Announcement

**Mark Goldberg** · 08-31-2019, 07:24 AM

The 540 engines approved are the ones with parrellel valve cylinders which have an empty weight right at 380-400 lbs. FYI. Maybe this diesel will turn out to be good. There have been various attempts over the years to come up with a suitable diesel engine. But no real options still. Mark

**bigrockcandymountain** · 08-31-2019, 01:24 PM

That is a fantastic design in my opinon. No valves, no blower.

Hopefully they get some going. And hopefully the effieciency is what they hope.

**Bigcat77** · 12-29-2019, 11:29 AM

[QUOTE=Trefftzs;n48998]Anyone know anything about the AC Aero (Higgs Diesel) stepped piston, Spark ignition, multi-fuel engine? http://www.ac-aero.com/jet-a/

Have been watching this engine for a while, and it looks like the first run of pre-production engines should be being tested / possibly being installed in aircraft.
(http://www.vansairforce.com/communit...d.php?t=167537)

The E330 (300 hp - Naturally aspirated version) could be an interesting future option for a Bearhawk 4 place.

The E330 is 301 lbs without accessories or cooling system. One estimate I read says 350 lbs. with accessories installed... Not sure if that includes the cooling system I would assume it does not. Just looking around at IO-540 weights, (which depending on where I look are listed as 400-480 pounds with accessories) the E330 may end up being about the same or even possibly less with the cooling system included.

I agree interesting engine, does anyone have more details?. Diesel should have great
potential, I just canâ€™t understand their fuel and ignition system. Kind of like getting half
pregnant. One pt of diesel is eliminates electrical ignition. Diesel doesnâ€™t evaporate,
mix with air and burn easily and efficiently in combustion chamber
like gas. Yes I know the air is compressed/heated in the step cyl, so should help but
to degree mixture is imperfect it will hate to start or burn efficiently. Because it will burn much slower (one flame front) like a gas it is not going to have the kind of fuel economy we know
and love from our direct injection diesels. Exciting but mostly uncharted territory.
Can imagine fowled plugs, black smoke, slobber, low power, etc,etc!
I want to see running engine and flying aircraft.

I went to their booth at Oshkosh, nice guys, good looking hardware but spark ignition
diesel????

**svyolo** · 12-29-2019, 10:51 PM

Google, Youtube, and their website show a bunch of CAD drawings, and the price of the engine. No picture, video, etc of a finished, let alone running, engine.

I would bet they are ready to take deposits, however :<)

**Trefftzs** · 10-08-2020, 05:14 PM

I got a little excited today thinking about my future build (Model 5 should fit my future plan perfectly) and decided to chat with AC Aero today.

Found out they have an RV-10 build in progress. Little is posted about it now but I was told to keep watching more information is coming... Possibly some videos of engines running /dyno numbers. It was implied that lots of information is being held back as they are not ready for full scale production yet.

http://www.ac-aero.com/hawkrv10/?fbclid=IwAR3UaRCutzdqSdgMkBfT4IYZk6ywnGav97qA2DGHQ0Qmn2wKRY0PMxON0EU

AC-Aero

https://www.facebook.com/acaeroengines/

AC-Aero. 990 likes · 1 talking about this. Manufacturer of the Higgs Diesel and Performance Aviation components for general aviation applications.

I was also told that someone (possibly more than 1 ) person has an Hawk 4V reserved for a Bearhawk. Fess up, Who is it?

Aaron

**Sir Newton** · 10-09-2020, 07:59 AM

So many engine choices, I would love to see an electric BH5! Time to get to work!

Attached Files

IMG_20201008_162620.jpg (1.49 MB, 394 views)

**svyolo** · 10-09-2020, 02:05 PM

The address listed for their corporate headquarters looks like an empty building at an airport. No company is listed on the map, no company signs on the building. The address listed as their "US distributor and build center" looks like a large construction contractor's location. 2 large multi-floor covered outdoor spaces. 10-30K square feet. No walls. Lots of construction materials.

They have a really nice website though.

**BradW1062** · 08-01-2021, 02:50 PM

I spent some time talking with AC Aero, mainly Karl Grove, at Oshkosh this last week. Here's my long-winded take on it, FWIW (i.e., this and about $2 will get you a coffee at your favorite diner).

