Free Web Hosting Provider - Web Hosting - E-commerce - High Speed Internet - Free Web Page
Search the Web

CRANKCASE VENTILATION SYSTEM OPERATION

The crankcase ventilation system has two important jobs to do. It must get rid of the fuel and water vapors that collect in the crankcase. It must also remove the "blowby" gases that would otherwise build up considerable pressure in the crankcase and cause oil seals and gaskets to leak.

We have already discussed how the fuel and water vapors condense in the crankcase and form sludge when mixed with the oil. Now let's discuss what "blowby" is and what problem it creates.

As you know, the piston rings and the lubricating oil have the job of sealing the high-pressure gases in the combustion chamber that are present during the compression and power strokes of the piston. The rings cannot form a perfect seal because they must have a gap so they can be installed on the piston and to allow for expansion when they become hot. They must also be free to move in the piston ring grooves. Otherwise, they could not expand out against the cylinder walls. The film of oil on the pistons, cylinder walls, and rings can help to seal the gases in the combustion chamber, but the film is not 100 percent effective as a seal. So part of the high-pressure gases in the combustion chamber escape by the rings and oil film into the crankcase. The gases escaping by the rings and into the crankcase are known as blow by gases.

The gases entering the crankcase as a result of blow by can build up enough pressure to cause oil seals and gaskets to leak if they are not allowed to escape. In addition, these gases contain unburned fuel, carbon, and other byproducts of combustion that will mix with and ruin the oil in the crankcase.

Now let's see how one type of crankcase ventilation system works to get rid of the blow by gases and the water and fuel that get into the crankcase.

 

FIGURE 64. DRAFT TUBE SYSTEM.

 

One of the ventilation systems used on automotive engines is shown. Air is drawn into the crankcase through openings in the valve cover. The air circulates around the crankcase and above the oil in the oil pan. The blow by gases and the fuel and water vapors are mixed with the air. This mixture is drawn out of the crankcase through the road draft tube and discharged into the surrounding air as the car travels down the road.

For this system to work, the air must be moved rapidly through the crankcase and out of the road draft tube. The lower end of the road draft tube is designed to act like a suction pump to get this rapid circulation of air. Notice that the lower end of the road draft tube is cut at an angle.

Have you ever stuck your hand out of the window of a fast moving car? The wind blowing past your hand was a pretty powerful force, wasn't it?

If a vehicle with a road draft tube was moving down the road, the wind would be rushing by the end of the road draft tube at a pretty good clip. With the road draft tube cut at the angle shown, the wind moving by it would create a suction (known as a partial vacuum) at the end of the road draft tube. This will draw the air and vapors out of the crankcase.

This system is known as a nonpositive crankcase ventilation system because it will not work unless the vehicle is moving forward.

A positive crankcase ventilation system operates anytime the engine is running and does not depend on vehicle movement. Notice that the air enters the crankcase through a breather and goes out through a tube leading to the intake manifold. Anytime the engine is running there is a low-pressure area in the intake manifold. The air from the crankcase containing the blowby gases and fuel and water vapors is drawn through the intake manifold to the combustion chamber. There, the combustible gases are burned and discharged through the exhaust system.

 

FIGURE 65. PCV SYSTEM.

 

When the intake manifold vacuum is high, too much of the vapors will be drawn into the manifold and will dilute (thin out) the fuel-air mixture coming from the carburetor if some steps are not taken to prevent it. This would cause a loss of power. To prevent this, a ventilator valve is located in the tube close to the intake manifold.

 

FIGURE 66. PCV VALVE.

 

This valve is normally held open by a spring, but high manifold vacuum will cause the valve to close. When the valve is closed, the amount of air entering the manifold from the crankcase ventilator system is reduced to prevent diluting or cleaning out the fuel-air mixture from the carburetor.

It is very important to remember that neither type of crankcase ventilation system can remove liquids. Gasoline and water in the crankcase cannot be removed unless they are changed from a liquid to a gas. For this reason, the engine must get hot enough to change these liquids into a gas. Engines that are operated without proper warm-up, or are used for short runs only, will soon have a crankcase full of sludge. If the thermostat in the cooling system opens too soon or if it is stuck open, the engine will not get hot enough to vaporize the water and gasoline. To keep the lubricating oil clean, the engine must get hot!

All military vehicles use the positive crankcase ventilation system. Since 1965, federal law requires that all new automotive vehicles manufactured or sold in this country use this system. The positive crankcase ventilation system is part of the emission control system for vehicles which reduces the pollution in the air we breathe.