Automotive Can a High RPM Gas Engine Move a Heavy Load Like a Low RPM Diesel?

[Moretorque]

cJl, thanks. will read over everything real closely. I have been spending time talking too drag racers and motor builders.

 

[Ketch22]

Moretorque, I would continue to applaud your search. This subject is ruled by a lot of good old boys that have been told stuff (and some if it close to correct) but not directly studied it. There is also a growing bunch of engine builders that are trying to change the popular conception to the accurate one. As you look I think that you would be well served to check out this article from a well backed engine build magazine.

http://dsportmag.com/the-tech/learning-curves-recognizing-a-race-friendly-dyno-graph/

 

[Moretorque]

What if the "car motor" had 15 speeds to work with?

Sorry I missed this, it would need a air shifter so the RPM would not drop off.

 

[Moretorque]

Moretorque, I would continue to applaud your search. This subject is ruled by a lot of good old boys that have been told stuff (and some if it close to correct) but not directly studied it. There is also a growing bunch of engine builders that are trying to change the popular conception to the accurate one. As you look I think that you would be well served to check out this article from a well backed engine build magazine.

http://dsportmag.com/the-tech/learning-curves-recognizing-a-race-friendly-dyno-graph/

Thanks.

 

[cjl]

Sorry I missed this, it would need a air shifter so the RPM would not drop off.

No it wouldn't. The only time shift speed would be important would be something like climbing a hill from a dead stop, and that's because the truck itself will slow down during the shift (and this is just as true with diesel as gas). On a level surface, shift speed really isn't that important for getting a load going, since the truck will continue rolling while the shift is happening.

 

[Moretorque]

If the motor is putting out 237 horsepower, it's putting out 237 horsepower. There's nothing particularly special about whether it's doing that at 1600RPM (like a truck engine) or 6500RPM (like a car engine), both will be able to do the same amount of work in the same period of time (since that's the definition of power). If it takes 237hp to move the truck at 70mph, then the car engine will maintain 6500RPM just as easily as the truck engine will maintain 1600, assuming they're both geared to hit those RPMs at 70mph.

Also, I'd be skeptical about 237hp and a 5 speed moving a 70klb load "no problem" - sure, it'll move it on flat ground OK, but it'll be awfully slow up hills, and the gearing will probably leave something to be desired compared to a more modern truck with a 10, 13, or 18 speed.



Horsepower tells you rate of acceleration under a given load, and also determines top speed. Torque on the other hand doesn't tell you much about the performance of the vehicle, just about what kind of gearing you'll need.

Thee Maxidyne 237 diesel engine is real and yes it is slow but can move 35 tons from 35 to 60 in 5th gear and that is no easy task. So you are saying a 237 HP 6000 or 7000 RPM Gas engine with a broad torque curve can do the same ? cjl, what is your experience in this ? I have been at a drag site learning a few things on this. Thanks for the input.

 

[cjl]

Thee Maxidyne 237 diesel engine is real and yes it is slow but can move 35 tons from 35 to 60 in 5th gear and that is no easy task. So you are saying a 237 HP 6000 or 7000 RPM Gas engine with a broad torque curve can do the same ? cjl, what is your experience in this ? I have been at a drag site learning a few things on this. Thanks for the input.

My experience is that I'm an engineer who has been interested in cars for a long time. I also work on my own cars (mostly, though I'll pay for someone else to do the really annoying jobs), and have also done some endurance racing (admittedly with a very low budget amateur racing team).

As for the diesel, I don't doubt that it's real, and as I said, I don't doubt that it'll go 60mph on flat ground (though I'd imagine it only barely can - I very much doubt it would do 70-80 though). I doubted its ability to climb hills at speed though, and the acceleration will be pretty slow (some back of the envelope calculations indicate that it would take more than a minute and a half to get to 60mph). A gas engine could move the same load with similar performance, though the fuel economy would be rather appalling. The advantages diesels have for heavy hauling lie largely in their substantially better fuel efficiency (and overall thermal efficiency) and their durability, not in their physical ability to pull the load.

