enginehistory.org

[gryan]

Surely we'll need to ensure the engine is balanced for forces and moments to, at minimum, second order. Even firing impulses would be important as well. It appears that this aviation engine will be operating at modest rpm (2000 to 3000 rpm) and it will have large swept capacity, so these issues will be most important.

A V-8 two-stroke engine is unbalanced. A 90-degree V-12 is the simplest layout required in order to achieve balance and even firing impulses from an in-line two-stroke engine.

It is different for four stroke engines. For an in-line you only require six cylinders for balance and even firing. A 90-degree V-8 with two plane crankshaft is balanced and has even firing impulses. A 60-degree V-12 gives full balance of all moments and forces to the sixth harmonic! It has even firing impulses as well.

For a radial I understand the minimum is five cylinders in a single row for balanced condition (well, not quite, as the eccentricity of the slave rods introduces some half order unbalances and suchlike).

Unless this is going to be a four stroke engine or a radial or we are prepared to accept unbalanced forces/moments/impulses, the minimum number of cylinders should be twelve arranged in a 90-degree Vee. Or is there something I have overlooked?

[kmccutcheon]

[gryan]

If the is the one that was published around 1920s by Tokyo University then yes, I have a copy of it.

[kmccutcheon]

[gryan]

"On the Balancing of Two-Stroke 12-Cylinder Engines" by Fujio Nakanishi, Tokyo, July 1932.

The paper deals with seeking balance of inertia forces and even firing intervals for two-stroke engines. It is stated that twelve cylinder V or W engines are the only practical types which meet the conditions for explosion at even intervals and perfect balance simultaneously.

A 90-degree V-12 and a 120-degree (outer included angle- each cylinder bank lies at 60-degrees to its neighbour) W-12 are examined.

For the V-12 there are four crankshaft arrangements that can be successfully balanced and also achieve even firing. They set the crank throws at 60-degrees to one another. Looking from the crankshaft snout end and proceeding at 60-degree intervals in a clockwise direction, the possible arrangements are 1 5 3 6 2 4 or 1 5 4 6 2 3 or 1 4 5 6 3 2 or 1 3 5 6 4 2.

For balance and even firing in the W-12 the crank must be a two-plane design, similar to the crankshaft layout used in the four-stroke V-8 engine. Throws are 90-degrees to one another. Looking from the crankshaft snout end and proceeding in a clockwise direction at 90-degree intervals the arrangement is 1 2 4 3.

[jrussell]

If a 90 degree v-12 ( two stroke ) is balanced in the primary and secondary, why did Napier choose the boxer layout? And how bad were the unbalance forces?
_________________
It runs best just before it blows!

[kmccutcheon]

[gryan]

Nakanishi does not discuss con-rod arrangements or types in any detail. The paper is a mathematical treatment of balance only. He sets out to show that should a designer require uniform explosion intervals and balance of inertia forces for an in-line two stroke engine, then he needs twelve cylinders in a v or w formation.

[gryan]

Gentlemen

I was recently emailed a response to the question that started this thread. It was provided by Alvin Lowi, a professional engineer who specialises in engine design. My comments are marked "Q" and his comments are indicated by "A".

Q/. The Ultimate Engine to fly

A/. "in general aviation aircraft"

Q/. would be (insert details)

A/. "the Lion aviation diesel"

Q/. and would feature

A/. "minimally cooled axial cylinders with inwardly-opposed pistons operated in a 2-stroke cycle carried out twice per shaft rev by twin double-harmonic face cams on a common axial shaft. A 9-liter turbocharged and inter-cooled compression ignition engine of this design can achieve greater power density and lower specific fuel consumption than any known conventional spark-ignition avgas-burning aviation engine of a similiar size."

Q/. (insert favourite attributes and reasons).

