Quote:
Originally Posted by DerAlte The ABS oil pump. Keeps the pressure up when the valves act, yes. You are right, without it the pedal will gradually go down. The pump works only when there is positive pressure on the pedal, not all the time.
It is also associated with EBD (it is assumed that ladies can't stomp on the brake pedal hard enough) to improve line pressure when it is required - in this case brake pedal is pressed 'fast'. |
Sorry for back-to-back posts as I am past my edit-time.
Der Alte
You seem to have misunderstood the role of the pump in the ABS hydraulic unit.
ABS functioning cycle is quiet simple.
It is
majorly* divided into:
* - I say majorly divided because there are many transition phases between the major phases which entirely depends on the software and adaptation of each ABS maker. For obvious reasons, I cannot get into those in a public forum as I love my job and would like to keep it for a long time to come.
1. Apply:
Pressure application - pressure applied on wheel brake is the pressure applied by the driver. No additional pressure is applied by ABS at any point of time.
2. Hold:
When this pressure is too much (causing wheel slip beyond a preset threshold), the solenoid valve in the inlet circuit (of that particular wheel) closes to avoid any increase in pressure.
Inlet circuit is from Master Cylinder Reservoir ---> Wheel Cylinder.
3. Release:
If even after holding the pressure, the wheel continues to go into slip, the solenoid valve in the outlet circuit is opened to enable return of brake fluid from the Wheel Cylinder (to a Low Pressure Accumulator). Since the pressure at Wheel Cylinder is higher than Accumulator, the fluid flows on its own to the accumulator.
Depending on the wheel slip condition, Apply - Release - Hold cycles are executed in varying sequence till that particular wheel is within acceptable slip behaviour.
The fluid from the accumulator needs to be pumped back to the reservoir / inlet circuit.
This is the ONLY function of the pump in an ABS unit. To drive this (reciprocating) pump, there is an electrical motor.
In a passenger car, there are four wheels, hence four brake circuits; one inlet valve and one outlet valve per circuit, and therefore total 8 valves.
Since two of the circuits are usually connected (X or H configuration), there are only two pumps (1 pump for 2 wheels) which is run by a single electric motor.
EBD:
EBD is Electronic Brakeforce Distribution and it is only a software function which substitutes a Proportioning Valve found in non ABS-equipped vehicles (some older vehicles like Estilo used to come with ABS without EBD, but proportioning valve).
Proportioning valve as you would know is used to reduce the brake pressure to the rear wheels since if equal pressure is applied to front and rear axles, the rear wheels will lock first (due to load transfer during braking) and the car will go into a spin. Front wheel lock is always preferred to rear wheel lock while designing passenger car brakes.
Interestingly, the pump motor has no function at all during functioning of EBD.
EBD usually works just by Hold cycle and very rarely goes to Reduce cycle. Therefore, no fluid is returned to the Accumulator and therefore,
the pump is not required to operate. EBD works purely by actuation of the valves (99% of the time).
Hence, even if the ABS motor fails, only the ABS warning lamp will come on and Brake Warning Lamp will not come on (meaning base brakes will continue to function properly). If valve failure occurs, base brake fails.
What you are talking about is EBA or HBA (depending on supplier) meaning Electronic / Hydraulic Brake Assist. It replaces the Mechanical Brake Assist provided in vacuum boosters (e.g. Swift Diesel). EBA / HBA is a function of ESP (Electronic Stability Program) and not ABS.
Quote:
Originally Posted by im_srini Besides, if the only pressurizing mechanism is the foot, then each time the ABS module releases pressure, won't the pedal too depress a bit ? |
im_srini
Pedal gets depressed if and when the driver presses the pedal. When the fluid is released, basically the circuit through which the fluid passes to the wheel cylinder (which is your input) is
temporarily closed. So you feel a pressure against your foot. Since this is done several times a second, it is felt in the form of a
pulsation.
The quantity of fluid released is so less (the total capacity of the low pressure accumulator itself is a few millilitres) that the pedal will not get depressed as you think.
Contrary to what many people believe, not all cars in all conditions give huge pulsations on the pedal every time the ABS kicks in. With the modern systems and analog controlled valves, the pressure pulses can be smoothed out to an extent where minor ABS interventions would go unnoticed by normal drivers. This is a 'comfort feature' of ABS and is demanded by most OEMs.
That is all I can say with respect to basic functioning of ABS without getting too technical.
Hope this helps in some way.
Cheers