What really happens while you are running-in your engine?

What I'm learning this time:
I've heard over and over about hard throttle for short periods produces the best run-in engines. The way lubricants and metals work together is why this works.

When an engine is new, all the metal bits inside it are new. A number of them are designed to touch each other during operation. You should already know a fair bit about which parts are being referred to here.

Now, the oil in the motor is meant to perform lubrication, primarily. But it also has other functions. It carries heat away from hot parts and washes away sludge (which is usually a slurry formed by oil additives designed to keep impurities, like metal shavings afloat till they can be trapped in the filter). Lubrication itself takes two forms.

The first is called hydrodynamic lubrication. This is the situation when a film of lubricant (thickness of the film usually is in inverse proportion to the pressure between the two surfaces. As in, the higher the pressure, the thinner the fil tends to be) separates two surfaces from each other without interrupting their interaction. The piston ring, for instance, will usually ride along the cylinder bore without touching it thanks to a film of oil that seals the two from each other. You can also think of hydrodynamic lubrication as a sort of planned hydroplaning of one metal surface over another, an altogether more desirable hydroplaning than the one we usually fear in the wet.

The other form of lubrication (there are more, but this should be enough for our purposes) is called boundary lubrication. This is the situation when the film becomes too thin to separate the surfaces fully. This can happen due to high loads, highs speed difference between the two surfaces or a change in the character of the lubricant itself. When boundary lubrication happens, contact betwen the two metal bits actually takes place.

The surfaces of even the most finely crafted metals have surface imperfections. Zoomed in close, you will be able to see a line of hills and valleys that are so tiny that to our eyes and fingers, they look like a uniform surface. Lubrication in this case becomes the responsibility of certain compounds in the lubricant whose task it is to have chemical reactions with the surface of the metal and form chemical bonds that create a lubricant surface of sorts and helps preserve the two metal surfaces. Most often, when you start a cold engine, the engine starts with boundary lubrication before the oil pump kicks in proper and full film lubrication, or hydrodynamic lubrication can take place.

We return to our brand new engine, then. When new, the piston ring and bore surfaces, no matter how well finished are all hills and valleys. The reason why most race technicians suggest short, hard pulls on the throttle lies in the lubrication aspect of the engine, and these imperfections.

The objective of the run-in process is to file these hills and valleys down, so that the hills become lower and flatter on top. Effectively, like plateaus. The valleys, which still remain serve to store oil should the film of lubricant give way, while the flat tops of the plateau make the film's job a lot easier. When done well, the plateau-ed surfaces allow better chamber seal and therefore more efficiency and power.

So, how to do this?

Under normal loads, the engine is able to maintain full film lubrication. Which means that the hills on both sides never actually touch. What is needed is to break the film lubrication up. The easiest way is to load the engine. Which is why, the suggestion is hard throttle for short periods. The short periods are because metal-metal contact generates heat. The short and periodical nature of throttle application gives the lubricants (and any other cooling systems available) to cool down the agitated bits and prepare the motor for another set of pulls.

Naturally, manufacturers, who have warranty costs to think of won't trust the ability of all and sundry to pull off these hard throttle applications without messing up. And a number of silent, decent-power, reliable engines are far better than a few extremely powerful ones and a vast number of blown ones from the warranty perspective, right?

You must also remember that just like hard throttle pulls help bed in the cylinder area, various other metal surfaces, like within the gearbox also need help to mate.

The other thing in this mix is the kind of oil you are using. Obviously, since our purpose in life is to break the lubrication up, using better oils would be a little counterproductive, right? My understanding now, is that many experienced race techs also suggest older oils (as in less effective, and therefore easier to break up) with oil changes scheduled relatively often rather than long periods of the latest and greatest full-synth oils. The explanation is pretty involved and I will return to it one day. The upshot is that the latest oils are engineered to generate films that are ever harder to break. So while their cooling, cleaning and anti-corrosion properties might be gobsmacking, their contribution to the run-in process is detrimental.

Also, older engines, like RD350s, which were designed to run a little loose should be run-in on mineral oils for best post-run-in performance, although after the engine has bedded-in you can do whatever you think is best oilwise.

Here's the link to the famous mototune run-in secrets page

Related links:

• Helmets: Some nifty MCN vids
• MC Tech: The four-stroke internal combustion engine
• Forewarning: complicated posts in the offing

Note:
The animation is from wikipedia