Variable Valve Actuation

Variable valve actuation can be used to positively influence the desired quantities for the combustion engine such as specific consumption, emission behavior, torque, and maximum output. Depending on their physical functional principle, variable valve actuation systems are divided into systems that are mechanically, hydraulically, electrically, and pneumatically actuated. Numerous such systems are known, and there is extensive research on both simple systems in which the control time can be varied between two positions and on more complex systems in which even the engine load can be controlled by variable control times. Figure 10-61 shows a detailed categorization of variable valve actuation. This categorization starts with the component of the camshaft.

Fig. 10-61. Categories of variable valve control

The camshaft criterion is the first of three selected categorization levels. Systems, whose energy is provided for valve actuation without a camshaft, are categorized according to their physical functional principle. This accordingly yields electrically, pneumatically, hydraulically, and mechanically actuated systems. With systems that use a camshaft for control, a distinction is drawn between the use of conventional and special camshafts. Those camshafts are termed conventional that have a conventional cam geometry, use common materials, and are created using familiar manufacturing procedures. The second categorization level deals with the site where variability takes effect. The third categorization level that describes the operational and functional principle of variable valve actuation is divided into 17 groups.

In this section, we describe only the individual systems. The series-connected systems are of particular interest. In the categorization in Fig. 10-61, the groups that use series solutions have a gray background.

The numerous types of variable valve actuation make it difficult for a developer to select a suitable type of control for his application. Such a wide variety of systems is used for cylinder heads that substantial adaptations are required when variable valve control is used. A new cylinder head generation usually has to be developed for a system to be used in stock engines. Usually a more complex design is required for variable control times in contrast to conventional engines, and this is expressed in higher costs.

In the future, the option of using variable valve actuation to control engine load will gain in importance. A basic goal of varying the valve lifting curves is to lower charge cycle loss under partial loads and, hence, reduce fuel consumption. The goal of many developmental activities is to dispense with the throttle valve in spark-ignition engines to control load solely by varying valve lifting. In comparison to pure throttle control (TC) with conventional throttle valves, Fig. 10-62 shows four load control methods that vary intake valve lifting.

Fig. 10-62. Possibilities of adjusting the valve lifting curves with variable valve actuation

The load control method “early inlet closure” (EIC) limits the amount of fresh gas by early closure of the intake valve after charging based on the set load. When the engine idles, the intake valve opening time corresponds approximately to a 60° crankshaft angle. With the control mode “late inlet closure” (LIC), the part of the charge that is not needed for the specified output is expelled from the cylinder.

This charge quantity passes through the throttle site of the valves twice with the corresponding loss. When the load is controlled using the “late inlet open” (LIO) method, the intake valve is opened only when the remaining opening time corresponds to the required amount of inflowing mixture. At the start of induction, a strong vacuum is in the cylinder that promotes mixture through turbulence. The cylinder charge is influenced by the control mode “variable maximum intake valve stroke” (VMI) by reducing the valve stroke with equivalent opening angles. Instead of the throttle valve, the valve acts as a throttle site that does not reduce the amount of charge cycle work. The valve friction can, however, be lowered, since the valve springs are only partially compressed.

The effects of the parameters on the valve lifting curve are familiar. An ideal valve gear is one that allows the valve lifting curves to be changed as freely as possible. It also makes sense to combine different load control procedures. Depending on the system, however, only a limited degree of freedom can be attained using the different types of variable valve actuation. In addition, a substantial amount of system engineering is required for valve actuation to approach the desired complete variability. When systems are used that turn the camshafts relative to the crankshaft position, the attainable improvements to the engine are substantial. These systems are widely used in stock engines, and we discuss them in great detail in the next chapter.

At this point, we can guess the degree to which variable valve actuation can improve consumption or emissions. In the professional literature such as Ref. [1], we find that improvements to consumption average between 5% and 15% within some engine mapping ranges. Frequently, however, the engines in the literature are optimized in other ways in addition to variable valve control so that it is difficult to directly identify the specific influence from variable valve actuation.

In comparison to spark-ignition engines, the potential improvement to diesel engines from variable valve actuation is limited. Relatively few investigations have been made into this.