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Subject: Odd question
buzzard    2/1/2011 5:00:06 PM
Ok, it is well known that the M1 is not well regarded when it comes to fuel consumption. Now of course it's damned heavy, and that certainly won't help, but it does use a turbine engine, and I've always been under the impression that such would help efficiency (not like they use piston engines for power generation on large scales). Is there some issue with turbine engines not scaling down well? Is there some other factor which makes these engines so inefficient?
 
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YelliChink       2/1/2011 5:12:48 PM

Ok, it is well known that the M1 is not well regarded when it comes to fuel consumption. Now of course it's damned heavy, and that certainly won't help, but it does use a turbine engine, and I've always been under the impression that such would help efficiency (not like they use piston engines for power generation on large scales).

Is there some issue with turbine engines not scaling down well? Is there some other factor which makes these engines so inefficient?


When idle, it eats too much fuel.
 
When moving at high rpm, turbine is fine.
 
Diesels are more practical, even though the shp/weight ratio isn't as good.
 
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buzzard       2/2/2011 1:03:11 PM
Can idling explain all the inefficiency of those engines? Certainly a turbine in a power plant is almost never idle, so that's a contrast. However if you are projecting the maximum range of a tank, would idling really figure into it?
 
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enomosiki       2/3/2011 12:46:06 AM

Can idling explain all the inefficiency of those engines? Certainly a turbine in a power plant is almost never idle, so that's a contrast. However if you are projecting the maximum range of a tank, would idling really figure into it?


Idling for extended period of time is what the APU is meant for. APU's that go into tanks are usually turbines, because of the size-to-power ratio advantage. Not enough to move the tank by any means (not that it's connected to the drive system) but more than enough to power vetronics and crank the turret. Even some other modern tanks that use diesel as their primary engines often come with turbine APUs.
 
And keep in mind that the Abrams can burn a wide variety of fuel, including gas, diesel and avgas. The Army just likes to use JP-8 for it because they use it for helicopters, simplifying logistics.
 
The downside is that turbines take time to reach peak output unlike diesels.
 
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Privateer       2/3/2011 2:19:02 PM
The Abrams has two idle speeds, a normal (870-950 rpm) and a (higher) hidle speed (1250-1350 rpm) (AKA Tactical Idle). Tactical Idle is supposed to be used on maneuvers or in combat (where you definitely don't want to experience any throttle lag). However, this setting certainly isn't good for fuel economy.
 
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JFKY    Fundamentally   2/3/2011 2:52:48 PM
what this involves is the fact that jet turbines operate most efficiently at a fixed and high RPM...they are NOT efficient at low RPM's nor in a variable RPM case.  In ships turbines are used to boost speed, and other engines are used to sustain speeds, and even in all turbine ships, there are "cruising turbines" operating at a fixed and economic set of RPM's.
 
Tanks don't have that luxury, the engine may be running at a low RPM's for long periods of time, and then only at an efficient output for a limited period of time...the result is that a turbine engine is less efficient than a diesel in engine in MOST regimes, but when you want acceleration, you want a turbine.
 
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