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Occasionally I come across the diesel-electric (DE) design for submarines and freight trains. They say something like "a diesel engine powering an electric drive lets the two operate at their most efficient RPMs, and the electric drive has very little losses in efficiency."

So the question hit me: Has there even been a DE helicopter? Is there something about the DE system that makes it bad for the aviation industry? Weight maybe? But note we do not need a bunch of batteries like on a submarine. We don't need to store the power, we just convert it to electricity which goes immediately into the electric drive for the rotors.

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    $\begingroup$ "Occasionally"? All diesel-powered submarines and almost all diesel railway locomotives are diesel-electric. $\endgroup$
    – David Richerby
    Commented Sep 3, 2015 at 7:22
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    $\begingroup$ @DavidRicherby Yes, these days they are. What I meant was, occasionally I read about them again. $\endgroup$
    – DrZ214
    Commented Sep 3, 2015 at 7:31
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    $\begingroup$ @Simon If you read the last two sentences in the OP, a DE helicopter does not need any batteries because we don't need to store the power. The power immediately goes into the rotors. $\endgroup$
    – DrZ214
    Commented Sep 25, 2015 at 3:38
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    $\begingroup$ @DarioP See my answer. Electric systems are just a very fancy gearbox if used without energy storage. $\endgroup$
    – Sanchises
    Commented Sep 25, 2015 at 20:28
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    $\begingroup$ @jCisco I'm referring to a traditional piston-driven diesel engine, which does not require spark plugs because the compression ratio is so high, it auto-ignites the fuel. This was the original meaning of the word "diesel", but today it could refer to only diesel fuel, so there is room for confusion. $\endgroup$
    – DrZ214
    Commented Sep 30, 2015 at 6:21

5 Answers 5

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Helicopters currently use lightweight but fairly inefficient gas turbines. Diesel-electric engines have great energy efficiency, in terms of mechanical energy delivered per kilo of fuel. However, they're quite heavy. This means that you need more lift, which in turn means more power is needed. This negates the initial advantage.

With the advent of lightweight materials, the latter effect decreases so this situation might change.

Diesel engines can throttle up and down quite a bit, unlike gas turbines, but this capacity is irrelevant to helicopters. Note that helicopters need a pretty constant amount of lift.

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  • $\begingroup$ "Note that helicopters need a pretty constant amount of lift. ", do you have a reference for this? It seems to me that helicopter need to vary their lift quite a lot (for example to achieve an upward acceleration?) $\endgroup$
    – ROIMaison
    Commented Sep 3, 2015 at 12:27
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    $\begingroup$ @ROIMaison: That's actually not a lot of variation. You typically don't climb at 2g in a helicopter, and you certainly try to avoid descent in freefall, so the trust you need varies by less than 100% from nominal. Cars often do accelerate at power levels several times what's needed for a street-legal speeds (which is why top speeds are so much higher). IOW, cars normally just battle air and rolling friction (not much) whereas helicopters normally battle gravity (much more). $\endgroup$
    – MSalters
    Commented Sep 3, 2015 at 13:00
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    $\begingroup$ Note that Airbus Helicopters is working on a diesel helicopter, but with a direct drive, not diesel-electric. $\endgroup$
    – Jan Hudec
    Commented Sep 25, 2015 at 8:23
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The main advantage of diesel-electric is that it offers a great deal of torque at low motor rpm [from Physics Forums]. In effect, the torque curve of an electric motor is highest at 0 rpm, and has a very flat decrease as rpms increase.

This is of great value for vehicles that have to pull a great deal of weight, and to overcome the inertia of moving it from a standing start. It is of little or negative value for aircraft because it is much less efficient. Anytime you generate power from one medium and then use that power to drive another medium, a lot of power is wasted. So converting diesel oil to mechanical energy, then further converting it to electrical energy, is far too wasteful and unnecessary for a helicopter, where every erg counts. With helicopters, which these days mainly use gas turbine engines, as much of the power as possible gained by burning fuel is converted directly into rotary motion. Note also that massive torque at low rpm is useless for rotor aircraft, since it is only at high rpm that such craft are able to fly.

Important, too, is the fact that dual-power systems like diesel-electric add a lot of weight, since you are dealing with two different types of engine. For any aircraft, especially the helicopter, that may even be a bigger drawback than wasted energy—though these things may be viewed as two sides of the same coin.

This just in: The Economist magazine now has published an article, Hybrid power will make helicopters safer and more productive, on the advent of helicopters that will use hybrid engines.

