EAA - Experimental Aircraft Association  

Infinite Menus, Copyright 2006, OpenCube Inc. All Rights Reserved.


Tools:   Bookmark and Share Font Size: default Font Size: medium Font Size: large

EAA Experimenter

[ Home | Subscribe | Issues | Articles | Q&A | How To | Forum Review ]
[ Hints for Homebuilders | Glossary | Polls | Around the Web | Submit an Article]

Spark-Ignited Heavy Fuel, Part Deux

By Anthony J. Liberatore, EAA 99484, aliberatore@comcast.net
Photos and images courtesy of Hirth, Orbital, Bob Skingley, and Anthony J. Liberatore

In the January 2009 issue of Experimenter, Anthony authored the article, “Is There a Spark-Ignited Heavy Fuel Engine in Your Future?” Never in his wildest dreams did he anticipate that an engine of this type would emerge so soon for unmanned aerial vehicle (UAV) applications, manufactured by Hirth-Engines, a name known to experimental aviation enthusiasts. With that in mind, what is the possibility we will see some advanced technologies like this become available for the homebuilt aircraft enthusiast? Before that question is answered, Anthony gives us some background on Hirth’s efforts and their journey into this arena.

In a press release dated February 15, 2005, Hirth-Engines announced they were launching a 45 kW (60 hp) twin-cylinder, two-stroke engine, capable of burning heavy fuels (JP-5, JP-8) to meet the 2010 NATO and U.S. military requirements to eliminate gasoline from the battlefield for safety and logistic reasons. A key enabler that Hirth chose to meet this requirement was to become a licensee of Orbital’s Air Assisted Direct Fuel Injection and incorporate this technology into these new engines.

Air Assisted Direct Injection Combustion System
Air Assisted Direct Injection Combustion System.

Orbital Australia Pty. Ltd., based out of Perth, Western Australia, developed their Orbital Combustion Process that is based around their highly patented Air Assisted Direct Injection, also referred to as AADI. These injectors are similar to current fuel injectors in terms of operating pressure but also utilize compressed air (supplied by an air compressor) to further atomize the fuel, preparing it for the combustion process. This air-assist atomizes the fuel droplets down to 6-10 micron SMD, which is the industry benchmark. (SMD = Sauter mean diameter: a way of comparing the atomization performance of different injectors).

Typical Orbital spray pattern
Typical Orbital spray pattern.

A cross section of an Orbital/Synerject Air Assisted Direct Fuel Injector
A cross section of an Orbital/Synerject Air Assisted Direct Fuel Injector.

This technology is utilized by a host of Orbital licensees such as Aprilia’s DiTech engines, Mercury Marine’s OptiMax outboards, and Tohatsu’s TLDI outboards, which are all gasoline two-stroke applications. The application of this AADI to two-stroke engines changes the emissions and fuel consumption of these engines dramatically in a positive way. In fact, fuel consumption drops some 40 percent (giving parity with four-strokes) and emissions drop in the order of 80 percent over a carbureted two-stroke. This is not only due in part to the direct injection, but the fact that the injection only occurs after both intake and exhaust ports are covered by the piston, therefore, not allowing any raw air/fuel mixture to escape out of the exhaust port as it would have in a carbureted two-stroke.

While the origins of Orbital experimenting with fuels other than gasoline may have their roots in their research and development (R&D), it is not commonly known that their engines could also burn heavy fuels. In a conversation I had with an Orbital team member at an SAE convention in Cobo Hall, Detroit, Michigan, in the mid 1990s, he noted that with Orbital’s AADI they could run heavy fuels, but it needed a sparkplug to make it work. Some 10 years later, this ability allows them to create this unique engine: a spark-ignited engine that runs on heavy fuel. However, there is a penalty for burning heavy fuel in these engines versus their gasoline brethren, that being a power reduction in the arena of 10 percent to 20 percent, depending on engine size.

Carbon piston as used in the Hirth 3053 HF, heavy fuel engine
Carbon piston as used in the Hirth 3053 HF, heavy fuel engine.

