The HN headline for the article is terrible. The Wave Disk engine is a combustion engine, albiet one of novel design. It is a commonplace among engine designers that a combined turbine-electric system will be higher efficiency than a reciprocating engine is capable of; the Wave Disk appears to be a turbine engine with some slick combustion dynamics. That's great, but fuel is being combusted in any case.
I apologize for that. This is pretty far from my area of expertice. I guess a more appropriate titled would be "more efficient than a traditional combustion engine".
Either way thanks for improving my understanding of the system.
"More efficient than an otto-cycle, reciprocating, piston-in-sleeve, internal combusion engine" probably would have put you over the title length limit anyway :)
I do feel like we've seen this movie before. The problem with rotary engines, usually, is that their round shape and operation, though efficient, can make sealing difficult. So they can be plagued by emissions issues when it comes to the wear-and-tear of everyday use.
But it would be sweet if this engine licked all of that.
Still, efficiency is good. It has been a lesser known fact that the standard multi piston engine model is hugely inefficient. Apparently this not only makes it more efficient fuel wise, but it makes it more efficient weight wise (20% less weight = 20% less energy wasted pushing it, not to mention cost and reliability)
> (20% less weight = 20% less energy wasted pushing it, not to mention cost and reliability)
Unless the weight of the competing engine is 20% of the weight of the car plus payload, reducing the weight of the engine can't get you to 20% less weight.
Also, cost and reliability don't necessarily go up as weight goes down.
An internal combustion engine based on rotation rather than reciprocation that reduces complexity for many advantages. I think I've heard this story before. Oh yeah:
The Wankel engine is by far the most mature of those, and it's a lot more complicated in 3d than in a 2d cross-section. Sealing and such are an issue. Mazda has really done a lot of good things with them, and their power-weight is way better than your average 4-stroke, but it is still a niche product.
I'm not saying the Wave Disk engine isn't going to be great, I'm saying you're not going to see one of these in your car in the next 20 years, and 20 years from now, who knows what the "average car engine" is going to be and how well the wave disk engine will compare to it.
There is an argument to be made that the shift to hybrid cars opens the door for a lot of engine design variations that did not work in the past. In a normal car, the engine drives the wheels directly which puts a lot of constraints on engine performance parameters (e.g. needs to be able to generate a lot of torque at slow speed, needs to operate over a wide RPM range, etc). Today's engines are a compromise to meet those requirements. If you decouple the engine from the wheels (as in some newer hybrids) and just use the engine as a generator, it can be run at its optimum RPM all the time, and does not need to be able to generate high torque.
So, I think it is feasible that we'll see some movement away from the 4 stroke otto cycle that is standard today, and some ideas that didnt work out in the past might work out better now.
It seems to use ceramic balls which have the same function as cranks in an Otto motor, which is probably a weak spot (much wear and tear). Otherwise the idea is elegant; an actual working prototype might be nice to see.
...and combustion engines in the future "could be" 3.5 times more efficient than they are now, as well.
How efficient are current wave disk engines? Any claim about a future, non-existent engine is just hot air. (In fact, the current wave disk engines probably generate way too much hot air already, or their current efficiency would be featured more prominently in the article.)
For every cycle and construction, there is theoretical efficiency limit inherent to it. Implementations will approach that, more or less asymptotically, but can't exceed.
A novel cycle and construction may leave more headroom for improvement, even if first implementation aren't particularly efficient.
IIRC turbines are usually highly efficient, but only in a narrow band, near maximum output; at low RPM they literally suck. Perhaps a novel construction could widen the gap considerably.
That's correct , but its possible that the time has come when narrow-band-efficiency is an appropriate technology.
A "typical" car with a piston engine directly mechanically connected to the wheels needs an engine with a wide rpm and power output range.
A hybrid electric drivetrain doesn't necessarily need that - if a motor that could run at 60% efficiency but only at 8725 +- 15 rpm existed, it'd be useless for a traditional car, but it'd be easy to integrate that into a hybrid electric vehicle...
It sounds like it has the same disadvantages as a gas turbine, then. It would have been helpful if they had contrasted it to a gas turbine instead of to a piston engine. Theoretically, a gas turbine has huge advantages over a normal ICE. Yet, there aren't any cars powered by gas turbines...
That's significantly because turbines have vastly worse throttle response than reciprocating piston engines though.
This does look like nothing more than a new turbine design, I agree, but frankly a turbine driving a generator and a battery is by far the most energy efficient means of running a vehicle from a combustion fuel source anyway; that's been the case for years, and if this tech helps get us to that point then all the better.
