Thielert Centurion 2.0

[Kevin Horton]

Originally Posted by Dave Cole

I don't understand the excitement. If the Centurion 2.0 is only 135 hp at sea level takeoff power, who would want one in an RV? That kind of defeats the "Total Performance" concept. In time, you may recover the extra cost of the engine with fuel savings, but the operational limitations represent an additional "cost" that would be unacceptable to me.

The Thielert engines are turbocharged. While the power available at sea level is lower than desired, 135 hp is still available at 6,000 ft, and 122 hp is available at 10,000 ft, and 97 hp at 17,500 ft. The cruise power available at typical cruise altitudes approaches that from an O-320.

It is a heavy and expensive engine though, so I agree that very few people would put one in an RV, even if a FWF kit was available.

[mdredmond]

Originally Posted by Dave Cole

I don't understand the excitement. If the Centurion 2.0 is only 135 hp at sea level takeoff power, who would want one in an RV? That kind of defeats the "Total Performance" concept. In time, you may recover the extra cost of the engine with fuel savings, but the operational limitations represent an additional "cost" that would be unacceptable to me.

Don't confuse horsepower with torque. A 135hp Thielert will perform as well or better than a 160+hp Lycoming AND do so up to the airplane's service ceiling.

[OldAndBold]

Can someone comment on what might be involved in transitioning an RV-7A QB wings/fuel tanks to allow for proper venting of kerosene jet fuel??? This consideration has kept me away from considering any diesels or the innodyn engine...


Thanks

John Babrick
RV-7A QB Empennage...

[Kevin Horton]

Originally Posted by mdredmond

Don't confuse horsepower with torque. A 135hp Thielert will perform as well or better than a 160+hp Lycoming AND do so up to the airplane's service ceiling.

Torque output alone tells you nothing. Power = torque x rpm (with appropriate conversion factors). So you need to know the rpm and torque to know anything useful.

The performance in a given aircraft is determined by the amount of power delivered by the prop. This is equal to the engine's power, times the prop efficiency. Or, if you really like starting with torque, it is equal to the torque, times the rpm, times the prop efficiency.

[mdredmond]

Originally Posted by Kevin Horton

Torque output alone tells you nothing. Power = torque x rpm (with appropriate conversion factors). So you need to know the rpm and torque to know anything useful.

I understand. Likewise, horsepower is meaningless when considered alone. BTW, the conversion factor is 'divided by 5252'.

Torque is the only thing the propeller feels from the engine. A given amount of torque made at one engine rpm will turn the propeller just 'as hard' at as the same torque made at a higher one, but the higher rpm will result in a larger horsepower measurement (with no difference in performance).

Diesels are designed to produce their torque at low rpms, thus lowering the measured horsepower. This doesn't necessarily translate into lower performance.

Don't get me wrong, I'm no engineer. My understanding is just the result of a bunch of reading about this stuff. Worth what you paid for it...

[Kevin Horton]

Originally Posted by mdredmond

A given amount of torque made at one engine rpm will turn the propeller just 'as hard' at as the same torque made at a higher one, but the higher rpm will result in a larger horsepower measurement (with no difference in performance).

Wrong. If you have the same amount of torque at a higher rpm, the additional power at the higher rpm will result in higher performance (this assumes that the prop efficiency does not have a significant decrease at the higher rpm).

I'm not sure how to explain this in simple terms. But trust me on it. I spent four years studying engineering, a year at test pilot school, and have been working full time as an engineering test pilot since 1988. Aircraft performance is determined by power delivered by the prop. Not torque.

Climb performance is determined by excess power (excess power = the power available - the power required to maintain level flight at a given speed). When climbing, you are lifting the weight of the aircraft upwards at a certain vertical speed. If we lift the weight of the aircraft a certain distance upwards, we have performed work, with the amount of work equal to the aircraft weight times the altitude gain. Power = the rate of doing work. The excess power = aircraft weight times rate of climb. So, the rate of climb is determined by the excess power. More power = higher rate of climb.

