# reasons not to use high-performance diesels

Discussion in 'Diesel Engines' started by Joris, Mar 25, 2013.

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### DCockeySenior Member

To determine the power flowing through a prop shaft you need to know rpm and torque on the prop shaft. The formula
Power = Rotational Speed X Torque (with appropriate conversion factor which is dependent on units used)​
is used to calculate the power from the torque and rotational speed/RPM of the shaft.

RPM (Revolutions per minute) is a unit of rotation speed. Other units of rotational speed include "Revolutions per second", "Degrees per second" and "Radians per second". Torque is the rotational equivalent of force. A torque applied at a rotational speed does work. The rate at which work is done is power and "Horsepower" is one unit of work. "Watt" and "Kilowatt" are other units of work.

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### Easy RiderSenior Member

Is the 3300lb definition not valid?

The power to lift a given weight a specific distance in a specific amount of time.

I believe that's basically the definition of HP. But to achieve that amount of work torque would be involved if twisting shafts and turning gears were part of the actual doing of work. But the work could be done w force and a wedge or water directed at an object or a reciprocating hand pump. There are many ways of doing work w/o involving torque. So how could torque be part of the definition of power?

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### DCockeySenior Member

Lifting 33,000 (not 3,300) lbs a distance of 1 foot at a speed of 1 ft / min equals a power of 1 HP.

Pushing horizontally with a force of 33,000 lbs at a speed of 1 ft / min equals a power of 1 HP.

There are many ways of doing work without involving torque. There are also many ways of doing work involving torque.

Consider a drum with a 1 foot in radius (2 foot in diameter) with a cable wrapped around it and a 33,000 lb weight hanging from the cable. If the drum is twisted at a speed of 1/(2*Pi) revolutions per minute the weight is lifted at a speed of 1 ft per minute which equals a power of 1 HP. The torque on the shaft is 33,000 ft-lb.

Twisting a shaft with a torque of 33,000 ft-lb at a speed of 1/(2*Pi) revolutions per minute equals a power of 1 HP independent of what type of work is being done by the shaft.

Power = Speed X Force (with appropriate conversion factor which is dependent on units used)

Power = Rotational Speed X Torque (with appropriate conversion factor which is dependent on units used)

And there are other equations for power for other ways of doing work.

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### JorisJunior Member

It isn't. The definition of power involves Force, distance and time. The use of torque in calculations is merely because in a lot of situations (for example with engines) it is easier to measure this accurately then Force and distance separately.

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### jonrSenior Member

I wonder how accurately one could estimate torque by measuring compression force (or distance) on an engine mount. The engine does attempt to rotate in the opposite direction.

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### Steve WSenior Member

All this theoretic stuff is beyond me, what I do know from personal observation, at least with road vehicles, is that torque is everything if you want to get work done, I don't even care about horsepower anymore, at least in vehicles it just for bragging rights. Why do I say this? well in 2005 gas prices were high and I made the decision to buy a diesel car (I already had a diesel pickup), I bought a VW tdi with a whopping 90hp at something like 5000rpm and 150ft/lbs of torque at 1900rpm, what an eye opener. We have a hill here with a 30mph speed limit that on any gas vehicle I have ever had I would need to be down to3rd gear to pass anything with the tdi I could loaf along in 4th at 30 and then punch it and pass cars without changing down. I love the driving experience of tdis. I have the unique experience of being able to do a a directly compare this vehicle with a vr6 gas version of the same year car as I wrecked the tdi and bought a vr6 to swap the tdi into,i put 60k on the tdi and 18k on the gasser, the vr6 is completely useless, it has twice the hp and more torque also but at much higher rpm so you really never get to use it, so as far as doing work the 90hp engine is much more powerful than the 195hp engine. Oh, the vr6 gets 26mpg at 58mph, the tdi, 55mpg.

Steve.

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### jonrSenior Member

> so as far as doing work the 90hp engine is much more powerful than the 195hp engine.

It might be more fun or convenient to drive, but 195 HP, properly geared, will do much more work, accelerate faster and after gearing, produce more torque at the same rpm.

