Auto Engines For Aviation

On the other hand automobile engines are turned out on a mass production basis and cost between $1000 and $1500 each.

So if we compare the price of a Lycoming 0. 360 at$13,412 with a Javelin converted Ford V6 at $3000, that’s a big difference. Incidentally, in all but single place, homebuilt aircraft, our DofA require a certificated engine, so don’t go rushing off to buy one just yet.

So much for the difference in price. Now let’s look at the problems associated with adapting auto engines for aircraft use.

It is apparent that there are two basic problems that one must overcome if you are to end up with anything practical when you try to convert some existing automobile engines into an aircraft powerplant. The first of these is the problem of having an adequate crankshaft. This is due to the fact that most automobile engines are designed by some pretty smart engineers who really know what they are doing and who are usually constrained by their management to come up with the least expensive design that will do the job. Thus, most automobile and motorcycle engine crankshafts are not designed to swing a high inertia member like an aeroplane propeller but are designed to spin an automobile clutch plate (with the aid of a flywheel), or, at best, a fluid torque converter which likewise doesn’t have too much of a moment of inertia. Since the forces developed by a piston engine crankshaft must act against the inertia of whatever is being driven by the engine, and the magnitude of the forces are determined by the displacement of a single cylinder and its compression ratio it is easy to see that a good engineer is not going to be designing any more material (and strength) into the crankshaft than is necessary for the engine to do what it is designed to do.

With materials costs on the increase, the automobile (and motorcycle) engine designer must make the crankshaft as lightweight as possible to conserve materials as well as keep the total engine weight as low as possible. This is becoming more and more important as automobiles get lighter and lighter in weight in an effort to improve fuel economy.

As a result, the crankshaft in most available engines just isn’t heavy enough to permit the engine to run at high power and swing a high inertia item like an aircraft propeller. Further, in their weight and cost elimination efforts, the auto engine people have gone to such devices as torsional vibration dampeners to help keep the weight to a minimum in the crankshaft. This is a particular problem with inline engines where the cylinder furthest from the power plant output end of the engine tends to ‘wrap up’ (twist) the crankshaft prohibitively, due simply to the torsional elasticity of the steel crankshaft member. One of the first things the engine conversion people want to do is to eliminate the torsional vibration dampener, and it is obvious that this is probably the last thing they should do. This is not to say that there are no automobile engines that will accommodate the inertia of a propeller on the end of the crankshaft, but this is the one big problem that must be overcome.

A further problem is how to attach the propeller itself to a flange which in all probability has been designed initially to accommodate the clutch assembly of an automobile. The bolt circle required to adequately drive a wood propeller such as is usually fitted to auto engine conversions is so large that some sort of solution to this problem is essential. Add to this the further fact that most auto engines are not designed to take any end thrust such as must be developed by a propeller and it is easy to see why there are so few dependable and practical auto engine conversions flying around.

While the crankshaft problem can be overcome, there is still one other major basic problem with the conversion concept and that is heat rejection.

An automobile engine is seldom called on to run for any great length of time at anything approaching its full power capability. Thus, you would have to drive up a very long hill at high speed at full throttle and in high gear to approach the condition you have with an airplane engine running at ‘cruise’ power. Even pulling a trailer can easily overheat a conventional automobile engine if the trailer is very heavy and the road is slightly uphill. Trying to do it at high speed is even worse and most auto manufacturers have special oil radiators and coolant radiators as well as special differential gear ratios available in an effort to make their products operate satisfactorily under these conditions. Anyone who has pulled a trailer in a mountainous country appreciates this problem and its limitations.

However, the same chap will go out and try to convert an automobile engine and operate it in even worse continuous conditions without recognising the problem. All automobile and motorcycle engines are designed for some limiting condition with regard to their ability to dissipate the excess heat that is developed under these types of operation, and there is an obvious limit at which you can ‘pull the engine’ without overheating and with most liquid-cooled engines, this will be more than enough for aircraft use. Increasing these limits can be done to some extent with liquid cooled engines by installing larger water pumps and bigger radiators, but eventually, some limit will be reached above which the engine just will not operate without overheating. Installing larger water pumps and bigger radiators in an aircraft can mean some real problems, and this can be a serious limitation that the builder must observe if he plans to install some converted liquid cooled auto engine in his ‘pride and joy’.

