Hi Friends,
can some one help me with understanding RPM, Speed and Gear Position. When my vehicle in idle the RPM is around 700. in the move even if am in 4th gear it is between 1500 to 2000 when the speed is aprox 30 to 40 Kmph. This goes up when the speed increased. I found out that it goes upto 3200 when am in 80-100 kms speek.
some where I read that when the RPM is lower, FE could be effitient. Is that true?
Pls explain.
Sethu
To understand the relationship between rpm, speed and gear position, you must understand a few technical terms first. I will try to explain these terms in extremely simple terms:
1. Engine output - Engines are made to propel vehicles. They are able to do this because they develop two things:
(a.) Power = Is the rate at which energy is transferred. The power developed by an engine is measured in Bhp i.e. Brake Horse Power.
(b.) Torque = Is the pulling power of an engine. Its measured in, Kgm (KiloGramsperMetre), Nm (NewtonMetres), etc.
Bhp is like a cheetah and Torque is like an elephant. A cheetah can run very fast but it cannot pull,push or twist stuff like an elephant can. Similarly, an elephant can pull, push or twist heavy stuff very easily bit it cannot run as fast as a cheetah.
Therefore, in a race between a cheetah and an elephant, the cheetah would win.
But if you were to conduct the same race after mounting 10 people on the back of both animals, the cheetah would just fall down and the elephant would easily win.
So torque is the ability of an engine to reach a given speed quickly and Bhp is the ability of an engine to maintain that speed for a long duration.
2. Gear Ratio: Is the ratio between two moving gears. It is calculated by the number of teeth on the meshing gears. For example, one gear set with in the transmission may have 40 teeth & the other has 10 teeth. You must know which one is the drive gear. In this case we'll say the drive gear has10 teeth. Then divide the larger number by the smaller number. 40÷10= 4 thus a "4 to 1" Ratio, also written as 4:1. This mean the smaller gear must turn 4 times for every one rotation of the larger gear.
If the larger gear is the drive gear, you would divide the lower number by the larger number. 10÷40=0.4, thus "0.4 to 1" or 0.4:1. If there is a decimal lower than 1 (0.4) then the this is referred to the gear set to be overdriven or overdrive. In this case the smaller gear would complete 4 revolutions to each revolution of the larger gear.
Similarly, A gear ratio of 1:1 means for every time the engine turns 1 revolution, the output of the transmission also turns one revolution. With a gear ratio of 2:1, the engine must turn 2 times for the output shaft to turn once. This goes on and on and the reason for it is as the gear ratio increases as in 10:1, output torque also increases but speed decreases. A gear ratio of 1000:1 will allow you to pull a house (if you had enough traction) but you would be moving so slowly, snails will pass you.
The power of the engine is transmitted to the driven wheels by means of a gearbox (and clutch). Due to various road conditions, one requires either more pulling power i.e. more torque (e.g. while negotiating an incline) or more horsepower i.e. Bhp (e.g. while moving along a straight road with less traffic).
Can you run while climbing a mountain? No. You have to reduce your speed and climb very slowly. Same is the case with a car. More load = reduced speed and more pulling power and that is why you have to downshift to a lower gear while you are negotiating, say a steep incline.
Different engines produce different amounts of Bhp and Torque at different rpm (revolutions per minute of the crankshaft i.e. the part of the engine that converts the linear motion of the piston into rotary motion).
Now to answer your question - the relationship between RPM in one gear and RPM in another gear is given as RPM * (gear2ratio/gear1ratio) at any particular vehicle speed. That is why your rpm at 40 kmph in second gear would be much more than the rpm at 40kmph in fifth gear.
And to answer your second question - yes, the best way to achieve higher fuel efficiency is to drive at a lower rpm in a higher gear (unless it strains the engine) i.e. if you can drive in 5th gear, please do not strain the engine in the fourth gear.
Got it?
