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What are some of the differences between piston engines used in aircraft and automobiles? It seems aircraft engines are much more expensive, I imagine some of that cost must be due to the more extreme environment (temperature, air density, roll angle) that an aircraft engine is expected to run in.

In particular:

  • Are aircraft engine parts expected to be less likely to fail?
  • What is done to allow the engine to function at extreme temperatures / altitudes / air densities?
  • Do changes have to be made to allow a piston engine to function while upside down (in a roll)?
  • What components are different on an aircraft vs automotive piston engine (magnetos vs. spark plugs, oil system)?
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The engine in a typical light airplane (say a Cessna 172 or a Piper Cherokee) has a lot in common with the engine in a classic 1960s VW Beetle (Type 1): Both engines are horizontally opposed four-stroke four-cylinder spark ignition gasoline engines. Their parts even have similar metallurgy, and broadly similar failure rates.
In fact if you remove the gearbox and install a magneto ignition system you basically have one of several "Volkswagen aircraft engines" that are popular in the experimental market, and have proven to be very reliable if well-maintained.

Certificated aircraft engines (the Lycoming and Continentals you would find in our hypothetical Piper or Cessna) have a few other features not found on a Volkswagen engine: They have dual ignition systems (two magnetos, two spark plugs per cylinder) for redundancy in case one fails, and an oil sump designed to hold at least twice the capacity of oil needed for safe operation. They typically also have a manual mixture control to allow the pilot to lean the fuel/air mixture as they climb (the VW Beetle generally didn't get too high above sea level, and when they did the local mechanic could adjust the mixture in the garage because those cars would probably stay at a relatively high altitude for most of their life - aircraft have an annoying habit of climbing and descending a lot, so either a manual mixture control or an altitude-compensating carburetor is necessary).

So what accounts for the price difference? Three things: FAA Certification, Volume, and Liability.

FAA Certificated engines cost quite a bit of money: The manufacturer needs to thoroughly document their design and manufacturing practices, and ensure the engine meets the design requirements and can pass the tests described in FAR 33. Doing (and documenting) all of this to the FAA's satisfaction is not a small burden, and it adds to the cost of the final product.

Volume is the next problem: As a reward for complying with Part 33 and getting your engine certified you now have the opportunity to sell your engine. To a very small market. There are far fewer piston aircraft than cars, and although piston aircraft engines are often "worn out" over the life of an airframe most owners overhaul their engines rather than replacing the whole engine outright. As a result the volume of new engine production is relatively low (and with relatively brisk competition in the overhaul market the manufacturer isn't even guaranteed a substantial amount of that business). As with any other product, when volume is small price goes up to allow a profit.

Liability is the last (and probably largest) factor affecting pricing: Jane Q. Pilot is flying along in his brand new just-off-the-production-line Cessna with her two kids and the family dog when the engine fails. She is unable to make a successful landing, and everyone dies, whereupon her bereaved and distraught husband John immediately sues the mechanic who last worked on the plane, Cessna (who made the airframe), and Lycoming (who made the engine).
Being ready to deal with this kind of high-profile and high-cost event requires a substantial legal team (the cost of which is built into each engine sold), and as the manufacturer is accepting a substantial risk the price gets bumped a bit more to provide a balancing reward (profit).


So to answer your bullet-pointed questions which I largely ignored in the explanation above:

  • Are aircraft engine parts expected to be less likely to fail?
    We may have this expectation given the price tag, but in reality they're probably about as reliable as any other well-maintained air-cooled 1960s engine design. A modern car engine has a number of design and technological advantages over the typical piston aircraft engine, and if we equipped them with dual independent ignition systems and gave them the kind of maintenance aircraft engines get they may well beat aircraft engines in terms of reliability. (Of course a modern car engine also weighs a lot more than a typical aircraft engine, so some money would have to be spent making it lighter without sacrificing that reliability.)

  • What is done to allow the engine to function at extreme temperatures / altitudes / air densities?
    Surprisingly little: The addition of a mixture control for the carburetor, dual independent ignition systems, and a healthy weight consciousness pretty much covers it.

  • Do changes have to be made to allow a piston engine to function while upside down (in a roll)?
    Yes (the fuel and lubrication systems for need to be designed to handle zero / negative G loads for an extended period of time), but your typical piston aircraft engine isn't designed to function while upside down: Aerobatic aircraft engines are a slightly different beast, and they cost a little more than their non-aerobatic counterparts.

