The problem with pistons
Piston slap, as previously mentioned is one of the inherent problems that have plagued engine designers for years and causes most of the wear in a cylinder. Increased wear in a cylinder reduces power output and consumes more oil. Before looking at the main issues caused by piston slap we need to understand what it is.
Piston slap is the as it sounds, the piston slapping or making contact with the cylinder wall. In a perfect world the piston is small in diameter than the cylinder and should therefore never maker contact with the cylinder. Unfortunately like most things in life, it isn’t as straight forward as that.
The piston not only moves parallel to the bore but also ‘rattles’ for lack of a better word in the cylinder. The piston travels in 2 directions which we’ll call up and down. On the downward stroke (in both 4 and 2 stroke engines) the piston is forced downwards by the expanding gas above it (the power stroke for 4 stroke engines only) When this happens the piston is tilted due to the outward movement of the connecting rod its attached to. Because the piston is attached to the connecting rod via a wrist pin the piston can slightly tilt around this axis.
Due to the fact that the piston rings hold the top of the piston away from the cylinder (acting like a buffer) its the piston skirt makes contact with cylinder. The image above is a massive exaggeration, but shows how this happens. The black arrows show the crankshaft rotation, the orange arrow represents the pressure forcing the piston downwards. As you can see the piston rings would act as a buffer and the piston skirt to the left hand bottom of the piston makes contact with the cylinder.
The same happens when the piston travels upwards towards the head, however the pressure against the cylinder wall is lower do to there no directly active force being applied to the piston. With this occuring thousands of times a minute at high rpm the cylinder begins to wear in an oval shape as show before. This oval wearing is always parallel with the wrist pins.
This happens on both 2 and 4 stroke engine, however to a lesser degree with 4 stroke engines as they have better oil film layers and have 1/2 as many power strokes compared to a 2 stroke engine at the same rpm.
Once the piston has been at this for years of running the issue becomes worse. The more oval-ing the more the piston can tilt and the greater increase in wear rate. This leads to ill fitting rings and gas blow by becomes a bigger issue. Just to be clear, you might hear people say that their engine is suffering from piston slap and that an ‘slapping’ noise can be heard. All current piston engines suffer from piston slap (apart from big diesel 2 stroke engine but that’s for later) What they mean by this is that the wear in the cylinder has progressed to a point wear you can hear it.
So engineers have spent countless man hours trying to rectify this problem, or should I say reduce the effects of piston slap. There are a few designs that have been floating around for years now with some new ones appearing in the last 10-20 years. I’m not going to go over every single one as some are debatable to whether they actually work. So we’re going to look as some design features that have been used with some success.
Wrist pin location
One of the simplest methods of reducing slap is to shift the wrist pin location off centre. Wrist pins over the years have always been located in the centre of the cylinder for balance considerations and manufacturing simplicity.
The image above isn’t the easiest to understand or illustrates the offset very well but it does show the tiny amount of offset required to make quite a difference, and the design work required to produce the desired effect.
What this achieves is to change the central pivot location of the piston, this in turn reduces the amount of tilt the piston can achieve on the power stroke (where most of the wear occurs) this does increase the amount of tilt on the upstroke, this however is where less of the wear occurs so its win – kinda win situation. As with most things in engine design, it’s a compromise.
Desaxe Engines
The French solution
Desaxe is French for ‘unbalanced’, the method is becoming more and more common. The Toyota Prius engine has desaxe cylinders. So what the hell is a desaxe cylinder? Normal cylinders, (if I can say that) are inline with the crankshaft pin. This means that the piston, con-rod and crankshaft centrelines are all inline, desaxe cylinders are slightly offset, this has the same benefits as the offset wrist pin without the inbalance the offset wrist pins. Its also recovers a tiny amount of force due to torque transfer, which we’ll cover in the crankshaft section.
The image above shows this offset in relation to the piston etc.
As you can see from the picture above, when the piston is on the down stroke the angle of the con-rod is a lot stepper than it would be if the cylinder and piston were aligned. This reduces the thrust loading on the side of the cylinder.
Asymmetric Pistons
Aftermarket solution
This all sounds good but what about engines that have already been built. Some aftermarket pistons have offset wrist pins, but this isn’t the only improvement that can be made. In conjunction with offsetting the wrist pin the piston skirt itself can be modified to help reduce the rate of wear.
This is achieved by increasing the surface area of the of the piston skirt, this spreads the thrust force against the cylinder wall, reducing the amount of wear compared to symmetrical piston.
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