Engine analysis and comparisons... apologies for long post

[mackattack]

On a completely different subject, I saw with interest that Cirrus swapped out the 6-cyliner Continental engine on the SR20 for a variant of the Lycoming 4-cylinder IO-390. No reported max cruise speed increase (still 155 knots) but the 215 hp engine will likely help climb performance some and be slightly more fuel efficient.  A lot of folks had wondered whether the next Cirrus generation would move to diesel engines/FADEC at least in part ... but so far, not yet.  That got me to thinking ... the diesels and the Rotax engines are the two groups of engines that actually burn fuels other than 100LL; how do they actually compare?  So I did a little research ... and it was surprisingly difficult to compare engines from posted specifications because manufacturers all do it a little differently.  But I pieced together a few things and feel free to comment if this topic interests you.  Or don't - it may interest only me!! *laughing*

I did not compare the avgas engines (I put in a couple facts from Wikipedia on the IO-390-X, a different 390 variant), just for information.  But what I found interesting is that the Rotax power-to-weight ratio completely blows away the diesel engines; they are all at least twice as heavy.  And it's also clear that the reported HP numbers only refer to peak power across all these engines, not max continuous power - only the Continentals report those as the same.  I think that KW at MCP may be a better comparison yardstick although max power is a good proxy for takeoff power.  Where the diesels shine, however, is torque - their torque is substantially higher than the Rotax engines.  Not being an aeronautical engineer, however, I'm not sure I understand how that correlates to performance.

Still, and while I didn't put the 912 ULS on the list, you can see why the Tecnam P2006T performs so well and is so light relative to the Diamond DA-42, for example. The latter's two diesel engines weigh 828 pounds, while the two 912S engines used in the Tecnam weigh only 249 pounds COMBINED.  The DA42 is a heavy plane, with a MTOW over 4400 lbs. The P2006 has a MTOW of just over 2700 lbs. 

Anyway, I find all of this very interesting in terms of how manufacturers source and select engines, and the Rotax stack up really well in my judgment.

---------------------------

Rotax 914:  115 hp, 100 continuous. 84.5 kw (which I think is at 115 hp).. 25 liters/hour at 5400, 26 at 5500. 106 ft-lb or 144 Nm of torque. 1211 cubic cm of displacement. Weight – 141 lbs.

Rotax 915iS: 135 hp, 120 continuous. 100 kw at max (guessing it’s a max continuous power number). 1352 cubic cm. Fuel consumption not listed, neither is torque. Weight 182 lbs.

Rotax 912is:  100 hp max, 98 hp continuous. 73.5/72 kw. Weight 140. Torque 132 Nm or 97 ft-lbs. 1352 cubic cm displacement. Didn’t see fuel on data sheet but it’s lower than the 914.

Continental CD-135 (proposed for new Mooney 10T): 135 hp max and continuous. 99 kw. Best economy is 97 hp and 74 kw. Torque is 410 Nm and 302 ft-lbs. 1991 cubic cm displacement. Fuel is 15-20 liters/hour (4-5.3 gph). 295 lbs.

Continental CD-155 (proposed for new Mooney 10J): 295 lbs. 1991 cubic cm displacement. 155 hp max and continuous. 114 kw. 97 kw best economy. Torque is 473 and 349, respectively. Fuel economy is 18-22 liters/hour, 4.7-5.8 gph.

Austro AE300 (used in Diamond DA40 and DA42): 168 hp max, 123.5 kw. MCP is 114 kw (same as the CD-155, a little more takeoff power). Best economy 97 kw and 21 liters/hour (5.5 gph). Displacement is 1991 cm, same as the Continentals. 414 lb weight. Note – Austro makes the 180-hp AE330 that Diamond uses on the DA62 twin but that engine is not yet available to other OEMs and its specs are not posted.

Lycoming DEL-120 (not certified for civilian use; only used on military Gray Eagle drone): 205 hp max, 180 MCP. 134 kw at MCP. Fuel consumption at cruise is .36 lbs/hp/hour; at MCP – approx. 10 gph. 1200 cubic cm displacement according to avweb video discussion. At economy cruise, it’s .345 lbs/hp/hour, which is shown at 60% (of 205, I’m assuming), which works out to 6.5 gph. 360 lb weight.

Lycoming IO-390-X (390-B series now used in the SR20): 308 lbs, 157 kw max power.  390 cubic inches displacement....

[jetcat3]

Love this! I literally think about this almost everyday haha. Great write up and very interesting comparisons! It will be exciting to see where Rotax goes in the future.

[mackattack]

Thanks! I obviously find it interesting too! I think if you have to have a diesel, the CD-155 is presently the best choice overall given weight, Mac continuous power output and fuel economy. The 915iS will outperform the CD-135 slightly and is 100 lbs lighter than the 135/155. Particularly if the 915 actually can do 127 max continuous power the. It really stacks up well to the Continental diesels.