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    $\begingroup$ It's also worth noting that in a diesel-electric locomotive or submarine, weight is often a good thing, since in either case if the diesel-electric power train and drive train didn't weigh enough it would be necessary to add ballast [a locomotive that's too light won't be able to pull very much, and a sub that's too light relative to its volume won't be able to submerge. $\endgroup$
    – supercat
    Commented Sep 24, 2015 at 18:28
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    $\begingroup$ Where do you still use ergs as a unit of energy? $\endgroup$
    – user20574
    Commented Oct 11, 2015 at 22:39
  • $\begingroup$ @immibis: In the Land of Metaphor and Colorful Writing. $\endgroup$
    – Robusto
    Commented Apr 11, 2016 at 2:37
  • $\begingroup$ @Robusto electrical engine dynamics confound simplistic models as presented. Contrary to generalization indeed eg shuntfield DC motors run at a constant RPM display a torque curve that behaves to the increase of RPM in the following manner. As RPM moves from 0 toward ~90% of the ideal and therefore targeted RPM. The Torque available is 300% of that available when at 100% RPM. Over the last 10% increase in velocity the Torque decreases to the 100% level. That graph Torque % (x) is a line straight up then bending to the left in 10% of rotation increase =y. $\endgroup$
    – jCisco
    Commented Jul 21, 2016 at 5:30
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Diesel-electric is chosen as a solution to a set of problems that are not really RPM related. The primary reason in the case of trains the choice was driven by the inability to effectively use mechanical linkage to drive a number of wheels.

There is a limit to the amount of power and torque that can be transmitted via a transmission before it becomes impractical to impossible. Trains with 2000HP diesel engines require a transmission that is excessive in size and weight in order to transfer 100,000lb feet of torque to the wheels. Further complicating this is the need to split the output accross multiple axles. It is complicated, heavy, unreliable and inefficient, and this was apparent on very early diesel engines despite not matching the steam engines they were replacing.

The solution to gain better tractive force was to drive multiple electric motors with each motor powering a single axle (sometimes more than one). This necessitated replacement of the mechanical transmission with a generator/alternator and the necessary power handling equipment.

There are of course some overhead losses in the order of 25% due to the conversion process of mechanical to electrical plus the need to power ancillary systems such as heat exchangers, charge battery banks etc , However the increase in flexibility, reliability, performance and simplification make the overhead a cheap tradeoff compared to alternatives.

Train and helicopter specifications:

  • A typical locomotive prime mover is 4,400 HP and 21,000 ft lb of torque operating at 1,100 RPM. The Generator/alternator will provide 2.1MW in electrical energy. The entire locomotive weighs 400,000lb. The electric motor within a typical set of six will have torque at around 6,200 ft lbs.
    Size: (Prime Mover, Generation and Drive gear): Massive.

  • A light helicopter such as the R22 has 124 hp and 241 ft lbs of torque at 2700 rpm MTOW 1,400 lbs.
    Size: (Powerplant & transmission) A large suitcase.

  • A heavy helicopter CH-53E has three engines delivering 4,380shp @ 14,280rpm with ~1,610 ft lbs of torque driving through multi-step transmission delivering nearly 600,000ft lb of torque to the main rotors shaft. MTOW 73,500 lb.
    Size: (Powerplant & transmission) 10,000lbs in the volume of the mini cooper.

Bottomline

  1. A locomotive requires immediate high torque to overcome the weight of itself and resistance from the load. The time to begin movement is critical, the train must be able to begin motion on demand. Whereas helicopters have time to spool the engine(s) and bring the rotor to operational RPM. The required torque rises from zero RPM to operating RPM at which point the rotor requires continued baseline torque with substantial overhead.

    • extremely different operating environment and envelope

  2. Making a helicopter diesel electric will require installation of a very heavy generator, shaft brakes/resistor banks, heat exchanges, switchgear, flywheel plus several hundred kilograms of copper wiring and electric motors. All in addition to the primemover(s), none of which has a pay off in terms of capability. The additional components consume valuable internal space and add significant weight far in excess of the very capable existing mechanical links which in multi-engine helicopters cannot be eliminated.

    • Complex, heavy and eats internal volume with no advantages
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    $\begingroup$ Another key consideration for railway locomotives is that weight is a good thing, since it improves traction but doesn't take much extra energy to carry around on wheels. By contrast, weight in a helicopter is a bad thing--every extra pound of engine that doesn't improve performance will cost a pound of payload. $\endgroup$
    – supercat
    Commented Jul 13, 2017 at 19:37
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    $\begingroup$ Yes absolutely the mass of a locomotive helps increase the maximum force that that can be applied to a wheel before it slips which actually aids acceleration and braking. Which are somewhat counter intuitive and aren't an issue as far as capability of a locomotive since space and weight aren't a problem and nor are there any implications to forcing feeding air to the heat ex changers, inter coolers and the load banks to sink current from brakes. High energy is space and weight consuming, something helicopters are sensitive too. $\endgroup$
    – jCisco
    Commented Jul 16, 2017 at 12:45
  • $\begingroup$ Interesting, I didn't realise that a heavy helicopter was about as powerful as a locomotive! $\endgroup$
    – Vikki
    Commented May 25, 2018 at 2:59
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One thing I don't see addressed yet in other answers:

The reason for diesel-electric systems is that diesel engines (in fact, all ICE engines) are only efficient at certain RPM values. For road vehicles that accelerate and decelerate a lot, this means that most of the time, your directly linked engine is not working efficiently. To alleviate this problem, you can either have a fancy gearbox (for example, dual-clutch or CVT - or even a normal gearbox will do), or have an generator and an electric motor: this is a very handy 'gearbox' since electrons are so easy to manipulate. Some (prototype) hybrid cars also use this system.

