Friday 17 August 2012

Anti-Lock Braking System (ABS) in Motorcycles

TVS has already made their ability by introducing Apache 180 with ABS. ABS in not a new concept for four wheeler and two wheeler as well. In 1988, BMW introduced the first motorcycle with an electronic-hydraulic ABS: the BMW K100. Honda followed suit in 1992 with the launch of its first motorcycle ABS on the ST1100 Pan European. In 2007, Suzuki launched its GSF1200SA (Bandit) with an ABS.

In 2005, Harley-Davidson began offering ABS as an option for police bikes. In 2008, ABS became a factory-installed option on all Harley-Davidson Touring motorcycles and standard equipment on select models. Now let us enlighten how ABS works on bikes.

Skidding Mechanism

Skidding of a vehicle leads to disaster in many cases. Skidding starts when force applied by driver on the brake lever is more than the required. Skidding results when friction in brakes become more than the friction exists between tyre and road surface. That means wheel gets locked and start skidding on road surface. Less force leads to poor braking and more force leads to skidding. So to avoid the skidding of vehicle, the braking force should remain in limit.
In normal bikes, the brake lever is directly connected with calliper. The force applied by the driver on lever is directly exerted on calliper & disc without any interrupt. In the case of ABS, this braking force is exerted through ECU and Hydraulic valve.
The ABS prevents the wheels from locking during braking. It does this by constantly measuring the individual wheel speeds and comparing them with the wheel speeds predicted by the system. This speed measurement is done by individual speed sensors.
If, during braking, the measured wheel speed deviates from the system‘s predicted wheel speed, the ABS controller takes over, correcting the brake force to keep the wheel at the optimum slip level and so achieving the highest possible deceleration rate.
This is carried out separately for each wheel. Controller is nothing but an ECU with appropriate programming. This program avoids the rotational speed of wheel to become zero (Locking). This is done by temporary releasing the brake force by shutting off the valve in oil reservoir.
The ECU constantly monitors the rotation speed of each wheel. When it detect that any number of wheel are rotating slower than the other (this condition will bring the tyre to lock), it moves the valves to decrease the pressure on the braking circuit, effectively reduce the braking force on that wheel.
The wheels turn faster and when they turn too fast, the force is reapplied. This process is repeated continuously, and this is causes characteristic pulsing feel through the brake pedal.

Figure show major parts of Antilock-Braking System. Basic of antilock braking system consists of three major parts;
  1. Electronic Speed Sensor: This sensor will measure the wheel velocity and vehicle acceleration. LOCATION: On wheel Hub
  2. Toothed Disc: It helps the speed sensor to read the speed of wheel. LOCATION: With Brake Disc
  3. Electrical Control Unit (ECU). ECU is a microprocessor based system contains program. LOCATION: Under the Driver’s Seat
  4. Electrically Controller Valve. This controller valve will control the pressure in a brake cylinder. LOCATION: With ECU
The following are the 3 major benefits of ABS

1. Stopping Distance

 

As the braking force is controlled and applied electronically, the stopping distance reduces considerably in comparison with normal bike.

2. Sudden Braking

In the case of ABS, the braking is intermittent in nature. So vehicle remains easily steerable during braking also. Below figure shows the comparison of normal bike and ABS bike at sudden braking.

3. Braking on Slippery surface

Most of the riders must have experienced this condition with their bikes and also know the results. ABS provides equal distribution of braking force on each wheel and provides straight line stopping of vehicle.

Some Interesting Facts about ABS

Donovan Green, United States, Department of Transportation had performed some experiments on bikes with and without ABS in 2006. Following bike were selected by him for his test.
  • 2002 Honda VFR 800 with ABS
  • 2002 BMW F650 with ABS
  • 2002 BMW R 1150R with ABS
  • 2002 BMW R 1150R without ABS
  • 2004 Yamaha FJR1300 with ABS
  • 2004 Yamaha FJR1300 without ABS
He had performed two types of tests: 1. Dry Surface Tests 2. Wet Surface Tests. Following are the results of his experiments.