> at much higher rpm so you really never get to use it

You can if you downshift.

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### Steve WSenior Member

On my vr6 if I were to get any use out of the torque I have to be above 3000 rpm, which as you say requires a lower gear which transalates to even more dismal fuel economy. The vr6 is silky smooth but is a guttless wonder until it gets over 3000 rpm at which time it comes alive but you simply don't operate in that range unless you are driving in a spirited, read, gas guzzling fashion. The big, 90hp tdi on the other hand was able to tow a 19ft beachcat through the mountains of west Virginia in 4th gear with the occasional drop to 3rd without the temperature rising at all while maintaining 30mpg, a much more efficient worker imho based on a lot of miles with each configuration, no theory. Actually the tdi never sees even 90hp as it is never turning at more than 3500rpm, and that's only merging at the on ramp.

Steve.

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### JorisJunior Member

I'm afraid i'd have to dissagree on that. The VR6-engines will be down 20-25lbs/ft around 2500 rpm compared to the TDI. But with the TDI the torques will start to drop off after 2500rpm and the VR6 will keep building torque up to 5000rpm, maybe even further. I found the VR6 a very nice engine and consumption wasn't to bad if you kept the it near 3000 without putting the pedal down...I now have a Peugeot 407 CoupĂ© with a 140hp HDI, best of both worlds

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### jonrSenior Member

It looks like you missed the fact that I was referring to the VR6. After gearing.

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### JorisJunior Member

Sorry if i misunderstood. I thought you meant the VR6 would beat the TDI on torque at any rpm.

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### Steve WSenior Member

The TDI torque may drop off after 2500rpm, I don't know, I am never that high unless merging onto the freeway maybe and I sure don't notice it, 1800 to 2200 is where it lives and is very powerful in this range, and this is also where the vr6 lives on our roads otherwise I would be getting speeding tickets all the time but unfortunately the VR6 has no torque in this range. There is a saying that you buy horsepower but you drive torque and this is very true. I currently drive a B5.5 TDI Passat and it is no where near as efficient as the old B4. I will be selling it once we have transplanted the Tdi from the wrecked car to replace the VR6. I envy you European guys with the choices you have.

Steve.

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### Easy RiderSenior Member

Thanks very much Steve, Joris, Jonr and DCockey.

Now getting back to the OP. Say I had a 30hp diesel that developed it's power at 3400rpm. Another fellow had the same boat w a 30hp engine and it developed it's power at 1500rpm.

I predict both boats will have the same top speed using the same prop at the same shaft speed.

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### AZboatbuilderJunior Member

? Is there any reason the light high performance diesel-engines
.... a 2.2L 4cylinder biturbo diesel that produces 170hp and over 300lbsft of torque seems very interesting. Thanks
_________________________________________________________________
Gas engines will always do better economy , per #/fuel .
They have greater variations , thus confusing ..Honda or Ford !
Honda has perfected the gas engine , highest economy , and highest
output , they always do high output above 6500rpm , and racing
output ( B18b or B18c ) is above 7500rpm ,
Diesels DONT DO rpm range , Even the 5cyl M.B. best in its class ,
has less rpm band than a Honda gas engine . Its their nature .

Study carefully . the effect of a turbo on a diesel !
toss the job of compressing , from the piston , to the turbo , NO GAIN ,
pumping losses are a bit WORSE .
But not on a gas engine , Turbo is idling , and NOT a load on exh gasses , up to 5000 rpm , where the Honda is breathing LESS ,
Turbo kicks in , carries the Honda all the way to 8500rpm ,
the economy is actually good at 8500 ,

I will reduce 4:1 to drive a big prop , i have 10 Honda B18b , DOHC

If i need it , i can call 2000 HP , at high economy .
Tom Scott from [ Roman Empire ] a 120' steel outrigger

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### AZboatbuilderJunior Member

Atlinson , experimented with BIGGER engines , and much lower C.R. ,

he succeeded ! Much better power , at same consumption , But in the

mines , Watts steam engine , burned cord wood , pleased the miners !

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