внесу своё КМК.
облегченный поршень нужен для снижения его кивающего момента относительно оси пальца и соотвественно, трения в верхней части цилиндра. частично это выполнено смещением оси пальца.
однако, сильно облегченные шатун и поршень нарушают динамическую балансировку системы "коленвал+шатуны+поршни". если кто видел, то КВ V-образных движков балансируют с дополнительными тарированными противовесами, имитирующими массы шатунов+поршней. как собираетесь решать вопрос балансировки?
Цитата:
Bobweight
When a crankshaft is balanced, the actual rods and pistons cannot be used in the balancing machine, so they must be simulated. This simulated weight is called the "bobweight". Once the bobweight is calculated, weights are bolted onto the rod journals to simulate the weight of the rods and pistons during the balancing process. Due to the configuration of a "V" type engine, just adding all the weights together does not work.
There are also some dynamic considerations to be made when balancing the crankshaft. Explaining those is beyond the scope of this discussion. If you want to study those topics further, contact a crankshaft balancing machine manufacturer and they can go into greater detail.
To calculate the bobweight of a particular assembly, the following formula and balance card is used:
For example, let's say we are balancing a Chevy 383 with the following component weights:
Piston 416g
Pin 118g
Locks 2g
Rings 35g
Rod big end 458g
Rod smal end 186g
Bearings 46g
The rod weight is seperated into "big end" and "small end". This is necessary because the small end has a reciprocationg (back and forth) motion and the "big end" has a rotating motion. This split weight is figured on a special scale fixture that supports one end of the rod while weighing the other end.
There are several things to note about this calculation. The "oil" value used on the left side of the calculation is an approximation of the weight of residual oil "hanging around" on the assembly. The number used here is a matter of preference. There is no solid "rule of thumb" for this. Eagle uses 5g for small block assemblies and 15g for big block assemblies. Since it is impossible to accurately represent this value, it is just an estimate. The actual amount of oil can change constantly and can even be different from cylinder to cylinder! We have found through experience that the numbers we use estimate this property well.
The second thing to note is the 50% value used for the reciprocating factor. This number deals with the geometry of the engine itself. A 90 degree bank angle "V" engine will use 50% here. A V6 or a narrow or wide bank angle "V" engine will use a different value (again, consult the balancer manufacturer). Some engine builders will perform what is call "underbalancing" or "overbalancing". They will use slightly differnet values here such as 48% or 52%. This is done to help compensate for dynamic effects at extremely high or extremely low rpm operation (again, beyond the scope of this discussion). Eagle uses 50% because this value is required for almost all common street or racing engines.
|