Why no multi engine diesels?

[AdamFrisch]

As I understand it, most serious flying over populated areas require 2-engine safety, either by regulation or by safety. Turbines were the perfect choice for this, but as we all know, they're very expensive to operate.

Now with the introduction of the new certified diesel engines like the Thielert's, wouldn't these be perfect for helicopters? The benefits would be that you get to fill up with Jet A1 which the industry has the infrastructure for, you also get a long TBO and most importantly, lower fuel consumption and a much lower servicing/overhaul/investment costs.

Imagine a R22-sized chopper with two Thielert 140hp - perfect for law enforcement or inspection where you need to be able to fly over dense population and have the safety of 2-engines, but don't really need the space of of carrying around 2 extra seats you never use. Or a larger corporate-type/oil rig helicopter using Thielert's new 350hp diesels?

The benefits would be obvious. After all, with all the global warming and environmental stuff going on, it doesn't make sense to waste such huge amounts of fuel right into thin air as a turbine does - it's not a very effective way to turn gas into shaft horse powers. And who wouldn't want to cut their servicing and fuel bill by lots (I'm guessing 1/5 - 1/6 of the cost of a turbine or something?).

What's wrong with this idea?

[GoodGrief]

too heavy.

[tangovictor]

the extra weight of 2 engines, + all the kit ( night sun / video etc ) + always having an observer / radio operator, even a R44 with 2 diesels wouldn't work

[Graviman]

Adam, single engine piston radials were often chosen on 40s vintage propeller fixed wing since they were very damage tolerant. The best way for a piston engine to go is to have seperate injector systems and controls - ideally one for each piston. As long as the big end is nice and robust, reliability is then not an issue. Failures often seem to be drivebelts, so better to design these out in favour of direct drive to gearbox (at some cost).

Turbine modes of failure tend to be catastrophic, so a small fault rapidly developes into a total engine failure. Perhaps the best way is to think of the crankshaft as part of the gearbox system, with multiple pistons....

Mart

[MrEdd]

There is one heli that is being developed in OZ that has a turbocharged diesel engine.
Think the heli right now is undergoing FAR part 27.
Read a bit about the engine they are going to use, sounded interesting.

[Kulwin Park]

Mr Edd, do you now what type of diesel engine is going into the chopper? DeltaHawk, Thielert, SMA, other ??

And what sort of helicopter?? and where??

Cheers, KP

[Shawn Coyle]

The main issue for putting two piston engines in any helicopter has got to be matching the power output and speed of the two engines.
Modern computers coupled to the fuel control will probably have to be used, and that introduces a whole new set of problems.
Just think of the issues - the engines have to be running at the same RPM, and then produce equal amounts of power. Pretty complicated, and certainly not the sort of thing I'd like to do manually.
(So we're lifting off to the hover, and I've got to manually match the engines - what parameters are we going to use for the matching? RPM first - so I have to fiddle with two throttles or maybe a 4 way beep switch, then torque (perhaps), all the time I'm trying to hover....)
Are we going to have contingency power ratings on the engines in the event that one engine fails?

Loads of issues that will probably stop the project from ever getting off the ground.

[Ptkay]

Mr Edd, I think, you mean this...

http://www.tgrhelicorp.com/home.htm

"TGR Helicorp Ltd is the only commercial helicopter manufacturer in the Southern Hemisphere"
"powered by a brilliant aircraft diesel engine, specifically designed by DeltaHawk Engines of Wisconsin USA."

[Ptkay]

The main issue for putting two piston engines in any helicopter has got to be matching the power output and speed of the two engines.
Modern computers coupled to the fuel control will probably have to be used, and that introduces a whole new set of problems.

Was solved long time ago in Ka-26 (with no computers).
:)

[Shawn Coyle]

ptkay:
Details of how this was solved in the KA-26, please.

[IFMU]

Shawn,

From before my time the S-56, and the S-60 were both dual engine recips. And, I think I remember reading somewhere that the Cypher-2 UAV had dual recips as well.

