With the urgent need to reduce CO2 globally, and it is essential to reduce the fuel consumption of vehicles by 50% over the next ten years, further improvement in efficiency of engines is required. Meanwhile, the pursuit of driving pleasure is a universal demand for cars. Although the variable compression ratio mechanism has attracted attention as a technology that can achieve both high efficiency and high performance, it has not been put into mass production use due to the complexity of its mechanism and the high level of technical difficulty.
In our company, we selected and has been developing a multilink method that has variable range that can achieve both compression ratio 14 with high efficiency and compression ratio 8 with 100 kW / L as well as affordable package. While this method achieves miniaturization, due to its structure, the bearing load is about twice, so we developed new technologies such as new bearing structure, high strength 16T bolt, and high load compatible actuator, succeeded in developing mass production
Equipped with this variable compression ratio mechanism in 2L inline 4-cylinder turbo engine, it achieved equivalent acceleration performance and 30% improvement in fuel economy against 3.5L engine.
2. Technical Contents
A variable compression ratio mechanism has been reported from several research institutes including our company. In order to achieve the ultimate high efficiency and high performance, we selected a multilink method that can obtain a wide range of variable compression ratio and is advantageous for practical application. As a result, while satisfying the conventional engine size (package, weight), it meets important requirements for mass production practical application that it can be established with basic engine parts (bearings, lubrication elements) as well as wider range( 8 to 14) which can realize high performance and high efficiency.
(1) Configuration of multi-link crankshaft rotation mechanism
Similar to the conventional system, this unique mechanism still has the piston and crankshaft, but they are connected in tandem by two links, namely, upper link (U-link) and lower link (L-link), instead of by a conventional connecting rod. One end of the L-link is connected to control link (C-link) and control shaft. The rotational orientation of the eccentric control shaft is controlled by an electric actuator via actuator link (A-link) (Fig1).
(2) Operating principle of VCR mechanism
When the eccentric control shaft, i.e., the swivel point of the C-link moves downward, the L-link turns in the clockwise direction around the crank pin. The included angle of the U-link and the L-link increases at top dead center and the U-link and the piston move upward, enabling the mechanical compression ratio of the engine to be raised. Conversely, when the rotational orientation of control shaft is changed in the counterclockwise direction, the mechanical compression ratio can be lowered. In the case of an inline multi-cylinder engine, one control shaft is shared by all the cylinders. Accordingly, the mechanical compression ratio in all the cylinders can be changed simultaneously by the change in the rotational orientation of the single control shaft(Fig2).
(3) Electric actuator for compression ratio control
A Brushless DC motor is connected and rotate A shaft via no backlash Harmonic drive reduction gear.
This reduction gear system can also realize to handle big input Engine torque with a reasonable motor size and satisfy necessary operating speed. A precision angle sensor is installed on the front side of this shaft to monitor the precise angle for realizing precise compression ratio control with a high precision dedicated electric control unit (Fig 3).
(4)Multilink Piston motion
In case of multi link crankshaft rotational system, it is possible to design piston stroke by link geometry. A normal conventional engine’s stroke speed is fast around top dead center, and bottom is slow due to normal connecting rod rotating crankshaft, it means piston inertia force has 2nd order and then in case of inline 4, such 2nd order inertia is overlapped with whole 4 cylinders, so it causes engine vibration.
But in case of this multi link, piston stroke is created as almost sine wave, simple harmonic motion, it means there is no 2nd order inertia, Therefore this engine does not need 2nd order balancer with realization of low vibration(Fig 4).
(5)Engine performance improvement on Variable compression engine
Equipped with this variable compression ratio mechanism in 2L inline 4-cylinder turbo engine achieved a large torque improvement against 3.5L engine and existing 2L turbo. In addition, the new premium SUV vehicle equipped with this engine was able to achieve a remarkable level in both vehicle fuel efficiency and power performance(Fig 5).
By combining the world’s first mass-production VCR mechanism with various technologies, it has realized a dramatic improvement in fuel economy performance while improving the power performance of a downsized turbo engine. We are confident that this engine can contribute to the sustainable society by playing an important role in reducing the environmental impact, without sacrificing driving pleasure.
Fig 1 Multi-link crankshaft rotation mechanism
Fig 2 Operating principle of VCR mechanism
Fig 3 Electric actuator for compression ratio control
Fig４Piston motion and vibration effect
Fig. 5 Outstanding Power and Fuel economy
*1 Member，Nissan Motor Co., Ltd. Powertrain and EV Engineering Division（〒243-0192 Okatsukoku 560-2, Atsugi-shi, Kanagawa）
*2 Nissan Motor Co., Ltd. Powertrain and EV Engineering Division（〒243-0192 Okatsukoku 560-2, Atsugi-shi, Kanagawa）