Why not use 2 automotive engines in single engine aircraft?

Factual Questions
1

A friend of mine was considering purchasing an airplane. However, he found that the plane needed an overhaul, and a Continental O-200 overhaul is about $13,000.

Ouch. In addition, engine failure is a major cause of accidents for single engine aircraft. I’ve seen several hairy emergency landing videos on youtube, where it’s quite clear that it’s half luck if you can find a place clear of obstructions to set down after an engine failure.

Well, why use overpriced engines at all. Build the aircraft to use 2 small, lightweight automotive engines. Even if the probability of failure is slightly higher for an engine not as “proven” as a Continental, if you use 2 isolated engines, the series probability of both of them failing is p1 * p2. If a lower quality automotive engine has a 2% chance of failing in flight per 10k hours, and an ultra reliable aircraft engine has only 0.5% chance (for example numbers), the chance of both engines failing is 0.0004, for a 12.4 times lower chance of failure.

Well, the Cessna my friend was looking at was made in the 1970s. At that time, automotive engines were extremely heavy for a 95 HP engine. So there is that.

Still, for new aircraft, why not do it this way?

2

There are some home builts that use air cooled VW engines.

3

I am sure your question is genuine but it is way off the mark. The simple answer is that is small plane engines are incredibly reliable especially if they are properly maintained and they have sufficient fuel. The single engine for any plane made before about 1995 uses what is basally 1930’s technology although there have been some innovations in recent years.

The two biggest problems with your question are:

  1. Regulation - that is the biggest reason why small, certified planes don’t tend to use the newest engine technology and the reason they stick with tried and true engines. It costs literally millions of dollars to get a major redesign approved for a market share that will generally be measured in hundreds to a few thousand installations in total. It usually isn’t worth it economically. However, that applies only to certified designs. There are experimental planes that anyone can build on their own. They could use a backup engine configuration if they wanted to but they generally don’t because…

2)…of weight and complexity issues. What you are proposing is building some type of bastardized twin engine aircraft. Either one engine is going to sit idle most of the time or it will be used as some type of inline thrust twin (the latter has been done with rather poor results). Both designs have huge problems. A backup engine design will cause weight and balance issues, decrease overall payload, introduce maintenance complexities and will probably never be needed. Small twins with engines that are both used continuously have more accidents than single engine aircraft if the engines are located on each wing because an engine outage causes serious control problems. An inline twin also has serious issues with noise because of turbulence interference with the aft engine.

In short, it isn’t a good idea in general.You are trying to solve a rare problem in a way that doesn’t have much practical purpose. Small aircraft engines rarely fail completely. When they do, it is usually because of fuel starvation or contamination which this design won’t help with. Even if it does fail completely, small planes can glide for an awfully long way and land in a survivable way as long as they are under positive control.

The reason that small plane engines are so expensive to buy and work on is because they are so incredibly regulated in the first place, not because they cost that much to manufacture. Your proposal will not help with that either.

4

Over the years there have been a number of experimentals with VW, Porsche and even (gasp!) Corvair engines. All of which are air-cooled flat 4 or 6 cylinder engines, much like the Continental and Lycoming aircraft engines common to small planes.

For a plane with a total lift capacity of only a few hundred pounds, a water cooled engine would add enough weight to pretty well eliminate any useful load. Remember these planes are surprisingly light. The engine design is to add lots of lightness and don’t lose too much reliability. That’s not easy.

As far as twin engines, the rule now is that one engine has to have enough power available to keep the plane flying. If one engine can do that (in a light plane like a piper or Cessna 172 or something similar), then what does the second engine gain you? Expense and weight.

Lindberg in 1929 intentionally chose a single engine for the Spirit of St. Louis with the same thinking: If one engine goes out in a twin (1929, remember), you’re hosed anyway. With two motors, the chance of failure is twice as high.

5

Fair enough. I was thinking (due to the control issues) of putting the engines inline. A front propeller and a rear propeller, since dual engines on the same driveshaft means potential interactions.

