ROTOR VERSES THE PISTON. By Paul Lamar
Lets analyze the motion of an aircraft piston engine rotating at 2500 RPM. Assume the bore is five inches and the stroke is five inches. At the three o'clock position of the crank the piston is moving down the bore at maximum speed. At the nine o'clock position the piston is moving up the bore at maximum speed. At noon and six o'clock the piston is stopped at top and bottom of the bore respectively.
The circumference of the crank is pi times five inches or 15.7 inches. That is 1.31 feet.
Since the RPM is 2500 the RPS is forty two revolutions per second. So the speed of the crank-pin is forty two RPS times 1.31 feet or fifty five feet per second moving in a circle. The piston speed is therefore the same as the crank pin speed at the three o'clock and nine o'clock positions.
Bear with me here as we are steadily getting to the point. It takes the crank one second divided by forty two or .024 seconds to rotate once. To rotate from noon to three o'clock it took one quarter of that or .006 seconds. Therefore the piston accelerated from a dead stop to fifty five feet per second or 37 MPH in only .006 seconds! The average acceleration is then 9200 feet per second in one second. Well now lets see. One G is only thirty two feet per second in one second so 9200 divided by thirty two is about 290 G's!!!!! That is just the average. Peak acceleration is probably twice that.
Let's assume for the moment the connecting rod, the piston pin and the piston with the rings weighs seven pounds. They all move the same way the piston does. Mr. Newton said, and he seems to be right, force equals weight times G's. The three pound piston alone looks like 870 pounds to the rod in the form of inertial load. Mr. Connecting rod has to push and pull that 870 pound piston back and fourth once every .006 seconds. As far as Mr. Crank-pin is concerned that seven pound collection of parts looks like it weighs about 2000 pounds! 290 G's times seven pounds is about 2000 pounds of inertial load. Mr. Crankshaft says "OUCH!".
This is a simplification of course. The compression and combustion pressure loads sometimes adds to and sometimes subtracts from these inertial loads. I cannot imagine a piston engine made without light weight pistons. The piston engine, as we know it today, would not be possible without aluminum pistons.
Metal fatigue happens when the force and therefore so too the stress repeatedly reverses on a part. The cylinder spacing, rod length, rod journal diameter, main journal diameter and cylinder bore all have a large effect on the fatigue life of a piston engine crankshaft.
According to Charles Fayett Taylor "The Internal Combustion Engine in Theory and Practice" Volume 2 page 493 the major problem with piston engine crank shafts is the fillet radius between the rod and main journals and the crank cheeks. The fatigue limit stress can be as little as 5% of the breaking stress of the crankshaft if these fillet radii are too small. The fatigue limit is defined as that stress level where the part will live indefinitely. Usually if the crank does not crack in two million revolutions or 13 hours at 2500 RPM max power it will live forever. Nobody does that test of course. The FAA engine certification test require full power at sea level for only five minutes at a time. Usually you are cautioned not to use max power for more than 5 minutes in any one flight. If you run an aircraft engine at mostly 75% power it will probably last about 2000 hours.
Unlike a real aircraft engine the automotive engine cylinder spacing is too close to allow adequate fillet radius in the crank cheeks. Engine size and length is everything to an automotive design engineer. This was a major factor in the downfall and bankruptcy of the Chev big block based Orenda V8 aircraft engine. If you run an automotive piston engine at 6000 RPM full power you will exceed two million cycles in about six hours. Auto engines only reach full power for a fraction of a second in a flat out acceleration run. There is some modest manifold pressure and HP level where the auto engine will live indefinitely however. What that is is unknown but most auto engines are designed to run indefinitely while generating only about 50 HP or less at modest RPMs. That includes the 400 HP Chev V8. What you wind up with is an engine that has a rather poor power to weight ratio or limited life for use in an aircraft.
This can be circumvented to a certain extent and the life increased by substituting better materials such as forged and heat treated 4340 for the cast iron typically found in automotive crankshafts and rods. That of course increases the cost of the engine almost to the point of a real aircraft engine while the TBO is unlikely to exceed 1200 hours. If you reduced the bore and consequently the displacement to limit the peak HP that too would help. If you shortened the stroke to reduce the displacement that would help even more as the rod journal to main journal overlap would be increased. Another known way of increasing the life of crankshafts. These techniques result in less displacement and less HP of course.
What if we increase the diameter of the rod and main journals to provide more overlap and keep the stroke the same? This will work up to a point. Plain journal bearings have a limited range of ratios of diameter to width in which they will work. If the diameter is too large relative to the width (read cylinder spacing) dynamic oil pressure in the space between journal and rod or between journal and main saddle will be lost and the bearing will fail. Large diameter journal bearings also add friction reducing the BSFC slightly. Of course it also adds a lot of weight to the crankshaft and rods. I believe this is what the original developers of the Orenda V8 did when they spent $100,000 on a new crankshaft forging die. Never the less they were never able to get more than 1200 hours TBO.
An over the road tractor trailer diesel engine on the other hand is designed to run at peak power almost indefinitely. Such a 300 HP piston engine would weigh in the neighbourhood of 2000 pounds. BTW I have always maintained such an engine installed much further aft in a crop duster airframe for CG reasons would make a great banner or glider tow air plane. Unfortunately it would be illegal in the eyes of the FAA.
