Torque and power quiz 4

[yoshimitsuspeed]

I spend a lot of time in the automotive community and a lot of time on car forums and groups.
Ever since I started to learn the actual relationship of torque and power it drove me crazy how few others in the automotive world actually understood this basic formula. People capable of building motors that make 500 hp per liter and who still think that torque is low end power.
I have gotten into enough arguments with people to learn that most would rather argue relentlessly cause that's what theys daddy taught them than sit down and think about the simplicity of the formula long enough to understand the relationship of the two.
I have decided to try a different tact and make a little quiz that gets people thinking about this from a different angle and maybe hoping they will get the point that torque and power can't be compared, and that torque does not mean low end power.
I just started on this tonight and it's 3AM. I want to do more to improve it but I also would love some input from others on ways I could improve it.
I would like it to be as detailed and informative as possible while still being interesting and keeping the person engaged and interested.

Tell me what you think of what I have so far.
What could I do better?
What are other questions I could ask or ways I could put things to get people thinking about the relationship without getting too bogged down in the math to loose too many people?
On that note should I focus more on the math or stay more with the basic relationship and principles?

http://matrixgarage.com/content/think-you-understand-torque-and-power

 

[BrianPetersen]

VW TDI torque peak near 2000 rpm, that's also about what they run at on the motorway. Need to accelerate? No it's not at peak power, but it's at least at whatever power it makes at peak torque, so something happens. On the one that I had, at 2000 rpm it had about 180 lb.ft of torque, so about 69 horsepower, which is not bad for a 105 hp engine, and that's without downshifting. Sure, you could downshift to get more if needed, but it at least had a decent amount right there.

Comparable non-turbo petrol engines might be geared a little shorter (say 2500 rpm highway cruise) but peak torque on those is well beyond that, and peak power beyond that again. A 1.6 petrol might have the same or a bit more peak power (at 6000 rpm) and it may have perhaps 120 lb.ft (at perhaps 4500 rpm), and less than that at highway cruise. Let's say 100 lb.ft for argument's sake. That's about 50 horsepower without having to change ratio. Want any more than that, and you will have to wait. If manual, for the driver to change gear. If automatic, first for the dim-witted electronics to realize that the driver wants more, then for the transmission itself to do the cog-swapping (or clutch-swapping or cone-shifting, as the case may be), all of which take a certain amount of time.

It's not all about how fast it will go, but for a daily-driver vehicle, it's also about how fuss-free it will go about its business.

The 1.6 petrol would probably have to downshift two gears to get what the TDI has right there ...

Sure, the petrol would win a flat-out drag race, but that's not how most people drive their daily drivers most of the time.

 

[140Airpower]

You must be a rather civilized fellow. "Sure, the petrol would win a flat-out drag race, but that's not how most people drive their daily drivers most of the time". Not admitting to anything, but "most people" does not include all people.

 

[BrianPetersen]

No, it most certainly does not include all people ... but I'll maintain that it includes most people, based on how easily I can outrun traffic using a 12 hp motorcycle or a 70 hp car.

The 12 hp motorcycle does get wrung out all the time. That's the only way you can ride something like that!

 

[Tmoose]

The penalty for driving around with lots of extra BMEP available for throttle response in "an emergency" is going to look pretty bad in the brochure's EPA fuel economy ratings.

I rented some small auto transed something or other for a few weeks in 2014. It's insistence on shifting into a high gear at part throttle caught me by surprise. At very light throttle around town it would make that poor little engine's individual power pulses noisily and maybe even tactily apparent. A smidge more throttle would quickly, but pretty smoothly go down a gear or 2, which I bet was necessary to create the sensation of even modest acceleration.

 

[BrianPetersen]

The penalty for driving around with a 5000 rpm torque peak available will be greater than that of driving around with a 2000 rpm torque peak available.

Just because a turbo Suzuki Hayabusa has more power than a 14 litre Cummins diesel in a 80,000 lb tractor trailer doesn't mean it's a suitable device for the job, no matter what gearing you use ... Can it move the vehicle, "yes", is it a good idea, "no" ...

 

[LionelHutz]

Apparently, this quiz is a theoretical exercise to help understand torque and power. In a theoretical context, you could say a Hayabusa engine will out-accelerate a 14 litre Cummins diesel.

I think you'd need a completely different quiz if you wanted to help people understand why the Cummins is a better choice in the real world. Or why an engine with a broader power curve is nicer to drive in the real world. It could also be noted in some text afterwards.