  • What components are different on an aircraft vs automotive piston engine (magnetos vs. spark plugs, oil system)?
    The biggest difference between a 1960s VW engine on a classic Beetle and a 2015 Lycoming O-320 is the dual magneto ignition system. The rest of the parts are largely identical, or at least analogous: Spark plugs look like spark plugs, cylinders look like cylinders, carburetors look like carburetors, etc.
    Aircraft engines are also designed for component swaps: Cylinders can be easily unbolted and replaced for example, as can the magnetos, the oil pump, the carburetor, the starter, etc.

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    $\begingroup$ As a 1960's VW engine veteran I found this answer great because I've often thought the same, but never dared say it in the company of a pilot! $\endgroup$ – Dan Jan 7 '15 at 23:39
  • $\begingroup$ @Dan If I were to build an airplane myself I'd probably go with one of the many VW conversion engines out there - there's very little modified in those designs, and having some experience with VW engines myself I'd honestly fly behind that before some of the newer whiz-bang experimental engines. Even though the newer engines have much more advanced technology I like knowing that I could field strip and reassemble the engine if I were forced to. $\endgroup$ – voretaq7 Jan 8 '15 at 2:44
  • $\begingroup$ Just as an anecdote, HondaJet claims to be using the first newly designed, certificated engines in the last almost 3 decades... $\endgroup$ – CGCampbell Mar 31 '16 at 0:30
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    $\begingroup$ I'd add a couple of quibbles to this. First, modern cars (pretty much everything built in the last quarter century) don't have carburetors, they have electronic fuel injection. Second, aircraft carbs don't look much like auto carbs. They're typically updraft carbs located on the bottom of the engine, while auto carbs are generally downdraft, mounted on top. Unless you're into '60s British sports cars, which usually have side-mounted SU carbs. $\endgroup$ – jamesqf Nov 29 '16 at 5:03
  • $\begingroup$ @jamesqf Thousands of small engines are still produced every year with carburetors (lawnmowers, chainsaws, generators), and up through 2011 all NASCAR engines were carbureted as well (they switched to fuel injection in the 2012 season). The position of the carburetor doesn't matter: It's a venturi, a throttle plate, and ports through which fuel is added to the air stream. It could be updraft, downdraft, or side-draft, but the direction the air moves and the plumbing that gets the fuel to the venturi & the mixture to the cylinders doesn't change the core technology. $\endgroup$ – voretaq7 Nov 29 '16 at 9:08
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To add some more aspects to voretaq7's answer:

Airplane engines have a very different operating point than car engines. This makes it impossible to simply plug a car engine into an aircraft.

  1. Aircraft piston engines have a max. RPM of 2700. Since they are normally directly coupled to the propeller, a higher engine speed would reduce propeller efficiency. Light aircraft are now fitted with small geared piston engines which reach up to 5000 RPM, but your regular Cessna or Piper will still have the conventional, ungeared engine. Note that the now almost 80 year old Rolls-Royce Merlin was run at 3000 RPM.

  2. Aircraft engines run near their rated performance for hours. Cruise setting is normally between 65% and 75% of maximum power, whereas your regular car engine will see this kind of power setting only for a few seconds at a time. Even when driving on the Autobahn, a car engine will maybe deliver 30% of its maximum power or less most of the time.

  3. Car engines have now plenty of computer control: The ignition point, the mixture and (in some cases) the valve timing can be adjusted for the specific operating point. All these parameters are either fixed, or, in case of mixture, need to be manually adjusted in aircraft.

Points 1 and 3 mean that you cannot compare car and aircraft engines of the same rated power output directly. The higher engine speed and the better adaption translates into much higher power ratings for car engines, but point 2 means that the rated power cannot be delivered when used in an aircraft. If a car engine is run near its rated power for an extended time, it needs a better cooling system. Even then, its components are not designed for these loads and will fail earlier than the corresponding parts of an aircraft engine.

