[John Robertson Posts: 5121 Status: Offline WWW]

I hadn't realised that either.  However they are 'simulated' Atkinson engines - not real ones.  The efficiency obviously suffers because of that (sucking charge into the cylinder then blowing half of it back out again can't be good!).  However, the advantage is the instant switching from simulated Atkinson to Otto cycle (or variations in between) - makes you wonder why all engines aren't like that (nearly all have VVT so it should be pretty easy to do). 

The other thought that crossed my mind is why not just reduce the valve open time and create a larger vacuum on the intake stroke (lower charge) - the energy used to create the vacuum would be recovered as it would help 'pull' the piston back up on the first bit of he compression stroke (all engines have a partial vacuum in the cylinder on intake anyway due to the volumetric efficiency being less than 100% (70-75% for a petrol engine).  And, of course that's exactly what happens normally when the throttle is closed - the compression stroke doesn't do any actual compression until the charge has reached atmospheric pressure again - at idle that must be almost at the top of the stroke so you could argue that an Otto cycle engine is already operating as Atkinson in that it's got a shorter effective "compression" stroke than the power stroke?

[Warby Posts: 1041 Status: Offline]

Toyota have recently produced a petrol engine 1.3 ltr I think with a thermal efficiency of just over 37%
YES 37% It uses the ATKINSON cycle (Invented in 1882!!) as opposed to the OTTO cycle.
What I hadn't realised until fairly recently was that Atkinson Cycle engines are used in many Hybrid Cars.
The electric motor helps with the short-comings of the Atkinson cycle ie when accelerating.

[Ian Brooks Posts: 3550 Status: Offline]

It’s worse than all that! What haven’t told you is that these electric systems run in excess of 600V!!! They have to otherwise the required current is enormous and the weight of copper for the windings is immense. The battery systems are potentially very dangerous presently, and have to have a battery management system to keep it all safe. It has to monitor individual groups of cells and be capable of isolating them if they get hot. A fire must be contained safely and must not propagate.

Interestingly a car only has to contain a battery fire for a short period- it is assumed that the battery can be isolated and occupants can escape quickly, whilst a plane must control fire AND continue to deliver current long enough to land! A hovercraft will have similar requirements of course. 

True long distance electric cruising is (I  would say) one generation of batteries away. It looks like Lithium Sulphur batteries have the potential to get there, but as yet no-one has successfully created a production cell that gives the energy density, discharge current AND lifetime needed to be practical. Several companies have demonstrated two of the three, but all are needed for a practical hovercraft, and no-one looks close yet.  Current Li ion batteries will just about be ok for the short of short hop that the Vertical aircraft is aimed at, possibly an hours cruise., but of course we want 3-4 hours cruising at least.

Even when these problems are solved, the cost will be way prohibitive for amateurs for a long time yet. So all in all we’re keeping ICEs for a little while yet I think.

However, if said millionaire would like to play then I’m happy to help!

The one thing that might fall out of these electric aircraft programs for us are better props. There’s research going on into quiet props for the first time in decades and that might spill over to us, which would be welcome.

Ian

[John Robertson Posts: 5121 Status: Offline WWW]

Probably correct though!

The thing that gets me is that the efficiency (and thus emmisions) of an internal combustion engine is unbelievably poor.  Less than 20% of the energy in the fuel is converted into movement - the remaining 80%+ is just thrown away as heat.  Compare that with, for example, a domestic gas boiler (also a fuel burning device) which is, nowadays, is in the high 90 percent efficiency (used to be 70-75 before heat recovery condensing designs).  It strikes me that it shouldn't be difficult at all to recover some of the waste heat from an engine, convert it to electricity and use it to charge batteries or for engine assist.  Peltier devices have been around for many decades and do just that (convert heat to electricity cheaply).  A typical car cruising on a motorway is probably only using around 20hp on average - that means it's dumping over 60kW of heat into the atmosphere!  Even if you could only recover/convert around 8kW of that heat (easiest from the exhaust which is where round 30% of it goes) you could half the emissions from the engine and also half the fuel consumption.  Loads of hybrid cars have online electric assist so all of the "tech" is already there from what I can see.  Instead government seem to be focussing on the most efficient (and least polluting) domestic boiler! <rant over>

[Ronnie L Posts: 507 Status: Offline]

I think electric and or hybrid cruising (Longish distance) hovercraft are things of fantasy.
If you have ever looked under the bonnet of a hybrid car, which as I see it is the nearest thing.
Trying to marinise that and then carry it would be impossible.
I think it will come at some point, but at the moment it’s a long, l o n g way away.
How’s that for an opinion…

[John Robertson Posts: 5121 Status: Offline WWW]

The hard mount issue depends on the hull construction - foam core composite is pretty good at absorbing vibration and noise (most of the engine-related noise on an Otter, for example, is airborne - and dealt with very effectively using sound dampening material inside the engine bay).

