Pros and cons of engine types?

[nrshida]

  You pair have completely ruined my awesome analogy to explain positive torque output 

If you don't pack it in I'm rolling out the graphs and the equations, then you'll be sorry 

Think Douglas Windsock going to the corner shop for tobacco on his old clunking 1920s Granddad Flyer™ and we'll be fine! 

[B3YT]

You pair have completely ruined my awesome analogy to explain positive torque output 

If you don't pack it in I'm rolling out the graphs and the equations, then you'll be sorry 

Think Douglas Windsock going to the corner shop for tobacco on his old clunking 1920s Granddad Flyer™ and we'll be fine! 

Can't help it , i run a bike repair business and my own brand of bikes .

[SIK1]

Shida I think your explanation was outstanding. The only issue I had was trying to visualize a 180* V.

My favorite aircraft piston engine of all time is the radial. I love all redials but especially the R-2800's. I love the way they look, sound and smell.

[Ardy123]

Shida I think your explanation was outstanding. The only issue I had was trying to visualize a 180* V.

My favorite aircraft piston engine of all time is the radial. I love all redials but especially the R-2800's. I love the way they look, sound and smell.

I wonder if the water cooled v12s had lower maintenance costs. I remember reading somewhere that the radials on the p-47 had a 'life expectancy' of 50 hours and after 50 hours they would just toss them and put on a new one (not exactly cost effective).

[B3YT]

According to my grand farther a hurricanes Merlin  was about 2 days to service after 30 hours flight time .

[SIK1]

I wonder if the water cooled v12s had lower maintenance costs. I remember reading somewhere that the radials on the p-47 had a 'life expectancy' of 50 hours and after 50 hours they would just toss them and put on a new one (not exactly cost effective).

Not sure I would believe that as just about every commercial airliner before the proliferation of jets was a radial.

[colmbo]

I wonder if the water cooled v12s had lower maintenance costs. I remember reading somewhere that the radials on the p-47 had a 'life expectancy' of 50 hours and after 50 hours they would just toss them and put on a new one (not exactly cost effective).

It was war, more expedient to just do an engine change than overhaul.  I bet the used engines were recycled however.

If I was flying behind one of those engines knowing that my life may depend on it's performance and knowing that the engines are being abused by combat flying I'd want the engine changed on a short service cycle as well.

When I was flying the B-17 and B-24 we were getting anywhere from 200 hours to 1200 hours on an engine....with several cylinder changes along the way.  The PW 1830s on the B-24 went through more cylinders than the Wright 1820s on the -17....don't know why.  The biggest difference between the two is that we used the turbos on the B-24 while the B-17 didn't have the turbos operational so maybe the slight extra boost used on the B-24 had an effect.  Usually we'd just notice a performance loss/rough engine and find low compression on a cylinder but sometimes they were more dramatic in their demise.  On both airplanes experienced the loss of cylinder heads...on the B-17 very rough engine, big ball of fire and smoke.  On the B-24 the only indication we had was some smoke that diminished with throttle reduction.

[earl1937]

Not sure I would believe that as just about every commercial airliner before the proliferation of jets was a radial.

You are correct! The first multi-engine commercial aircraft was the Ford Tri-motor in 1925, which had 3 Pratt & Whitney C-9 "Wasp" engine of 9 cylinders. Then the Boeing 247 was the next "big" airliner with 2 P & W S1H1-G Wasp engine which came in with 550 horse power. Then Douglas came onto the commercial airline business in 1934 with their DC-2, with a P & W GR-1820, a 9 cylinder engine, the fore runner of the famed DC-3/C-47/RD-1 aircraft. All these engines had their good and bad points, but overall, they were the ones which led the way for development of the more powerfull engines such as those used in WW2 on bombers and fighters. The largest engine ever mass produced was the P & W R-4360, which 4 of them powered the Boeing Stratocruiser, a international carrier aircraft, which was quite popular with the traveling public back in the 50's.

[SIK1]

Don't forget the Lockheed Constellation, and the Douglas DC-7.
I don't remember it, (because I was like a year old) but I'm told that my first commercial flight was on a four engine piston powered plane.