Gas turbines have by far the best power-to-weight ratio of anything short of a rocket motor. And large gas turbines can be extremely efficient.
But they're expensive and the efficiency advantage largely disappears as they're scaled down to a size appropriate to power a car.
Probably the fundamental reason we don't see production gas turbine cars is that there are cheaper and less technically risky ways to remove weight from a car, and weight savings often isn't even the cheapest way to improve efficiency.
Sadly, they don't seem to have taken the market by storm even though they claim to double to fuel efficiency of a 40 ft bus. Unless fuel prices go up significantly, factors other than fuel efficiency will probably stand in their way (questions of reliability and supply chain are mentioned).
It actually reminded me of another turbine-like engine I had seen a while back. They had tested it on chainsaws where the reduced vibration was desirable. I guess hybrid applications are mentioned because its trendy. I'm sure there are tons of other potential applications, just can't chase everything.
sounds like a variation of gas turbine in disguise. Any implementation that is cheaper and simpler that existing gas turbines, yet reaches its high efficiency and power/weight ratio would be just great.
>The resulting sudden build-up of pressure in the chamber generates a shock wave
that makes me doubtful about longevity of the engine if the shock wave touches the metal.
Gas turbines actually have varying efficiencies against reciprocating engines, depending on the designs being compared. In the overlap area of small turbines and large pistons, pistons win out significantly in HP per fuel rate. Turbines do have higher power/weight as you say though. This Wave engine's pretty different as it features centrifugal versus axial flow (ironically, the first turbines developed by the US/UK attempted centrifugal designs - the germans gave us our modern axial flow jet engines).
A shock contacting the metal isn't a big deal. A typical jetliner will have a standing shock on the upper surface of its wing during cruise flight, visible if the lighting is just right. With this I doubt there will be significant stresses caused by the pressure gradient because the thing is pretty much a compact and flat disk (short moment arms).
> that makes me doubtful about longevity of the engine if the shock wave touches the metal.
Sure. But what about the common reciprocating engine style. There's a lot of mechanical strain with the pistons being yanked back-and-forth 1000s of times/second not to mention the energy lost in counteracting momentum.
considering the upvote to your comment, there is at least another HN reader who thinks that there are 60K+ rpm reciprocating engines and who don't know what detonation sensor, octane number or cavitation are about... Hope you know your Python better :)
Furthermore, the ideal cycle efficiency is 1-T_low/T_high, where T_low would be the exhaust temperature and T_high the combustion temperature for this engine, in deg K.
T_high is limited by chemistry and materials to abt 1400 K, T_low depends on a whole pile of stuff, but in practice cannot be lower than 700 K or so for a simple (as opposed to compound) cycle that this engine uses. So the absolute maximum theoretical efficiency, using perfect insulators, frictionless bearings and ideal gases is about 50%.
Modern car turbo-diesel engines get abt 40% efficiency, stationary diesels reach 50% efficiency. Note these are compound cycles.
tldr: 3.5x efficiency improvement claim is bullshit.
The inefficiency is reduced, and making it 'twice as efficient' would be .7*.7=.49 inefficiency. 3.5 would be about .29, or a 71% efficiency. Though it's cute that you tried to make it come out to more than 100%.
They really cheat with that number. They are using engine-to-wheels efficiency, but then talk about the wave-disk in a hybrid presumably with no drive-train losses. A gasoline engine running over a much smaller RPM range will run much more efficient than 15%.
An efficient car may run at about 18% on average (hence the 15% after external losses), but quite possibly have a peak efficiency of 30%
Also, they compare to a gasoline Otto cycle engine. The diesel cycle is more efficient than the Otto cycle. A cars turbocharged diesel may run nearly 30% average efficiency, and peak efficiency in the 40s. (The ratio of peak/average eefficiency in diesel engines tends to be closer to unity than in gasoline engines).
There are actual operating now diesel engines operating at over 50% efficiency, but they are not the sort of thing you would put in your car (more the sort of thing where your car would fit inside the engine).
[edit]I wanted to find info on the prius before posting this, but was unable to. I finally did, and the complexity of the drivetrain is to (among other things) maximize the amount of the time that the engine is in it's peak efficiency area of 230g/kWh which is roughly an efficiency of 33%. Since the article specifically talks about running the engine in a hybrid comparison, this is apples-to-apples, and it is less than a 2x improvement, not 3.5x
[edit2]The Prius is not a vanilla Otto cycle, but it's pretty close. I really think of it as more of a Miller cycle than an Atkinson, but in any event it's a 4-stroke spark-ignited reciprocating piston engine.