As previously mentioned, this is not Orbital’s first foray into the heavy fuel arena with one of its licensees. Mercury Marine has created a 185 hp heavy fuel variant of their Optimax outboards for the U.S. Navy. The United Kingdom’s E.P. Barrus Ltd has developed under license from the U.K. Ministry of Defence a prototype 40 hp “Orbital-equipped” outboard. What is unique about the E.P. Barrus entry is that it can burn multiple fuels (gasoline, kerosene, and diesel). This is currently handled via a selector switch, but Orbital is looking at the feasibility of adding knock sensing and automated fuel sensing capability to be able to adjust the calibration for various blends of gasoline, kerosene, and diesel. Primary interest for this technology is on outboards and land-based vehicles, as well as auxiliary power unit (APU) applications.

While the current 3053 HF and S1200 engines presently have representation in the form of brochures on the Hirth heavy fuel (UAV) website, it may not be a true representation of the inroads Hirth has made in this arena in terms of the number of engines that have been or will be developed. In fact, due to the sensitive nature of the mission of these engines and at the request of some customers, additional development programs may or may not be under way. At this point Hirth’s product array in this arena is fairly customized with each engine designed for a specific application.

So why did Hirth team up with Orbital? Orbital’s AADI system allows an engine to be calibrated to compensate for the poor burn qualities of heavy fuel. The fine atomization and full mapping of the engine calibration improves transient operation and fuel consumption and allows the engine to start at cold temperatures without additional heaters or additives. This provides a solution that is more fuel efficient than a turbine engine and lighter than a compression ignition diesel. Orbital’s AADI heavy fuel solution can be applied to two-stroke or four-stroke engines.

Another key technology that the Hirth heavy fuel engines employs in these innovative engines is the use of carbon (graphite) pistons. These pistons have thermal expansion that is virtually nil and allow piston-to-cylinder wall clearances of 0.0005 compared with a typical 0.005 for aluminum pistons. The composite unit utilizes two piston rings, with the design intent of being utilized for centering purposes only. While being shown this remarkable piece of engineering, Jason Wright of Recreational Power Engineering www.RecPower.com, U.S. distributor of Hirth-Engines as well as Powerfin Propellers, demonstrated that by dragging the skirt of the piston across a piece of paper, it left a mark just like a pencil!

With all these advances Hirth has made with their heavy fuel engines for the UAV arena, when will these advances ever trickle down to the average homebuilder? Before we answer that question, here’s some background on Hirth’s current line of engines available to homebuilders. For years at air shows, aviation enthusiasts have approached Matt Dandar, also of Recreational Power Engineering, and have asked him when Hirth was going to introduce the four-stroke engine. Hirth took a different approach to this request; in many ways they have felt the two-stroke was superior, and even though for years in terms of R&D, four-strokes have garnered the lion’s share of attention and dollars. Hirth decided to take a completely different approach with their two-strokes’ inherent weight advantage. They asked themselves, why not strive to make our two-strokes with the fuel consumption and reliability of a four-stroke? It was with this mindset that Hirth embarked on their journey to do just that.

As Hirth continues this venture, here is a glimpse of how it may unfold in the near future. First, the carbon pistons are now a reality and in production, and their cost will come down as production rates increase. To complement the carbon pistons, you could add as an option the currently available electronic fuel injection (EFI), which would bring a two-stroke offering to the table that would have an air/fuel mixture continuously optimized via the “black box.” With the EFI working in concert with the antiseizure properties of the carbon pistons, for example, on Hirth’s current 3202 engine, you may have fuel consumption and reliability stats that may win over even the most fervent four-stroke enthusiast. With the combination of EFI and carbon pistons potentially being offered in concert to the current line of Hirth experimental engines in the near future, Matt notes there has been discussion of a new warranty policy to accompany these teamed options. Without stealing Matt’s thunder, keep an eye out for one-year full warranty with no hour limit and a three-year prorated warranty, with a time between overhauls (TBO) in the arena of 1,000 hours when these features are purchased in concert.