[nrshida]

Shida I think your explanation was outstanding. The only issue I had was trying to visualize a 180* V.

My favorite aircraft piston engine of all time is the radial. I love all redials but especially the R-2800's. I love the way they look, sound and smell.

You are very welcome. I have done a lot of research for a long time into piston engine design. Sorry there are no references but it is easy to check what I've said and of course read further.

A 180° V12 looks externally the same as a boxer engine. These are Porsche racing car engines but they will suffice for explanation.

Here is the crank from a 911 six-cylinder boxer engine. You can count the big end bearings, six in number (plus seven mains):



and a close up of the crank:







As you can see from the hole which supplies oil to the big end and the width of the bearing only one con rod attaches to each bearing



Now the famous Porsche 917's engine, here it is assembled, sans fan shroud and so on:





Looks no different externally from a boxer engine, however the crank is fundamentally different:





Each main bearing carries two connecting rods. This makes this engine a 180° V12 and not a boxer engine. This is significant because as stated earlier, only a 60° or 180° V12 has perfect primary and secondary balance, while any boxer engine (with an even number of cylinders) above six have this attribute. Why did Porsche then make their engine a 180°? Well it's a practicality issue basically. A high performance boxer engine usually has big end +1 number of main bearings, and the webs want to be as thin as possible. This raises crank flexing issues. I believe no one has built a boxer unit with more than 8-cylinders.



Just an addendum, the Rolls Royce and the Daimler Benz V12s have what is now an unusual con rod arrangement, instead of identical con rods which sit side by side, they have two different types one of which inserts into the other. These are often referred to as fork and blade rods:



This means each bank of cylinders aligns when you view the engine from above, whereas with the Porsche 180° V12 in the 917 and many automotive engines each bank is offset slightly. This is very apparent when peering into the engine bay of some Ferraris.



About radials, if you sit down and try to design one and pay attention to an even firing order you will see why a single bank radial has to have an odd number of cylinders. Of course two banks makes it even again. Staggering the banks allows the rear bank to be cooled through the gaps of the first but after that you have problems. Increasing the number of cylinders does make maintenance very difficult but the quest for compact power generally negates those concerns.

Regarding cooling it is interesting to note that 40% of a piston engine's heat escapes through the exhaust, regardless of whether the rest of the engine is air / oil-cooled or liquid cooled.


I wrote an article over at the Bristol Beaufighter Wishlist thread which used the Bristol Hercules radial engine. This engine used sleeve valves and apparently had three times the service interval of a conventional pushrod engine. Additionally because of the porting the engine became more fuel efficient at maximum power, something which cannot be achieved with poppet valves.

Take a look at the article and please support the Bristol Beaufighter Wishlist thread, those guys have worked very hard to gather informaiton.
Here is the thread:

http://bbs.hitechcreations.com/smf/index.php/topic,255408.885.html

Here is a preiew of the remarkable crankcase and valve gear:





I'll stop boring you all now  

 

[earl1937]

You are very welcome. I have done a lot of research for a long time into piston engine design. Sorry there are no references but it is easy to check what I've said and of course read further.

A 180° V12 looks externally the same as a boxer engine. These are Porsche racing car engines but they will suffice for explanation.

Here is the crank from a 911 six-cylinder boxer engine. You can count the big end bearings, six in number (plus seven mains):



and a close up of the crank:







As you can see from the hole which supplies oil to the big end and the width of the bearing only one con rod attaches to each bearing



Now the famous Porsche 917's engine, here it is assembled, sans fan shroud and so on:





Looks no different externally from a boxer engine, however the crank is fundamentally different:





Each main bearing carries two connecting rods. This makes this engine a 180° V12 and not a boxer engine. This is significant because as stated earlier, only a 60° or 180° V12 has perfect primary and secondary balance, while any boxer engine (with an even number of cylinders) above six have this attribute. Why did Porsche then make their engine a 180°? Well it's a practicality issue basically. A high performance boxer engine usually has big end +1 number of main bearings, and the webs want to be as thin as possible. This raises crank flexing issues. I believe no one has built a boxer unit with more than 8-cylinders.



