A Simpler Explanation Of The Measurement Of ONE HORSE POWER

jharrell wrote:

4x4ord wrote:
Keep in mind that reving the engine from 6000 rpm to 13000 rpm is only increasing the vehicle speed by a factor of 2.17. The Cat in my Peterbilt makes 400 HP by 1100 rpm, reaches a peak of 500 hp by 1300 rpm and holds the power flat to 2100 rpm. So even a big slow turning diesel can double the speed of the vehicle while maintaining 80% of its peak HP.

So real RPM matters here not just rpm factor. A Tesla model 3 goes 0-140 mph with a single speed 9:1 gearbox. That means its torque is multiplied 9x to the wheels across all rpms. Your diesel cannot do that, the gear ratio needed to run at highway speed would not multiply the torque enough off the line to get the vehicle moving and stall the engine.

Your diesel has 0 power and torque below what 600-700rpm because below that it stalls. The Tesla motor has max torque from 0-6000 rpm with usable power all the way.

Also you are comparing a 3000lbs diesel engine to a electric drive that weighs maybe 300lbs with inverters. The Tesla semi has 4 Tesla model 3 motors for 1000hp and 2000lb-ft of torque which weigh maybe about a 1000lbs with inverters. I would imagine the gear ratio is lower than 9:1 since they don't need to go to 140mph meaning even greater wheel torque multiplication.

Diesel are better than gas for sure when it comes to power curves, but they are heavier and more expensive to make due to the more robust design needed to take the high compression ratios and consequently torque numbers (along with direct injection and force induction). Electric still easily surpasses it though when it comes to power curve.

I agree that there are reasons to appreciate the capabilities of an electric motor. The only point i was making is that stating an electric motor has the ability to generate 80% of its peak power over a 7000 rpm operating range doesn't mean much when that operating range is from 6000 to 13000 rpm.

4x4ord wrote:
Keep in mind that reving the engine from 6000 rpm to 13000 rpm is only increasing the vehicle speed by a factor of 2.17. The Cat in my Peterbilt makes 400 HP by 1100 rpm, reaches a peak of 500 hp by 1300 rpm and holds the power flat to 2100 rpm. So even a big slow turning diesel can double the speed of the vehicle while maintaining 80% of its peak HP.

So real RPM matters here not just rpm factor. A Tesla model 3 goes 0-140 mph with a single speed 9:1 gearbox. That means its torque is multiplied 9x to the wheels across all rpms. Your diesel cannot do that, the gear ratio needed to run at highway speed would not multiply the torque enough off the line to get the vehicle moving and stall the engine.

Your diesel has 0 power and torque below what 600-700rpm because below that it stalls. The Tesla motor has max torque from 0-6000 rpm with usable power all the way.

Also you are comparing a 3000lbs diesel engine to a electric drive that weighs maybe 300lbs with inverters. The Tesla semi has 4 Tesla model 3 motors for 1000hp and 2000lb-ft of torque which weigh maybe about a 1000lbs with inverters. I would imagine the gear ratio is lower than 9:1 since they don't need to go to 140mph meaning even greater wheel torque multiplication.

Diesel are better than gas for sure when it comes to power curves, but they are heavier and more expensive to make due to the more robust design needed to take the high compression ratios and consequently torque numbers (along with direct injection and force induction). Electric still easily surpasses it though when it comes to power curve.

jharrell wrote:

valhalla360 wrote:
Correction, the horsepower band for a gas motor is typically wider than an electric motor.

It has to do with the fact that typical electric motors can generate peak HP from 0 to max RPM...the result is the higher the RPM, the higher the HP (sometimes at the top there are some oddities in the torque but they typically are outside normal operating ranges). The result is a linear increase in available HP as RPM increases.

On the other hand, typical gas engines (and diesel) start with relatively low torque and the torque increases as the RPM increases ...but as you near peak RPM, the torque often starts to drop off. The result is the actual HP being generated (not the rating) for a similarly rated engine starts lower but builds more quickly before dropping off around the peak RPM...

This results in a range of ideal RPM where you can produce peak (really near peak) HP for a particular engine. For an electric, peak is generally only at the peak RPM.

Not sure what electric motors you are looking at but this is not my understanding. An induction motor such as on a typical electric pump will pull current based on load exceeding its rated hp if needed at any rpm (perhaps tripping a breaker or overheating). The locked rotor torque and pull up torque can be much higher than rated torque in order to bring the motor up to operating speed.

I guess one issue is the case of a set speed induction motor is how hp is rated between gas and electric, gas is peak, while the electric is operating hp. A 3/4 hp electric motor may have a peak of well over 2 hp from zero to operating speed.

