A question about horsepower and torque

[rca2000]

I'm pretty good at electronics, and such, as I have been in this field for more or less 30 years now, since i was just a kid. But, some mechanical stuff still puzzles me. For instance, the difference between horsepower and torque. For example, say you have a big rig, with an engine of 12 or more liters, but only rated at 250 or so horsepower. At the same time, you have MANY new six-cylinder cars and vans, rated at this much or more. Is the way the horsepower output of a big diesel engine is rated different from that of a small gas engine, of say 3.5 liters or less?
How come those big diesels don't produce much hp.?

Second, if you have two 250 horsepower engines, and one is rated at 250 hp/240 ft/lb. of torque, at 4000 rpm, and another one, of 250 hp, is rated at 350 ft/lb/ torque, also at 4000 rpm, , isn't the one with the 110 ft./lb. torque advantage going to "seem" to be a LOT more powerful, EVEN THOUGH THE HP. RATINGS ARE THE SAME?

I hope someone can give me a concise explanation to this, as I have wondered about it for many years now.

 

[Andyman]

I think it has a lot to do with rpms. Those litle engines, especially mototrcycles, spin like 14,000+ rpm to get their horespower, but the big Harleys have the torque down low. They are both equal at something like 5400 rpm, but basically, torque is low end grunt and hp is top end power.

I'm sure others will expand on this.

 

[EchoWars]

(Horsepower x 5252)/Rpm = Foot-Pounds of torque, or...

(Torque * RPM ) / 5252 = Horsepower

Horsepower is a measure of the work done, wheras torque is the measure of the force providing the actual work (kinda like Watts & Voltage).

You can see that in order for an engine to do a lot of work (I.E., horsepower), you need either huge amounts of torque, or a lot of RPM's. Big diesels redline at like 2500 RPM (ever see a connecting rod or a crankshaft from a big diesel? No wonder 2500 RPM is about all it can turn), so it's no suprise that the horsepower figure looks low.

Nice writeup here.

 

[Butuz]

Hmm ive yet to have this explained to me well to me in layments terms but as your an electronics guy ill try and explain.

Firstly power and torque are seperate entities that affect performance. Think of it as voltage and current - high voltage, and high current combine to give the most "power".

The way i see torque - is that if you are going on a straight road at 2000rpm and you then hit a steep hill, an engine with little torque will lose RPM's and hence you will either slow down, or change gear. An engine with high torque will not slow down as much or at all, and would just stay at 2000rpm. Torque is kinda like the engines ability to keep itself revolving under increasing load, wheras power is the ability to increase its revs from 1000rpm to 6000rpm at any given load.

Turbo Diesil engines generally are more torquey than non turbo pertrol engines - at any given power - so you may have a 2.0 petrol engine with 140bhp and 120 ft/lb torque wheras the diesil may have 140bhp but 180 ft/lb torque. if you are towing the extra weight of a caravan and you go up a hill - the petrol car will probably slow down, and the diesil could be fine - that is the effect of having an extra 60 ft/lb of torque.

Does that above make any sense?? I have no idea :

Butuz

 

[Butuz]

Lol looks like my explanation is totally wrong according to EchoWars link :

I think i'd better stick to what i know - computers!

Butuz

 

[rca2000]

Thanks a lot guys

Now, things are a lot more clear. thanks to your input, and that GREAT BMW article, I THINK i understand things a lot better. I have made a few conclusions-(feel free to correct any that I have wrong):

(1) Torque is a twisting motion, i.e, the ability to turn or pull something.

(2) Horsepower is a COMBINATION of torque AND rpm. ( I did NOT realize this before).

(3) Horrepower is measured over TIME. if 1000 ft/lbs. of troque is generated, BUT ONLY for a couple of seconds, and then , the torque drops off, to only 100 ft/lbs, little horsepower is produced.

(4) a bunch of torque at a LOW rpm , is NOT a lot of HP.

(5) - given this, it stands to reason ,why a big diesel doesn't seem to be very powerful- the torque peak is probably under 1500 rpm. BUT, they can pull a heck of a lot of weight.

(6)- Here is the reason that a car with a multivalve engine does better at top end, MUCH BETTER, than a pushrod engine, where the torque curve peaks at a LOW rpm, say 3000, or so.

So then, the ideal engine would be one with a FLAT torque curve, from 2000 rpm, to redline. I know this does not exist, but if it did, you would have that low-end torque(for a quick launch), and then torque to keep building that speed.

This kind of stuff is really enjoyable to me- learning things I did not know before- instead of wasting time making fun of the current, or former occupant of the White House.

 

[RuSsMaN]

You're on the right track. Most of the big deisel 6's hit max torque at 1500-1800 rpm. Don't forget about compression ratios either. Take a look at a typical gas engine, and a similarly displaced diesel, and tell me what you see.

Think about a Locomotive ELECTRIC motor, yeah, the ones that have Deisel engines JUST to generate the electricity to make them go. Torque un-equaled via internal combustion.

