Why Do Diesel Engines Need A Crankcase Breather?

Put simply, the crankcase breather is a system designed to vent gases out of an engine's crankcase safely. The crankcase is a sealed, lower half of an engine that houses the crankshaft, which is what converts the pistons' vertical motion into rotational power to turn the wheels. During typical use, it's inevitable that small amounts of fuel will slip past the piston rings and into the crankcase — this is called blow-by. These gases carry various combustion byproducts and unburnt fuel, and if they aren't dealt with, pressure builds up within the crankcase. That pressure is then free to force oil past various seals and gaskets, which leads to leaks and contamination, in addition to emission and efficiency issues. 

Older engines would simply dump these gases into the atmosphere, but thanks to legislation brought forward in the 1960s, modern engines instead route those gases back into the intake system to be burned again. The end result is that the engine's internal pressure remains in check, without harmful emissions finding their way out. These systems are necessary for all modern cars, but diesel vehicles use a special type of crankcase ventilation system, and for good reason. Diesel engines have higher compression ratios in order to facilitate combustion without spark plugs, and these higher compression ratios can lead to increased levels of blow-by. Because of this, crankcase ventilation systems are especially vital for these types of vehicles.

There are two primary types of crankcase breather systems: open crankcase ventilation and closed crankcase ventilation. As covered above, older engines will sport an open crankcase ventilation system, in which the unwanted gases are simply let out into the atmosphere. In comparison, modern gas and diesel engines will have a closed system. 

In a closed system, the vapors are dealt with internally, rather than simply being let out into the air. A diesel engine's closed crankcase ventilation valve is commonly referred to as a CCV, and it's quite a complex beast, capable of oil-gas separation. Fine oil mist is caught in a CCV filter and is then free to drain back into the crankcase, maximizing lubrication, while the gases are sent back into the intake system. Gas engines tend to use a variation of the closed system called a positive crankcase ventilation (PCV) system, which features a valve that can adjust airflow based on engine conditions.

Sending those vapors back through the intake system helps sidestep the reliability concerns that arise when gases are kept within the crankcase. However, this process is also what enables modern diesel engines to continue meeting strict emission regulations. 

Crankcase breather systems generally need very little care and attention. However, they can cause grief as cars begin to age, and some diesel specialists suggest filter changes every 50,000 to 60,000 miles. Fortunately, if a crankcase breather is beginning to fail, there will usually be an array of telltale signs. These include blue smoke from the exhaust, an unstable idle, discolored engine oil, a check engine light, unusual noises — like a high-pitched whistle or hissing sound — and oil leaks caused by excessive pressure that damages seals and gaskets. As mentioned, a small amount of blow-by is normal during regular operation, which is why crankcase ventilation systems are necessary in the first place. However, too much blow-by is what can cause many of those issues above, in addition to real performance and efficiency problems.

At the more serious end of the scale, excessive blow-by can even lead to diesel runaway. In this situation, unburned diesel mixed with oil finds its way into the combustion chamber, essentially providing the engine with a continual and uncontrollable source of fuel.