The valve determines the fuel efficiency: specifically targeting fuel efficiency to reduce costs (I)

Variable valve mechanisms have become a key technology for improving fuel efficiency. Previously unattainable functions such as improving combustion in the partial load range and changing the compression ratio are now becoming practical. Diesel engines that have nothing to do with variable valve mechanisms are also using variable valve mechanisms to improve fuel efficiency.

Although the powertrain systems of automobiles, such as hybrid systems, are becoming increasingly electrified, the mainstream technology for improving fuel efficiency is still improving the engine. The key technology that has emerged recently is improving fuel efficiency through variable valve mechanisms.

The main variable valve mechanisms that have been put into use are variable valve timing mechanisms and variable valve lift mechanisms. The variable valve mechanism has expanded its operating range from the initial opening and closing to continuous operation. The variable valve mechanism has been developing in the direction of expanding the degree of control freedom (Figure 1).

Figure 1: The development process of variable valve mechanisms and the changes in their usage methods
Variable valve timing mechanisms (VTC) and variable valve lift mechanisms were originally mainly open-close mechanisms, and their original main purpose was to increase output power and improve exhaust gas purification performance. Recently, the number of continuously variable systems has increased, and their main purpose has become to improve fuel efficiency.

As the control freedom of variable valve mechanisms increases, there are more and more cases in recent engines where fuel efficiency is improved by increasing the Atkinson cycle in which the expansion ratio exceeds the compression ratio, reducing pumping losses, and increasing the combustion speed.

A typical example of a greatly improved control freedom of variable valve mechanisms is the "Uni-Air" (Figure 2, Fiat calls it "MultiAir") developed by the German Schaeffler Group and used by Italy's Fiat on the "Alfa Romeo MiTo 1.4T Sport".

Figure 2: Structure of the hydraulic variable valve mechanism "UniAir" of the German Schaeffler Group
The roller follower converts the cam's driving force into hydraulic pressure to push the valve. By controlling the hydraulic pressure with a solenoid valve, the valve lift can be changed.

This is the world's first hydraulic valve drive system, which is characterized by the ability to control valve lift, valve operating angle and valve timing through a single system. The Schaeffler Group said that this mechanism can improve fuel efficiency by up to 10% and torque in the low-speed zone by up to 15%. In addition, by combining supercharging and reducing engine displacement (so-called miniaturization), fuel efficiency can be improved by up to 25%.

Use oil pressure to push the valve

The working principle of UniAir is as follows. There is a roller follower connected to the cam in the system. When the roller follower is pushed, oil pressure is generated in the oil pump. The oil pressure is transmitted to the oil pressure plunger through the solenoid valve, pushing the intake valve. Since the middle solenoid valve is open to release the oil pressure when it is not powered, the intake valve does not move.

When the power is on, the solenoid valve is closed, the hydraulic oil has nowhere to be released, and the oil pressure of the oil pump is transmitted to the intake valve. By controlling the opening and closing time of the solenoid valve, the opening and closing timing, valve lift and valve working angle of the intake valve can be changed.

Uni-Air has four main working modes as shown below (Figure 3). From this, we can see the actual benefits of using Uni-Air.

The first mode is the mode without operating the solenoid valve (Figure 3 (a)). In this mode, the valve lift and valve operating angle are both at their maximum, the most air is inhaled, and the maximum output power can be obtained.

The second mode is a mode that efficiently generates the necessary torque in the partial load range (Figure 3 (b)). The intake valve opening timing remains unchanged, but the intake air introduced into the engine is controlled by changing the closing time. The intake valve is used to control the intake air volume instead of the throttle valve to reduce pumping losses.

The third mode is similar to the second mode in that the intake valve controls the intake volume, but is suitable for lower load conditions and idling conditions than the second mode (Figure 3 (c)). By delaying the timing of the intake valve opening, the valve opens only after the cylinder becomes negative pressure, so the intake speed is increased, which can improve combustion in the low-load range.

The fourth mode is used in the ultra-low load range with even lower loads. Its characteristic is the "double opening" of closing the intake valve that was once opened and then opening it again (Figure 3 (d)). By introducing intake air at high speed when the valve is opened for the second time, the air flow in the combustion chamber is promoted, thus achieving good combustion even in the range of small intake air volume.

Figure 3: UniAir's valve lift control mode
Four modes are used depending on the engine load status

Specially designed to reduce costs by focusing on fuel efficiency

Although UniAir has a high degree of control freedom, it is undeniably more complex than the previous valve drive mechanism, so the cost will also increase. Although other companies cannot obtain the same degree of freedom as UniAir, they are improving the control freedom by improving the structure of the original variable valve timing mechanism and variable valve lift mechanism.

Hitachi Automotive Systems, which uses both the variable valve lift mechanism "VEL" and the variable valve timing mechanism "VTC", estimates that by combining VEL on the intake valve and VTC on the intake and exhaust valves, fuel efficiency can be improved by about 10% (VTC is the company's name).

[page]

Hitachi Automotive Systems' VEL is currently being used in Nissan Motor's V6-cylinder engine "VQ37VHR". VEL uses a motor to rotate the eccentric shaft to shift the fulcrum of the connecting rod mechanism that drives the valve, allowing the valve lift to change continuously. In order to further popularize this system, Hitachi Automotive Systems is developing a VEL for 4-cylinder engines that is lower in cost than the VEL for V6 engines (Figure 4).

