Auto specifications: Power & Torque [and other questions]

In summary: In addition, engines are more efficient at lower rpm, so it makes sense to use a lower gear to keep the engine working at its most efficient point, especially when climbing hills or hauling heavy loads. In summary, auto specifications cards mention both 'bhp' and 'torque @ a certain rpm' as separate specifications because the peak power and peak torque of an engine occur at different engine speeds. When the accelerator of a car is pressed, more fuel and air are put into the engine, allowing for more power and speed. Lower gears have a lower gear ratio, providing more torque to the wheels and allowing for better acceleration at lower speeds. This is necessary for moving off from a standing start and for efficient engine operation at lower speeds.
  • #1
rohanprabhu
414
2
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
 
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  • #2
rohanprabhu said:
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 let's 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.
 
  • #3


I would like to clarify some misconceptions and provide answers to your questions regarding auto specifications.

Firstly, bhp (brake horsepower) is indeed a measure of the power produced by an engine. However, it is important to note that this is the maximum power output of the engine under ideal conditions and does not account for any losses in the transmission or other components. The actual power output of an engine can vary depending on various factors such as engine wear, temperature, and altitude.

Secondly, torque and power are related but are not the same thing. Torque is the rotational force produced by the engine and is measured in units of force multiplied by distance (such as Nm or lb-ft). Power, on the other hand, is the rate at which work is done and is measured in units of force multiplied by distance divided by time (such as horsepower or watts). Therefore, while torque and angular velocity do contribute to power, they are not interchangeable.

When you press the accelerator of your car, you are opening the throttle and allowing more air and fuel into the engine. This increases the power output of the engine, which in turn increases the speed of the wheels and propels the car forward.

You are correct that lower gears have a lower gear ratio, which means that for every rotation of the wheels, the engine has to rotate more times. This results in a higher torque being produced at lower speeds. This is necessary because a greater force is needed to overcome the inertia of a stationary object (such as a car) and get it moving. As the car gains speed, the gear ratio can be increased, and the engine can rotate at a lower speed while still maintaining the same speed of the wheels.

I hope this helps clarify some of your questions about auto specifications. It is important to understand these concepts when looking at car specifications and choosing the right vehicle for your needs.
 

1. What is the difference between power and torque in a car?

Power and torque are often confused, but they are two different measurements. Power is a measure of how quickly work can be done, while torque is a measure of the twisting force applied to an object. In simpler terms, power is the rate at which a car can accelerate, while torque is the force that enables it to accelerate.

2. How are power and torque related?

Power and torque are related through the equation P = T x RPM, where P is power, T is torque, and RPM is the engine speed. This means that if two cars have the same power, the one with higher torque will have a greater ability to accelerate at lower speeds.

3. What is the significance of a car's horsepower?

Horsepower is a unit of measurement for power and is commonly used to describe a car's engine performance. The higher the horsepower, the more powerful the engine and the faster the car can accelerate. However, horsepower is not the only factor in a car's performance and should not be the sole consideration when choosing a car.

4. How do power and torque impact a car's fuel efficiency?

The relationship between power, torque, and fuel efficiency is complex and depends on various factors such as engine size, weight of the car, and driving habits. Generally, a car with higher power and torque will consume more fuel, but technological advancements have made it possible to improve fuel efficiency without compromising on performance.

5. Can a car have high power and low torque, or vice versa?

Yes, a car can have high power and low torque, or vice versa. For example, sports cars typically have high power and low torque, as they are designed for high-speed performance. On the other hand, trucks and SUVs often have high torque and lower power, as they are designed for hauling and towing capabilities.

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