Piston

Subjects:

  • General
  • Piston bottom
  • Materials
  • piston rings
  • Piston ring lock clearance
  • Piston pin
  • Desaxation of the piston pin
  • Piston deformation
  • Tilting piston
  • Cooling

General:
The pistons make up and down movements in the cylinder. The cylinder is stuck in the engine block and does not move. The piston is constantly moving from the ODP (Lower Dead Center) to the TDC (Top Dead Center) in the cylinder. The combustion takes place at the top of the piston (which is called the piston bottom). As the inlet valves open and the piston moves towards it, a vacuum is created in the inlet section. This vacuum draws air (or a fuel mixture) into the cylinder. In a supercharged engine (by means of a turbo or compressor), the intake air is pushed into the cylinder with a certain overpressure.
De inlet valves close and the piston moves up. The air (or fuel mixture) is compressed (compressed) then at a petrol engine , working online and earning money via candle and at a diesel engine ignited by adding diesel fuel.
As the mixture ignites, the piston is pushed downwards with great force. Then the exhaust valves open and on the upstroke the piston pushes the burnt gases into the exhaust.

Pistons must meet the following characteristics:

  • The lowest possible mass to keep the mass forces in TDC and ODP as low as possible. Small mass forces put less stress on the bearings and allow higher rotational frequencies.
  • Good heat conductivity; the temperature of the piston bottom can exceed 400 degrees Celsius. In order to prevent the temperature of the piston bottom from rising too high, it is constantly cooled with an oil jet against the bottom. The lower thermal load results in less wear and less oil consumption.
  • Sufficient mechanical resistance.
  • Low coefficient of friction.

Piston bottom:
The top of the piston is called the “crown” or the “piston bottom”. Often recesses are ground for the valves in the piston bottom.

With direct injection diesel engines, the piston bottom is often still part of the combustion chamber. A special cavity is then ground in the piston, which serves to swirl the air. The air in that space will make a swirling movement, so that the diesel fuel immediately mixes well with this air during injection.

The image shows a direct injection diesel engine with a pre-swirl chamber in the piston. With an indirectly injected diesel engine, there is a separate pre-swirl chamber in the cylinder head. There is then no question of a combustion chamber in the piston bottom.

Materials:
Pistons are usually made of aluminum or magnesium alloys. Sometimes forged aluminum pistons are used, the piston bottoms are chrome-plated. These are very strong and have a low weight. The advantage is that due to the low weight they also have a lower mechanical load in the cylinder walls (and therefore less wear), plus that they can be used in engines with a lot of power. Due to the specialist production, the price is a lot higher than normal aluminum pistons.

Small grooves have also been made in the side of the piston, similar to the honing grooves in the cylinder wall. These serve to sort of "take along" the oil when moving up and down. If there were no small grooves, the oil could simply move past them and enter the combustion chamber.

Piston Rings:
Piston rings must provide the best possible gas seal between the piston in the cylinder. Leaks along the piston rings cause, among other things:

  • Loss of compression (thereby also loss of power).
  • Oil loss through the combustion chamber.
  • Premature aging and contamination of the oil; because leakage gases get into the oil, those gases can mix with the oil, causing the oil to age.

There is always a layer of oil between the piston ring grooves and the piston rings (see picture below). It is not possible to have only the piston rings take care of the sealing. The oil also plays an important role in this. It goes like this:

  • As the piston rises, the piston rings move to the lower portion of the piston ring groove. (see picture)
  • Oil on the cylinder wall, penetrates between the piston ring and piston ring groove. The piston is thereby pressed against the cylinder wall.

When the oil scraper rings are worn, the oil can get between the cylinder wall and the oil scraper ring, causing it to enter the combustion chamber. The oil is then burned, resulting in blue or black smoke from the exhaust. Blue smoke is from engine oil that enters the exhaust directly, unburned and evaporates. With black smoke, the oil has participated in the combustion process and the burnt oil residues leave the exhaust in the form of (black) soot.

Piston ring lock clearance:
The clearance is the space between the two ends of the piston ring. When the lock clearance is too small, the piston ring has no room to mold to a smaller diameter. This can damage the cylinder wall and break the piston ring. If the lock clearance is too large, there is too much space between the ends; the piston rings do not seal sufficiently and can cause loss of compression or increased oil consumption.

