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

[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]

Closed for moderation

 

[russ_watters]

Thread will stay locked. It is getting too disjointed and rambling to have any value. If there are specific questions, they can be asked in new threads. But please: be specific.