Toyota: targeted cooling of upper cylinder liner effective way to improve anti-knock quality and thermal efficiency
Engineers at Toyota Motor Corporation in Japan have determined that cooling the upper part of the cylinder liner on the exhaust side is an effective way to improve anti-knock quality—opening the way to improved engine thermal efficiency. The team presented a paper on their investigation at the recent SAE 2012 World Congress.
In theory, a higher compression ratio and leaner combustion are effective ways to increase the thermal efficiency of an engine; however, higher compression ratios also lead to a higher knock tendency. Thus, the team of Daishi Takahashi, Koichi Nakata and Yasushi Yoshihara noted, it is important to improve an engine’s anti-knock quality. Typical technology approaches for this are reducing the combustion period by utilizing squish, direct fuel injection, and improving engine cooling to lower the temperature of the air-fuel mixture.
Enhancing the engine cooling is very effective for improving knocking. Consequently, engine cooling technologies have been enhanced to improve anti-knock quality over many years. However, excessive improvement of engine cooling means an unnecessary increase in cooling heat loss. To find a good balance between anti-knock improvement and cooling heat loss reduction, this study investigates the effect of temperature of cylinder-head, cylinder-liner, and piston on both knocking and cooling heat loss.—Takahashi et al.
For the study, the team used computer-aided engineering (CAE) to predict the effects of each part of the engine on engine knocking and cooling heat loss, first calculating the amount of heat energy that the air-fuel mixture receives from the engine cylinder head, liner and piston during the intake stroke. The results showed that the cylinder liner contributes the largest amount of heat energy to the air-fuel mixture, especially on the exhaust side.
They then calculated the amount of heat energy discharged from the combustion gas during the expansion stroke to the head, liner and piston. These results showed that the cylinder liner receives the least amount of energy from the combustion gas.
Overall, the CAE results suggested that cooling the upper part of the cylinder liner on the exhaust side would be an effective way to improve antiknock quality.
The team then used a singe-cylinder engine equipped with a total of 14 independent cooling water paths and about 150 thermocouples to validate the CAE results. Eight of the cooling paths were placed in the cylinder head side, and six cooling paths were placed in the side part of the cylinder liner, on both intake and exhaust sides. Intake and exhaust sides were divided into an upper, middle and lower part. They also switched the material for the cylinder liner from cast iron to aluminum.
Test results showed an agreement with the CAE results, indicating the effectiveness of the targeted cooling of the liner on anti-knock quality.
The current study did not take engine friction into consideration; that effect will be considered in a future study, as well as the application to inline engines.
Daishi Takahashi, Koichi Nakata and Yasushi Yoshihara (2012) Engine Thermal Control for Improving the Engine Thermal Efficiency and Anti-Knocking Quality. (SAE 2012-01-0377) doi: 10.4271/2012-01-0377