This compilation began as an attempt to list all horizontally-opposed aircraft engines (HOAE) that have been built in the United States. It has been extended and now includes engines built by non-US manufacturers, as well as automobile engine conversions. There is more work required to enlarge upon the information already presented and the compiler is continuing the effort.
Both two-stroke and four-stroke cycle engines have been included. The material here has been obtained exclusively from published sources. There has been no direct input from the engine manufacturers other than to gather information from their advertisements, material released to the aeronautical press, and their Websites (to which links are given both in the individual company listings and in the External Links summary listing). Horizontally-opposed engines often are referred to as "flat" engines, or "boxers" because of the opposing motion of the pistons. These latter descriptions generally will not be used here.
Many of the companies that have produced engines of this type have undergone several changes of ownership and name. The historical "Big Three" of US HOAE development and production, namely Continental, Franklin and Lycoming, are uniformly described by those names. Their corporate ownership and preferred contemporary names are discussed in the preliminary paragraphs for each of them.
An important reason for including HOAE from all of the world is the increasing globalization of the aircraft engine business. Early examples were engines developed in the US, but manufactured elsewhere under license or under foreign ownership. Conversely, engines developed in non-US countries were licensed to US manufacturers. Special note should be taken of the Rolls-Royce engines developed and/or produced in conjunction with Continental under a broad agreement that began in 1960 and lasted through 1980. These R-R engines are included here under the Continental listing with details of the design, development, and production cooperation between the two companies. Also, many of the Franklin engine designs and type certificates were sold in the mid 1970s, eventually to WSK-PZL in Poland. Production of some engine series continued in Poland until 2002, when WSK-PZL was purchased by Pratt & Whitney (Canada) and Franklin engine production was ended. The engines built in Poland were exported worldwide and were built under FAA Type Certificates.
For each company, the listings are in the order of increasing engine displacement. The basic US DOD method for designating horizontally-opposed engines has been used for uniformity. With engines that do not have military designations, the basic idea of those DOD designations has been used. Namely, the displacement (in cubic inches) is rounded off to the nearest multiple of five; e.g., the Continental C-75 and C-85 with displacement of 188.0 cu. in. are listed as the O-190. Engines with displacements generally (but not always) less than 120 cubic inches are rounded off to the nearest cubic inch. The only exceptions to this are various VW conversions, the Continental A-40, and the Franklin 2A-110 and 2A-120, the latter three of which are rounded off to O-110, O-110, and O-120, respectively. In all cases, the manufacturer's designation is given as well. Engines built by Continental, Franklin, and Lycoming since the 1960s generally follow the DOD format. Lycoming has used this designation system from the beginning of their HOAE development. Continental designations were initially based on engine horsepower instead of displacement. Franklin used both displacement and horsepower at various times. Franklin designations through WWII were by displacement. In the early post-WWII years, horsepower was used, but in the 1960s, Franklin designations by displacement were resumed and continued to be used for WSK-PZL production.
Most of the HOAE engines are air-cooled. However, liquid-cooled HOAE have been developed since about 1911 and are denoted uniformly in the text with an OL- prefix, e.g., the Continental TSIOL-550 (Voyager 550) series. For manufacturers other than Continental, use of the OL- prefix is the compiler's convention and must not be construed as official in any sense.
Continental has also begun using the OF- prefix, e.g., the IOF-240, for engines with Full Authority Digital Electronic Control (FADEC). Again, use of the OF- prefix for the products of other manufacturers is strictly the compiler's convention. It should be noted that FADEC is also used by various manufacturers as the acronym for "Full Authority Digital Engine Control" and "Fully Automatic, Digital Engine Control".
The horsepower range for each engine type has been obtained from the FAA Type Certificate Data Sheets (TCDS) information available online for engines with US type certificates. Otherwise, the data are from the published sources referenced in the compilation for each engine. The maximum continuous rated power is used for the engine performance; the short-time maximum take-off power is also listed, if different. The FAA TCDS can be found on the FAA Web Site. After entering the web site, select and click on "Type Certificate Data Sheets", and proceed from there to find what you are seeking.
The type-certificate dates given in this compilation are for the earliest certification for the specific model series under that TC Number; e.g., O-235. For those engine series that have been developed over many years and are still in production under the original TC, the early production models may have had their approval canceled many years ago. The cancellation information is sometimes given on the FAA TCDS. A prime example is the Lycoming O-235 series, which was initially certified in 1940 and still is in production. Later models are still produced under the original, but greatly amended, type certificate. Some of the early models have been canceled from use in aircraft which received type certification after the cancellation date.
The Tables listed at the end of the Contents are also cited here for added convenience to the user of this Compilation. The Continental, Franklin, and Lycoming engine families provide a quick illustration of how these manufacturers have developed and expanded their products over the years. Moreover, four tables of all four-stroke and two-stroke cycle engines are listed, by increasing displacement, at All Engines (4-S): (O-20 through O-109), All Engines (4-S): (O-110 through O-195), All Engines (4-S): (O-200 through O-1815), and All Engines (2-S). These facilitate comparisons among all the engines for which displacement data are available. Horsepower range, chronological information, and the numbers of cylinders are given for added utility. Finally, five tables of all four-stroke and two-stroke cycle engines are listed, in chronological order, at All Engines (4-S): 1905-1930, All Engines (4-S): 1931-1950, All Engines (4-S): 1951-1980, All Engines (4-S): 1981-present, and All Engines (2-S). These facilitate a glimpse of the development history of all HOAE.
United States Army Air Force (USAAF) and United States Air Force (USAF) designations of the various pre-1950 engine models are from data in the Model Designations of USAF Aircraft Engines, USAF Air Materiel Command, 1/1/49; revised 1/1/50 (referenced in the compilation as AMC). This useful document has been scanned by others and can be found on the Website of the Aircraft Engine Historical Society under their "Reference" heading. All of the USAAF and USAF designations have an odd final dash number. Similar information for the even final dash numbers used by the United States Navy (USN) are as yet unavailable.
This compiler wishes to reiterate what has been said elsewhere in the various engine-model write-ups. Namely, users of this compilation should go to the FAA Type Certificate Data Sheets for US type-certificated engines to obtain data on specific engine versions of interest. Different sub-types of the same engine can have features such that the same rated horsepower can be achieved at different rpm, or different hp can be achieved at the same rpm, for example. These result from different versions having different fuel octane requirements and/or different compression ratios, etc.