EVAP TOYOTA EVOLUTION
As the theory teaches, about 20% of the emissions of hydrocarbons (CH) from cars are produced by the evaporation of fuel. The evaporative fuel collection system (EVAP – from “evaporative”) serves for their accumulation and utilization, preventing it from entering directly into the atmosphere. Vapors are formed in the tank with increasing fuel temperature (especially in schemes with fuel return lines from the collector), with increasing pressure, they begin to flow into the adsorber with activated carbon. In modes where the engine can normally perceive the additional enrichment of the mixture, these accumulated vapors are “blown” into the intake manifold and added to the charge of the air-fuel mixture for combustion in the cylinders.
The first fuel vapor recovery systems appeared on Toyota as early as the carburetor era and were actively used until the first half of the 1990s.
The early system included the following components:
– fuel tank,
– throat cover with vacuum check valve,
– adsorber with a set of check valves,
– thermopneumatic valve (BVSV, TVV),
– EVAP port in the throttle body.
In some cases, there should be a slight overpressure in the tank, reducing the likelihood of the fuel pump cavitating. This pressure is created by the fuel return line and is maintained by the adsorber check valve 2 and the check valve in the cap. When the fuel level in the tank falls, a vacuum may be created in it, which in the worst case leads to its collapse. To prevent this, atmospheric air enters the tank through the non-return valve 3 of the adsorber or the valve in the neck cap. Thus, EVAP prevents the creation of excessive pressure or vacuum in the tank.
When the engine is running, the coolant temperature is above ~ 54 ° C and the throttle is opening behind the purge port, accumulated fuel vapors from the overpressure zone in the adsorber pass through the check valve 1 and the thermopneumatic valve, entering the vacuum zone behind the throttle valve. Atmospheric air enters directly into the adsorber through a filter in order to ensure proper through-flow when vacuum is applied to the adsorber. At temperatures below ~ 35 ° C, the thermopneumatic valve closes and blocks the supply of vacuum to the valve 1.
To provide a more accurate control of the effect of EVAP on the composition of the mixture and the operation of the engine, the first electronically controlled circuit appeared, with an electropneumatic valve instead of a thermal pneumatic valve. Fuel vapors likewise come under the action of vacuum into the intake manifold (when the coolant temperature is above ~ 54 ° C and work in closed-loop mode), while the ECM can change the width of the pulses applied to the valve, adjusting the vapor flow rate.
The simplest self-test here refers to the electrical part of the valve:
P0443 – Evaporative Emission Control System Purge Control Valve Circuit
No appropriate response to ECM commands
This scheme has been successfully working on models for the Japanese and European markets for more than 20 years, almost without creating problems in operation.
Electronically controlled, with monitoring function / 2VSV, vapor pressure sensor
But the ancestors of OBD did not stop at what had been accomplished and supplemented the next scheme with yet another electric pneumatic valve and a fuel vapor pressure sensor.
When the sensor valve is closed, the ECM has the ability to measure the vapor pressure in the tank; when open, the vapor pressure in the canister. When the purge valve is opened, the vapor pressure in the canister should decrease, but if this does not happen, the ECM determines the “sticking in the closed position” of the purge valve. In the next step, the ECM checks the vapor pressure in the canister when the purge valve is off, if the pressure remains low, the ECM determines “sticking in the open position”.