 

[NormPeterson]

You must be a rather civilized fellow. "Sure, the petrol would win a flat-out drag race, but that's not how most people drive their daily drivers most of the time". Not admitting to anything, but "most people" does not include all people.

Maybe Brian's description of average daily driving doesn't fit all the people all of the time, but almost certainly covers all of the people most of the time.

It hardly matters which of my cars I'm driving; on the street I'm rarely at wide-open throttle . . . let alone way up there where peak power is developed.

When I'm out on the track, there is simply no point in running the engine at peak power rpm with barely 50% throttle going through a corner, as doing so would then force a potentially upsetting shift while there is still a lot of tire grip being demanded laterally. Being at or around peak torque rpm at the apex works much better, if only to help ensure that I stay on the black stuff and don't dig up any of the track's green stuff. Maybe all of my "enthusiastic driving" is closer to this than to the wheelspinning start, three shifts and coast kind of driving . . .


Norm

 

[NormPeterson]

Apparently, this quiz is a theoretical exercise to help understand torque and power. In a theoretical context, you could say a Hayabusa engine will out-accelerate a 14 litre Cummins diesel.

I think you'd need a completely different quiz if you wanted to help people understand why the Cummins is a better choice in the real world. Or why an engine with a broader power curve is nicer to drive in the real world. It could also be noted in some text afterwards.

From earlier,

Geared appropriately, running at max HP will provide maximum acceleration at any given speed (traction permitting and ignoring rotational inertias and other secondary effects). But the maximum acceleration with that same gearing will still occur at max torque.

so what needs to be done is to get people to recognize the difference.

The "torque is everything" crowd only sees max acceleration in some [fixed] gear occurring at peak torque rpm and rests their case. The power people see max acceleration at some given speed in terms of using the most that the engine can provide . . . and ditto.


Norm

 

[140Airpower]

Norm,
"Geared appropriately, running at max HP will provide maximum acceleration at any given speed (traction permitting and ignoring rotational inertias and other secondary effects). But the maximum acceleration with that same gearing will still occur at max torque".

The resolution of this "duality" of the IC engine characteristic is infinitely variable gearing (by whatever means, including electric drive). Without that we have what we have always had, a small number of gear ratios and the task of trying to get the most flexibility and performance -necessitating sacrificing maximum engine efficiency.

 

[NormPeterson]

As a "fix", an infinitely variable drive (typically a CVT) is at least a reasonable option in some cases. Including most folks' daily driving.

But in the context of this thread, it side-steps around the value of understanding torque and power. You're still stuck with the matter of what rpm do you want the engine to flash up to on any given demand for acceleration - do you want it to hit max power rpm for max acceleration or peak torque rpm for adequate and more economical acceleration (given that BSFC more or less mirrors the torque curve) . . . or somewhere in between? IOW, as a powertrain engineer/developer/aftermarket tuner you'd still have considerable interest in the topic of this thread even if your expected end users couldn't care less about it.


Norm

 

[GregLocock]

The Prius is a good, and well documented, example of how you use a stepless CVT. It works out the demand power, accelerates the engine to that point on its ideal operating curve (minimum bsfc for a given power) and then lets the trans do its magic.

Scatter plots of rpm vs cruising speed (hence power) obtained by users overlay very nicely over the bsfc map.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[140Airpower]

Norm, obviously, for max acceleration you go to the max power condition. You would operate the engine differently for max economy.

 

[tbuelna]

The Prius makes good use of its chain CVT. First, chain and push belt CVTs are friction devices that are more efficient than traction type CVTs. And the electric motor of the Prius eliminates the need for a torque converter for starting, which is normally a large source of losses in the drivetrain. The electric motor also reduces the total drive ratio required from the CVT variator. It can be difficult to get the optimum amount of drive ratio required from a single stage chain or push belt CVT variator. Chain or push belt variators are also very sensitive to slippage, so they require very conservative torque margins. Using an electric motor to supplement torque is a good approach to regulate torque transfer thru the variator.

 

[BrianPetersen]

Prius does not use a chain CVT ... it is a planetary gear set used as a torque splitter, and the CVT function is done through the electrical system. All Toyota and Ford hybrids are like this, and the Volt is sorta like that.

The chain CVTs are in the non-hybrid Toyotas (and many others).

The planetary gear set arrangement means the Prius arrangement doesn't have to wait for the CVT to change ratio. If the driver wants instant torque, it can be delivered through the electric portion of the drivetrain. It only works because it is a hybrid.

