RPM : Speed : Gear Position


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sethumurugan

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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
 
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RPM is Revolutions Per Minute.
Its the measure of frequency of rotation in a fixed axis.

It is the speed at which the engine shaft rotates.

Diesel engines normally operate at low RPM and they provide high low end torque.
Its advisable to keep the RPM of Diesel engines below 2500
Same way for Petrol engines, it should be kept below 4500.

During idling, if you rev the engine (applying accelator while in neutral), RPM suddenly shoots up.

Same way if you accelerate more in lower gears, RPM shoots up.

You need to maintain a minimum rpm level for the vehicle to move. If the rpm is less than 1000 your car may stall.

For extracting FE, one need to maintain an optimum rpm.

As per Diesel car manufacturers in general, it is 2000 rpm.

As per Narayanan Menon from Coimbatore who created a record by achieving a spectacular FE of 42 km per ltr for his Manza, the ideal rpm is 1400.
 
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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?
 
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sethumurugan

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Thanks guys.

As per Narayanan Menon from Coimbatore who created a record by achieving a spectacular FE of 42 km per ltr for his Manza, the ideal rpm is 1400.
How do we control the RPM? Does this means, u try accelerate slowly? cos as soon as I reached 100Km speed in 5th gear the RPM is 3200.

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.
I like this explanation. Very simple and good for the people who does not know much technical about motors.


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.
Eventhough this part is little bit tough, I think the crux of it and understood the basic.

Based on the above two replies, I have couple of doubts to be cleared.

I have LPG/Petrol vehicle. So can it be considered in par with petrol vehicle?
How do we control the RPM?

Thanks
Sethu
 
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How do we control the RPM? Does this means, u try accelerate slowly? cos as soon as I reached 100Km speed in 5th gear the RPM is 3200.
For a petrol car 3200 RPM at 100 kmph in 5th gear is normal.

Recomended RPM for various gears would me mentioned some where by the manufacturer - like on the front wind shield or in the manual or some other places.

To control RPM, you need to shift gears.

Shift upwards if your RPM goes up and shift downwards if the RPM goes down.

Don't give too much imprtance to that and just concentrate on your driving.

Try to keep it around 2800 to 3000.
For that you need to be at around 80 - 90 kmph in 5th gear.
So far that is the ideal speed in Indian Highways for a safe journey.
 
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sethumurugan

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For a petrol car 3200 RPM at 100 kmph in 5th gear is normal.

Recomended RPM for various gears would me mentioned some where by the manufacturer - like on the front wind shield or in the manual or some other places.

To control RPM, you need to shift gears.

Shift upwards if your RPM goes up and shift downwards if the RPM goes down.

Don't give too much imprtance to that and just concentrate on your driving.

Try to keep it around 2800 to 3000.
For that you need to be at around 80 - 90 kmph in 5th gear.
So far that is the ideal speed in Indian Highways for a safe journey.
Thanks.

As you have mentioned I drive mostly in 90. Just when am sure the coast is clear then go upto 100-110 [ very occational ].
 

TSIVipul

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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
Well,buddy let me explain it my way:-
RPM:Revolutions per minute,simple it is how many times the crank turns 360 degree per minute or say how many times a piston moves up-down-up(complete cycle) in a cylinder per minute.

RPM=Gear=Performance:
Every engine behaves a different way,like you can say all the turbo diesel engines performs best above 1500-2000 revs.Now if you select a gear such a way that if your car is above 2000 rpm in any gear then whatever the speed may be,however the road may be the car will perform at its best.And if you are above or below the main powerband in any gear then you will see a dip in performance.
Example:If the engine is below the main powerband(the RPM range where engine gives its best power output)then you will get bogged down and in case the engine rises higher than the main powerband,then there is a sudden dip in power.

To get the best FE out of the car,its best to keep the car in the main powerband,else the FE dips.

BTW,which car do you drive buddy?This will help me explain better.
 
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In a spark or any other petrol vehicle the ideal shift point for FE would be 2100 RPM.

To extract maximum FE shift as soon as you can to a higher gear ( when you have enough speed), try coasting the vehicle or ease off the throttle whenever you feel you have enough momentum.

The overdrive gear is designed for efficiency, never hesitate to use it!

And as for spark the car would consume more at a 100 or 120 KMPH, but already you mentioned you drive at 90, so try being at a constant speed.
 
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To get the best FE out of the car,its best to keep the car in the main powerband,else the FE dips.
BTW,which car do you drive buddy? This will help me explain better.
Sir, for the benefit of me and possibly for others in this forum, will you please explain what is the optimum FE in general, for all cars having MPFI engines?
 
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I like this explanation. Very simple and good for the people who does not know much technical about motors.
I cannot take credit for this. This explanation is a twisted rendering of many such explanations by the two-wheeled guru of India - Dilip Bam.


Based on the above two replies, I have couple of doubts to be cleared.

I have LPG/Petrol vehicle. So can it be considered in par with petrol vehicle?
How do we control the RPM?

Thanks
Sethu
The thermal co-efficient (= heat generated by a particular fuel/material when it burns) of LPG is lesser than that of petrol. Therefore, LPG burnt in any engine (in your case, a 1000cc engine) will produce less power than petrol burnt in the same engine. That's why you have to stay in lower gears more when yo drive on LPG (lower gears = more rpm = more pulling power)

RPM is controlled by gearshifting. Let's not get too technical on this. Just remember to always keep your car in a gear at which the engine feels smooth and comfortable for a given traffic/road condition..
 
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sethumurugan

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I cannot take credit for this. This explanation is a twisted rendering of many such explanations by the two-wheeled guru of India - Dilip Bam.
..
Thats is great of you. Let us thank Mr. Dilip Bam.


The thermal co-efficient (= heat generated by a particular fuel/material when it burns) of LPG is lesser than that of petrol. Therefore, LPG burnt in any engine (in your case, a 1000cc engine) will produce less power than petrol burnt in the same engine. That's why you have to stay in lower gears more when yo drive on LPG (lower gears = more rpm = more pulling power)
..
Yes. I sometimes felt that. unknowingly I would reach top gear in low speed and I will find the car cruze with little bit difficult till it reaches speed for the gear.

RPM is controlled by gearshifting. Let's not get too technical on this. Just remember to always keep your car in a gear at which the engine feels smooth and comfortable for a given traffic/road condition
..
I think, I under stood. Thank you very much for the explanantion.
 
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RPM and Fuel efficiency

What should be the ideal RPM value for a particular gear to get the most fuel efficiency ? Does it vary in Diesel and Petrol model ?
 
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drniladrisaha said:
What should be the ideal RPM value for a particular gear to get the most fuel efficiency ? Does it vary in Diesel and Petrol model ?
Yes it depends on all factor like engine type , no. Of cylinder, fuel type, gear box design, load that car is carring ( no. Of passenger) and many

So the best rpm will always different in all cars.
 
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Re: RPM and Fuel efficiency

My car is Ford fiesta
Diesel
My car ideal RPM which i felt to fuel effient is 2000 RPM
company recomends to drive in between 1500-2000RPM
 
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