 

[Moretorque]

Your saying a 237 Hp gas engine which spun a good degree of RPM could do it with a 5 speed ? like a 350 CI or something ?

 

[cjl]

If it has a similarly broad power curve as a percentage of peak RPM to the diesel, then yes it could.

(Specifically, the breadth of the power curve determines how many gears the engine will need, and the peak power determines its actual ability to haul the load, given an ideal gearbox)

 

[Moretorque]

I am just trying to understand this, Your HP is just the amount of work being done at a given RPM. It is not how you got there and how quickly you got there under a load the amount of torque { ability to spin up and build power under a load } you have at the operating RPM determines this. When you drop the clutch with a load of 35 tons and the RPM drops way down your torque out of the crank shaft determines how well you spin up and recover your RPM HP MPH back. You can clutch it but ultimately if you have more torque the motor will move up the RPM range much better and faster under load.

The 237 HP Mack can move 35 tons like it does because it has the torque of a healthy Pro Stock drag car at the crank. When I tell motor buiulders there are people out there saying you can move 40 tons down the highway at a good clip with a small block car motor HP being equal they practically laugh. They are the ones saying it would have to have a tranny with many many gears and be electronically or air shifted if it had any chance of working and they still say it would not work well at all. You cannot get 40 plus tons rolling from a dead stop and moving up the RPM range unless you have the nuts coming out of the crank shaft to do it. The drag boys know their stuff when it comes to HP and I applaud them for there input in setting me straight. They do not care what the HP # is for the most part they want to know how much torque and at what RPM, when they look for more power they look for more torque to get it and not more RPM spin. Trading torque for HP will not get them down the track faster in most instances.

Torque is to HP what AMPs are to electricity, a 350 small block V8 is a 15 pound 200 watt Kraco car amp and a Semi engine is a 200 pound 200 watt Krell you can weld with. I have come to the conclusion in my book HP is not HP and a motor that does it's work at lower RPM is more powerful HP for HP..

Thanks for all the help.

 

[jack action]

OMG!

They are the ones saying it would have to have a tranny with many many gears

You don't need many gears to get the torque, you need many gears to extend your speed range.

They do not care what the HP # is for the most part they want to know how much torque and at what RPM

If you need to know at what RPM is your torque, that means that you are looking for HP; Because Power = Torque X RPM. If they didn't knew the RPM, torque alone is a useless number to determine performance (and vice versa). HP is a unit that was created just for that: Evaluating the effect of the torque combined with RPM.

when they look for more power they look for more torque to get it and not more RPM spin.

Tell that to a professional Top Fuel dragster or a Formula One engine builder; Then see who will laugh ...

Torque is to HP what AMPs are to electricity,

Actually, torque is to HP what VOLTs are to electricity. AMPs is related to RPM. And - look at that - in mechanics:

Power = Torque X RPM

and in electricity:

Power = VOLT X AMP

And in fluid mechanics:

Power = Pressure X Flow

and in mechanics (linear):

Power = Force X Velocity

See - in any case - power is proportional to a force applied (Torque, VOLT, Pressure, Force) AND the rate at which that force is delivered (RPM, AMP, Flow, Velocity). And 1 mechanical HP = 1 electrical HP; Because power is a unit of measure that represents the same thing, no matter where you apply it.

I have come to the conclusion in my book HP is not HP and a motor that does it's work at lower RPM is more powerful HP for HP..

Again, HP is unit of measure, it's a definition. Let me show you how silly the sentence you wrote sounds like:

I have come to the conclusion in my book a pound is not a pound and a motor that have more volume is heavier pound for pound..

If you keep the same density, yes, a larger motor will be heavier. But if we assume that they have the same mass (in pounds), then two motors can have different volumes if they have different densities (for example, one is made of aluminum and the other one made of cast iron). This is true because:

Mass = Volume X Density

which is the same mathematical relationship as

Power = Torque X RPM

1 lb of aluminum is 1 lb of cast iron is 1 lb of water is 1 lb of air is 1 lb of ... anything! The volume occupied by this mass depends of the density of the material.