A/. "The compression ignition combustion process is not only thermally tolerant but also thermophilic -- the more heat the better for speed and stability. The barrel configuration is compact and offers a minimum of frontal area for reduced aerodynamic drag generation. The inwardly opposed piston/barrel cam layout is perfectly balanced. The recip-to-rotary motion conversion mechanism is all rolling contact, which eliminates dependence on hydrodynamic bearings and critical lubrication requirements. The rolling bearings generate less friction and heat, and require less oil, oil control and pumping power. The Lion engine has no cylinder heads to attach, clamp and seal. It has no valves to cool. Its cylinders are free of tensile and thermal stresses, all pressure and inertia loads being handled in tension in a single simple structural member -- the tubular shaft. The engine is ebullient cooled with an inflammable organic liquid to minimize both temperature gradients and heat extraction required for proper temperature control. Coolant ebullition amplifies the coolant heat capacity and film heat conductance to minimize temperature differences and coolant flow rates resulting in a substantial reduction in line sizes, pumping power, radiator size and overall bulk and weight of the heat rejection appurtenances."



Q/. Let's build a paper engine. I guess the best way to do this would be to start with an open to all contenders invitation. Given a fresh sheet of paper and the challenge to design the ultimate engine, what would evolve in 2005?

A/. "The Lion engine already exists in an advanced state of paper construction (see SAE Paper No. 2000-01-1686). This design is projected for a FAR Part 23 (12,000 lb. GW) single-engine aircraft application to give the following sea level take-off performance:

Displacement, l 9.0

Frontal area of bare engine, sq. ft. 2.8

Box volume of bare engine, cu. ft. 12.6

Weight of complete engine, lb. 755

Shaft power, hp 980

Shaft speed, rpm 1710

BMEP, psi 207

BSFC, lb/hp-hr 0.36

Power density:

hp/l 109

hp/lb. 1.3

hp/cu ft 78




Q/. What better venue to ask this question at than the AEHS? Given all the knowledge that has been assembled in the last 50+ years and with a healthy dose of hindsight we should be able to think up some really impressive big piston engines. The design weaknesses and problems of past efforts can be avoided (as far as possible) and we can use to advantage of all that hard won experience from previous engines. Cooling troubles, combustion chamber issues, rod/stroke/bore ratios, RPM, balance and so on; all these matters can be addressed and the issues properly solved. Or can they?

A/. "The answer to this question depends on the ultimate purpose of the "ultimate aero-engine design." If the purpose is to pay tribute to the esteemed founders of the profession by extrapolating their inspiring accomplishments into the present as if the social environment for innovation had not changed, the answer is yes. However, if the purpose is to actually advance authentic piston engine technology to serve a bona fide aviation application in the existing social environment, the answer may be otherwise. Meaning no disrespect to the founders and their aficionados, I follow the latter path perhaps because I am not as steeped in the tradition or knowledgeable of the history. I prefer to think I am being realistic and entrepreneurial. I am concerned that by simply projecting the traditional design approach into a fictitious future, combining all the cumulative expertise with all the additional expertise I might bring to bear, I might just end up in an idle academic exercise and make no history. For example, if no knock-resistant aviation fuels will be widely available with which to implement the traditional design approach and employ it in practice, the result will be academic.

Realistically, the disappearance of high PN aviation fuels was inevitable once the big air-cooled radials lost out to kerosene-burning turbines for commercial transport aircraft propulsion applications. It is now a fact that the mainstream petroleum industry no longer produces quantities of such fuels. This fact of life has nothing to do with the purity of aero engine design. It is the ascendancy of fuel-thirsty aviation turbine power plants (props and jets) is a result of demand for airspeed, payload and passenger comfort that produced this outcome, such that nowadays all major air transport operations burn simple, high-yield petroleum distillates that contribute more the industry bottom line. When spark-ignition aero engines represent only a niche market, volume availability of synthetic (lower yield), leaded-alkylate fuels is doomed. Avgas has now been relegated to boutique status consistent with the vintage aero engines that require it.

Consequently, at the risk of subverting your contest, I suggest "the ultimate aero engine" might be more realistically defined as one that can attain the power density of the aviation turbine while utilizing the common fuel to achieve the operating and ownership economies of the traditional positive displacement internal combustion engine. However, the design of such an engine may be traditional only in the sense that it utilizes pistons, positive displacement and internal combustion. Otherwise, it will be unprecedented.

The thermally-tolerant compression-ignition Lion engine has few aero engine precedents. But it does have a few. One is Junkers' opposed piston diesel, the turbocharged Jumo 207. This is the only diesel ever to see substantial aero service. Another is Herrmann's Dynacam barrel cam engine. It got CAA (FAA) type certification but no production."




