Say I am driving my car (Hyundai i10) at 2nd gear. My clutch is engaged. As I increase my throttle position, rpm will increase and so will speed of the car till a point when the speed wont increase with increase in rpm as I increase my throttle.

Now 2nd case, I keep on increasing my throttle with my foot on the clutch pressed halfway. So I have disengaged my clutch half. What will happen in this case? What are the changes I can see in the car/engine? Will load increase? How much? Will the pattern be the same for Speed vs Rpm graph (speed increasing with rpm till a point and then becoming constant)?

P.S. plots to explain concepts are most welcome.

  • Driving the vehicle in 2nd case for any extended length of time will damage the car. It is not a valid scenario.
    – vini_i
    Feb 5 '16 at 15:26
  • @vini_i Can you please elaborate? Why is it not a valid scenario? I have seen people driving with constantly pressing the clutch while driving. Will there be any change in the plots?
    – Soumya Sen
    Feb 5 '16 at 15:38
  • If it's actually pressed halfway then clutch slippage might or might not occur. If it does - you will see RPMs rise without speed increasing. If it doesn't - you won't feel anything, but the throwout bearing will wear at an accelerated rate. Feb 5 '16 at 15:43
  • Also, I'm not sure why you say that speed will stop increasing with RPM at some point in the first case. It will stop increasing when RPM stops increasing. In the case of fully engaged and not badly worn clutch, that is. Feb 5 '16 at 15:44
  • 1
    What your asking is akin to; what happens if i drain out the engine oil and fill the engine with noodles. The answer is; it will damage the engine. If you see someone putting noodles in their engine you would stop them. If you see someone constantly pressing the clutch tell them that they will damage their clutch.
    – vini_i
    Feb 5 '16 at 15:46

To get us on the same footing, I'm going to assume by saying "clutch pressed halfway" you are suggesting the clutch be halfway engaged (meaning, you are still getting power from the engine, but it's not fully engaged).

What you are talking about is called slipping the clutch. It is a process whereby you can get the engine within the power/torque band without the vehicle itself being at the normal speed for the desired gear. By doing this, you get the benefit of the higher power/torque which allows the vehicle to accelerate quicker without lugging the engine. Once the vehicle has sped up to the point where it would normally be within the power/torque band, then the clutch would be allowed to be entirely engaged so the vehicle can speed up to the point where it should be upshifted to the next higher gear.

When the clutch is fully engaged (no slippage occurring), in any given gear, the engine RPM to vehicle speed ratio will be constant and very predictable. Here is a chart which basically describes how it is for a 5-speed Mitsubishi Mirage (other vehicles will be similar, but different):

enter image description here

If you are slipping the clutch, engine speed will probably stay relatively constant while the vehicle speeds up. This means it is non-linear and not predictable. You can again assume the vehicle speed will increase, but by how much and how quickly all depends upon how engaged the clutch is as well as how much power/torque the vehicle you are testing has.

Calculated engine load will be unchanged, as that is a factor of throttle position and engine speed. External load will be unchanged ... but I don't believe this is the type of load you are talking about.

It is of note it is very bad to slip the clutch for an extended period of time. This creates a very large amount of frictional heat buildup within the clutch assembly (friction disk, pressure plate, and flywheel). This has a tendency to cause premature clutch failure and heat cracks within the flywheel. It also puts an enormous amount of stress on the throwout bearing as it is being used continuously while the clutch pedal is depressed to any extent.

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