Mechanical Engineering - Automobile Engineering - Discussion

Vapor lock is a problem that mostly affects gasoline-fueled internal combustion engines.

It occurs when the liquid fuel changes state from liquid to gas while still in the fuel delivery system. This disrupts the operation of the fuel pump, causing loss of feed pressure to the carburetor or fuel injection system, resulting in transient loss of power or complete stalling. Restarting the engine from this state may be difficult.

The fuel can vaporize due to being heated by the engine, by the local climate or due to a lower boiling point at high altitude. In regions where higher volatility fuels are used during the winter to improve the starting of the engine, the use of "winter" fuels during the summer can cause vapor lock to occur more readily.

Braking generates considerable heat and to resist these high temperatures without boiling, brake fluid is designed with a high boiling point. But brake fluid is also hygroscopic, which means it absorbs water. In any hydraulic brake system, the brake fluid gradually absorbs moisture and this reduces the brake fluid's boiling point. Once this moisture absorption reaches a certain level "under prolonged, hard braking, especially in hot weather or heavy loads " the heat can cause the brake fluid to start to boil, creating gas bubbles. As gas is far more compressible than liquid, the driver will experience no pressure at all on the brake pedal, resulting in total loss of brake power known as vapour lock.

This is a term used when the brake fluid in a vehicle brake system is overheat and loses efficiency.

First we will explain what brake fluids usually consist of there are three main types of brake fluids, Mineral brake fluids, Glycol brake fluids and silicone brake fluids. The GLYCOL brake fluid is most common and used in 99.5% of modern vehicle brake systems.

There can be up to 2 pints of brake fluid used in a vehicle brake system and this is piped around the vehicle using copper or rubber hoses to feed the fluid from the operator to the brake system.

Operating the brake forces the fluid into the master cylinder and then brake calipers or wheel cylinders to actuate the brake.

During the life of a vehicle or even a drum of brake fluid sitting on the floor of a workshop things happen to Glycol fluids because they are what we call "Hygroscopic". They absorb water even through the walls of the (would you believe) slightly previous rubber brake hoses and open top on cans or vehicle master cylinders.

This water vapour drawn into the fluid will of course boil at somewhere around 100 degrees (or a little more under Pressure) so any water content in a brake fluid is bad news. It also causes system internal parts to corrode.

Basically Brake Fluid are of mainly three types:

1) Mineral.
2) Glycol.
3) Silicone.

Mainly in all of the automobile Glycol is used as Brake Fluid.

Glycol is Hygroscopic in nature, So it absorb Water from the environment during its life from the Master Cylinder, Rubber Hose etc.

Due to Severe Braking or Continuous Braking the temp of the Fluid Rises and Water will convert into Vapour at 100'C.

Being Compressible in nature it lead to loss in Braking Effect or termed as Vapour lock in Brake System.

It is possible when the whole brakes lines are unfilled, with the full filled the brakes lines a small quantity of air is in it because brakes lines have narrow size. To comes out the air from brake line, first filled the brake line, then the end of the brakes lines is slight loss the applied the 3or 4 time the air from the lines comes out. There is no another case locking of vapour or air in brakes lines.

When we applied brake, brake fluid helps to transmit brake power to the brake shoe. Due to frequent use of brake, temperature of the fluid present in it raises. When fluid temperature get raised above boiling point, it converted into vapor. Vapor is more compressible as compare to fluid resulting in auto locking of brake which may cause accident.

The vapour lock may occur because of high temperature in brake system due to any reasons either environmental or brake applications but in rare cases because brake fluid has the quality to withstand very high temperatures, in my career I have not come across any brake fluid vapour lock though it may occur in fuel systems.

Most of the braking fluid contains glycol fluid. It is hygroscopic fluid. The conversion of water in the brake fluid to steam when the brakes are heated significantly which causes compressibility in the brake system. This vapour lock leads to increased pedal travel and can result in a significant loss of brake effect.

The Hygroscopic nature of the glycol which is often used in now days modern vehicles can easily absorb water from any available surrounding.

This water so absorbed boils at 100'c (even far away from the fluids temperature). Vapour formed is compressible at high pressure hence making the brake to loss it's effect.