ACTIVE VALVE TRAIN (AVT™)
Lotus's Active Valve Train (AVT) research system is an electronically controlled, hydraulically operated system that provides the control of individual valve lift profiles by a digital signal processor based controller. It provides our clients with a tool for faster research into advanced combustion performance and fuel economy.
The AVT system was developed to support research into advanced combustion techniques including Low Temperature Combustion, Controlled Auto Ignition and Homogeneous Charge Compression Ignition (HCCI). Where there is a need to develop camshaft and valvetrain designs, AVT simulations eliminate the need to change engine hardware thus dramatically speeding up the testing and development of engines and combustion. The Lotus AVT system is used on single cylinder engines in the research department of vehicle manufacturers and universities around the world who benefit from the ability to independently control each valve with cycle to cycle variation and operating speeds up to 8,000 RPM. The system can be adapted to fit on a variety of single cylinder research engines, including Lotus own single cylinder research engines.
In addition to supplying AVT, we offer support and training as well as in-house research for clients using our own system.
The Lotus AVT system is an electronically controlled, hydraulically operated system that allows the control of individual valve lift profiles by a digital signal processor based controller. AVT allows the user, via a PC to specify the desired valve lift profile per valve. The user can edit and save pre-defined valve-lift profiles from a selection of profiles stored on hard disk, or generate, edit and save user defined profiles. Thus, it provides a flexible research and development tool with which to control the lift and timing of the valves, whilst still allowing conventional valve acceleration and velocity characteristics to be obtained.
Conventional camshafts are replaced with hydraulically operated actuators and proportional electro-hydraulic servo valves. A double-acting hydraulic actuator is fixed to the cylinder head and co-axially aligned with the engine valve. The actuators hydraulic piston is directly attached to the engine valve and a displacement transducer is connected to the top of the piston enabling the position of the valve to be accurately monitored.
A hydraulic power pack supplies pressurised hydraulic fluid to the electro-hydraulic servo valves which proportionally control the flow of hydraulic fluid to either the top or bottom of the actuator piston. This enables control of the velocity, timing and lift displacement of the engine valves for each individual crankshaft degree of rotation.
The control parameters of the system are accessed via a graphical user interface program on a PC which communicates with the AVT Controller.
The system employs a phase advanced PID controller to detect and correct automatically the actual measured displacement towards achieving the desired displacement. The system operation is monitored continuously to identify potential malfunctions such as hydraulic pressure loss, crank or valve position signal loss, and potential valve to piston or valve to valve clash scenarios. A desired valve lift profile can be defined via the Lotus engine valve profile generator software (EVPG) which is communicated to the AVT Controller via a PC based user interface. It is possible to vary the opening and closing points of a profile individually in one-degree increments, and the maximum lift is adjustable to any valid figure between 0.01 mm and the maximum travel of the actuator. Each engine valve is controlled individually.
A library of many thousands of user specified lift profiles can be established by the user for use in their engine test programs. From this library the user can select up to 128 lift profiles, that can be applied to any engine valve, for use in any given engine test session and each of these profiles can also be phase advanced or retarded +/- 128 degrees. If required, whilst the engine is running, a lift profile may be changed or modified every engine cycle and can contain as many lift events as are physically possible to implement. For example an engine can be configured to swap between two and four stroke combustion on alternate cycles if required. This allows the user to change a profile quickly and to observe its effects on exhaust emissions and engine performance, using standard test cell monitoring equipment. For a four stroke cycle a lift demand is specified as 720 lift points. ie. one lift increment for every degree of crankshaft rotation.
A crankshaft encoder provides the crank position and LVDTs provide the individual engine valve displacements to the AVT controller, these signals are compared to the demanded profile, approximately every 100 microseconds, as part of the feedback control loop and actuator demand signals are sent to the servo valve.