@LeSving, FWIW my choice in the Lycoming O-320 line for a new plane that is being optimized for fuel economy would be an IO-320 with a Whirlwind constant speed prop. The 160 HP IO-320 with 9.0:1 CR burns about 7.5 US GPH in cruise (or 28 liters per hour) and Whirlwind makes the most advanced props currently available for any engine IMO.
The specific fuel consumption of the IO-320 as I mentioned above is probably a bit better than the 135 HP Rotax and it makes more power, but it is heavier. I think it’s plenty small enough for a compact installation (the twin Comanche cowling below being a good example), and air cooling makes it better in that regard. It’s also notably easy to own and maintain by a private owner with a budget, there aren’t any better in the same power range: 2000 hr TBO followed by $25K engine overhaul for another 2000 hrs and so on, the engine core lasts likely 6000 hrs.
The application for which I would choose the turbo Rotax instead would be a high altitude focused operation in a plane designed specifically for that, hopefully operated by an organization that could maintain it and replace it every 1200 hrs or so. UAVs have been using the turbo Rotax in that role since circa 1988, although less so today than in the past due to limited power and the ever growing requirement to carry increased weight.
Lycoming 320s are about €9000 -€10,000 more than the Rotax 915iS.
Average Fuel consumption in a APM41 Simba about 27l/p/h
Average fuel cosumption of the 320 in a Cessna 172 is around 30l/p/h IIRC.
An MCR4S with 915iS certainly looks to have better performance than both in all figures.
Silvaire wrote:
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.
gallois wrote:
What negligible capability. I can’t see much difference if one makes like for like comparisons.
Eg the newTecnams v Cessna 172s or PA28s.
A 140 HP 915 with turbo and CS prop will perform much better overall than a similar 150-160 320 with fixed pitch prop. CS and turbo obviously drastically increases the efficiency of the engine installation in an aircraft. But, they are also complex, expensive and adds maintenance time- and cost.
What negligible capability. I can’t see much difference if one makes like for like comparisons.
Eg the newTecnams v Cessna 172s or PA28s.
4 seaters with negligible capability though; not enough HP. Especially for the average person’s weight today.
There are more and more Rotax engines popping up in 4 seater aircraft nowadays.
IMO 3 reasons
1/ Engine weight
2/ Cost of engine both new and to overhaul
3/ Fuel consumption
In the USA cost of fuel has been historically unimportant in GA aviation. This is not the case in Europe and with Avgas on and around €3/litre it is a significant portion of GA hourly costs.
Diesels are great for fuel consumption but they are expensive to purchase and to overhaul if you can overhaul them. Plus they are heavy. The women at our club will not pull the DA40D put of the hangar if there is not going to be someone there to help push it back up the slope into the hangar at the end of their flight.
An electric tow is on the club’s shopping list as soon as there is spare money.
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.
IMO if the Lycoming 360 is the optimal engine for GA both now and for the forseeable future, GA is doomed.🙂
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.
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.
