Toyota implements Atkinson cycle in non-hybrid ESTEC engine; up to 38% thermal efficiency and improved fuel economy
|Performance of the new ESTEC engine (1NR-FKE) compared to the current 1NR-FE engine applied in the Yaris. Yamada et al. Click to enlarge.|
Engineers at Toyota have developed an approach to applying the Atkinson cycle—used in the engines in Toyota hybrids since 1997—for engines in conventional, non-hybrid vehicles. The Atkinson cycle with a high compression ratio is a common approach that hybrid vehicle engines use to enhance thermal efficiency. However, the drawback of the high compression ratio is a reduction of engine torque; in a hybrid, the motor torque compensates for this reduction in engine torque. Thermal efficiency at low load areas is relatively more important with conventional engines than with hybrid engines and how to overcome these issues is significantly important with conventional engines, the Toyota developers observed.
The result of their work, presented in papers this spring at the SAE 2014 World Congress, the Vienna Motor Symposium, and the JSAE Annual Congress, is the new 1.3-liter ESTEC (Economy with Superior Thermal Efficient Combustion) in-line 4-cylinder Gasoline Engine (1NR-FKE). The ESTEC 1NR-FKE achieves 73 kW of output (3 kW higher than the 1NR-FE used in Toyota A and B segment vehicles such as Yaris, iQ, etc.) with thermal efficiency of up to 38%—equivalent to the engines in hybrids. Further, at low loads, the ESTEC engine improves fuel consumption by 11% in the JC08 mode.
Improving the vehicle fuel economy is a must due to the climate change and energy issues. Enhancing the engine’s thermal efficiency contributes to lowering the vehicle fuel economy, manufactures, suppliers, and most research institutions are making strong efforts to improve this. … Engine thermal efficiency for conventional vehicles is now around 36% [and] the engine thermal efficiency for HVs is raised to more than 38%. In regards to the engine technologies for HVs, the Atkinson cycle, cooled EGR, electric water pump and low friction technology play an important role for enhancing the engine thermal efficiency.
In the future, it is expected that technologies used for HVs will be applied to the conventional vehicles and the engine thermal efficiency of both engines will be raised to more than 40%. Since these technologies also improve the engine thermal efficiency at low load areas, it is being considered whether these technologies will overcome the weakness of NA [naturally aspirated] gasoline engines. As for the future direction, large amounts of cooled EGR and lean burn technologies are essential to achieve over 40% engine thermal efficiency. This means that the role of combustion gets more important for the engine development. In addition, the improvement of fundamental technologies such as low friction and valve train system need to be looked at.
The engines with technologies contributing to lowering the vehicle fuel economy are described as ESTEC (Economy with Superior Thermal Efficient Combustion) engines from now on.—Yamada et al.
|Trend of engine efficiency. Toyota indicated the placement of the new ESTEC engine. Yamada et al. Click to enlarge.||Future directions in engine efficiency. Yamada et al. Click to enlarge.|
Basic components of the ESTEC. The ESTEC engine combines the Atkinson combustion cycle with a geometric compression ratio of 13.5:1 and water-cooled EGR. (The conventional 1NR-FE has a compression ratio of 11.5:1 and internal EGR.) An electrical Variable Valve Timing system (VVT-iE) is a key element for implementing the Atkinson cycle. VVT-iE enables quick and precise control of the phasing of the intake valves, and avoids restrictions that might result from oil temperature and pressure variations in cold starts.
The efficient EGR cooler uses a quick response EGR valve. Additionally, the intake manifold, EGR cooler and EGR valve are directly connected to reduce the chance of condensation from the cooler.
An intake port with a high tumble and flow volume enables rapid combustion and helps reduce knock. To meet both performance and fuel consumption requirements, a 4-2-1 pipe exhaust manifold has been designed to reduce residual exhaust gas in combustion.
Friction reduction technologies also play a role.
Performance recovery. Increasing the CR to 13.5:1 dropped torque from 104 N·m to 96 N·m. To recover the torque, Toyota used a combination of a modified exhaust manifold shape to reduce residual gas to compensate for the increase of gas temperatures with the CR increase; optimized the temperature on the cylinder surface through the use of new water jackets; and optimized injection timing. The combination of these (with the modified exhaust manifold producing the bulk of the recovery), brought torque up to 105 N·m.
At low load, cooled-EGR results in excessive torque fluctuation. To address this, at low load, Exhaust VVT is advanced and internal EGR is used. At the middle to high load areas, Exhaust VVT is retarded and the EGR valve step is advanced.
While cooling works as a countermeasure against torque reduction with engines with high compression ratios, improved cooling can also have a negative impact on fuel consumption because of the increase of friction and cooling losses. The new water jacket spacer with Expad controls temperature on the cylinder surface. (Earlier post.) The temperature of the middle section warms up more quickly, while the top and bottom nearly keep the same temperature. With the same water temperature, the Expad increases engine torque because of less friction loss. Ignition retarding is also minimized due to maintaining the temperature of the top section.
Yamada, T., Adachi, S., Nakata, K., Kurauchi, T. et al. “Economy with Superior Thermal Efficient Combustion (ESTEC),” SAE Technical Paper 2014-01-1192 doi: 10.4271/2014-01-1192