Moss Motors, Ltd

In most internal combustion engines, there is a certain amount of combustion gasses which force their 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 which vents the crankcase into the airstream under the engine. Due to the angle cut on the end of the pipe, a slight vacuum is created in the crankcase when the car is in motion. This vacuum pulls fresh air into the engine through a wire mesh oil filler cap, and sucks out the blow-by gasses and vapors through the draft tube.

This atmospheric venting of blow-by produces about 20% of a vehicle's total hydrocarbon emissions. The easiest way to keep these gasses out of the atmosphere is to route them to the engine's intake system to be burned within the cylinders. To do this, Positive Crankcase Ventilation (P.C.V.) systems were developed. Since merely running a hose from the crankcase into the intake manifold would create a massive vacuum leak in the intake system, P.C.V. systems use either a calibrated restriction in the line, or, more commonly, a P.C.V. valve to control the air flow in proportion to the breathing requirements of the engine and the ventilation requirements of the crankcase. The valve is calibrated so that it permits maximum air flow under high speed and heavy load conditions, when blow-by is normally at its worst, and when this "extra" air will have a minimal effect on the running of the engine. During low speed and idle operations, when blow-by is usually light, the valve opens to allow only restricted air flow, to avoid a too-lean air-fuel mixture. The valve will also open whenever there is a condition of positive crankcase pressure.

In 1970, MG and Triumph introduced evaporative loss control systems, the most notable feature of which is the large black plastic vapor adsorption cannister and its associated hoses. These evaporative loss control systems are the most physically complex and difficult to understand of the various emission control systems. Their function is to collect, store, and recycle fuel vapors which would otherwise get into the atmosphere. These vapors account for approximately 18% of a vehicle's hydrocarbon emissions. Since this system also incorporates the crankcase breathing system, it handles almost forty percent of a car's total hydrocarbon pollution output. (See illustration)

Fuel vapors are collected from the fuel tank and carburetor float chambers, along with any fuel overflow from the carburetors. These vapors, along with any small amounts of raw fuel, are adsorbed and stored in a thick bed of charcoal granules within the vapor cannister. When the engine is started, the vacuum in the crankcase breather system draws fresh air into the cannister through the air vent pipe (#5) and the running-on control valve (#9, when fitted). Accumulated vapors in the vapor cannister are picked up by this air flow, and are pulled into the breather system. They then join any crankcase blow-by, enter the intake system, and are burned in the engine's cylinders as part of the normal combustion process. P.C.V. valves are not used, and the crankcase vent line is run into the constant depression chamber of the carburetor(s), between the air valve (piston) and the butterfly valve, instead of being connected directly to the intake manifold. All of this requires a non-vented oil filler cap. Use of a vented filler cap will prevent this system from working. Proper functioning of this system depends on tight, leak-free connections, and properly sized hoses, connectors, fittings, and calibrated orifices to properly handle and control a careful balance of pressures within the system.

Later MGs and TRs have an anti-run on valve connected between the vapor cannister and the intake manifold. Controlled by the ignition switch and the oil pressure switch on the engine, the anti-run on valve operates in the brief time between when the ignition is switched off and the oil pressure in the engine drops. During this time, the valve shuts off its normal air intake, and opens to allow manifold vacuum to act on what is normally the air intake of the vapor cannister. This prevents (or at least minimizes) running-on (dieseling) by putting manifold vacuum to the air space in the carburetor float chamber(s), preventing any further flow of fuel into the intake system.

Also found on some later cars are fuel cut-off valves and capacity limiting fuel tanks. The fuel cut-off valves shut off fuel supply to the carburetor in the event of sudden impact or roll-over. The capacity limiting fuel tanks contain a chamber into which no fuel may be put, which prevents fuel overflow due to thermal expansion.