Torque vs kW

[Pox]

Here is a real world example. Me and colleague. He kept boasting about power. I told him that torque and power band is more important.

And power to weight ratio is even more important.

 

[TheTwo]

If we go back to basic physics then it makes sense:

An object that is accelerated needs to gain energy. Energy is measures in kw.

Given an infinitely long road, the engine with higher power will accelerate the car to the higher speed (if mass and drag are equal for both cars). This is only logical, because the engine with the lower energy output in kw can't possibly add more energy to the car than the engine that produces more kw.

Now to go a step further: speed results from the integral (or "sum) of all the energy added over time to a car.

The typical diesel engine has more torque than a petrol engine. But it can only add energy in small "bursts" in a rev range of 1500rpm to 4000 rpm and then a gear change is required, where no additional energy is delivered. Also, once a gear is changed, then diesel is at lower rpm and must build up revs to make more power (since power = revs x torque).

The petrol engine, on the other hand, can add energy over a much wider rpm band from 1000 to even 7000rpm. And when the gears are changed, then engine rpm drops to 4000 or 5000rpm maybe, which produces a lot more power than the diesel at 1500rpm.

These are just examples. But basically to sum it all up: The diesel adds energy in strong bursts or "lumps" to the car. The gives you that "thrust" in your back feeling. But the petrol car adds energy in a gradually increasing, consistent slope. This gives less of a push in the back feeling, but ultimately the amount of energy the petrol car adds to the car is higher when summed up.

This is why a car with more kw will ultimately win the race. It has to, all else being equal. But over shorted distances, the car with more torque might "feel" faster and win up to a point, until the energy needed to accelerate the diesel, cant beat the energy added to the petrol car.

Very well explained thank you. I could finally understand the difference easily

 

[Suspect99]

Very well explained thank you. I could finally understand the difference easily

Sort of right. But energy is not measured in KW. It's Joules. KW is power. Rate of energy

 

[TheTwo]

Sort of right. But energy is not measured in KW. It's Joules. KW is power. Rate of energy

I suspect you may have 99 problems but power aint one

 

[Suspect99]

I suspect you may have 99 problems but power aint one

what can I say? I'm an engineer

 

[LinuxMan]

Well summed up there.
I understand this much better now.

If we go back to basic physics then it makes sense:

An object that is accelerated needs to gain energy. Energy is measures in kw.

Given an infinitely long road, the engine with higher power will accelerate the car to the higher speed (if mass and drag are equal for both cars). This is only logical, because the engine with the lower energy output in kw can't possibly add more energy to the car than the engine that produces more kw.

Now to go a step further: speed results from the integral (or "sum) of all the energy added over time to a car.

The typical diesel engine has more torque than a petrol engine. But it can only add energy in small "bursts" in a rev range of 1500rpm to 4000 rpm and then a gear change is required, where no additional energy is delivered. Also, once a gear is changed, then diesel is at lower rpm and must build up revs to make more power (since power = revs x torque).

The petrol engine, on the other hand, can add energy over a much wider rpm band from 1000 to even 7000rpm. And when the gears are changed, then engine rpm drops to 4000 or 5000rpm maybe, which produces a lot more power than the diesel at 1500rpm.

These are just examples. But basically to sum it all up: The diesel adds energy in strong bursts or "lumps" to the car. The gives you that "thrust" in your back feeling. But the petrol car adds energy in a gradually increasing, consistent slope. This gives less of a push in the back feeling, but ultimately the amount of energy the petrol car adds to the car is higher when summed up.

This is why a car with more kw will ultimately win the race. It has to, all else being equal. But over shorted distances, the car with more torque might "feel" faster and win up to a point, until the energy needed to accelerate the diesel, cant beat the energy added to the petrol car.