But, if you think it through, what's the big deal? Pretend some mechanic mis-rigs the throttles in your dual engine recip such that one of them has 25% of the displacement compared to the other. Now head off into the wild blue yonder. As you pull pitch, the engine with its throttle more advanced will do all the heavy lifting. You have significant collective pitch, and significant throttle on both engines even though they are mis-matched. What happens to the engine with the retarded throttle? It doesn't just stay at idle. It will rev up until it hits the brick wall of the rotor system, then it won't accelerate any more. Everything will stay at the same RPM, both engines and the rotor system. Equivalent rotor RPM, that is. So none of your 3 needles will split. But, the engine with the retarded throttle is adding a lot less torque to keep the rotor turning, the other engine is adding a lot of torque. Not good for the hard working engine, or the input shaft/gears/bearings it is hooked to, but it will work.

A good analogy is a tandem bike. Similar overrunning clutches and everything. The guy in the front works hard, pedals up the hill. The guy in the back spins his pedals just as fast, but doesn't put any effort into it. Works fine until the guy in the front has a coronary.

How do you balance out the torque between the engines? I don't know how they do it, if the pilot has 2 throttles or if they do it in rigging. But either a torque measurement or manifold pressure should be close enough of a measurement to keep everybody happy.

-- IFMU

[MrEdd]

Kulwin park the link Ptkay posted is spot on the one i ment.
So the answere would be DeltaHawk.

I coulden´t find the link so thanks Ptkay for posting it.

They have a bit more to do before well see the little buggar in action.
But it´s going to be interesting to hear how that diesel will performe.

Soon it´s Pattys day lads, not having a beer then is a crime:ok:

[Graviman]

How do you balance out the torque between the engines? I don't know how they do it, if the pilot has 2 throttles or if they do it in rigging. But either a torque measurement or manifold pressure should be close enough of a measurement to keep everybody happy.


IFMU, diesels are not as bad as avgas because there is no throttle. Like a turbine the power is a direct function of fuel flow (assuming it burns well), so torque is unlikely to go far out of alignment. I could see Shawn's point if there was a differential between the engines, or each was connected by a torque converter (fluid equivalent of free power turbine). Easiest is just to mechanically connect diesels to same transmission input and calibrate the injector pumps. Direct measurement of torque is difficult because you need a strain guage in the system (often on a shaft). With a differential input, rpm control would work well since torques would be balanced.

What i do find amazing is that the TGRHelicorp machines are full fly-by-wire! They may only be CAD renderings, and no mention of price is made, but it does make the dream of FBW in a light heli closer. I wonder what sort of additional safety features they are thinking about putting into cyclic & collective control. The system may be a simple gyro feedback rate control system, or there may actually be control software. One to watch for sure.

Final note on Deltahawk (http://www.deltahawkengines.com/) is that quoted BSFC is 0.38 lb/HP-hour, which to my metric mind is 190 g/kW-hour. Using the figure i ealier worked out of BSFC 83.7 g/kW-hour for 100% efficiency (most aviation fuels), this makes the engine 44% efficient when running at optimum rpm.

Mart

[RVDT]

Graviman,

I agree with your comments to Shawn, I would guess in the current multi-engine turbine fleet most just use RPM. How diifficult would it be to do? OK some FADEC controlled engines can be Q or T matched but only because they can. Torquemeters these days are pretty simple as well if you wanted to go that way with digital electronics, phonic pickup measuring phase shift

I'm currently based on a ship that has 8 engines the same, diesel electric, doesn't seem to be an issue.

As for FADEC, when I did the PW 206/207 course the instructor mentioned how the PT6 FADEC in the S-76B became rapidly obsolete because it was basically over engineered and in a short period of time you could not purchase the components in the FADEC any more! His comment was that the FADEC in the 206/207 engine was probably very similar to what was in a Fuel Injected Korean car. The box it was in and the screws were made in the US by Hamilton Standard and that was about all.

There was a similar issue with Concorde near the end of it's life. They were having difficulty supporting the analogue and steam powered electronic systems in it.