Due to the contractual nature of the development and specificity of each engine to its application, the marketing and reselling of existing Hirth heavy fuel AADI engines for homebuilders is not allowed. There are currently no plans to develop an Orbital AADI gasoline or heavy fuel engine for the manned aircraft market due to the high insurance cost and liability concerns with supplying fuel system hardware into this market. Orbital is focusing efforts with Hirth on unmanned applications only.

But the story does not end here; in fact, other possibilities exist on the horizon for Hirth engines. For instance, charging in the form of turbocharging post the tuned exhaust is a possibility up to 0.7 bar, via research conducted at Wright-Patterson AFB. When asked if a six-cylinder engine could be developed from the existing flat four architecture, Matt noted current bearing journal diameters might not be suitable for that configuration. If a six-cylinder were to be developed, the configuration of interest would be that of a V-6.

Much like WW II accelerated the development of the piston engine, so today the war on terror and its utilization of UAVs may be advancing the piston engine once again. In fact, one could argue as compared to other arenas where engines are pushed to their limits, UAVs may actually be the cutting edge. To many experimental, amateur-built aircraft enthusiasts, to have one of our own in this arena and making great strides is a source of pride. Perhaps soon the advances being made in this realm will start filtering their way back to the civilian side of the ledger so that experimenters can take advantage of these technological achievements as well.

The author would like to thank Matt Dandar and Jason Wright of Recreational Power Engineering for their time and hospitality extended during our visit to Rec Power at their facility in Tiffin, Ohio. I would also like to thank Orbital’s U.S. Representative Bob Schmidt and Hirth-Engines’ Siegfried Gobler for their technical review of this article, and many thanks to Bob Skingley for his assistance during the photo session of the engines. All pictures, unless otherwise noted, are by Anthony J. Liberatore and Bob Skingley.

For further information:
Factory authorized U.S. distributor for Hirth aircraft engines: www.RecPower.com
Hirth UAV Engines: www.Hirth-UAVEngines.de
More info on Orbital: www.OrbitalCorp.com.au
Orbital’s United States Representative Bob Schmidt: bschmidt@orbitalcorp.com.au

Hirth 3503 HF (heavy fuel) AADI 62 hp engine with attached 2 kW generator
The Hirth 3503 HF (heavy fuel) AADI 62 hp engine with attached 2 kW generator on the left side of the engine in this photo. Also, note some perspective of the engine’s size relative to the tape measure.

From the brochure: The 3503 heavy fuel is a water cooled, reed valve controlled two-cylinder, inline, two-stroke engine with electronic direct injection fuel system and Nikasil coated cylinders. It has one of the highest power-to-weight ratios available on the 60 hp engine market. Ideally all types of propeller applications with direct drive or gear reduction and all applications where the power-to-weight ratio is an issue. Factory-recommended TBO is rated at 1,000 hours at 75 percent power. The warranty of the crankshaft is three years.


 Technical data


Two-cylinder, two-stroke


 625 cm3 (38,1 cu in)


69 mm (2.72 in)


76 mm (2.99 in)


45 kW (62 hp) at 6500 rpm Specification with 194° F coolant

Max. Torque

67,5 Nm (50,0 ft-lb) at 6000 rpm


Air Assisted Direct Injection (Orbital)


CDI programmable


250 W, 20 amp, 12 volts


Liquid cooling


Oil injection


30 kg (66,0 lb) with exhaust and coolant

Start device

Recoil starter


Counterclockwise, view to output shaft


 5 (F 44) / JP 8 (Jet A / F 30)

Two Hirth horizontally opposed UAV engines
Two Hirth horizontally opposed UAV engines. The S1200 heavy fueled AADI 13.6 hp engine on the left, the 4102 EFI gasoline powered 8.2 hp engine on the right.

Jason Wright and Matt Dandar
On the left, Rec Power’s Jason Wright, and on the right, Rec Power’s President Matt Dandar.

Copyright © 2014 EAA Advertise With EAA :: About EAA :: History :: Job Openings :: Annual Report :: Contact Us :: Disclaimer/Privacy :: Site Map