The Secret Horsepower Race by Calum Douglas (and piston engine discussion)

F119Doctor

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I keep wondering why the early gas turbines failed to accelerate as quickly as late model piston engines and, why the hybrids failed to bridge the gap. I understand materials were a problem until quite late on but I reckon a lot of it came down to political decisions and strategic investment.
To accelerate a gas turbine engine, you have to add fuel to the combustor to generate the additional energy to the turbine necessary to turn the compressor faster to increase airflow.

When that additional fuel is first introduced at the low power setting, it increases the combustor temperature, which challenges the turbine heat resistance, and the combustor pressure, which challenges the compressor stall margin at the low airflow condition. Early engines had low stall margin and low turbine temp capability, so they were limited on how fast the fuel flow could be increased for acceleration.
 

steelpillow

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I keep wondering why the early gas turbines failed to accelerate as quickly as late model piston engines and, why the hybrids failed to bridge the gap. I understand materials were a problem until quite late on but I reckon a lot of it came down to political decisions and strategic investment.
Gas turbines are at their best at high speeds and altitudes. When stationary on the ground, they are less effective. On the other hand a piston engine is far more at home at zero feet, while a variable-pitch prop is equally at home with zero airspeed. Result; the jet takes time to build up full thrust.
Jeremy Clarkson once raced a Eurofighter from a standing start (can't recall what supercar he was driving, a McLaren or Bugatti Veyron or something like that). He shot ahead at the gun, but the jet overtook him comfortably before it reached rotation speed.
 

Calum Douglas

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They were getting there its just that the development curve was quite behind the USA, as we had pretty much ignored turbochargers in the 30`s. Fedden was very keen on turbos, going back a long time, but - as Fedden also had a talent for annoying almost everyone he was dealing with, I suspect this may have retarded their development in Britain to some extent.

Also, I think we probaby didnt really have the capacity to really develop more than one engine with full national effort at a time, so its also probably a case of us having all our eggs in the "Merlin" basket.

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I keep wondering why the early gas turbines failed to accelerate as quickly as late model piston engines and, why the hybrids failed to bridge the gap. I understand materials were a problem until quite late on but I reckon a lot of it came down to political decisions and strategic investment.
Gas turbines are at their best at high speeds and altitudes. When stationary on the ground, they are less effective. On the other hand a piston engine is far more at home at zero feet, while a variable-pitch prop is equally at home with zero airspeed. Result; the jet takes time to build up full thrust.
Jeremy Clarkson once raced a Eurofighter from a standing start (can't recall what supercar he was driving, a McLaren or Bugatti Veyron or something like that). He shot ahead at the gun, but the jet overtook him comfortably before it reached rotation speed.
With a car, you lose massively in transmission and drivetrain. While those so called tests are interesting to some I have to admit to boredom.
 

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I keep wondering why the early gas turbines failed to accelerate as quickly as late model piston engines and, why the hybrids failed to bridge the gap. I understand materials were a problem until quite late on but I reckon a lot of it came down to political decisions and strategic investment.
Gas turbines are at their best at high speeds and altitudes. When stationary on the ground, they are less effective. On the other hand a piston engine is far more at home at zero feet, while a variable-pitch prop is equally at home with zero airspeed. Result; the jet takes time to build up full thrust.
Jeremy Clarkson once raced a Eurofighter from a standing start (can't recall what supercar he was driving, a McLaren or Bugatti Veyron or something like that). He shot ahead at the gun, but the jet overtook him comfortably before it reached rotation speed.

That shows the mismatch between the high-speed exhaust from the jet and the surrounding air, making power transfer inefficient. That's what makes jet engines sluggish from a standstill. High-bypass turbofans and turboprops are better in this regard.
 

steelpillow

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That shows the mismatch between the high-speed exhaust from the jet and the surrounding air, making power transfer inefficient. That's what makes jet engines sluggish from a standstill. High-bypass turbofans and turboprops are better in this regard.
Most authorities regard bypass turbofans as a form of jet engine, and for that matter ramjets too. What you are thinking of is more correctly described as a turbojet. And of course all those early jets were turbojets.
But one must be careful over the velocity mismatch; a rocket generally creates the highest thrust of all, yet has the greatest velocity mismatch - in fact, the bigger the mismatch the higher its thrust. So the issue is about more than just thermodynamic efficiency.
 

