It must always be remembered that vibrations are absorbed by allowing controlled movement of the engine using resilient engine mounts. This can also be used to dampen some of the movement when viscoelastic materials such as rubber are used in the mount. Generally speaking, the more the anti vibration mount lets the engine move the better the vibration isolation. There is a limit to this alternative movement. The selection of an engine anti vibration mount can therefore become a compromise between a number of competing criteria.

Vibrations in engines come predominantly from the movement of pistons and connecting rods up and down (mostly) and the rotation of the crankshaft. There are also vibrations from crankshaft flex and from the combustion process. Generally the movement of pistons and the rotation of the crankshaft have the greatest amplitude and are the lowest frequency. The lower the frequency the harder the vibration is to isolate so we will keep the discussion focussed upon the vibrations from pistons, connecting rods and crankshafts as these are normally the largest and lowest frequency.

Single cylinder engines are more difficult to balance as a single piston and connecting rod make balancing more difficult. So for single cylinder engines the vibrations generated of most concern are RPM/60 hertz (first order).

Twin cylinder in-line engines have both pistons moving up and down in unison and so represent the same case as a single cylinder engine. So for in-line twin cylinder engines the vibrations generated of most concern are again RPM/60 hertz (first order).

Twin cylinder engines can be better balanced for piston movements by using a V-twin arrangement and this is often adopted for medium horsepower engines although the result is an uneven firing pattern. Due to the ability to better balance V-twins they are commonly used for twin cylinder engines.

Three cylinder in-line engines are inherently balanced for the piston movements by the 120 degree arrangement around the crankshaft. In lower cost engines there are often end to end vibrations of the crankshaft but these are addressed by the addition of balancer shafts in more expensive engines. First order vibrations are generally not of great concern although the second order end to end vibrations of the crankshaft in lower cost engines may be an issue. Second order vibrations are (RPMx2)/60 Hz.

Four cylinder in-line engines have two pistons moving up and two cylinders moving down and most of the effects of piston movement are balanced out. Due to piston velocities at certain parts of the cycle this cancelling out is not complete and in more expensive engines a balancer shaft can be used to cancel out this second order (RPMx2)/60Hz effect.

In all engines there are vibrations generated from firing. These vibrations increase with torque output (load). In one and two cylinder in-line engines these vibrations are first order and are cumulative to the piston and connecting rod effects. In three and four cylinder in-line engines the firing effects are second order.

So what does it mean in a practical sense when installing a small diesel engine. Firstly, one and two cylinder in-line engines vibrate more noticeably at lower frequencies than the other in-line types. For two cylinder in-line engines the firing frequency also adds to the first order vibrations, especially under higher torque. One and two cylinder in-line engines therefore more frequently require anti vibration mounts with increased rebound capacity. Mackay MD mounts, which are especially designed with increased rebound response, ideally suit these applications. Higher rebound capacity mounts allow for more lifting forces from piston movements hence the engine may move about a small amount more than with other general purpose anti vibration mounts.

For three and four cylinder engines the second order (and higher) vibrations are more noticeable only because the first order vibrations are reduced. These higher frequency vibrations are more easily isolated and the need to accommodate rebound is reduced.

The lower frequency vibrations are more “difficult” to isolate as they require an anti vibration mount that has more flexibility. For example, a motor running at 1500RPM has a first order vibration of 1500/60=25Hz and a second order vibration of 50Hz. If an anti vibration mount is selected that has a static deflection of 1mm due to the supported mass then the isolation efficiency will be approximately 60% for the 25Hz vibration and approximately 90% for the 50Hz vibration. To achieve 90% isolation efficiency for the 25% vibration a static deflection of about 4mm would be required.

Often when CBC is approached about engine vibrations in existing installations the culprit is the installation of anti vibration mounts that are too stiff to effectively isolate the first order vibrations. The reasoning for the stiffer mounts being in service may be to stop excessive engine movement and problems with space envelopes for the engines and ancillaries. Unfortunately, movement equals isolation. There are a number of ways in which excessive engine movement can be limited which allow basic general purpose anti vibration mounts to be used but in general these mounting arrangements need to be selected via an appropriate calculation criteria at the initial design stages.

Idle speeds are an area where significant problems with vibration can be encountered. For example for an engine idling at 500rpm a mount static deflection of just over 10mm gives only 50% isolation efficiency for first order vibrations and transmitted vibrations may cause significant issues. Few available anti vibration mounts give this level of static deflection so increasing idle speed marginally to 750rpm allows a mount with around 6mm of static deflection to give around 65% isolation efficiency with first order vibrations.

Fortunately most small engines tend to run at higher constant speeds over 1,500 rpm where the first order vibrations are 25Hz and an anti vibration mount with 3mm static deflection will give around 85% isolation efficiency. Engines which are run at both idle and power settings, such as medium and larger sized marine engines, are either three or more cylinders’ with relatively good first order balance or are twins with a vee arrangement which also have good first order balance and hence the problem with low frequency vibrations is avoided in most cases.

CBC provides a range of anti vibration mounts for a wide variety of applications. Application engineering services are available from our engineering team and are supported by our suppliers Mackay Rubber, Rosta etc.

The CBC supply package for anti vibration mounts and isolators includes ranges from the following manufacturers – Mackay, Paulstra, Fruedenberg, Silent Block, Rosta, Lord and Trelleborg.