 

[werner]

hp = torque x rpm /5252. It is a calculated value.
To accelerate, you need torque to come in early (because that means your calculated hp is high too at the biginning). Dynos measure torque, and mathematically calculate hp, that is why the two lines of the graph WILL always cross at 5252rpm.
turbo petrol is the best bet, with small turbos. That will make a car drivable, streetable and usable and fast. A nice flat torque curve, staring early, like 1500-2000rpm and staying strong till rev limiter is what is ideal.
Stay away from 0-100kmh....(i didnt shift right..too much wheelspin, my tires are cold blah blah bull****)
Use something understandable like 60-160. As long as both cars are in a similar rev range at the beginning it'll be a fair test.

you cant really go wrong by having a read here
http://rubydist.com/Family/Power.html

 

[SauRoNZA]

hp = torque x rpm /5252. It is a calculated value.
To accelerate, you need torque to come in early (because that means your calculated hp is high too at the biginning). Dynos measure torque, and mathematically calculate hp, that is why the two lines of the graph WILL always cross at 5252rpm.
turbo petrol is the best bet, with small turbos. That will make a car drivable, streetable and usable and fast. A nice flat torque curve, staring early, like 1500-2000rpm and staying strong till rev limiter is what is ideal.
Stay away from 0-100kmh....(i didnt shift right..too much wheelspin, my tires are cold blah blah bull****)
Use something understandable like 60-160. As long as both cars are in a similar rev range at the beginning it'll be a fair test.

you cant really go wrong by having a read here
http://rubydist.com/Family/Power.html

But wouldn't Torque be what you want for roll-on/overtaking acceleration?

Say 80-120 you could leave the Diesel in a higher gear which is what makes it more driveable, whereas the petrol requires you to shift down to catch the revs?

Then again as you say a turbo'd petrol solves that problem.

 

[HeftyCrab]

Also which one would be taking a harder knock with the short trips? Petrol or Diesel? Or does it amount to a paper value with no real world difference?

I could be totally wrong here, but I believe that a petrol is better for short distance and a diesel better for long distance because the diesel can build up deposits over regular short distance travel.

Dont know if this is just an old story or only applicable to older cars. Maby someone can chime in.

 

[killerbyte]

What I love is when the car makers get clever.

The Alfa Romeo Giulietta 1.4T Distinctive has something called a Multiair unit. The standard 1.4T produces 88KW while the MA version kicks out 125KW and 250NM @ 2500 RPM.

 

[silver6933]

KW is how fast you are going when you hit the wall,
Torque is how far you take the wall with you after you hit the wall.



Understeer is when you see the tree you are about to hit

Oversteer is when do not see the tree you are going to hit

 

[silver6933]

Say 80-120 you could leave the Diesel in a higher gear which is what makes it more driveable

Yes but the use of higher gear in the diesel(rather than down shifting), means more unburnt diesel puffed out the back and also more carbon deposits. It is better to shift down.

This is called throttle tip in enrichment and usually this requires over fueling. over fueling results in bore wash and increased wear. I suspect it is the reason so many diesel cars have engine problems.

Sit behind any diesel without a DPF and you will see a cloud behind it when it accelerates, this is the unburnt fuel.

 

[SauRoNZA]

Yeah I see that cloud often on diesel Jettas.

 

[kripstoe]

Yes but the use of higher gear in the diesel(rather than down shifting), means more unburnt diesel puffed out the back and also more carbon deposits. It is better to shift down.

This is called throttle tip in enrichment and usually this requires over fueling. over fueling results in bore wash and increased wear. I suspect it is the reason so many diesel cars have engine problems.

Sit behind any diesel without a DPF and you will see a cloud behind it when it accelerates, this is the unburnt fuel.

Sort of. For a diesel, white is unburnt and black is where the fuel didn't combust completely (due to insufficient air/oxygen afaik), and formed black soot (carbon) particles.

 

[Gaz{M}]

Sort of right. But energy is not measured in KW. It's Joules. KW is power. Rate of energy

You are correct. I dun goofed. kW is the amount of energy (in Joules) that the engine can add per second to the car.

 

[SauRoNZA]

What I love is when the car makers get clever.

The Alfa Romeo Giulietta 1.4T Distinctive has something called a Multiair unit. The standard 1.4T produces 88KW while the MA version kicks out 125KW and 250NM @ 2500 RPM.

Not quite

The same engine without the unit makes 114kw.

Sounds like fancy VVT to me.