The engine I had in mind was the ford ecoboost I-3. It apparently weighs ~200lbs and develops 123 HP. Although, come to think of it, this is probably dry weight - the coolant in the radiator would add more mass. It has approximately the same power : weight ratio as the much older Continental design.

This plane would have triple the power of a small Cesna - that in theory means you could make the wings larger and increase the cargo capacity.

I would assume you could use the second engine for a big power boost on takeoff? You wouldn’t be able to take off with a heavy load on only one engine, but you would be able to perform all the other flight maneuvers.

6

Aircraft engines are very low reving compared to auto engines.
So before you brag about the power from an auto engine look at the amount of power produced @ 2200-2500 RPM.

7

Its not exactly an airplane, but there is this.

The method of flying doesn’t exactly instill confidence in the reliability of the engine :).

GA aircraft are stuck in an unfortunate technology vortex. It’s obvious that more modern engines would be vastly superior to the stuff they have. But car engines themselves aren’t that good a fit for reasons listed already (and more). They need a custom, modern engine design–but the money isn’t there. The money won’t be there without greater volume, but the volume won’t come (in part) unless costs come down. And so on…

I’ll bet that one could take an existing auto engine, make a relatively small number of tweaks for reliability, etc., and end up with a pretty nice aircraft engine that could be certified. The problem is that the supplier is going to change the engine in a couple of years (for a new car model) and then you have to recertify from scratch. You need a source that will be around for a long time.

Maybe someday, rapid prototyping will reach a point where we can have “open source” engines that can be printed out on demand. They’ll still be more expensive than normal mass production, but possibly cheaper than traditional GA suppliers like Lycoming.

8

Well there is progress of this type. The Centurion engines, are based upon a Mercedes car engine. They are modern, diesel, highly efficient. But they weigh more than an equivalent output engine from 1960, are much more complex, including the need for a gearbox. Where they win is in a 30% improved range.

After some serious financial shenanigans, which resulted in their founder in gaol, they are now owned by Continental. But the engines are being made, and are going both into new planes, and retrofitted to old ones.

In addition, there is at least one new engine. Albeit with a very conventional general design. OTOH, it isn’t clear that there is anything at all wrong with the conventional horizontally opposed aero-engine. It is hard to see how you would improve it. The new engines do however use much more advanced component designs.

I think people tend to underestimate the old engine designs. Thinking of them in terms of the rubbish that came out of Detroit at the same time, and expecting advances of relatively similar nature is naive. Aero engines were very highly developed, and used to best metallurgy and materials of the time. Car engines were designed to be made as cheaply as possible, and to operate with very loose tolerances, and to run when essentially no maintenance was done. Aero engines are also very different in terms of their use profile. They run at essentially one speed, and can be optimised for most efficiency at that speed. Probably the first advance from the most basic design of a light aircraft is to add a constant speed prop which allows the engine to run at the same speed independent of power output. Also, aero engines run at high power all the time. Even at cruise an engine will typically be run at 75% of rated maximum power. This is why twin engined light aircraft are a problem. One engine out can’t be fully compensated for by maxing out the other, and the aircraft may become marginal to fly. It ceases to be able to climb at rates that may be needed to maintain safety (or even climb at all), and is very much at risk from adverse weather.

9

There were a couple of promising engines in development about 15 years ago that would have worked as the op described. Very high output for their size and weight. They could have been clutched together to produce 200+ hp which would have been great for a lot of the homebuilts. It would have given both speed and redundancy to the airplane.

Not sure what the engine overhaul would have been but it would have produced a small twin engine plane without all the parasitic drag.

Alas, neither of the engines succeeded.

10

How can you have an aero engine with a gearbox? Won’t the propellor be unpowered while the clutch is engaged?

11

You don’t say what kind of airplane. Cessna 150s used Continental engines. (They switched to Lycoming engines and the airplane became the 152.) Cessna 172s also had Continental engines until they switched to Lycoming for the 1968 year model (Cessna 172I – that’s ‘I’ as in ‘India’). The Continental O-300 is 145 hp and had six cylinders. The Lycoming O-320 is 150 hp (160 hp in later versions) and has four cylinders, so you save a little money by not having two cylinders to overhaul.

