Cub Cadet MTD 70 Series - Manual (Page 56)

61/65/70/75 Series Horizontal Shaft Engines
50
Evaporative (EVAP) emissions system
NOTE: Gasoline is made from the graduated distillation of crude oil. It consists of a multitude of individual
hydrocarbons and has a boiling range of 86 - 410 o F (30-210 o C) 1 . The large quantity of hydrocarbons
and the low boiling range makes gasoline an ideal fuel for spark ignited, internal combustion engines.
However, the hydrocarbons are not good for the environment. To reduce or eliminate the release of fuel
vapors into the atmosphere, an evaporative (EVAP) emissions system is used. Starting with the 2008
season, an EVAP system has been offered in areas that require it. All fuel caps must be tethered to the
fuel tank as part of the EPA tier III emissions. A broken tether on the fuel cap must be repaired before
the unit can be put back into service.
This charcoal canister system consists of:
• A charcoal canister
• The fuel tank with a roll over valve
• An unvented fuel cap
• Vacuum lines
This system operates as follows:
1. The gasoline evaporates, letting off vapors.
2. The vapors exit the fuel tank through a roll over
valve.
3. The vapors are carried to the charcoal canister by a
vacuum line (colored orange in Figure 4.18.
4. The activated charcoal inside the canister absorbs
the hydrocarbons allowing the air to pass through
and out to the atmosphere.
5. When the engine is running, the vacuum between
the air filter and the carburetor is used to draw the
vapors out of the charcoal canister through the
purge line, colored green in Figure 4.18. This tem-
porarily enriches the fuel/air mixture, and is used in
the combustion process.
1. Dr. Ullmann, J, Fuels, Automotive Handbook, seventh edition. Bosch, Robert distributed by SAE Society of
Automotive Engineers, 2007. 320.
Figure 4.17
Charcoal canister
Vacuum lines
Roll over valve
Figure 4.18
Purge Line Vent Line