The next couple of years will be very interesting with the new Part 23 consensus standards and all these choices. Avgas really does not appeal to anyone outside North America.

[cole505]

Awesome! Write up! You should be a columnist 👍🏻✈🇺🇸😎🐾 very nice work......

Ray & Lucy

[mackattack]

Thanks Ray! I just think it's interesting to understand the engines that will drive future platforms ... you can see how the weight gap between Rotax and normal engines, and normal engines and diesels, along with a big HP gap with the commercially available diesels, will drive everything from whether we will see a 915iS-powered LSA (I think not) to whether Cirrus will produce a diesel or FADEC airplane (Not for a while yet).  For example, it's really interesting to me how the Austro 300 seems to be a stronger engine (170 vs. 155 HP) but in fact, when measured in terms of KW, the two are essentially indistinguishable (although the torque numbers for the Austro were not published) and the fuel consumption is essentially the same too.  But the Austro weighs an extra 120 lbs ... 

Cheers!

[Cluemeister]

I can't imagine any of those engines would go in an LSA, unless the LSA weight limit is pushed to 1700 lbs, (which the EAA says is one of their major pushes this year).

But the market is hungry for aircraft that is as fast as an SR20 but at a lower price point imho.

[trainpilot]

I think civilians should push Rotax and Bombardier to market their two-stroke direct injection diesels for more applications. The E-tec technology uses high-performance piezo injectors and spark plugs in gasoline and diesel versions. These engines are so versatile they can run on gasoline or diesel. Evinrude (BRP) makes a 50 hp diesel outboard for the navy with a recoil (pull-rope) starter. I owned a boat with a 60hp gas version of this engine. What a great motor! Rotax makes a 2-cylinder snowmobile engine of 850cc that develops over 150 horsepower. It weighs less than 80lbs w/o gearbox. I have no doubt that it could be run on diesel with a similar power output. They run very clean. Since the fuel is injected directly into the combustion chamber, very little oil is injected into the crankcase and power valve assemblies. It is not diluted by the fuel. As these engines are, by nature, dry-sump and fuel injected, they could be operated at any orientation. BRP gets about three times as much for their diesel outboard version, but I rather suspect there are only minimal differences in the engines, and that it's mostly a matter of programming the injection and ignition for the particular fuel. I think that if they were better promoted, they could sell more than enough engines at a reasonable price to make up for letting go of the "military markup."

[Ed Midgley]

To put things in perspective, my 914 with 1211cc's and 115hp is the equivalent of am IO550 Continental putting out 1000hp. The Continental would explode in a three seconds.

 

Ed

[jetcat3]

It's simply amazing what kind of output Rotax gets out of their engine's footprint.

[Cluemeister]

When my mom saw my 914 for the first time, she said "that little engine is going to get your plane in the air?"

[mackattack]

They are amazing engines. But I don't know if I'd want to fly for an extended period of time behind a two-stroke power plant, regardless of horsepower ... those are not particularly smooth engines generally. I've seen a few in operations (e.g., some powered parachutes at my airport) - would not want to do a long cross-country behind (or in front) of one ...

Still, no question but that the Rotax engines have the best power-to-weight/volume efficiency in the entire industry!

[rainstorm]

Great post. I am still pursuing the forum and appreciate the data. Here is the detailed data on the 912iS with the sport upgrade:

www.rotax.com/files/project/userdata/downloads/rotax_broschuere912iS_SPORT_FINAL_18032014.pdf

The 912iS was projected to save 20% in fuel over the ULS but in flight tests at a flight school on a P92 tricycle gear airplane it came it at an impressive 30% reduction of fuel usage in a 250 hour flight test on the same exact airframe. It is particularly interesting as the P92 and P2008 are so similar and this translates directly to range increase of over 30% . This is the link to that video:

www.youtube.com/watch?v=mgrP4JkSUsI

This is a promo video by Rotax about the 912iS but covers the Rotax aircraft engines decently. Of note, two stroke engines are not likely to come from Rotax for aircraft due to emissions, which in Europe are something you get taxed on and the efficiency of the 912IS makes it by far emit the least pollutants. While this may not be as important in the US market we are a just a piece of the small aircraft (LSA level) market globally. For all of us lucky enough to fly Rotax and Tecnam they are both likely to be around for the long haul in a very turbulent aircraft market.

www.youtube.com/watch?v=azXdDXWpZ_I&feature=youtu.be

[mackattack]

Thanks - just in discussions on the forum, the range difference between the 912iS and the 914, for example, is apparent and significant. However, the 914 also keeps full power up to 15,000 feet which makes a big difference where density altitude or climb-to-cruise is important... definitely a player's choice among those two engines. The ULS has the advantage of being much lighter and less expensive, but no performance advantage that I can see.

The 915iS should be very interesting indeed.

Cheers