20th Century power

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Posthumane
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Re: 20th Century power

Post by Posthumane »

Apart from the many automotive powered homebuilt/experimental aircraft, there are also "in between" solutions for modernizing aircraft engines. Some of the major innovations in automotive engines have been the ancillaries - fuel injection and electronic ignition systems, and these can be retrofit onto existing lyc/cont engines in the non-certified domain (E-mags/P-mags, various EFI systems, etc). Mechanically, modern car engines are not that much different than older car or aircraft engines. Better flowing head designs and higher compression ratios due to better fuel/ignition controls have allowed higher power outputs from smaller displacements, but aircraft engines are typically thermally limited rather than airflow limited. You could make a high compression, high flow lycoming (like some experimental guys do) and the torque will be significantly higher than the stock model, but the heat dissipation requirements and stress on the engine will be higher which certainly increases your chance for mechanical failure.
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lupin
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Re: 20th Century power

Post by lupin »

Ame213 wrote: Mon Sep 10, 2018 4:24 pm One thing to consider is that an aircraft engine is designed to make its max horsepower until overhaul reliably, you car is on average running at what? 1/3 to 1/2 hp? Try running any car let alone a high compression turbo engine at full throttle for 2000 hours and see how long it lasts then. A horizontally opposed air cooled engine is the by far the best option for an aircraft piston engine, there is a reason they haven't changed much in the last 60+ years and I'm sure it wasn't for a lack of trying.
As quoted above, an automotive engine in an auto at 100km/h is likely producing 20-30Hp. If you ran that same engine at 75% power continuously, they probably would not last. Car engines only see max power for very short periods of time. If you are looking for high HP with a continuous duty cycle, you would be looking at the industrial Diesel engines and then the weight issues would be your problem.

The other issue (mentioned in an other post) is RPM. Since you don't want the prop to spin so quickly you need an engine capable of producing HP at low RPM or a gear box.
With an engine spinning below 2700 rpm you really limit HP.
With a gear box, you add weight, complexity and have all sorts of vibrational harmonics issues unless the proper engineering is done.

Look at the homebuilt (experimental)aircraft world. They usually use Lycoming, Continental or Rotax engines (4 stroke) due to reliability. Any alternative engine drops the resale price of the aircraft. It's not ideal to be the one testing reliability while flying when you are operating single engine aircraft.
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Posthumane
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Re: 20th Century power

Post by Posthumane »

I think the idea that an automotive engine won't last at high continuous power is a bit of a red herring. The OP talked about derating an automotive engine and running it at 50% original rated power for takeoff and 40% for cruise. That's the kind of power used cruising at high speed on a highway (like 150km/h) and any modern engine will handle that just fine.

Even if you're not using a prop speed reduction unit, you still need something to handle the propeller thrust loads. You can't just bolt a propeller to an automotive crankshaft, as car engines do not have a thrust bearing.

To make power you need airflow. You get that either through displacement, RPMs, or high cylinder pressure (turbo charging). High RPMs require a gearbox (not insurmountable) and high cylinder pressures require very careful fuel and ignition management to prevent detonation, which is why many of the traditional aircraft engines use high displacement for their modest power output.

I suggest if you want to use a Subaru engine in your aircraft, use one of the PSRUs on the market and make it run on higher RPMs rather than relying on excessive turbocharging at low RPMs, as it's a fairly proven system. Stock subaru motors don't make peak torque until well north of 3000 RPMs.
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lupin
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Re: 20th Century power

Post by lupin »

There's an interesting write up about the Subaru engines at the following link.

http://jdfinley.com/what-happened-to-th ... ft-engine/
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rigpiggy
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Re: 20th Century power

Post by rigpiggy »

Waaaayyyy back i read an article that equated longevity to piston speed. To get power use plan pressure, length of stroke, area of piston and number of power strokes. To make more power move more air, or increase the efficiency. Figure out the number of rpm a 2.2l subie needs to make 180 hp, then figure out relative piston speed vs a lyc O360
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anofly
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Re: 20th Century power

Post by anofly »

I think you will find that Lupin is on to it , as are others. the duty cycle of a car , while it may have "280" hp available , when it is tooling along the 401 (or pick your favourite hiway) the average car is using about 25-35 hp continuous, a lycoming say 0360 of nominal 180 hp is using 135 hp and it will do it all day every day....
the marine world is a world sort of similar to aviation, boats do not loaf along on a fraction of the available horsepower, they work hard , all the time the boat is moving (picture a 30 foot sea ray at 23 knots) those engines are converted "car engines" they are almost always getting overhauled at around 650 to 700 hours. If your car needed an overhaul after 700 hours (it would be every 2 years based on 6 hours a week. that would not be acceptable.
the computers on many modern cars shows the average speed , i have never reset mine, it sits at 56 km/hr, and that is nowhere near 135 hp all day...
what we are all on about , i think called specific fuel consumption, pounds (or kilos?) of fuel per hour per hp (watt?)
that is the only reason car engines look attractive, but i think you will find that if you drove your subaru at 135 hp all day it would not last like you think it might, and it would burn way more than 2 gallons an hour (30 mpg at 60 mph?). because at 60 mph it is not using 135 hp or anything near it.
what would be interesting is if you drove your subaru on an empty highway at 110 knots for a few hours, how much fuel would it burn then? (at cessna speed)
that asked, it probably has a better specific fuel consumption than a lycoming.
diesels have better specific fuel consumption than air cooled gasoline engines, they are expensive, and unless you fly a whole lot, you will never save enough in fuel, to pay for the overhaul difference, let alone the purchase price premium that currently exists.
i think in general , converted car engines do not go the distance , when put in planes... there may be exceptions....
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Posthumane
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Re: 20th Century power

Post by Posthumane »

Well there's a number of Subaru and VW conversions flying around with significantly more than 1000 hrs on the original engines. High continuous power outputs don't actually increase the wear on the engine significantly, as long as the heat is properly managed. But even if you had to replace the engine every 700 hours, a full EJ25 long block can be purchased for less than $5k. No different than all the planes running 2 stroke Rotax engines. The big cost is in doing a proper conversion, not to mention the weight.

Fuel burn in auto conversions is comparable to fuel burn in an aircraft engine designed with electronic ignition and FI. BSFC is a bit better than the old Lyc and Contis, but not orders of magnitude better. A 165hp Subary conversion running a typical 65-75% cruise will burn maybe 10-20% less than an equivalent powered lycoming with fixed timing and a carb. Having P-mags with variable timing and running lean of peak with tuned injectors will close up that gap to almost nothing though.

As was mentioned, piston speed IS a good indicator of engine longevity when discussing piston wear. And EJ25 making 160hp at 5600 rpm and 79mm stroke has about 60% higher piston speed than a Lycoming O-320 making 160hp at 2700rpm with a 98mm stroke.
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