Figure 4: Variable valve lift mechanism "VEL" for 4-cylinder engines
Hitachi Automotive Systems is developing VEL for 4-cylinder engines. In terms of structure, it is not much different from the VEL for V6-cylinder engines currently used, but the speed limit is lowered and the cost is reduced, and the focus is also on work control and fuel efficiency.

According to Hitachi Automotive Systems, VEL for V6 engines is aimed at sports car engines, so it uses performance parameters as high as 7500rpm, which increases the cost. However, if it is used for ordinary engines, it does not need to support such a high speed. If the engine speed is increased to about 6000rpm, the load on the components can be reduced by 40% to 50%, the strength and rigidity of the material can be reduced, and heat treatment can be omitted. This can significantly reduce costs.

On the other hand, the widely used VTC is also being improved. As for the hydraulically driven VTC, which is now the mainstream, a model with a wider working range than the original has been developed. It is called "intermediate locking VTC" (Figure 5). The original VTC is equipped with a mechanism that locks the valve timing at the latest position in the low speed range of 800 to 1000 rpm where sufficient oil pressure cannot be obtained. Therefore, it is required that the engine can be started when the VTC's working timing is at the latest position, which becomes a constraint on expanding the VTC's working range.

Figure 5: The intermediate locking VTC
achieves the Atkinson cycle by expanding the operating range compared to the original VTC. The appearance is the same as the original VTC, and the intermediate locking device has not changed much.

However, recently, vehicle manufacturers have requested that the Atkinson cycle, which increases the expansion ratio over the compression ratio, be used to improve fuel efficiency by delaying the valve closing timing. When the engine is started, if the valve closing timing is delayed in this way, the engine startability will be reduced, especially at low temperatures.

The intermediate locking VTC is equipped with a mechanism to lock the valve timing between the fastest angle and the slowest angle, which can expand the working range without reducing the engine's startability and realize the Atkinson cycle. Component manufacturers other than Hitachi Automotive Systems are also developing this type of VTC. For example, Fuji Heavy Industries uses the intermediate locking VTC (called AVCS by Fuji Heavy Industries) manufactured by Borg Warner in the United States in its newly developed horizontally opposed engine "FB type".

Figure 6: The electric VTC
works faster than the hydraulically driven original VTC and can operate even when the engine is stopped

In addition, Hitachi Automotive Systems predicted the trend after the intermediate lock VTC and developed an electric VTC (Figure 6). The electric VTC uses a motor to change the valve timing instead of oil pressure, and can control the valve timing with high precision even in the low speed range with low oil pressure. In addition, the working speed is also very fast, "about three times that of the oil pressure VTC" (Hitachi Automotive Systems). Therefore, the working range can also be expanded.

Although electric variable valve timing mechanisms are already used by Denso for Toyota's V8 engines, Hitachi's electric VTC improves responsiveness and reduces power consumption. Hitachi plans to adopt electric VTC in all vehicles starting in 2014.

The above-mentioned VEL and electric VTC can also be used in conjunction with an idle system and a hybrid system. In the idle system and the hybrid system, the vehicle repeatedly stops and moves forward, and the engine repeatedly stops and restarts. When restarting the engine as described above, if the electric VTC is used to delay the valve closing timing, the engine starting torque generated by the starter motor can be reduced. It can achieve a quick restart of the engine and reduce the cost of the starter motor. In addition, if the hybrid vehicle uses VEL for cylinder intermittence when decelerating, it can reduce the resistance of the engine and increase the regenerative energy.

Diesel engines also use variable valve lift mechanisms

As seen with UniAir and VEL, variable valve lift mechanisms have been used primarily to reduce pumping losses. Therefore, diesel engines, which already have low pumping losses, have rarely adopted this mechanism.

Against this backdrop, Mazda announced that it will use a variable valve lift mechanism on the exhaust valves of the new generation diesel engine "SKYACTIV-D" currently under development. Mazda has not yet announced the specific parameters of the mechanism used in this engine, but it basically uses a cam switching method. Although it is not the continuously variable type introduced above, the case of a variable valve lift mechanism being used in a diesel engine has attracted much attention.

The company's purpose of using a variable valve lift mechanism in a diesel engine is not to reduce pumping loss but to ensure startability. The biggest feature of SKYACTIV-D is that the compression ratio is reduced to 14, which is the lowest in the world among diesel engines. This can extend the delay time from fuel injection to ignition, so the fuel vaporizes and burns evenly.

This not only reduces soot generation, but also avoids local high-temperature combustion, thereby reducing NOx emissions. In addition, fuel efficiency is also improved because there is no need to delay fuel injection to reduce NOx, and the maximum combustion pressure is reduced, which reduces the weight of moving parts and mechanical losses.

However, if the compression ratio is lowered, the engine startability will be reduced, especially at low temperatures, and the warm-up operation after starting will also be unstable, which is prone to semi-misfire. Therefore, SKYACTIV-D is equipped with a variable valve lift mechanism on the exhaust valve to slightly open the exhaust valve during the intake stroke (Figure 7). By allowing the exhaust gas in the exhaust port to flow back into the cylinder, the intake temperature is increased and the temperature rise during compression is promoted. This improves the stability of ignition. (To be continued, reporter: Yoshiro Tsuruhara)

Figure 7: SKYACTIV-D variable valve lift mechanism
The exhaust valve is opened during the intake stroke to allow exhaust gas to flow back into the cylinder to increase the compression temperature