When you get into variable speed electric motor such as those used in cars the power band is still far superior. For instance Tesla has peak hp at 8000 rpm but produces more than 80% of peak from 6000-13,000 rpm, a 7000 rpm range that is an rpm range greater than the entire operating range of many gas engines. Hence the need for only a single speed transmission:

Keep in mind that reving the engine from 6000 rpm to 13000 rpm is only increasing the vehicle speed by a factor of 2.17. The Cat in my Peterbilt makes 400 HP by 1100 rpm, reaches a peak of 500 hp by 1300 rpm and holds the power flat to 2100 rpm. So even a big slow turning diesel can double the speed of the vehicle while maintaining 80% of its peak HP.

valhalla360 wrote:
A similar way to think about this is:
- Torque (at the wheels) determines...IF you can climb the hill.
- HP determines...how FAST you can climb the hill.

Unless I'm not interpreting the above correctly ... what you say seems to support my thinking that goes something like this: "Horsepower is horsepower - so given the same weight to haul and given the proper gearing for each - a 400 HP gas engine should be able to haul anything at the same speed that a 400 HP diesel engine can".

(Not considering altitude of course, since so many modern diesel engines are turbocharged but so many modern gas engines aren't.)

valhalla360 wrote:
Correction, the horsepower band for a gas motor is typically wider than an electric motor.

It has to do with the fact that typical electric motors can generate peak HP from 0 to max RPM...the result is the higher the RPM, the higher the HP (sometimes at the top there are some oddities in the torque but they typically are outside normal operating ranges). The result is a linear increase in available HP as RPM increases.

On the other hand, typical gas engines (and diesel) start with relatively low torque and the torque increases as the RPM increases ...but as you near peak RPM, the torque often starts to drop off. The result is the actual HP being generated (not the rating) for a similarly rated engine starts lower but builds more quickly before dropping off around the peak RPM...

This results in a range of ideal RPM where you can produce peak (really near peak) HP for a particular engine. For an electric, peak is generally only at the peak RPM.

Not sure what electric motors you are looking at but this is not my understanding. An induction motor such as on a typical electric pump will pull current based on load exceeding its rated hp if needed at any rpm (perhaps tripping a breaker or overheating). The locked rotor torque and pull up torque can be much higher than rated torque in order to bring the motor up to operating speed.

I guess one issue is the case of a set speed induction motor is how hp is rated between gas and electric, gas is peak, while the electric is operating hp. A 3/4 hp electric motor may have a peak of well over 2 hp from zero to operating speed.

When you get into variable speed electric motor such as those used in cars the power band is still far superior. For instance Tesla has peak hp at 8000 rpm but produces more than 80% of peak from 6000-13,000 rpm, a 7000 rpm range that is an rpm range greater than the entire operating range of many gas engines. Hence the need for only a single speed transmission:

There is such a thing as insufficient torque. It's kinda like funds.

Rpm at the wheels tells you whether you can climb the hill and how fast.:)

All I could afford wrote:
My high school physics teacher explained it this way... you can exert as much force on an object as you want, generating lots of torque, but if the object doesn’t move, you didn’t accomplish any measurable work, and thus didn’t generate any power, even if you are covered in sweat.

A similar way to think about this is:
- Torque (at the wheels) determines...IF you can climb the hill.
- HP determines...how FAST you can climb the hill.

jharrell wrote:

Gas engines put out their rated peak horsepower in a relatively narrow range compared to an electric motor.

Correction, the horsepower band for a gas motor is typically wider than an electric motor.

It has to do with the fact that typical electric motors can generate peak HP from 0 to max RPM...the result is the higher the RPM, the higher the HP (sometimes at the top there are some oddities in the torque but they typically are outside normal operating ranges). The result is a linear increase in available HP as RPM increases.

On the other hand, typical gas engines (and diesel) start with relatively low torque and the torque increases as the RPM increases ...but as you near peak RPM, the torque often starts to drop off. The result is the actual HP being generated (not the rating) for a similarly rated engine starts lower but builds more quickly before dropping off around the peak RPM...

This results in a range of ideal RPM where you can produce peak (really near peak) HP for a particular engine. For an electric, peak is generally only at the peak RPM.

turbojimmy wrote:
Although electric motors have near flat torque curves (which is why train locomotives are diesel/electric).

Actually, diesel-eletric trains are using the electric part to replace the transmission.

Imagine trying to build a mechanical transmission that would handle 10,000hp each from 5 engines. Try to release the clutches without stalling the diesels at the same time...this is especially bad as these large slow turning diesels have a very narrow RPM band, so you would likely need dozens of gears to go from 0 to 60mph.

With Diesel-Electric, it's easy as the front engine can have controls that ramp up the diesel motor power output and then gradually start taking that power via the electric motors and apply it to the wheels.

It's actually slightly less efficient than direct mechanical drive but because it solves the transmission issue, it's worth it.