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10/01/2006

  2.3 Engine Pt 3

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2.3 Engine Pt 2
2.3 Engine Pt 3

 
So we have established, then, that the larger the capacity of the four-cylinder IL engine, the greater the Secondary vibration and the less the ability of flexible engine mountings to suppress it.

The greater weight and complexity of a six cylinder engine, especially in the lower capacities, militates against using it simply for its refinement - what the vehicle manufacturers need is a four cylinder engine which is light, powerful, economical - and very smooth.

Oddly enough, the issue of the Secondary imbalance was addressed as long ago as 1911. The Lanchester cars of that year used secondary balance shafts which did the impossible - it exerted a force upward at the right time, then downward, and did nothing in between. The figure below demonstrates the principle of the secondary balancer:

The diagram at A shows the cleverness of the design. Two counter-rotating shafts carry weights and rotate at twice crankshaft speed. They are timed to coincide with two pistons when they are TDC, and balance out the secondary force. In as little as 45' later, B, they are no longer required but they have already turned and in opposing each other the weights cancel out. In C at 90' Crankshaft angle they apply an upward force and then cancel out again in D when their force is no longer required. Spinning in this way in light low-pressure bearings and timed to the crankshaft, the secondary balance shafts were very successful in the Lanchester but were not adopted generally. 

In 1975 Mitsubishi Motors produced their 'Silent Shafts' design and in many ways it is similar to the Lanchester principle except that the two balancing shafts have been set at different centres so that the shafts can also damp out the rolling force produced by the combustion stroke:

The shafts are set with centres equidistant from the crankshaft axis and their centres are 0.7 of the length of the connecting rod. This system was used in the 2.4L Porsche 944 engine in the 1980s and was claimed to reduce the engine noise by 20Db.

Ford has used a Secondary Balancing system, apparently of their own design, in the 2.3DOHC engine. The two shafts are set at the same height but below the crankshaft, in bearings in the sump immersed in the oil. Plastic covers over the counterweights prevent foaming in the oil, and the shafts are driven by helical gears and a simplex chain from the crank sprocket. It is in this way that they have produced a very refined, smooth engine with all the advantages of weight saving, size and simplicity over a 6. The resulting engine is mechanically similar to the DOHC2000 but with a larger bore and with an increased height because of the balancer module.

Used first in the Scorpio, the engine produced enthusiastic reports from the motoring press which generally commented on the smoothness and refinement of the engine. Producing 145bhp and with enhanced torque, it was perhaps the obvious choice for the new Galaxy. In this vehicle the engine mountings and manifolds have been redesigned for a transverse layout.

 

 

 

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