The 1^st In A Series Of Engines Utilizing A Combination Of Advanced Technologies Keeping The Two Stroke Alive

By Anthony J. Liberatore

At Sun and Fun this year, the Higgs HAWK V4 E330J/G engine made its public debut in front of the aviation community. I was able to discuss this innovative engine with Karl Grove, CEO, AC USA.

Karl is a United Kingdom expat with a background in marketing, operations, and professional motocross racing. Thereafter; Karl made a segue into the air racing world in 2010. Fast forward to 2019 and a fourth place finish in the Sport Class Gold Sunday race flying the Grove Aero RS540TT AC AERO powered Super Legacy.

However; Karl is not alone in this effort. The aforementioned Lycoming based race engine was the brainchild of the team at AC AERO Engines, specifically engineer extraordinaire Andrew Higgs with his MotoGP, F1, Indy, and LMP1 background.

For the last few years, press releases and photos of CAD 3D models have been released, denoting a whole new family of engines utilizing stepped piston technology. Also, a few years ago, a video surfaced on the web of a Lycoming crankcase maintaining the 4 stroke “bottom end” while being converted to a two stroke burning JP-4 -- running on a test stand swinging a propeller.

What?

Yes, stepped pistons are what made it happen. Even before the press releases and the video, some of us in the aero-writing arena were getting wind of the project. I tried to get an interview at OSH a number of years ago, however it was too early in the development cycle for a story.

If you go back even further than AC development cycle, a lot of us with a penchant for the internal combustion engine (be they land, sea, air)  have been following a host of technologies. If you include two strokes in that mix, design features such as external scavenging, direct injection, semi-direct injection, and stepped pistons have been on our radar screens -- even if they were prototypes that never made it to production. 

Stepped piston engines have been around since pre-WWII times with the Dunelt motorcycle, the Elmore car circa 1908, General Motors 1915, Burnett 1926, a Russian Radial, and in more recent times the Bernard Hooper Norton Wulf 500 motorcycle (which never made it to production). The easiest way to describe stepped pistons, is to image a two layer wedding cake with the smaller diameter on top for combustion, and the lower (both concentric) larger diameter acting as an air-pump to feed the air charge into the adjacent cylinder. This eliminates using the crankcase as an air-pump and mixing of fuel and oil for bottom end lubrication, as it sports a four stroke type bottom end. This has a one two punch of simultaneously increasing reliability, and lowering emissions. If you added direct or semi-direct injection for lower fuel burn and lower emissions, it is easy to see how this might be a candidate for Gen-Av with competitive power to weight ratios, and low specific fuel consumption numbers.

I interviewed Karl in front of AC-AERO's Hawk V4 E-330J/G – soon destined to be installed on a RV-10. AC-AERO hopes to have this combination ready for Oshkosh 2021.  He elaborated on the design history which is a springboard for this family of engines -- that range from 115hp up to 1600hp. 

“You know, there were many attempts to commercialize the stepped piston design, but no one has ever really done anything with it. My partner at AC-AERO, Andrew Higgs, has been developing this technology, with the Higgs Engine. The original patents have long since expired, however we have patented and patents pending for design features with the Higgs engines. Andrew has been working on this technology for the last twenty years. We have come to a fruition point, to where we are now ready to market.  It's solid, and we have a lot of testing completed at our facilities in  Japan, so you know now is the time.”

Karl continued to explain the benefits of configuration: “Absolutely, that's where you get such Lightweight Engines, the Hawk for example. It's this Combined Cycle Technology (CCT) with a Spark Ignited Cylinder, and we are not using glow plugs, so there are no overly excessive pressures inside the engine (8.5:1 compression ratio). We can make everything a little thinner (think engine block, cylinder heads). As an example, the Hawk is 306 lbs, 350hp, with 870 foot pounds of torque, and it is a very very impressive engine. If you compared that to an IO-540 Lycoming, you’re 100 lbs lighter, and 100 more horsepower -- so it is a very interesting concept -- liquid cooled, as well, obviously.”

Karl continued to describe the Hawk V4 E-330J/G's technology as we talked about the bottom end,  which is a wet sump with some dry sump characteristics. While looking at the display engine in AC-AERO booth, I noticed what looked to be the fuel injectors nestled in the “V” of the V4's engine block. This correlates with the CAD animation video, on their site, describing the injection system. With that said, perhaps the 300J/G might be utilizing a semi-direct injection system injecting into the intake ports.  I asked Karl about not seeing an expansion chamber type two stroke exhaust, and that might be a hint that there was no fuel escaping via the exhaust port -- and that something interesting must be going on with the fuel injection.

I also inquired if this was proprietary and can't be discussed. He said with a hearty laugh, “a little bit of both!” Karl continued, “we have our own proprietary ECU's and ignition, and can only use the engine with them. With our products and ECU's, we are only going to launch Jet A capability initially (Experimental Amateur Built, EAB) as a quality assurance play for us to make sure that the product succeeds, and customers have a good experience with the product.

So, initially, even though it is a multi-fuel engine, we will launch as a Jet-A Engine, and slowly introduce the other Fuel options.” In regards to fuel consumption, AC-AERO's literature denotes a brake-specific fuel consumption (BSFC) number of 0.398 lb/hp for the 330J/G. As a reference, it is not atypical to have a certified Gen-Av engine with a  BSFC in the .45 to .48 range.

When asked about other markets for this high power-to-weight ratio multi-fuel engine, Karl said: “There's lots of applications: marine, generators, and we are talking to a lot of E-VTOL companies in that community for utilizing our engine as a hybrid gen-set. The Falcon and the Swift for example, (115hp and 250hp respectively) are ideal power-sources for generators, and for the E-VTOL community, so we are very excited. We have a lot of different products to talk about, and to pitch to different industries, different markets -- but initially we are focused on EAB. We are all pilots, and we want to succeed in EAB, and then we will seek Type Certification down the road.” I said goodbye to Karl after the interview, however I look forward to a follow up interview with him at Oshkosh 2021 to get an update on the Higgs Engine and its installation in a RV-10.

FMI: http://www.ac-aero.com/