However, due to the much higher number of car engines in operation, we have much better data on the long-time behavior of all parts and much more R&D effort is spent on optimizing them. When run as intended, car engines are now much more reliable than ungeared aircraft engines, even though they have a higher parts count (see point 3 above). Basically, ungeared piston aircraft engines are living museum pieces. The much smaller market size for new aircraft engines is one reason, but the other is the very extensive certification effort which is needed before a newly developed engine can be used in an aircraft. This is necessary to ensure a low failure rate, but has at the same time effectively stopped technological progress to trickle down to aircraft piston engines for half a century.

Electronic ignition systems allow any modern car engine to run upside down, but the oil system needs gravity for the oil to be collected and kept in circulation. For a short time, modern car engines need no modification to run upside down, only the fuel system would need to be adapted.

The temperature extremes for which car engines are tested are more severe than those of airplane engines. Car engines are expected to work at all elevations, so they are designed for similar density variations. Only in extreme cases will an aircraft engine fly higher than the elevation of the highest roads.

  • Regulation

  • Lawyers and

  • Market size

drive the price of aircraft engines up. If they would be expected to work at the environmental extremes that car engines are designed for, they would be even more expensive.

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    $\begingroup$ It's my understanding that the Subaru EJ25 (2.5 liter, flat-4, optionally turbo-charged) is very popular in the GA world. It is a fairly modern, electronically controlled motor that will happily run to around a 7500RPM red-line on the ground. What accounts for the differences between those specs and the ones you specify, and how do those who use them in aircraft account for it? (Or do I need to make this a whole new question?) $\endgroup$ – FreeMan Jan 7 '15 at 14:38
  • $\begingroup$ The controller needs to account for the altitude to adjust the fuel mix, and the (water) cooling needs to be able to keep up $\endgroup$ – ratchet freak Jan 7 '15 at 14:53
  • $\begingroup$ @ratchetfreak, my use of "accounts for" was not not looking for a system to overcome the differences, but why is the EJ25 so popular since it seems to differ so much from Peter's description of an aircraft piston engine. I hadn't thought of your interpretation at all! Seems you hadn't thought of mine. ;) $\endgroup$ – FreeMan Jan 7 '15 at 15:04
  • $\begingroup$ @FreeMan: Power is proportional to RPM, so the same performance can be gained from less displacement. The engine becomes smaller and lighter, but now needs to be coupled to a gearbox to reduce RPM to something between 2000 and 2500 for propellers. Notice that I sprinkled "ungeared" all over the place in my answer. $\endgroup$ – Peter Kämpf Jan 7 '15 at 16:04
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    $\begingroup$ @voretaq7: The VW engine is even older than the Lycomings and Continentals, so it should not be a surprise if the RPM range is similar. $\endgroup$ – Peter Kämpf Jan 8 '15 at 7:28
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One point that seems to be missing from excellent answers provided so far is carb heat. Carburetor icing is a concern for most if not all carbureted aircraft. The small planes I've flown (Cessna 152/162/172) all have a manual carb heat knob. When you engage it, the engine's air intake is switched over to air that has been warmed by the exhaust. This is designed to melt ice that may be blocking the carburetor or to prevent the ice from forming in the first place.

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    $\begingroup$ Carb heat is also used on car engines. Older cars still need a manual change, where the inlet air is ducted over heated engine parts for the winter setting. Modern cars do this automatically. $\endgroup$ – Peter Kämpf Jan 8 '15 at 7:29
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No or very limited emissions control on the aircraft, at least compared to new cars.

Aircraft engines have a rough time with shock cooling. Cruising at 75% power for hours, then slam the throttle shut and descend, fast. Shock cooling can crack engines. Auto engines would not survive very long. It would be like running an auto engine at full blast on a dyno for a couple hours (as if an auto engine would survive that first step, LOL) and then blasting it with icy fire hose water, like taking hot plate out of oven, toss into icy dish washing water, it shatters. Even carefully engineered aircraft engines have problems with this.

With respect to fuel pump vapor lock on fuel injected engines, aircraft are about as bad as 80s cars. Its not really a significant problem under most conditions for most aircraft most of the time, but its sometimes pretty hard on a hot summer day to get liquid gas into the engine to start it. Ground vehicles work around the problem by having the refinery blend winter or summer gas, but planes can fly from the arctic to the tropics in hours, so its not so simple for planes.

