) can effectively reduce the surface area to volume ratio of the combustion chamber, further minimizing heat transfer losses
Split cooling system - By setting up independent cooling circuits for the cylinder block and cylinder head, the optimal working temperature of the cylinder head and cylinder block can be maintained. Low-temperature cylinder heads can prevent knocking and support high compression ratio operation. High-temperature cylinder walls can reduce heat transfer losses and lower friction. The split cooling system can also accelerate the preheating of the combustion chamber, enhance combustion stability and reduce emissions during cold starts.
The thermal barrier coating, through a composite structure of ceramic coatings (such as YSZ yttria-stabilized zirconia) and metal bonding layers, can reduce the wall temperature of the combustion chamber by 150-300°C and decrease the conduction heat loss through the cylinder block and piston (accounting for 25-30% of the total energy loss).
Summary
The following figure summarizes the influence of different technologies on each link of engine operation. The green part indicates positive effects, while the red part presents negative impacts. For instance, the cooling EGR technology can enhance the specific heat capacity of the mixture in the cylinder, reduce heat transfer losses and thereby improve engine efficiency. However, at the same time, this technology will have an adverse effect on combustion stability and combustion rate. Therefore, combining it with a high-energy ignition system and a low bore ratio engine design will have more advantages. Some weak interactions are not shown in the figure, such as the influence of the compression ratio (under the same cylinder bore ratio) on parameters like heat transfer.
