Why do we need Gears

[Harish Kumar]

I would like to know the need for Gear Boxes. Can't we just couple the car engine to the wheels directly without using gears? If we change the RPM of the engine by controlling fuel injected, the speed of the car is automatically controlled. Why do we need gears which cause loss of efficiency while transmitting power due to friction, etc.

 

Likewise, cant we connect the steam turbine shaft directly to the screws of the submarine, instead of routing it through a gear box? Aren't gears bringing down the efficiency of the engines which consume a lot of power?

 

Can anybody clarify?

[ydoaPs]

Likewise, cant we connect the steam turbine shaft directly to the screws of the submarine, instead of routing it through a gear box? Aren't gears bringing down the efficiency of the engines which consume a lot of power?

It would cavitate like hell. As for slowing down the turbine to match the screw speed, the speeds at which the screws turn, the turbines are ridiculously inefficient.

[Mr Skeptic]

It's easier to make an engine that runs at a certain speed than a variable speed, so your idea would also lose efficiency.

 

Get yourself an electric car. Electric engines can run at variable speed much more easily, and in fact some designs call for having the motors right on the wheels to dispense with the drive shaft as well.

[insane_alien]

in case you haven't noticed, cars idel at about 500-800rpm. this means that if you were to weld the engine to the wheel, you'd need to be going at a speed where the engine could reach 500-800rpm before you could start it.

 

depending on ratios this could be 2-30mph.

 

the gears are used to allow for a wide range of speeds and torques that are required for moving vehicles. its no good if you stall every time you stop.

 

on the other hand, many of the electric cars are currently made with no gears(some with independant engines inside the wheels). but this is due to the greater flexibility of the electric engine over the internal combustion engine. There would still however be advantages to gears although these are more likely to apply to performance driving(off road, racing, haulage) than the casual commute as large torques and speeds can be required.

[michel123456]

(...) Electric engines can run at variable speed much more easily, and in fact some designs call for having the motors right on the wheels to dispense with the drive shaft as well.

 

Emphasis mine.

And not only electric engines. Remember the velosolex.

"The Vélosolex has a small 49cc motor mounted above the front wheel. Power is delivered via a small ceramic roller that rotates directly on the front wheel by friction to the tire."

[TonyMcC]

Think of yourself riding a bicycle which has gears. I'm sure you would agree that different gears would allow you to climb slopes more easily and go downhill more quickly. Now think of replacing yourself with some sort of engine. A fixed gear would not allow your vehicle the range of possibilities that it would have with multiple gears.

[Harish Kumar]

I wish to thank all the guys here for clarifying my doubt. Thank you once again!

[Charlie]

"Why do we need gears which cause loss of efficiency while transmitting power due to friction, etc."

 

 

Gears are not a loss of efficiency. They massively increase efficiency for combustion engines by keeping the engine revving in a band that has the best fuel efficiency.

[John Cuthber]

in case you haven't noticed, cars idel at about 500-800rpm. this means that if you were to weld the engine to the wheel, you'd need to be going at a speed where the engine could reach 500-800rpm before you could start it.

 

depending on ratios this could be 2-30mph.

 

the gears are used to allow for a wide range of speeds and torques that are required for moving vehicles. its no good if you stall every time you stop.

 

on the other hand, many of the electric cars are currently made with no gears(some with independant engines inside the wheels). but this is due to the greater flexibility of the electric engine over the internal combustion engine. There would still however be advantages to gears although these are more likely to apply to performance driving(off road, racing, haulage) than the casual commute as large torques and speeds can be required.

 

The OP was talking about eliminating gears- not the clutch.

 

Though it would still be a pig to start off with a fixed gear system and not stall unless the wheels were really small (in which case you would need to race the engine to get any decent speed).

It's not a viable idea..

[SMF]

Many cars are running 1:1 through the transmission in the highest (lowest ratio) gear that is not overdrive. There is another gear reduction in the differential. SM

[steevey]

I would like to know the need for Gear Boxes. Can't we just couple the car engine to the wheels directly without using gears? If we change the RPM of the engine by controlling fuel injected, the speed of the car is automatically controlled. Why do we need gears which cause loss of efficiency while transmitting power due to friction, etc.

 

Likewise, cant we connect the steam turbine shaft directly to the screws of the submarine, instead of routing it through a gear box? Aren't gears bringing down the efficiency of the engines which consume a lot of power?

 

Can anybody clarify?

 

Gears provide mechanical advantage, which in the classical level is using the potential energy of gravity and weight distribution to make an action more efficient. Many machines do this in different ways, and they allow energy to be transfered across systems and used.

 

If you have a teeter totter, there's two big ends which are sticking out. Normally to lift one end up, you'd have to use your arms and really push up. But, because there's a focal point (or center), the force of the big ends caused by gravity is being put on that focal point, allowing you to put energy into making a stick go up and down without having to worry about that extra weight yourself.

Edited February 10, 2011 by steevey

[lemur]

Generally I would agree with the post about trying to bike uphill without gears. However, if you were wanting to build an extremely light, high-efficiency car I could imagine building it with an engine and clutch but no gear box to save weight. That way, you could just have the engine tuned to optimum efficiency and sacrifice acceleration. You could release the clutch slowly enough to prevent the engine from stalling and then slowly reach target speed and then avoid braking, e.g. highway travel. This also seems like a good idea for rail transit, though I know nothing about how such vehicles accelerate and manage power in terms of gearing.

