Ironing out the not so good vibrations
I drive a 1993 Mazda Rustler bakkie which is giving me excellent service.
Occasionally I drive a more modern vehicle, and then I'm struck by how much smoother some modern engines feel. What did manufacturers do to get the engines so smooth? Is it fuel injection that made the difference? - Just Curious
JC, fuel injection can make a contribution to engine smoothness by ensuring the optimal air/fuel ratio in all cylinders.
But its most important benefits are seen in other ways - better idling and performance during engine warm-up, better control over mixture strength, and better reliability (generally) than a carburettor.
The main reasons why the engine on many modern cars feels noticeably smoother , are to be found in improved crankshaft design, the use of balance shafts, advances in engine mountings, and the use of lightweight pistons and low-friction components.
The study of the various types of vibration is too technical to enter into here. We can only discuss in broadest outline how the advances have influence engine smoothness.
- Improvements in crankshaft design centred on the placement of the balance weights on the webs opposite each crankpin.
These are needed to balance the mass of the piston and conrod, attached to the crankpin. For years most in-line four-cylinder engines had only one counterweight for each crankpin. This design is still found on some lower-priced new cars, and it still gives perfectly acceptable service.
But it does have a small problem in that the mass of conrod and piston acts in the plane of the cylinder centre line, while the balance weight is offset from this plane.
This causes the crankshaft to jiggle ever so slightly, contributing to the "buzz" at higher revs. The more modern crankshaft design splits the counterweight into equal halves, one on each of the two webs opposite the crankpin. Their combined effect is to create a weight balance.
People who test a variety of new cars claim they can tell, when driving a car with a four-cylinder engine, which type of crankshaft it has.
I cannot, and, frankly, I couldn't care less. A little bit of vibration has never worried me in the slightest. It's the engine's way of talking to you. Who wants to kiss a statue?
- Balance shafts are used to cancel out secondary vibrations, which result from the fact that a piston travelling downwards from top dead centre reaches maximum acceleration at a different point in time than another piston simultaneously travelling upward from bottom dead centre.
Secondary vibrations become especially troublesome on big in-line four-cylinder engines.
Thus 2-litres was for years regarded as the unofficial displacement limit for in-line four-cylinder engines.
The idea to cancel the secondary vibrations with a pair of eccentric-weighted shafts, rotating in opposite directions and at twice crankshaft revs, originated with the British engineer Frederick Lanchester in 1904. In the modern era the concept was revived by Mitsubishi with their "silent shaft" Astron engines of 1975.
As always, there's a price to be paid for refinement: more complexity, higher production cost, and in this case, a small but unavoidable loss of power (and resulting sacrifice in fuel economy) since some engine power has to be used to drive the balance shafts.
- Engine mountings used to be fairly simple components, consisting basically of two metal brackets bonded to opposite sides of a block of rubber. One of the brackets was attached to the engine, the other to the car's bodywork. The rubber block prevented some of the engine vibrations (and noise) from being transmitted to the bodywork. However, engineers decided they could improve on this crude design.
So they came up with "hydraulic" mountings which have hollow chambers filled with glycol or hydraulic fluid. These mountings act like a jelly-filled doughnut to absorb vibrations.
Some of the hydraulic mountings even have internal valving and a solenoid to change the damping characteristics as the engine speed changes, in order to better tune out vibrations.
On certain late-model vehicles, the onboard computer energises a solenoid to apply engine vacuum to chambers inside the engine mountings. In this way the stiffness of such an "active" engine mounting can change in response to changes in engine speed.
It doesn't stop there either. Recently, mountings appeared that contain a fluid which has iron particles suspended in it.
When a magnetic field is applied to the fluid by a coil, the particles line up, and this has the effect of making the mounting firmer.
So there you have it, JC. Engineers have certainly made significant progress in the field of engine smoothness. Some advances came from improved design, which I admire. But some came from fancy stuff which I view with scepticism. NVH has become one of the buzz (if you'll excuse the pun) words in modern automotive technology. Noise, vibration and harshness have been turned into the demons of car design.
Funny, I can think of worse demons; being overpriced, and being nightmarish to work on, for example. Appreciate your Rustler for what it is - an unpretentious, no-frills, honest-to-goodness, cheap-and-cheerful little buggy that will probably give you yeoman service for years to come.