First, since I've been away from this forum for a while as life has gotten in the way of my Bearhawk project, I will give a little background on myself, again FWIW, so you know where I am coming from:

I'm building a Bearhawk 4-place original "model A" QB kit that I got back in 2008/9 from Mark G & co. I met Bob in 1996 at Sun N Fun, and was so happy with the experience that I wanted to purchase plans on the spot and start scratch-building. But for some reason I didn't, and planned to do it as soon as I got home. Again it took a little more of "life" before I got back around to getting the plans, and that was along with my QB fuselage order 12 years later. I am a long-term gear-head, tearing things apart and putting them back together since childhood. I am especially interested in engines- piston, turbine and rockets, and have a degree in mechanical engineering to fuel that fire. I have some graduate-level work in aerospace engineering as well, to scratch the airplane and spaceship bugs. Basically, I am crazy about all kinds of engines, and have a good bit of experience messing with them. I have said for years that the common misconceptions by pilots in our aviation community about piston engines are the main things preventing acceptance of better ways. Of course, all the experimental shade-tree auto engine conversions haven't helped either. Anytime I hear a pilot explain how automotive engines are not good enough for airplanes because aircraft engines were "designed" for aircraft and auto engines were not - I just roll my eyes, bite my tongue and go the other way. They really have no idea what they are talking about. The only real differences are the prop flange on the crank, the pendulum dampers to keep the impulses out of the propeller, and the low rpm for all the obvious reasons - oh, and the miles of FAA red-tape attached. The certified air-cooled piston engines in our airplanes are based on 1930's technology for the most part, and the air-cooled cylinders and heads are the biggest problem they have, with the second problem being magnetos. I have always been a proponent of changing to liquid cooling, and have been carrying the idea around in my head for years of making a bolt-on set of liquid-cooled cylinders for a Lycoming or Continental. Well, look what we have here at AC Aero...liquid-cooled cylinders!

As usual, visiting their display at Oshkosh, it took me a while to work through the marketing-speak (read "B.S.") and to find the person who actually had some technical knowledge or understanding of what they are selling. Example; when I asked the first guy why they were calling their engines diesels when they clearly have spark-plugs, I got a lot of uncomfortable stammering and he asked me to come back later to talk to the technical guy who could explain it better. That turned out to be Karl Grove, who is the sport-racer pilot - not necessarily the "tech guy", but he knows enough to talk about it plainly. It seems that AC Aero's CTO, Andrew Higgs, wasn't around, or at least not when I was there.

I told Karl that I appreciated what they are doing and that the parts and pieces all look great and it seems that they have super ideas, but that I had some specific technical questions about their engines and parts. Karl, openly and honestly, said he was most experienced with the Legion stroker kits and the Gladiator/Centurion cylinder kits for Lycoming engines since he races with those, but not the AC Aero engines specifically as that was really Andrew Higgs' area. AC Aero is a sponsor/partner of Grove Aero. But he offered to answer questions as he could. OK, cool.

We talked about the Lycoming kits first (no Continentals). Even though they have the brochure saying you can combine the two to get the best features of both, they actually can't do that yet. The large-displacement stroker kits are not currently compatible with the liquid-cooled cylinder kits. Since the cylinders are designed as a bolt-on to the stock O-360 and O-540, the long-stroke of the Legion kits are about 0.25" too long for the new cylinders. He expects the new cylinders for the combined package within a few months. The cylinder bank they had on display was clearly a parallel-valve, but the ports were well done so I don't think the typical benefits of the angle-valve is a factor on these. I asked if they were selling the parts as kits for me to install, and he said no. They are requiring an authorized build center to do the engine work. But, they do not want to do firewall-forward installations with the radiators and everything - they are leaving that up to the builder. They had an RV-10 V-4 Hawk package on display, though - mainly just a motor mount with the integrated nose gear fitting, but I didn't ask about it specifically. It appears at present that the only authorized builder is Aircraft Service and Parts in Bakersfield CA. You can send them your core engine and they will return it ready to go, or they can find a suitable core for you. He said the combined package would run about $52K for an O-360, and about $69K for an O-540, if I remember correctly. A good bit more than a new Lycoming engine (I may be out of date on their prices), but liquid-cooled power is probably worth it for the longevity and power. They have the prices of the individual components on their website, as brochure downloads. It looks like they are charging about $12K for the building service. I thought that the build quality of the cylinder parts was outstanding. Definitely latest technology, modern processes. They are using 3D-printed sand molds and pressure casting to get high detail and low porosity. A company in Taiwan, who was also at the booth, is doing that work. I didn't spend a much time on the crank, rod, and piston parts - that's almost off the shelf racing stuff these days and pretty common - but it all looked fine at first glance.

