Wood Head Cam: Single or Double

Distribution of 4-stroke engines Part 2

Last week we saw valve control mechanisms and their evolution towards ever more efficient systems. Now let's take a look at the Dual ACT, which is currently the quintessential valve engine.

Sus to intermediaries ...

Despite the appearance of an overhead camshaft, there are still slopes for valve control, which is not optimal. By placing the 2 camshafts above the valves, they can work with little or no intermediaries. An idea presented at the very beginning of the 20th century, over 100 years ago. A term that translates to the DOHC acronym in English as "Dual Overhead Camshaft".

Signature: On a dual ACT engine, cams operate the valves using tappets without the use of dump trucks.

There are pushers ...

However, the absence of an intermediate piece is not complete, as it is important to adjust the valve clearance (see frame). Therefore, pushers with thick plates were inserted to adjust the clearance. But the more power we want, the more and therefore the faster the occurrence of the camshaft. Part that displaces the cam / thrust point of contact. And the faster you go, the more this movement, so the larger the diameter of the push rod should be. As a result, it becomes heavy !!! Hell, this is exactly what we wanted to avoid by eliminating the rocker. We walk in circles.

Adjustment tablet

The adjustment plate comes out onto the black handle (at the end of the screwdriver). It can also be implanted underneath it, then it is lighter, but the camshaft must be removed to replace it, making it difficult to adjust.

Did you say linget?

Therefore, the end solution is to use tiny, rounded levers that reinforce valve displacement without having to tilt heavily. Thanks to the rounded contact surface, the movement of the contact point is reduced, which minimizes parts and gains weight. Here is the top of the top, which can be found on F1, on GP bikes and on top performing production bikes (like the BMW S 1000 RR) ...

Situated between the camshaft and valves, the lingivators eliminate the push rod and save valuable grams for distributing high-performance engines.

What's next?

Can you do better than the double ACT system? Yes and no, because today all four high-performance times use this technology. However, if the ACTs are not removed, the springs that make up the Achilles' heel of the mechanism are removed. To see your systems in action, you still need to look to the GP motorcycle, formula one ... or the road! Indeed, to get around the valve panic mentioned last month, the springs are being replaced with either mechanical rockers, as Ducati does with its Desmo, or pneumatic return systems. A kind of version of the Fournalès suspension applied to the engine. No more spring breakage, no more panic, less weight and ultimately more productivity. Dedicated to very high speeds (17/20 rpm). However, it would also support the very “harsh” cam laws operating in lower modes.

legend: The ultimate evolution in distribution: pneumatic recall. It replaces the mechanical spring with a cylinder filled with pressurized air.

Box: Why adjust valve clearance?

Over time, the impact of the valve against the seat eventually leads to a settlement. This leads to a gradual lowering of the valve into the cylinder head. In fact, the stem is lifted and the initial gap is reduced until it disappears completely. As a result, the valve, which expands with heat, is constantly pressed against the camshaft and no longer closes the air duct completely tightly. Under these conditions, the mixture escapes during combustion, burning the seat, which wears out very quickly and becomes even less waterproof ... In addition to the valve no longer landing on the seat, there is no more contact with the outside world to evacuate calories. So it gets even hotter. Engine performance deteriorates, consumption and pollution increase at the same time. Cold starts also get very difficult. At the same time, the constant friction of the tappets on the camshaft causes wear on the distribution, which will eventually bond. Then it is necessary to replace the pushers and the camshaft .... It's best to adjust the play on the valves before the trouble starts!