Page 9 - June 2020 Newsletter

June Return to
2020 `Triumphs Live On’ Start
Page 8
www.tswf
l.org
Tech Corner-continued

and regulation of vehicular emissions has tightened substantially. Most of today's petrol engines continue to use PCV
systems.

Breather

In order for the PCV system to sweep fumes out of the crankcase, the crankcase must have a source of fresh air. The source
of this fresh air is the "crankcase breather", which is usually ducted from the engine's intake manifold. The breather is
usually provided with baffles and filters to prevent oil mist and vapor from fouling the air filter
.
PCV valve

Intake manifold vacuum is applied to the crankcase via the PCV valve. The airflow through the crankcase and engine
interior sweeps away combustion byproduct gases. This mixture of air and crankcase gases then exits, often via another
simple baffle, screen, or mesh to exclude oil droplets, through the PCV valve and into the intake manifold. On some PCV
systems, this oil baffling takes place in a discrete replaceable part called the 'oil separator'. Aftermarket products sold to
add an external oil baffling system to vehicles, which were not originally installed with them, are commonly known as
"oil catch cans". The PCV valve controls the flow of crankcase gases entering the intake system. At idle, the manifold
vacuum is high, which would draw in a large quantity of crankcase gases, causing the engine to run too lean. The PCV
valve closes when the manifold vacuum is high, restricting the quantity of crankcase gases entering the intake system.[9]
When the engine is under load or operating at higher RPM, a higher quantity of blow-by gases are produced. The intake
manifold vacuum is lower in these conditions, which causes the PCV valve to open and the crankcase gases flow to the
intake system.[10] The greater flow rate of intake air during these conditions means that a greater quantity of blow-by
gases can be added to the intake system without compromising the operation of the engine. The opening of the PCV valve
during these conditions also compensates for the intake system being less effective at drawing crankcase gases into the
intake system in these conditions. A second function of the PCV valve is to prevent positive pressure from the intake
system from entering the crankcase. This can happen on turbocharged engines or when a backfire takes place, and the
positive pressure could damage the crankcase seals and gaskets. The PCV valve therefore closes when positive pressure
is present, to prevent it from reaching the crankcase. The crankcase air outlet, where the PCV valve is located, is generally
placed as far as possible from the crankcase breather. For example, the breather and outlet are frequently on opposite valve
covers on a V engine, or on opposite ends of the valve cover on an inline engine. The PCV valve is often, but not always,
placed at the valve cover; it may be located anywhere between the crankcase air outlet and the intake manifold.

Carbon build-up in intake systems

Carbon build-up in the intake manifold may occur when blow-by gases are allowed to permanently contaminate the intake
air because of a failing PCV system.

Auto Emissions and Solutions – Part II Control Systems

Identifying the separate emission control systems within the seemingly hopeless tangle of vacuum lines, hoses, pipes, and
mysterious boxes and cans in your engine compartment is the key to understanding how these systems operate. These
systems fall into three groups: crankcase emission control, fuel evaporation emission control, and exhaust emission
control. This article will give a brief explanation of crankcase and fuel evaporation emission controls as found specifically
on your MGB and TR6, although the basics also apply to most other cars that have these systems.

In most internal combustion engines, there is a certain amount of combustion gasses that force themselves way past the
pistons and into the crankcase. Unless the crankcase is vented, this blow-by builds up pressure in the crankcase, and would
blow oil out of every possible place in the engine, as well as forming acids within the engine. Until the early 1960s,
crankcase blow-by was simply vented into the atmosphere. TR2s and TR3s, for example, have a draft tube that vents the