Air cooled engine conversions have the same problem of heat rejection, but there is little that the engine converter can do except increase the size of the oil radiator, or add such a radiator in an effort to use the engine oil to assist in overall cooling. This solution also has definite limits although most successful conversions of the VW engine into aircraft engines do rely on the use of oil coolers to solve the heat problem. Other solutions to the cooling problem for the VW engine range from the installation of special aluminium cylinders with more fins to the use of integral cooling fans. While improved baffles and cooling ducts can help, it is usually the basic heat rejection limitations of air cooled engines that gives the greatest problems. Even production aircraft engines with all of their massive cooling fins still must rely on such solutions to excessive power operation(as in take-off) by using an extremely rich mixture to help get rid of the excessive heat.

Other problems of converting an auto or motorcycle engine to an aircraft powerplant are pretty routine. Obviously, anything that can be done to reduce the installed weight of the installation must be done. This can include the elimination of heavy intake manifolds and the substitution of a fabricated intake system and exhaust headers instead of heavy cast manifolds. Some engines can be lightened by the use of sheet metal oil sumps (of welded aluminium) instead of their usual steel sumps, and, of course, any bosses normally used to hold the transmission or fly-wheel housing should be eliminated. Fabricated sheet metal (steel) engine mounts can sometimes be made to reduce weight. While it is desirable to eliminate the flywheel usually found in most engines, this cannot be done completely(although the flywheel may be lightened) since the starter ring gear is usually on the flywheel. However, as we mentioned earlier, the flywheel has a very valuable additional function with regard to the suitability of the crankshaft. Similarly, the torsion dampeners found on some engines may have to be retained in order to get a durable installation.

Carburettors are another of the big problems for the engine converter. Aircraft carburettors are very expensive, and the one thing that they have that isn’t available with usual auto carburettors is the mixture control. This can be overcome to some extent by use of the CV or CD-type auto carburettors which have constant displacement features which tend to make them have usable automatic altitude compensation. This is particularly true if the engine to be converted is going to use some sort of turbo installation since the constant displacement feature is sensitive to the total airflow through the carburettor rather than the density of the air. However, carburettors must be matched to the engine in terms of engine displacement and rpm(total airflow) so that they give proper mixture throughout the usable engine speed ranges. One word with regard to turbos- do not expect turbo to increase the available power for most engine conversions. The turbo should only be used to maintain effective sea level manifold pressures. Thus while someone may tell you that a 2 litre Escort engine will put out 140hp, he forgetting that those ratings are considerably higher manifold pressures than the engine was ever designed for and at very high rpm(which entails the use of gearing to drive a propeller at reasonable speeds) where the engine life cannot be expected to be anywhere near as good as you are going to want for your airplane. Further, turbos can give you at least sea level manifold pressure at altitude, but you must then have some sort of propeller installation that can deliver that power to the thinner air you are going to be flying in Thus, you should have a controllable (or constant) speed propeller if you are going to be using a turbo. In fact, a controllable propeller can be used to advantage even without a turbo since such a propeller will let you turn your engine closer to maximum permissible rpm for take-off and climb, whereas a fixed pitch propeller will seriously limit take-off rpm (and available power) if you select a proper fixed pitch for good cruise speed. Broadly speaking, a fixed pitch prop should let your engine turn up to the maximum permitted rpm only in level flight at sea level. Thus, at any higher altitude, the propeller probably is going to let the engine overspeed, particularly if you have mixture control and you fly around at full throttle. In that regard, one of the most useful instruments you can have in a homebuilt aeroplane when you are playing around with the engine and propeller is a good manifold pressure gauge. Very few home-builders have any real idea of the power they are pulling from their engines, and the physical position of the throttle lever has little bearing on what actual manifold pressure you are running your engine.

Other accessories such as the starter, alternator and ignition system are also problem areas for any conversion. Starters can usually be lightened by use of a small gear train between the starter motor body armature and the starter drive pinion. Usually, a three-to-one gearing between the motor and the Bendix drive will let you use some small foreign car motor to easily crank your engine and save you many valuable pounds of weight plus giving you far more positive starting and longer battery life due to the lessened current drain on the battery. There are some very small foreign car alternators now available at low cost that let you save additional valuable pounds and still give you an adequate charge rate for your homebuilt light-plane.