Most aircraft have multiple, usually manually selected, fuel tanks. Many pilots have died by being overly cavalier about toggling from tank to tank in circumstances where a failure would be fatal. Very few cars or trucks have more than one gas tank. Fuel tank piping can be extremely complicated on larger aircraft.

Aircraft fuel is not blended with ethanol so water does not mix with the ethanol like it does in automotive E90 type fuel. What that means is part of preflight is draining a little (like a cup) of fuel out of the tanks, until no water comes out of the fuel tanks. You're not suppose to just dump that fuel out on the ground to evaporate, but ...

There is nothing like the ODB-II car standard for planes. There is no universal plug on all aircraft currently flying where all computer test sets ever made can talk to the engine computer about engine performance. Such things DO exist for specific engines and specific aircraft but a universal world wide standard does not exist for engine test sets and engine computers in aircraft.

Many aircraft engines are air cooled and its not unusual for performance stats to vary such that the cylinders in back run hotter than the cylinders in front because they sit in air heated by the cylinders in front of them. People used to water cooled car engines are sometimes surprised by this.

Often variable pitch props are operated by engine oil pressure. Usually car/truck transmissions use special transmission fluid in their transmissions and power steering/brakes. I think some (all?) motorcycles use engine oil in their transmissions. There do exist variable pitch propellers that use hydraulics not engine oil.

Car engines heat the car using circulating water. Aircraft engines, being air cooled, usually heat the aircraft using a heat exchanger gadget off the exhaust. Obviously a leak would be rather dangerous.

There is a stereotype that all cars use multi-grade engine oil, and all aircraft use single grade oil. That is strictly false but true enough, often enough, that it'll be claimed as fact.

(I edit to add in a meta commentary that all 2015 car engines are virtually the same, and the diversity level of aircraft and aircraft engines is dramatically higher. If you look at a random 2015 car, its engineering is probably roughly identical to any other randomly selected 2015 car. But seemingly all aircraft are different. So its easy to talk about car engines but hard to nail down aircraft engines)

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    $\begingroup$ Not all cars use liquid coolant - air-cooled cars have similar heat exchanger muffs around their exhaust for winter heat (the VW Beetle from my example being one such vehicle -- Fun VW fact, the heater channels in the Type 1 Beetle are also structural elements of the vehicle) $\endgroup$ – voretaq7 Jan 7 '15 at 19:11
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    $\begingroup$ Another point is that there really is no ECU/PCU in most aircraft piston engines -- this may change as emissions tighten and more manufacturers find FADECs worthwhile, though. $\endgroup$ – UnrecognizedFallingObject Jan 8 '15 at 0:44
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    $\begingroup$ Water-cooled car engines survive shock cooling much better than air-cooled engines. Porsche converted a car engine to aircraft use (the PFM 3200), which had water-cooled cylinder heads. It was very popular for glider tow aircraft, because it allowed a steep dive after release without problems. Car engines are also better in this regard! $\endgroup$ – Peter Kämpf Jan 8 '15 at 7:33
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    $\begingroup$ For a lot of aircraft, shock cooling is not the problem that a lot of folks make it out to be. See this, and this. In my years of operating and maintaining 320 to 550 sized aircraft engines I can confirm this as well. $\endgroup$ – J Walters Jan 11 '16 at 13:05
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An aircraft is pulled thru the air by the crankshaft of the engine, and automotive engines aren't designed to have any load pushing or pulling on the crank.
Most if not all aircraft engines have a big thrust bearing to handle this, so auto engine conversions use a reduction drive with a thrust bearing for pulling/pushing the aircraft, or if direct-drive change out the bearings in the engine with one designed to handle the forces involved.

Another major difference between automotive and aircraft engines is the power curve: The Horsepower and Torque value of an aircraft engine run almost parallel lines, whereas in most automotive engines the torque peaks in the low RPM range and horsepower in the high RPM range (for stop-and-go driving, varying speeds, etc.). An aircraft engine is run more like a stationary application like you would have with a generator or irrigation pump etc.
If an automobile engine were utilized in this fashion it would far exceed the life of most aircraft engines. Why? Air cooled verses liquid cooling. A Liquid cooled engine maintains a constant temperature at all times. An air cooled engine does not. You will not witness a dive from a high altitude in a general aviation aircraft where you chop the power and point the nose at the ground. If you were to try this There is a very good chance you would have some stuck valves when you tried to come back in with the power. Could turn into a bad day.

