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How the Soap Bar works

Posted on 8 August 2012 | 0 Comments

Throughout the development of the Scuderi Engine technology, there have been many discoveries and new inventions. A special one is what we affectionately call the “Soap Bar,” a system within the variable valve actuation (VVA) system.

Besides enabling the Scuderi Engine high-speed valves to do what they need to do, the system could also be a key element for various alternative applications, such as:

• In engine start applications;
• In trucks using the engine as an air compressor (therefore doing away with an external compressor);
• An engine brake;
• A temporary boost for acceleration aid (while the turbocharger comes up to speed); and
• As a gas injection device (e.g. natural gas direct injection).

How does it work? The Soap Bar is a retractable metal element disposed between a dwell cam and a rocker within the VVA system for the Scuderi Engine’s outwardly opening crossover valves. Actuation of a crossover valve occurs when the eccentric (or dwell) portion of the cam engages the fully extended metal element, which then pivots the rocker to lift the valve.  The opening timing of the valve is controlled by a cam phaser.  The closing timing of the valve is controlled by retracting the metal element to disengage from the dwell portion of the cam at any time during the valve lift event. 

A key advantage of the metal element is the capability to provide a very stiff connection between the cam and the rocker, due to its compact design, with a comparatively small added mass.  This in turn enables extremely fast valve actuation rates and the extremely short opening periods.

A hydraulic system is utilized to control the retraction and extension of the soap bar.  The compliance that may be introduced into the VVA system from the hydraulics is minimized by positioning the actuator at an almost perpendicular angle to the direction of the main actuation forces, which run generally perpendicular to the cam and rocker surfaces at their point of contact with the soapbar.

The design of this VVA Soap Bar actuation system enables the valve lift event duration to be accurately controlled from cycle to cycle over the entire length of the dwell portion of the cam at any engine speed.  In the particular case of the crossover valves on the Scuderi Engine, the valve event can vary from about 6 to 100 crank angle (CA) degrees from cycle to cycle since the dwell portion of the cam is approximately 100 CA degrees wide.  The control can be performed in open loop or, for enhanced timing accuracy, in closed loop.

Another aspect of the design is the use of a gas spring in place of the more conventional coil springs.  This is necessary to enable the fast actuation rates that are needed to achieve short valve opening events.  Gas springs, although not necessary for lower actuation rates, can advantageously vary their filling pressure during engine operation, therefore reducing parasitic losses when the full extent of the spring force is not needed.  This is particularly useful in loss motion valve trains like the Soap Bar system.

Furthermore, the VVA Soap Bar actuation design may be utilized in other more conventional engine applications as well, by applying it to the actuation of intake or exhaust valves in gasoline and diesel engines.  These valves typically open inwardly. 

The figures below show the soapbar valvetrain applied to an end pivoted follower design and to a center pivoted rocker design.