Q/. There are likely to be some debates as preferences for particular design details come to the fore. Which way is better? Of course all the usual design decisions are there to be made and all the options (including the way-out ones) are available as well. Should the engine be an in-line or radial, air or water cooled, two-stroke or four-stroke, compound or not compounded? It could be opposed piston. Perhaps it could be spray cooled or use constant loss evaporative cooling, if either of those provide enough of an advantage. Should it have two, three, four, six or even seven valves per cylinder? What about sleeve valves; surely they are the best solution? There is nothing to prohibit oval pistons (as used by Triumph or more recently Honda). Rotary valves such as NSU's discs, Cross's, Aspin's, Negre's or Coates rotary valves are allowable (but how to make them work?). We could use Wankel's rotary engine as a valve (he did and he even employed it as a supercharger). The poppet still has a lot going for it (desmo?) and is a well known quantity which flows gas well. Then there's the issue of carburetion versus fuel injection versus direct fuel injection. Should the engine be integrated into the airframe (what about having it as part of a ducted fan system with waste heat recovery?) or should it be a power egg? Counter-rotating props; is that part of the deal? Reduction or direct drive? Plenty from which to choose. Perhaps we should stick with what is known and perfect it or is that too limiting?

A/. "The uniflow-scavenged, direct-cylinder-injected, two-stroke cycle, inwardly-opposed piston, axial-cylinder (barrel) Lion engine has no valves or ignition system. Its low-frontal area, low heat rejection design facilitates either a ducted fan/jet arrangement (extremely high bypass ratio if you want to compare with turbofan propulsion) or tight low-drag nacelle installation mounting a large propeller for lower speed and altitude applications. Instead of crankshafts with articulated pistons and connecting rods, it uses twin double-harmonic face cams with fully guided rigid pistons that provide perfect balance and a 2:1 torque amplification enabling the direct drive of large propellers without gear boxes. The face cams do not limit the piston motion to a simple sinusoid but allow optimization of piston motion to maximize charging and combustion performance. Slider-cranks do not offer this option. "



Q/. Here's a basic requirement (we can tighten it up as we go if necessary). We are looking for the ultimate piston engine design. The fuel is free although MOGAS might be preferred. Water injection is fine and power enhancers such as N2O and methanol are OK, but as they must be carried in the aircraft for the duration and as this one is not a Reno racer there may be a volume & mass issue right there. Let minimum cruise power be 1000 bhp continuous (more would be better, but this is here to avoid the use of automotive engines). TBO shall be minimum 1000 hours (although routine maintenance like plug checks etc. are OK, we just shouldn't be pulling down whole sections of the engine to deal with issues prior to 1000 hours- I guess the turbines have spoiled us!). SFC should be as parsimonious as possible in cruise and not too shocking at max power. Let's expect an endurance of six hours per flight and state the aircraft is a single-engine two-seat (tandem) monoplane requiring outstanding climb and acceleration, not forgetting good turn of speed (400+ mph). OK so it's a fighter (or a hot plane). An alternative use for this engine may be to have four of them in a larger plane. Whatever. It's a paper engine at this stage, a paper airplane is for another day.

A/. "Regardless of power and drag, airspeed depends on propulsion efficiency. The best of props will crap out as the tip speed approaches Mach 1. There is nothing that says an internal combustion engine can't drive a ducted fan as well as a gas turbine (turbofan). See 1940 Campini Caproni Isotta Fraschini and 1948 Klimov MIG 13."

_________________________________________

I've obtained a copy of the SAE paper describing the Lion engine. It's a very innovative design with all the details carefully thought through. Recommended reading. This would be an exciting engine to see in the air!

Does anyone have any other ideas or suggestions for an ultimate engine?

Regards

Gerald

[gryan]

Jrussell

The Napier Nomad flat-12 is really a 180 degree vee and not a boxer. Therefore it should be balanced. But there is no substitute for doing the maths to know for certain. I'll check it out and post the answer in due course.

Regards

Gerald

[jjuutinen]

And what is a 180-degree vee? I checked several engine design textbooks and none of them knew this mysterious engine. An engine with a bank angle of 180 degs is allways called a boxer in these books. And I don´t give a damn what auto people call these things, only real engine people may comment on this.