On another issue with diesels that are out there in FW the penalties aren't as great as you would first assume. The lower SFC trades into not having to carry so much fuel. With the current cab 1 hours fuel = 2.5 average sized people!

Also I think you would need to address the torsional issues well on a diesel.

[Graviman]

RVDT, i thought about the phase shift torque measurement technique - i've never seen it in use, so can't comment about it's effectiveness over the design life of an aircraft powertrain. Electric drive is also an area of interest to me, the larger contruction equipment uses it, and i've whiled away many an hour discussing heli applications with Dave Jackson. There are several promising contenders for direct fuel to electricity conversion, so i wouldn't rule out the electric helicopter. My best guess is i'll be making my move into the heli industry at about the same time. :)

Diesel torsional resonance is not such an issue now as it might once have been. Finite element analysis techniques allow accurate predictions of powertrain vibration modes, allowing design out on the "drawing board". One of the more obvious things highlighted is the need to put the camshaft drive near the flywheel, for accurate timing. Various solutions can be examined to keep crankshaft modes away from drivetrain, and actually this is why the old LandRover TD5 was a 5 cyl design - the 2N (2x crank speed) inertial of 5 pistons acting on crank balances out the diesel knocks (i've got an interesting internal paper somewhere). There are also emerging laser ignition/combustion techniques which i believe will cure the diesel engine of any disadvantage regarding RPM and vibration.

I am convinced that high boost turbodiesels will be the future of helis in the near future. Basically the turbine handles the airflow for power, while the piston handles the pressures for efficiency. Whether i can get back into the development arena is another question. :ouch:

Mart

[rudestuff]

Interesting thread - not so long ago I was kicking about a hangar looking at a 300 with no engine - and the mechanic working on his motorbike on the side. Obviously the conversation drifted toward the fact that there is enough room under a 300 to easily fit 2 large motorcycle engines side by side, both driving the MRGB through a triangular pulley system (with curved edges, ovbiously)
A 1000cc motorcycle engine develops 150bhp or so and can be fitted with turbochargers. (Put a Lancer turbo on a Hayabusa and you get 350bhp)
Obviously you would have two stressed out engines, but they could be replaced every 500 hours or so and still be cheaper to overhaul, plus you'd have a spare engine if and when one does go 'pop'.
Any thoughts? or are there not enough of those 'Tork' things?

[AdamFrisch]

Fadec would make 2 engines harmonize easily.

But even without modern things like that, he venerable Sikorsky CH-37 Mojave proved that a 2-engine piston helicopter is fully workable. The Mojave had 2x Pratt and Whitney Wasp engines hanging in pods outside the main fuselage - driving the main blades through a shaft entering the fuselage and into the gearbox from each side.

[simonwalther]

Graviman, you wrote that .38 lbs/hph equals 190g/kWh. Now if I do the maths, I end up with a different figure:
1 lb = 454 g
1 kW = 1.36 hp
so:
0.38 lbs/hph = (0.38*454*1.36) g/kWh = 235 g/kWh

Or did I make a mistake?

However, 235 g/kWh would still be very good, especially compared to our pre-war avgas guzzlers... Very good truck engines can get as efficient as around 190g/kWh, but they are "somewhat" heavier than what can be accepted for aircraft...

Cheers,
Simon

[Graviman]

My mistake Simon, you are correct. I was using the program "convert" and hit "pounds (troy)" not "pounds (lb)" - this is why i much prefer metric!
So 0.38 lb/hp * 454 g/lb / 0.746 kW/HP = 231 g/kW (using BHP not PS).
Then using BSFC 83.7 g/kW-hour for 100% efficiency (most aviation fuels), makes the engine 36% efficient when running at optimum rpm.

I was suprised at my original figure since the most efficient small 4-stroke diesel is 45%, and 2-stroke is never as efficient (even with low bore/stroke ratio designs). I was designing a 2-stroke turbodiesel for fun, but dropped it when i realised that the slight power advantage of 2-stroke disappears when you look in detail. The scavenging process of even a uniflow design (like the Wilksh aeromotive) is not as good as 4-stroke, and at high turbo boost makes up for power. CAT have even developed hybrid generator turbos to take off excess blowdown power directly. Besides oil always gets into exhaust with 2-stroke.