Probably one of the stickiest problems for anyone trying to convert an auto or cycle engine to an aircraft application as far as hardware is concerned is the ignition system. Here the modifier is faced with current DofA regulations which require the use of dual mags and some sort of separate ignition system that does not rely on the electrical system of the aircraft. The DofA will accept the use of one magneto and one distributor, but you must have two plugs per cylinder. Installing two spark plugs per cylinder is almost impossible with some engines. However, do not overlook the possibility of heli-arcing additional material into some space in the head and then machining a suitable plug recess. There are several ways of installing dual plugs in some cylinders such as the standard VW heads and some of the custom heads available for hot rod installations offer a suitable solution to this dilemma. Don’t overlook this problem when you are considering the idea of making your own conversion. This can be a real stickler.

While some homebuilders content themselves with single ignition systems for their engine conversions (and there are many single place homebuilt flying around with only one magneto), the fact remains that dual ignition is very desirable, and most engines will develop additional power with two plugs firing over what they will put out with a single ignition system. Note the drop in engine speed at full throttle on one mag as compared with “both’ operations even with usual lightplane aircraft engines. Anyone aspiring to develop a conversion for eventual DofA certification must, of course, have a dual ignition installation. There are many arguments as to the merit of dual vs single ignitions and we doubt that this article is going to help solve anything on this score.

Not the least of the conversion problems is the matter of the oil pump and its capacity. This is particularly important if you are going to be needing to install an oil radiator. Be sure that you are using the largest possible pump (if there is a selection available for the engine you have chosen to convert). Or, if possible, convert the oil pump to give you as much oil flow as you can get. You are probably going to need it to help keep the engine cool during that long-pull condition that it is going to be operating in all the time.

There are other problems such as being sure that the engine is properly balanced-mostly that the pistons and rods are of equal weight. Piston rings can also give you fits if you try to use standard rings and end up running the engine at excessive pressure and heat in an effort to get the desired power from the installation.

Remember that air-cooled engines must be set up for different piston ring clearances than you use with fluid-cooled engines so specifications in this regard must be closely followed if you are to end up with a dependable installation.

All in all, there are many existing engines that probably can be successfully converted to aircraft engines, but there are many problems. Just one little thing like finding suitable shielded spark plugs can result in your engine conversion falling short of being truly practical. In that regard there are now suitable shielding installations available for use with CB radios which will let you shield standard ceramic spark plugs satisfactorily, so look into that solution if you run into the problem.

Any conversion must be well thought out before it is started, and then it is not something that is going to be easily accomplished. If you think you can run down to the corner wrecking yard, get a $350 engine and make it into a usable, dependable aircraft powerplant-forget it! You should start out with a good test stand that you can easily make from angle iron that you can buy at the local machine shop, and you are going to need a lot of patience as well as lots of money and time if you are planning to go this ‘easy (?) route’. It is far better to turn to some experienced engine conversion outfit that has spent years finding out how to make some particular type of engine work than it is to try to do it yourself. However, even then you must be aware there are many people who are trying to sell converted powerplants as well as designs and instructions for such conversions who have never really ever built a successful one, much less flown such an installation for the hundreds of hours that it takes to really prove their suitability.

We would be remiss in this discussion if we didn’t mention the other big problem that anyone trying to convert an engine must face- and that is the finding of suitable engines to convert in the first place – in volume. It may be easy enough to find a single engine and make it into a suitable lightplane powerplant, but what do you do for the second one, or the third? Many engine manufacturers won’t even give you the time of day when you tell them that you plan to use their engine in an aeroplane. They are afraid of product liability and any mention of aviation scares the daylights out of them.

My thanks go to Molt Taylor for much of the input for this first edition of ‘Auto Engines for Aeroplanes’.

In the next issue of Australian Aviation, we will list a number of manufacturers in the USA who are successfully converting auto engines with a fairly comprehensive description of each- their weights, output and cost and availability.