Why Does IC Engine Torque Change with RPM and Gearbox Mechanics?

[koolraj09]

Hi guys!
This is a question related to IC engine characteristics(performance curves, torque-speed curve).
Why is the Starting torque of IC engines small? Also why does it decrease after a maximum?
Secondly,
In one of the lectures, our professor asked us that in a gearbox form where does the extra torque comes from? Like the input torque of a gearbox is T and speed is reduced by 9 times, the output torque is 9T. So from where does the extra 8T come from?
The answer which he gave was that extra torque comes from reaction(reaction torque) provided by foundation, bolted joint.
I didn't understand this answer.
Any help would be greatly appreciated.

 

[brewnog]

The torque characteristic is broadly a function of breathing; torque begins to drop above a certain engine speed because as speed increases, you have less and less time to fill the cylinders with air (and you need air to burn your fuel). At very low speed you can fill the cylinders, but you have fewer combustion events per second, this increases with speed until the breathing effect takes over. There are many other factors but this gives you an idea of the basic characteristic.

Through a gearbox, power is conserved, and is equal to the product of torque and angular speed. So if the input shaft rotates at 1,000rpm with a torque of 50Nm, and the output shaft rotates at 500rpm, the output torque will be 100Nm. There are some losses, but this is the premise.

 

[Lsos]

An automobile ICE engine, by design, has to be moving to make power. It has to suck in air, compress it, and expel it, and it's not going to do that at 0 RPM. It's not going to do it at 10 RPM either, or even 100 unless it's some huge marine engine. Remember, a piston only makes power 25% of the time, and this power has to pay for the remaining strokes, including compression, which slows it WAY down. So in general, it has to spin at a decent speed to make enough power/torque to be able to even pay for itself. Once it gets to that speed, it has to spin faster to make extra power, and even faster to make the kind of power you want. Depending on design, you'll usually end up being in the thousands of RPM before you get to that point...


At the same time as the rpms start to rise, the air is also flowing through the engine faster and faster. Any imperfections: curves, valves, anything that gets in the way starts to impede the flow of air more and more. The engine has to work harder to pump it in and out. Compounding this is that you have less time to fill the cylinders. Then there's increased friction. Coolant flow. These factors and more conspire to decreased power/ torque. Eventually you reach an rpm where they overwhelm the factors that increase power, and so power/torque starts to drop.

All these things and more play their part in the engine's torque curve.