If you are going to design a V8 or V12 aircraft piston engine you need to start with a clean sheet of paper. V engines (and all other in line piston engines) are trying to shove their crankshafts through the bottom of the blocks. The extra main bearing structure required to support this is heavy. A horizontal opposed engine will weigh less due to the more efficient structural layout. The load from one side is partial balanced by the load from the opposite side. Lycoming built some horizontal opposed air cooled 8 cylinder engines and they too had problems with the crank despite the wider cylinder spacing.
It is no wonder your aircraft engine crank shaft and other parts are a mass of cracks at over haul time. Its a miracle that piston engines last as long as they do. It has taken over 100 years of minute detail development and billions of piston engines to make them viable. Here is a picture of a broken aircraft engine block. This happens so frequently there is a name for it. It is called windowing the block.
The beauty of the Wankel on the other hand is the eccentric shaft is straight through yielding no concentrated stress problems up through four rotors and maybe more.
The loads on the eccentric shaft are always in one direction. They do not reverse. Reversing loads are what causes metal to fatigue. Consequent the life of an eccentric shaft is not limited. The same thing can be said of the rotor. If the wear of the seals and the sliding surfaces can be reduced to very low levels then the life of the rotary could be 10,000 hours or more. There is some evidence that silicon nitride ceramic apex seals moving on the chrome plated rotor housings may have a life of 30,000 hours.
All other things being equal, like intake breathing, the HP of an ICE engine is directly proportional to the RPM it can be run. The Wankel engine breaths extremely well at high RPM's due to to the lack of poppet valves getting in the way of the intake and exhaust ports.
The idea of an aircraft engine turning 6000 RPM takes getting used to, however only the E-shaft turns this speed, everything else is 2000 RPM. At these rotational speeds the gas pressure and centrifugal force are balanced and it is actually easier on the engine. George Grimes
BTW The Achilles heel of the aircraft piston engine is the exhaust poppet valve. Typically it runs at 1200 to 1400 degrees F. Aircraft engine exhaust valves are constantly on the verge of total failure. These valves are made from the finest materials available and in the case of Lycoming are also sodium cooled internally. This brings a high price as a Lycoming exhaust valve is over $250. A slight mis application of leaning the mixture by the pilot can easily lead to exhaust valve failure.
Skyranch Engineering Manual.
Skyranch Engineering Manual.
Skyranch Engineering Manual.
Unlike any and all piston engines the Wankel rotary is in complete and precise balance. Just like a turbine or electric motor. Despite commonly held beliefs it is impossible to precisely balance any piston engine as there are always reciprocating parts that oscillate back and fourth.
You might surmise from this the Wankel rotary is able to revolve at unlimited RPM's because there are no reciprocating parts. You would be wrong. The Wankel rotary also has its RPM limits. Centrifugal force rears its ugly head. The weight of the rotor spinning around the center of the e-shaft creates a terrific centrifugal force on the rotor bearing at speeds of 10,000 RPM or more. By the way the turbine engine is also subject to the same centrifugal force limits on its compressor and turbine blades.
Never the less the Wankel rotary can rotate far faster than an equivalent HP piston engine adding to its intrinsic power to weight ratio and small size advantages. Also this centrifugal load due to the weight of the rotor never changes direction on the e-shaft at high speeds so a cracked or broken e-shaft on a Wankel rotary is unheard of. This is why the Wankel rotary engine manufacturer can afford to make the rotor out of low cost and extremely durable but heavy cast iron.
Further development of the rotor is possible of course. Better casting techniques are used by Mazda to reduce the weight of the rotor in the RX8 engine. The RX8 engine is rated at 250 HP at 8500 RPM. Not bad for a 200 pound engine twelve inches wide, fourteen inches high and seventeen inches long. This is an exceptional power to weight ratio and power to size ratio that exceeds any piston aircraft engine despite the use of low cost cast iron rotors and end housings.
Rotors made from other material with a better strength to weight ratio than cast iron may eventually be used. A more costly aluminum rotor is possible with steel inserts in the high wear areas of the apex seal slots. A titanium rotor is also possible. So too would be a sheet steel rotor perhaps laser welded together.
Curtis Wright failed to take advantage of this weight reduction not realising that someday the RPM of the rotary would be limited to 11,000 RPM by the weight of the rotor. Modern material, like Beryllium aluminum alloy, would make aluminum rotors even better. Be/Al alloy is three quarters the weight of just aluminum. My guess is the RPM of a two Be/Al rotor rotary (weighing 180 pounds) would double to around 22,000 RPM and the HP would also double to about 1600 HP for an all out p-port turbo charged racing version. That would be a power to weight ratio much better than a pure turbine with about 1/2 the fuel burn and 1/10th the cost in a turbo compound rotary configuration.
When this happens it is going to be an earth shaking event. A 300 HP car engine could shrink to below the size of a soccer (foot) ball. The empty weight of the car would also decrease as it is largely a function of the engine weight. The light aircraft industry would never be the same if these super power to weight ratio rotary engines were used in VTOL designs.