If your friend is looking at a 150, then it has a Continental O-200 of 100 hp. So you wouldn’t need two Ford Ecoboost engines; only one. There’s also a certified Rotax 100 hp engine. One of these engines would not have enough power for a 172, but there are other automotive engines that could be used instead. Of course, you have to have a certified engine if you want to keep your plane in the Normal category. If you’re going Experimental, there are other, more efficient airplanes out there.

No clutch. The ‘gearbox’ is a reduction gear so that the prop turns ~2,400 RPM while the engine is turning ~3,500 RPM or whatever. So the prop turns with the engine.

12

Ah, that makes sense. Thanks.

13

Incidentally, helicopters have a disconnecting clutch. It’s called the ‘freewheeling unit’ or ‘sprag clutch’. You start the engine and then gradually engage the clutch, because the rotor system has considerably more mass than a propeller. In case of an engine failure, the freewheeling unit allows the rotors to continue spinning without having to drive the dead engine. More trivia: In an airplane the prop works as a flywheel. This can be useful for restarting a stopped engine. There is no flywheel effect in a helicopter, so restarts must be accomplished with the starter alone.

14

Actually the rotor in a helo would give you one hell of a flywheel effect, there are just two problems with that

  1. The one way (sprag) clutch won’t transmit power in that direction.
  2. The slowing of main rotor to try and start the engine would cause the helo to stop flying even faster than it already was. Your auto-rotation glide path would that of a well thrown brick.
15

That’s the whole reason the rotor isn’t used as a flywheel… I’m not sure the glide would resemble the path of a thrown brick; well-thrown or not. The key to surviving an engine failure in a helicopter is immediately entering autorotation. There’s only so much inertia in the rotor system, and it goes away quickly from drag alone. Trying to turn an engine would slow it down even more. In a Robinson R22 (which has a low-inertia rotor system) you have 1.1 seconds to lower the collective and enter autorotation. That’s all the inertia you have.

16

Rotary engines like that used in the Mazda rx7 are sometimes used in aviation applications. Low weight, small size, high power output (for their size), smoothness and awesome reliability makes them a great choice. The biggest downside is not to great fuel economy.

17

The question is mainly answered by the fact that ‘proven’ Continental/Lycoming type general aviation engines are so reliable twin engine, if not needed to get a bigger heavier hauling plane with those same engines, is somewhat a solution in search of a problem. Although as also noted, there are car (and snowmobile etc) derived engines on small planes which don’t seem to cause a lot if any more engine failure accidents.

Also traditional twin engine (separated on the wings) causes control issues for inexperienced pilots which make it uncertain if that’s the best approach to redundancy, as opposed to say a parachute for the whole plane, as Cirrus has successfully used, though not everybody else is flocking to that idea. OTOH if the two engines are in the nose driving one prop, the clutch system to allow that would also be a point of failure.

Not that it’s here or there, but Continental is now owned by a Chinese company, as is Cirrus.

18

I recall seeing something about using the more modern, high efficiency, high reliability auto engines - like 4-cylinder or 6-cylinder aluminum block Honda or Ford engine. they are quite lightweight for the power, and the weight of the coolant does not add that much. the fact that modern car designers are obsessed about weight too is a plus. Modern engines are far more reliable, and incredibly lighter, and more fuel efficient, than a 302ci iron block from decades ago.

Another point is that traditional auto design puts the crankshaft and output (and oil sump) at the bottom of the engine.

Aircraft engines must be able to run at more interesting angles than auto engines, considering fuel and oil flow. With modern auto fuel injection systems, this is less of an issue. Likewise, electronic ignition is much more reliable than it was a few decades ago. Aircraft engines had (have?) dual ignition systems, two spark plugs per cylinder, to ensure that a bad cable or distributor does not kill he engine.

