Horsepower increases with rpms

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I am wondering what it is about an engine that gives it so much more power and acceleration capabilities with high rpm torque versus low rpm torque.

Say an engine with 400lb-ft at 2000 rpm vs an engine that puts out 400lb-ft at 6000 rpm. What is going on inside that engine to make it accelerate the vehicle so much faster?



Does the rotating assembly respond better at a higher rpm?

Or maybe once an engine is already spinning over pretty fast it is easier for it to go even faster.
 
Torque is the rotational 'force' applied by the engine, and powers a product of torque and engine speed.

You could provide 100ft lbs on a torque wrench, and be lucky to do 2rpm, generating nearly no power... Do it at 2,000 rpm, and 1,000 times the power.
 
It all comes down to bore and stroke.

You pretty much nailed it with the last line. For spirited driving and track use the engines that peak in power and torque towards the end of the RPM range are more desirable and as long as you manage to keep it in the "power band" you are gold.

Think at it like a turbo...if the RPM is on the low side, there isn't enough momentum in the turbine to boost the intake instantaneously.
 
I think you mean seat of the pants feeling from acceleration with higher RPM ...

Very few un-blown engines make 400 pound feet at 2,000. But that is usually well above the floor on the torque curve (in the bulge). It'll often flatten out at around 4,500 for an OEM built engine. But 400 pound feet at 4,500 is no slouch. You are burning a lot of fuel and volumetric efficiency might be as good as it gets for that engine, so it's making power and accelerating hard
laugh.gif
 
Are you asking why does a gasoline engine have a powerband, unlike a steam engine of electric motor?
 
Originally Posted By: Shannow
Torque is the rotational 'force' applied by the engine, and powers a product of torque and engine speed.

You could provide 100ft lbs on a torque wrench, and be lucky to do 2rpm, generating nearly no power... Do it at 2,000 rpm, and 1,000 times the power.



Big thanks! I mean kapow. Teacher grade answer right there...especially the second line. W2G Shannow.
 
Shannow nailed it early on...
The equation I found is
HP = (Torque)X(RPM)/5252
for English units. This basically jibes with Srt20's numbers (I think his first result should be 152 HP, close enough).

But, basically horsepower is proportional to torque and RPM.
You see lots of diesels with monster peak torques but more modest peak HPs because their torque peaks early in the RPM band...I'm sure there's a good explanation for this, but I don't know what it is.
 
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Think of it in terms of volumetric efficiency. This is a calculation based on the amount of air ingested with RPM and displacement as two variables.

At 100% volumetric efficiency (VE) You can calculate how much air an engine will be ingested according to RPM and displacement.
(RPM X displacement in CI)/3456

Stock, N/A, gasoline automotive engines these days will be in the high 90's for VE at peak torque and then taper off somewhat to low to mid 90's at peak horsepower

example) at 95% VE a 350 CI engine at 4000 RPM would consume 384.8 SCFM (standard cubic feet per minute) lets take the same engine at 4500 RPM but now the VE has tapered down to 93% that engine would now consume 423.8 SCFM

That engine is consuming more air at 4500 RPM than it was at 4000 RPM even though VE was lower. More air = more fuel = bigger boom! = more power!


Now what makes engine different from each other is how VE can be maintained at various RPM's That depends on the camshaft, cam timing, intake port design, exhaust flow, and dozens more!
 
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Originally Posted By: Virtus_Probi

You see lots of diesels with monster peak torques but more modest peak HPs because their torque peaks early in the RPM band...I'm sure there's a good explanation for this, but I don't know what it is.


My understanding is ignition lag. Squirt the D2 in, and it takes too long for it to ignite--the fuel has to be heated up from--well it's not room temp, probably 150C? (its pretty high compression to hit the pressures)--up to the ignition point. Takes time to absorb that heat. Whereas the spark will kick off ignition instantly.
 
Motor Trend used to have well written articles on subjects like this, but anymore, it's just inane chitchat about high priced vehicles most of us don't own or even want to own. I've taken the magazine for fifty some years, but lately, there's nothing between the covers that I can relate to.
 
The ability to do work is clearly illustrated in propeller driven aircraft. More HP means better climb (work) rate. My Cessna climbs far better at 2700 rpm vs 1700. Same torque at each setting too!
 
Originally Posted By: KrisZ
Shannow pretty much nailed it as far as simple analogies go. Of course there is much, much more to HP and Torques than this.


In terms of what ARE hp and torque, that's all there is.

In terms of HOW to make torque, and move it around the powerband...lots lots more I agree.
 
Originally Posted By: Virtus_Probi

You see lots of diesels with monster peak torques but more modest peak HPs because their torque peaks early in the RPM band...I'm sure there's a good explanation for this, but I don't know what it is.


Diesel is a long burn, it will continue to push that piston down long after a petrol burn has gone out, that doesn't work so well at high speeds.
 
Originally Posted By: Silk
Originally Posted By: Virtus_Probi

You see lots of diesels with monster peak torques but more modest peak HPs because their torque peaks early in the RPM band...I'm sure there's a good explanation for this, but I don't know what it is.


Diesel is a long burn, it will continue to push that piston down long after a petrol burn has gone out, that doesn't work so well at high speeds.


Here's a pic of the pressure an volume curves of a couple of theoretical engines (otto and diesel) - note, of the SAME COMPRESSION RATIO.

p-V-diagram-of-Otto-Diesel-and-Dual-cycle-for-same-compression-ratio.png


Ideal otto, the fuel is compressed to TDC (minimum volume), then all burns instantly, which is the vertical spike on the curve 1, 2, 4, 7.

Ideal diesel, it gets to the same point, then the fuel is introduced slowly over time, and forms the curve 1, 2, 6, 7...and as this takes heaps of time, as silk says, limits speed.

The area within the curve is the energy available per cylinder power stroke to make power.

Real life diesels, the Compression ratio (point 2) is much higher, so the diesel is not as much hindered as the area within the curve suggests.

Real life ENGINES, the combustion process is neither instant (theoretical otto), nor flat (theoretical diesel), but takes closer to the 1, 2, 3, 5, 7 approach egardless of design, and as Mazda are demonstrating, otto C.R.s are going up, and diesel going down, and with 7+ separate injection events per cylinder for modern diesels, they are nearly the same.

My Nissan ZD30 (EGR delete, as it couldn't do it stock) will pull away at idle, and accept a 1-2 shift on the flat with no throttle movement at any stage...wet grass, just let the clutch out quickly in 1st at idle, then chnge 2nd, and it will never flinch, nor gain traction.
 
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