Level flight performance is also determined by the amount of power. In level flight the thrust = drag. Thrust times speed = power. More power = higher thrust = higher speed.

[Deuskid]

Originally Posted by Dave Cole

I don't understand the excitement. If the Centurion 2.0 is only 135 hp at sea level takeoff power, who would want one in an RV? That kind of defeats the "Total Performance" concept. In time, you may recover the extra cost of the engine with fuel savings, but the operational limitations represent an additional "cost" that would be unacceptable to me.

others have just commented about torque and that is key.

at about 2xxx rpms [depends on the particular diesel] you'll have all the torque you'll need. That is part of the beauty of diesels no PSR needed ... it naturally functions at optimal rpms for props.

as far as weight, yes, diesels weigh more... but you need to carry less fuel for the same range [because of the efficiency] so its weight disadvantage is offset...

here is a pilot's report on a recent 1.7 flight :

http://canardaviationforum.dmt.net/s...ghlight=diesel

She was flying a spam-can but gives a good comparison between a gasser and the T-plant.

I've lived with auto diesels and a turbo'd 90 hp diesel provides all the power I need to be pleased [I also drive sports cars too and although the turbo-D is not as performance oriented, my diesel satisfies and at > 50 mpg it is a joy to use on the highway].

I also like the thought of using biodiesel [something we can do here in the US] and being energy independent. [I worry about AVgas going away [I guess a 150 hp Lyc or an auto gasser are options too hedge this concern]]

Thielert is too pricy but there are some promising substitutes [Subie is coming out with a boxer opposed 4 turbo-D and Toyota/Lexus has a dandy 2.2 4 banger already out].

Diesels aren't going to please those who want optimal speed but I'm thinking a 135 hp turbo-diesel is the ideal engine for a -9/9A.

YMMV

John

[Dave Cole]

When you are crusing, drag is a force, in pounds, and you need an equal quantity of thrust from the engine/prop combination to maintain altitude. The power required can be calcualted as the product of the airplanes velocity and the thrust, or ft/sec * lbs, more commonly expressed as ft-lbs/sec. For reference, 1 HP = 550 ft-lbs/sec.

The point is, what you need to offset drag is power. While engine power is a function of engine torque, high torque does not necessairly mean high power...it depends on the rpm at which it is produced.

A 135hp Thielert simply will not perform as well as 160hp Lycoming under sea level takeoff conditions (and up to about 8,000 ft pressure altitude). However, at higher altitudes, the turbochargers allow the Thielert to produce more horsepower than the Lycoming.

For my money, and for the kind of flying I do, takeoff power is far more valuable than cruise power at higher altitudes. YMMV.

[Kevin Horton]

I just thought of an automotive example that might help shed some light on the confusion of torque vs power.

Imagine you are driving a car that has an engine with a very flat torque curve - i.e. it produces the same torque over a wide range of rpms, like many big V-8 engines. We are going to measure the acceleration at two different rpms, starting from the same speed.

Run 1 - The engine is turning 1,500 rpm. You push the gas pedal to the floor and time the acceleration.

Run 2 - Now, imagine you slowed down to the same speed as before, but now you shift down a gear or two, so the engine is turning 3,000 rpm. Same torque as before, but twice the rpm. You push the gas pedal to the floor again, and time the acceleration.

Anyone who has a driver's license should intuitively know that the car will accelerate much better in the second run. But the torque was the same in both cases.

The car accelerates better in Run 2 because performance is determined by power, not torque. Or, if you like, performance is determined by torque times rpm, not torque alone.

[mdredmond]

Originally Posted by Kevin Horton

Wrong. If you have the same amount of torque at a higher rpm, the additional power at the higher rpm will result in higher performance (this assumes that the prop efficiency does not have a significant decrease at the higher rpm).

I am ignoring prop efficiency because I'm assuming the same prop for both installations.

We're talking about engine RPM, not propeller RPM. Horsepower for these engines is measured at the crankshaft (i.e., at engine rpm), not at the propeller.