This may sound crazy, I know nothing about engineering, but I was wondering. A rotax operates at higher RPMs and then uses a reduction drive. Other engines, same horsepower, use direct drive. All things equal does one have an advantage over the other (not concerned about added moving parts). Another way to put it is if the rotax is gearing down does it have a torque advantage? Other performance advantage?

dplunkt wrote:This may sound crazy, I know nothing about engineering, but I was wondering. A rotax operates at higher RPMs and then uses a reduction drive. Other engines, same horsepower, use direct drive. All things equal does one have an advantage over the other (not concerned about added moving parts). Another way to put it is if the rotax is gearing down does it have a torque advantage? Other performance advantage?

Smaller displacement = less thirsty, but will require more RPM to make the same HP.

The Rotax is considerably lighter weight.

Higher RPM and a reduction bow also reduces the TBO of the engine. A continental O-300 has a TBO of 1800 hrs., a GO-300 (same engine with reduction box) only has a TBO of 1200 hrs.

Rotax's TBO is 2000 hours now. My last Rotax was a 2005 model and the TBO was still only 1500 hours. I put 1500 hours on it and based on leak down, compression and oil analysis, my Rotax mechanic said that is like a motor that has 100 hours on it.

Steve
Wild West Aircraft

To have endurance, a high rpm engine has to be better engineered with regards to tolerance and balance. When Honda introduced their small engine bikes back in the 60's no one thought they would stand the test of high usage. Best small engine ever. Rotax will have some of that same engineering advancement but the engineering alone is not what makes their success. They have the advantage of water cooling which eliminates hot spots and unequal cooling reducing stress loads and metal fatigue. Look at the difficulties of baffling the O-360 in homebuilts. Cooling or lack of it to critical areas of the heads and cylinders affects the "real life" of the engine, not TBO, from one airframe design to the next even with the same engines.

For low end torque pulling a big load there is a theoretical advantage to having large displacement pistons and crank providing the momentum to carry load changes by swinging around more mass. But unless that engine is in a tractor lugging a plow through a surprise gumbo spot, that more steady momentum required by an aircraft prop can be just as well supplied by swinging a smaller crank and pistons at a higher rpms. Centrifugal force at higher speeds can provide similar effects of larger mass (but will drop off from peak Hp sooner under increased load or as rpm drops). This idea works in aircraft because of the lack of gumbo spots needing to be powered through at the prop and that reduces the sudden changing torque load requirement of the engine in order to maintain rpm's and hp. Gearing down a high speed engine works better in aircraft than with tractors and long haul trucks.

Same effect as having a 5 speed car that is driven in 4th gear all the time.... Been working great in my plane so far. 400+ hours and about 60,000 miles...

Here is the hp and torque chart for the Rotax 100 hp.

http://www.rotaxservice.com/documents/912Sperf.pdf

The following is a listing of hp/torque values for many 100 hp aviation engines including the Rotax redrive. An interesting note, above the chart it states that the Rotax factory numbers assume no friction loss through the redrive but in real life performance there is a drop of 10%.

https://sites.google.com/site/joesarcst ... scriptions

Air cooled motors run a richer mixture in order to help cool the engine. It's far less efficient than liquid cooling. Still, it all comes down to something in the neighborhood of .4 pounds of gasoline per horsepower hour. The Rotax is lower than that. The Continental of similar power is higher. It's about changing heat into horsepower. When EPA gets involved you need to run it all hotter and efficiency goes down. I read something recently that the overall fleet MPG of US new cars is lower now than it was in 1980. Or thereabouts.

EB

dirtstrip wrote:For low end torque pulling a big load there is a theoretical advantage to having large displacement pistons and crank providing the momentum to carry load changes by swinging around more mass. But unless that engine is in a tractor lugging a plow through a surprise gumbo spot, that more steady momentum required by an aircraft prop can be just as well supplied by swinging a smaller crank and pistons at a higher rpms. Centrifugal force at higher speeds can provide similar effects of larger mass (but will drop off from peak Hp sooner under increased load or as rpm drops). This idea works in aircraft because of the lack of gumbo spots needing to be powered through at the prop and that reduces the sudden changing torque load requirement of the engine in order to maintain rpm's and hp. Gearing down a high speed engine works better in aircraft than with tractors and long haul trucks.

Horsepower is the only real requirement. Any required torque is accomplished through gearing. OTR tractors use large displacement engines because they have better long-term durability by making large amounts of power a low RPMs. You can pull the same load with a 450hp 350ci V-8 as you can a 450hp CAT - provided you adjust the gearing to match the engine RPM you'll be making that power at. The big difference is that the 450HP CAT will be able to do it for half a million miles. The 350ci might be good for half a dozen . Rotational inertia plays a role, but is better gained through weighted flywheels. Heavy pistons and CRs require lots of energy to stop and start and stop and start throughout the engine's rotation.

I'm no engineer, but I'm pretty sure that an inline 6 cat at 450hp will make a lot more torque then my V8 Chevy 350 making 450hp. I could gear that 350 down as much as I wanted, but still have trouble pulling a set of super b grain trailers loaded at 63500 kgs nearly as efficiently or easily as the cat. But again, I'm no engineer, just a driver.
David

Remember, horsepower is just a calculation of the relationship of torque to rpm. It means virtually nothing in regard to power.

Rotational Horsepower = (Torque x RPM) / 5252

It's more an issue of power to weight. If weight and fuel economy are not issues, then there is no replacement for displacement. But in a light sport aircraft, a dirt bike, snowmobile or FI race car, a high rpm small displacement light weight engine has the advantage.
Even the new O-200D is an anchor compared to a Rotax ULS. And with a 2000 hour TBO, they are just as reliable.