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Turbomachineries have high rotational energy and low torque, this will make tham less responsive (even if variable guide vans were used). The Chrysler turbine car suffered from slow response despite it was really optimized for reacting quickly. In aircraft application, the response time is not as important as in road vehicles, so this drawback was simply accepted.
 

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What low torque? E.g. the gas-turbine of the Abrams tank has the highest maximum torque and torque rise among its power class's tank engines.
 

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In a car vs. fighter jet drag race, the reason why spool up time comes into play at all is the lack of a "clutch", so to speak. The brakes cannot hold the jet on the line at full thrust if T/W approaches (let alone exceeds) 1.0, so it can only throttle up to full reheat after brake release. Even in static condition a EJ200 engine develops 90kN, so a lightly loaded Typhoon (empty weight 11t, say 2t of fuel) would otherwise accelerate at almost 1.5g straight off the line, comfortably beating any car short of a top fuel dragster.

This is the reason why in time-to-climb record attempts the aircraft is tied down until the engines achieve full thrust.
 

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Let’s take a look on the PT6 112:

https://www.easa.europa.eu/downloads/16560/en

The PT6 112 has a shaft horsepower of 373 kw at a shaft speed of
31914 rpm (power turbine); this can easily used to calculate the torque:

M = P/(2*Pi*n) = 111.6 Nm

This is about the torque of a natural aspirated engine from a small car (1,2 L displacement). Of course, shaft torque is nothing you can feel and it shouldn’t be overestimated, but this example makes clear, that turbines have high rotational energy (due to the high rpm) and little torque to spool them up, so they are needing a lot off time for changes in power output
 

Pasoleati

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Let’s take a look on the PT6 112:

https://www.easa.europa.eu/downloads/16560/en

The PT6 112 has a shaft horsepower of 373 kw at a shaft speed of
31914 rpm (power turbine); this can easily used to calculate the torque:

M = P/(2*Pi*n) = 111.6 Nm

This is about the torque of a natural aspirated engine from a small car (1,2 L displacement). Of course, shaft torque is nothing you can feel and it shouldn’t be overestimated, but this example makes clear, that turbines have high rotational energy (due to the high rpm) and little torque to spool them up, so they are needing a lot off time for changes in power output
Take a look yourself of real torque curves.
 

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Nicknick

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these are torque curves behind the gearbox, they give no indication of the amount of torque on the shaft. All it takes is just a simple calculation (P=M*n*2Pi)
 

Pasoleati

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these are torque curves behind the gearbox, they give no indication of the amount of torque on the shaft. All it takes is just a simple calculation (P=M*n*2Pi)
The torque available on the turbine shaft is irrelevant as a reduction gear is always fitted in turboprop/turboshaft installations. And for vehicle installations you miss the key point, torque rise (maximum torque available at the turbine shaft is irrelevant as one can manipulate it by reduction gear ratio). Torque rise is the key figure because it affects the number of speeds in the gearbox required for a given speed range. The higher the torque rise, the less gear shifting is required to accelerate/drive.
 

Nicknick

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You shouln't forgett, we are talking about the amount of torque which can spool up the turbine. For simple reasons, there is no free torque given in any chart for the free spinng gas generator shaft, so we have to use the torque of the the drive turbine to get the right magnitude. The surpluss torque on the gas generator shaft will be even lower than that of the drive shaft. How much torque we have in the final drive is abslolutly irrelevant, the load change depends soley on the spool up time of the gas generator shaft.
 

Pasoleati

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You are making absolutely no sense. By your logic the lowest throttle response (in piston engines) would be in high-revolution engines with light crankshafts while the fastest response would be in large marine diesels because their crankshafts have the highest torque.
 

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The engine load in natural aspirated Diesel engines (and lean burn Otto engines) can be increased from one cycle to the next, just the speed increase takes some time (not much). In turbocharged engines, the load changes depend on the turbocharger which needs some time to spool up.
 
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