Saturday, November 28, 2015

Grommet vs Connector In Power Brake Booster Design

(Though i have a degree in mechanical engineering since 1986 from Romania, it is in Machine Tools and not Automotive Engineering. After i graduated, most of the work i've done was in computer programming though at the beginning related to engineering CAD after which i swicthed to accounting, payroll and medical insurance. Also my level of English when i came in the United States, especially in the technical area was much lower than today, after i started writing on social media. Thus it is only recently when i understood the below described phenomena and also became capable of describe it.)

A rubber grommet is a ring with a groove on the outside that fits in a hole and helps with the passage of a pipe, cable or connector through that hole, preventing the last one touching the edges of the hole. Most common use of grommets, as described in this Wikipedia picture, is for passing cables through holes.

In automotive industry they are also used for passing and sealing of ribbed connectors and holes, mostly in PCV valve or power booster applications. Here is the image of a power booster check valve with two connectors on each side (in and out). It is a one way valve with a spring and seal inside that allows the flow of air in one direction only and it ends with two ribbed connectors, of which one goes into the power booster grommet and the other to the power booster vacuum hose that at the other end is connected to engine manifold or throttle body on the manifold side. The function of this valve is to make sure that vacuum does not escape from the booster or air it's not getting in immediately after the engine stops, ensuring one or two brake cycle in case of engine failure or sudden stop while the vehicle is driven. For the purpose of this discussion it does nothing more than an L shape connector.

A brake power booster is a big cylinder with one or two diaphragms inside that helps with braking by adding extra force to the master cylinder when a smaller brake force is applied on the pedal. The extra force is generated by the suction power of the engine on the admission cycle, which in automotive terms is named manifold vacuum, or the difference of pressure between the inside of the manifold and atmospheric pressure applied on the opposite sides of the diaphragm.

Many don't understand. In combustion engines the air volume necessary for running the engine is small, at least at idle and under no load conditions. Thus, the flow of the air in the engine is restricted by a round shape valve called throttle that turns inside a cylindrical air passage with the same diameter with the throttle called throttle body with the pressing of the "gas" pedal. At idle the throttle is in perpendicular position with the air flow and closing the passage. A small amount of air (by)passes usually through a parallel passage that is also controlled by the computer through a solenoid or actuator, that is open only at idle. On the rotation axis of the throttle there is a potentiometer like sensor (similar with those in older audio devices) that turns with the throttle, thus transmitting to the computer a variable voltage that tells the computer how much the throttle is opened. Because of this severe restriction of the air flow inside the engine, vacuum is created on the intake side of the throttle. This vacuum is available and engineers long time ago figured they could use it for powering different devices and actuators in the vehicle as if would come from a vacuum pump), as the brake power booster.

In a normally running engine, at idle, this vacuum generates a difference of pressure with the atmospheric pressure of or above 22 inches mercury, that is about 11 psi and can go a little higher but obviously not higher than the atmospheric pressure itself that is about 14.7 psi since the difference it's generated by vacuum, or suction, or lack or pressure that can obviously cannot go lower than zero. 11 psi is equivalent with about one third to almost half of the pressure inside a tire that is above 32 psi.|

Here is a picture with a power booster that uses a grommet for passing the connector of a check valve.

Another type of design for solving applying manifold vacuum to the power booster described in the picture below uses instead of a grommet a connector that is part or welded or pressed on the power booster in the same place where the grommet hole is in the grommet solution, connector that is similar to the connector used on the other end of the hose as in both solutions, in which case the check valve is placed somewhere else like inside the booster.
Due to the way it is created, by the reciprocating motion of the pistons inside the cylinders that follows a time dependent sine pattern, engine vacuum has a pulsating nature that is partially smoothened by the volume of (low pressure) air inside the manifold and hoses (there are other pneumatic devices that are also fed with the pressure difference of pressure between inside the manifold and surrounding air that are connected to the manifold side of the throttle body though smaller and less important that also hold a certain volume of the same low pressure air in all the hoses and devices).

In a normally running engine, at idle, vacuum is  about 22 inches, relative or lower than atmospheric pressure. When the gas pedal is slightly pressed and throttle opens a little, around 2000 rpm, the vacuum gets even higher, closer to zero pressure or absolute vacuum or 14.7 psi relative to atmospheric pressure due to increasing need of engine for air because of faster movement of the pistons and the throttle's opening resistance. When the throttle is opened even more, thus allowing more air freely into the manifold the vacuum starts to decrease again.

The more vacuum in the manifold, the more pulsating is the pressure difference due to increasing amplitude of the sine wave due to reciprocating motion of the pistons.

Also. Due to various dynamic forces the engine has a tendency of moving or vibrating on its rubber mounts, mostly following the cycle of pistons' reciprocating movement, but also the rpm. There are certain rpms that make the engine move more or at higher amplitudes than others due to resonance phenomena.

Also, all the hoses, including power booster vacuum hose are not rigid and move around together with the engine having their own moving pattern and resonance with certain rpms.

The grommet in the grommet solution in all different design variations (i've seen so far) have the length and diameter of the groove comparable with the sheet metal and the hole they seal, design that cannot generate a sealing force on the wall of the booster. Thus the sealing is being insured by the difference of pressure between inside and outside of the booster applied on the outer side of the grommet that presses against the booster's wall.

