What is Clearance Volume?
Clearance volume is a volume between the cylinder head and the piston top when the piston is at top dead center (TDC). It can also be defined as the volume of cylinder that is not swept by the piston. This volume is contained in the cylinder head gasket usually made of copper and the combustion chamber. Each time the piston goes up, it compress the fuel air mixture in the clearance volume before ignition takes place.
Why to Find the Clearance Volume of a Cylinder?
During engine maintenance, we remove the cylinder head for overhauling and inspecting the cylinder liner, piston, lubricating quills, and the cylinder head itself. After overhaul or inspection when we put back on the cylinder head, it is often advisable to change the copper gasket between the cylinder head and the cylinder head. If we change the copper gasket, the clearance volume of a cylinder also changes, and it is the engineer’s duty to ascertain the change in clearance volume in order to ensure that it is still within allowable limits. If the clearance volume of a cylinder is more than the normal limit, then the compression ratio of cylinder will decrease and this will result in power loss. If the clearance volume is less than normal value, then the compression ratio will increase which will cause engine knocking. Therefore it is very important to maintain correct clearance volume to avoid the above problems.
Clearance Volume of a Cylinder with Flat Piston Crown
There are basically two ways to find out the clearance volume of a cylinder. One is a find the clearance volume of a cylinder with a flat piston crown, and another way is to find the clearance volume of a cylinder with a convex or concave piston crown.
In case of flat cylinder head face and flat piston crown, the clearance volume can be calculated from the clearance space height and bore of a cylinder.
- First remove the cylinder head of the cylinder that you have to measure for clearance volume.
- Bring the piston slightly off top dead center by using a turning gear or turning bar.
- Place a soft non-plastic lead ball over the highest point on the piston and ensure that the ball is slightly larger in diameter than the expected clearance of a cylinder. This expected clearance space can be taken from the engine manual to avoid any mistake.
- Also ensure that the ball is not sliding or rolling to the shallow portion in the piston.
- Now put back the cylinder head of a cylinder and torque the cylinder head studs as per the manual.
- Then crank the engine manually by means of a turning bar or turning gear motor so that the piston pass over top dead center pressing the lead ball on the piston crown.
- Remove the cylinder head and measure the thickness of the lead balls. This thickness will give you the clearance space height (H) between the cylinder head and the piston crown at TDC. As we know the bore of the cylinder, the volume of a clearance space can be calculated by the following formula
Clearance volume= (pi/4 )*D2 * H
Where D is the bore of a cylinder
H is a clearance space height (thickness of a cylinder)
Clearance Volume of a Cylinder with Concave or Convex Piston Crown
In the event that the piston crown and the cylinder head are of different shape and with a pre-combustion chamber arrangement, it is not easy to calculate the clearance volume of a cylinder. In this case the piston crown may be either concave or convex shape to increase the turbulence inside the combustion chamber. To calculate the clearance volume of a cylinder with concave or convex piston crown, follow the steps below.
- Keep the cylinder head intact with cylinder jacket and check for proper tightening. If it is not properly tightened, tighten it with a torque wrench as per the instruction manual.
- Open the fuel injector or fuel valve from the cylinder head.
- Bring the piston slightly off top dead center by means of a turning bar or turning gear motor.
- Take a known quantity of thicker oil and pour it in to the cylinder head till the cylinder is full of oil up to the fuel injector hole or any convenient level. Always use thicker oil because use of very thin oil may leak through the piston ring clearance.
- Record the quantity of oil poured in to the cylinder and slowly turn the flywheel so that the piston approaches TDC. Now the oil will start coming out from the cylinder through the injector hole exactly till the time piston reaches TDC.
- Carefully collect the oil without spillage and measure the expelled quantity (L) of oil. The difference of this measured quantity of oil with the initial volume of oil poured will give the clearance volume of a cylinder including the volume of the injector hole. Now the clearance volume of a cylinder can be calculated by subtracting the fuel injector hole volume, which can found mathematically by the diameter and the height of the hole. Thus the clearance volume of a cylinder with an irregular shape of a piston crown and cylinder head is calculated.
You may also be interested in how to find top dead center for timing or other purposes. There are several methods that can be used.