Engine torque output

GrauGeist · Feb 5, 2023

I'm not sure if equating automobile engines to aircraft engines, in regards to torque, is a good comparison.

With a vehicle, the engine is propelling a mass on a surface, so a smaller displacement engine that takes advantage of torque through gearing can perform with impressive results.

With WWII aircraft, torque doesn't seem to as much of an in issue unless the engine was driving a shaft, like the P-39/63, Do335, etc.
In these cases, the shaft needed to be robust enough to handle the output of the engine under full load.

GregP · Feb 13, 2023

Equating horsepower to torque applies to all internal combustion engine regardless of their intended use. The Horsepower to torque relationship is by definition, not by application, as you know. HP = (torque x rpm)/ 5252 as shown above.

The main issue with aero engines is the fact that we want to keep the propeller tip subsonic, which pretty much means aero engine turn slowly or will by necessity have reduction gears. Most but not all WWII aero engines, be they radials or inlines, had reduction gears. If they did, then it doesn't make a lot of difference what type engine is attached to the reduction gears, other than wanting two spark plugs per cylinder in case one fouls since you can't pull over and park once airborne.

Other than that, aero and auto gasoline engines are pretty similar, with the aero engine mostly turning rather slowly, mainly due to reliability considerations. That doesn't stop them from developing very good power. The BMEP for WWII aero engines was very good, as you no doubt already know, too.

GregP · Jul 13, 2023

ddwhitney said:
HP = (Torque * RPM)/5252. At 5,252 RPM Torque and HP values are the same.

QUESTION: For Automobile engines the peak torque is usually higher than the torque at HP peak, and at a lower RPM. How is it for Aircraft Engines?

ANSWER: Aircraft engines have to deliver Torque to the propeller. It takes power to do this.

Below is a Torque and BHP versus RPM curve for an Allison V-1710-73(F4R) engine, having 8.8:1 supercharger gears and rated at 3,000 RPM to deliver 1560 BHP using 60 inHgA (14.8 psi Boost) manifold pressure. The torque data was obtained 5-11-2001 on the ACE Allisons calibrated dynamometer, and the horsepower values calculated per the above relationship.
See V-1710-73 Chart

The above chart covers the full range of useful power from the engine. Of significance, is the linearity of the shown data. For comparison, similar curves for the Chevrolet LS-1, a V-8, are shown below. This chart clearly shows the intersection at 5,252 rpm where torque and horsepower have the same values as well as the "linear" range from 1,000 to just over 4,000 rpm.
See LS-1 Chart

This linear range is where the cylinders are being fully filled on each intake stroke, i.e., Volumetric Efficiency is not changing as rpm increases, for both naturally aspirated and supercharged engines. When supercharging, intake manifolds and/or the valves have reached a flow rate where they can no longer deliver a full charge to the cylinder torque begins to drop off, as does the produced power. This is very apparent on the Torque/Power curves for an engine capable of reaching 5,252 rpm, or above.

Large aircraft engines, due to their relatively long strokes and heavy pistons are not capable of operating at speeds in the area of 5,252 rpm. As such, aircraft engine power curves do not show the peaking and subsequent drop-off. Instead, they operate in the very linear range where, for the most part, horsepower is a direct function of rpm.

Note that supercharged aircraft engines are rated, at each available rpm, by identifying the manifold pressure at the point of incipient detonation, as established by the fuel and mixture strength being used. This then describes their "available power" curve, which is always going to be in the "linear" range shown on the Torque/Horsepower versus RPM curve.

Another interesting aspect of the above formula is the origin of the 5252 factor. It is simply the definition of horsepower, 33,000 lb-ft/minute, divided by 2(pi). The 2(pi) comes from the conversion of RPM into Radians.

Dan Whitney

Orangevale, CA 2-4-2023

rpm doesn't convert into radians. It converts into radians per revolution or radians per minute mif multiplied by rpm.

2* pi radians in one reevolution, multiplied by x revolutions per minute gives you 2 * pi * x radians per minute. Of course, you can define whatever time slice you want to use ... seconds, minutes, hours, etc. Most formulas use radians per minute because the rpm time base is one minute.

WARSPITER · Jul 14, 2023

Going on the equation above torque will be horsepower x 5252 / RPM so 200hp x 5252 / 2000 RPM = 525 foot pounds.

It isn't really a factor for aircraft as such as engines were made larger to give plenty of power at lower RPM. Keeping the
tip speed of the propeller below certain speeds was one reason plus the engine would have to run at 75% + capacity
for long periods whereas a motor vehicle gives higher RPM when needed as it spends most of it's time using less
than 25% of its capacity.

AMCKen · Jul 16, 2023

And 200hp at 4000rpm is just 263ftlbs.
Also, dynamometers don't measure horsepower. They measure torque and the results are then converted to horsepower.

GregP · Jun 10, 2024

HP = (Torque * rpm) / 5252
when:
1 HP = 33,000 ft-lbs / min (SAE hp)
Torque = lb-ft
rpm = revolutions per minute

Engine torque output

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GrauGeist

Generalfeldmarschall zur Luftschiff Abteilung

GregP

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GregP

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WARSPITER

Tech Sergeant

AMCKen

Senior Airman

GregP

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