The check valve's connector on the grommet side has a slightly bigger diameter than the hole of the grommet and it is ribbed, and when is inserted into the grommet it causes very hard to predict deformations of both parts which in this case are static but asymmetrical due to the weight of the hose but at least at the beginning of its life and at idle it seals the vacuum or at least partially, up to a certain pressure difference or pulsating amplitude at certain rpms.

Thus there are two sealing surfaces regarding the grommet. One is between outer rim of the groove and the sheet metal around the hole and one is between the hole and the connector.

As i said the vacuum hose has its own weight. This weight is applied on the grommet side as well but with dynamic variations following the different relative motions between the grommet and the hose, generating variable forces that are impossible or very hard to predict that are in the end applied onto and sustained by the grommet, causing it to slightly change shape many times a second, or causing dynamic deformations, following the relative motion of the engine and the hose. Those changes of shape are asymmetrical due to direction of the variable force being applied on the grommet and very hard or impossible to predict, in the end changing the sealing surface, in a dynamic way and increasing it on one side and decreasing on the other.

In conclusion, both the pulsating nature of the engine manifold vacuum and the forces generated by the hose moved by the engine moving on the mounts deform the grommet dynamically, combined can change the sealing areas and pressures on the sealing surfaces of the grommet, possibly leading to small, pulsating leaks that might appear only on certain rpms and engine loads.

The power boosters are oftenly painted on the outside to check corrosion. The sealing surface between the grommet and the power booster is also painted. Paint thickness can be irregular causing unpredictable variations on the final shape and smoothness of the sealing surface and can also degrade in time or even be peeled or chipped due to age causing irregularities to grow in time and sealing to decrease, thus increasing  the pulsating vacuum leak.

Peeled, eroded paint an a small indentation at the power booster grommet hole
The grommet solution compared to welded connector solution brings several more areas than need to be sealed in the vacuum transmission assembly between throttle body and booster, that is throttle body connector-hose, hose-check valve, check valve-grommet and grommet-power booster.

Grommets are made of rubber. Rubber is a material that often has irregularities but also changes properties in time due to aging. Constant pressure like from the ribs of the connector can cause grooves inside the hole of the grommet thus in time decreasing sealing capacity on that side. Also rubber looses elasticity with age thus the initial installation pressure and sealing capacity between grommet and check valve connector and capacity of following the irregularities on the painted area.

The welded connector design is much simpler, and cost comparable solution, it follows the solution on the other side of the hose, has less sealing surfaces and much less like to cause vacuum leaks.

Uncontrollable vacuum leaks in the end translates by air flowing in the engine in uncontrollable ways, bypassing the throttle body and throttle.

Small intermittent vacuum leaks on carburrated engine can cause trouble by changing in unpredictable ways the fuel/air ratio.  The computer responds promptly by the reading of the oxygen sensors, thus adding more fuel.

But on injected engines even small vacuum leaks are critical because in injected engines fuel/air mixture is adjusted by a computer based on reading from one or more oxygen sensor inside the exhaust system, among others. The more oxygen the sensors read due to a vacuum leak or air escaping uncontrollably into the manifold, the more fuel it adds. Also air/fuel ratio is adjusted by the computer using readings from other sensors, including throttle positioning sensor and pressure sensors inside the manifold and intake hose. Air flowing into the manifold due to unpredictable, intermittent leaks translates into reading in the sensors than cause the engine running erratically and mainly increasing fuel consumption with all its consequences, like more fuel burning with the extra air inside the exhaust and catalytic converter, shortening the life of the catalytic converter and overheating other components of the vehicle, like floor, transmission and clutch. Extra fuel consumption also translates in more fuel being used, extra cost, pollution, etc.

The following pictures are done with my old check valve and hose and a new original grommet.

All the deformations that remain permanent due to aging of the grommet, abrasion of the paint, inevitably lead in time to more important vacuum leaks that can lead to decreasing braking capacity, but that is not obvious since it occurs very gradually and the driver gets used to. Though vacuum is present in the booster, it is diminished due to vacuum leaks that appear only in this design up to a point when the computer finally sets an error that usually indicates a lean condition (too much air in the air/fuel mixture), because through its oxygen sensors it actually measure the amount of oxygen left after the burning cycle inside the cylinder that is increased in case of vacuum leaks or more air bypassing the throttle thus unaccounted for by the TPS sensor inside the intake manifold. But when usually that happens that engine has been functioning abnormally for too long and other components have been damaged by overheating. Also the vacuum leaks even at the beginning of the life of this vehicle that are due to the grommet solution are intermittent, and the computer is programmed to put a check engine light only after a contentious malfunctioning or a high reading on one of the oxygen sensors of a certain period of time, cycles and events, that only happens when the vacuum leak grows bigger and/or gets permanent.

All these consequences can be averted if the pressed or welded connector solution is used instead of the grommet one.

It is hard for me to estimate the amount of fuel being used by multiplying this type of failure due to a faulty design to the the number of vehicles that still use it, but i estimate it in the enormous area.