Rotax engines - more efficient than Lyco or Conti?

gallois wrote:

They are just a lot more efficient than their 1950s counterparts.

How do you measure efficiency in an aircraft engine? (I’m seriously wondering). The only efficiency I can think of from the top of my head is thermodynamic efficiency. Everything else are just ratios. But super high thermodynamic efficiency doesn’t mean all that much if a 100HP engine weighs 1 ton Clearly HP per kg is an important ratio. So are HP per volume (size of the engine), HP per liter (of burned fuel) and HP per kg of burned fuel, HP per needed cooling area, as well as HP per needed mass flow of cooling air and probably several other factors. HP per liter of burned fuel is proportional to the thermodynamic efficiency, and just as meaningless on its own. For an electric aircraft, that ratio goes to infinity

Brake Specific Fuel Consumption is how engine efficiency is measured, with units in e.g. grams of fuel burned per kilowatt-hour, or equivalent. The link shows that the efficiency of a Rotax 914 and Lycoming O-235 are almost identical.

Engine power density is not the same thing as efficiency. Volumetric and weight based power densities are measured in kilowatts per unit volume or mass, or equivalents.

Efficiency and the two power densities often compete in design. Efficient designs tend to heavier and bigger. Light, compact engine designs tend to be inefficient, think two stroke engines. The Rotax is pretty good at power density while maintaining reasonable efficiency, although the liquid cooling system components effectively reduce the power density of the overall installed package.

At the aircraft system level, light, compact inefficient engines tend to promote efficient, high performance airframe designs, creating another set of competing trade-offs between engine selection, aircraft performance and practicality.

What is interesting to me is how much better some aircraft designs are at maneuvering though these trade-offs and others (like serviceability over a long service life) to achieve maximum effect in terms of ‘balanced performance’. Vans aircraft are very good in this regard.

One question that pops up in my mind when reading your post, Silvaire, is if Rotax may have hit a sweet spot with the 915iS? I mean, we’re talking horse power close to an O-320 but with a significantly lower weight of the powerplant.

If I got your power density concept right, it’s about pairing an efficient enough and, most of all, light engine with an equally light and slick airframe. Sounds simple enough. The thing is, most ultralights would seem ridiculously overpowered with the 915iS. Heck, my 655 kg Europa seems almost overpowered with the 914. So what can be done with those 41 extra horses (or even 61 extra compared to the original 912)?

I keep returning to the Sling TSi as an example of what can be done with the 915. Four-seat, decent payload, cruise speed and consumption that beats all spamcans and even most “Arrow” level aircraft (retractable UC, CS prop, bigger than O-360 engines). I just wish they would make a certified version…

Yes, the Rotax 915iS is an electronically fuel injected, turbocharged and intercooled engine, as well as being geared like a 912, with resulting relatively high power density. That’s a good thing especially since it maintains power at altitude. However it also makes only 135 HP continuous due to small displacement and it’s obviously much more complex than a typical low power aircraft engine. That’s not a problem until you have to diagnose and fix the thing when it’s 10 or 30 or 40 years old, then you might well care.

I understand fuel consumption isn’t outstanding but isn’t awful either, at 7-9 US GPH in cruise – being turbocharged means the 915iS has a relatively low compression ratio, which impacts efficiency to a degree. I’d guess a 160 HP-version IO-320 Lycoming is more efficient, with 9.0:1 compression and lower RPM, if heavier.

I wouldn’t choose to own and maintain one, but for a very expensive new aircraft like the Sling I could see the 915iS making sense, for their market. Neither the plane nor the engine appeals to my sense of enduring value but other people have other purchasing priorities and good luck to them.

It’s not the Rotax engine, it’s the low horse power and the light airframe it is used in.

Next step in cruise fuel burn is achieved with Diesel engines, that get real efficient in low power modes..

Silvaire wrote:

Brake Specific Fuel Consumption is how engine efficiency is measured, with units in e.g. grams of fuel burned per kilowatt-hour, or equivalent.