The production chain/push-belt CVTs that I am aware of, still use a torque converter for starting off from a stop.

 

[GregLocock]

Quick amplification - the Prius transmission is essentially a differential, or mixer. One input comes from the engine and the other input is from a motor gen. The notional output shaft which is direct coupled to the wheels also has a motor gen on it.

Since a differential is a torque balancing device, appropriate changes in the torques applied by the 3 torque generators/absorbers can be used to hold any one shaft at zero speed, or any other speed, and the torque balance can also be moved around.

It is a very flexible solution, and was first suggested and built in the 70s.


Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[rancherman]

I found this thread, after getting pretty disgusted at exactly what the OP said..
All the conjecture, urban myths, and just plain old misinformation either online, or down at the local pub is quite amazing!

I'm not schooled in mechanical engineering, but had a pretty good teacher.. 'Dad'. Dad was a mechanical engineer, and an ex-Marine officer.
45 years ago, I burst home from school, with 'new found' information! I just learned my buddies fathers 'dyno'd' their race cars and had 'x' amount of power.. I was sure to impress the old man!
After the lecture of the proper pronunciation of 'dynamometer'.. and a heated impromptu algebra lesson, I walked away with newfound information; HP= Torque X RPM/ 5252. What 3rd grader needs this kind of information??? Shoot, I was struggling with long division!

Anyhow, my career path took me away from the hows and whys of engine principles, and well, here I am!

I also am a Cummins nut these days, and had some pretty good bench racing sessions both recently, and in my youth with my old big block Dodges.

BTW, I took your test, and got 7/8. The one I missed, was the first question that had 3 different engines, and you wanted to know which one would reach redline first. I chose the one with 700+ torque.. the correct answer was 'not enough info'.
Perhaps I mis read the question?
I am going to list a series of statements here, ones that I believe to be true. Please correct me If I am wrong!
1. 2 things are easily measured coming out the end of the crankshaft: Torque, and RPM.
2. HP is 'calculated'.. it 'quantifies' what X amount of torque can do at 'Y' rpm.
For the life of me, I cannot understand why some will state " Torque launches the vehicle, but the 'horsepower thing' kicks in and pulls the car across the finish line". Since when is HP a 'thing'??
3. Isn't crossing the finish line and still accelerating just an extension of torque? (More torque is being applied to the rear axle- final drive- than what the car is trying to hold back? ie; a 500 ft. lb cummins crossing the line in 'direct drive' is putting 500 lbs to the ring gear, say 3.54 ratio... which multiplies it to 1770 at the end of axle. Our 32 inch tall tires then 'reduces' torque by 50% (16 inch radius) for a grand total of ~885 lbs. to the ground.
So, as long as the truck needs less than this to over come all drag.. it should still accelerate? (least until the engine starts dropping off the torque chart.. which is getting close via the rpm chart.

4. Perhaps the HP and torque debate was long ago conceived.. back in the caveman days of hot rodding. We didn't have much in transmissions. All were either 2,3,4 speeds, and pretty wide ratio. The big 3 had pretty much the same configuration. Same with rear gear ratios. It was easy peasy to see the difference in performance of a 340 Plymouth cuda, as opposed to it's Hemi brother..(stock form)
The 340 cars COULD compete, but needed very high numerically rear gears, (which increased the torque to ground).. but the engine needed to be built to turn 8,000 rpm or more, to get MPH. At the time, I never connected the change of gears actually increased the amount of torque to the ground.. but took away from a 'stocker' engine. We built the engine to handle more rpm.. better breathing, lighter components, stronger valve springs.. And our trap speeds actually increased.
Nowadays, we can custom build just about any ratio needed to 'fit' a particular power band, for a particular type of running.
I did some tractor testing for JD engine works..(work study program) I was just the goof that drove the mules around the track all day long.. but in the late 70's and early 80's, they did what was quite common industry wise: Increase engine rpm. They got claim of 'new improved' powerful tractors.. and sold a pile of them in the process. All they did was change up the governor springs, and perhaps a little modification to the fuel plate, and of course adjusted the final drive ratio. viola'.. 15-20% increase in HP.. Torque really didn't change though.. They sold a LOT of new green paint for just a few dollars parts change.
5. People were, and still are suckers for the HP 'thing'.. It's sexy, It commands attention.
6. on any engine graph, HP continues to build even if torque is pancake flat..(such as my cummins with near flat torque from 1600-2600 rpm) because of the 'math' in increased rpm.. And will even continue upwards even when torque decidedly starts dropping off.. again due to the math, But HP will soon follow (drop) as the torque no longer sustains the 'formula'.