Horsepower and torque.

The two terms are related, but for simplicity, torque is referring to an engine's ability to do work or how much force it can apply, while horsepower is the amount or rate it can accomplish work.

A big long stroke heavy diesel has a lot of torque and is better suited to starting and hauling heavy loads.

bumper

OK, I'll bite. Now that I spend more time managing engineers and projects than doing them myself I can let the repressed engineer-speak loose a little here.

Horsepower is horsepower and torque is torque. They are mathemathically interrelated. If a tractor trailer going down the road requires 450 hp to go 65 mph on a constant grade, road surface, etc., the exact same amount of torque is being delivered to the drive wheels, regardless of what engine it has. The tractor trailer at a steady speed and load couldn't care less what power source is providing the horsepower to maintain that steady state.

The difference is the gear reduction that would be required to achieve this horsepower requirement with the gas and diesel engines; the gas engine in the aforementioned example is likely going to deliver this at 4000+ rpm, while the diesel engine at 1500 - 2000. What the diesels have going for them is a rising to consistent level torque curve in its operating range, meaning that as more throttle is applied, you get the same or more torque with the increase in rotational speed, whereas the gas engine will be on the backside of its torque curve in the land of diminishing returns. This fact along with their inherent efficiency, reliability and durability make the diesel the best choice for consistent load applications.

I spec equipment on a regular basis that is rated at 25,000 to 50,000 foot pounds of torque, which is driven reliably in 20 year continuous service with a 1 hp to 2 hp motor. This is accomplished through triple reduction gearing, with a final output of in the neighborhood of .03 rpm. Using the formula mentioned above, this gives an extreme example of how horsepower and torque are directly related, only modified through rotational speed.



Flynengr

A1Skinner wrote:I'm no engineer, but I'm pretty sure that an inline 6 cat at 450hp will make a lot more torque then my V8 Chevy 350 making 450hp. I could gear that 350 down as much as I wanted, but still have trouble pulling a set of super b grain trailers loaded at 63500 kgs nearly as efficiently or easily as the cat. But again, I'm no engineer, just a driver.
David

Gearing multiplies torque. I've done the math, but suffice it to say that yes, two 450hp engines - regardless of displacement - will put the exact same torque down at the wheels if you gear them to make the same wheel speed at peak hp. Wheel torque is all that matters in this equation.

I wouldn't trade your CAT for a 350 if you want to make the trip more than once though, lol. There's a definite reason the industry uses the equipment they use, and it's because of efficiency, durability, and longevity like you talked about. It can be done as "easily" with a smaller displacement engine though, as long as both engines make the same power, they can both do it.

Av8r3400 wrote:Remember, horsepower is just a calculation of the relationship of torque to rpm. It means virtually nothing in regard to power.

Rotational Horsepower = (Torque x RPM) / 5252

Power = the rate that work is performed. Horsepower is a unit of measurement of power. Horsepower means virtually everything in regard to power (in the rotational context we're talking about)

746 watts per each horse.

Emory Bored wrote:746 watts per each horse.

But can you check one like a 9 volt? ewwww......

CamTom12 wrote:
Gearing multiplies torque. I've done the math, but suffice it to say that yes, two 450hp engines - regardless of displacement - will put the exact same torque down at the wheels if you gear them to make the same wheel speed at peak hp. Wheel torque is all that matters in this equation.

I hope I have not deleted so much of your post that I am replying out of context of your meaning but I don't think so.

I will expand on "Wheel torque is all that matters in this equation". This needs to be given the same perspective on the diesel truck engine geared down through transmission and axle ratios to required wheel speed torque as it is on the high rpm aircraft engine geared down for the required torque making for the most efficient prop rpm. As Bumper stated earlier its no good to run in fourth gear all the time, so the torque delivered to the wheels must match the torque requirement of the wheel at desired speeds. Likewise, the high rpm geared aircraft engine is limited by the requirement of efficient prop speeds. The higher rpm of the engine runs through reduction gears to deliver torque at the correct prop speed like the correct transmission/axle ratio delivers required wheel rpm. This is more torque than the geared engine could deliver if it was direct drive since it would need to slow down out of its peak hp rpm. This is regardless of the fact that the engine is capable of making the same horsepower with or without gear reduction. Spinning the prop too fast to make hp results in inefficient or dangerous prop tip speeds so the reduction gears in both allow each engine to run at their peak power rpm at the required final drive speeds of either the prop or truck wheel.

dirtstrip wrote:I hope I have not deleted so much of your post that I am replying out of context of your meaning but I don't think so.

I will expand on "Wheel torque is all that matters in this equation". This needs to be given the same perspective on the diesel truck engine geared down through transmission and axle ratios to required wheel speed torque as it is on the high rpm aircraft engine geared down for the required torque making for the most efficient prop rpm. As Bumper stated earlier its no good to run in fourth gear all the time, so the torque delivered to the wheels must match the torque requirement of the wheel at desired speeds. Likewise, the high rpm geared aircraft engine is limited by the requirement of efficient prop speeds. The higher rpm of the engine runs through reduction gears to deliver torque at the correct prop speed like the correct transmission/axle ratio delivers required wheel rpm. This is more torque than the geared engine could deliver if it was direct drive since it would need to slow down out of its peak hp rpm. This is regardless of the fact that the engine is capable of making the same horsepower with or without gear reduction. Spinning the prop too fast to make hp results in inefficient or dangerous prop tip speeds so the reduction gears in both allow each engine to run at their peak power rpm at the required final drive speeds of either the prop or truck wheel.

Agreed, and that's a great analogy! For example, 2x 100hp aircraft engines, say an IO-233 and a 912S.