Corrosion is inevitable if you are near salt water (salt is in the air!).  Replacing with better materials is the only real option - luckily there isn't all that much external to an air cooled engine that would affect reliability (water cooled engines are also affected by corrosion to a very similar degree in my experience - and you've now got an external cooling system to corrode).  Marine stuff is designed from the outset with almost everything inside oil-filled casings thus avoiding the problem (although many smaller outboards still used points up until the late '80s - instant corrosion if unused for any period of time!).  Then, of course there's internal damage and blockages cause by the salt cooling water (I know you are supposed to rinse with fresh but the vast majority of engines never get that (bit difficult on a moored boat!).

It's perfectly feasible to reduce the effects of corrosion to a large degree on hovercraft with the correct choice of materials and some minimal maintenance (just like a boat).  Certainly to the point where it becomes a maintenance inconvenience rather than a failure - again just like with boat engines.

[Warby Posts: 1041 Status: Offline]

[




We suffer simple corrosion, (water and metal)
then theres erosion, electrolytic corrosion, moisture+ differing metals - (different positions on the electro chemical series), then added problems with differing electrical potentials ie add a battery and sprinkle lightly with sea salt, just to get the energy potentials popping!

I still think corrosion is a big problem with AIR COOLED engines, particularly on a design like the Otter where all the "Lift" air passes (complete with moisture) over an engine and ancillaries not designed for the marine environment. This makes it difficult to completely marinize, and prevent ALL the corrosion.
Outboard motors were designed to be "Hung" on the back of a boat, had TOTALLY sealed electronic ignition systems (Even back in the 1970's ) and were generally made from corrosion resistant metals.
 BUT certain items still corroded!! (In my experience) So I think it will always be a problem.
How to overcome this problem? A water cooled engine in a separate "Engine Box" ? probably only practical on a larger HC like an Prospector or Explorer.


Regarding HC noise. I think one of the major problems is that most engines are too solidly mounted to the hull, resulting in the hull acting as a "soundboard" I realise this is a compromise as a "Soft" mounted engine would require extra shafting with UJ's  etc so again it comes down to weight. AND fan tip clearance would be a problem with "Soft" mounts
I also thought of a completely separate "Thrust frame" supporting engine and all drive components ( inc Prop/fan) mounted to the hull on "Soft" mounts with a double UJ shaft to the lift fan(s). This would be fairly simple on a large Sev.?

Many years ago I was involved with "Old Wooden" speed boats with inboard marine engines 4cyl, 6cyl & V8's, directly coupled to the prop shaft, all were virtually rigidly mounted and the prop thrust was taken by the rear gearbox bearing. Noise wasn't a major problem as I think wood tended to absorb noise.

On one occasion I tried an experiment with a 4cylinder engined boat, I made up and fitted a separate thrust bearing and frame for the prop shaft , then mounted the engine on "Soft" mountings and fitted a short double UJ shaft between engine & prop shaft. The difference in sound level was amazing, no resonance through the hull at all!!
 
Just some of my thoughts!!!!!

[Nick Flint Posts: 4853 Status: Offline]

Knowing so little does not help my optimism here.
The same way that ICE  ignition systems have transported marine engine situations,,,,,
I feel these engines "perhaps" need to become "solid state" also.
Whilst ever moving parts remain open they will fail. Protection will never work due to the ability of moisture to find any way in,
The size of a water molecule gap is big enough! We are talking small!


To avoid confusion, "perhaps" all aspects will require encapsulation as our marine environment is such a test,
that "Marinisational Encapsulation" --- (ME)---will be the moment eVTOL and eHOVER takes off.
I presume their present systems ARE already designed for outdoor use, but as we  understand---
the marine environment is well harsh----   
We suffer simple corrosion, (water and metal)
then theres erosion, electrolytic corrosion, moisture+ differing metals - (different positions on the electro chemical series), then added problems with differing electrical potentials ie add a battery and sprinkle lightly with sea salt, just to get the energy potentials popping!
I reckon the developers will already be jumping straight to the holy grail of ME - see above.


In ascending order--------
1 A hovercraft to operate in dry conditions
2 Then operate well,
3 Then operate reliably and well in marine situations,
4 Then all of the above and economically,
5 Then all of the above without an ICE --- eHOVER


My feeling is that lift fan noise attenuation to still well within our grasp
and within my daily reducing time line! 

[Ian Brooks Posts: 3550 Status: Offline]

This is a pointer to the future of recreational hovercraft. We are seeing rotor development for the first time in decades that will result in lower noise, and we are seeing development in batteries and motors that will make electric hovercraft a real possibility, These developments are far more important to light hovercraft development than electric cars for example, which have a much lesser emphasis on weight.

It will be a fair time, though, before any of this filters down to use (unless there are any millionaires out there who fancy a project, get in touch!)

https://evtol.com/q-and-a/tim-williams-vertical-aerospace-va-x4-evtol-progress/

Ian