[gryan]

A 180-degree vee and a boxer are not the same thing.

In a vee engine connecting rods from pistons on opposite banks usually share the big end journal. This may be accomplished by using master and link rods, fork and blade rods or side by side rods.

If the vee is opened out to 180 degrees the main feature to notice is that opposing pistons reciprocate in the same sense. That is, when the left piston is approaching the crankcase (going "down" the bore), its opposing "mate" on the right side cylinder bank is moving away from the crankcase and approaching the right side cylinder head (going "up" the cylinder bore). This means that when the LHS piston reaches BDC, the RHS piston reaches TDC and visa versa.

To build an engine of the boxer type it is necessary for opposing conrods to be provided with separate big end journals phased 180 degrees apart. For the boxer engine both pistons approach the crankcase at the same time and approach their respective cylinder heads at the same time. This means that both pistons reach BDC simultaneously (and also TDC simultaneously).

It is most unusual for a boxer to be built with 12 cylinders. The crankshaft becomes an exercise in brinkmanship; a spindly, torsionally soft thing! A true boxer twelve would need a crankshaft with 12 big end journals and 13 main journals! Such a crankshaft would likely be an unsafe design unless it was very massive indeed. Torsional problems would be difficult to resolve. The safest route for a flat engine is to have the opposing rods share big end journals as in a vee design. This design only requires six big end journals and seven mains (same as the in-line six). That's why (as I understand matters) the Nomad was really a 180 degree vee and not a boxer.

BTW if you imagine the motion of the big ends as the crankshaft rotates you can see where the boxer engine gets its name. The motion of the big ends is reminiscent of a boxer punching.

Some examples of boxer engines

Porsche flat-6: 7 main journals, 6 separate big end journals
Porsche flat-8: 9 main journals, 8 separate big end journals

Some 180-degree Vees

Ferrari 512 flat-12: 7 main journals, 6 separate big end journals
Porsche flat-12: 7 main journals, 6 separate big end journals
Ferrari 312 flat-12: four main journals, six big end journals

Porsche abandoned the boxer design when they built their twelve cylinder engine. They understood the trouble they were heading into with torsionals and the like. The crankshaft would have been very spindly and not stiff at all had they persevered with a boxer layout. Ultimately the Porsche 12 developed 1150 bhp reliably. They also built a sixteen cylinder engine by adding cylinders at each end of their twelve. Projected outputs approached 2000 bhp. This one was built but never raced. As may be imagined it was a 180-degree vee.

The Ferrari 312 engine is a most clever design. This unit revved beyond 10,000 rpm yet had only four main journals. Starting from the drive end of the crankshaft, the first pair of opposing conrods shared big end journals, as in vee engine practice. The next pair of conrods were accommodated on a big end journal phased 180 degrees from the first pair. A main journal is located at the next station along the crankshaft. The next four rods are accommodated in the same manner as the first group but are phased at 120 degrees in relation to them. Likewise the final group of four is phased a further 120 degrees in advance. The pattern is best imagined as three modules of four cylinders. This engine has elements of both vee and boxer design; a hybrid. It is really a 180-degree vee since opposed conrods share big end journals.

In conclusion, a boxer engine allocates every conrod a separate big end journal (opposed conrod pairs are phased 180-degrees apart) while the 180-degree vee engine provides a single big end journal for opposed pairs of conrods (which reciprocate "in phase").

Regards

Gerald

[gryan]

In the examples above I should have stressed that b/e journals are shared for 180-degree vee engines.

Some 180-degree Vees

Ferrari 512 flat-12: 7 main journals, 6 big end journals shared by opposing conrod pairs

Porsche flat-12: 7 main journals, 6 big end journals shared by opposing conrod pairs

Ferrari 312 flat-12: four main journals, six big end journals shared by opposing conrod pairs

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[jrussell]

gryan - Thank you for that one. I learned something.
_________________
It runs best just before it blows!

[jjuutinen]

In that case I have interest in boxer engines. I have also no interest in V-engines that don´t have the opposite conrods acting on the same crank throw with either articluated or fork and blade rods. I.e. most car engines disqualify.