Turbo charging the rotary is a very viable thing to do. So far the very robust Wankel rotary engine has outlasted the turbos.
LIMP HOME MODE
I've been thinking about the limp home mode of the 13B. I know we try to design in redundancy, but say you end up with a dead rotor due to a fuel or ignition problem. Do you have any idea how many hp would be absorbed in driving the dead rotor at 6k rpm to overcome pumping/compression/friction losses? I'm trying to figure how many hp would be left to drive the prop in that situation. And would peripheral vs side ports make a significant difference?
I don't know but having the throttle wide open would help I think. What gets compressed de compresses and feeds the torque back into the e-shaft. Friction is half of what a piston engine is. BTW losing one rotor is like losing two cylinders of a four cylinder piston engine.
I did some extensive test on the matter while I was at Mistral. A Mistral N/A engine produces approximately 170 hp per rotor, gross. Then you lose ~65hp to pumping losses, compression losses, friction losses (from seals as well as from the PSRU) and accessories losses for a net power of ~105hp per rotor.
If you lose ALL power on one rotor on a 2-rotor engine, then you are down to 40hp on the prop, not enough to stay aloft unless you are in a very light aircraft or are benefiting from strong uplifts! It would just allow you to stretch the glide, not to limp home.
On a 3-rotor engine, the arithmetic look better and, with a fully failed rotor, you still have a good 140 hp on the prop, or 45 % of full power, enough to stay aloft, at least at lower altitudes and lowest drag airspeed.
The good news is that we NEVER lost the full power of one rotor accidentally (we made the missing-rotor tests by cutting ignition and/or injection to that rotor). Detonation will damage your apex seals, but actually, damage steel seals don't leak too much as long as they stay hot. The worst power loss we saw with steel seals after a major detonation event (which always happened on a single rotor, always the front one because of less optimal chamber wall cooling) was a loss of ~70hp, leaving 65% power to the engine and very little unusual vibrations.
We never damaged a single ceramic seal however (and we tried hard, believe me)! Even if you were to crack one such seal, you would keep perfect compression on one chamber of the damaged rotor, and probably some compression on the two chambers separated by the damaged seal. The worst case scenario is, however, a ceramic seal breaking away from its slit and pieces of it breaking the remaining seals. Such scenario would obviously make you lose all power on the affected rotor. But believe me, it would take an awfully violent detonation event to break those ceramic seals; possible only on a quite highly boosted turbo engine and most improbable if not impossible on a N/A engine. By the way, Iannetti makes 3mm thick seals (instead of the normal 2mm ones) which he recommends for high-power applications. We never felt the need to test those.
Of course, all I just said is mute if you don't have reliable dual ignition and injection like Mistral engines have. Then, you could lose 100% of a rotor's engine because of a failed injector... and THAT does happen!
Conclusion: rotary engines are impressively robust and forgiving, they really are! Some of Mistral's technicians, coming from the piston engine side of the industry, could initially not believe how much abuse a rotary engine could survive.
Best regards to all,
NEW CHEVY LS7
In December 2008 I saw the Chevy LS7 and LS9 at the PRI show in Orlando. Both have forged crankshafts and titanium rods. Sodium cooled exhaust valves and titanium intakes. They did this to get the red line up to 7000 RPM. Peak power happens at 6300 RPM.
All this is a first for LSx engines. Crate engine price is about $15,000 for the LS7 with injectors, intake and exhaust manifolds less computer. A real bargain but you better buy one while you can still get them. HP is 500 for the normally aspirated and 600 for the, unsuitable for aircraft use, supercharged LS9. (Burns too much fuel)
Of course the duty cycle is still very low. How often are you going to go 200 MPH in a Corvette? In a quarter mile drag you may hit 500 HP for a fraction of a second and not make 200 MPH. I have no idea at what HP level you can run these engines in an aircraft environment and get 1000 hours TBO let alone 2000 hours typical for AC engines.
If I were guessing the 500 HP normally aspirated engine might be a good 400 HP aircraft engine for 5 minute take off and cruise at 375 HP for 1000 hours. It might be a good Reno engine at 500 HP for 15 minutes but you would still risk blowing it up in the hand grenade mode. The road racing versions have undisclosed modifications and are good for a 24 hour race but cost well over $100,000
Weight without gear box is about 400 pounds. No PSRU. The only Chevy PSRU that I know about that might take 400 HP on take off but it weighs about 120 pounds. There may be others.
The displacement on this engine is 7 liters. No replacement for displacement :)
FOUR ROTOR MAZDA
By comparison an all aluminum 5.2 L P-port four rotor bare block would weigh about 220 pounds and put out 690 HP at 9,000 RPM with cast iron rotors. It had undetectable wear after winning the Le Mans 24 hour race outright.
I think one could build one for less than $15,000. It would be comparable to the racing version of the LS7 but there is no doubt in my mind it would last ten times longer. I think the Bell 47 final drive like Doug's would be OK for the PSRU. It weighs about 47 pounds.
I hope I have adequately got the point across of why the Wankel rotary is a better engine than the piston engine.