I don't know your level of interest, but i believe that high boost turbodiesel has the potential to revolutionise the light helicopter market. The concept is that the turbine does all of the compression and expansion work, with a much smaller piston core replacing what would be the high pressure stage. Either the combustion pressure would be very high, or the engine would run very low compression ratios. This greatly reduces engine size and weight for a given power. There are also emerging technologies which overcome the combustion ignition rpm limitations of diesel engines. I have also considered alternative layouts and crank mechs to reduce the higher piston skirt wear which would occur.

Mart

[simonwalther]

Mart, 231 g/kWh makes it look even better than 235, doesn't it? That's a good figure when you compare it to car engines. No comparison with "Continencoming" :-D
I am neither a technician nor an engineer, just an interested layman, by the way :-)
You wrote that 2-stroke engines are always less efficient than 4 stroke engines. That's what I thought , too until lately. But by now I know that the most efficient piston engine of the world is a 2 stroke turbodiesel. Look here:
http://en.wikipedia.org/wiki/Wärtsilä-Sulzer_RTA96-C
163 g/kWh at best economy... just unfortunately this (ship diesel) baby would be too heavy to be built into even an A380 :-) It has valves, however. Maybe you can tell me how that works...?
But anyways, the problem with traditional 2 stroke engines is that air (and with gasoline engines: fuel) have to go through the crank case and than push out the burned gases of the cylinder - with more or less losses and a lubrication in which the oil is lost/burned. The deltahawk diesel, however, has an oil filled crank case, just like a 4 stroke engine. The fresh air is being pumped in by a supercharger (in the deltahawk case combined with a turbocharger). This overcomes the oil loss. Fuel loss isn't an issue, anyways, because a diesel injects its fuel only when the piston is up. I think this kind of engine does have some potential. Especially in aviation. Why?
1. It is simpler than a 4 stroke turbodiesel --> less error sources, cheaper to build and overhaul (important with our low numbers of engines sold in aviation, which makes us always more expensive than the road traffic folks!), possibly easier certification
2. It (should) have a better power/weight ratio than a 4 stroke. Thus you can possibly (as in the case of delta hawk) also save the reduction gearbox (with airplanes) and run the diesel engine at rpms that are more efficient for diesels, anyways
3. There are no exhaust emission standards in aviation (yet) which might be a bit tougher to fulfill with these engines
What do you think?
How about that Wilksch diesel? Do you know, how that works? I just browsed through their homepage, but I didn't really understand it. It has overhead valves like that ship diesel, doesn't it? :confused:

Anyways, 2 stroke or 4 stroke, I quite agree with you that diesel engines have good potential for aviation. They may be heavier than turbines, but then, much more efficient.
Simon

[Graviman]

Simon,

That Wartsila-Sulzer engine is a uniflow scavenge 2-stroke design. There are ports uncovered by the piston rings, but only for inlet. The single huge valve is for the exhaust, and is uncovered at the same time as the inlet ports. MAN B&W have recently done a lot of work on hydraulic actuation of this valve, for improved efficiency by making engine run closer to Atkins cycle (exhaust expanded more than starting volume of compression stage). The efficiency comes from the large volume:surface_area possible with such a huge piston, since it reduces heat rejection to the cooling system. The turbo can then capture more of the exhaust stream power.

For a while truck diesels (eg Detroit Diesel 71 and 92 series) worked on the 2-stroke uniflow scavenge principle, but ever tightening emission standards forced them to 4-stroke. The Wilksh Airmotive is also another example of this type of engine, while the Deltahawk is a Snurl type loop scavenge 2-stroke. For some additional mechanical complexity uniflow offers a less turbulent, hence more efficient, charge scavenge. There is actually an arguement for putting the inlet valve at the top with cylinder wall ports as exhaust, because the combustion fluid is naturally moving downwards at the end of the expansion process. Mercedes and AVL have also done a lot of development on automotive diesel 2-strokes, but found that they offered little benefit over a well designed 4-stroke. This is particularly true once you depend more on the turbine for compression/expansion.