[SMF]

Lemur. You could accomplish what you suggest with an automatic transmission. That is, a fluid coupled torque converter between the engine and transmission in place of a manual clutch. However, the real problem is that gasoline (and propane and methane) ICEs produce very little torque at low RPMs, thus the transmission. Electric motors and steam engines produce maximum torque at 0 RPM and wouldn't need a transmission, while diesel engines are somewhere in between with higher torque, than gasoline, at low RPM, but limited high RPM horsepower. If I were going to make an extremely light, high efficiency car I would either do what the X-Prize 100 MPG winners did with a gasoline car, or make an electric car with a very low weight, high RPM, ICE for occasional charging and include a solar panel in the package. SM

[lemur]

Lemur. You could accomplish what you suggest with an automatic transmission. That is, a fluid coupled torque converter between the engine and transmission in place of a manual clutch. However, the real problem is that gasoline (and propane and methane) ICEs produce very little torque at low RPMs, thus the transmission. Electric motors and steam engines produce maximum torque at 0 RPM and wouldn't need a transmission, while diesel engines are somewhere in between with higher torque, than gasoline, at low RPM, but limited high RPM horsepower. If I were going to make an extremely light, high efficiency car I would either do what the X-Prize 100 MPG winners did with a gasoline car, or make an electric car with a very low weight, high RPM, ICE for occasional charging and include a solar panel in the package. SM

Ok, but any transmission, automatic or manual, adds weight and mechanical complexity. Assuming acceleration and top speed are not issues (only efficiency), why not design a vehicle with a top speed of @40mph and accelerate it using auxiliary power, e.g. pedaling (think Flintstones)? It would basically be a bicycle with long-distance capability by engaging the motor at @20 (top human-powered speed, I think). I think this would involve all horsepower and no torque, and require maybe 10-20hp for a 500-1000lb vehicle, no? Such a vehicle could be handy for moving large amounts of vegetables around town in some extreme future devoid of all but minimal fuel availability.

Edited February 10, 2011 by lemur

[ewmon]

Why do we need gears?

(Please note that the graphs shown below are not for the same engine or vehicle.)

 

You will find the answer in an engine's power and torque (versus rpm) curves. Engines produce usable torque and power over a limited rpm range, so the different gear ratios allow the usable power and torque range to be applied over the entire range of vehicle's range of speed.

 

 

So, using the diagram above, let's say you wanted the ability to apply at least 90 ft-lb of torque at all speeds. This means you will want to shift when the engine reaches 9,500 rpm, and you want engine at 6,800 rpm at the "bottom" of the next gear. This dictates that each gear ratio will be about 70% of the previous one. That is, 6,800/9,500 = 0.70. Note that I chose 5th gear to be "direct drive" and I based the other ratios from there.

 

1st 4.00

2nd 2.80

3rd 2.00

4th 1.40

5th 1.00 (ie, "direct drive")

6th 0.70 (ie, "overdrive")

 

You would shift at the vehicle speeds shown below. The first shift point is part of the design (I arbitrarily picked 40 mph), but the other shift points follow a ratio based on the torque curve, which means that the next shift point is about 1.40 (which is 9,500 rpm/6,800 rpm) times the previous shift point.

 

1st to 2nd at 40 mph

2nd to 3rd at 56 mph (that is, the engine reaches 9,500 rpm in 2nd gear at 56 mph)

3rd to 4th at 80 mph (etc, etc)

4th to 5th at 112 mph

 

When you plot the torque at the wheels in each gear, you get something like this (These curves don't match my numbers above, but you can see where the torque curves crisscross and you'll get more torque in the next gear ... for example, shift into 4th gear at 100 mph):

 

 

If you used only 5th gear, your car would not have much acceleration at low speeds. If you used only 1st gear, you'd never get past 40 mph, because the engine won't rev that high.

[Xittenn]

I just thought this was noteworthy.

 

There was mention of using epicyclic gearing in direct coupling of electric motors to their respective wheels in electric vehicles when electric vehicles were becoming a higher priority on developers lists but I find no current mention. I guess that would just mean they each get their own automagik transmission but still, it was somehow different in that the gears had a uniquely fashioned locking mechanism that could be actuated for varying conditions :/

Edited February 10, 2011 by Xittenn

[Harish Kumar]

It's easier to make an engine that runs at a certain speed than a variable speed, so your idea would also lose efficiency.

 

Get yourself an electric car. Electric engines can run at variable speed much more easily, and in fact some designs call for having the motors right on the wheels to dispense with the drive shaft as well.

 

From what I have been reading there is a trade-off between speed(rpm) and torque, and how does an electric motor, starting from zero rpm able to produce the necessary torque to turn the wheels which are standing still. Wouldn't even the electric motor need to be running at certain rpm to produce the torque required to turn the wheels from standstill?

[Pantaz]

 

From what I have been reading there is a trade-off between speed(rpm) and torque, and how does an electric motor, starting from zero rpm able to produce the necessary torque to turn the wheels which are standing still. Wouldn't even the electric motor need to be running at certain rpm to produce the torque required to turn the wheels from standstill?

 

Electric motors produce maximum torque at zero rpm.