From this........

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It is often said that going beyond limits result in extraordinary outcomes. But there are some which do not want to follow this and if followed, the result come out in the form of damage. One particular thing is automobile engine. Whatever is going on inside the engine is displayed on the instrument cluster. Engine temperature, instantaneous speed, fuel level, rpm etc are some of them. Most space on a cluster is occupied by the rpm display. The tachometer has a scale of numbers starting from 0-10*1000 or in performance m/cs even up to 20*1000. Kawasaki Ninja has rev marking upto 20*1000 rpm....

.....whereas Bajaj pulsar RS is limited to 12*1000. Rpm or revolution per minute is the count of number of times an engine rotates per minute. If the needle points 3 then it means the engine is rotating 3000 times in one minute and same for others. The scale is divided into 3 parts. Green is marked adjacent to 3000-5000 rpm which means that if the engine is running in this scale then the m/c would give an optimum mileage. Moving on to the next division is yellow which means the engine is now running at a speed which is giving optimum power output. The last division is marked with red and as known to us that red is a sign of danger and on the cluster it indicates the same. Running the engine in this red zone no doubt would give out best power and speed but also damage the engine parts. The red marking adjacent to the number gets thicker with each increasing number. Range depends from model to model. This is called redlining or over-revving and if done often then it seriously wears out parts like valve, cylinder,piston,piston head,cams. Redlining unintentionally occurs if a gear is missed, e.g. when a driver shifts down from 5th to 3rd directly. Intentional redlining occurs when a driver adapts long shifting method. Engine parts are produced to work within a certain speed, stress and temperature range. Redlining increases the chance of a catastrophic event when valve and piston crown collides into each other resulting into an irreparable damage.

                   CREDITS: FORD VS. FERRARI(2019)      

If you have watched the movie FORD vs. FERRARI then just recall the scene when Ken Miles in his FORD GT40 goes head to head with Lorenzo Bandini in his Ferrari 330P3. They both actually push their cars too hard(redlined). Soon Bandini’s Ferrari faces a blew up line and shoots out flames with huge cloud of smoke. The movie showed the best example of why redlining was and is harmful even for high performance racing cars. You might be imagining that when both pushed their cars hard then why only Ferrari faced the damage and not Ford. Well...that is a different and long debate and I will explain that in my later articles.

 Rev limiter

Rev limiter is a pre-set controlling device that stops the engine from redlining even if the driver wants to intentionally do it. It is fitted to a manual transmission engine and restricts the engine from reaching red marking. In automatic transmission, the engine cannot be redlined in drive mode as the transmission shifts to next gear automatically as soon as the engine approaches redline. Yes, only when the engine is in neutral gear, redlining is possible.

 A rev limiter consists of a speed sensor, ECU(electronic control unit)and a solenoid. It works in accordance with ECU to shut off either fuel or spark. Speed sensor is connected to transmission gear and detects increase in the speed. Sensor informs ECU by sending electric signals. ECU commands the solenoid to limit air flow. This is basically how a rev limiter works. Rev limiters are classified as hard or soft cut depending on the rate of cut-off.

SOFT-CUT LIMITER

ECU in a soft-cut limiter softly or gently or partially cuts the fuel or spark when the rev approaches red mark region. It even makes the rev stay at a pre-set rpm. This often helps in perfect launch of a stationary car. It works together with launch control when the car quickly takes off but without any wheel spin. The rpm gets fixed to low which is somewhere around 3-4*1000 rpm depending on rear or front wheel drive.  

HARD-CUT LIMITER

On over-revving, ECU completely cuts off spark or fuel supply. This supply remains cut till it reaches green marking or in safe revving range. If the throttle is pressed open, the limiter turns itself off and allows the engine to reach high rpm and the cycle repeats. This is called bouncing or oscillating and often could be guessed by the fluctuating engine sound and flame shots.

Classification based on combustion components cut-off:-

Spark cut-off

In this limiter, ECU shuts off ignition or spark on reaching red mark. This is rarely used in production cars as fuel injectors continue to circulate fuel in the cylinder irrespective of its combustion. This sends unburned fuel out to the exhaust pipes which may damage the system due to high temperature.  

Fuel cut-off

The cons of spark cut-off limiter are solved by fuel cut-off limiter which is why it is widely used. Limiter working with ECU shuts off fuel supply by turning off fuel injectors. This results in filling the cylinder with lean mixture i.e. a mixture containing more air and less fuel. It is widely used in production cars.

 Feel free to drop your queries in the comment box.Thankyou!!!!  

by: AutoVogue