The slot play is measured with a feeler gauge. With the above measurement, the slot play should be between 0,35 and 0,55 mm. The feeler gauge with a thickness of 0,5 mm could be moved through it with some resistance. So the final play is OK. For more information, see the page “piston ring measurementsunder the heading Measuring diagnosis mechanically.

Piston pin:
The piston pin is rotatably attached to the connecting rod. The piston pin is (theoretically) mounted in the center of the piston and is secured with a circlip. In reality, the piston pin is mounted off-center, which improves performance. More information about this can be found below in the next chapter: Desaxation of the gudgeon pin.

Desaxation of the piston pin:
Offset of the gudgeon pin means that the gudgeon pin is not fully centered (as shown in the picture). These pistons must of course also be mounted in a certain direction. The direction is indicated by an arrow, which is marked in the piston bottom. This arrow points to the distribution side.

Positioning the gudgeon pin off-center serves an important purpose; reduce cylinder wall wear and reduce piston noise when changing cylinder walls. When moving the piston upwards, it is pressed against the left side of the cylinder wall and when moving downwards to the right side. With each power stroke, the piston will therefore be struck from the left side with an enormous force against the right side.
Because the piston pin is off-centre, the connecting rod is already upright before TDC. The piston moves to the right side of the cylinder before the power stroke. When the power stroke now takes place, the piston is already in the correct position and can now go straight down in one movement. Due to the off-centre piston pin, the piston is therefore no longer struck against the cylinder wall by the power stroke, so that noise and wear are reduced.

Piston Deformation:
The piston takes on a different shape when the engine is warm than when it is cold. The material expands due to the heat. The piston is constructed in such a way that the expansion only takes place in one direction. Otherwise, the piston could get stuck in the cylinder.

On the far left of the image, the piston can be seen in normal condition. The center plate is of the piston in the cylinder viewed from above when it is at operating temperature. The engine has therefore been on for a while, so that the material of the piston has heated up and expanded. The right picture is of the piston in cold condition. It is now oval in shape. The arrows above and below indicate the difference in size. The piston in the right-hand picture is reinforced in width and deliberately constructed in length so that it has room to expand here. The reason for this is that any material expands when heated. The piston must also be given the space for this.

The side that does not expand, i.e. the left and right sides of the piston in the illustration, is pressed against the cylinder wall during the power stroke. This side absorbs the slide force (see the image in the chapter below “tilting piston”. This is of course constructed in this way, because otherwise the space between the piston and the cylinder wall is too large with this enormous force. engine against the cylinder wall and will therefore have a short life.

Despite this, the sound can be different when the engine is cold than when the engine is warm. When the engine is cold, there is so much more play between piston and cylinder that a slight ticking sound can still be heard. This is not a problem at all, as long as the warm-up phase of the engine runs smoothly. By this I mean that the engine should be warmed up gently (not too high revs and certainly not too much throttle in the low revs). If this does happen, the piston is not yet fully expanded and the oil is not yet at the operating temperature of at least 60 or 80 degrees. The motor will then have a considerably shorter life. The cylinder wall will wear in faster, as will the side of the piston which will wear hard. The sound of the piston can also be reduced by the manufacturer by applying “desaxation”. (See chapter above).

Tilting Piston:
While moving up and down, the piston also moves slightly in the width direction in the cylinder wall. If wear occurs in the cylinder wall due to incorrect use of the engine (think of driving hard / making high revs when the engine is cold), the part of the cylinder wall (marked red in the illustration) can wear hollow. A poor choice of material by the car manufacturer can also play a major role in this (think of certain 1.4 16v engines from the VAG). This means that the width of the cylinder wall increases and therefore the piston gets more freedom of movement as a result of the slide force. In that case, we speak of “tilting pistons”. The illustration shows that the piston is drawn slightly twisted in the cylinder. A slightly exaggerated situation, but this is how the concept of "tilting piston" is clearly visible.

The result of tilting pistons is that the engine makes a lot of ticking noises. For example, it can sometimes almost be compared to the noise that a diesel engine produces. The sound is purely the hitting against the cylinder wall due to the extra space that the piston has in the cylinder. As a result, oil consumption often increases (due to the poor seal) and wear will often also increase. The only thing that can be done about this is to overhaul the engine.

Cooling:
The piston is cooled because engine oil is sprayed against it at the bottom. This can be done with an oil sprayer (see picture below), or via a hole in the connecting rod. This, along with more information on cooling and lubrication, is described on the page lubrication system.

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