Similarly, 1 hp is 1 hp, no matter where it comes from. The torque needed to produce that 1 hp depends on the RPM at which it is produced.

This very fundamental mathematical concept holds everywhere, in every field. Hydroelectric power stations, satellites sent in space, ballistics, name it: They are all parts of science based on that principle. Do you really think engine building is the black sheep of science that do not follow the proven laws of physics?

When I tell motor buiulders there are people out there saying you can move 40 tons down the highway at a good clip with a small block car motor HP being equal they practically laugh.

If they do laugh, it is because they don't understand physics. They usually can build only one type of engine (V8, I4, Chevy, Honda, etc.), because they mostly monkey what others around them have done more than they understand what they are doing.

It doesn't mean they don't know how to build good and powerful engines, but they cannot explain why and how in terms of physics. And people used to build a lot of great stuff before understanding what we know now. But we were able to improve ourselves once we begun understanding what we were doing.

Don't fear the knowledge, it will open your mind to a wonderful world.

Read (and re-read) carefully the link by @Ketch22 in post #92. Make it your bible, because that is engine building based on an understanding of physics.

 

[cjl]

I am just trying to understand this, Your HP is just the amount of work being done at a given RPM. It is not how you got there and how quickly you got there under a load the amount of torque { ability to spin up and build power under a load } you have at the operating RPM determines this. When you drop the clutch with a load of 35 tons and the RPM drops way down your torque out of the crank shaft determines how well you spin up and recover your RPM HP MPH back. You can clutch it but ultimately if you have more torque the motor will move up the RPM range much better and faster under load.

Nope. If you just dump the clutch and the RPMs drop, what determines how fast it will accelerate is still the horsepower. Again, the gearing will need to be dramatically different to make this equal, but the acceleration of the vehicle is directly related to the horsepower the engine is producing at all times. I suspect you keep thinking that both drop down to near idle when the clutch is dropped (although I don't know why you would "drop" the clutch - you get much better results by feathering it at a desired RPM), but there's no reason that needs to be true.

The 237 HP Mack can move 35 tons like it does because it has the torque of a healthy Pro Stock drag car at the crank. When I tell motor buiulders there are people out there saying you can move 40 tons down the highway at a good clip with a small block car motor HP being equal they practically laugh. They are the ones saying it would have to have a tranny with many many gears and be electronically or air shifted if it had any chance of working and they still say it would not work well at all.

And they're wrong. They don't have practical experience with this because nobody actually does this (for a number of good reasons). It's not necessarily intuitive, but horsepower really is all you care about.

Let's look at it this way. Suppose I handed you two black boxes with a throttle. One of them contains a diesel engine (let's suppose it's a 10L turbodiesel I6), with 300hp at 1800rpm (which means it makes 875 lb-ft at 1800rpm) and a torque peak of 1100lb-ft at 1000-1300rpm. The other one contains a small gas racing engine (say, a 1.5L V8) with 300hp at 18,000rpm (thus making 87.5 lb-ft at this speed) and a torque peak of 110lb-ft at 10k-13krpm. This second box also contains a 10:1 reduction gearbox, so the output shaft spins at 1/10 the speed of the engine. This means that the output shaft will spin 1800rpm with 875 lb-ft when the racing engine is at its power peak, and 1000-1300rpm with 1100 lb-ft when the engine is at its torque peak. For the purposes of this thought experiment, let's say that they both weigh the same (because I've added a bunch of lead or something to the second box), and you have no way of telling which is which. Why would you think that box 1 would power a truck any better than box 2? How would you even be able to determine which is the gas and which is the diesel?

You cannot get 40 plus tons rolling from a dead stop and moving up the RPM range unless you have the nuts coming out of the crank shaft to do it. The drag boys know their stuff when it comes to HP and I applaud them for there input in setting me straight. They do not care what the HP # is for the most part they want to know how much torque and at what RPM, when they look for more power they look for more torque to get it and not more RPM spin. Trading torque for HP will not get them down the track faster in most instances.