Just an addendum, the Rolls Royce and the Daimler Benz V12s have what is now an unusual con rod arrangement, instead of identical con rods which sit side by side, they have two different types one of which inserts into the other. These are often referred to as fork and blade rods:



This means each bank of cylinders aligns when you view the engine from above, whereas with the Porsche 180° V12 in the 917 and many automotive engines each bank is offset slightly. This is very apparent when peering into the engine bay of some Ferraris.



About radials, if you sit down and try to design one and pay attention to an even firing order you will see why a single bank radial has to have an odd number of cylinders. Of course two banks makes it even again. Staggering the banks allows the rear bank to be cooled through the gaps of the first but after that you have problems. Increasing the number of cylinders does make maintenance very difficult but the quest for compact power generally negates those concerns.

Regarding cooling it is interesting to note that 40% of a piston engine's heat escapes through the exhaust, regardless of whether the rest of the engine is air / oil-cooled or liquid cooled.


I wrote an article over at the Bristol Beaufighter Wishlist thread which used the Bristol Hercules radial engine. This engine used sleeve valves and apparently had three times the service interval of a conventional pushrod engine. Additionally because of the porting the engine became more fuel efficient at maximum power, something which cannot be achieved with poppet valves.

Take a look at the article and please support the Bristol Beaufighter Wishlist thread, those guys have worked very hard to gather informaiton.
Here is the thread:

http://bbs.hitechcreations.com/smf/index.php/topic,255408.885.html

Here is a preiew of the remarkable crankcase and valve gear:

(Image removed from quote.)



I'll stop boring you all now  

 

What an outstanding post!!! I knew that somewhere in here, we had some engine experts that could explain things better than an old throttle pusher like me! Thanks again for a great, great post!! 

[LCADolby]

 I believe no one has built a boxer unit with more than 8-cylinders.

Subaru made a boxer 12 for an F1 car. If I remember right it was for Coloni, who found it to have a terrible power to weight, they decided to scrap it for v8 Fords.

[nrshida]

What an outstanding post!!! I knew that somewhere in here, we had some engine experts that could explain things better than an old throttle pusher like me! Thanks again for a great, great post!! 

    

Subaru made a boxer 12 for an F1 car. If I remember right it was for Coloni, who found it to have a terrible power to weight, they decided to scrap it for v8 Fords.

I'm fairly sure that one was also a 180° V12 Dolby. Take a look at the small offset between the banks. With a boxer the centreline of one cylinder usually aligns exactly with the gap between two on the opposing bank. This is the problem when people (not necessarily you Dolby) see a flat engine and automatically think it's a boxer. Another interesting flat 12 was the Mercedes-Benz C291 engine. They put the induction system between the cams and the exhaust out of the top allowing the engine to sit very low in the car (normally the exhaust system needs a lot of clearance).

Boxers and flat 12s are pretty wide. When the aerodynamics became a big thing in F1 it was more advantageous you use narrower Vs.

[BaldEagl]

This has been a very interesting and informative thread.  Thank you Shida for your research and insights.

What I haven't seen discussed yet is the differences in torque and power curves among the various engine types and how that plays into aircraft design.

From a very high level both are affected by displacement which is a combination of piston size (diameter) and stroke.  Inline engines typically produce maximum torque at very low RPM where V types typically produce torque at much higher RPMs.  The reason is that an inline engine can, by it's design, allow fo a much longer piston stroke.  I suspect though am not sure that both flat and radial engines would also produce tourque at higher RPMs due to stroke limitations.

That makes an inline engine an odd choice for an aircraft where it would seem moving the tourque curve up the rpm spectrum would be a better choice.

I've also not seen any discusson of valve design and am curious what types of valving were used in the various aircraft engines of WWII.  I suspect it was kept very simple in the radial engines but am curious as the v types were introduced.

[SIK1]

More good stuff Shida. 
I get the 180* V now thanks to your explanation. It's basically an opposed arrangement but the opposing cylinders share a common crank journal. Where a true opposed design each cylinder has it's own crank journal.