I have no idea what I'm talking about, but when I drink, it sure FEELS like I do.

Cheers,
Russ

 

[EchoWars]

Horsepower is a COMBINATION of torque AND rpm. ( I did NOT realize this before).

Indeed, on the right track. Just remember that horsepower is a calculated value, and is a measure of the work that has been done, wheas torque is the force providing the means to perform the work...

...and now when you look at a graph of horsepower and torque, you know why the graph lines always cross at 5252 RPM

 

[Lefty]

Originally posted by EchoWars
Indeed, on the right track. Just remember that horsepower is a calculated value, and is a measure of the work that has been done, wheas torque is the force providing the means to perform the work...

...and now when you look at a graph of horsepower and torque, you know why the graph lines always cross at 5252 RPM

But we all know that two engines that might measure the same will not sound the same. The sound stage will always favor the the engine using pure copper ignition wire and deisel engines with no ingition wires don't stand a chance! RPMs, torque, horsepower are all good and dandy but it's all about the SOUND :

Lefty

 

[Sandy G]

I'll wager if you painted a diesel w/that wondrous C-37 stuff, you'd get rid of the "Pocketapocketapocketa" noise they make....& it would probably help alleviate the smell, too. -Sandy G.

 

[MCS Guy]

In drag racing, Torque moves the car off the line. Torque is the most important rating for a street car. It is what gives you passing power.

Horsepower in drag racing is to keep your momentum moving at its rate of acceleration. horsepower alone will not move the car very well. NASCAR at the super speedways is all horsepower depended. The HP keeps the car moving at its velocity.

 

[LBPete]

Sorry to come late to the party, but I can’t resist jumping in on this one.

Horsepower VS Torque, it’s a little like Ford VS Chevy. They both have advantages and disadvantages. As has been pointed out in some of the posts, Torque is a measure of the force applied or “work” done by the engine. Horsepower is Torque over time. In other words, how fast you can have it. You can think of it as the difference between a punch and a shove. Both may have the same net force, but the punch hurts more because it concentrates that force in a very short time. In terms of the two engines in the original post, the one with the highest torque is the more powerful engine.

Which is better, Torque or Horsepower? That depends totally on what you are trying to accomplish. If you need to move a freight train, speed becomes secondary to force. If you only need to move a little rice rocket, horsepower is where it’s at. There are references to drag racers. Here you need to move a relatively light vehicle as fast as you can. There is another factor, gear ratio. A top fuel dragster needs to be geared for 300+ MPH so you can imagine how tall a ratio is required. (Think about starting you bicycle in its highest gear). You need enough torque to get off the line pulling that gear ratio, then tons of horsepower to get the low ET.

As far as RPM for maximum horsepower or torque, that is dependent on several factors, charge density, fuel density and the length of the crank throw. Charge density refers to how much air and fuel you can get into the cylinders at a particular RPM. Assuming your fuel metering system keeps the air fuel ratio correct, in a naturally aspirated engine, (no turbo or supercharger) you are totally dependent on the difference in air pressure between the ambient air and the cylinder. A lot of design time is spent on maximizing this transfer efficiency. The reality is, the lower the engine speed (RPM) the longer the time to charge the cylinder. At higher engine speeds, there is just too little time to get much air into the cylinder so the power produced drops off.

An engine with forced air induction, (turbo or supercharged) has an auxiliary air pump pushing air into the cylinders. This improves the transfer efficiency and greatly improves the charge density, but the dynamics outlined above still hold.

An engine’s maximum Torque will occur at the engine speed that provides the best cylinder fill. This occurs at a low RPM because there is simply more time to fill the cylinder. Maximum Horsepower will occur at some balance point between cylinder fill and RPM.

There are a lot of references to diesel engines. These engines produce higher Torque for a lot of reasons. First is fuel density. Diesel fuel simply has more BTUs per unit than gasoline. In other words, all other factors being equal, burning a pound of diesel fuel produces more power in the form of heat, than burning a pound of gasoline.

Next there’s compression ratio. This is the volume of the combustion chamber divided by the volume of the cylinder. Diesels use the heat generated by compressing the air in the cylinder to ignite the fuel. To do so, they generally have compression ratio twice to three times as high as a spark ignition gasoline engine. Higher compression ratios improve the amount of power produced. (The high compression ratios also contribute to the characteristic clatter of diesel engines.)

Another factor is the length of the crank throw. Industrial diesels tend to have long strokes and relatively small bores. The longer crank throw is simply a longer lever arm working to turn the crank. A long crank throw produces more Torque than a short throw but generally can’t turn as fast. There are short stroke, high rev diesels but they are generally used in passenger cars.

One last thing and I’ll get off the soapbox, Volts and Amps. I’ve always thought of Amps as the quantity of electrons flowing through a circuit and Volts as the force moving them. It’s a little different than Torque and Horsepower because the time factor is fixed at the speed of light. So no matter how much voltage you apply, you can’t increase the speed, but you do increase the overall power, (Watts).

- Pete