If drag cars look for more torque, not more RPM, why do the fastest drag cars spin 500ci V8s at almost 9000RPM? Horsepower is what gets you down the track faster, and I'm rather skeptical that you're actually talking to "drag boys" at all, since drag racers should really know this.

Torque is to HP what AMPs are to electricity, a 350 small block V8 is a 15 pound 200 watt Kraco car amp and a Semi engine is a 200 pound 200 watt Krell you can weld with. I have come to the conclusion in my book HP is not HP and a motor that does it's work at lower RPM is more powerful HP for HP..

Thanks for all the help.

Actually, this is another example where power (watts) really is all that matters. A 200 watt car amp will drive speakers just as well as a 200 watt Krell, as long as the following conditions are met:
1) They are both rated with similar methods. A lot of car amps are somewhat dishonest, and promise ludicrously high "peak" power levels that they could never deliver continuously. It also matters what the distortion threshold is for the rating - an amp rated 200W @ 1% THD is not the same as one rated 200W @ 0.01% THD.
2) They are both rated to drive the load. If the car amp is rated 200W into 4 ohms, and the Krell is 200W into 8 ohms, obviously the Krell will be better if you're trying to drive an 8 ohm speaker.

At the end of the day though, watts are watts. If both amps are actually delivering 200W to the speaker (with low distortion and no other flaws), the speaker will perform the same, just as a truck with 200hp will accelerate the same so long as the engine is actually delivering 200hp.

 

[Ketch22]

Moretorque, i appreciate your search. It is a little confusing to me who you are talking with. I deal a lot with racers and truck drivers and they seem to get what we are talking about. I do also see where you are listening to other inputs than the ones here.

Please also consider what you just see in normal life. Two points of reference, a relatively plain family sedan of approximatley 4000 lbs will require somewhere between 16 and 20 hp. to maintain 65 mph on level ground. A typical non-aero tractor trailer fully loaded will run between 200 and 240 hp. to maintain 65 on level ground. It is really interesting that many of the car drivers (that pay attention) and all of the truck drivers know that when you need more power to go uphill you need to downshift. The action of downshifting increases rpm and thus increases power. It most of the time reduces torque as it moves the engine past its peak. If torque is as you say more important how does it play into your observations that even drivers who are not engineers know to do the predictable thing when they need it?

 

[Moretorque]

Sorry about not getting back and I appreciate everybody's time and knowledge on the posts. I understand all the math and what everybody is saying but I guess I am getting bad info but I have talked to top engine builders and they have told me the engine that makes HP earlier is more powerful at building and maintaining it under load in real world app. Top Pro Stock engine builders have told me this and they live and die by the dyno. Most of what I may have been missing is the " power under the curve " which allows you to throw more gear at a motor than one that is more peaky but makes more HP on top.

You guy's back it with math and none of the engine builders have done this so you all are probably right.

Here is what a sled operator told me who operates the sled at a tractor pull. He said one class will be rated at less HP more Torque and will be say 700 HP semi engines then another class will be 1000 plus HP car engines with way less torque and he says the higher HP engine cannot pull close to the load the higher torque semi engine can with less HP. He says the sled is setup for the box to move up slower on the 1000 HP motored car engines. He claims the 1000 HP engine class will get out of the gate faster and run more MPH at first but then it gets choked down way faster. Is this more than likely the power under the curve ? the diesel engine makes way more average HP throughout it's narrow operating RPM range ? Thanks...

