Preventing roller lifters from rotating
An important element in improving engine performance is the optimisation of the valve lift curve to maximise the mass of trapped charge in the engine. In an engine with an overhead valve (pushrod) valvetrain, the change from flat-faced lifters (also known as flat tappets) to roller lifters has given design and development engineers greater freedom when synthesising new lift profiles. It is true that the fastest NASCAR class, Sprint Cup, has engines that are equipped with flat tappets, but the performance would be even better if roller lifters/tappets were not specifically banned by its governing body.
With flat tappets, the lift velocity is limited by the size of the tappet, which again is limited by NASCAR. A roller lifter allows higher lift velocities, and within a given valve event duration this extra latitude can be used to maximise lift and the area under the lift-angle curve. It is little wonder that roller lifters are almost always used in passenger cars, and the same applies in racing where allowed. Even though roller lifters are more complex and expensive, very few passenger cars have continued to use flat lifters in recent years
One disadvantage with the use of roller lifters though is that, for the roller to be properly aligned with the cam lobe, the lifter must be prevented from rotating within its lifter bore. There are several ways of achieving this.
Chevy’s ‘lifter tray’ solution is a one approach, where a flat on the lifter body locates on a corresponding feature on a plastic lifter tray. One problem sometimes reported with lifter trays though involves oil collecting in the trays, so additional holes are often added to ensure oil cannot pool in the tray.
Another solution, often known as a ‘dog bone’, is where a metallic plate is bolted to the engine block, and is equipped with flats which locate on corresponding flats on the lifter body. It is similar to the lifter tray approach, but the dog bone is a much smaller component than the lifter tray, and suffers none of the oil drain problems reported with trays. In common with the lifter tray solution, the dog bone gives the lowest possible lifter mass, as no reciprocating mass is added.
The lifter bore can be machined to allow an anti-rotation ‘key’ to slide within it. This key is formed as part of the lifter, and this is possibly the most compact solution to the problem. It does add a little mass to the reciprocating follower though, and requires a more complex lifter bore.
Another idea, although much heavier in terms of reciprocating mass, is to link a pair of lifters on one cylinder with a relatively loose articulating link known as a tie bar. The link itself is quite a simple component, although it has the disadvantage of adding to the reciprocating mass of the components on the pushrod side of the rocker. However, only part of the tie bar’s mass is ‘seen’ by each follower, as the mass centroid will move only half of the average lift of the two valves on any cylinder.
Written by Wayne Ward