Torque Power etc
Posted: Thu Jan 15, 2009 10:12 am
About time this forum had something interesting posted?
Torque, power and performance
The facts are:
The torque developed by the engine is multiplied by the gearbox and axle ratios to produce the drive wheel torque, which when then divided by the tyre rolling radius gives the tractive effort.
To drive the car at a constant speed requires a certain tractive effort, to overcome wind and rolling resistances, the major part of which varies as the square of the speed. If the applied tractive effort exceeds the required effort the car accelerates at a rate equal to the available effort minus the required effort divided by the mass of the vehicle. At maximum speed, or at any lower constant speed at part throttle, applied effort equals required effort and the acceleration is zero.
Engine torque times engine speed equals engine power, which, apart from transmission losses, is the power driving the car, which equals the tractive effort times the vehicle speed. It therefore follows that max acceleration at any vehicle speed is achieved when the engine is developing its max power.
Changing up when over the peak of the torque curve to pick up at the same level on the rising torque curve in the higher gear isn?t the route to best performance. Although the engine torque remains high, in the higher gear it has been multiplied by a lower number to produce the prop shaft torque; a higher prop shaft torque and therefore tractive effort is maintained by hanging on to the lower gear up to at least max power speed.
In fact, to stay as close as possible to the max power and acceleration condition as the car accelerates means actually going over the peak of the power curve until the power has fallen to the same level as on the rising power curve in the higher gear, mechanical sensitivity or a deep enough pocket permitting!
What this simple analysis does not take into account is the engine torque required to accelerate the rotary inertia of the engine and flywheel. This can be expressed as an "additional equivalent mass", the value of which is different for each gear. It is probably only significant in reducing the torque reaching the prop shaft in the lower gears, when, for example, the engine may have to be accelerated from 700rpm to 4000rpm whilst the car goes from walking speed to 24mph.
Apparently...
Torque, power and performance
The facts are:
The torque developed by the engine is multiplied by the gearbox and axle ratios to produce the drive wheel torque, which when then divided by the tyre rolling radius gives the tractive effort.
To drive the car at a constant speed requires a certain tractive effort, to overcome wind and rolling resistances, the major part of which varies as the square of the speed. If the applied tractive effort exceeds the required effort the car accelerates at a rate equal to the available effort minus the required effort divided by the mass of the vehicle. At maximum speed, or at any lower constant speed at part throttle, applied effort equals required effort and the acceleration is zero.
Engine torque times engine speed equals engine power, which, apart from transmission losses, is the power driving the car, which equals the tractive effort times the vehicle speed. It therefore follows that max acceleration at any vehicle speed is achieved when the engine is developing its max power.
Changing up when over the peak of the torque curve to pick up at the same level on the rising torque curve in the higher gear isn?t the route to best performance. Although the engine torque remains high, in the higher gear it has been multiplied by a lower number to produce the prop shaft torque; a higher prop shaft torque and therefore tractive effort is maintained by hanging on to the lower gear up to at least max power speed.
In fact, to stay as close as possible to the max power and acceleration condition as the car accelerates means actually going over the peak of the power curve until the power has fallen to the same level as on the rising power curve in the higher gear, mechanical sensitivity or a deep enough pocket permitting!
What this simple analysis does not take into account is the engine torque required to accelerate the rotary inertia of the engine and flywheel. This can be expressed as an "additional equivalent mass", the value of which is different for each gear. It is probably only significant in reducing the torque reaching the prop shaft in the lower gears, when, for example, the engine may have to be accelerated from 700rpm to 4000rpm whilst the car goes from walking speed to 24mph.
Apparently...