 

[Ketch22]

[QUOTE=" Is this more than likely the power under the curve ? the diesel engine makes way more average HP throughout it's narrow operating RPM range ? Thanks...[/QUOTE]

This is more realistically a case of "invalid test." In all classes of tractor pulling the track is 100 meters long. The Sled is also the same including the weight. The operator is correct in that the rate of transfer and positioning of the weight is different. The engineers consulted by the regulating committee decided on a transfer protocol that can use approximately the tractive force developed by each class limiting them to the same "full pull" distance. When a person says I ran two different engines on the same track with entirely different protocols I say you can't compare apples and oranges. My experience in the tractor pulls has me seeing more consistent full length pulls with the modified fuel motors than diesels. I know how closely the operators adjust their weight to meet class regulations. These two things only tell me that the motors in the class are pretty close and drivers are consistent with each other. The short and missed pulls are all obviously a result of loss of traction or steering causing loss of distance. It is the same as going to the drag races. It is almost certain that when I see a racer smoke his tires at any point before the finish I know who is going to loose.

 

[jack action]

the engine that makes HP earlier is more powerful at building and maintaining it under load in real world app.

The phenomenon described is real, but the wording is misleading:

more powerful

If 2 engines are at the same (low) rpm, and one has more torque than the other, then it necessarily has more power too (hp = rpm * torque / 5252).

building and maintaining it under load

This is the wording that is the most hurtful. This is not a consequence of a better vs worst engine, it is a consequence of well-match vs mismatch engine/tranny combo.

If you need a certain amount of power at a certain wheel rpm, you can either increase the engine power at the rpm it is in OR you can change the gear ratio such that engine is at rpm where it already produce more power. From the point of view of the wheel -> same power, same thing.

The transmission modification generally implies more gear ratios, which have other consequences (complexity, cost, weight, more shifting, ...) that can also affects performance (and finance!). Usually, it is a lot easier and cheaper to have a fixed transmission design and to play with the engine. When an engine performs better than another (giving same average hp), all it means is that it is better suited to the drivetrain.

You can do an experience with your own vehicle with a manual gearbox. Drive at certain initial speed and, by being in the appropriate gear that set your engine in its high HP rpm range, give it all you can, shifting upward as you need. You will attain a certain maximum speed while being in your last gear. Why can't it go faster? Because the power needed the fight the aerodynamic resistance is equal to the maximum power of your engine.

Now repeat, starting at the same initial speed, but in your last gear. In theory, you should get to the same maximum speed, since you will have the same maximum power at the same wheel rpm. The only difference should be that it will take a longer time because your initial power is lower (the engine is in its lower power band). But the reality is that you will probably never be able reach the same maximum speed; why? Because at some point, before you can reach your maximum power output, the power produced by the engine at a lower rpm will be equal to the (lower) power produced by the aerodynamic resistance. At this point, the vehicle won't be able to accelerate, hence you will be stuck at that speed forever, never being able to get that maximum power.

Of course, the more power you have in the lower rpm range, the less likely this will happen (or the higher will be your final speed). The point I want to make is the following: Even with your own car, you can see how much a difference a proper gear ratio makes. This is also true with race cars. For example, some race teams will slightly alter the gear ratios depending on the tracks; why? Because, on one track, with one gear ratio, at the big curve before the long straight, the car will be at, say, 80 mph with the engine rpm at slightly lower than its max hp; So the driver snaps it and he gets everything he can out of the car. On the other track the curve is more pronounced and the exit speed is at 60 mph, which would put the engine rpm in its lower range, well under its desired power range. Downshifting is a possibility, but then it puts the engine at the end of its power range; That would make 2 gear changes almost back-to-back. But if he changes the gear ratio for that particular gear, @ 60 mph he will be right at the beginning of its power range and will performs at its best (at least for that particular location on the track).

So you should understand by now that what is important is the average power you get in your usual rpm range that matter, while also being transmitted appropriately by the drivetrain. If you engine needs to go in the low rpm range, you will need more power (or torque) to have better performance. If it doesn't go there, or very briefly, than nobody cares how much power it produces, as long as you have enough to have an adequate acceleration to get to the higher rpms. (Does anyone really care how much power an engine makes at idle?)

Stop thinking in terms of the best engine and the best transmission; Look for the best powertrain.

 

[Moretorque]

Yaa thanks for the help, I will go email the tractor pull sled operator and get more details on this subject. I understand all of it now but a lot of people have told me I am missing something in real world app.

