Long Stroke vs Short Stroke 4x4 Engine, Why?

[luthj]

Its hard to visualize for those who don't have background in physics, but HP is Power (kilowatts). Torque x RPM = Power. Torque by itself has no value in propelling a vehicle.

Power out must equal power in minus losses. The gear does not affect this. All it does is change the engine RPM to speed relationship, which allows for the ideal power output for a given road speed. Its not intuitive, but most of engineering isn't.

I can put a 5ft lever arm on a transmission input shaft. Then put all my weight on that arm to make 1000lb-ft of torque. Does that mean I can now move the vehicle at 60mph by myself? Nope.

An M1 Abrams tank is powered by a gas turbine engine. This engine makes a measly ~200lb-ft of torque. However it makes 1500 shaft horsepower (something around 10k rpm operating speed). Does that mean its not suitable to move a 63 ton tank? Of course not. There is a transmission, including a significant gear reduction as part of the design.

The reason everyone is focused on low rpm and high torque is because big commercial diesels have no other choice than to make big power at lower RPM. They simply cannot rev high enough due to limitations of diesel combustion.

When it comes to driving a low speed and off road, a properly geared low range or low gear can easily be chosen to provide good control and power at any engine RPM. Though there are cases where its desirable to have a significant power off idle, but thats more to do with loss of control due to rapid revving.

A wide and flat powerband/curve ensures that the engine can be used without 30 gears to deal with heavy loads. This often means sacrificing peak power. But with no sudden drops at either end of the powerband, you can operating inside of it without needing to downshift to increase the available power for that road speed.

 

[nickw]

20 years around trucks and over a million miles in the mountains, I'm just going to quit.

Like I said, there can be a number of practical limitations why you'd pull a hill at a lower RPM like gear ratio limitations or fuel efficiency that we can't quantify, but outside of that, the math is the math.

I'm 100% willing to admit practicality does not always = math, but given the hypothetical argument, HP is king.

 

[nickw]

An M1 Abrams tank is powered by a gas turbine engine. This engine makes a measly ~200lb-ft of torque. However it makes 1500 shaft horsepower (something around 10k rpm operating speed). Does that mean its not suitable to move a 63 ton tank? Of course not. There is a transmission, including a significant gear reduction as part of the design.

Great example, I had to check, from Wiki:



The numbers above are with a 10:1 reduction, so the engine at peak HP is 30,000 RPM and 275 ft/lbs of torque at the crank. Max torque is 10,000 RPM and 400 ft/lbs at crank.

 

[luthj]

One watt is equal to 1 Newton-meter per second (Nm/s)

Torque in the SI system is measured as newton-meters (Nm).

It would be an interesting work of magic to convert torque to watts with dimensional analysis. Its akin to saying that volts is the same as watts.

Instantaneous torque is not power, torque over time (RPM) yields power. So when someone talks about torque at a certain rpm, they are actually referring to power. Which unfortunately leads to the confusion that torque numbers are all that matters, but they only tell a portion of the story.

 

[toylandcruiser]

almost every engine, gas or diesel, horsepower peaks near redline, torque peaks in midrange.
with a class 8 engine horsepower might climb to 600 HP at 2200rpm.
but torque will plateau at 1800ftlb at 1600rpm, slowly dropping off as you accelerate.
so when cruising at 2000rpm and entering a 10% grade as the rpms fall, the available torque increases til you hit 1600rpm.
and the quickest way to climb that grade is to keep the rpm between 1500rpm and 1900rpm.

Diesels live for mid range rpms because they make way more torque than HP.
But gas engines perform best at redline since they make so much less torque.

The 2f peaks at 1800 I’m pretty sure.

 

[nickw]

The 2f peaks at 1800 I’m pretty sure.

Yes, torque peaks at that number.