To try to answer your questions about loop scavenge 2-stroke, Nick Lappos style: :ok:

1. It is simpler than a 4 stroke turbodiesel --> less error sources, cheaper to build and overhaul (important with our low numbers of engines sold in aviation, which makes us always more expensive than the road traffic folks!), possibly easier certification

Slightly, since you do not need expensive valve and cam mechanisms. Although the castings and cylinder liners require additional complexity and machining to get the ports right. Since casting development cost is ideally a one off for the entire production run, while machining costs are per engine, there is an arguement for simpler castings and more machining for low volume designs. There is also a good "parts bin" for valve train components.

2. It (should) have a better power/weight ratio than a 4 stroke. Thus you can possibly (as in the case of delta hawk) also save the reduction gearbox (with airplanes) and run the diesel engine at rpms that are more efficient for diesels, anyways

Since the scavenging process is far from ideal the power/swept volume advantage is maybe 1.5 over a 4-stroke. In practice with turbo charging this advantage is eroded, since you can find other ways to use additional blowdown pressures - overcoming flow losses or even driving ancilliaries, with a combined generator/turbine. This means that the pressure difference between inlet and exhaust possible in a 4-stroke is advantageous.

Any engine will offer better power/mass at higher RPM. Ships use low RPM diesels for better matching the propeller speed, in the same way that aircraft do. A gearbox will always weigh less than increasing engine swept volume for low RPM, at some cost increase. This is particularly true for helicopter applications where RPM reduction is a given. Belt drive is also a reduction possibility, albeit not as reliable,

3. There are no exhaust emission standards in aviation (yet) which might be a bit tougher to fulfill with these engines
What do you think?

That is true, and was also recently true for motorbike production. Once motorbikes became subject to emission standards, then all moved from 2-stroke to 4-stroke. For the costs associated with a new engine design, there is an arguement for going with 4-stroke from the outset. The Wilksch already has a valvetrain mechanism, so the cost is already there. The real problem is oil leakage into the exhaust, from the lubricated piston skirt to the ports, which is inherently reduced in a 4-stroke - although still subject to much development. In fact Detroit Diesel carried over a clever crosshead skirt piston design to seperate out the functions of load bearing and sealing in the 4-stroke S60 engine (no, that one is not a Sikorsky design).

Hope that helps!

Mart

[Graviman]

Interesting thread - not so long ago I was kicking about a hangar looking at a 300 with no engine - and the mechanic working on his motorbike on the side. Obviously the conversation drifted toward the fact that there is enough room under a 300 to easily fit 2 large motorcycle engines side by side, both driving the MRGB through a triangular pulley system (with curved edges, ovbiously)

A 1000cc motorcycle engine develops 150bhp or so and can be fitted with turbochargers. (Put a Lancer turbo on a Hayabusa and you get 350bhp)
Obviously you would have two stressed out engines, but they could be replaced every 500 hours or so and still be cheaper to overhaul, plus you'd have a spare engine if and when one does go 'pop'.

Any thoughts? or are there not enough of those 'Tork' things?


Rudestuff, i meant to answer this post but got distracted and forgot! :\

Ah those hanger conversations on a rainy day - distant, but fond memory...

The big problem here is reliability. Very true you could replace the engines every 500 hours, although the novelty might wear off after the 4th or 5th time. In general the increased wear is an indication that the engine is seeing more stress, so will be less reliable in difficult circumstances. If the engines were at 490 hours and you pulled collective in a VRS demo there might be an inconvenient clunk sound, with a resulting auto demo. This might not be in a demo. :oh:

In principle you could put in any engine, but i would rank heli engine requirements as:

1. Reliability
2. Power-to-weight
3. Efficiency

Not putting cost there, because the objective of any designer is to achieve the above at the minimum outlay and running costs... :8

Mart