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Super-compact internally rotating combustion engine packs a solid punch

Super-compact internally rotating combustion engine packs a solid punch
Avadi MA-250 engine cutaway side view
Avadi MA-250 engine cutaway side view
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The prototype Avadi MA-250 engine completed
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The prototype Avadi MA-250 engine completed
Avadi MA-250 engine front view showing its rotary valve system
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Avadi MA-250 engine front view showing its rotary valve system
Avadi MA-250 engine cutaway side view
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Avadi MA-250 engine cutaway side view
Avadi MA-250 engine cutaway 3/4 view
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Avadi MA-250 engine cutaway 3/4 view
Avadi MA-250 engine rendering showing combustion
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Avadi MA-250 engine rendering showing combustion
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An innovative new combustion engine eliminates half the guts of a traditional engine, and uses a fascinating internally rotating piston and sleeve arrangement, making it lighter, simpler and more efficient while still making strong power and torque.

Michael Arsenaeu designed the Avadi engine 20 years ago in the hopes of creating an entirely new engine design to be efficient as well as reduce emissions. In 2015, Avadi began the build and development.

Rotary might be a good name for this sort of engine, if the name wasn't already taken. Everything inside the crankcase rotates, but unlike a Wankel-style rotary engine, this design uses a piston with two connecting rods that have a scissor-like movement attached to pinion gears at the back of the engine.

All of this rotates on a fixed ring gear connected directly to the output shaft. It uses a valve disk that also rotates to complete a four-stroke cycle of intake, compression, combustion, and exhaust as everything rotates internally.

It's dizzying. Check it out:

Avadi Engine MA-250 Animation

According to Avadi's website, "two connecting rods and their corresponding pinion gears reside in what we call a 'halfshaft.'... the halfshaft can be likened to the heart of the Avadi design, it is essentially the housing where up-and-down piston motion is translated to rotational motion."

Unlike a Wankel rotary engine, the Avadi eliminates the traditional crankshaft entirely, as the cylinder and piston rotate within the case during its stroke. Everything very much looks and functions like what you'd see in a rear differential – if differentials had pistons connected to planetary gears.

Looking at it in motion does make me wonder if the low mechanical advantage from the connecting rods to the pinion gears might constitute a long-term reliability issue from the torsion factor on a narrow, short stroke. I also wonder how efficient it could potentially be with a better mechanical advantage in transferring the up-down motion from the piston to the rotation of the output shaft.

The prototype Avadi MA-250 engine completed
The prototype Avadi MA-250 engine completed

Either way, Avadi demonstrated a prototype some years back. The MA-250 engine was producing 15.8 hp (11.8 kW) at 3,700 rpm and 22.3 lb-ft (30.24 nm) at 3,500 rpm weighing a mere 23.6 lb (10.7 kg) and 250 cc.

For comparison, the Honda GX240 single overhead cam commercial-grade 242-cc engine might suffice; it's around the same displacement and doesn't have a gearbox adding to its weight like a single-cylinder motorcycle engine would. It produces 8 hp (6 kW) at 3,600 rpm and 12 lb-ft (17 nm) of torque at 2,500 rpm, weighing in at a hefty 77.2 lb (35 kg).

Avadi claims a 42.12% brake thermal efficiency as well. To put that in context, Toyota celebrated a breakthrough when its 2.5-liter, 4-cylinder A25K-FKS engine got to 39% thermal efficiency, and typical gasoline engines come in at closer to 20% according to Green Car Reports.

Avadi MA-250 engine cutaway 3/4 view
Avadi MA-250 engine cutaway 3/4 view

We say the prototype "was producing" those figures, because after Avadi made an appearance at CES in January of 2022, little has been heard from the small Washington, USA company. A single blog-style post on its website dated May 28th, 2023 tells us that Avadi has replaced Arsenaeu with Dimitrios Dardalis as Chief Technology Officer, and the rotary valve design had "unsurmountable deficiencies in airflow," which Avadi believes it has solved with a reworking of the valve design, which the company expected to demonstrate with a new prototype slated for testing by the end of 2023.