I did a little 'test' on my towing rig just the other day. I was told my cummins could 'top a hill' easier at 2600 rpm, rather than at 1600.
This is in direct drive, and the test started at the bottom of the upcoming hill.
What I found was: Yes. 'easier' on me... I didn't have to shift! easier on my engine? ummmm, no. As I hit the bottom of this test hill, I watched my tach drop from 2600 rpm in 100 rpm increments per second..
Next run my tach dropped from 1600 rpm in 50 rpm increments per second. ( I was going slower, so less wind resistance)
Where's the Horsepower 'thing' here?? I should've had almost double the HP to the wheels to fling me over the top. The engine struggled just as bad.. The one thing that 2600 rpm gave me was 'time'. I told myself I'd shift when I hit 1400.. I just cleared the hill as I reached the magic number. Whereas the 1600 start point I only made it halfway up before I was forced to shift.

Guys, I am not trying to re invent the wheel here, just a 'fmi' friendly update to my noggin! Let me know if I am in the right path of thinking, or need to adjust! I am all ears!
I won't be alive for too many more years, I'd sure hate to have to wait until I meet up with dear ol Dad and have to explain myself again how I never 'got it right'!

 

[rancherman]

dang.. I messed up. The final drive ratio in tire size isn't 50% reduction! should be 33%??

 

[gruntguru]

Note the speed you were doing at 1600 rpm. Now select a gear that gives the same speed at 2600 rpm. Hit the hill at 2600 rpm in that gear and the speed doesn't drop at all. Same task, more hp, more performance.

The two tasks you were comparing in the same gear are totally different tasks. Going up a hill at twice the road speed is storing gravitational potential energy at twice the rate so requires twice the power (neglecting wind resistance.)

je suis charlie

 

[LionelHutz]

On the JD tractor - if they increased the final drive ratio and ran the engines at a higher rpm then they did get more HP and the tractors would do more work per hour on the field. The change in final drive ratio DID put more torque on the axles and more force to the rear wheels. It wasn't just a number game to fool consumers. Well, the only game might have been trading engine life for HP.

You don't really put "torque to the ground". You have a force at the wheel to road interface.

As for your truck - as already pointed out you need to change the drive ratio to get 2600rpm at the same speed then test again. You proved it works because downshifting lets you start accelerating again.

I'm not sure you read many of the previous comments because you seem to have missed how a gear ratio change to allow a higher rpm will affect HP which affects the work the engine can do, even if the torque curve is flat or slightly falling off. You seem to think that people who follow HP because it increases with rpm are suckers since the torque remains the same or drops off.

 

[rancherman]

I Should've pointed out that in the Dodge test, 5th gear was used. (direct) in both instances. The point I was trying to prove to the guy who insisted that HP gets us over the hill 'easier'.. *without shifting*


both you guys missed the point here.
"Overall vehicle ratio"

Sure, I could've downshifted and put the engine in the 2600 rpm at the bottom of the hill. The ratio I was in was 1:1 (5th)
4th is 1.39:1 3rd is 2.09:1 Stabbing 3rd would instantly put me to a tad over 3200, 4th would've been about 2224.
This wasn't the point. My point was to show 'engine ONLY' scenarios. Not what happens to the torque AFTER it leaves the engine, and subsequently 'modified'.

My experiment here was to show how 'power' NOT torque fails at maintaining speed. Both trials showed clearly that the gear I was in, along with the load I was carrying, overwhelmed the output 'applied to the ground'.
By downshifting to 4th, the transmission.. *aka 'torque multiplier* now sends 1.39 times MORE torque to the axle than when it was in direct (5th)
In 5th, I was putting (theoretically) 500 ft. lbs out the rear of transmission. Which is now converted via ring and pinion by a factor of 3.54
(1770) to the end of the axle shaft. 'Felt' pressure to ground with 32 inch tires should be 1185 ft. lbs.
By downshifting, the same 500 ft. lbs, now is 695 out the rear of trans.. which is converted to 1648 ft, lbs *to the ground*


This only proves my point. Increasing torque to rear axle either helps us maintain speed going up a hill, or accelerating.
Same with the JD. They went from a final drive ratio of about 180:1 to just a tick over 200:1. Same forward speeds, same transmissions. We could now pull a larger plow, because of the 'torque multiplication' factor.