I know, but that has never been efficiency. It’s just a ratio that by coincidence is fairly proportional (probably inverse proportional in this case) to the thermodynamic efficiency (if the same fuel is used). Fill the tank with liquid hydrogen, and the BSFC will reach unprecedented values but the thermodynamic efficiency will not Uranium works even better for BSFC, by orders of magnitude, but won’t make a difference at all for the thermodynamic efficiency.

But that wasn’t the point. The point was to have the efficiency of the engine in the aircraft. It is an aircraft engine after all. BSFC could be super good. It doesn’t matter when the installation weighs a ton and produces lots of drag. Clearly less drag and less weight increases the overall efficiency. The ability to run the prop at lower RPM also increases the overall efficiency. A Rotax has lower mass per HP, it is less in size per HP, it can use a more efficient propeller. The conclusion can only be that a Rotax is a much more efficient engine in an aircraft. But how to put a number on it ?

In physical terms, efficiency is a measure of what you get out of the thing in a specific unit of measurement compared with what you put into the thing, using the same specific unit of measurement. Usually it’s power or energy, but it could be anything. With aircraft engines, I don’t think it’s possible to get a specific measure. Partly because there will be no agreement of what should be measured. It’s probably also a combination of lots of things, and the relative importance of those things changes with changing environments. It’s more like a Darwin-efficiency thing IMO, survival of the fittest. The company selling most engines, does so because it has a product that simply fit well in the environment at that specific period in time, more so than engines from other companies. A 912/915 with two more cylinders will have everything from 120 to 200+ HP, and could potentially replace every O-320 to O-390. It wouldn’t be very difficult for Rotax to put on two more cylinders, and it certainly could make all C-172s in the world much more efficient than today (lower drag, less weight, better props, and all of them could run on 10% ethanol mogas ). But would those engines be efficient in terms of surviving in the market? Probably not at the moment, but it could change fast.

The company selling most engines, does so because it has a product that simply fit well in the environment at that specific period in time, more so than engines from other companies.

Indeed, practically matters. Design for service and widespread parts and service support can create a practical environment. However, in an open market for engineered products it’s not a ‘specific period of time’ plus inspiration, luck and fashion that creates that market, it’s a long period of development and optimization that includes a lot of people and companies in forming what trendoids like to call an ecosystem. Short lived genius is only the start of a very long process.

would those engines be efficient in terms of surviving in the market? Probably not at the moment, but it could change fast.

If I had a C172 (or a Tecnam P2010) with a six cylinder liquid cooled EFI turbocharged geared Rotax making 200 HP, I would be looking to replace it with something more like a four cylinder O-360 Lycoming.

I laid out above in post 5 in a series of simple statements in how design is a series of trade offs all the way up to the ‘system’ level, as you’ve reiterated. System Engineering as a discipline attempts to find an optimum solution to the top level objectives by working the interactions of the trade-offs at component and subsystem levels and evaluating their combined impact at the system level. However IMHO it generally doesn’t work unless there is a well defined single problem to solve with a huge budget – SE as a discipline was created to manage e.g. the Apollo program. Otherwise the optimum product solution gets worked out in the open market by competing products and design iterations. Vans Aircraft seems to have found an optimized volume GA product by that method, over a period of many decades. It amazes me that the result is at once conventional in appearance while being exception in all around performance, practically and cost.

Another example is how every airliner looks almost identical after 65 years of development since the 707, which was the initial push in what turned out to be the right direction. However the ones we now travel on are much optimized by comparison.

Of course… the optimal engine is an IO540-C4

But seriously, to a first order approximation, all petrol engines make the same power when peak-EGT or LOP, for a given fuel flow and a given compression ratio.

The CR is the biggest thing you can play with which is why diesels are so good; my 2 litre VW fuel tank lasts long enough to cover the duration of the longest likely UK fuel tanker strike. Probably wouldn’t stretch for long enough to cover a French fueller strike however

There was some work being done to burn petrol at a 15:1 CR; not sure how far that got. Petrol avoids the diesel particulate issue with its EGR / DPF problems. But we will never see any such developments in GA.

The lower CR is why turbo engines do less MPG e.g. a TB21 has less range than a TB20.

Rotax engines do well because they cover the small 2-seater market which Lyco/Conti don’t really address. They also withstand long periods of non running much better, which is vital for a market where in Europe most activity shuts down for several months every winter.