That is why I theorized a ship engine vs a car engine for comparison and a load pulled with a cable for complete opposites of the spectrum. It would be a good real life test to see if the math is correct, when I talked to Reher Morrison and my friend who has built engines for over 60 who also built dyno's they told me the higher torque engine also along with other people is more powerful under load. I cannot see how the math is wrong. I was told by a drag site it is the drive coming off the crank which is the amps and higher torque engines had more of it.

Again thanks...

 

[cjl]

What do you mean "more powerful under load"? Power is always measured under some kind of load - any engine producing peak power is (pretty much by definition) also under high load at that point. Also, it kind of feels like we're going in circles - you've posted almost this exact same thing a number of times, and it has been responded to with math, examples, and lots of detail. What do you not find convincing in the responses?

 

[Moretorque]

It has more flywheel effect like in drive, as in when you clutch it on and off wide open it will maintain the RPM better under load. I have had some serious engine people tell me Peak HP is just your MPH and your amount of torque coming off the crank determines your ability to maintain it under load.

These people have told me the math crunchers do not understand HP in some ways, it is probably they are not explaining the power under the curve which is very very important as a back drop to spring off of like a slingshot. The more you have the more gear you can feed and use it as a catapult to go forward faster. This is what the drag racers were telling me, they need a broad torque curve.
When you shift and the RPM drops and the engine with more torque where the RPM drops to will rebound better driving back up the RPM range, so you just add more gears to the peakier engine to compensate ? they have rules however. So an engine with less HP and much more average HP can get down the track faster ?

Another thing is diesel are at a big disadvantage racing because of rev gain lose because of the long stroke ?

This is what one of the guy's at Reher Morrison told me and said the ship engine was a perfect example. I asked him if a ship engine pulled a 2500 hundred pound car and made 2300 HP at 270 RPM with 44000 foot pounds of torque and was geared correctly would it accelerate the car to 300 in less than a second and he said it would but from what I can see on looking at the drag race times on diesel powered cars he is wrong. The diesels are slow compared to the higher HP lower torque cars.

Thanks for hanging in there CJI.

 

[cjl]

It has more flywheel effect like in drive, as in when you clutch it on and off wide open it will maintain the RPM better under load. I have had some serious engine people tell me Peak HP is just your MPH and your amount of torque coming off the crank determines your ability to maintain it under load.

"Flywheel effect" is purely a matter of rotational inertia. It's true that large diesel engines (on account of being large) tend to have higher inertia, but I could change this for a small gas engine just by adding a whopping great flywheel.

These people have told me the math crunchers do not understand HP in some ways, it is probably they are not explaining the power under the curve which is very very important as a back drop to spring off of like a slingshot. The more you have the more gear you can feed and use it as a catapult to go forward faster. This is what the drag racers were telling me, they need a broad torque curve.
When you shift and the RPM drops and the engine with more torque where the RPM drops to will rebound better driving back up the RPM range, so you just add more gears to the peakier engine to compensate ? they have rules however. So an engine with less HP and much more average HP can get down the track faster ?

When you shift, the engine with more power where the RPM drops to will rebound better. If the RPM ranges are similar, this will also be the engine with more torque, but if the engines are substantially different in their operating RPM, this is a very important distinction. Power is the rate at which energy is being added to the vehicle, so acceleration is a function of the power (minus losses).

Another thing is diesel are at a big disadvantage racing because of rev gain lose because of the long stroke ?

This is what one of the guy's at Reher Morrison told me and said the ship engine was a perfect example. I asked him if a ship engine pulled a 2500 hundred pound car and made 2300 HP at 270 RPM with 44000 foot pounds of torque and was geared correctly would it accelerate the car to 300 in less than a second and he said it would but from what I can see on looking at the drag race times on diesel powered cars he is wrong. The diesels are slow compared to the higher HP lower torque cars.

Thanks for hanging in there CJI.

