I recently heard that the recent Abrams' GT is as fuel efficient as a Diesel engine. Is this correct, and if so, what would ye see as advantages/disadvantages?

Depends on the particular diesel engine you're comparing it to. The main advantage of the GT is it's multi-fuel; it can be used with basically all grades of diesel, including jet fuel. With tuning it can run on alcohol, but we don't crank out the volume to do that yet.

graylion said:

I recently heard that the recent Abrams' GT is as fuel efficient as a Diesel engine. Is this correct

Short answer: no

In theory a turbine can be more efficient compared to a Diesel engine but this is only the case if the turbine is continously used at its optimum. In a static fashion. As is the case in a power plant. In this case the second main advantage would be that other fuels than diesel can be used. Especially gas.
For liquid fuels the most efficient internal combustion engine in reality is a high displacement, super long stroke, two-stroke Diesel. The ones used in modern large ships are close to the thermodynamic optimum.

The advantage of a turbine are its comparable small size and and low weight for its power output. Which is why its common in aviation applications.
Disadvantages are high fuel consumption per power output especially if not used at the optimum, high exaust temperature, large moving air volume.
Turbines are particulary bad when idleing. Which unfortunately is very common in AFVs.

The multi fuel capability of a turbine is not better compared to a military diesel. A diesel can use jet fuel or kerosine as well. If so its usually dilluted. Even dillution with water is possible to a degree. Diesels commonly use alcohol mixed into the diesel base as standart nowadays.
The funny thing is the use of jet fuel for tanks is not an advantage of turbines but a requirement. To ge the best performance it has to use jet fuel. With common diesel it looses performance. This is why the US now delivers jet fuel to its turbine equiped tanks. They did not equip the tanks with turbines to be able to use jet fuel. They now pretend the bug is a feature.

A modern diesel engine for AFVs has a specific consumption in the 220 g/kWh range when running high output. A turbine is in the 290 g/kWh minimum specific consumption range.

There are good reasons why nobody really conciders turbines as power plant for AFVs anymore.

The main issue with the turbine is you can't really idle it efficiently, Turbine has to maintain the airflow and pressure so it idles at more than 60% of the max speed, and when idling uses up to 4x the fuel of a diesel. 

As for Idle efficiency is probably 80+% of tank engine life, the turbine could only be ''efficient'' as a power generator for an electric drive where it could outperform a diesel generator.

Another thing is diesel had 50+ years of development since last turbine that powered a tank was developed,while turbines for ground vehicle use had practically no development which is an important factor , other similarly sized turbines (aircraft/helicopters) do not use heat exchangers/recuperators like you would want in an MBT . And heat recuperator is where the major gains are to be had . An additional bolt-on add on recuperator on M1 can for make 80kw without any engine mods. Energy that is for now lost .

Mr. T (MrT4) said:

As for Idle efficiency is probably 80+% of tank engine life, the turbine could only be ''efficient'' as a power generator for an electric drive where it could outperform a diesel generator.

For this to be possible you would need a full scale hybrid with significant battery capacity to buffer out fluctuations in power demand. Which brings an entiry new set and level of problems. If the entire setup is more efficient compared to a diesel direct drive in an AFV is questionable.

BTW diesel generators are extremly efficient. Most of the consumption of Diesel or Otto engines is because they are not used at their optimum. While during idleing piston engines are better compared to turbines they still are inefficient. If you put the same hybrid system to a Diesel or Otto engine it is absolutely possible that they would perform similar to a turbine. At the very least they would offer the ability to go full battery during idle and low requirement cycles because they can be started up fast if required.

Yes and no , the battery pack in such drives can be relatively small (probably not much over 100KWh for MBT , and maybe 50KWh for a IFV )but most importantly you could have a relatively small generator turbine, 700-900hp turbine, and 1500hp electric drive with unrivaled torque, same would be the case with diesel in the electric drive you would have much smaller diesel than is now and likely no transmission at all or something with 2 gears. The same battery would also negate the need for APU and vehicle would have lots of electrical capacity for systems and possibly even laser weapons. I imagine an armored battery pack would need to be external to the main armor possibly jettisonable. 