Rotary valve systems are not a new concept, having been used extensively in the 1970s in 2-stroke engine configurations. They were later dropped in favor of reed valves.

We reached out to CEO and Co-Founder, Landon Wilkinson, to see where things stand in June 2024. "The revised engine design is coming along nicely," replied Wilkinson. "We achieved positive compression results last week with the new valve design and anticipate run tests in the near future."

We look forward to the results. It's gotta be tough trying to break through into a rapidly electrifying world with a new combustion engine, but at the same time there's plenty of ICE engines out there, and there will be for some time yet.

Meanwhile, enjoy the video below. Especially the part at 1:56, where you can wonder together with us whether Avadi really meant to leave a shot in the video where somebody appears to either arc out a spanner on the engine, or grind it on a moving part...

Avadi Engines Introduction

Source: Avadi

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20 comments
20 comments
Chase
Lol, yep. That's where the starter's electrical connections are, so my money is on arcing out on the metal test bench. Classy.
Charles Butcher
Damn good idea one for each wheel of the car
KaiserPingo
So old school and hopeless.
michael_dowling
KaiserPingo: Agreed. Why on earth introduce yet another combustion engine design when the world is moving towards eliminating ALL such engines?
Treon Verdery
Its recently published (2023ish) that adding 100 PPM ozone at the air utilized at an ICE engine causes 17% higher fuel efficiency, and cleaner less polluting exhaust. The exhaust might be cleaner from ozone caused higher combustion velocity and greater warmth. Supporting long engine lifespan is a new cylinder alloy, based on valve alloys, developed for high-warmth hydrogen fuelled engines. I am also wondering if volutes on the cylinder head surface could cause more rotational energy, noting the cylinder rotates. I think a rotating engine like this will have flywheel/gyroscopic effects. Perhaps the mounting position and angle can be optimized to minimize vibration, with the mounting position of wankel engines as a guide. At the videos I would have very much appreciated hearing the sound of the engine. I think it is possible to tune the engine sound to always sound good. One way for the engine sound to be optimized for consumer preference is to utilize a -30dB acoustic metamaterial curved form I recently read about. That -30dB form was 3D printed from polymer but could be easily made with metal. -30dB is 1000 times less noise produced at narrow sound wavelengths. An ICE engine 1000 times quieter could get their company lots of attention and publicity, supporting their main engine product strongly.
Karmudjun
Thanks Joe, this is almost as innovative as the Wankel engine, we'll see if the development to efficient production models results in a long lasting useful engine. In all the electric vehicle cost benefit analysis, most calculations lean toward hybrid technology as the long term lowest cost option. I certainly don't want a full electric vehicle that cannot be recharged easily on the road, and if a small engine - or bank of engines of whatever model - can provide efficient recharging I can see even diesel trucks using this to increase load capacity mpg. It clearly is not built for a heavy diesel fuel, but with the balance and output range, it could run while a long haul trucker was getting their sleeper time in, and possibly complete charging the battery pack before the next shift. I know in family rancher pickup trucks, I'd like an option that would make a 4x4 a real efficient workhorse everywhere. My key needs, efficient use of rare earth minerals, efficient use of fossil fuels without requiring fossil fuel consumption on a daily basis, and a dependable vehicle when out in a rainstorm attempting to cross rainstorm washes to get home to the charger. Just keep us apprised Joe, as incremental gains in efficiency and reductions in greenhouse gases can only result in a reduction of our current trends!
CraigAllenCorson
If you watch the first video at ¼ speed, you can see it throw a rod at about 0:06. 😁
Username
Looks like the design limits the engine to one piston.
DBK
So, nice compact design, but it is still a traditional four stroke engine. Maximum theoretical efficiency? 50% Pumping losses drawing intake charge and discharging combustion material.
jerryd
They claim. The problem is it is too late as by the time it hits the market if it does, EVs and E will have taken the market with 90% efficiency and much cheaper fuel, especially if you make your own.
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