As for the "Higgs Diesel" engines; that's a bit of another story. I actually think they are a great design, and the parts they had displayed, again, are premium castings. The E-1000 V-12 is a pretty impressive sight. They are claiming up to 1600 hp when turbocharged - a replacement for turboprops that uses less fuel. The V-4 Hawk (on the RV-10 set up) equally was very nice looking. And the inverted direct-drive V-4 Swift or Falcon they had displayed was very compact and clean, reminds me of the DeltaHawk, too. A power to weight ratio of 1:1 or more is almost twice that of an IO-540, and even more considering take-off numbers. But, I had to ask the specific tech questions I wanted to know from the start. They are advertising them as a 2-stroke Diesel, and CCT (Combined Cycle Technology) and are obviously using marketing language (read "BS") instead of real technical details. So I asked, how are they scavenging - is it blown, or what, how are they getting the charge in the cylinder? Karl told me it was a proprietary design, where they use the piston shape to force the air through the cylinder. So I said - it's loop-scavenged then? He said, yes basically. At least that was a straight-up, honest answer so that's good. Loop scavenging is where the exhaust ports and intake ports in the cylinder wall are arranged so that the intake air flows up and spirals around the cylinder while helping push the exhaust out the other port. Not new technology. So I asked, what do you mean by Combined-Cycle Technology, it's either Otto, or Diesel, no matter if it's 2-stroke or 4-stroke? He said it's not actually a Diesel cycle, they call it that because it can run on diesel fuel or Jet-A or other fuels, but it is not compression ignition it is "spark-assisted" ignition, so that's the combined part. It has about an 7:1 compression ratio. This is about the same range as old-tech auto engines and most aircraft engines today. That means that it is NOT a diesel, diesel is compression ignition of the mixture with compression ratios in the 12:1 and higher range. I still don't see what has been combined here. I guess it's using a spark plug to ignite diesel fuel. I don't know why they find it necessary to introduce this kind of BS marketing into a solid idea - but whatever. These are spark-ignition 2-stroke Otto cycle engines, and they can run on just about any fuel you can spray in the chamber - so just say that, please. The Hawk engine rated 350hp take-off and 300hp at 85% continuous, is $76K, and the 500hp (425hp cont.) is $96K - pretty steep for us home-builders, I think. The Falcon, at 210hp (178hp cont.) is $52K, and maybe a better Bearhawk application, and especially for a Patrol. This can be a really important engine consideration if you are somewhere that fuel availability is a problem, or you are worried about the future of 100LL.

What I found out later after a little research (not through the Q&A) is that the key thing that Higgs has done is to use what is called a stepped piston design, where the bottom of the piston has a larger diameter than the top, and it is used as the air pump in a separate chamber below the combustion chamber (cast into the block below the cylinder) that scavenges an adjacent cylinder. It's not an entirely new idea - naturally. It is a good method, but is a bit more complex than just using a roots blower, which is dirt simple. Maybe blowers are patented? This design is how they are keeping the air out of the crankcase. The stepped-piston concept was patented by Bernard Hooper, and was used on the Norton motorcycles of the late 1960's. It appears that Hooper also was probably the designer of the engine, or at least the concept, that AC Aero is using in their Swift and Falcon engines, since he was also promoting a UAV engine of a very similar design. Unfortunately, most people commonly think that all "2-cycle" engines have to run oil in the fuel because it goes through the crankcase. Well, that's simply not true - it's just another common misconception, based on the fact that most 2-strokes people are familiar with are crankcase-scavenged engines on model airplane, chainsaws, string trimmers, leaf blowers, dirt bikes, etc. But the Detroit Diesels and the Electro-Motive Diesels form the 1930's were roots-blower-scavenged two-stroke diesels that used uniflow cylinder design - the cylinder ports admitted the blown intake air, and the standard poppet valves in the head were the exhaust ports. These old engines did not circulate the intake air through the crankcase - they had blowers to move the air instead. And they were direct-injected diesels. BTW, if you ever wondered, a "6-71" blower on a hemi dragster likely came off of a GM Detroit Diesel 6-71 engine, an in-line 6-cylinder 426ci 2-stroke mechanical fuel injection diesel. Since these old engines had blowers for scavenging, the crankcase was a normal, oil journal bearing case like common 4-stroke engines. Not exactly new technology here at AC Aero, but because of the common misconceptions, Higgs is capitalizing on the somewhat hyped marketing idea that he has solved the oil problems of the 2-stroke engine.