Diesels are at a big disadvantage in racing because they tend not to produce a sufficiently high power to weight. This is because of their low revving nature, yes, but fundamentally, the problem is that given a clean sheet, unlimited engine design that must weigh 200lb, you'll get more power out of a gas engine. Diesels can kind of gain some advantage back under certain specific circumstances (endurance races especially, where fuel weight and time between refuelling is a significant concern, hence the success of Audi's diesel LMP1 cars). Again, how fast a car accelerates (given appropriate gearing) is really just a matter of power to weight ratios.

 

[Moretorque]

The thing I heard about the Endurance cars but this could be wrong was they actually pulled hills better and accelerated better than the gas engines at high speed. I am talking like top end close to peak MPH. I also read because diesels have long strokes they build power slowly and this hurts them.

One thing you did not consider when you put a large flywheel on a gas engine the engine with more torque can spin it up to speed faster because of the stronger rotational power but as I stated it could be a wash compared to a diesel because the longer stroke...

I found a place in California who builds diesel race engines and will talk to them tomorrow. I have just had to many people who build race engines tell me a semi can do what it does because it makes the torque of a Pro Mod, it can't do the work as fast but has the drive off the crank to do the work where as a 400 hp car engine cannot drive that load from lack of twisting force at the crank and it would need many more gears to work.

More than likely you all are right and math does not lie and they just need to look at gearing closer...

 

[cjl]

I have just had to many people who build race engines tell me a semi can do what it does because it makes the torque of a Pro Mod, it can't do the work as fast but has the drive off the crank to do the work where as a 400 hp car engine cannot drive that load from lack of twisting force at the crank and it would need many more gears to work.

Can we go back to my black box example from a few pages back? I hand you 2 black boxes. Inside one is a diesel engine, which makes 1000ft-lb of torque from 1000-1800rpm. Inside the other is a highly tuned racing engine that makes 100ft-lb of torque from 10,000-18,000 RPM. The second engine is also hooked to a 10:1 gear reduction, so the output shaft is spun at 1000-1800RPM with 1000ft-lb of torque. What experiment can I do to figure out which box is which, assuming I can't tell by sound, size, weight, or anything like that? Why would one box power a truck any better than the other?

 

[Ketch22]

asked him if a ship engine pulled a 2500 hundred pound car and made 2300 HP at 270 RPM with 44000 foot pounds of torque and was geared correctly would it accelerate the car to 300 in less than a second and he said it would but

This is a fascinating exercise of the mind. Ship engines are truly at the far end of spectrum. Weight is of consideration but not high on the list. Horsepower and long service life are near the top along with economy. As previously mentioned the drive train as a system is an integral part of which engine is best suited for an application. In a ship the Hp requirements are well established and stable. The cost of a "custom gearbox" is negligible as ship parts are generally one off in nature.

So if one works the math backwards a little and looks at the identifiers involved. Hp as a rating of work performed is Torque (Foot / Pounds) multiplied by time applied multiplied by a correction factor for uniformity. Let's be real simple and restate Hp as Pound per foot per second. With that said we can reverse calculate your ship engine as producing 12,079,600 Lb/Ft/Sec. Dividing this by the car weight shows the work done on that vehicle would be 4,831.84 Ft/Sec or 3.66 seconds for a 1/4 mile pass. All of this is possible if the engine was anchored to a perfect immovable anchor and the connection was via some kind of perfect no loss winch system which could spool at over 300 miles per hour terminal velocity assuming that the car can hold together getting jerked that hard. I hope you notice that all of the iffy statements come in the application and not in the possibility.

Say I had a 2000 Hp engine that did not require the perfect anchor or the perfect connection system. This would only be providing 10,504,000 Lb/Ft/Sec. This obviously is providing less work however, this one can do it all day right now while the other one must find perfection. Please notice that it is just as much about the rest of the drivetrain and how well it works. Can it be beat, Yes but at what cost.

 

[Moretorque]

Thanks I understand all that through the math but something is not adding up, each time I have posted here I learn and thanks a lot. There are 2 camps here, one says the math does not lie and one says it does and more than likely the one that says it does is not explaining it is the power under the curve is what I am looking for.