One last thing: I don't believe a modern well designed engine needs two ignition systems. In all my years of flying I never or have ever heard of any type of ignition failure. When a pilot is doing a run up before takeoff and does a mag check, the reason you get that 50 rpm drop is because that combustion chamber is not very well designed. You don't need two magnetos.
One thing is certain on any power plant be it auto or aircraft: the less moving parts you have, the better off you'll be. As stated before,the aircraft engine manufactures are held back by the cost of being STC'd from FAA regs. Which in turn hinders development. Many new ideas have come from the home built crowd.

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    $\begingroup$ You made a number of excellent points, but on the subject of the need for dual ignition I have to disagree with your assessment: I've heard more than one story of a magneto's timing slipping badly enough to make the engine extremely rough, and the ability to shut off the offending magneto is what allowed the flight to be completed successfully. I'm not sure I'd want to trust my life to a single-engine single-ignition engine... $\endgroup$ – voretaq7 Jan 8 '15 at 2:58
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    $\begingroup$ The dual ignition feature is not merely a redundancy issue, but is a necessary part of full flame front propagation across the wider cylinder bore of aircraft engines. The 50 rpm drop at rich mixture becomes much more noticeable (150-200+ rpm) at leaned out settings. Can fly is very different than producing rated takeoff power. Perhaps there could be a better design, but I'm not aware of one for a big bore IO-550. $\endgroup$ – J Walters Jan 11 '16 at 13:12
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    $\begingroup$ Power and torque curves can't run parallel, since the power (in W) is simply the torque (in Nm) multiplied by the speed (in radians per second) (A completely flat torque curve results in a power curve that increases linearly with engine speed) (Torque/power curves for cars are peak values though) $\endgroup$ – Gert van den Berg Jan 5 '17 at 9:19
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My credentials are became the youngest ASE Master Tech when I was 20 years old. I was a mechanic as a sergeant in the US Army, I'm a VW and Audi Master Tech and I modify the spark, fuel, and boost maps in factory ECMs. So I'm really familiar with all the numbers and stuff like that.

Firstly a modern automotive engine is FAR more reliable and efficient than than these aircraft motors I think that because the EPA isn't all over these guys they haven't had to increase their efficiencies.

I googled the specs on the Lycoming IO 540, and I found it to be kind of pathetic. An 8.7 liter motor, running on 100 octane gas, making 300 hp!?! Compression ratios in the 8s, KILLING fuel too just due to displacement size.

The engine is a mix of old, inefficient, mechanically unreliable parts, that are high maintenance, and then new high performance tech.

For example, it has a direct gear driven overhead camshaft, which robs hp. The valvetrain is ANCIENT and sloppy.

It also runs a fuel injection system that seems to be like the Bosch L and K Jetronic, and CIS Motronic. So, instead of electrical injectors you have braided steel hoses with an orifice in the intake, those go to a fuel distributor, which varies the fuel pressure by modulating a plunger.

VW, Audi, BMW, Porsche, Volvo, ETC ran this system in the 80s and early 90s. When it runs, it runs well, but the fuel pattern isn't very good, and in my experience the nozzles clog and are unable to be cleaned, and the fuel distributor can cost a couple grand with the differential pressure regulator. I'd be VERY interested in seeing the spray patterns of one of them. They generally don't seal well when closed, and they will leak fuel out of the tips, spiddle out rather than a good solid cone shape.

Then, the cylinder heads have only 2 valves per cylinder, so they definitely flow like crap, and depending on their size if they're close to the cylinder bore it can block air flow. The solid tappet camshafts which will flatten if you aren't running high zinc oil. I figured it caused issues, googled Lycoming tappet failure, and what do you know, stories about it along with a roller upgrade.

Running the cam off the crank via a gear causes backlash forces on the bearings and friction parasitic draw of hp, so do flat tappet lifters, the roller lifters are a must imo. The lifters are hydraulic, so they don't need adjustment. One thing scared me, they have you push the lifter down to compress the hydraulic portion and then check the clearance between the lifter tappet and cam. The spec is .028 to .08...You'd think that maybe because they're hydraulic it doesn't matter. Then they state, if you're out of spec you install a longer or shorter pushrod, and to beware because the decreases valve to piston clearances. AKA, if you have a .028 and a .08 gap. Then one of your valves is opening .052 of an inch more than the other. That makes cylinders run unevenly.