Mr. T (MrT4) said:

Yes and no , the battery pack in such drives can be relatively small (probably not much over 100KWh for MBT , and maybe 50KWh for a IFV )but most importantly you could have a relatively small generator turbine, 700-900hp turbine, and 1500hp electric drive with unrivaled torque,

No army would currenlty accept that. It also violates one of the most fundamental priciples: Murphies Law.
What if you need full power and the batteries are empty or out of action for some reason?

The power generator needs to be able to drive the vehicle at full capacity on its own. So neither the turbine nor the piston engine would get any smaller. Using a direct drive (mechanical connection) for high speeds and acceleration also is very desirable in addition to being required for redundancies sake. So the electrical part can only be in addition. In this case a small buffer battery would suffice.

Going more electric with the engine only functioning as a charger means the layout needs to be very different. If the electric motors would be the only thing connected to the final drive, the only option to power the vehicle a 100 kWh battery is not nearly enough. The Tesla semi truck has ~850 kWh capacity for ~1000 hp. This is more likely the size we are talking about. Depending on how quickly you want to be able to chage the battery you know how to size the engine. From a engineering point of view a small engine charging allmost constantly would be preferable for efficiency. But the way AFVs are used makes this not really a good option. Ideally the user would want an engine that can charge lots very fast for short run times. Which kills efficiency.

I am not convinced that with the current storage technology a heavy weight hybrid AFV is a viable option.
 

Mr. T (MrT4) said:

and possibly even laser weapons.

If these would be viable because of the physics involved in the fist place

Mr. T (MrT4) said:

I imagine an armored battery pack would need to be external to the main armor possibly jettisonable.

That again is not such a good idea. The battery pack is so integral to the function of a hybrid drive train that a significant part needs to be well protected to preven easy mobility and mission kills.

Ultimately wast majority of MBTs and IFV's have highly explosive ordinance in crew space, the battery would be the same type of hazard

SEP hybrid drive had no  battery just capacitors, but the battery adds efficiency, electricity storage,silent movement capability, increased power output capability etc so will likely be considered

SEP E-drive was mainly able to offer any engine placement irrespective of the drive motor location , SEP dual motor in hull sides above the tracks or wheels opened up internal space

Mr. T (MrT4) said:

Ultimately wast majority of MBTs and IFV's have highly explosive ordinance in crew space, the battery would be the same type of hazard

Yes exactly. This is a huge drawback for the current technology. The batteries are more dangerous than fuel.

Mr. T (MrT4) said:

SEP hybrid drive had no battery just capacitors, but the battery adds efficiency, electricity storage,silent movement capability, increased power output capability etc so will likely be considered

I am aware of the SEP. The important thing is that it failed. Or in nicer words it proved why the hybrid technology currently does not offer any advantages for military applications that would be worth the increase in cost and complexity. Also very important: SEP is a lightweight vehicle. Its not nearly in the weight class of a modern IFV or MBT.

The setup also has been rather wastefull with internal volume. Since it was optimised to get a single large payload volume in the center. This can be done with light protected vehicles its not applicable to anything with substancial armor though.

The problem is the modern combustion engines and geatboxes are so efficient and compact that its hard to come close with anything else. In addition they are super reliable and rather uncomplex and cheap. As soon as you start messing with this setup drawbacks will appear and compromises have to be made. Since the main driving force behind electric powered vehicles does not apply to militaries there is really zero incentive for them to move in this direction since it simply makes no sense to do so.

Well SEP came at the time when no major Euro project really got any traction and Sweden was hardly the mayor player.

And 15+ years of development of electric and hybrid vehicle drives makes SEP technology look like Dinosaur

Powerpacks are growing together with the weight of the wehicles, IFV's used to have 500-600hp now some are pushing 1000hp

I can only imagine how much space could be gained with in wheel electric hub motors in wheeled vehicles

quite typical powerpack space allocation.