Question regarding peak torque in first gear

[Emre]

Hello,I have been trying to wrap my mind around this question for a long time.So I thought it is best the way to share it with you.

For a given car speed,the greatest acceleration comes when we are at peak horsepower RPM.This is true for any gear except the first gear.Because in any other gear,you could downshift to get the greatest acceleration but you can't do this in first gear.

Since the wheel torque is engine torque x gear ratio x differential ratio,I think the best acceleration in first gear comes when we are at torque peak.

So it seems the best and the fastest way to launch the car is to rev until torque peak RPM and launch at this point.(You can neglect tire spin for now.)

Once the car moves along,the car having more horsepower wins the race,assuming the gear ratios are appropriate for both cars.

So here is my question.
For many diesel engine cars the max torque comes at 1500-1750 RPM.When you want to launch at that RPM from a dead stop,tire don't spin very much.But if you rev higher(like 3000-4000 RPM).,the wheel spin becomes too much.Why do this happen?Don't tires theoretically have to spin too much when RPMs are close to torque peak,since in first gear this is the point where the most wheel torque comes?

 

[miltos]

The wheel spin you mention happens due to engine rotational inertia. When realising the clutch pedal the engine will deccelarate to lower rpm producing extra torque.

 

[Emre]

The wheel spin you mention happens due to engine rotational inertia. When realising the clutch pedal the engine will deccelarate to lower rpm producing extra torque.

Could you enlighten me a bit more please?

 

[OldYat47]

No, peak acceleration occurs when the torque at the driven wheel or wheels is at maximum. Power is not related to acceleration. Power is work over time, or force times velocity. For any given rear wheel horsepower there is an infinite number of possibilities for acceleration (just change the velocity). You have to take the time (or velocity) factor out, leaving you with torque (or force).
Wheel spin happens when the torque supplied to the driven wheel or wheels is great enough to overcome the static friction between the tire and the road surface. Once breakaway occurs the tire will keep spinning since the coefficient of sliding friction is lower than the coefficient of static friction, or until the tire surface speed and vehicle road speed match up.

 

[CWatters]

Another way of saying this is that what matters is the maximum _usable_ power, not the maximum available power from the engine.

 

[OldYat47]

I want to expand on my comment above about wheel spin and breakaway torque. The faster the engine turns the greater the inertia. That inertia can be a greater influence on tire breakaway than engine torque. Launching a vehicle without spinning the tires is a balance between available tire to road friction, engine torque and engine inertia. It's not a simple problem, there are many variables that must be considered. For example, weight transfer to the rear wheels in a rear wheel drive car helps, but in a front wheel drive car hurts.
This statement, "When realising the clutch pedal the engine will deccelarate to lower rpm producing extra torque" (sic), is not necessarily correct. It depends on the shape of the torque curve.
Also note that sitting at a stop the engine is producing zero net torque and zero net horsepower regardless of RPM until the drive train is engaged. Inertia does the initial movement unless you use something like line lock brakes.

 

[Emre]

No, peak acceleration occurs when the torque at the driven wheel or wheels is at maximum. Power is not related to acceleration. Power is work over time, or force times velocity. For any given rear wheel horsepower there is an infinite number of possibilities for acceleration (just change the velocity). You have to take the time (or velocity) factor out, leaving you with torque (or force).
Wheel spin happens when the torque supplied to the driven wheel or wheels is great enough to overcome the static friction between the tire and the road surface. Once breakaway occurs the tire will keep spinning since the coefficient of sliding friction is lower than the coefficient of static friction, or until the tire surface speed and vehicle road speed match up.

You confused me a bit.Let me ask the question by illustrating my car.
It has

I want to expand on my comment above about wheel spin and breakaway torque. The faster the engine turns the greater the inertia. That inertia can be a greater influence on tire breakaway than engine torque. Launching a vehicle without spinning the tires is a balance between available tire to road friction, engine torque and engine inertia. It's not a simple problem, there are many variables that must be considered. For example, weight transfer to the rear wheels in a rear wheel drive car helps, but in a front wheel drive car hurts.
This statement, "When realising the clutch pedal the engine will deccelarate to lower rpm producing extra torque" (sic), is not necessarily correct. It depends on the shape of the torque curve.
Also note that sitting at a stop the engine is producing zero net torque and zero net horsepower regardless of RPM until the drive train is engaged. Inertia does the initial movement unless you use something like line lock brakes.

My car produces 90 hp at 4000 RPM and 200 Nm torque between 1750-3000 RPM.If I wanted to haul some timber with my car and increased the weight,would it be better for me to launch between 1750-3000 RPM since this produces the most wheel torque?

 

[OldYat47]

In theory, if your torque curve is flat, yes. A load of lumber will increase the force required to get the whole mass moving, so now the real world considerations of tire friction come into play.
And I must reiterate, power and acceleration are not related. Acceleration is (force / mass). Power is (force X distance / time), or (force X velocity). So keeping mass constant, for any value of power there are an infinite number of values of force (change the velocity), therefore an infinite number of values of acceleration. power and acceleration are not related.

 

[ABINASH KUMAR]

i have a question that why acceleration measured below max speed in particular gear i mean why gear shift happen below 10% less speed of actual speed.