The Semi has gobs of low end torque to get the load moving so I probably have not been figuring into how big a role this plays when getting 50 tons moving and how this makes the power always there because it produces so much off and just above idle...

Thanks!

 

[Moretorque]

Thanks a lot Ketch 22,

 

[jack action]

a 400 hp car engine cannot drive that load from lack of twisting force at the crank and it would need many more gears to work.

Isn't that what we are saying since the beginning? A 400 hp car engine CAN drive that load, if it has the proper transmission.

 

[cjl]

Isn't that what we are saying since the beginning? A 400 hp car engine CAN drive that load, if it has the proper transmission.

Though to be fair, it wouldn't even necessarily need more gears. Just dramatically different ones. The number of gears required is determined by how wide the power band is, the ratios required determined by what RPM that powerband falls at.

 

[jack action]

Though to be fair, it wouldn't even necessarily need more gears. Just dramatically different ones. The number of gears required is determined by how wide the power band is, the ratios required determined by what RPM that powerband falls at.

But it will be most likely the case as the power band is usually narrower (percentage-wise) at high rpm than at low rpm. Because a ±1500 rpm range at 6000 rpm (±25%) is actually narrower than a ±1500 rpm range at 3000 rpm (±50%). For example, if the low-revving engine could do the required work with 3 gears, the high-revving engine would need 5 gears - maybe 6 - even if they both have a 3000 rpm range. The math would be:

N_{hi} =1+(N_{lo} - 1)\frac{ln\left(\frac{RPM_{max-lo}}{RPM_{min-lo}}\right)}{ln\left(\frac{RPM_{max-hi}}{RPM_{min-hi}}\right)}
5.3 =1+(3 - 1)\frac{ln\left(\frac{4500}{1500}\right)}{ln\left(\frac{7500}{4500}\right)}

And that is what most dyno people fail to see when they look only at the engine.

 

[Ketch22]

Moretorque, One other thing that I think is confusing the issue. Your focus on the area under the curve is possibly one
When we look at an internal combustion engine of the otto cycle type a critical item is in the throttle plate position. The horsepower developed is directly related to the volumetric efficiency. When the throttle plates are at any position other than wide open there is introduced an artificial restriction. An engine that runs " at the curve" may be capable of producing 350 Ft/Lbs (at 4000 RPM) and 360 Hp (at 6000 RPM). However if it is run part throttle open, Let's say at even a slight reduction such that only 90 volumetric efficiency is achieved. That engine will make 315 Ft/Lbs ( and that not at the same RPM due to complex intake path alterations due to the plates) and 324 HP.
A similar action takes place in a Diesel cycle. Although these get full air at all times and the fuel is controlled by some type of Governor relating to the accelerator pedal. The same basic problem in that the smaller the difference between actual RPM and requested RPM the smaller the overfuel rate from Stoichiometric. Even so a lower RPM engine that produces say 500 Ft/Lbs (at 2000 RPM) and 306 Hp (at 3500 RPM) will still if it stays in the 90% range of fueling be closer as it will be 450 Ft/Lbs and 275.4 Hp.
It must be conceded that the 35 Ft/Lb reduction of torque compared to the 50 Ft/Lb appears to indicate when compared to the 36 Hp loss to the 48.6 makes it seem that Hp makes less difference than Hp. This is in reality a case of the part throttle open condition having notable effect in various parts of the application.
In my early days, even while I was crew on a blown fuel funny, I also raced a Pro Bracket of my own. It was obvious to me the huge effect if I "pedaled a little." The reality being that restraining the motor ever so slightly kept me in the bracket and If I needed to open it up I was holding a lot in reserve by doing so.
In your exploration of the peak performance (the area "at the curve") it is good to remember that you may be speaking about what some pepes feel comfortable putting to the ground ( any spot "under the curve") while engineers deal with the possibilities " at the curve"

 

[jedishrfu]

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