Dry Surface Tests

On the ABS-equipped motorcycles, the operator was tasked with braking sufficiently to assure the operation of the ABS. The measured stopping distance values were corrected to compare data from the speeds of 48 km/h and 128 km/h, except for the BMW F650 data, which was corrected to 48 km/h and 117 km/h, the latter figure limited by that model’s top speed of 157 km/h (i.e. 75% of 157 km/h).
In the ABS-enabled mode, for each load/speed/brake combination, the stopping distances were very consistent from one run to another. In this mode, the braking force was applied in a controlled and consistent manner by the ABS mechanism. With the exception of having to react to the possibility of the rear wheel becoming airborne under high deceleration, the rider did not require significant experience or special skill in order to achieve a high level of performance.
In the ABS-disabled mode, the stopping distances were less consistent because the rider while modulating the brake force, had to deal with many additional variables at the same time. Up to six runs were allowed for the rider to become familiar with the motorcycle’s behaviour and to obtain the best stopping distance.
Test results from non-ABS motorcycles were noticeably more sensitive to rider performance variability.
Despite being compared to the best stopping distances without ABS, the average results with ABS provided an overall reduction in stopping distance of 5%.
The stopping distance reduction was more significant when the motorcycle was loaded (averaging 7%). The greatest stopping distance reduction (averaging 17%) was observed when only the rear foot pedal was applied to stop the motorcycle from 128 km/h.

Wet Surface Tests

The original test procedure called for wet surface braking tests to be conducted at 48 and 128 km/h. However, for safety and stability reasons, all low-friction surface tests were performed in a straight-line maneuver, from an initial speed of 48 km/h. The tests were repeated with and without ABS. The test track was wetted by a water truck, and the wetting procedure was repeated every three stops.
With ABS-equipped motorcycles, the rider was instructed to brake sufficiently to assure that the ABS was fully cycling by applying as much force as necessary to the brake control device (no restrictions on force application).
The front and rear wheel brakes were operated simultaneously when the initial test speed was reached and then were operated individually when the front wheel and rear wheel were tested separately. During braking, the engine remained disconnected from the drive train.
A steering operation was allowed to keep or correct the running direction of the motorcycle during the test. Below vehicle speeds of 10 km/h, wheel locking was permitted.
For motorcycles not equipped with ABS, the test procedure was the same except that the rider was instructed to apply as much force as required on the brake control device in order to get the shortest stopping distance without losing vehicle control or having any wheel lockup above a speed of 10 km/h.
As with the dry surface tests, practically no learning process was required for the operator to achieve the best performance with the operation of ABS. In the ABS-disabled mode, the stopping distances improved as the rider became more familiar and comfortable with the braking system.
On the wet surface, the overall average stopping performance with ABS improved on the best non-ABS stopping distance by 5.0%. The stopping distance reduction with ABS was more significant when both brakes were applied, with an overall improvement averaging 10.8% over the best stops without ABS.
The greatest stopping distance reduction with the use of ABS was observed when the motorcycle was loaded and both brakes were applied, averaging a 15.5% improvement over the best stops without ABS.
In general, the test results demonstrated an improvement in braking performance with the use of ABS, whether braking on a dry or wet surface even compared with the best stops obtained without ABS.

COOLING SYSTEMS OF IC ENGINES


Introduction:

We know that in case of Internal Combustion engines, combustion of air and fuel takes
place inside the engine cylinder and hot gases are generated. The temperature of gases
will be around 2300-2500°C. This is a very high temperature and may result into burning
of oil film between the moving parts and may result into seizing or welding of the same.
So, this temperature must be reduced to about 150-200°C at which the engine will work
most efficiently. Too much cooling is also not desirable since it reduces the thermal
efficiency. So, the object of cooling system is to keep the engine running at its most
efficient operating temperature.
It is to be noted that the engine is quite inefficient when it is cold and hence the cooling
system is designed in such a way that it prevents cooling when the engine is warming up
and till it attains to maximum efficient operating temperature, then it starts cooling.
It is also to be noted that :
(a) About 20-25% of total heat generated is used for producing brake power
   (useful work).
(b) Cooling system is designed to remove 30-35% of total heat.
(c) Remaining heat is lost in friction and carried away by exhaust gases.
Objectives
After studying this unit, you should be able to
• understand the methods of cooling of IC engine,
• explain the air cooling system, and
• know the water cooling system of IC engine.

 AIR COOLING SYSTEM:


 

There are mainly two types of cooling systems :
(a) Air cooled system, and
(b) Water cooled system.
Applied Thermal
Engineering
Air Cooled System
Air cooled system is generally used in small engines say up to 15-20 kW and in
aero plane engines.
In this system fins or extended surfaces are provided on the cylinder walls,
cylinder head, etc. Heat generated due to combustion in the engine cylinder will be
conducted to the fins and when the air flows over the fins, heat will be dissipated
to air.
The amount of heat dissipated to air depends upon :
(a) Amount of air flowing through the fins.
(b) Fin surface area.
(c) Thermal conductivity of metal used for fins.
Fin
Cylinder
Figure 5.1 : Cylinder with Fins
Advantages of Air Cooled System
Following are the advantages of air cooled system :
(a) Radiator/pump is absent hence the system is light.
(b) In case of water cooling system there are leakages, but in this case
   there are no leakages.
(c) Coolant and antifreeze solutions are not required.
(d) This system can be used in cold climates, where if water is used it
   may freeze.
Disadvantages of Air Cooled System
(a) Comparatively it is less efficient.
(b) It is used in aero planes and motorcycle engines where the engines are
   exposed to air directly.