HOWEVER one thing good, the engine has molybdenum nickel coating on the cylinder, which reduces piston and ring friction losses, modern cars just started using this. It's been used in racing a long time. Molybdenum is used in those friction reducing oils you see advertised, and they mix it with steel to increase machinability.

Otherwise it's a lot like a Beetle in ALL the bad ways. A split engine case that can leak, removeable pushrod tubes you have to install and seal between the block and head, that leak, uneven cylinder temps.

There's a lot of unknowns too, like how close do they match the factory pistons and rods for weight difference to balance best as possible?

There's NO WAY that engine is more reliable than a modern car that adjusts nearly everything. Soon cars will have electric solenoids to operate the valves, they're already doing it in test cars. This will allow infinitely variable lift and overlap. Once that happens, camshafts are obsolete, along with the timing belt or chain draws.

This motor has way too many metal on metal, non adjustible, wearable parts. You CANNOT make the cylinders make roughly the same hp due to the variations in the pushrods and the heat differences. Anybody who ever was into Beetles knows of the infamous 3rd cylinder meltdown issue. The heat differences between the cylinders cause massively different power output and cylinder balance.

I had many BMW Volkswagen customers with over 300,000 miles on their original engines with no issues and I had decent guys with close to half a million period in eight years of owning a shop I don't think I can say that I ever seen one engine fail that wasn't due to neglect like leaking coolant or not changing the timing belt. Crappy oil.

As far as the torque part numbers people are listening at 60 miles an hour on the freeway the torque load sense by the ECM is usually more like 20 to 25%if you could remap an engine computer like I can it's turbocharged you could call for Boost and stuff down lower then what it's from the factory.

I would find a way to run it off the cams that are the timing belt. But your engines are a joke my sisters 03 Jetta 1.8 Turbo puts out 300 horsepower to the wheels with the stock Turbo. There's no reason to have that big ass V8 that weighs a ton and makes no power. My sister's car gets 20 miles to the gallon when driven nicely.

Your engines have missed out on all the updates that that the car makers are had to do to keep up with the fuel mileage for example running a distributor takes horsepower semi hemispherical combustion Chambers adds horsepower, variable valve timing adds horsepower turbochargers with individual pipes that are separated all the way to the turbo to keep the exhaust pulses group add horsepower.

I don't have my license yet but I've flown a couple times and I was honestly a little scared before I went and took my first lesson but I was astonished at how shitty the technology was on the Cessna 172 and how it basically looks like 1940's automotive technology.

I would run a VW diesel engine or the 2.0 Turbo found in the 05 and up chat is golf's and A3.

The 2.0 T is direct gasoline injection like a diesel because of that and the fuel being injected right at top dead center it allows you to run extreme amounts of boost and ignition timing. There's literally not enough time before the spark event for the gasoline to detonate. Those engines run 16 to 1 air fuel ratio on the highway. They also make them up 210 horsepower stock and with just the tune they make 300 or more.

Quality Automotive electrical systems or absolutely nothing to fear they're redundant about the only thing that will keep one sideline from running would be a bad crankshaft sensor or bad fuel pump comma if you ran joule crankshaft sensors and dual fuel pumps there would be absolutely nothing to worry about.

So, take a look at this here. This is a 1.8t dyno, and a mild one. If you take a 2:1 reduction drive you end up with 200 hp at like 3000 rpm. enter image description here

I don't get it, maybe it's like the reason truck engines need to be 6, 8 cylinder or more, because they need the increase in bearing size and rod size and strength for load?

But I ran a ton of computation, it doesn't seem to be under a VERY heavy load.

Anyway, Cessna was doing diesel car size motors that seemed fine.

I think it's the old guard not feeling comfortable with computers....I'm supposed to get a settlement soon. Maybe I'll build myself a Cessna with one:)

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    $\begingroup$ Diamond makes diesel aircraft engines which are modified Mercedes car engines. The main driver in aviation is not horsepower or efficieny, it is avoiding the unknown. $\endgroup$ – Koyovis Jun 14 '17 at 23:39

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