How Do Electrically Controlled Engine Mounts Work?

by Richard Rowe

Simplicity and evolution are typically exclusive concepts; when something evolves to do something better, it rarely gets simpler in the process. Such is the case with electronic or adjustable motor mounts, which offer a whole new level of comfort, versatility and performance compared to any static mount. It might be one more expensive part to break, but the electronic motor mount does work quite well while it works.

Mount Basics

All engines emit a certain amount of vibration; motor mounts serve as a sort of buffer to isolate engine vibration from the chassis. Yes, you could just bolt the engine to the chassis using metal plates (as is often the case in professional racing), but the resultant vibrations would prove more than annoying over the long run. Constant high-frequencies snaking through the chassis can loosen bolts and body panels, damage electrical connections (including the engine's control computer) and shred weatherstripping around doors and trunk lids.

Hard vs. Soft

Motor mounts have traditionally been made of hard rubber in a sandwich-like construction. One of the mount's metal plates bolts to the engine, the other bolts to the chassis and a layer of rubber separates them. This rubber will keep engine vibrations out of the chassis, but they'll also allow the engine to expend some of its energy rocking back and forth instead of accelerating the car. As mentioned above, some performance enthusiasts and racers will replace these rubber mounts with solid mounts made of metal or polyurethane; such motor mounts can significantly sharpen throttle response, but do so at the expense of driver comfort and (to some extent) chassis longevity. On a unit-body car, solid motor mounts can help to drastically increase chassis stiffness and handling precision by making the engine a fully structural member of the chassis.

Hydraulic Adjustable Mounts

Liquid-filled (or, more accurately, silicone-filled) motor mounts have been around for decades, and offer a whole new level of vibration control compared to solid motor mounts. These motor mounts are functionally identical to shock absorbers. They utilize two liquid-filled chambers separated by a membrane with two holes in it. One of the holes is small and the other is very large. Under acceleration, a valve keeps the large hole shut, forcing fluid to flow from one chamber to the other through the small hole. The small hole restricts flow, which makes the mount stiffer. At low rpm and under certain other conditions, the large-hole valve opens and allows fluid to flow more easily, thus making the mount softer.

Control Mechanisms

The simplest way to control such an arrangement is to connect the mount's valve to a vacuum diaphragm. Engine vacuum is high under idle and cruise conditions, but drops when you nail the gas. Engine vacuum pulls the big orifice's valve open to soften the mount, and a lack of vacuum shuts it. Many modern cars utilizing adjustable mounts use some sort of computer control to either modulate vacuum or to control the valve directly with an electronic servo. The computer may opt to leave the mounts permanently soft if it detects an engine misfire, or to leave them hard if the car is in a driver-selectable "sport" setting.

Other Types

Non-hydraulic adjustable mounts use a mechanical system to stiffen the mount. Mechanical systems rely on an eccentric ("cam lobe") inside the mount; this mount can turn either toward the engine or away from it, taking up space inside the mount and reducing its damping effect. Magnetorehological (MR) mounts -- found on some high-end sports cars like the Porsche GT2 -- use a metal-impregnated fluid to control mount stiffness. When subjected to an electromagnetic field, this fluid immediately thickens and increases the mount's firmness. An MR mount or shock absorber can respond to changes in demand within milliseconds, far faster than any vacuum, hydraulic or mechanical system. The computer can even use input from the throttle position sensor to solidify the mounts before the engine has a chance to compress them, which is something that no other system can do.

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