 WATER COOLING SYSTEM:

 

In this method, cooling water jackets are provided around the cylinder, cylinder head,
valve seats etc. The water when circulated through the jackets, it absorbs heat of
combustion. This hot water will then be cooling in the radiator partially by a fan and
partially by the flow developed by the forward motion of the vehicle. The cooled water
is again recirculated through the water jackets.
Types of Water Cooling System
Cooling Systems
of IC Engines
There are two types of water cooling system :
Thermo Siphon System
In this system the circulation of water is due to difference in temperature
(i.e. difference in densities) of water. So in this system pump is not required
but water is circulated because of density difference only.

Applied Thermal Engineering:

Water cooling system mainly consists of :
(a) Radiator,
(b) Thermostat valve,
(c) Water pump,
(d) Fan,
(e) Water Jackets, and
(f) Antifreeze mixtures.
Radiator
It mainly consists of an upper tank and lower tank and between them is a core. The
upper tank is connected to the water outlets from the engines jackets by a hose
pipe and the lover tank is connect to the jacket inlet through water pump by means
of hose pipes.
There are 2-types of cores :
(a) Tubular
(b) Cellular as shown.
When the water is flowing down through the radiator core, it is cooled partially by
the fan which blows air and partially by the air flow developed by the forward
motion of the vehicle.
As shown through water passages and air passages, wafer and air will be flowing
for cooling purpose.
It is to be noted that radiators are generally made out of copper and brass and their
joints are made by soldering.

Thermostat Valve:
It is a valve which prevents flow of water from the engine to radiator, so that
engine readily reaches to its maximum efficient operating temperature. After
attaining maximum efficient operating temperature, it automatically begins
functioning. Generally, it prevents the water below 70°C.


Antifreeze Mixture
In western countries if the water used in the radiator freezes because of cold
climates, then ice formed has more volume and produces cracks in the cylinder
blocks, pipes, and radiator. So, to prevent freezing antifreeze mixtures or solutions
are added in the cooling water.
The ideal antifreeze solutions should have the following properties :
(a) It should dissolve in water easily.
(b) It should not evaporate.
(c) It should not deposit any foreign matter in cooling system.
(d) It should not have any harmful effect on any part of cooling system.
(e) It should be cheap and easily available.
(f) It should not corrode the system.
No single antifreeze satisfies all the requirements. Normally following are used as
antifreeze solutions :
(a) Methyl, ethyl and isopropyl alcohols.
(b) A solution of alcohol and water.
(c) Ethylene Glycol.
(d) A solution of water and Ethylene Glycol.
(e) Glycerin along with water, etc.

 Advantages and Disadvantages of Water Cooling System

Advantages

(a) Uniform cooling of cylinder, cylinder head and valves.
Specific fuel consumption of engine improves by using water cooling
system.
(b) If we employ water cooling system, then engine need not be provided at the
   front end of moving vehicle.
(c) Engine is less noisy as compared with air cooled engines, as it has water for
   damping noise.
Cooling Systems
of IC Engines

Disadvantages

(a) It depends upon the supply of water.
(b) The water pump which circulates water absorbs considerable power.
(c) If the water cooling system fails then it will result in severe damage of
   engine.
(d) The water cooling system is costlier as it has more number of parts. Also it
   requires more maintenance and care for its parts.

SAQ:

(a) Why is cooling necessary for IC engine?
(b) Explain in brief the methods of cooling of IC engine.
(c) Differentiate between air cooling system and water cooling system.
(d) What is the purpose of the fins in an air-cooled system?

 SUMMARY

Most internal combustion engines are fluid cooled using either air (a gaseous fluid) or a
liquid coolant run through a heat exchanger (radiator) cooled by air. In air cooling
system, heat is carried away by the air flowing over and around the cylinder. Here fins
are cast on the cylinder head and cylinder barrel which provide additional conductive
and radiating surface. In water-cooling system of cooling engines, the cylinder walls and
heads are provided with jacket through which the cooling liquid can circulate.

 KEY WORDS

Cooling System : A cooling system in an internal combustion engine
              that is used to maintain the various engine
             components at temperatures conductive to long
            life and proper functioning.
Air Cooling System : In this system, heat is carried away by the air
                  flowing over and around the cylinder.
Water Cooling System : In this system, the cylinder walls and heads are
                    provided with jacket through which the cooling
                   liquid can circulate.