Simple question regarding cylinders.

[Mohit_C]

I'm a little confused over the concept of power that a cylinder has. When they talk about one cylinder being more effective than another (or let's say one of 200 HP and the other of 150HP) what parameters determine this? Is it the actual volume of the cylinder that is displaced during the combustion, the rate at which the cylinder moves up and down (speed), the relationship of compression of the fuel, the efficiency or what?

I'm talking about aviation piston engines.

Thanks.

[matt_hooks]

That's one big subject you're looking at!

Effectively what determines the ability of an engine to produce power from the chemical energy in the fuel is how much fuel it can take in, and how efficiently it can burn it.

There are two main ways of getting more fuel into a cylinder.

Firstly increase the volume, either by increasing the diameter of the bore or by increasing the stroke of the piston.

Secondly you can increase the ability to burn fuel by forcing more air into the cylinder, using a supercharger or turbocharger. The larger amount of oxygen available means you can then inject more fuel and get more power from the same volume.

(I'm not even going to go into other ways of getting more oxygen into the cylinder, such as nitrous injection, not really relevant to your average aero engine anyway)

So, if we choose to use a larger volume, we have several problems. First is weight. A larger cylinder and piston requires greater mass of metal to support it adequately. Of course, for a given volume, one large cylinder will usualy carry less of a weight penalty than several smaller ones, but that's a different story.

Some of the problems with a very large cylinder include (for an Avgas/mogas system) getting the charge to burn evenly, getting all the fuel to vapourise properly (ok I know, not ALL the fuel) and moving the fuel/air mixture in, and the burnt gases out, of the cylinder. Hence large cylinders tend to run at a much lower speed than smaller cylinders.

To get round the first of these, aero engines almost without exception use twin sparking systems. Hence the flame front starts from two points simultaneously and you get a more even burn. Hence the slightly rougher running you expect when switching to single mag systems during engine testing. In order to get the fuel/air mixture to work properly you need either a large bore carburettor or on more modern engines an injection system. With very large pistons, the speed of airflow required to ensure a proper mixture in the cylinder can get very high, and the manifold pressure (actually a partial vacuum for normally aspirated engines) can get very low. This brings increased risk of carb/inlet manifold icing and if the air supply cannot be maintained sufficiently, fuel starvation.
Even with most fuel injected systems (direct injection systems are slightly different, but not a great deal) you are dealing with very low pressures in the inlet manifold which, added to the heat absorbed by the fuel in vaporising, can leave you at severe risk of carb/manifold icing.

Of course most engines tend to use several smaller cylinders to provide the same swept volume as on larger cylinder. Now despite the weight penalty incurred in supporting structure, it is possible to get far higher power outputs from a many-cylindered engine than a single cylinder of similar volume. As I'm sure you can imagine, it reduces the vacuum in the inlet manifold, requires less of an air velocity to provide proper fuelling and can rotate much faster as each cylinder is much lighter than in the singl cylinder case.

Now in the case of turbo charging/supercharging (effectively the same thing, just the method of driving the compressor is different) you actually force the inlet manifold pressure up, so much more air can flow from the manifold into the cylinder while the inlet valves are open. This allows for much more power to be produced per cycle (four strokes as I'm sure you know, induction (suck) compression (squeeze) ignition (bang) and exhaust (blow)).

So back to the original question, what effects the power output of a cylinder? Firstly it's highly unusual to consider a single cylinder, as the combined effect of four, six, eight, ten, twelve, sixteen or even greater numbers of cylinders in the case of some of the realy large rotary/radial engines, is greater than the sum of the power-production ability of a single cylinder.

Now the power output of an individual engine is affected by the factors outlined above, as well as a few others such as the design/manufacture of the air intake system, the shape of the crown of the piston head, the shape of the cylinder top, the design of the exhaust system, the method of fuel supply, the type of fuel supplied..... the list goes on forever.

Hope some of that is helpful.

[Brian Abraham]

If you go to the library and draw out Sir Stanley Hooker's book "Not Much of an Engineer" (he developed the superchargers for the RR Merlin) he will lead you step by step through the process as to how you can calculate the horse power of an engine. Need to understand maths of course.

[matt_hooks]

Brian, indeed, an excellent book and well worth a read!

[Mohit_C]

Thanks for the replies guys, I more or less understand this now.

[Junkflyer]

An increase in rpm will mean more power as long as the air/fuel mix and valve train can keep up. Prop speed (among other things) limits rpm on piston engines. There are some geared engines (TGIO-520 in Cessna 421 and others, this one is also turbocharged) which redline at 3200 rpm if memory serves. Most similar engines redline around 2700 rpm.