With all that said, I was impressed by the design and quality of their engines, and I like the idea that these are very simple engines. They don't have camshafts, or a valve train of any kind. The only moving parts are the crank and pistons, and on the higher powered ones also the gear-reduction and accessory drive on the front. That means less to fail, and also means more efficiency by eliminating the cam and valve-train loads. And they are liquid-cooled, which I think is the right answer, too. I hope they prove successful in their reliability. I don't doubt the benefits of their design, even without the marketing BS. I agree this is the way to go for future piston engines. It just makes great sense - the ability to burn a wide range of fuels is enough to make it viable in my opinion with the impending 100LL phase-out, and the 2-stroke allows more power in the same displacement. Some people will scoff at the gear-reduction, but spur gears are well proven technology, and as long as they have done their homework on torque pulses and vibration harmonics this should not be an issue at all. I didn't ask about that, since Andrew Higgs wasn't there, but I'm thinking this has probably been well taken care of. Elimination of the valve train allows for the engine to turn 5-6000 rpm long term with no worries, so I don't see a problem. Gear reduction is the best way to get additional power and drive a propeller the required lower rpm. As usual, it's just all a little out of my price range, unfortunately.

Hope you enjoyed my dissertation!
Brad

**zkelley2** · 08-01-2021, 03:14 PM

Absolutely no one would ever replace a 1000hp turboprop with anything piston. That's insane. Almost no one running a turboprop's first concern is fuel economy. First is dispatch reliability.

**Nev** · 08-01-2021, 05:54 PM

An enjoyable read Brad. I used to have a 12V71 - aka the â€œRocky Mountain Hummingbirdâ€. Great reliable engine with a sweet sound ! (Not that Iâ€™d want it anywhere near an aeroplane, but still a great engine). It was the first time I had come across a 2-stroke diesel.

The rumour was that you could do an oil change without shutting it down. I never tried. But I had an old fire pump connected via a valve to the sumpâ€¦..oil changes were very quick.

**Hewko** · 08-02-2021, 09:21 AM

This doesn't apply to airplanes, unless you wanted to modify a 200 or 300 hp outboard to use. BRP uses the ETEC 2 stroke in their skidoo's and used to use it in their Evinrude outboards. I have both and love them. They are the most technologically advanced 2 stoke available, using high pressure direct injection across the indexed spark plug to create a non-homonigized fuel charge, which reduces wasted fuel. There is no fuel charge washing oil off the cylinders or crank bearings, they use very little injection oil and have great fuel economy. They run the fuel through the ECU to cool it and have proven to be very reliable. Very cool technology.

**PaulSA** · 08-03-2021, 01:26 PM

Bottom line is that the Lycomings work. Add in some sort of electronic ignition (my choice is PMag) and you have a pretty fireproof solution.

The problem with any new modern design (which everyone hankers after) is getting the numbers and flying hours to get proof of reliability,

**Bcone1381** · 08-04-2021, 06:39 AM

It is not the critic who counts; not the man who points out how the strong man stumbles, or where the doer of deeds could have done them better. The credit belongs to the man who is actually in the arena.....

DeltaHawk diesel engine....found in 1996, was not at Airventure this year, two years ago had a huge booth. At one point they were "working towards 2012 FAA certification" and later set a goal of 2015. Today their website says "the wait is almost over." I've been interested, rooting, and waiting since about 1997.....seriously. They've spent tons of money. I'm grateful. Its awesome that they have not given up. Their effort inspires me.

But yes, Lycoming's work, I want to fly some day, and doing something other than standard on my firewall forward is another "airplane size" project, in time, money and risk.

**zkelley2** · 08-04-2021, 06:58 AM

If you really want a diesel the best bet is to probably find a diamond twinstar in a salvage yard, buy an engine and overhaul it. They're not enough hp for the 4 place. Might work on the patrol.

Announcement

AC Aero - Experimental Diesel Engine

AC Aero - Experimental Diesel Engine

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