Can a three-cylinder engine's vibration problem be solved by a balance shaft alone?

The three-cylinder engine has now become an option or even the only configuration for many small cars, and has already entered the C-class platform. First of all, from an NVH point of view, we don't care how big the vibration source itself is, we care about the frequency. There are many ways to address vibration, roughly divided into three categories: source attenuation, path attenuation and attenuation at the receiver.

To answer this this question, it must first be clarified, compared with the traditional four-cylinder machine, three-cylinder machine vibration problem is difficult in the end? The main excitations of a three-cylinder are: 1.5 order Pitch excitation (firing order dependent); and 1st order roll or Yaw excitation. That's right, there's more than one 1st order excitation, and that's the heart of the triple-cylinder problem!

Let's talk about this 1st order stuff. Let's take an idle example, condition: engine 750 RPM. 1.5th order: 750/60 x 1.5=19.5Hz; 1st order: 750/60 x 1.0=13.8Hz. 3-cylinder engines exhibit low-frequency (1st order) longitudinal and/or side-to-side excitation, which is extremely problematic for modal configurations. The susceptibility to **** vibration with other sub-components is the first thing to take into account when designing NVH for a three-cylinder. Why? We do decoupling rate calculations for the entire powertrain suspension system between doing modal matching, calculating the intrinsic frequencies of the six degrees of freedom of the powertrain rigid body.

Generally speaking, the frequencies of the yaw and roll modes will reach about 13Hz, and if the rotational speed is a little low, this 1st order vibration will cause *** vibration in the yaw and roll direction of the powertrain at idling speed, resulting in outstanding NVH problems. So how to solve it? Well, add balance shafts, there are add one, there are home two.

In short, the balance shaft only solves the first-order excitation problem of the engine! Note that if there is only a counterweight block and no balance shaft, the first order excitation can be altered but not eliminated, it will just shift between roll and yaw! However, balance shafts increase fuel consumption due to weight and friction, . The need for mounting space and increased cost is not something that low-end cars generally do, so go for suspension, which of course can be worked around.

In general, the three-cylinder machine frequency avoidance is difficult, resulting in large idle vibration, easy to use in the user's common engine speed range and important components modal **** vibration, manifested in high-grade position when accelerating large loads steering wheel vibration.