Auto specifications: Power & Torque [and other questions]

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Main Question or Discussion Point

On all the auto specifications card that I read, they mention 'bhp', which i guess is the power provided by the engine, when none of the transmission losses are counted. But, is the power of a engine constant?

Secondly, the also provide a specification 'Torque @ a certain rpm'. But, isn't Torque x Angular Velocity = Power? Then why are there 2 separate specifications given?

Also, what exactly happens when I press the accelerator of my car?

Also, lower gears have a low gear ratio. Hence, for every single rotation of the wheel, more torque is provided on lower gears than for higher gears [am i right?]. Why is a higher torque needed at lower speeds?

Thanks,
rohan
 

Answers and Replies

brewnog
Science Advisor
Gold Member
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On all the auto specifications card that I read, they mention 'bhp', which i guess is the power provided by the engine, when none of the transmission losses are counted. But, is the power of a engine constant?
No, the power of a given engine at full throttle depends entirely upon engine speed.

Secondly, the also provide a specification 'Torque @ a certain rpm'. But, isn't Torque x Angular Velocity = Power? Then why are there 2 separate specifications given?
The peak power and peak torque occur at different engine speeds. For example, a particular engine develops 150bhp at 4,000rpm. This is the power peak. However, the torque peak for the same engine is 300Nm at 2,000rpm.

Also, what exactly happens when I press the accelerator of my car?
It depends on the type of car. The all-encompassing answer is that more fuel and air are put into the engine, allowing more engine speed/load to raise. The accelerator of a carburetted spark ignition engine simply opens a valve (the throttle) which lets more air into the engine; the carburettor then adds more fuel. A spark ignition engine with electronic fuel injection will also open a throttle, but also electronically squirt more fuel into the inlet tract, and also modify ignition timing.

A simple diesel engine will typically just squirt more fuel into the cylinders, but a more advanced one will also modify the injection timing profile and (if fitted) the turbo boost characteristics.

Also, lower gears have a low gear ratio. Hence, for every single rotation of the wheel, more torque is provided on lower gears than for higher gears [am i right?]. Why is a higher torque needed at lower speeds?
More torque is provided to the wheels in a lower gear. This is to facilitate moving off from a standing start, and to give high acceleration from low speeds.
 

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