Now lets turn to the blow-by gases that are created into the engine crankcase.
Fig 1: Blow-by Gases
Blow-by occurs when combustion gases get past the piston rings and into the crankcase. A slight amount of blow-by can be expected. This is why crankcases are usually ventilated.
However severe blowby can create real problems:
1.Pistons are overheated and expand. scoring the piston and cylinder wall.
2.Compression is lost & reducing power.
3.Crankcase oil is contaminated causing wear.
Causes of oil consumption and blow-by.
1. Piston rings installed wrongly.
2. Stuck rings or plugged oil ring.
3. Top ring broken or top groove worn.
4. Overall wear in piston, rings, and cylinder wall.
a) Abrasive wear.
b) Scuffing and scoring.
c) Corrosive wear.
5. Physical damage to piston.
Let’s look at each of these causes in detail.
1. Piston Rings Installed Wrongly.
For a good seal in between the piston and cylinder wall, the piston rings must conform to the cylinder wall and have plenty of tension.
If rings are installed upside down, are the wrong type of or size, or are stretched or even broken by bad installation, more oil and gas vapor get by the piston.
For correct procedures, see ‘Installation Rings on Piston’ later in the chapter.
Fig 2 Stuck Rings that have broken / Fig .2a Plugged Oil Control Ring
2. Stuck Rings or Plugged Oil Ring.
Deposits caused by too much heat, unburnt fuel and excess lubricating oils, collect in the piston ring area. Ring failure usually occurs when these deposits harden and freeze the rings in their grooves.
Deposits in the top ring groove can cause sticking, scuffing, and scoring because they keep out oil and trap metal particles that wear off the piston.
Sludge deposits in the oil control ring can cause it to plug. (Fig 2a).
This means that oil control has been lost.
Other conditions that lead to stuck or plugged rings are.
. Top. groove failure
. Cylinder liner distortion.
. Combustion knock.
. Pre-ignition.
. Overloading.
. Cooling system failure.
. Improper lube oil.
. Cold engine operation(stop & go service)
Top Ring Broken or Top Groove Worn
The top piston ring act as both a compression and final oil control ring.
It must form a seal between the sides and the ring groove and between its face and the cylinder wall.
A poor seal will allow oil to by-pass the ring seal and be lost.
A poor seal will allow blow-by to contaminate the crankcase oil, forming sludge which leads to ring sticking. Clogging and possibly scuffing.
The top ring and grooves wears most since this. area is exposed to the most heat, pressure and abrasives and gets the least lubrication.
Fig 3: How a New Ring and New Groove Affects Oil Control
Fig 3 shows new compression rings in new grooves. The sides of the grooves are flat, parallel and smooth. The ring also has the correct side clearance. Combustion gases in the power stroke force the ring down against the lower side of the groove. At the same time, the gases pass behind the ring and force it out against the cylinder wall. The result is good seal.
Fig 3a: How a New Ring and Worn out Groove Affects Oil Control
Fig 3a. We can see a new ring installed in worn out groove. The groove permits the ring to sag. This causes the upper corner of the ring face to the cylinder wall, resulting in oil being wiped up into the combustion chamber. The flat new ring cannot mate with the worn sides of the ring groove. The result is bad seal at both the face and sides of the ring.
Top ring groove wear result in broken top rings or damaged piston.
The following conditions can cause top groove failure
1. Abrasive entering the engine through through intake system.
2. Combustion knock
3. Pre -Ignition
4. Installation of new ring in worn groove.
5. Assembly of new rings without ridge reaming the cylinder.
6. Use of wrong size piston or ring, result in too much or too little side or end clearance.
Next article we will continue on Main Engine Parts and Parts Function: Part 5