Helicopters almost invariably operate at a fixed RPM set by the governor. This means that whatever engine is employed, you can just let it run at the best RPM using a simple fixed geared transmission. Actual variations in lift are done with blade pitch, not motor RPM. The whole point of having an electric system, (which is just a very fancy transmission like some sort of CVT), is then negated, which is why this system will never be used in both propeller fixed-wings and helicopters alike.

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  • $\begingroup$ The same thing can be said of diesel-electric trains. I'm sure the engine RPM of a cruising train is nearly constant too. The real question is, is a typical turboshaft engine most efficient at the RPM needed for a helicopter? If so, then yes the diesel-electric idea loses some incentive. $\endgroup$
    – DrZ214
    Commented Sep 25, 2015 at 20:41
  • $\begingroup$ I agree with DrZ214, this sort of system is already being studied and used in fixed-wing aircraft (which are not all fixed RPM). I don't think your point is completely wrong, I just don't agree with all of the assertions. $\endgroup$
    – fooot
    Commented Sep 25, 2015 at 21:03
  • $\begingroup$ @fooot, the point of hybrid is that there is a battery in the circuit. Much easier in fixed-wing, since they need lower thrust/weight ratio. $\endgroup$
    – Jan Hudec
    Commented Sep 29, 2015 at 18:05
  • $\begingroup$ @JanHudec yes, but the last sentence seems to lump all propeller/helicopter vehicles together saying they will never use it, which is where I disagree. $\endgroup$
    – fooot
    Commented Sep 29, 2015 at 18:15
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    $\begingroup$ @DrZ214. cruising train has nearly constant RPM. But trains have friction bearings (because they are too heavy for roller bearings) lubricated with axle-driven pumps. So when the train stops, oil pressure drops, the oil gets squeezed out and the static friction is much higher than rolling. So train engine needs high torque from standstill and that is what electric engines are great at. $\endgroup$
    – Jan Hudec
    Commented Sep 30, 2015 at 4:51
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Advantages of using electric drivetrain with diesel engines are:

  1. High torque from zero RPM. This is extremely important for trains, because trains have much higher rolling friction in the first few feet before the wheel-driven oil pumps manage to sufficiently lubricate the bearings. Diesel trains that use mechanical drive train even often have additional short-term power sources like steam turbines to help with that.

    Note that this reason does translate even to cars, which use diesel engines, but with mechanical drive trains. And for helicopters, they don't even need a gear box, because they run the rotor at constant RPM anyway.

  2. Batteries can be easily included. This is important for submarines, because they can't run diesel engine below periscope depth, because it needs a lot of oxygen (at periscope depth they can use snorkel). But this is specific to submarines.

    Other vehicles may have use for that as well, for example hybrid cars use it to get higher peak power with smaller engine that can run more efficiently. But helicopters need fairly constant power, so they don't have much use for that either.

So helicopters with diesel engine are possible, and at least Airbus Helicopters are working on it, but the electric part simply has no use. They did not exist so far, because diesel engines were too heavy, but they are becoming light enough that they should be usable in future.

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  • $\begingroup$ When you say "they are becoming light enough", are you talking about diesel engines like those in a truck, like a piston engine? Or are you talking about a turboshaft engine running on diesel fuel? $\endgroup$
    – DrZ214
    Commented Sep 25, 2015 at 20:43
  • $\begingroup$ @DrZ214, “Diesel engine” means compression-ignition reciprocating engine and that is what I am talking about. Turbines running on diesel fuel would not be noteworthy, because they all do—jet fuel is basically the same thing as automotive diesel with stricter limits for purity (and aviation diesel engines usually run on Jet-A/Jet-A1). $\endgroup$
    – Jan Hudec
    Commented Sep 26, 2015 at 16:35
  • $\begingroup$ @JanHudec: Jet fuel is kerosene, not diesel. $\endgroup$
    – Vikki
    Commented May 25, 2018 at 3:00
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    $\begingroup$ @Sean, standard automotive diesel fuel is slightly heavier fraction than Jet-A1, but the ranges overlap and neither engine cares too much anyway. $\endgroup$
    – Jan Hudec
    Commented May 26, 2018 at 20:46

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