As mentioned, auto engines are designed for a lower duty cycle and higher RPM. There is no stop-and-go traffic in the sky, small aircraft typically run for hours at 80% to 90% of full power; tootling down the highway at 65mph is the heaviest use of an auto for extended periods, probably uses about 50% of a typical auto’s full power.

Car engines at peak horsepower turn much faster than aircraft engines, so a gear system is needed. Propellers tend to be efficient in the 2000 to 2500rpm range. Fortunately, this conveniently solves two problems. The “gearbox” is a pair of aluminum wheels and a cogged rubber belt, large wheel above the small one. This reduces the RPM and raises the center of the propeller higher above the ground.

Aviation tends to use a different octane of gas than cars. Not sure what the process is for changing this, and not sure if small airports sell auto gas nowadays.

Aircraft engines meet incredibly tight manufacturing and maintenance standards. Your engine must be inspected every 50 or 100 hours of use, often a top overhaul after 1000 hours and a rebuild after 2000. One of the tests is an oil analysis to see if metal particles are being created inside the engine, but the rebuilds are a preventive measure- they can’t wait until it fails, the typical fix for cars.

The rebuild for aircraft engines involves replacing the cylinders, since in a VW-like flat-4 each (aircooled) cylinder jug just bolts on. Not sure what the equivalent would be with an auto-like engine - rebore and fit bigger cylinders and throw away after 2 rebuilds?

Also, given the cost of maintenance, a single engine makes more sense than a twin. the Cessna Skymaster 337 is a push-pull configuration; any other setup, like one engine on each wing, adds flying complexity. Control in the event of an engine failure is tricky, since the plane wants to turn. A coaxial drive (one driveshaft around the other, two propellers out front) is an unacceptable level of complexity in a small plane. Since each engine must be capable of flying the plane by itself, why bother? Just put in one good engine and make it as reliable as possible,

As a side note, a large number of engine failure set-downs are due to fuel supply problems not maintenance. Pilots manually switching tanks and failing to flip the lever all the way used to be a notorious killer.

Another side note - aircraft parts are required to be manufactured, tracked and certified under strict controls. The manufacturing processes are heavily regulated. Which batch of steel, quality of the processes, etc. are carefully checked. If NAPA Auto Parts sells you a drive shaft or valve stem that breaks, or an alternator that burns out too fast- well, pull over to the side of the road, get towed and have it fixed. Can’t “pull over” in the sky.

Unfortunately, this has resulted in a “counterfeit parts” problem. Bigger, more expensive aircrafts’ parts can go for tens of thousands of dollars. (Even a Cessna 152’s carburetor, IIRC, was thousands). As a result, shady business make part that look like and are packaged like the original, with fake serial numbers, but are made with whatever unreliable materials are at hand. IIRC, Gary Powers, the famous U2 pilot, was allegedly killed in a helicopter crash in LA because a fake part in his helicopter failed.

19

Do some helicopters have sensors to detect uncommanded loss of engine power, and a system to automatically engage autorotation? This sounds a lot safer than expecting a human to react in 1.1 seconds. Also, if the engine stops unexpectedly (the engine controls are set to run and the engine is no longer producing power) why would you ever not want autorotation?

20

I’m not aware of any helicopter that automatically enters autorotation. (The freewheeling unit disengages automatically.) If the engine stops, you definitely will want to enter autorotation. There’s quite a bit of training so that it becomes an automatic reaction. One point one seconds is not a lot of time, but it’s enough. Actually, a second is a long time in a hovering autorotation where you have to wait one second before pulling collective to cushion the landing. Otherwise you rise up and then come down hard.

Note that the R22 is a very small helicopter. Its engine is only rated to 124 continuous horsepower. (There’s more power available, but they put a red line on the gauge.) The rotors don’t have a lot of inertia in them. I’ve heard/read about pilots chopping the power on a UH-1 sitting on the ground, raising it up, rotating it 180º, and setting it gently down again. Big, heavy blades have more inertia. The critical time limit will vary depending on the helicopter. It’s just not a very long interval in an R22.