[Brian Abraham]

Mohit_C - taken from Sir Stanley Hooker's book.

A=K*B/C(D-E/F)

A= Charge Weight (pounds) Comprises 93% Air 7% Petrol. For every pound/min of charge consumed 10.5 horsepower is produced. From this to find the rated power of the engine you have to subtract the horsepower consumed in overcoming frictional losses and power needed to drive ancillaries (eg supercharger).
B= Revolutions per Minute
C= Charge Temperature (degrees Kelvin)
D= Boost Pressure (inches Hg)
E= Ambient Air Pressure (inches Hg)
F= Compression Ratio
K= Constant applicable for the particular engine in question. Is related some how to the bore, stroke, number of cylinders (ie capacity) of the engine. Unable to deduce (too dumb) how the number is derived exactly, but the number for the RR Merlin is .422

[Junkflyer]

Higher compression will also yield more power from the same displacement cylinder. I believe it is stated in the previous equation, however its above my not particularly bright brain power. Higher power tends to lead to less reliability or reduced recommended tbo times.

[matt_hooks]

Junk. Not ALWAYS true that an increase in compression ratio will increase power. You get to a point where you have a maximum, and then it starts to drop off rather rapidly as you get problems with detonation. This is where the compression of the fuel/air mixture causes the mixture to explode, prior to the spark that's meant to ignite it at exactly the right time.

This gives several problems. Firstly it means that some of the power generated is actually working against you, pushing against the cylinder before it reaches the top of it's travel (TDC). Secondly when detonation occurs you have an explosion, rather than the controlled burn that you normally expect. In a normal cycle, the rate of flame propogation is much slower than that which occurs during a detonation. This means that when a detonation occurs all of the energy hits the piston in a big lump, often with disasterous results. I've lost count of the number of con-rods I've seen snapped clean in half due to detonation. And even in it's mildest form it makes for a hugely rough running engine.

[nomorecatering]

Ah the old chestnut about why are there two ignition systems and pugs per cylinder. The books say increased efficiency and redundancy.

The real reason is that Americans dint know how to;

a) design a cylinder head that actually propagates a resonable flame front

b) they cant design/manufacture a reliable ignition system, so they have to have two.

christ they cant even make engine parts that conform to their own drawings.

Messors Porsche, BMW, Mercedes, Ferrari, Lamborgini, Audi, Volkswagon etc etc have never had to fall back on flimsy engineering practices like Lycomming and Continental. Have you ever driven an American car lately??

So the text books are more of a marketing coverup, smoothing over a lack of engineering talent.

[matt_hooks]

Watchit NMC, you'll be accused of American bashing soon!

Have to agree though, design a decent cylinder head and piston crown, so that you get decent squish, and the flame propogation sorts itself out.

[411A]

Fact.

Location, San Juan Puerto Rico, some years ago, airline name, Prinair.
Aircraft, deHavilland Herons re-engined with TCM IO-520 engines.

FAA approved TBO...4,000 hours.

Now, if those RR engines that came originally with the Heron were so reliable, they wouldn't have needed to be replaced with a different manufacturers design.

Case closed.

[enicalyth]

r-r reprint

Besides Sir Stanley's book, the R-R heritage Trust do a facsimile of an internal paper drawn up in 1941 to explain why the largely empirical RAE formula did not accurately predict perfomance leading to friction between propeller designers, aerodynamicists and mechanical engineers. Most of the writers' names are well known save one poor gentleman whose tastes and predilections did not suit the times and he left. A waste of talent. I look on the pamphlet with great affection, it is easy to draft into an Excel spreadsheet allowing you to jiggle parameters and see the results. I met Bill Practice who used to design propellers and was astonished to find it was him, not perhaps his son, when I found the address by chance in a phonebook. Not far from the airfield that was his workshop and where D O Finlay, the English captain at the 1936 Olympics and later Spitfire pilot did much flight testing. Practice himself was a very gifted engineer and unassuming. After enjoying his hospitality and listening to first-hand accounts of people like Fedden, Hooker and so on I noticed a picture in the hallway which I recognised as a famous Aussie rocketry range. "Oh yes" he said "Rockets came along later, I thought you were only interested in Merlins".

[Brian Abraham]

Now, if those RR engines that came originally with the Heron

Never Sir. RR build damn fine engines, but never fitted to the Heron Sir. de Havilland Gipsy Queen 30 Mark 2 rated at 250 bhp were the original fit if you please. Later re-engined with characterless 290-hp Lycoming IO-540s, Continental IO-470s,520s, or powered by two 750-shp Pratt & Whitney Canada PT6A-34 whining turbines. Heavens Sir, you do slight us, begone with you.

had quite a bit to do with the original with Gipsys - sigh